Half a croissant, on a plate, with a sign in front of it saying '50c'
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Thermite dry ice rocket

Lots of hot - meets lots of cold.
  (+8, -1)
(+8, -1)
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Rocket fuel is scary because it blows up and catches on fire. Dry ice rockets are less scary in that respect. Dry ice is a nice way to handle a compressed gas because it is not runny or reactive. But a dry ice rocket is really not much better than a pressurized air / water rocket - the pressure built up on the ground is what the rocket gets to launch with. Just as a flying water rocket expels the water behind it, I suspect that a flying dry ice rocket expels unsublimated dry ice behind it. The problem is that the relative slowness of sublimation on the ground (which is good, because it is safe) is too slow to sustain flight in the air.

What is needed is some way to accelerate sublimation in the air and turn a large mass of dry ice into hot hot gas in flight. The hotter it is, the move volume it occupies, and the more propulsion you get. I propose thermite. Thermite is cheap and packs a lot of heat into a small area. It does not require oxygen to react and can be jiggered so it does not react explosively. A thermite reaction in a dry ice rocket will turn that dry ice into nice hot CO2 for propulsion. The thermite and dry ice should not be loaded willynilly in the rocket (but if you try, please post video). I think a central core of dry ice filling the rocket with the sides of the rocket lined with thermite would work best. Maybe the thermite could be seperated from the dryice core with a ceramic mesh, but this is getting a little Tom Sawyer.

If the thermite was doped with something to slow the reaction, it could start at the bottom of the rocket and proceed upwards, sublimating the adjacent dry ice as it went.

A problem is if the rocket explodes and showers bystanders with reacting thermite, which would hurt. The standard 2L soda bottle would be inadequate for this use because it would melt. I think an aluminum scuba tank might be used as the rocket body.

bungston, Sep 15 2008

Thermite http://en.wikipedia.org/wiki/Thermite
Thermiterrific! [bungston, Sep 15 2008]

Piston Rocket Piston_20Rocket
got me thinking. [bungston, Sep 15 2008]

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       It sounds to me as though you throw some fun parties. I'll donate a scuba tank and bring my own safety glasses, but you need to provide the barrier.
normzone, Sep 15 2008

       Thermite is wonderful stuff; you can use it to burn through anything, which begs the question what you are going to use to conduct the heat of the thermite to the dry ice which won't be destroyed by the high temperature thermite burn.
FlyingToaster, Sep 15 2008

       This sounds cool, though I'd put the thermite in the center, so the Al tank doesn't become part of the reaction mass.
MisterQED, Sep 15 2008

       Thermite in the center is good. But it will get windy there. There would need to be a ceramic cage of some sort.   

       Al tank / reaction mass - if it does then this converges on the piston-powered rocket idea of a rocket which consumes itself in flight.
bungston, Sep 15 2008

       Mmmmm .... Thermite doesn't have an amazingly good energy density; that's why rockets tend to use liquid fuel/oxidiser systems. The shuttle SRBs use perchlorate .... "The propellant mixture in each SRB is made up of 69.6% ammonium perchlorate oxidizer, 16% aluminum fuel, 0.4% iron oxide catalyst, 12.04% polymer binder and 1.96% epoxy curing agent"   

       This is a much more energetic system than thermite, and the working fluid (CO2) is effectively "dead" mass in terms of propulsion.   

       The key to newtonian-reaction rocketry is to rapidly and efficiently convert a (cold) dense fuel system to a (very hot) fast-moving flow of working fluid ....   

       The math required is to compare the mass/velocity characteristics of the proposed thermite/CO2 system with an equivalent mass of LH/LOX.
8th of 7, Sep 15 2008

       I am incapable of creating the math but willing to appreciate some elegant math. I would like to think that thermite / CO2 is safer too.
bungston, Sep 15 2008

       // thermite / CO2 is safer too //   

       Tautology: "thermite" and "safer" in the same sentence ?
8th of 7, Sep 16 2008

       It occurred to me that a way to use the dry ice without spattering scary thermite on every one would be to freeze it with flakes of metal foil within, then detonate it using a microwave.
bungston, Dec 07 2009


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