CEV and SM Safety Issues

[rsp1202]

Crew Exploration Vehicle safety is improved over shuttle because of, among other things, use of safer RCS propellants, more redundant systems, lack of fuel cells and APUs (more SM related than CEV), disposable heat shield. That leaves parachutes and landing bags as things that must absolutely work first time. Any other components that will either guarantee astronaut safety or need to be addressed?<br /><br />As for service module, it seems to come down to the engine (or engines) used for de-orbit burn and getting out of lunar orbit. That means pumps and pressurization issues, as well as fuel mixing and combustion. What all is involved here?<br /><br />In 1964, Martin Caidin wrote the novel "Marooned" in which the Apollo SM engine didn't fire, leaving astronauts stranded in earth orbit. Caidin had Wally Schirra critique the manuscript, and Schirra claimed Caidin missed one vital element that would have ensured the engine would ALWAYS fire -- but he wouldn't tell Caidin what it was so he could always remain "one up" on the author. Anyone know what Schirra knew?

 

[tap_sa]

<font color="yellow">". That means pumps and pressurization issues, as well as fuel mixing and combustion. What all is involved here?<br />"</font><br /><br />CEV SM main engine will be pressure fed LOX/methane. This is very simple and reliable technology, the only moving things will be the valves. There is no special pump, pressure inside propellant tanks pushes propellants into the combustion chamber. This same engine will be on the lunar ascend stage.<br /><br />Apollo had similar setup but main difference was that Apollo had hypergolic propellants ie they spontaneously ignite when fuel and oxidizer are brought together. The new CEV must have special igniter (sort of a spark plug). I believe the hypergolic effect is what Schrirra was referring to. Apollo engine will fire if you just manage to open the valves.

 

[henryhallam]

Exactly - and the valves were in a redundant series-parallel arrangement:<br /><pre><br /> /- />(VALVE)->(VALVE)-<br />Tank-| |- />Engine<br /> - />(VALVE)->(VALVE)-/<br /></pre><br />This way, a valve can fail either shut or open while retaining full control. Even two valve failures is often survivable depending on which ones they are.<br /><br />I think the Apollo SPS might even have had two whole sets of valves like that for the fuel and two more sets for the oxidiser. A very safe and redundant system. Of course, you are still in trouble if the engine or tanks sustain serious damage such as what happened with Apollo 13.

 

[darkenfast]

Regarding the airbags and parachutes: of course they must work, but their safety is enhanced if there are more than one. For example, Apollo had three chutes, a nice arrangement because all three served a useful function and were not dead weight (as a reserve chute would be). The design was such that failure of one resulted in only a slight increase in descent velocity. I imagine the the airbags could be designed the same way.<br />As for the heat shield: monolithic ablative heatshields have an extremely good track record. Does anyone know of a failure of one? <br />Reliability of the new engines will probably be the area that gets a LOT of attention. Triple ignitors?

 

[dobbins]

The heat shield failed on Cosmos 140 in 1967. This was a unmaned test flight of the then new Soyuz capsule. The failure had deadly consequances. It increased the presure on a faulty parachute sensor causing it to work when it shouldn't have. Not catching the faulty sensor in the test flight resulted in the death of the Cosmonaut in Soyuz 1 a couple of months later.<br />