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What is involved in 'safing' a shuttle?
"Jeff Findley" writes:
I'm not so sure. When I was a kid, I flew a small RC airplane which had a glow-plug on top and seemed to fire quite a few times a second. Certainly a non-toxic RCS thruster based on combustion of some fuel with LOX would be a challenge, but from a physics point of view, it doesn't seem impossible. Well, there are lots of things that aren't exactly impossible from a physics point of view, but still really complicated. Repeating all the uncertainties of ignition (and to do this with changing temperatures of the systems) hundreds of times a second is surely not going to give you more accuracy, I think. And for things like docking to a space station or other hardware you really, really need that. A pulsed system with liquid fuels in which all the fuel ignites is quite perfect here. Another interesting possibility is resistojets (or similar) which at one time were proposed for use on the space station. A non-toxic propellant like LN2 or even LH2 is heated by some means (resistive heating element or other means) and then the resulting hot gas is expelled through a nozzle. This sort of a thruster could be pulsed many times a second to achieve the sort of attitude control you're talking about. ISP may not be extremely high (LH2 would be better than LN2 in this case), but the toxicity would be zero. For nothing but fine attitude control this would be nice, yes. If you want to have the very same system double as an OMS in a pinch (as the shuttle does) this is a bit limited, though. A hypergolic RCS can be pulsed for really fine control and additionally used in a continuous mode for getting some real work done (as long as you have enough fuel) while being less precise then. As a backup for de-orbiting your craft this can be very handy to have. And it still can be quite simple. There are good reasons for using such fuels for RMS. They suck for things like quick turn-around, but as long as this is not a real hard requirement... Jochem -- "A designer knows he has arrived at perfection not when there is no longer anything to add, but when there is no longer anything to take away." - Antoine de Saint-Exupery |
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What is involved in 'safing' a shuttle?
On 2/28/2010 12:44 PM, Jochem Huhmann wrote:
Well, there are lots of things that aren't exactly impossible from a physics point of view, but still really complicated. Repeating all the uncertainties of ignition (and to do this with changing temperatures of the systems) hundreds of times a second is surely not going to give you more accuracy, I think. RC planes are fueled with a nitromethane/methanol mixture, and pure nitromethane can be used as a monopropellant, just like hydrazine: http://en.wikipedia.org/wiki/Nitromethane Simply heating it will cause it to decompose, and it's a lot more benign to work with than hydrazine. Another monopropellant is Otto Fuel II, such as is used in torpedoes; it is mildly toxic: http://en.wikipedia.org/wiki/Otto_fuel_II Pat |
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