Saturday, November 18, 2006

A little message to the Chair Force Engineer

Engineering is mostly data plumbing. What matters is that the right people understand the right bits of the problem. All those meetings, all those revisions of all those specs, it's all about exploring the problem and getting the right bits to the right guys and gals. Everything is specialized. For the most part, that means you aren't ever going to get it all. Those windbags probably mean well.

The good news is, you don't have to get most of it. You need to understand who needs to know what, and how to get them what they need. Know what is the critical path. There will some small part that's your specialty. Make sure you're on top of that. The rest is plumbing. You're smart enough. Relax.

As for the whole nihilistic thing... What drives you? I'll tell you what drives me: what is the coolest thing I can pull off and actually make work? I've long since realized that a team can make stuff happen that I could never do myself. Right now I'll take the trade of having a smaller part in achieving a more audacious goal.

For the next 30 months, concentrate on getting experience, especially practical experience. You're going to be around for a very long time. This idea that you were going to figure out by 30 what you are going to do in life is a (boring) fantasy. One of the best CPU hackers I know started doing serious astrophysics, did CPUs, moved on to chip assembly tools, and now does something completely different. You know what my dad remembers from hacking on accelerators at Berkeley in the 60s? Plumbing. He's awesome with solder and 3/4" copper.

It sounds like you're motivated by Serving America. Very noble. You can probably see lots of ways of projecting force better than we are now. You don't need to implement that right now. You might take a 20 year detour first. Hell, you might try raising kids. That'll throw a wrench in things, I promise you.

Oh, and when you get out, give me a call.

Friday, November 17, 2006


I noticed that got messed up somehow, probably when Google moved to their Blogger in Beta infrastructure. So I switched my account to Blogger in Beta. As part of the process, I realized that I had comments!

This is like an unexpected birthday present! These are great comments, written with care by people who know. Thank you!

This calls for two apologies.

First, for not getting these things published before. Some of these are a year old.

Second, of 42 comments there was one that was inappropriate. In rejecting it, I accidentally erased 7 other comments. They aren't recoverable (which is a bug, btw, and now that I'm a Googler I can file a bug against that!) I know you folks take care writing those, and I'm really sorry about dropping them.

Anyway, thanks a ton for all the great comments.

Wednesday, August 16, 2006

New job, less postings

I'm now working at Google. Specifically, I'm working on Google Earth. The project is irresistably cool, I work with interesting and talented people, I'm learning a lot, and there is plenty of money and resources to get the job done. I'm pretty happy about the whole thing.

But my posting should be about techie stuff. So, along that vein, I'm sure you are all wondering, does water evaporate from the pool faster or slower on cooler nights? I think I have a counterintuitive answer: cold air speeds evaporation.

The pool water holds vastly more heat than the air above it, so the air in contact with the water will be at the water temperature, pretty much regardless of the ambient temperature. Lower ambient temperatures usually come with lower dew points, which means that once the air has been heated to the pool temp, initially colder air is dryer. More water can evaporate into this colder air.

There is a second effect as well. On a windless night, the convection over the pool is driven by the temperature difference between the pool and the ambient air. As ambient gets colder, the convection gets faster and more dry air is brought to the pool surface, evaporating more water.

The net result is that on a cool night, the area all around the pool gets quite damp. I'm fairly sure this is pool water recondensing out of the air as the air moves away from the pool and cools back down.

Saturday, May 27, 2006


Listening to NPR yesterday, I heard a guy from the New Orleans Aquarium describing the mess they found after Katrina. The equipment there had stopped working, so the interior temperature had risen into the 90s, most of the tanks had gone anoxic, the fish had died and were rotting in the heat. The tanks were a filthy, smelly, bacteria-laden mess.

The alligators, however, loved it. Somehow, perhaps because I'm the owner of two Labrador retrievers who will eat anything, my heart warms to the thought of unadulterated joy in the midst of what would turn a human stomach.

Monday, May 08, 2006

Renewables vs Nuclear

There is a great discussion over at EnergyPulse. The article suggests renewables can replace nuclear. A number of good comments that follow show the claim to be wrongheaded, but perhaps not entirely wrong.

The claim that renewables (primarily windpower) can replace nuclear is not entirely off base. We are building windpower right now faster than we are building nuclear, and the EIA predicts this will continue to be true for 30 years, even with the new subsidies. The existing U.S. nuclear plant fleet is huge, however, and renewables are not forecast to challenge the capacity of the existing nuclear base.

I think the debate between renewables and nuclear is pointless. Both are domestic supplies. I think the more interesting comparison is between domestic and foreign energy supplies, and what we can do to move to more domestic energy. Secondarily, I think there is an interesting comparison between the amounts of CO2 produced by various energy supplies.

We use oil and gas-fired turbines and hydro to follow variations in electricity demand. Our dependence on foreign oil and gas supplies is a major threat to our national security. In the U.S., hydro is all built out. I see two strategies that will help move the U.S. from foreign energy sources to domestic sources. Both could use some legislation.

  • Dynamic pricing. If the price of electricity varies minute-by- minute, many customers can time-shift their loads. This reduces variations in electrical demand and allows domestic sources (coal, nuclear, renewables) to replace foreign sources (oil, gas). Examples are ice-storing commercial cooling plants and aluminum smelters.

  • Plug-in hybrids. These gasoline and battery powered cars can charge up at night. They actually help twice: they directly shift their energy source from a foreign one (crude oil) to a domestic one (coal, nuclear, renewables). But also, the extra demand at night makes it economic to build more domestic-source generation instead of foreign-source generation.

  • As for the CO2 problem, natural gas was once sold as a cleaner, greener substitute for coal. The cost in blood and money is now clear. We should use all the windpower we can get, and conservation can help too. But I think the big answer here is hundreds of gigawatts of new nuclear generation over the next 30 years.

    I foresee the costs of coal going up in the future, from the additional costs of CO2 sequestration. I foresee the costs of nuclear coming down in the future, if many similar plants are built and regulation and operation become more standardized. So I see a second benefit to a massive nuclear buildout: a drop in the price of energy here in the U.S. I think that drop, along with a change to domestic energy supplies, could make a big difference to our balance of trade and national security.

    Tuesday, May 02, 2006

    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


    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


    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

    LEOextra stage 1

    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.

    Tuesday, March 28, 2006

    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.

    Thursday, March 23, 2006

    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.

    Wednesday, March 22, 2006

    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.

    Tuesday, February 14, 2006

    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.

    Enginethrust per chamberIspVehicle
    F-17740 kN265 - 304Saturn V
    RS-683312 kN365 - 420Delta IV
    RS-242278 kN363 - 453Space shuttle
    RD-1802076 kN311 - 338Atlas V
    RD-1711976 kN309 - 337Zenit
    NK-331638 kN297 - 331N-1/Kistler
    Vulcain 21300 kN318 - 434Ariane 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.

    Wednesday, February 01, 2006

    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.

    Thursday, January 26, 2006

    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.
  • Wednesday, January 25, 2006

    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.

    Monday, January 16, 2006

    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.

    Tuesday, January 10, 2006

    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?