Three Stage to Orbit

I'm going to try to convince you [hi Jon!] that a three stage to orbit rocket might be significantly cheaper than a two stage to orbit system lifting the same size payload. My argument centers on the idea that the engines are the expensive portion of the rocket, and that tanks and structure are cheap.

To get into a 200 km orbit, a two stage rocket generally has a first stage which burns for about 150 seconds, followed by a second stage which burns for around 500 seconds. Second stage engines deliver much more impulse per dollar, primarily because the engine burns so much longer, and also because the engine has a larger expansion ratio and so a higher specific impulse.

The second stage burn time cannot be extended greatly beyond 500 seconds when going to low earth orbit, because the initial acceleration becomes so low that the stage falls back to earth before it achieves orbital velocity. Similarly, the first stage burn time cannot be pushed much past 150 seconds (assuming LOX/kerosene, for which the Isp is around 300 seconds) because the rocket must start with significantly more than 1 G of acceleration to get off the ground. This is easy to see: if initial acceleration is exactly 1 G, and the vehicle was all fuel and no structure, then the burn time would be exactly the Isp.

The point of a three stage rocket is to significantly extend the first LOX/kerosene stage burn time, and improve the expansion ratio, with the use of an earlier and much cheaper stage. To yield a cost improvement, the entire new stage must cost less than the first stage engines it is replacing. Also, the development cost of the new stage must be low.

Typically, strap-on solid rocket motors are used as cheap extra launch thrust on American or European launchers. LOX/Kerosene launchers, such as the SpaceX Falcon series and many of the Russian launchers, are so cheap that such solid rockets and their handling and cleanup are too expensive to help. I suspect that steam rockets (such as the rocket that hurled Evil Kneivel's "motorcycle" over the Snake River Canyon) may be cheap enough. Unfortunately, steam rockets have such low specific impulse (about 50 seconds) that any benefit must come from just a few hundred m/s of delta-V (realistically, around 300 m/s). The rest of this post will show that there is a very large benefit to having just a few hundred m/s of initial boost.

One way to measure the benefit of the extra stage zero is to compare the payload of a similarly priced rockets, one with and without the extra stage. Ideally, the rockets would be seperately optimized, but that makes cost comparisons looser. Instead, we will imagine that they have the same engines and differ in smaller details like expansion ratio and tank size. The baseline rocket I have chosen is the SpaceX Falcon 1, which is promised to take 570 kg to a 200km orbit for $6.7M. My simulations show SpaceX has a significant sandbag in there, and that the rocket might deliver 670 kg at the edge of its performance envelope, and it is this larger number that I compare against when calculating the benefit of extra boost.

The first comparison is between a series of identical Falcons, some with a stage zero delivering various burnout velocities, and one without. As shown, a 300 m/s boost allows the three-stage rocket to lift 41% more payload.

Falcon 1 with various starting boost

	LEO
boost	payload
(m/s)	(kg)
0	670
100	760
200	866
300	945
400	1009
500	1065

Because a three stage Falcon does not require it's LOX/kerosene engines to lift it off the ground, the engines can be configured for larger expansion and higher vacuum Isp (and thus lower sea-level Isp). Elon Musk at SpaceX has claimed that a vacuum-optimized Merlin would deliver 340 seconds Isp. In the table below, I've assumed 330 seconds, and a drop in sea-level Isp to 190. Now the three-stage 300 m/s boost lifts 61% more mass to orbit. This change carries the reliability penalty that the engines may have to be started in flight to avoid thrust instability due exit pressure being much less than sea level pressure. Note that a Falcon with no stage zero boost cannot get off the ground with higher expansion engines.

Falcon 1 with various starting boost, Isp=330 s

	LEO
boost	payload
(m/s)	(kg)
0	dead
100	778
200	946
300	1077
400	1173
500	1255

The full gain of the stage zero is realized by extending the burn time of the first LOX/kerosene stage by extending the tanks. Here I've assumed that the extra tankage and pressurant and so on weighs 4.3% of the extra propellant added. With extended tanks, the three stage 300 m/s boost lifts 138% more mass to orbit.

Falcon 1 with various starting boost, Isp=330 s, extra propellant

	LEO	extra stage 1
boost	payload	propellant
(m/s)	(kg)	(kg)
0	dead
100	866	6000
200	1288	12000
300	1595	18000
400	1814	14400
500	2039	24000

You might wonder if these massive payload increases require the use of a bigger and more powerful upper stage. The answer is no, even hefting a 1595 kg payload, the existing falcon upper stage delta-V drops from 4973 to 3454 m/s. I did not look at moving propellant from the lower stage to the upper stage, which might improve the payload numbers a bit more.

138% is a large payload increase, but we have to compare this to the cost of simply scaling up an existing rocket. Fortunately, SpaceX has done some of this work for us. Interpolating between their prices, the 138% payload boost might be sold for $1.8M more.

Which leaves the question, can a suitable steam rocket be built to heave the Falcon 1 to 300 m/s straight up for less than $1.8M per shot, and minimal development costs?

Here's what you'd need:

Mass Fraction = 80% (Shuttle SRB mass fraction is 85%, it holds 60 atmospheres pressure)

Mass, fuelled = 62992 kg

Mass, empty = 12600 kg

Volume = 100 m^3

Size = 3.6 m diameter x 10 m long

A 12600 kg welded stainless steel tank might cost $250,000, but can probably be recovered and reused with minimal work. I'm going to claim the expensive bit isn't the flight hardware, but rather the ground support, and I'll get to that in another post.

Testing New Rockets

Now that a respectful period for SpaceX's loss has passed, it's time to begin the enthusiastic but uninformed Monday morning quarterbacking. Don't be shy. Here, I'll go first.

First, I don't see why people are at all sad the thing blew up. It was a test article, and test articles break, generally with tons of instrumentation to tell you how. If you want to feel sad, feel sad for the upper stage testing guy, who won't see a video of his machine airstarting for another six to nine months. And that's what motivates the rest of this post.

Why does the vehicle have to be tested all-up? Testing all-up is gutsy and smart when your expectation of failure is very low, but wasteful if you are pretty sure you have multiple problems to find and fix. Dwayne Day has pointed out that about half of all new rockets succeed on their first launch, but this launch was not like the first launches of most of those other rockets. Those rockets had very expensive Big Government testing programs behind them. This rocket did not. That's good (because it can be more efficient), but it means that an all-up launch is not likely to yield a lot of testing data for the amount of money and time invested.

At this point, SpaceX has already paid for plenty of expensive lessons at Kwaj, so incremental flight testing might not seem necessary anymore. I'd do it anyway: they'll need it for the Falcon 9 too, and nobody knows how many gremlins remain to be flushed out of the Falcon 1 design, because they didn't get to test much of the vehicle.

If each stage carries a full load of LOX but only enough kerosene for about 100 m/s delta-V, we should get a short, locally recoverable hop, without the need for an intercontinental missile range. A set of large floats attached to the tail of the rocket might keep the engine from a complete dunking on splashdown, if that is perceived to be a problem. The lack of a range is a huge deal -- they could have done quite a bit of flight testing in parallel with getting onto Kwaj, and my guess is they're going to need plenty of launch experience before Vandenberg will let them launch there.

So the test plan, then, would be to launch two or three rockets perhaps a dozen or more times from a small island in the middle of a small uninhabited lake in the continental United States. Start by launching single stages by themselves (first and second), and then move on to two stage launches. I've read that Wisconsin wants to have a spaceport, perhaps they'd be willing to cough up the necessary permits.

While short hops are not going to test the vacuum and high-speed portions of the flight, they will test all sorts of other good stuff, many of which were tested for the first time at Kwaj (where it was more expensive):

Launch procedures, except those relating to the interface with the range and the recovery vessel. This would include things like discovering how many shitloads of LOX it takes to load the thing up.

Launch in high winds, heat, etc, by picking the time of year and using ballast. Granted, this takes time, but expanding the launch envelope is only needed once you are trying to support a high flight rate.

First and second stage structure under some but not all flight loads. Again, ballast necessary.

Payload environment in the lower atmosphere.

The staging event -- shutdown, seperation, propellant settling, ignition, interstage seperation. This is huge.

Fairing seperation, unfortunately with an aerodynamic load. The load could be mitigated by blowing the fairing at the top of an almost vertical flight, perhaps by adding enough delay between first stage engine stop and seperation that the vehicle coasts to a stop.

Some of the first stage recovery hardware, obviously not including the re-entry sequence.

Recovery of first stages -- water handling, floatation, etc.

The flight termination system in various stages of flight. Note that since flight termination is nondestructive, they can really test the hell out of it by using FTS to shut down half the time.

Reuse of first stages. This could be really big, since some lessons that might otherwise be learned might be erased by reentry.

Recovery and reuse of second stages. Yes! They are going to try to do this with Falcon 9, so why not start now? Testing this out with moneymaking operational launches sounds cheap, but you'll never get the same instrumentation or number of tests as you can have with low-altitude test flights.

I'll stop here, the list is endless. The point is that a lot of confidence can be built doing cheap flight tests away from the U.S. Government's test range.

Communication Lasers

Jon Goff notes that MIT has developed a new, more sensitive infrared receiver. The article mentions that data rates to space probes might improve as a result. Brian Dunbar wonders why I think pointing (and though I didn't mention it, antennas) are a big problem.

The location of Mars is known to great accuracy, and the location of the spacecraft is well known also. The direction the spacecraft is pointing is less well known, and the direction that the laser comes out of it is less well known also, and that's the pointing problem in a nutshell.

Think in solid angles. An omnidirectional antenna spreads its transmitted energy evenly over the whole sphere: 4*pi steradians. So, if you transmit 100 watts across 4 x 10^8 km (max distance to Mars) to a 70 meter dish at Goldstone, the most signal you can receive is (10 watts) * (goldstone aperture) / (transmit solid angle) / (distance^2). That's 100 * (pi*35^2) / (4*pi) / (4*10^11)^2, or about 2 * 10^-19 watts. You can see why the folks who built Goldstone wanted it a long way from the nearest 100 kilowatt AM radio station.

A 4 meter high gain antenna might transmit 100 watts at 20 GHz. 20 GHz is 1.5 cm wavelength, so the planar beam spread might be about (1.5cm/4m)^2 = 1.4*10^-5 steradians. The Goldstone dish will receive 7*10^4 times as much power from this dish as from the omnidirectional antenna (still a piddly 1.3*10^-14 watts). The downside is that you have to point the antenna to within 1 part in 530, about 0.5 degree accuracy. That's not too hard.

Now suppose you transmit with a 100 watt 1.55um infrared laser, with an aperture of 4cm. The beam spread is 100 times smaller than that old high gain antenna (and the solid angle is 10,000 times smaller). The Goldstone dish is now receiving 1.3*10^-10 watts, which is much better, but you'll have to orient the spacecraft that much more accurately. Which means eddy currents in the propellant swishing around in your tanks will knock the beam off. Sunlight pressure will knock it off. Worse still, differential heating of your optics will cause the beam to steer relative to the spacecraft. The fact that the spacecraft is in free-fall means that the structure between your star sensors and your maser has changed shape somewhat since you calibrated it on the ground before launch. All of which means when your computer thinks it's pointing the beam at Earth, it's acutally illuminating the Mare Iridium, and Goldstone is getting nothing.

Except the trouble is that Goldstone can't handle the infrared radiation your laser produces, so you'll need something more like an infrared telescope, which might have a 2 meter aperture instead of Goldstone's 70 meter aperture, and so you've lost three of your four orders of magnitude improvement.

Now if someone shows me a spacecraft with a multimeter telescope used to transmit infrared to Earth, I'll get seriously impressed. Courtesy of the fiber optic revolution, we now have an awesome amount of experience with high-data-rate micron (i.e. infrared) lasers. I can imagine a satellite's telescope where the same mirror is used to transmit to and also take pictures of earth. The pictures taken are used for pointing calibration, and now the stable optical structure is just the picture sensor and laser transmit head, and doesn't include the mirror at all. As a side benefit, the same telescope and imaging system can be used for very nice pictures of the probe target, i.e. Mars, although not at the same time, of course. You might even be able to do high-precision lidar (radar with lasers) surface terrain measurement.

Crackpots and Rocket Science

There's been an uptick in talk of space elevators. Here's Rand Simberg going at it. I get annoyed when I read this stuff, and lately I've been trying to figure out what it is about space elevators that I find so alarming. Unfortunately I have figured it out and don't like the answer.

I'll get the tedious bit out of the way first.

Space elevators from anywhere in the Earth's atmosphere are not going to be built for a very long time, certainly not in my lifetime, probably not ever. In short, they require engineering miracles (cheap large scale carbon nanotubes and megawatt lasers), and they do not have realistic return-on-investment (a proposed $5 billion elevator would lift one 8-ton cargo per week. 5% interest and 5% maintenance is about $10 million per week, or $625 per pound lifted, and does not include the cost of actually lifting the cargo).

And, should the above engineering miracles occur, they would enable other ways of getting to space that would be better than an elevator. You could, for instance, build a fully reusable single-stage-to-orbit rocket from large-scale carbon nanotubes that would certainly be cheaper than an elevator. All sorts of things from la-la land are possible if you make unreasonable assumptions. It's sort of like trying to figure out if a Tyrannosaurus Rex could win a fight with King Kong....

Back to the bit that alarms me.

Much of the breathless discussion of elevators is conducted by the same folks who discuss something quite important to me: cheap rocket launches. These people are clearly unable to sanely evaluate engineering propositions. In short, they are dreamers or crackpots.

(An aside: researchers who are developing carbon nanotube materials are most definitely not crackpots. That's R&D, which is a great thing. It's common for folks working on new materials to suggest outlandish uses. That's fun and harmless so long as they concentrate at their day job which is figuring out how to make the material in the first place. CNT materials, if developed, are likely to be as popular as carbon fiber is today, and find all sorts of good uses.)

Anyway, here's the bit I don't like at all: how is someone who does not know a thing about engineering (my mom, for instance) supposed to tell the difference between me and one of the aforementioned crackpots? I'm working on an upcoming post which will suggest that hot water first stages and a little aerodynamic lift could cut LEO launch prices by a factor of about 2. Like the aforementioned folks, I don't work in this industry, and am unlikely to. My suggestions are unlikely to be picked up by others in the industry. Why am I burning my valuable time on this stuff?

The answer is that I find the engineering entertaining, and I post the bits that I do because I think they might be entertaining to a small group of people who I don't bump into day-to-day. Part of the entertainment is the thought that if I noticed something really useful, I'd act on it. But it's just a thought. I like to think I have a reasonably good sense of the difficulty of making technical progress in a few fields, this one included. Frankly, I decline to make the big investment here (i.e. change careers) because I think the difficulty is too high. It is rocket science, after all.

Why Merlin 2?

SpaceX does not need to design and qualify a third, much bigger, engine in order to become a profitable launch company, or to take over payloads intended for the Shuttle, or to fly people to either ISS or a Bigelow hotel. SpaceX should concentrate on execution, development of parallel staging (Falcon 9S9), and on a regeneratively cooled Merlin. Execution includes things like recovery of first stages and getting to one launch per month, with at least one or two Falcon 9 launches each year.

In this context, a regenerative Merlin 1 (hereafter refered to as Merlin 1x) may make sense because it may improve safety, reliability, and operational costs, and the cost of those improvements may be realistically amortized over dozens of launches. As the engine cycle is more efficient, a small increase in Isp and perhaps thrust is likely.

So why Merlin 2?

Elon keeps saying that Merlin 2 will be the biggest single thrust chamber around, but smaller than the F-1. Presumably, he is implying that it will have less total thrust than existing multiple thrust chamber engines, in particular the RS-180 (2 chambers, 4152 kN). That puts a fairly tight bound on the size. Here's a list of the biggest current engines, by thrust per chamber.

Engine	thrust per chamber	Isp	Vehicle
F-1	7740 kN	265 - 304	Saturn V
RS-68	3312 kN	365 - 420	Delta IV
RS-24	2278 kN	363 - 453	Space shuttle
RD-180	2076 kN	311 - 338	Atlas V
RD-171	1976 kN	309 - 337	Zenit
NK-33	1638 kN	297 - 331	N-1/Kistler
Vulcain 2	1300 kN	318 - 434	Ariane 5

Taking Elon at his word, Merlin 2 will have between 3312 and 4152 kN of thrust. Call it 4000 kN, 17% more than the Falcon 9 first stage.

What can SpaceX do with this engine that cannot be done with the Merlin 1B?

Improve Falcon 9 LEO lift to 11000 kg. That's not even close to lifting stranded Space Shuttle payloads, which is the largest obvious market in terms of LEO mass in the next four years. I'm not sure what payloads are enabled by a 9300 to 11000 kg capability improvement.

Cost-reduce the Falcon 9. I can't see how this can pay for development, unless the Falcon 9 is launching monthly by 2010. And a further (fractional) price decrease doesn't seem like it would stimulate the market any more in the near-term than the existing, bold, price statement.

Improve Falcon 9S9 LEO lift to 30000 kg. The promised Falcon 9S9 lift capacity (24750 kg to LEO) is almost exactly that of the Shuttle (24400 kg to LEO), but the shuttle payload bay supports its payload better. The additional weight of a frame inside the 9S9 fairing might make some Shuttle cargoes too heavy to lift. A Merlin 2 based first stage for the Falcon 9S9 would fix this. The trouble is, a modest increase in thrust from the Merlin 1x would also fix the same problem. And it seems the latter change would have to be less costly than a whole new engine.

If the Falcon 9S9, improved with either Merlin 1x or Merlin 2s in the first stage, took over a dozen Shuttle ISS launches, I can imagine that would be a high enough flight rate to pay for Merlin 2 development. The trouble is that if I were paying to lift a billion-dollar ISS segment, I'd prefer to go on a machine with engine-out capability at launch. Three Merlin 2s don't give you that capability, and 4 implies some kind of vehicle (and its development cost) other than a Falcon 9S9 with single Merlin 2s on the first stages.

Build a 100,000+ kg to LEO launcher. This is what you get if you want engine-out capability with Merlin 2s. This kind of capacity is not necessary for exploring the solar system with robot probes, or even for building big orbital telescopes. It makes sense only if you want to send people a long way for a long time. Elon Musk passionately wants to build this rocket, that's why it's called the BFR.

The only organization with any credibility talking about using such heavy launches is NASA, for use in sending people to the Moon and maybe Mars. The BFR is Elon Musk's statement that he wants to take over the U.S. manned space program's launches. The business case for Merlin 2 and BFR must fundamentally rely on the U.S. government privatizing a critical, and the most public, portion of the manned space program. A program which from its outset has been about national pride.

A more likely scenario is that NASA will spend billions developing its own HLLV in competition with SpaceX, in the process abandoning the Space Station and strangling SpaceX, and will end up being able to afford just two or three launches to the Moon before abandoning VSE for the next thing. The history of heavy launchers is not reassuring. The Saturn V (118,000 kg to LEO, $2.2B per launch in 2004 dollars) was launched 13 times. Energia (85,000 kg to LEO, $1.4B per launch) was launched twice.

If all this sounds dire, a more reassuring comparison can be made by considering what SpaceX intends the Merlin 2-based vehicle to cost. Mr. Musk has stated several times that the point is to get costs below $1000/kg. A 100,000-kg-to-LEO vehicle priced at $500/kg would be $50M per launch. The history of $50M launchers is much more attractive. The various Delta incarnations (~1300/kg to LEO, ~$50M per launch) were launched hundreds of times. Soyuz (7200/kg to LEO, $45M per launch) has been launched 714 times.

A big rocket at such prices would clamp Falcon 9S9 prices to $30M and also reduce the 9S9 flight rate, at which point the parallel-staged rocket might cost too much to fly profitably. So SpaceX faces a fork in the road: develop either the Merlin 2 or parallel staging, but not both. Parallel staging will cost less to develop, implement, and support, and the BFR will cost more but enable people to get out of low earth orbit, and perhaps convert the VSE from a boondoggle into a success.

And thus we get to the vision thing. Elon has at various times stated that SpaceX is out to make money, and at other times stated that the goal of SpaceX is to enable the colonization of space. Let me point out that you can make money first, and enable colonization second, but not the other way around.

Catapult Gain

The Chair Force Engineer doesn't think much of a catapult start for rockets. His point is that if your catapult gets the rocket going fast horizontally, it just makes the max Q problem and drag losses worse.

As the rocket accelerates, the dynamic pressure on the front of the craft gets larger, until it gains enough altitude and vertical velocity that the atmospheric density drops faster than the velocity component rises. The maximum pressure experienced is max Q. Max Q is a problem because that drag turns the rocket's forward velocity into heat, which the structure has to deal with somehow, usually with a thin layer of ablatives on the nose.

A study done at UC Davis, A Study of Air Launch Methods for RLVs looked at dropping the rocket from an airplane, as is done for the Pegasus and SpaceShipOne rockets. They found that the high starting altitude helped, as did releasing the rocket with some significant vertical velocity component.

The thing that air launch and (vertical) catapults have in common is they both allow the rocket to delay fighting gravity.

As I pointed out in an earlier post, a rocket coming vertically off a launch pad is losing nearly all of its delta-V to gravity losses. Later on in the flight, once a good bit of propellant has been burned off and the acceleration has improved, a much smaller portion of the delta-V is lost to gravity. Note that the total vertical impulse required is a function of the time spent getting to orbital velocity, and is insensitive to the actual flight path.

The situation is much different for a rocket firing horizontally. Delta-V expended horizontally adds to the rocket's final velocity regardless of what the current acceleration is. Unfortunately, horizontal velocity added early in the flight adds a lot of drag loss.

So to minimize gravity losses, the rocket should start out firing at least somewhat horizontally, losing vertical velocity, and only later recover that vertical velocity. There is some limit to this pattern, since too much horizontal velocity too low in the atmosphere will exacerbate drag losses, and since we don't want the rocket to smack into the ground. Starting at high altitude helps, as does starting with some vertical velocity.

This leads to a concept I'll call "catapult gain", because I haven't read about it and don't know what it's really called. If I start the rocket off with a couple hundred m/s of vertical velocity, I increase the velocity at first stage burnout by more than the initial velocity boost, for two reasons. First, the gravity losses are lower, since the rocket can postpone some of its vertical impulse to when it is more efficient. Second, the first stage rocket can start out with more gas in the tanks, burn longer, and deliver more delta-V, because it doesn't have to have positive vertical acceleration right from the start.

Catapult gain is limited to the first few hundred m/s of velocity. The first effect is limited because we're reducing gravity losses, and increasing drag losses. There is only so much gravity loss to be mined, and drag losses rachet up quickly. The second effect is limited for the same reason that upper stage minimum accelerations are limited: eventually the extra delta-V is stretched over so much time that the gravity losses start to overcome all the extra delta-V. More starting velocity is always good, of course, but the point is that it reverts to a gain of one. To give a rough idea of catapult gain, some spreadsheet calculations indicate that a 250 m/s jump reduces the upper stage delta-V requirements of a SpaceX Falcon 9 by 500 m/s, for a catapult gain of two. Lest that seem small, note that it would allow the dead weight of the upper stage to increase by around 18%, and I'd guess the payload would increase by at least twice that.

So now that I've established a motivation for a vertical catapult, let me suggest one: a steam rocket. This is a very big bottle of very hot water at very high pressure, which partially flashes to steam as it exits. It has really crappy specific impulse (about 50 seconds), but can produce really large amounts of thrust very cheaply. For a Falcon 9 stage 0, it would be an 80 tonne steel tank (HY 100, safety factor 1.5) holding 400 tonnes of water, starting at 300 degrees C and 86 bar. The thing would produce around 26 MN of thrust for about 7.5 seconds, boosting the Falcon to 280 m/s. A 25 meter pipe, inserted up the throat of the nozzle, would add another 40 m/s by acting as the piston in a cylinder, allowing the Falcon to react against the Earth instead of mere propellants.

Let me point out how simple this system is. It has no moving parts on the vehicle, no flight-operational valves, no sequencers. The nozzle is bolted down onto a seal while the water in the tank is heated (pumped through an external heat exchanger). Fire the explosive bolts and it goes. There are no gimbals, and no guidance system. The LOX-Kero first stage is started while on the pad, and fires directly onto the top of the tank, which being solid steel backed by water is unaffected. The interstage is a truss which the engines fire through. There is no recovery system: the thing sails through the air for about a minute, coasting up to 4 kilometers high, and then comes crashing down into the sea, where it eventually resurfaces, nozzle down, until it's dragged back to shore. If we wanted to be sure it doesn't sink, we could inflate a balloon inside the casing, which would passively inflate as the internal pressure dropped during boost, and would require no pyrotechnics or interface of any kind. The exhaust is hot water and steam, has mild overpressure, requires no water suppression, nor cleaning of nasty chemicals afterwards. I will guess that the tank and nozzle can be built for $500k. Fuel costs for heating it up are about $10k, so an extensive program of test-firings with dummy rockets atop would be cheap.

I recommend tilting the launch pad slightly to ensure that the thing comes down in the ocean and not back onto the pad.

The thing vents 53 tonnes of propellant per second, which is orders of magnitude more than any chemical rocket ever built, and will produce a singular launch spectacle. So long as pictures of parboiled parrots can be avoided, the PR value alone should be immense. As a side benefit it should allow the Falcon 9S9 to put more payload into orbit than a Delta IV Heavy, and maybe enough more than the Shuttle to allow it to lift Shuttle ISS cargoes with an added strongback.

EDIT NOTE: In an earlier version of this post I claimed a doubling of throw weight. Math error. That's what I get for posting after midnight.

Good news

Delta IV has been canned in favor of Atlas V. I predicted it would go the other way, because the DoD has yet to launch on Atlas V. I had presumed there was some sort of issue with the Russian (Ukranian?) origin of the engine. I now presume the issue has been dealt with somehow, and that's good news all around.

So now, does the Air Force think it's safe from a Boeing/LockMart monopoly because SpaceX is coming along well enough to scare them into better prices? Or is the Air Force figuring that keeping the Delta IV on life support is going to be more expensive than monopoly overcharges on Atlas V?

And, for those who might protest at me treating the whole thing as a spectator sport, let me point out that it costs me something like the cost of a movie ticket every time they launch the Shuttle or Titan IV.

Other good news, might be old though: John McCain is being clear about our foreign energy dependence problem, and how to deal with it: build nuclear power plants. Of course, McCain is also in favor of hydrogen-fuelled vehicles, which is a boondoggle as far as I can see.

Men in Space, II

I thought I'd learned a long time ago that if you're going to write in public, you need to start with a large backlog of already written pieces, so that when a dry spell comes (christmas vacation or whatnot), you can post the prewritten bits and not lose your audience. Well, you have to remember to post those prewritten bits.

I just read Giving future human space explorers the credit they're due. I think it nicely, if accidentally, summarizes the pitiful reasoning behind manned space exploration.

Mr. Kendall's main point seems to be that humans can cover more difficult terrain, and more of it, than robots. Fine, we'll build a rover with legs. This is orders of magnitude cheaper than sending people to Mars. Note that people on Mars are going to need vehicles to get any great distance anyway, so I suspect there is no real advantage for people here.

The Mars rovers have driven only kilometers because they are small, low power vehicles. If we want to spring for more payload for a faster vehicle, we can do that without also spending orders of magnitude more for a person to ride on that vehicle.

Second point: people can be more detailed than robots with the same instruments. Given the vastly superior stay-time of robots, I find this unbelievable. His point about people staying 500+ days on the Martian surface presumes a very expensive mission.

Third point: Excavating partially buried rock, drilling hundreds of meters... These seem like things not done by people but by their equipment, and are an excellent argument for robots.

Fourth point: repairing broken parts, creating new tools. How? With what? In a machine shop? How much will that weigh?

Fifth point: pilots can step in if mission control miscalculates. No way. No pilot is going to override a reentry burn successfully. What data would he base his intervention on, that mission control does not already have?

Sixth point: cultural importance, human triumph. Now we're back to "footprints in the dust". This is bogus. Footprints are symbolically important when they are the first footprints of many. When we have a reason to put people on Mars, the first footprints there will be (temporary, due to wind) symbols of our purpose there. What are the lunar footprints symbols of? Peace?

Seventh point: educational importance. How do we make Mars interesting enough for a fourth grader to pay attention? Step 1: put in a sense of scale by placing a person in the picture. This helps, but it's not worth the money. I have a better suggestion: concentrate on movie cameras that take pictures that people care about. Make sure the mission definition team has an experienced, professional nature photographer with sufficient clout to overrule science goals if necessary and ensure that the mission returns great TV.

I think we're missing movies of robots in action. Taking pictures of the exploration itself is what communicates the triumph to the taxpayers footing the bill. We need movies of rovers drilling rock and driving around. We need movies of assembly robots piecing together large orbiting telescopes. We need movies of Cassini cruising through the rings.

Movies aren't usually included because the bandwidth required is appalling, and wouuld swamp the science returned. This is not a problem anymore. The cameras and recording system can be lightweight and solid state. They should be standard NTSC resolution, and use compression like any non-science TV show uses, as the point isn't to enable some scientist to make verifiable conclusions by closely examining the pixels. Finally, the cameras can shoot dozens of hours of footage, store it locally in a few hundred gigabytes of flash memory (lightweight and cheap nowadays), and the probe can send back a tiny fraction of that selected by a movie director.

The other problem with movies is that there hasn't usually been a vantage point from which to film the robot. But this need not be the case either. The Deep Impact mission actually had two probes. The one that survived could have sent back a nice high-res, slow-motion movie of the moment of impact in the months after the mission was over. The Mars rovers all had landing vehicles that could have shot movies of them leaving the lander. Heck, I think it would be reasonable for the rovers to cart around a monopod with a remote camera on it. Find a nice rock, stick the monopod nearby, then drive over and look at the rock while being filmed from the monopod. When done, pick up the monopod and drive on.

Slow Launch

Every time I see a rocket vault off the pad, I wonder why the engineers didn't make the propellant tanks bigger. Adding propellant to any stage always increases the delta-V delivered, so long as you ignore gravity losses.

It takes a lot of thrust and energy expended just to get the rocket to hover stationary above the pad. The delta-V lost is called the gravity loss. As the rocket gets closer to orbital velocity, gravity losses drop. At low velocities, gravity losses are just time * acceleration by gravity.

Gravity losses, as it turns out, are a big deal. As you add propellant, you logarithmically increase the delta-V, but you linearly increase the gravity losses. At some point, gravity losses overtake the increased delta-V.

The tradeoff between the two is different for different kinds of rockets. In a solid-fuelled rocket, the entire stage is a big combustion chamber, that must contain the gas pressure used to accelerate the vehicle upward. Large pressure vessels are heavy, and so the fuel container is a large fraction of a solid rocket's mass. Pressure-fed liquid-fuelled rockets don't actually have the combustion chamber in the tanks, but the tanks must hold higher pressure than the combustion chamber, so the mass penalty is similar.

Pump-fed liquid-fuelled rockets hold their propellants at a small fraction of the combustion chamber pressure, and so their tanks are a small fraction of the weight of a similarly-sized solid rocket.

Let's take a look at the two extremes. First, a kerosene/LOX liquid-fuelled rocket, like the Saturn V or Falcon 9. I've made up a table to show the decreasing performance return of steadily larger and larger tanks. Here I've presumed a base rocket with an Isp of 290, a first-stage thrust of 750,000 kg, a Gross Lift-Off Weight (GLOW) of 500,000 kg, and a first stage burnout weight (this includes the upper stages) of 150,000 kg. This rocket has an initial acceleration of 1.5 G (but remember you lose 1 G to earth). Incremental tankage weighs just 2.5% of the incremental propellant stored. I'm assuming that the engine thrust stays fixed. Delta-V numbers are in meters/sec. Tower clearance times are a little high, as they assume no acceleration beyond the initial acceleration.

Isp	290
Ve	2842
Tankage	0.025
Thrust	750000
Burn rate	2586
Tower	50
			inc	inc		delivered	tower
G	Mf	Me	delta-V	time	G-loss	delta-V	clear
2	375000	146875	0	0	0	0	3.2
1.9	394737	147368	136	7	73	63	3.4
1.8	416667	147917	143	8	81	62	3.6
1.7	441176	148529	151	9	91	60	3.8
1.6	468750	149219	159	10	102	57	4.1
1.5	500000	150000	169	12	115	53	4.5
1.4	535714	150893	179	13	132	47	5.1
1.3	576923	151923	191	16	152	39	5.8
1.2	625000	153125	205	18	178	27	7.1
1.1	681818	154545	221	21	210	11	10.1
1	750000	156250	240	26	252	-12	N/A

You can see why the Saturn V initial acceleration was just 1.13 Gs. You can also see why launching the Saturn V in a strong wind could have been a problem: it takes a long time to get past the tower.

Now let's look at the other extreme, a solid rocket first stage, like the Titan 4 or the Stick proposal. (Because most of the Shuttle's liftoff thrust is from it's solids, it fits in this category too.) Here the Isp is a bit lower, but more importantly the tankage fraction is far higher: 12%.

Isp	242
Ve	2371.6
Tankage	0.12
Thrust	750000
Burn rate	3099
Tower	50
			inc	inc		delivered	tower
G	Mf	Me	delta-V	time	G-loss	delta-V	clear
2.5	300000	126000	0	0	0	0	2.6
2.4	312500	127500	69	4	35	34	2.7
2.3	326087	129130	71	4	38	33	2.8
2.2	340909	130909	73	4	41	32	2.9
2.1	357143	132857	75	5	45	30	3.0
2	375000	135000	78	5	50	28	3.2
1.9	394737	137368	80	6	55	25	3.4
1.8	416667	140000	83	6	61	22	3.6
1.7	441176	142941	86	7	68	18	3.8
1.6	468750	146250	90	8	77	13	4.1
1.5	500000	150000	93	9	87	6	4.5
1.4	535714	154286	97	10	99	-3	5.1

You can see why a Titan 4 gets off the pad with nearly 1.5 Gs of initial acceleration.

Solid rockets are a good match for first stage engines. It takes relatively little engineering work to produce a large amount of thrust from a solid motor, which is the biggest cost driver for first stage engines. But because the casing weighs so much, and also because solid propellants have lower Isp, the delta-V of a solid first stage is never going to be as good as a liquid-fuelled analog. That leaves more work for the second stage, which means thinner engineering margins and more desire for high-Isp propellants, like liquid hydrogen. And so, cheap solid rocket first stages drive more cost into the upper stage.

Launching in 50 MPH winds

I was a little curious about the idea of launching in 50 MPH winds. Could the vehicle be blown into the tower?

The Falcon gets off the pad with something like 1.3 Gs of acceleration, so to clear a 21 meter tower will take about 2 seconds.

Plain horizontal translation is not enough to worry about. I'm not sure what the horizontal drag coefficient of Falcon is, but it might be around 0.5. Assuming that, a horizontal 50 MPH wind would accelerate the vehicle laterally at about 0.3 m/s. By the time it gets off the pad, that's a little over half a meter.

One wonders how much the vehicle sways under wind. If the nose was deflected a meter and a half towards the tower when the holddown clamps release, that would add a meter of translation by the time the rocket clears the tower. I think the tower is more like 2 meters away, and I find it pretty hard to imagine pushing the launch button on a vehicle whose nose is whipping around anything like the complete distance to the tower.

So, no problem, but I thought the numbers were interesting so I'm posting them.

P.S. I think the SpaceX LOX boiloff solution of insulated panels velcroed onto the rocket, that get ripped off at launch, is cheap and cute. But this isn't going to change their procedural problem that they need to unload a bit of kerosene if they need to sit on the pad for any length of time. It was an error during the unloading of kerosene due to a launch delay that caused the tank damage.

And I'll bet those panels are going to get blown to bits when the rocket blast hits them. Not that it matters a lot, but they are going to get polystyrene bits all over their lovely tropical isle. If they ever want to clean that up for a VIP visit it's going to take a lot of time, and where are they going to find low-wage workers out in the middle of nowhere?

And finally, I think SpaceX gets my vote as best nerd entertainment of the decade. At some point, some producer is going to realize that real-time updates on spectacular engineering projects are better than "reality" TV programs, and then I'm going to have to start watching TV again. I wonder how much the buzz they're getting is worth, in term of popular support when they need to negotiate with the government?

Interplanetary Superhighway

A nice read about orbital mechanics.

Solar Heating Lowers Construction Costs

Solar Energy is a grim business. Usually, folks who decide to buy a solar system make some sort of projection about their fuel or electricity costs and then figure out how much the solar system is going to cost. The trouble is, these comparisons take thought, and are dependent on unknowns like the rising future cost of power, effective interest rates in the future, and structure depreciation. No wonder solar systems are a hard sell.

Well, things are changing in sunny California. Our Title 24 law sets energy efficiency standards for most buildings. A new revision of this law has come into effect as of October. Among other things, the law sets standards for window thermal resistance.

Essentially, California has mandated that you are going to save energy, relative costs be damned, but at least you get to pick how to do it. I could write a blog entry on how I feel about this kind of regulation versus simply changing the price of energy... but no. Today I want to talk about the solar industry.

The big news for the solar industry is that instead of comparing solar collectors to just burning fuel, people now get to compare solar collectors to better insulating windows. It's a much easier comparison, since you pay for both things when you build the house, and solar collectors come off looking very good in comparison. When the tract home developers figure this out, they're going to slap solar panels on any place they'll fit.

How's that? Well, consider a standard 4.5 foot by 3.5 foot sliding glass window.

A good aluminum-framed version will cost a few hundred dollars, and have a U-value of 0.61, which means that it leaks 0.61 BTUs an hour, per square foot, per degree F temperature difference. In San Francisco, where we have 3458 degree-days of heating each year, that's 797,345 BTUs a year -- about ten bucks.

As of October 1, California requires a U-value of 0.38 on most windows. These are pretty nice windows. To get that U value, you need double panes of low-E glass with argon fill, wood or vinyl frames, or high-end aluminum frames with thermal breaks. This window will cost a couple of hundred dollars more than the aluminum-framed window, and it'll leak about 300,000 BTUs a year less than the aluminum-framed version.

So here's the thing. A rooftop solar thermal system can supply those 300,000 BTUs a year for about $120. (That $4068 kit with 2 Gobi 408 collectors will pull down 80 to 120 therms a year.) And California will let you use inexpensive aluminum windows if you have a solar thermal collector to supply the extra heat.

Now some people are going to want the wood-framed window look, and so for those people this comparison doesn't much matter. But developers are the kind of folks (cheapskates, and I use that word endearingly) that will save a couple of bucks per house by using skinnier electrical wire. Saving $100 a window while being able to sell the resulting house as "green" is a fad they can embrace with enthusiasm.

But developers are also the kind of folks that prefer to suceed doing things they've seen other people succeed at. So, if you live in California and are building a new house, do your part and stick on a solar thermal collector. It won't take long to get the fad going.

Why Merlin?

My anonymous commentor is getting at a very interesting point. Why does SpaceX do their own engines when clearly better engines are available from Russia? The Russian engines have far better Isp, burn the same practical fuel mix, are available for known and probably reasonable amounts of money, and are known to work now (which takes a lot of schedule uncertainty out of SpaceX's plans).

Using Russian engines was Kistler's plan. Kistler spent five times as much money as SpaceX has without building a complete vehicle. Maybe it was their recovery system. Maybe it was their basing plan.

I suspect that using a Russian engine puts a big Chinese wall in the middle of the company, for both intellectual property and ITAR reasons. Enough of the vehicle design is tied to the engine design that what you end up with is a company that, to a noticeable extent, resells Russian launch services at the whim of international relations.

How big a deal can this be? The Atlas-V uses Russian engines, and is intended to be used by the military for sensitive launches. If Lockheed can do it, why not SpaceX?

According to Astronautix, Atlas V has launched 4 communication satellites, only one of which was a U.S. satellite, which was commercial. By comparison, Delta IV has launched 2 U.S. military commsats and a european commsat. My sense is that the U.S. military is averse to relying on a launcher using unsubstitutable components from a major overseas competitor, and funds the Atlas solely as a backup to the Delta.

I think SpaceX knows it will be dependent on launching U.S. military payloads, and knows it can't do that with Russian engines.

If SpaceX is sucessful, I expect an EELV, probably Atlas, to get cancelled by 2010. Delta will become the backup launcher, subsidized by the military and flying in very low numbers.

And seven years from now, I think the boost competition will not be United Space Alliance versus Ariane versus SpaceX. I think it will be SpaceX versus at least one Russian company (perhaps marketed by a western company e.g. SeaLaunch) versus the Chinese. SpaceX will get the U.S. military business by default. To win the international competition, SpaceX may need to figure out reuse.

Dry Launch 2: LH2 vs Kerosene

I had been expecting that LOX-LH2 would be less expensive than LOX-kerosene for a LEO-out boost.

At some goading by "anonymous", I reran the numbers assuming a different mission profile. (I really appreciate good criticism, by the way. Thanks.)

This time, lets assume SpaceX sends up Falcons whose top stages have been stretched to carry just extra LOX. The first mission leaves the top stage in orbit, later missions lift extra LOX to it, and we end up with a big LOX tank in orbit.

The LEO-out booster is launched from an EELV, e.g. Atlas. The idea is to use something that uses the dual-engine Centaur upper stage. You might want to consider a Falcon-Centaur, but then you have to deal with paying for all that integration and LH2 infrastucture when Boeing and Lockheed have already done it for you. My guess is that until there looks like a solid market for LEO-out boost, SpaceX will have nothing to do with Centaur.

Here are the numbers: An Atlas 551 can put 20,050 kg into LEO. If we assume an incremental tankage ratio of 25 (like the Space Shuttle external tank), we get a modified Centaur with enormously stretched tanks, 5.9x for the hydrogen tank, 4.9x for the oxygen tank. The Centaur launches with a full hydrogen tank and a nearly empty oxygen tank. After achieving LEO, the oxygen tank is empty and the hydrogen tank still has 83% of it's initial load. The Centaur picks up 86,240 kg of LOX from the Falcon tanker. This Falcon would have loaded LOX from 9 other Falcon flights. The stretched Centaur would then be able to add 4000 m/s delta-V to a 58680 kg payload. Burn time: 73 minutes.

That's a bit more than a third of the max payload of my earlier LOX-kerosene all-SpaceX proposal. Cost would be $270 M (SpaceX), perhaps $200 M (Atlas). Interestingly, the cost per kg ($8000/kg) isn't different from the all-SpaceX proposal ($8035/kg).

This didn't come out like I thought it would. First, note that the LEO-out booster performance is not terribly sensitive to the controversial tankage ratio, because the empty booster doesn't weigh all that much compared to it's payload, even if the tankage ratio gets cut in half. That's because 4000 m/s is not an amazing amount of delta-V if you have all day to burn your propellants. Maximum payload is affected, of course, and that has a big impact on the LOX-LH2 cost.

The fully fuelled LOX-kerosene LEO-out booster is heavier than the LOX-LH2 booster, but this big disadvantage is neatly balanced by the high cost of ground-to-LEO launch of the hydrogen.

The LOX-kerosene proposal assumes transfer of both LOX and kerosene, where the LOX-LH2 proposal requires the transfer of just one. My assumption is that the extra bother of kerosene transfer is not a deal breaker.

Here is a deal-breaker though: SpaceX uses an ablatively cooled combustion chamber and throat. Regenerative cooling is probably a requirement for this application, due to the ridiculously long burn times. I'm sure that regenerative cooling would be a major development headache for SpaceX. This makes a LOX-kerosene LEO-out booster unlikely unless SpaceX wants to develop long-burn engines for some other reason.  [Update: SpaceX has switched to using regeneratively cooled Merlins, so no deal-breaker any more.]

A LOX-LH2 LEO-out heavy booster would probably be better if a high-volume operator (like SpaceX hopes to be) were to pursue it. Assuming just a few uses, the choice is trickier. The LOX-LH2 option has less technical risk, as it requires no engine development and transfers just one propellant.

Dry Launch

Jon Goff has posted the idea of launching a LEO-to-wherever booster into LEO, and then fuelling it with subsequent flights. This makes me wonder how big a dry launch / on-orbit refuel booster can SpaceX put up, with just Falcon 9?

First, we need a bunch of assumptions.

Let's assume the complete but empty booster has to go up in one flight, so no seperate launch of fuel and oxidizer. Seperate tanks should almost double the resulting booster size and launch/fuelling cost. Using a Falcon 9S9 with the big strapons would nearly triple the booster size and launch/fuelling cost.

Assume insulated tanks are 2% of fuel mass. Payload size is highly sensitive to this number, which I just pulled out of a handy nearby orifice.

Assume that both the fuelling rockets and the LEO-out booster are just Falcon 9 upper stages outfitted with larger tanks and (in the booster's case) lots of insulation.

Assume that without the fairing and payload interface bits the Falcon 9 can boost 10,000 kg to LEO. Assume 300 kg for fuel transfer hardware.

The resulting super-stretched Falcon upper stage / LEO-out booster should hold 475,000 kg of propellant, making it about twice the size of the Falcon 9 lower stage. If built with the same tank diameter (3.6m), it would be about 56m long. At launch, the whole thing would be about 89m long, with an aspect ratio of 25, which is pretty skinny but probably doable.

Such a booster could give 4,000 m/s delta-V to a vehicle with a empty mass of 168,000 kg. That's enough delta-V to get out of Earth orbit to L5, low moon orbit, and at least near Mars, see this handy cheat sheet. It's also just a bit more mass than the Saturn V rocket put into Low Earth Orbit -- not excessive for a manned excursion to anywhere dramatic.

It would require 49 fuelling flights, with a total cost (just for the booster) of $1.35 billion dollars with SpaceX's published prices. SpaceX's prices are for single launches, assuming no hardware recovery/reuse. 50 total launches would get them a lot of experience with recovery, and change the whole cost dynamic. It would also take at least of year of fairly amazing launch activity.

An equivalent booster, launched with the Shuttle-Derived Heavy lifter, would require about 3 launches, as it uses liquid hydrogen, which really helps this application. Shuttle launches are hard to account for, but probably around $500M each, and if the SDH is similar, it would cost $1.5B for this booster, not counting amortized development costs.

I don't think anyone would start an Artic expedition that required 50 plane trips just to build a cache of dog food, so I don't think this version of the idea is going to fly. Perhaps liquid hydrogen is inevitable for this application. Alternatively, SpaceX's Merlin 2-based launcher may get launch prices down enough that Kerosene and LOX make sense even for a big out-of-LEO booster.

[Note: the next post answers comments.]

Gun Launch

It's said that people never publish negative results. Well, in response to a comment on the previous post, here's mine.

Most every engineer who has looked at a rocket launch has been appalled at the horrible efficiency of a rocket coming off the pad. At the moment of launch, the rocket is using as much propellant per second as it can, throwing away nearly all that energy in gravity losses, and getting very little velocity with the rest. Almost anything can convert propellant energy into projectile velocity more efficiently than a rocket just off the launch pad.

One of the best things, it turns out, is a cannon. A cannon with reasonable muzzle velocity uses the earth or the cannon as the reaction mass, where a rocket just uses the propellant itself. More mass means more efficiency. So many folks have wondered, why not use a cannon to launch a rocket?

It's been done, of course. Back in the 1960s, and 1970s, and finally 1980s, Gerald Bull looked into using cannons to launch rockets. I've looked at Gerald Bull's work. His is a sad story -- the guy got so taken by the promise of his research that he eventually let his ethics slide. When the U.S. military cancelled his funding, he started doing work for nearly anyone who would pay for his research. Near the end, he was doing work for Saddam Hussein, at which point he was assassinated, maybe by Israel, but maybe not.

But nevermind about Mr. Bull, what about gun launch?

Cannons convert propellant energy into projectile kinetic energy well when the muzzle velocities are well below the Ve for the propellant combination -- around 2000 m/s for solid propellants, and 3000 m/s for liquids (liquids have better oxidizers, see wikipedia). Most guns have muzzle velocities much smaller than that, so you see very good payload/propellant ratios, as my commenter "fred" stated. (Fred should check his numbers, though. A 1000 m/s rifle round has a payload/propellant ratio closer to 2 than 100.)

Low-energy gun launch has been done too. An MX missile launch from a silo starts by blowing the missile 100 feet out of the silo with compressed air, and then air-starting the rocket. This is done to make the silo cheaper, not to give the missile any substantial delta-V. One could conceivably boost a satellite launcher this way as well, but the delta-V is probably limited to 100 m/s or so, which isn't enough to help the rocket much. And it's certainly less safe than starting a set of liquid-fuelled engines while bolted down, checking for correct operation, then releasing.

High-energy gun launch only makes sense for bulk payloads like fuel. High value payloads like comsats and people are never going to take the accelerations from any cannon anyone can afford to build. But still, one wonders, maybe just for bulk loads....

Guns really get expensive when muzzle velocities approach Ve. Essentially, the propellant gases run out of energy by the time they expand enough to act on the base of the projectile. To even get close to Ve, the gun has to accelerate a good fraction of the propellant before it burns, and now we're talking about a rocket inside a gun barrel. And we enter the realm of rocket-like payload/propellant ratios, but with a very nasty twist.

Lots of propellant isn't a problem by itself (the propellant is cheap). The problem is processing all that propellant. More propellant requires more stuff (plumbing, pumps, thrust chambers, or gun barrel) to contain and process it. Launch guns process all the propellant in a very short period of time (tenths of a second), which means they need two orders of magnitude more stuff than a rocket to process the same amount of propellant. Sure, it's simple stuff (a gun barrel), but there is just too much of it.

A few years ago I thought I'd had a clever idea here. Since a gun doesn't have to contain the blast for very long, I figured you might be able to build a thin barrel and then immerse it in the mud at the bottom of a lake (or any other high density fluid). When the gun fires, the barrel attempts to explode, but to do that it has to move the mud out of the way. This is inertial confinement: the mud can't move away fast enough, so instead it compresses. The gun propellant deflagration turns into a pressure wave in the mud, which can be arranged not to crush the barrel after the barrel pressure returns to normal. You get to make a big cheap gun, that is horribly difficult to point at different things, and so pretty much worthless as a weapon. Sounded good to me.

I wrote some simulators, and faced the following choice: after gun launch, would the projectile have one stage or two? (I assumed liquid propellants. Bull used multiple-stage solid propellant rockets.)

If the projectile is a single stage, then you need a muzzle velocity of at least 2500 m/s. That's fast enough that the round inside the gun ends up looking like a big solid rocket -- one that burns it's fuel in a fraction of a second. Since solid fuels burn at centimeters per second, you end up with a highly perforated fuel grain and a very nasty ignition problem (all that surface area has to simultaneously jump a thousand degrees C in a few milliseconds). Also, this fuel grain itself has to survive hundreds or thousands of Gs of acceleration, and to do that you end up using carbon fibers as the fuel rather than HTPB or something more standard (and higher energy).

Then comes the brutal comparison. If you were going to do all that work to put the rocket in the gun barrel, how does it compare to just having an ordinary solid rocket first stage? Answer: the ordinary stage has way lower capital costs, isn't much bigger, and doesn't subject the payload and avionics to brutal accelerations.

Okay, so what about a medium-energy gun lauch of a two-stage rocket? Starting with 1000 m/s or so really does help the payload quite a bit. But the massive acceleration requires building that rocket to be very sturdy, so most of the payload ends up being structure. And a two-stage liquid-fuelled rocket can get to orbit just fine without a gun launch to start, while being so much simpler.

In short, rockets may seem horrible coming right off the pad, but they don't spend much time at low speeds. They get up to canon-muzzle-velocity-like speeds in 30 to 50 seconds, and then they get to keep using that engine hardware to go a whole lot faster.

The best optimization yet found for the first 30 seconds of flight is strap-on solid rocket motors: they're cheap, fairly reliable, small, and require few changes to the support infrastructure. If the rocket has some delta-V margin and positive acceleration even if the strap-ons fail to ignite, then failure of one of the strap-ons can be survivable.

False choice

You don't have to be an Evangelical Christian to be fed up with the way science is taught in the primary and high school classroom these days.

Science is different from Belief. There is a process to science. Some folks propose hypotheses, with which anyone can make predictions (otherwise the hypotheses are useless and mostly ignored). Those predictions are then tested.

Over time, we get some confidence in old theories that haven't been proved false. Confidence comes from checking theories on your own, and it comes from confidence in other people checking theories. We develop confidence in the journals where their papers are published from the occasional well-publicized mistakes. People who cheat are found out, and that news gives you confidence that the system is checking claims being made. Some scientists make great names for themselves, but their theories are still checked and contrary evidence is received with skepticism, but if validated, fanfare.

New science has not yet earned this confidence. For example, string theory is not yet well accepted, but much of quantum mechanics is. It's fun, and the continuous stream of claims and counterclaims is not surprising once you understand that these are younger and untested theories.

Belief, on the other hand, is not about making falsifiable predictions. Belief is about understanding yourself, what you should and should not do. It's about morality and emphathy.

To science, there is no difference between Mendel experimenting on peas and Mengele experimenting on Jews. It's just data (in both cases, unscientifically falsified to try to prove a point). There is a huge moral difference, of course, and that's where Belief comes in.

The trouble I see is that most science education doesn't teach science. Instead, science education teaches something like Belief in scientific theories. The result is neither science nor religion, but a glib materialism that assaults religion and cherishes a lack of critical thinking. I think many conservative religious leaders are right to think that science is being taught as an alternative to religion. Many people, me included, would like to see this kind of teaching banished from the classroom. I'd like to see reporting with the same mindset laughed out of the mainstream media while we're at it.

That doesn't mean I want to see equal time in the classroom for Intelligent Design. I know Intelligent Design is not well accepted. More worrisome, I have heard of no actual hypotheses with which independent labs can make testable predictions. The alternative theory, evolution, has a long productive history, during which many papers have been written and a great deal of elaboration and independent verification has taken place. There is no such productive history in the scientific literature for Intelligent Design, and so I have no confidence in it.

Religious leaders appear foolish when they attempt to construct alternative science, because they appeal to authority rather than verification. Usually this happens when people interpret ancient texts too literally. Genesis says the world was created in 7 days several thousand years ago, and all the animals were created at one time. Unfortunately for literalists, we have a lot of physical evidence that says this planet was created 4.5 billion years ago, and has experienced an evolving population of fauna subject to periodic mass extinctions. I have confidence in much of that evidence and the generally accepted scientific interpretation of it.

On the other hand, Jesus is reported to have said, "do unto others as you would have them do unto you". This shows up in the mathematical literature under game theory, which frankly hasn't given me a whole lot of insight or motivated my compassion for other human beings. But I've always enjoyed listening to sermons on this kind of thing, I love the singing in church, and I often find I have a refreshed appreciation for the condition of others after a good service.

As an aside, I don't really understand this need to interpret texts so literally. The need for continuous discussion and contemplation of morality is not based on the number of days it took for the world to form, either according to physical laws or divine will expressed any other way. The need for morality comes from our being intelligent social creatures living in crowded conditions.

If we are to have a debate about the seperation of Church and State in the classroom, let's have it about what gets taught in Civics. Civics teaches what you ought to do and ought not to do in civil society. It requires us to agree on a common set of beliefs. Since many religious leaders around the world regard the existence of other religions as anything from irritating to blasphemy to a justification of murder, this is clearly an area deserving of robust public debate and continuous refinement.

Meantime, let's see some real science being taught in the science classroom. Kids should be taught to check theories, but they should also be taught to check primary sources in the literature. They should learn about the process, read reasonable-sounding theories that were disproved, and how. This way we can grow their confidence in older and more accepted theories. This way they can learn to listen with skepticism to the endless litany of new claims brought forth by medical researchers testing small groups. This kind of education can help prepare them for the world.

Men in Space

I read the Washington Post editorial. They don't like the costs and lack of results from putting people in space. I think they don't much like space exploration at all.

So, why astronauts?

Science. Some claim that people can do things that robots cannot. It's true. But it's also true that robots have staying power, and people do not. Right now, the balance is overwhelmingly on the side of staying power. Michael Griffin himself has admitted that space science not focussed on people is best pursued without people.

Consider: Apollo used a noticeable fraction of the U.S. GDP to put six pairs of men on the moon for a total of a couple of man-weeks. For three to four orders of magnitude less money, the Spirit and Opportunity Mars rovers have gone much farther, and seen more, and are still working away more than a year after they landed. Apollo returned samples, which we have not yet done from Mars (but have from deep space). The thing is, sample return does not require people.

The choice is not robots versus people, it's robots versus robots-and-people. Consider just the fingers of a glove of a space suit suitable for use on the moon. The stiffness from the pressure difference is so high that the hands of serious athletes are cramping and exhausted within hours. Next generation space suits may use mechanical hands outside the suit, operated from within. Such waldoes are already standard practice in many kinds of surgery, deep sea research subs, and some hazardous materials operations. Notice that the control for that waldo can go in the space suit, or in a vehicle, or in a spaceship, or in an office in downtown Houston. One of those doesn't require a mission-critical life-support system.

In fact, there is no choice. There will be robots now, and there will be robots and people later. In between, we can launch people if we like, but they won't do much.

Prestige. Which event gained more international and domestic attention: Shuttle mission STS-113 to the ISS (which installed the P1 truss), or the Deep Impact comet probe rendezvous? Which cost more?

The problem with manned spaceflight right now is that the next step is really hard. It will be a long time before we are able to do something obviously new and exciting. But for unmanned probes, there are dozens, perhaps hundreds of interesting places to go, all within reach of rockets we have today.

Destiny. Sigh. The claim here is that humans will eventually populate other places, so we should do it now. I'm really bothered by this kind of reasoning. Karl Marx said that communism was the inevitable end of the evolution of nations. When the inevitable seemed like it was taking too long, Lenin and Stalin tried to hurry it up a lot, eventually by starving the peasants, destroying their economy, and nearly destroying the Russian culture.

Our destiny will arrive on its own schedule. People will work in space when it makes economic sense to do so. Eventually those workers will have significant needs and desires of their own, best satisfied in space, and... off they go.

NASA's Mission. NASA's original mission was not to launch people into space. NASA's original mission (as NACA) was to do basic research on atmospheric flight. The goal of this research was to develop technologies that, while potentially useful to the U.S. as a whole, would not be pursued by private companies that need predictability and exclusivity to satisfy shareholders.

When the U.S. and the Soviet Union began competing for world mindshare by pulling ever-larger stunts with rockets, NASA got the job of putting men on the moon (and returning them safely).

The prestige mission of men in space can be retired, just as the B-1 bomber fleet was retooled to carry conventional weapons instead of nuclear ones. It was useful, and it is over. NASA should get back to doing basic space science. Telescopes, nuclear power and propulsion, extraterrestrial geology, that sort of thing. They will eventually find the killer app up there. They have not yet, but there is no need to be impatient. Meantime, we should enjoy the pretty pictures and exciting discoveries.

P.S. If Michael Griffin ever reads this blog, can you please point out to your mission designers that the most important instrument on every probe is the high-resolution camera? The images returned are the data best interpreted by the people who are paying for all this science -- the U.S. public. The other science is important too but none of it matters if it doesn't matter to Joe Sixpack. Mission designers who do not agree should feel free to get private funding of their probes.

LH2: Love it or hate it?

Jon Goff has an absolutely fantastic post here at LH2: Love it or hate it?

LH2 seems most clearly stupid for a first stage fuel. Hydrogen engines have low thrust-to-weight, low thrust-to-cost, and big tanks which can cause a lot of atmospheric drag. Hydrogen first stages also see larger gravity losses than hydrocarbon first stages.

Both the Shuttle and the Delta IV use LH2/LOX engines off the pad. The Shuttle uses hydrogen because those engines are ground-ignited second stage engines, so it has at least some excuse. But the Delta IV is a simple two stage rocket with both stages being LH2/LOX. Why? Did they need to be different than the Atlas V?

Opportunity

I just read a fantastic article in The New Republic. In it, Husain Haqqani & Daniel Kimmage comment on a series of 430 short biographies of Iraqi insurgents. At the end, they have a suggestion I really like:

Imagine how the biography of the "hero" Al Shammari would read if it were juxtaposed with the biographies of the people he killed? What might readers in Saudi Arabia, Syria, and elsewhere in the Arab world make of a companion volume to "The Martyrs" in which each suicide bomber faced his victims, not as statistics in a war against the infidels, but as individuals in their own right?

Falcon 9 Upper Stage Recovery

The SpaceX annoucement is fluffy. Their big engine is late, and they're going to try to push their Merlin 1 based product a bit past its economic sweet spot.

Significantly, SpaceX has not specified the upper stage. The upper stage for a plain Falcon 9 to LEO lift will require at least two and perhaps three Merlin engines. The upper stage for a Falcon 9-S9 to LEO lift (required to deliver a Space Shuttle cargo to the ISS) will require at least five Merlins, which means it will actually be a Falcon 5 first stage, but with large expansion ratio nozzles and some ability to do in-space restarts (propellant settling, multi-use igniters, RCS without gimballing the main engine). If they are going to recover that stage, though, they'll need more than just the F5's parachutes, they'll need some really wonderful ablatives, somehow spread under all the delicate engine bits.

If you ignore the upper stage reuseability, the business scheme seems wonderful. NASA needs 20+ heavy LEO launches to lift the rest of the ISS. SpaceX builds a fleet of identical airframes and engines. Four airframes and 32 engines go up, three and 27 come back. They build about 25 airframes and 140 engines over several years, and each airframe gets an average of three flights, and each engine gets four or five. They might charge NASA $1.5B for the whole launch set and make a ton of money, and save you and me (i.e. the average taxpayer) about 75 bucks apiece.

But what's this about Full reuseability? Full reuseability is not a press mistake. The SpaceX press release says explicitly that

Falcon 5 and Falcon 9 will be the world's first launch vehicles where all stages are designed for reuse.

(Except Kistler's design, of course.)

This is crazy. If they take over lifting the ISS segments, they'll have a nice steady stream of launches. Over time, they're going to learn how to manufacture their engines better (more thrust, better reliability, lower manufacturing costs), and the later engines will be better than the earlier ones. The steep learning curve makes the early hardware depreciate fast, which makes spending more money to recover it less attractive. Throwing away the upper stage is a great way to clear out the obsolete inventory.

Heck, at some point (2007?) they're going to deliver the Merlin 2. My guess is that this will be a 600,000-lbf-thrust engine. Their next-gen EELV-class launcher will lift 25,000 kg to the ISS without strapons, with four Merlin 2's as the bottom stage and four or five Merlin 1's as the top stage. Why use the Merlin 1's in the upper stage? Because their business plan requires each engine to get used a bunch of times. The last thing they want to do is introduce a new rocket which obsoletes $60 million dollars worth of engine inventory. Instead of recovering a brand-new engine from the upper stage, it will be cheaper for them to use and expend their Merlin 1 inventory in the upper stages of their launches.

I bet the upper stage reusability verbiage is just there to appease some group at NASA. SpaceX may actually attempt a controlled upper stage reentry with ablatives when launching some slightly more lightweight ISS segment. They may get some interesting data from such experiments. But I can't see them pushing upper stage reuseability very hard. They need to concentrate on the easier and more lucrative problem of recovering their lower stages first.

My Own Vision for Space Exploration

Since I think Bush's Vision for Space Exploration is screwed up, I thought I'd offer my own.

Unmanned exploration

I would initiate a steady program of planetary probes, a common interplanetary communication system, and space-based observatories.

Budget: $4B/year for about 6 probes a year over 2006-2016
about 30 heavy and 30 medium launches
Budget: $8B for 3 observatories over 2009-2016.
about 10 heavy launches

Exploration infrastructure

Most probes are limited by their ability to transmit data back to Earth. That ability is limited by the power available to transmit. For the planets beyond Earth (or perhaps Mars), power is limited because solar cells do not have significant yield. I would fully fund the Prometheus project, which is developing a small nuclear reactor for use in space.

Beyond finishing the development of nuclear reactors in space, existing technology is already comfortably close to the limits of what can be transmitted for a given amount of power. It makes sense, then, to have multiple probes in close proximity use the same communication system. This is already done to some extent at Mars, with Mars Odyssey and Mars Global Surveyor relaying signals for ground rovers.

I would initiate a program to launch a nuclear-reactor-powered long-range communication satellites into orbit around each of Venus, Mars, Jupiter, Saturn, and Uranus, and (probably) one roving satellite in the asteroid belt. These satellites would primarily be responsible for relaying high data rate bit streams to earth from local probes. They would be built for a useful lifetime of 25 years. They would also carry a modest scientific payload.

Budget: Prometheus: $4B over 2006-2010
Budget: Comsats: $12B for 6 comsats over 2011-2016
about 20 heavy launches

Orbital Assembly Development

Observatories and comsats, and to some extent probes, have one thing in common: very large communication dishes or imaging reflectors. Observatories and especially the extraterrestrial comsats will also be very heavy. All of these missions can benefit from orbital assembly. I do not forsee intricate assembly of the sort requiring people, nor do I see the ISS as a good place to do this assembly, as it will likely involve close proximity to very large amounts of propellant. I would fund one assembly robot in low earth orbit.

Budget: $2B for one assembly robot over 2006-2010.
2 heavy launches

Transportation development

The extraterrestrial comsats in particular and long-range probes in general have large propellant requirements. To avoid the need for seldom-used massive launchers and their infrastructure, these probes should be boosted to their destinations by the same stage that put them into low earth orbit, but refueled in orbit by other rockets, so that multiple launches can assemble the mass to be launched out of LEO.

To help the nascent boost industry, NASA would be required to fit all payloads into the standardized EELV payload masses and sizes. Payloads intended to fit in the Shuttle for ISS delivery will fit into Heavy EELV boosters.

The Shuttle program would be summarily dumped. If ATK thinks that solid rocket boosters are a good match to LEO delivery (as they may well be), they are welcome to collaborate on an EELV booster to compete with the existing 2 (Delta 4 and Atlas 5) and potential 1 other (Falcon 9).

NASA would develop a Crew Transfer Vehicle. This vehicle would be a capsule carrying about six that would ride as one of the smaller EELV payloads (perhaps the 9000 kg class). If t/Space thinks they can drop-kick people to the ISS for less money, I'd entertain a proposal.

Budget: $2B over 2006-2008 to develop the CXV.

Manned spaceflight

The International Space Station would be completed with EELV Heavy launches over the next several years.

Budget: $4B over 2006-2010 (just for cargo launches)
about 20 heavy launches (ISS segments cargo only)

As well as assembling the station, crews would be sent up fairly often to perform science experiments.

Budget: $1B/year over 2006-2016 (just for the launches)
5 medium launches per year.

Manned exploration

There isn't going to be any of this in the next 20 years. Instead, NASA would spend some portion of it's budget learning how to live, work and do science in space. In the meantime, all the probes launched would find out what we actually want people to look at.

Budget: $2B/year over 2006-2016

Budget

My total yearly budget ends up about 70% of what NASA spends today. Probably I have no idea how much things cost. In particular, I left out ISS operational and future segment build costs.

I don't see any massive increase in the launch rate. Over the next
11 years, I see about 160 launches with a total price tag of about
$12B. A billion dollars a year seems like enough money to keep
perhaps two players going -- with just 1000 employees each.

Heavy: 82 @ $100M/each
Medium: 80 @ $ 40M/each

SpaceX choices

SpaceX has just announced a new, bigger launch vehicle. It has a number of configurations, some of which are roughly the size of some of the Evolved Expendable Launch Vehicle (EELV) sizes. The EELV program was a Defense Department-funded program to develop two launch vehicles independent of the Shuttle for military payloads, started after the Shuttle fleet was grounded after the Challenger accident.

Q: What is the Falcon 9 intended for?

A: The Falcon 9 is intended for three missions:

Steal payloads from the other two EELV boosters: Delta 4 and Atlas 5. These payloads are mostly military, and the Pentagon would very much like to have a reasonable fallback if they have to cut funding to one of the two subsidized EELV launchers. The Pentagon likes this idea so much they are spending real money on it: Falcon 9's first launch is a defense department satellite.

Replace NASA's "Stick" booster.

Lift the remaining ISS sections.

Q: Why has SpaceX announced Falcon 9 now?

A: SpaceX wants to stick a crowbar into the public debate over NASA's two new boost vehicles. I'm sure the competing camps at NASA have been fully aware of SpaceX's Falcon 9 plans for a while, and probably driving them to some extent. But NASA's decisionmaking is not entirely internal -- they have to convince policymakers to fund their plans, and quite a lot of that convincing is done by attempting to frame the public's perception of those plans.

The ATK/Morton Thiokol lobbyists, and their camp within NASA, have been pushing two new mostly expendable launch vehicles, one to launch a 21 - 26 metric ton crew capsule and resupply ship to the International Space Station, and one to launch Very Heavy Things (110 metric tons) needed for manned lunar or Mars expeditions. Both would use solid rockets derived from the Shuttle SRBs. The big launcher might throw away four reusable SSME engines per launch, which is expensive.

The push isn't going well right now. Developing two vehicles will cost more than NASA is spending now, and that looks very bad to legislators spending hard-won tax dollars on the non-pork-barrel Iraq war and the $700 million/day cleanup of Katrina.

Jumping into the fray now lets SpaceX grab some of the public's mindset before it gets solidified by the ATK camp. They have a launch coming up soon. If it succeeds, John Q Public is going to wonder why NASA can't use the reliable SpaceX booster instead of developing a nearly brand-new launcher.

NASA's designs for a Shuttle-derived Shuttle replacement center around the idea that the technology development, supply chains, and infrastructure it has developed for the Space Shuttle are valuable in their own right. Because the Shuttle is nearing retirement, all three of these things are in danger. For instance, if there is no future vehicle to use a Shuttle/SRB-like solid rocket booster, ATK will have to shutter its plant for assembling these monsters. NASA is absolutely right in realizing that a unique capability that only exists within the U.S. will vanish if that happens.

But the hard question is: are those capabilities actually useful? The ATK boosters, for instance, produce 3.3 million pounds of thrust each, and cost about $40 million per launch. A simplistic analysis of SpaceX's launch prices put an end-user cost of $2.25 million per launch on each Merlin engine, each of which produces 85,000 pounds of thrust. The solid rockets are a much better deal for straight liftoff thrust, at $12.12/pound rather than the $26.47/pound of the Merlins. (The price difference is mitigated significantly but not reversed by the better Isp of the Merlins.) The as-yet-unfulfilled promise of SpaceX is that they are going to recover their engines or even most of the vehicle after each launch, and then will reduce their prices. And, of course, prices in the launch market are flimsy: I read wildly different estimates for costs and have not done my own accounting of NASA's costs, which I assume are public record.

I think the SpaceX entry is great for SpaceX. I had lamented earlier that the Falcon V was just not big enough for ISS resupply. Falcon 9 solves that. This gives SpaceX an obvious and fairly large launch market (I'm guessing three-plus launches a year), which should give them an operating profit with which to fund future development. It should also give them a launch history. It's up to them, of course, to make that launch history one to be proud of.

One hopes that launch market is also an elastic market: since SpaceX's prices are quite low, NASA might get to eventually rotate more of its astronauts through the ISS and actually do some science up there.

I think the SpaceX entry is great for NASA. It gives them the excuse to retire the Shuttle early. It gives them a cost-effective way to assemble the ISS without the Shuttle. With some foresight, they may be able to focus on the in-space aspects of putting people on the Moon or Mars instead of spending all their money on ground handling of launchers.

Lingering problem #1: How does NASA kill off the Shuttle-derived Heavy Launcher? NASA's standing army can't be dismissed until that thing is dead, gone, and maybe replaced. Or perhaps, the folks at NASA will let go of the need for big heavy launchers. I can see this happening for missions with large fuel requirements (launch the fuel seperately).

Lingering problem #2: SpaceX needs to launch a bunch of Falcon 9s before anyone should be confident that they are safe enough for people. What are they going to launch? Possible answer: ISS cargo-only resupply missions. Stretch goal: launch astronauts on Soyuz, and launch ISS sections on big Falcons. It might take a lot longer to assemble the ISS with just 4 people on board, but it might still be doable.

Finally, SpaceX's F9-S9 launcher has no less than 27 2.25-million-dollar engines at the bottom, and their launch prices are about $3000/kg to LEO. Maybe those engines are going to get cheaper if they start cranking them off the production line. But it seems to me SpaceX needs a bigger engine (and a larger diameter standard fuselage), because the current scheme isn't going to scale up much larger. Elon Musk promised that such an engine is in development, but it must not be well enough developed to enter into the current debate, as SpaceX is going to have enough difficulty getting credibility for even the Falcon 9. SpaceX development is late, and they are starting to reap the costs of being late.