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In Project Gemini, the Titan II GLV space rocket used as fuel hydrazine and nitrogen tetroxide, so safety concerns over its toxic propellants were present. But along with the special care that was needed due to its toxic fuel, something else that should guarantee safety was the launch escape system.

Usually there are two types of escape systems:

-First, the capsule abort system with rockets mounted on a tower above the capsule. (Dragon V2 uses a different concept, but still the role is to “push” the capsule away from the rocket.)

-Second, ejection seats as used in military aircraft: each crewmember returns to Earth with an individual parachute. But such systems are effective in a limited range of altitudes and speeds.

In human spaceflight programs, a possible failure would mean crew loss and a possible termination of the program or, in the best case, postponing it for years until everything would be 100% safe. But Project Gemini was like a stepping stone for the manned moon program. Time was very important because it was a space race in those years, and to achieve the goals, everything should be successful, at the right time, and safe. Of course, taking the risk could lead to success, but why did NASA choose that kind of technology, the ejection seat, instead of a system with rockets mounted on a tower above the capsule, which was tested previously in Project Mercury and used in later capsules like Apollo and Soyuz. The tower escaping system is used even in this days in Soyuz spacecraft and will be used in the future missions of Orion spacecraft. Since the ejection seat system is effective and safer in a limited range of altitudes and speeds, the risk is higher by using this system because the failure of the rocket could happen at any time in different altitudes and speeds. The tower escape system had been developed and tested as a technology before Project Gemini, why wasn’t it chosen?

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  • $\begingroup$ Hypergolics or not, the main concern would be giant explosions, not the toxicity of the fuel. $\endgroup$ – Hobbes Apr 7 '16 at 20:30
  • $\begingroup$ Toxicity of the fuel has been one reason to stop the development of some previous rocket engines because of the higher chances for engine damages or failures which would cause that giant explosion and risking the crew. Anyway LR87-AJ-7 engine was a good one with good records for important missions and that's why the question is concentrated more to the launch abort system even if there would be a possible explosion since rocketry is not perfect, this system should be as good as possible or perfect so the crew would be safe always. $\endgroup$ – Mark777 Apr 7 '16 at 21:38
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While a number of sources say the motivation for using ejection seats instead of LES is to save weight, a tower launch escape system can be jettisoned relatively early in flight, while ejection seats have to be carried all the way into orbit. Depending on the exact masses and time of tower jettison, this can be a wash or even slightly favorable to a tower LES.

A down side to ejection seats is that they offer less protection to the crew members and are more likely to cause injuries (if a crew member has an arm outstretched in a way that doesn't clear the hatch during the ejection sequence, for instance). Thus LES is generally preferable.

For Gemini, according to Max Faget (page 38), there were two major considerations that made ejection seats more attractive.

One, at one point in development, Gemini was going to land as a controlled glider, touching down on a runway under a parasail instead of splashing down at sea under a parachute. If something went wrong during the glide approach, having ejection seats would be nice, but a LES would be overkill (and would require another parachute besides), and carrying the LES through the entire flight would be too large a mass penalty.

Two, unlike the launchers for the Mercury and Apollo programs, the Titan used hypergolic propellants which burn on contact; while that sounds dangerous, in a major failure situation where the propellant tanks ruptured, they would start burning as soon as mixing began, which would tend to disperse the remaining propellants and give an overall less energy-dense, slower-burning fireball. With the non-hypergolics in the Mercury and Apollo launchers, it would be possible for there to be extensive mixing of the propellants before ignition, which would yield an explosion, in which case the extra protection afforded by the capsule would be very important.

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  • $\begingroup$ Thanks for the link to that Max Faget oral history; that looks like fascinating reading. $\endgroup$ – DylanSp Apr 8 '16 at 15:12
  • $\begingroup$ Ejection seats are lighter, but cannot be staged like the escape tower is. Are you sure the penalty is still lower than the escape tower ? $\endgroup$ – Antzi Sep 27 '16 at 4:16
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    $\begingroup$ Good point, and by my estimates, an escape tower is preferable to the tune of about 50 m/s of delta-v, if jettisoned at the same time as primary staging. $\endgroup$ – Russell Borogove Sep 27 '16 at 8:06

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