h a l f b a k e r yClearly this is a metaphor for something.
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Cheap Turbo Pumps for small liquid fueled rockets
Solid fuel rockets only get you so far. Great lifting force but not
great at high speeds.
Large liquid fueled rockets use expensive centrifugal pumps to raise
fuel and oxidizer pressures up to the point where they can be injected
into the rocket
motor. The higher the pressure of the pumps the
higher the pressures you can run the motor and the more efficient the
system is.
The general solution for small rockets is to eliminate the pumps by
using pressurized tanks. The down side of this is the weight. The
more pressure the tanks can take the more they weigh.
I saw a re-post on Space Fellowship called “Unreasonable Rocket -
Lessons Unlearned” (Link #1) about using electric motors to drive
hydraulic gear pumps to feed a H2O2/RP1 rocket. This has two
problems, it adds a lot of weight for the batteries and the electric
motors and the gear pumps add pulsation in the flows from the gear
meshing. I started wondering if there wasn’t a better way. XCOR is
using gangs of piston pumps run by electric motors and also has some
kind of free piston pump (link#2) design pushed by Helium. (Link #3)
I had heard about the latter on a show about the RRL (Rocket Racing
League).
My solution dates back to the steam engine. In fact it would be
recognizable as a steam engine. (Link #4) The basic idea is to bleed
pressure from the rocket and pump it into a double-acting steam engine
“power” piston which turns a wheel (add generator here if required).
The piston also operates a pair of smaller double acting pistons that
pump the fuel and oxidizer. The fuel and oxidizer are then pumped
into twin piston accumulators that are pressurized by a volume of CO2
or other gas/liquid mixture whose phase change occurs within reachable
temperatures to create the pressures.
The height of the pistons in the accumulators would be monitored and
the engine throttled to keep them within the center range. Throttling
would be performed by heating or cooling the liquid CO2 to vary the
injection pressure.
Link#1
http://unreasonablerocket.blogspot.com/
[MisterQED, Nov 07 2008]
XCOR piston pumps
http://www.xcor.com...ts/pumps/index.html
[MisterQED, Nov 07 2008]
XCOR free piston pump
http://www.google.c...AAEBAJ&dq=xcor+pump
[MisterQED, Nov 07 2008]
Steam engine
http://en.wikipedia...ngine_in_action.gif
[MisterQED, Nov 07 2008]
[link]
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I've not had much to do with rockets, but if I read right then you're basically wanting to use a 'turbocharger' type syspem to pump your (I assume liquid) fuels, smoothing the fuel/oxygen mass flow with accumulators? |
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Fuel/Oxygen
Tank -> Pump -> Accumulator -> Injector -> Combustion chamber with bleed off to feed turbine side of pumps -> Nozzle -> Thrust |
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The idea sounds good, although I think your throttling idea mey need to be changed, I think you will find that heating/cooling the CO2 would simply change the volume within the accumulator, rather than the pressure. I guess the best way to control it would be the pressure of the gas feed to the pistons, with a PID (or similar) controlled throttle. |
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I would suggest you'd want 2 or 3 preferably double acting pistons (90 or 120 degrees apart respectively) to give you a constant torque supply to your pumps, as this would remove the need for a flywheel in the system. |
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As a secondary boost - could you use the 'exhaust' from the pistons to heat your tanks, either with a coil inside the sphere or some kind of physical connection on the outside? |
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//heating/cooling the CO2 would simply change the volume within the accumulator, rather than the pressure.// The CO2 is in transition between liquid and gas. (link) That transition takes place at a specific temperature and pressure. If you heat the liquid, then the pressure will rise, push harder on the accumulators, The higher pressure will cause more fuel/oxidizer to go into the rocket, throttling up the rocket. The average change in piston position will throttle up the "steam engine" to correct the position of the accumulator piston. |
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//you'd want 2 or 3 preferably double acting pistons//Yes and no. You are absolutely correct that more pistons would smooth out the flow, but it also raises the complexity. The pumps are in phase with the torque of the power piston, so the wheel doesn't need to act as much of a flywheel, it just is there to order the motion of the piston and prove an easy place for a small generator. I wanted to keep this as simple as possible. I could replace the piston pumps with screw pumps, but that seemed more complicated and now the torque requirement is constant, so I'd need to have a flywheel and guarantee that the pump was turning the right way. |
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//As a secondary boost - could you use the 'exhaust' from the pistons to heat your tanks// Yes you could, but you don't want to. The idea is to keep the pressures in the tanks as low as possible to make them cheap and light. Also the oxidizer is rarely something you want heated before combustion. |
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// The more pressure the tanks can take the more they weigh // |
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You're considering that the tank must be pressurised to the maximum by a propellant gas, which - by the time the tank is empty - still has enough residual pressure to expel the last of the propellant. |
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The answer could be found by using a pyrotechnic gas generator. These are used in fuel-air explosives (see under "poor man's atom bomb), a bulk overpressure weapon, also used for minefield clearance. |
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On launch, the gas generator is fired electrically. The hot gas is bled at a controled rate through an orifice plate into the propellan tank. As propellant is forced out, more gas enters to amintain a steady pressure. |
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There's little risk of an internal explosion, as the gas, while hot, is effectively inert; when compressing fuel, there will be too little oxygen for ignition (despite the heat) - with the oxidiser,there will be nothing left to oxidise. |
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Pyrotechnic gas generators are a simple, well understood and extremely reliable technology. they are small, lightweight, and cheap. |
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Ahh, no. I'm not going to dispute the gas generation system, but I will dispute it solves anything. Your primary supposition is incorrect, pressurization is usually performed by a separate high pressure helium tank, so that is not the problem. The problem with any pumpless solution is the difference in weight between a plastic 5 gallon gas tank and a 5 gallon fiberglass wrapped aluminum high pressure tank. If we need 1000 psi then it pays to pump the fluid up to that pressure outside the tank vs. building a tank that can contain the pressure. |
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Is there a pressure reducer between the helium tank and the propellant tank ? |
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Yes, a pressure regulator. I'm sure the helium is at thousands of pounds pressure as the tanks are always a tiny percentage of the size of the fuel tanks. The gas generator is s good idea though to power the pumps. LLNL seems to have created "the smallest" pump for rockets using one to power the pump. It's the ASTRID project. Their site has a much better explanation of the big picture. Just Google "LLNL rocket" and you can find it. I have to read up on gas generators, my biggest worry is bleeding the pressure from the rocket. |
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