One of the reasons I haven’t blogged much about my reactions to the President’s NASA budget proposal is because I’ve been doing a lot of commenting over at NASASpaceflight.com. I just realized that my latest post could actually serve pretty well as a blog post too, so I’m copying it over here for discussion. One area that there’s been a lot of argument about is the value of the HLV/Propulsion R&D budget. A lot of people have been saying “we know how to build HLVs, we don’t need more R&D”. Here are my thoughts:
I’ve been a bit busy to weigh in on this for a while, but being a propulsion engineer myself, I can see *several* potentially interesting technologies that could be invested in. To me, I don’t think the reason they’re suggesting investing in HLV technologies instead of building an HLV right now has anything to do with whether or not we can build one with existing technology. My guesses at the reasoning are more along the lines of:
1-If they build an HLV right now and keep Orion going, they would either need to greatly increase the budget, stretch timelines out so far that it would drive the costs way up, or sacrifice the other parts of the plan that are what they’re actually interested in (tech demonstrators, ISS utilization, commercial crew maturation).
2-If they punt on actually building an HLV until after this first five-year chunk, then they’re no longer conflicting with the most immediate tech demonstrators or the commercial crew development efforts, which would allow them to build such vehicles without requiring a big increase in NASA’s budget.
3-By punting for a while, they both may have better technology options to start with, allowing for a cheaper, lower manpower HLV, but there will also be a much better baseline to start with for a new Orion vehicle. By that point there will likely be at least two commercial capsule providers out there, and possibly some new tricks up their sleeve, which may make Orion a much smoother, better vehicle overall.
4-By punting on the actual HLV development until after the technology demonstrations, it may be clearer what capabilities you really need for the HLV. Going ahead with one right now either has to assume that the tech demonstration will work (risky) or assume it won’t work (risky via overconservatism). Waiting until you know more about if depots and high-Isp in-space propulsion systems are reasonable allows you to pick an HLV better matched to the new technology.
Going back to the technologies, if you understand that this isn’t a question of “we can’t do it with the technology we have today”, but more of a “we don’t want to do an HLV for other reasons, but what stuff could we do to make an HLV more affordable when we do have the budget to move on it”, then it becomes a lot clearer. To me, the goal of any of the booster engine R&D should be to take technologies that have potential for drastically better cost/performance ratios, and mature them to the point that they could be rolled into an HLV effort when it is time to move. TAN is one option, since if done right you could get enough thrust that a shuttle derived HLV wouldn’t need SRBs anymore to takeoff, and commercial EELV-class vehicles could either reduce their engine size, or increase their propellant load drastically for the same weight booster engine. It might also allow you to get good performance with a lower pressure booster engine. Other options include stuff like pistonless pumps, or some of the other pump concepts I’ve seen that while they may not have quite the performance of staged combustion, have most of the performance at a tiny fraction of the complexity/cost. Flow separation control is another cool trick that can allow you to get more performance out of lower chamber pressure engines. Imagine being able to build an engine that had better T/W ratio than an existing staged combustion engine, had no turbomachinery, and similar or better mission averaged Isp, but cost about a tenth as much? That would be a game-changing set of technologies in my book.
There may be some other work relating to getting domestic high-thrust LOX/hydrocarbon engines ready. There’s also the high thrust expander cycles you mentioned, which would be beneficial to EELVs as well as HLVs. Depending on the engine size, it might even be interesting to further develop the Mid-Air Recovery idea that LM was investigating for their Atlas V engines. Being able to recover the engines from an HLV launch and reuse them a few times without having to deal with salt-water corrosion issues is a great way to reduce some of the big marginal costs of a flight. For upper stages, putting some money into ACES/Raptor related technologies might not be a bad investment either–find something that can give you some commonality between your HLVs and other stages flying (or make it so your HLV is just a family member of an upgraded EELV-class launcher family). Or doing at least a little funding on reentry technologies could also be interesting (to allow for easier reuse of currently expendable booster stages, low the hurdle for true-RLVs, and also possibly make Orion-like vehicles easier to build and more capable down the road). Some concepts like transpiration cooling and electromagnetic TPS/aerobraking are both really interesting.
Anyway, I’m an ideas guy and a propulsion engineer, so I may have a different view of this than most, but I for one think this is a reasonable approach. To me, cutting back drastically on the amount of people who are needed to launch an HLV, while also using new technologies that allow you to get better system-level performance out of lower-cost, lower-complexity hardware is potentially game changing in my book.
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