Chair Force Engineer

Tuesday, August 11, 2009

Shuttle-Derived Devil's Advocate

The Augustine Commission, appointed by the president to chart the future of NASA manned spaceflight, is looking at a variety of missions and boosters for the future direction of Project Constellation. One dark-horse concept that's gotten a lot of attention lately is a side-mount shuttle-derived rocket, which I think of as "Son of Shuttle-C." It's definitely an improvement over the existing Ares designs in terms of its development costs and schedule. But much like the rush to Ares, in which issues like thrust oscillation and air-starting SSME's were swept under the rug, it would be helpful to look at all of the challenges that will face the team developing the new rocket.

It's important to distinguish the new concept from the original Shuttle-C proposal. instead of flying the shuttle orbiter, Shuttle-C made use of a "cargo element" which looked much like an orbiter stripped of its cockpit, wings and tail. But the new design utilizes a much larger payload fairing, nearly as wide as the external tank. With a much wider cross section, the new rocket will be a "draggier" design than the existing shuttle or Shuttle-C.

Another thorny issue will be the separation between the new rocket's fairing and external tank. The fairing is supposed to jettison during ascent, with the exception of the aft thrust structure and the connected structure which supports the payload. There should be some concern about achieving a clean break when the fairing jettisons, although it shouldn't be much more dicey than when the SRB's are cast off. In the block I version of the new rocket, the thrust structure and payload structure would separate in the same fashion as the shuttle orbiter. I would guess that the payload would eject parallel to the vehicle's yaw axis, similar to the way the shuttle deploys its payloads.

The separation issue gets riskier on the block II rocket, where a live upper stage is carried inside the fairing. The upper stage ignites suborbitally at a speed just under Mach 17. I would be very concerned about recontact between the upper stage and either the payload structure or the ET.

The "Son of Shuttle-C" promises low development costs for as long as we have spare Space Shuttle Main Engines to throw away when the orbiters are retired. But the new challenge is creating a version of the SSME which can be economically thrown away after every mission. While promoted as an upgrade of the SSME, it's really an extensive new development (moreso than developing the RS-68A from the existing RS-68.)

There already was an expendible Space Shuttle Engine: it was called RD-0120, and it was developed by the Soviets for their own shuttle system. It pioneered the "channel wall" nozzle concept which would drive the production cost of SSME down if it were adopted. But do the Russians still have the tooling and industrial knowledge to build more RD-0120's? Is it practical and politically feasible to buy RD-0120's from Russia? The answer to both questions is probably "no." But it would be worth consulting with them when designing a new SSME nozzle. RD-0120 achieved a similar specific impulse to the SSME, but it required a higher expension ratio nozzle and produced less thrust.

A channel-wall nozzle, designed to the same expansion ratio as the existing SSME, can’t be developed overnight, although Wayne Hale has stated that the idea has been given some consideration in the past. Channel-wall nozzles are far easier and cheaper to manufacture than the existing shuttle nozzles, which use thousands of tiny tubes welded together to form the nozzle wall. Instead, the coolant channels are milled into a solid piece of metal, with a thin sheet of brazed over the top of the channels to seal them off.

Current Shuttle PM John Shannon also stated that the turbopumps would have to be redesigned for making a cheaper, throwaway SSME. Again, this isn’t a trivial modification. I have to wonder aloud whether it might be possible to substitute the expendable turbopumps from the RS-68 (which are designed for higher flow rates than those on SSME.)

When it comes to "Son of Shuttle-C," I don't think it's a bad idea, if your goal is to preserve the "Central Florida Economic Stimulus" that the shuttle program provides. But it will not be a trouble-free or inexpensive development; as long as NASA and the taxpayers keep that in mind, it should result in a workable launch vehicle. Everybody should be warned that an expendible SSME will be a very different beast from the current SSME, and developing the new engine may force the taxpayers to look back on the program with bitter feelings about what was supposed to be a quick and economical program to close the post-shuttle gap.