Post by
VimyJ »
https://forums.nicoclub.com/vimyj-u238.html
Sat Feb 01, 2003 6:08 am
My father (the retired fighter pilot and flight safety expert) sent me this from one of his circle of friends:
"I received this from a friend of a friend, it is worth a read.
Subject: Shuttle Abort Modes
To All, I got this message from my Naval Academy classmate, Steve Coester. He worked on the space shuttle for NASA for most of his career. He is now retired, but still in touch with many who are still active in the program. This message answers my questions about a possible abort saving the crew. It makes interesting reading.
Mike Cronin ----------Friends,
I had questions from some of you whether NASA should have aborted after seeing the debris affect 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 around 180,000 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 200,000 feet rather than returning from space above 400,000 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.
Hope this clears up the question a little.
Steve"
