Chair Force Engineer

Monday, May 18, 2009

The Heat is On

If you want some interesting technical reading, do yourself a favor and check out the DIRECT Launcher rebuttal to NASA's review of their concept. I'll go into depth about the review tomorrow, but the most exciting (or shocking, depending on your point of view) development comes on Slide 112 of the presentation:

DIRECT has come to the conclusion that the ablative nozzle of the RS-68A/B will not be sufficiently robust for a cluster application in such close proximity to the exhaust from a pair of SRB’s, and a regeneratively cooled nozzle is necessary to survive this extreme base heating environment.

The takeaway: RS-68 isn't going to cut it for DIRECT or for Ares V without some MAJOR modifications. The DIRECT team believes that a regen nozzle is necessary, and they're advocating the Space Shuttle Main Engine as a replacement. NASA is conducting a trade study between SSME and a regenerative RS-68 for Ares V. This is consistent with reports from earlier this year that SSME was back in the trade space.

We've been down this road before. During the days of ESAS, before the "Ares" name was official, there was Cargo Launch Vehicle (CaLV.) Much like the Jupiter 232 of DIRECT, it used Shuttle-derived tankage and an upper stage. It also used five SSME's on the core. Over the next year, the RS-68 replaced SSME because it would be too expensive to throw away five SSME's per flight. The consequence was a wider, all-new core with more propellant to compensate for the lower specific impulse of the less-efficient RS-68.

NASA faces the choice of switching back to SSME, or trying to create a regen RS-68. Both choices are fraught with many unknowns. How easy will it be to restart SSME production? Can any incremental changes to the SSME result in cost savings? After all, Wayne Hale has said that if the shuttle program continued past 2010, the next upgrade might have been a channel-wall nozzle to replace the thousands of welded coolant tubes in the current SSME nozzle. But a regen nozzle for RS-68 won't be trivial, and it will add to Ares V schedule and development costs. And if NASA is going to pay for a regen nozzle on RS-68, it should also reconsider the expansion ratio of the new nozzle to ensure an optimal balance between thrust level and specific impulse.

When I look at the design problem created by SRB heating of the core engines, I wonder whether "SSME vs. RS-68 Regen" is a false choice. For starters, could an ablative RS-68 be viable if the outer nozzle was thicker and absorbed more heat? For that matter, could RS-68 work if its position on the booster changed? Remember that on the shuttle, the main engines aren't mounted between the two SRB's. A similar arrangment could work on Ares V if the six engines were mounted in two separate pods. If the base of Ares looked like a clock with SRB's mounted at three and nine, one engine pod would mount at twelve and the second pod would attach at six.

Just when it might have seemed like the design of Ares V was set in stone, it's all open for debate again. Perhaps the sixty days of ESAS studies weren't enough to thoroughly review all of the underlying assumptions behind the study. At least the DIRECT guys deserve credit for laying all of their assumptions out in the open. Let's hope that NASA gets it right this time around.