Chair Force Engineer

Tuesday, April 19, 2011

An End to Escape Towers?

Since the earliest days of human-rated space capsules, it's always been a challenge to safely tear the capsule away from a launcher that was exploding or boosting off-course. The system devised by the great NASA engineer Max Faget was a tower equipped with a powerful solid rocket that would attach to the front of the capsule. Escape towers debuted during Project Mercury and were reused for the Apollo & Soyuz spacecraft (still used today by the latter!)

During the pad & early launch phase, the escape tower is a heavy but effective method to pull a capsule and crew to safety. The engines on the capsule's service module or the other rocket stages lack the thrust or the response time to act in situations where the capsule needs to accelerate from a standstill to escape its booster, or where it needs to boost downrange and reorient itself into an attitude where the heat shield bears the brunt of the aerodynamic heating before the capsule can deploy its parachutes and land.

After the early portion of ascent (usually around or shortly after stage II ignition,) an escape tower is no longer needed. The service module or upper stages can get the capsule safely downrange, or even all the way to orbit. The abort mode is determined by the phase of the launch where the failure occurs. During Soyuz 18a, the bad separation of stage 2 & 3 led to an abort using the service module engine. Soyuz T-10 used the escape tower after the rocket caught fire on the launch pad prior to launch. One space shuttle mission even aborted to orbit after a main engine shut down early.

Engineers have recognized the limitations of launch escape towers, particularly how they are quite heavy for a piece of equipment that is only useful for less than two minutes during ascent. Gemini capsules got around this using ejection seats, but only because the aerozine-nitrogen tetroxide fireball of an exploding Titan rocket would be more contained than the boosters of Mercury. Former NASA administrator mike Griffin promoted his Max Launch Abort System, a shroud which housed four solid rockets and fit over the capsule. Now SpaceX will demonstrate a new launch escape system under a $75 million contract with NASA. The escape rockets would be completely contained within the Dragon capsule. It could very well work, but it does bring two concerns to mind. For starters, the mass of the escape rockets would make it all the way to orbit. And the use of multiple escape rockets reduces the system's reliability when compared with the single-rocket designs of the past. But it will be interesting to see if SpaceX can come up with an effective solution to this old problem.