Wednesday, October 26, 2005

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.

Saturday, October 22, 2005

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.]

    Saturday, October 15, 2005

    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.

    Wednesday, October 05, 2005

    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.

    Monday, October 03, 2005

    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.