Chasing the Challenge
by Mark J. Trulson
|The big problem with higher altitude suborbital flight is that when you come back in a nearly vertical trajectory from these higher altitudes, you hit the atmosphere fast and have a very short amount of time to slow down. The peak heating loads can actually exceed those seen in a ballistic orbital reentry, and the peak G’s can get high really fast.|
Goff: It’s interesting to note that our vehicle’s aspect ratio (the length divided by the base diameter) is actually not that different from DC-X. That said, I’m not the aerodynamicist, but as I understand it, the shape we chose happens to be a very well studied shape that has a lot of previous data on it, making our design a bit easier. Also, it makes manufacturing the tanks and innards of the vehicle substantially easier, at least for XA-1.0. For other vehicles, we may have to do things differently, but this should make our lives easier the first time around.
The vehicle is slightly unstable on the way up, but we have plenty of control authority from the vernier engines. On the way down we use a few deployable drag brakes to help insure that the vehicle is ballistically stable. That’s about all I can really say about the aerodynamics.
TSR: With expected turn-around time between launches of a couple hours and the reuse of the engines measured not in single digits or low 10’s but in the high hundreds of uses, what performance compromises such as payload sizes had to be made to reach those benchmarks?
Goff: I’ll address the engines first; as that is one of the areas we’ve been doing a lot of work in recently. While we don’t have the final numbers for turnaround time and engine lifetime, we’re actively working toward those goals. The engine fabrication style we’re using (a variant of the Chamber-Saddle-Jacket approach used by Swiss Propulsion Labs and XCOR) is a bit heavier than more traditional tube-wall or milled channel type chambers, but there’s good reason to believe that they can deliver much longer operating lifetimes and lower fabrication costs. We’re also going with a relatively low chamber pressure compared to traditional aerospace groups, which tends to help enhance engine lifetimes by quite a bit.
Both of these choices lead to engines with lower T/W [thrust-to-weight] ratios, and slightly lower Isp numbers than traditional rocket engines. The lower engine performance requires you to carry more propellants for the same mission profile, but for suborbital vehicles, it really isn’t a big hassle. The mass ratios needed to deliver 100 kg to 100 km really aren’t that challenging by aerospace standards. More importantly, it’s a lot easier to take a reliable, easy-to-use engine and make it lighter than to take a complicated, ultra-light-weight engine and make it reliable and easy-to-use.
As for the overall vehicle operations, the biggest tradeoff is development cost and added dry mass. We’re trying to keep the development cost down by doing things in an incremental build-a-little, test-a-little, fly-a-little fashion. While our first XA-0.1 vehicle might not be the easiest thing in the world to work with, you can be sure that by the time XA-1.0 is finally flying that it will have benefited from dozens of flights worth of operational experience. The dry mass hit does mean that we will likely tend to have a much smaller payload-to-GLOW ratio compared to most expendable vehicles, but that really isn’t what matters. What matters to a customer is cost, services, ease-of-use, and flight frequency.
TSR: What sort of control systems will the XA vehicles have? Are you looking at having a pilot in-the-loop (if not on the vehicle), or full autonomy? What influenced the choice?
|If they ask for 1600 km for the high-altitude suborbital contest, for example, that’ll probably require us designing a reusable upper stage: 1600 km requires about as much energy as getting to orbit, and the reentry environment is probably heinous. If they ask for something closer to 500 km or so we should have a good shot at it.|
Goff: As I understand it, for XA-1.0, we’ll have a pilot in the loop, but not onboard the vehicle. The computer will be given a preprogrammed flight plan, and it will execute that, unless the pilot takes over. Even then, the computer will still perform stability augmentation stuff, but the pilot can make decisions on where to steer toward, and what the right thing to do about that flashing light is. Down the road, we’d like to put the pilot onboard the vehicle. We do eventually plan on doing manned flights with the XA-1.0 or the XA-1.5, and the XA-1.0 is specifically sized to fit someone the size of my boss comfortably. However, we want to make sure that the rest of our system is working safely and reliably before we try to put people on board.
TSR: Is MSS fully funded for XA-1.0 development?
Goff: We are funded through a significant part of our R&D development path but we will need to bring in outside capital to finish XA 1.0 on schedule and with the performance we need. Michael Mealling, (our Vice President for Business Development) is the one to speak with if anyone wants further information about that.
TSR: With none of the principal management team exactly what you’d call “crusty gray-beards,” is the environment more of a “California laid-back” style? How would your describe the work atmosphere?
Goff: We’ve got a rather eclectic bunch working here, but yeah, I’d say it is a very laid-back work environment compared with traditional aerospace companies. We’re still learning how to strike that critical balance between having just enough structure to make sure things work smoothly but not enough to stifle innovation, however, we’re getting more and more into a decent groove for development. It is a challenge some days to make sure we don’t get too far off onto tangents about arcane economic theories, polycentral legalism, space development, or foreign policy, but we tend to have a lot of fun and, more importantly, make decent progress.
TSR: As we currently understand the two new Centennial contests, (pending the actual release of the contest rules), do you see any additional engineering challenges facing the XA-1.0 design, to meet the requirements of the contest?
Goff: Depends a lot on the contest rules honestly. If they ask for 1600 km for the high-altitude suborbital contest, for example, that’ll probably require us designing a reusable upper stage: 1600 km requires about as much energy as getting to orbit, and the reentry environment is probably heinous. If they ask for something closer to 500 km or so we should have a good shot at it. It’ll take some serious TPS [thermal protection system] development work, and maybe even some redesign of the aeroshell (to lower the ballistic coefficient), and maybe even some other tricks, but it’s something we’re now planning on giving a shot.
As for the lunar lander prize, a lot also depends on the rules. We would love to eventually work on lunar landing equipment, and something like this prize would definitely encourage us to move that up on the development schedule. As a side note, I’ve spent a lot of time over the past several years studying and advocating commercial development. Michael Mealling has also been heavily involved in the leadership of several lunar-oriented groups, like the Artemis Society and the Moon Society. So, yeah, we have a lot of interest in commercial lunar development here at MSS, and we’re really thrilled that NASA may actually be doing something smart to help make commercial lunar development more of a reality.
As for engineering challenges, unless there are a bunch of requirements in the final rules that haven’t been mentioned yet, it should only be a bit more challenging than what we’re already planning on doing for XA-1.0. It’ll require some work or some cleverness (or likely a lot of both), but I feel it’s not too far from what we’re trying to do for XA-1.0 and XA-1.5.