lwe3207@acf4.UUCP (04/18/84)
Nf-From: acf4!lwe3207 Apr 17 19:43:00 1984 [] Someone noted that the Shuttle doesn't use its main engines for de-orbit burn. I thought it would because I thought the RMS was used primarily for direction-changing. Which is used for "station- keeping", i.e., in-orbit altitude changes? How much fuel is actually on-board for the main engines, and what does that translate to in full-power burn time (or some other useful metric)? What percentage do the main engines contribute to the thrust required to achieve orbit? (I assumed that the solid rockets provided the majority.) Someone told me that the Shuttle can't fly level, and that it doesn't have enough lift to take off horizontally, even if it had the necessary thrust. Is this true? If not, what are the actual "back-of-the-envelope" estimates for the amount of main engine fuel required for the Shuttle to take off horizontally under its own power? Does it "break even"?: i.e., would the poundage of fuel required to make it take off make it too heavy to take off? Just curious, Lars Ericson ..cmcl2!acf4!lwe3207
henry@utzoo.UUCP (Henry Spencer) (04/19/84)
Once in orbit, the OMS is used for *all* maneuvering, orbit changes, station-keeping, de-orbit burn, etc. (Well, anything that's heavy enough that the attitude-control engines can't handle it.) The SSMEs (Space Shuttle Main Engines, for anyone not in on the alphabet soup) are just along for the ride. There is *no* fuel on board for the SSMEs, and they are not restartable in space. Building a restartable engine isn't nearly as easy as building an engine that gets, so to speak, "re-primed" between starts. In general, the only engines that are restartable without manual maintenance intervention are those that have to be, like the OMS. The F-1 engines in the first stage of the Saturn V, for example, were not restartable. The main engines contribute quite a modest fraction of the thrust during ascent; I don't recall the numbers, but the SRBs are definitely doing most of the work during the early part of ascent. Given that the SSMEs are non-restartable and that there is no fuel for them on board, flying the Shuttle level is sort of academic. Once upon a time, the Shuttle was supposed to have jet engines for in-atmosphere flight. That was scrapped due to weight constraints, in favor of a clip-on jet kit. The latter would not be available during landings from orbit, but would permit the orbiter to fly under its own power for ferrying between landing and launch sites. The clip-on kit was then itself scrapped, a combination of complexity and budget problems I think. The current Shuttle would need significant modifications for atmospheric flight. For one thing, the landing gear is not retractable in flight. (Once you extend it for landing, it *stays* extended!) -- Henry Spencer @ U of Toronto Zoology {allegra,ihnp4,linus,decvax}!utzoo!henry
lwe3207@acf4.UUCP (04/20/84)
Nf-From: acf4!lwe3207 Apr 17 19:45:00 1984 [] Oops...for "RMS" read "OMS". ("Orbital Maneuvering System" vs. "Cherry Picker".)
jerry@oliveb.UUCP (Jerry Aguirre) (04/23/84)
I don't have the article in front of me but I remember on reading the thrust vs. weight figures that the shuttle engines can lift the shuttle and its payload into orbit. The solid fuel boosters lift themselves and the strap on fuel tank. Jerry Aguirre {hplabs|fortune|ios|tolerant|allegra|tymix}!oliveb!jerry