De-orbit question

Got a question from a neighbor here that I can't answer, but I'm sure
someone can help me out:

Question: Once the shuttle does the de-orbit burn for say a KSC
landing what are the options to NOT land? My impression was that once
you had done the OMS de-orbit burn you were coming down about 60
minutes later, be it on runway, house, farm, whatever, you WERE coming
down period.

IS there any ability to say re-boost and remain in some type of orbit
at all, say even if you cut off the OMS burn short of a full 2:32 (or
so)??

Thanks,

George

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George R. Kasica wrote:
Got a question from a neighbor here that I can't answer, but I'm sure
someone can help me out:

Question: Once the shuttle does the de-orbit burn for say a KSC
landing what are the options to NOT land? My impression was that once
you had done the OMS de-orbit burn you were coming down about 60
minutes later, be it on runway, house, farm, whatever, you WERE coming
down period.

IS there any ability to say re-boost and remain in some type of orbit
at all, say even if you cut off the OMS burn short of a full 2:32 (or
so)??


In theory, up until the drag exceeds the OMS thrust, there is no reason
you cannot get back to a stable orbit, if you have enough fuel. Just
point the other way and fire 'em up, then stabilize your orbit at
apogee.

In practice, the've used up almost all their maneuvering fuel at this
point, so unless they cutoff the de-orbit burn in the first few
seconds, they won't have enough to re-orbit, then de-orbit again. Up
until half way through the burn, there is still enough fuel to
re-orbit, but it would be a really bad idea since they would not have
enough fuel to de-orbit. So in practice, once they get very far into
the burn, they *are* coming down....

Lou Scheffer

On Tue, 09 Aug 2005, Bruce Palmer wrote:

wrote:
In theory, up until the drag exceeds the OMS thrust, there is no reason
you cannot get back to a stable orbit, if you have enough fuel. Just
point the other way and fire 'em up, then stabilize your orbit at
apogee.

In practice, the've used up almost all their maneuvering fuel at this
point, so unless they cutoff the de-orbit burn in the first few
seconds, they won't have enough to re-orbit, then de-orbit again. Up
until half way through the burn, there is still enough fuel to
re-orbit, but it would be a really bad idea since they would not have
enough fuel to de-orbit. So in practice, once they get very far into
the burn, they *are* coming down....

FWIW Jim Kelly just said in the crew post-flight press conference that
he had a brief moment of trepidation just before Collins hit the button
that confirmed the OMS deorbit burn. He said once you hit it there's no
turning back. You're coming down.

That's the way I always understood it. Barbree's assinine comment today
appears to have set this topic in motion. I'd rely on the astronauts'
word before anyone else's, but it would be nice to get the OMS
propellant reserve numbers to settle it once and for all.

Unless there's a need to leave extra propellant on board (such as might be
required to manage the c.g. if the payload configuration has driven it too
far forward) they typically burn the engines until there's almost no usable
propellant remaining. Once the burn is over, the orbiter is coming home.

The real breakpoint during the deorbit burn is known as Safe Hp. This is
the perigee at which you can stop the burn and still be considered to be in
a safe orbit. Once you get beyond this point the procedures have the crew
finish the burn by whatever means are available. From an ISS orbit Safe Hp
typically occurs about two thirds of the way through the deorbit burn.

--
Dean Lenort