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.