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First some background, liquid rockets take fuel and oxidizer, mix them together, ignite them, direct the super-heated gas out a nozzle and the reaction to the mass of gas going out the back pushes it forward. The faster the gas goes out the back the more efficient the rocket is. Raising the pressure
inside the rocket causes higher exhaust velocity and higher efficiency, but needs the fuel and oxidizer to be pumped in at even higher pressures than the pressure inside the rocket. There are two main methods of pressurizing the fuel and oxidizer, pressurized the storage tanks and pumps. Pumps give higher pressures, but are CRAZY expensive, to the point where their cost is a major part of the whole rocket.
There are several oxidizers used for rockets, all are nasty in one way or another, but for this idea I will use liquid oxygen (LOX). Rockets use LOX because it is 860 times denser than gaseous oxygen (GOX). LOX is hard to deal with because it has to be stored cryogenically at ~-190 Celsius. This temperature makes use of most pumps hard or impossible.
My idea is similar to one from Flometrics (see link) to store the LOX in a low pressure vessel (50psi) and then pressure feed a quantity into a second small pressure vessel through a one way valve. Then boil the LOX to create pressure, then bleed that pressure out the bottom, hopefully taking most of the remaining LOX first. Then vent the remaining gas into a intermediary vacuum (low pressure) chamber to cool the boiler. Then pump the GOX back to the main LOX tank from the vacuum chamber and start the whole process again. Use two chambers alternately for a constant feed rate.
Heat seems to be the quantity most available on rockets, so it makes sense to use it to pump the fuel.
If you use Propane for fuel, then a similar system can handle fuel pumping.
(?) Flometrics Pump
http://www.flometri...R7JYCFQOjFQodZkaIrA
Uses heated helium for pressurizing gas [MisterQED, Jul 21 2009]
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Annotation:
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There is something distinctly Dr Suess about using a LOX to GOX to LOX pressurising box. It knocks the blocks off NOX, buy the stocks! |
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We need lots and lots of LOX and GOX, low the GLOW... |
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Oh well, I am not Dr. Suess, but yes I have that book and my tongue does hurt when I read it. |
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It has got to be one of my most hated favourite things. My kids request that I read it, they say, oh say, can you say, 'cause mommy can't. |
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So we have a lot of LOX, in a "not so cold" cold box. Let's call it a "Hot" LOX box. We feed some of this LOX into a locked box (the second chamber with valves) We burn some LOX (in the reaction chamber with nozzle) with some stocks (of hydrogen) to create thrust and some more hots for boiling LOX into GOX in the locked box. We take the GOX, under pressure and feed it back into our second box with LOX from the "Hot" LOX box. Using the heat to create a pump from the "hot" LOX box to the "hotter" reaction chamber, under pressure from our locked GOX box. |
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The switching and valving seems complex. How high a
pressure do you need, and why can't you just pressurize the
main LOX tank in this way? Especially for a small tank, the
extra weight to withstand the pressure might be less than
the weight of the additional valving and switching stuff. |
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//The switching and valving seems complex.//
Yes, but not too much the tough part is putting in just the right amount of heat for the LOX and the timing the GOX release. The other two functions could be just pressure release valves. With correct design the heating could be self regulating. In the long run even if it is complicated: except for the vacuum pump the system would be powered by waste heat, so it's "found money". |
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// How high a pressure do you need//
As high as I can get, though it is diminishing returns as you go up. LLC teams are running rockets with ISPs of around 200s using pressure fed systems of varying pressure between 300-500psi. I want to double the pressure to get near 280s. There are theoretical maximums based on every fuel oxidizer pairs at a given pressure. What an Isp pf 280s means in real terms is 280N of thrust for every 1kg of LOX/propane. That extra thrust means you need less fuel, which means you need less thrust, which means you need less fuel, etc. |
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//why can't you just pressurize the main LOX tank in this way?//
You could but the weight and expense of a tank that can handle those pressures and volumes in itself exceeds the N-Prize limit. We NEED to REALLY drop the price of getting stuff to orbit, that is the real prize I am looking for. |
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Slightly off topic but how is LOX usually kept liquid? Are the storage tanks giant vacuum flasks? |
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//how is LOX usually kept liquid//
Outside the rocket it is kept in a dewar, which is like a vacuum flask except the boil-off is used to cool the remaining liquid.
On the rocket it is just kept in insulated tanks. |
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//What an Isp pf 280s means in real terms is 280N of thrust for every 1kg of LOX/propane// I thought it meant 280N.s of impulse for every 1N (about 0.1 kg) of LOX/propane. |
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// So we have a lot of LOX, in a "not so cold" cold box. Let's call it a "Hot"
LOX box. We feed some of this LOX into a locked box (the second chamber
with valves) We burn some LOX (in the reaction chamber with nozzle) with
some stocks (of hydrogen) to create thrust and some more hots for boiling
LOX into GOX in the locked box. We take the GOX, under pressure and feed
it back into our second box with LOX from the "Hot" LOX box. Using the heat
to create a pump from the "hot" LOX box to the "hotter" reaction chamber,
under pressure from our locked GOX box. // |
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I was just now finally able to read that quickly without getting
tongue-twisted.
I think it helped me to be extremely tired and listening to music
that I can sing along to on autopilot (though I obviously wasn't doing so while
reading it), somehow. Maybe the first part is something to do with tiredness
and alcohol having similar effects (apparently—I've never tried alcohol), and
alcohol in low doses apparently improving performance on certain tasks, and
maybe the second part is because the music distracts one part of my brain
that would otherwise distract another part of my brain that needs to be
undistracted to read it properly? |
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Edit later: I tried a couple of other tongue twisters off the top of my head earlier ("toy
boat" and "she sells seashells by the seashore"), and was able to do a lot better than usual
on them too. Still haven't slept. |
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I'm not clear, from this, whether the oxygen is a working fluid,
being cycled around a closed system, or a reaction mass, being
pushed out behind the rocket, or a reagent, binding
exothermically (to, say, propane, or whatever), or some
combination of those three. If it's a combination, then
presumably the system will have to deal with a rapidly dwindling
amount of working fluid. Would anyone care to clarify? |
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