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Space Forum / Space Flight / August 2005Why SSME for SDLV?
The shuttle-derived heavy-lift vehicles under discussion would for the most part be powered by the SSME. Why use the expensive SSME on an expendable vehicle when the cheaper RS-68 is available? As a bonus, the RS-68 provides more thrust. It's true that it's specific impulse is not quite as high, but the ultimate objective here is to keep costs down rather than to maximize technical performance. Proponent@gmx.net wrote in news:1123238450.135965.65810 @f14g2000cwb.googlegroups.com: > The shuttle-derived heavy-lift vehicles under discussion would for the > most part be powered by the SSME. Why use the expensive SSME on an [quoted text clipped - 3 lines] > specific impulse is not quite as high, but the ultimate objective here > is to keep costs down rather than to maximize technical performance. The SSME's Isp is _significantly_ higher, it's lighter and that translates to more payload. But if the higher performance isn't needed, I don't see why the RS-68 couldn't be used--if we handwave away interface issues and who knows what else? It's not like the RS-68 was ignored; the studies did consider it. A less expensive alternative might be COBRA, which uses the improved SSME turbopumps but is otherwise a simplied design (single preburner, milled wall combustion chamber/exhaust nozzle, considerably less plumbing. Still looks kind of expensive... The SSME itself will need some modifications to be used as an upper stage engine. Wonder if there will be a nozzle extension to maximize vacuum Isp? http://www.pratt-whitney.com/prod_space_cobra.asp --Damon > The SSME's Isp is _significantly_ higher, it's lighter and that > translates to more payload. But if the higher performance isn't > needed, I don't see why the RS-68 couldn't be used--if we handwave > away interface issues and who knows what else? It's not like the > RS-68 was ignored; the studies did consider it. There may be a substantial number of SSMEs with at least one launch worth of life left at the end of the shuttle program. Using those up would certainly make sense... > > The SSME's Isp is _significantly_ higher, it's lighter and that > > translates to more payload. But if the higher performance isn't [quoted text clipped - 4 lines] > worth of life left at the end of the shuttle program. Using those up > would certainly make sense... There are only 14-15 active SSMEs now. The others have been retired. Some of them are in museums. - Ed Kyle >...Why use the expensive SSME on an >expendable vehicle when the cheaper RS-68 is available? >As a bonus, the RS-68 provides more thrust. It's true that it's >specific impulse is not quite as high, but the ultimate objective here >is to keep costs down rather than to maximize technical performance. Uh, no, the ultimate objective here is to maximize continued employment for the shuttle workforce.  SignatureNo, the devil isn't in the details. | Henry Spencer The devil is in the *assumptions*. | henry@spsystems.net > The shuttle-derived heavy-lift vehicles under discussion would for the > most part be powered by the SSME. Why use the expensive SSME on an [quoted text clipped - 3 lines] > specific impulse is not quite as high, but the ultimate objective here > is to keep costs down rather than to maximize technical performance. The shuttle-derived launchers will not be used very often or for very long, so it's better to minimize development risks and time, not operating costs. This is why the cargo SDLV will be basically Shuttle C (SRBs and ET unchanged, cargo pallet = orbiter with no wings or crew cabin). > ... This is why the cargo SDLV will be basically Shuttle C > (SRBs and ET unchanged, cargo pallet = orbiter with no wings or crew > cabin). Actually... according to space.com the SDLV will be in-line... no side mounted cargo pallet, just SRBs connected to a modified, flattened external tank with an upper stage and a payload shroud on top. > > ... This is why the cargo SDLV will be basically Shuttle C > > (SRBs and ET unchanged, cargo pallet = orbiter with no wings or crew [quoted text clipped - 3 lines] > mounted cargo pallet, just SRBs connected to a modified, flattened > external tank with an upper stage and a payload shroud on top. Yes, that is what space.com says. We'll see. |
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