|Posted on Nov 23, 2008 02:31:51 PM | Wayne Hale | 16 Comments ||
I keep meandering around on this topic and if you get confused, I'm sorry about the writing style.
Just to review the story: Mercury, Apollo, Soyuz, Shien-Zou, and the Orion all use launch escape towers for crew safety. Gemini used ejection seats. The shuttle famously does not have a crew launch safety system although it had ejection seats for early flights for problems during entry and now has a bail-out pole and parachutes for problems resulting in not being able to reach a runway.
I will discuss re-entry safety in a later post; I know a lot of folks are interested in that, too.
Of course a launch escape tower does not provide complete safety. For example, off the pad or very early in the launch of a Saturn rocket, the launch escape system would get the astronauts away from the rocket, but the Apollo capsule would land on the beach. Those capsules were not rated for anything other than a water landing so crew injury potential was high. Similarly, there was a tremendous concern about running into the launch umbilical tower shortly after liftoff. In some of those scenarios, the launch tower might not be effective in getting the capsule away.
At a certain point in the launch sequence the escape tower is jettisoned. Survival here depends on several factors. First of all, that the launch vehicle failure still results in the spacecraft being pointed in the direction of travel. As we saw in the Soyuz 18A story, the third stage firing with the second stage attached resulted in large attitude excursions and a subsequent flight direction that resulted in far higher loads than a controlled abort at that point should have. A real spin up of a launch vehicle would probably overwhelm any of the launch escape systems ever designed.
Second, after launch escape tower jettison, for multi-engine rockets, the capability must exist for all the running engines to be shut down. So if you have an engine out case and everything is still holding together but you cannot shut the remaining engines down perhaps due to an electrical fault, generally the capsule cannot get away. There must be a rocket engine to separate the capsule from the failed launch vehicle, but many times these are relatively small. For example, the Gemini Orbit Adjust Maneuvering System (OAMS) provided such slow acceleration that even with one engine out on the Titan II second stage, the capsule could not get away. Mercury used very small separation rockets with the option to fire the solid retro rocket package, but these could not overcome the acceleration of even the Atlas sustainer engine burning alone. So attitude control upsets which in themselves can be caused by electrical faults, coupled with the inability to shut down the upper stage engine(s) - again could be the same electrical fault - could lead to very bad outcomes. On the other hand, Apollo with its huge Service Propulsion System (SPS) engine -- designed to launch the CSM off the moon when direct flights were envisioned -- had enough oomph to get the Apollo capsule out of almost any circumstance.
All that being said, a capsule with moderate rocket engines and a launch escape tower on top of a long slender rocket is much safer than any tandem design like the space shuttle.
The space shuttle is safe if any single SSME prematurely shuts down and both of the other engines keep running AND the attitude control system is functional AND there was no debris generated in the engine shutdown that affects the orbiter's heat shield. That is quite a bit of difference. One of the reasons for the difference is that the cargo - up to 30 tons - goes everywhere the crew goes.
In the current design for Ares/Orion, the cargo goes up on a separate rocket. This allows for improved crew safety, but at some operational cost -- the crew capsule must rendezvous with the cargo on orbit before any work can commence.
Two other quick points. One person wrote a comment that the SSMEs are so reliable that we could quit worrying about one shutting down in flight. While the engine designers and builders are justifiably proud of the extraordinary reliability of the SSMEs, nobody that has studied them in detail rests easily. Too many parts are rotating at too high speeds, combustion is taking place at too high temperature and pressure for anybody to not keep their fingers crossed for the entire duration of engine firing.
Second, a number of folks have wondered why we did not put a crew escape system on the shuttle. Various systems have been proposed, a number have been studied to a high level, and a couple of designs have been looked at in detail. Ejection seats have been rejected for reasons I mentioned earlier. Putting the crew in a capsule in the payload bay that could be separated has received a detailed examination. Some of the limitations associated with this proposal can be easily imagined. Another idea was to separate the crew module with large solid rockets away from the rest of the shuttle in a launch emergency. And there have been others. The problem with designing in these solutions to an already flying vehicle is that none of them are as reliable as we would like; all of them are very heavy and drive the center of gravity out of an acceptable region, and they all cost an incredible amount of money to retrofit in. So, none of them have been implemented.
Any winged orbital vehicle under consideration needs to have a serious capability for crew escape designed in from the beginning. As to vehicles which are carried aloft by other aircraft for their launch; crew (and passenger) safety in that environment has its own challenges and is going to be neither a simple nor cheap capability to design into that type of vehicle.
I think this rounds out my discussion on Black Zones for launch and how they affect spacecraft design. All you guys out there working to design a spacecraft, keep these points in mind.
Tags : aborts, launch escape systems