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The Shuttle's Abort Options. People have been discussing the options that were available to Columbia had the crew known of a problem earlier in the flight. Here's a message originally written by Steve Coester, who worked on the space shuttle for NASA for most of his career. This message answers questions about a possible abort saving the crew. It makes interesting reading. [Source: Mike Cronin via Carmine Pecoraro, on the Beech-Owner's mailing list]
I had questions from some of you whether NASA should have aborted after
seeing the debris impact on the left wing. I was unable to immediately give
a good answer so I did a little research.
First remember that during STS107 ascent nobody knew that a piece of debris
had hit the Shuttle. This was only discovered after the Shuttle was in
orbit when the Launch Team was reviewing the films taken during launch. No
one had anyway of knowing there was a problem real time.
If a problem is immediately discovered such as Main Engine failure then
abort is an option invoked by Flight Rules. Preference is to either Abort to
Orbit or Abort Once Around which takes the Shuttle around the earth at
almost orbital altitude and then landing in normal mode. If the problem
occurred too early to achieve ATO or AOA either RTLS (Return to Launch
Site) or TAL (TranAtlantic Abort Landing) would be called with TAL almost
always being preferable if possible. If an observer saw something that
looked bad at liftoff such a a big section of tile falling off I'm sure they
would call an RTLS. Remember that on STS107 nothing was seen until the film
was reviewed so nobody knew there was a
problem.
RTLS has always been considered the abort of last resort. The stresses on
the orbiter are extremely high. The Shuttle needs to continue to fly until
the solid Rockets are expended and jettisoned and the Main Engines need to
be used until it has gained sufficient altitude to glide back to the launch
site (KSC). Also they have to be able to reach a point where they can
jettison the External Tank without danger of it
hitting the Shuttle. At some point while racing along it has to pitch around
and flying backwards use the Main Engines to reduce its velocity to zero.
See photo There is a point in the trajectory where the shuttle's velocity
away from KSC goes to zero. This is the "VREL=0" point (pronounced as
"vee-rel-zero"). At Vrel zero, the shuttle begins heading back toward KSC.
This is a dicey place to be. Flight control is performing a balancing act.
The vehicle is not moving forward or back but is falling at about 1000 -
1200 feet per
second. The plunge does not level out until the shuttle is around180,000
feet. After that, the shuttle climbs toward her separation altitude of
200, 000 feet.
I don't have a heat analysis on RTLS but it would be lower than the regular
reentry starting at 200000 feet rather than returning from space above
400000 feet. At least up to this disaster no one would call for RTLS except
under known engine loss or immediate pending disaster. The safest thing is
"almost" always to try to get to orbit. The stresses are
just too high and the unknowns too many to try RTLS
Next option is TranAtlantic Abort Landing. The TAL trajectory carries the shuttle to a normal MECO altitude (360,000
feet) but shuts down the SSME's sooner. External tank separation occurs
like it does nominally, but the shuttle quickly flies to an entry attitude
afterwards. The entry trajectory itself is similar to that flown at
end-of-mission except that it starts lower and ends at a landing site
overseas.
You see that the final altitude in TAL is very close to the 400000 feet
nominal reentry interface when air molecules begin to impact the
Shuttle. I would suppose that reentry heating is almost as severe for TAL
as for a normal reentry, but I don't have that data.
--Steve
Posted Tuesday, February 04, 2003 9:40:06 AM 
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