The basic assumption for the shuttle design is that the shuttle would be like an airliner: no ejection seats, no parachutes (except for the first test flights) — crew safety consisted in total vehicle safety and the crew riding the vehicle down to a runway.
In retrospect that was a very poor assumption.
Adding crew escape to the space shuttle has received tremendous attention over the years and there are actually some methods that might work. Not to put too short a discussion on it, the problem with all of the best methods is the additional weight. After adding the crew escape system (capsules, rockets, whatever) and ballast to get the center of gravity right, there is no payload capacity left. The shuttle would become a huge crew transportation device with no capability to carry much of anything else up or down. Not to mention the pricetag to develop some of these devices! Wow. So, in the final analysis, the best way to make the shuttle safer is to retire her as soon as possible and go to a different type of vehicle. Sorry but there it is.
Actually, the shuttle does have a minimal crew escape capability. If the shuttle gets to a straight and level glide (actually not very level since the shuttle glides mostly like a rock), then down at 30,000 feet or less the the crew can jettison the side hatch and bail out with parachutes like some WWII bomber crew. This is better than ditching in the ocean or rough terrain. All studies show touchdown “off-runway” would not be survivable. So the subsonic, aircraft-in-control, bailout is all there is. And in most cases the crew probably winds up sitting in a tiny inflatable rubber raft in the middle of the North Atlantic waiting for somebody to pick them up. Not a lot of fun.
But the shuttle does have a remarkable capability that most other rockets do not. In virtually all expendable rockets, if any one of the booster engines shut down prematurely — even if that shutdown is benign — the mission is over, the payload is going into the ocean somewhere, and the Flight Control Officer is going to “send functions”. On the other hand, the shuttle is designed — required — to be able to safely return the orbiter, crew, and payload to a runway landing following the benign shutdown of any one of the three SSMEs.
A word about “benign”. High performance liquid rocket engines have a tendency to come apart in a hurry if something goes wrong. The SSMEs have been extensively instrumented and tested. Their computer control brain has a number of ways to detect an impending failure and turn the engine off before it comes apart. The system is not completely foolproof, but should prevent an explosive catastrophe in most cases. The only SSME premature shutdown in flight history occurred in 1985 on STS-51F when faulty temperature sensors erroneously indicated a problem with the engine and the computer shut that engine down. This occurred late enough during the boost phase that the mission continued to a completely successful conclusion. After that flight we spent a lot of time building more reliable temperature sensors.
So if any single SSME shuts down prematurely at any point in the launch phase, a safe return of the shuttle and crew will result. All the various options have been examined, simulated, and verified by computer analysis, wind tunnel testing, etc., etc., etc.
From launch to about 4 minutes into flight the shuttle can perform the scariest type of abort – a Return to Launch Site abort (RTLS). Prior to the first shuttle flight, somebody proposed that we do an RTLS on purpose as a test — they called it the “Sub-Orbital Flight Test (SOFT). Capt. John Young, the chief of the astronaut office and the commander of STS-1 was noted for his colorful memos that he would regularly send on topics of the day. The SOFT proposal drew a classic response: “RTLS requires continuous miracles interspersed by Acts of God to be successful” John wrote in 1980. And in fact, on STS-1, a trajectory bug lofted the shuttle trajectory higher than expected and an RTLS probably would not have been successful.
Since those days, RTLS has been significantly improved and would most likely work — but I’d just as soon not find out. In particular the separation from the External Tank is very tricky. ‘Nuff said on that subject.
From about 2 1/2 minutes into flight until almost orbital insertion loss of an SSME could result in a Trans-Atlantic Landing abort (TAL). The shuttle keeps going forward but aims for Europe rather than orbit. The entry is very similar to a normal end-of-mission entry and the landing would occur at a prepared runway in Spain or France (in the early days we also had landing sites in west Africa).
Later in flight, from about 4 1/2 minutes on, loss of an SSME would result in an Abort To Orbit (ATO) where the shuttle presses forward and we try to scavenge out all the propellant in the External Tank to go on to orbit. Sometimes a dump of propellant from the Orbital Maneuvering System is required, sometimes other adjustments to the trajectory are required, but ATOs can range from landing after a few orbits on launch day to having a fully successful mission depending on many variables. The longer the main engines run, the closer to normal the shuttle can get.
The Abort Once Around (AOA) mission – which is exactly what it sounds like – is basically not used these days except for problems like a big air leak from the crew cabin.
Now all of that is fine as long as two of the three SSMEs continue to operate and the shuttle remains under control. If control is lost, then all is lost since the shuttle does not fly sideways very well. A capsule might right itself, but the shuttle will break up.
If two of the SSMEs quit but one remains running, there are some options to steer toward the east coast of the United States and land at an emergency airfield somewhere on the Atlantic Coast of North America. However, many of these trajectories result in entry conditions that exceed the capability of the shuttle orbiter either thermally or structurally: black zones. The possibility of executing a successful East Coast Abort Landing (ECAL) is far from guaranteed, but in that situation it is worth a try. What is the other choice? If the shuttle doesn’t break up or burn up on the steep ballistic trajectory for an ECAL there is every reason to believe that a safe landing will occur. That is sort of a big “if”, however.
If three SSMEs quit all at once, there is real trouble. There is little to no way to control trajectory and the black zones get immense. In some lucky cases a successful ECAL might result but then you are not really having a lucky day if all three engines quit, are you?
My least favorite abort is a low alpha (low angle of attack) stretch to try to cross the Atlantic and make it to Ireland or someplace. These multiple-engine-out aborts result in extreme heating on the wing leading edge and the RCC panels are likely to fail. Another thing to try if there are no other options.
And of course, if the whole stack comes apart, its game over. Don’t even talk about a failure of a Solid Rocket Booster, either.
So the shuttle has a lot of capability compared with other rockets — and a lot less capability than any capsules.
More black zone discussions tomorrow.