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Liquid Rocket Engine


georgexiro
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Me and my friends are building an amateur liquid rocket engine with kerosene and liquid oxygen. But we have roblems with the fuel flow pressures.. we need them to be over at least 14 bar so the fuel does not fail going into the combustion chamber... does anyone know how to solve this?? waht pump could we use?

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For a small rocket, I would go with a blowtorch design. Tanks are heated up by the engine and generate their own pressure. Of course, you need tanks that can actually hold 14 bar, then, but for LOX you probably want that anyways. BP temperature at 14 bar is quite reasonable for both. Make sure you have relief valves, though, or you might have serious problems. And I hope you are good with thermodynamics. This thing is not easy to design to make sure it can maintain the pressure at correct flow rates nor exceed the maximum pressure. Then again, if you don't know how to do these things, you probably shouldn't be building a liquid fuel engine either. These things tend to explode for very similar reasons.

The more classical solution is a centrifugal pump, but these things don't scale down very well. Also, heavy, pressurized tank is a much greater disadvantage for a large rocket. So pumps make sense there. For a small rocket, it's more trouble than it is worth, generally.

If you do go with pressurized fuel, you might also consider switching over to liquid propane. The main reason Kerosene was used in V2, etc., is because it is liquid at ambient pressure. You don't need that feature with pressurized tanks.

If you do end up actually building this thing, make sure you have a proper bunker for testing and that you can fire it up and kill the fuel flow remotely. If you actually get the sustained burn, the main failure mode for a rocket is a catastrophic explosion. And liquid rocket means metal parts that become shrapnel. This means you can't be just standing a few meters away or have a bit of plywood in between. That won't protect you. If you have nothing you can use as a proper bunker, at very least, dig out a ditch and make sure there is no line of sight from the engine to you, any other person, any houses, etc. If you need to watch it, get a web cam set up nearby, or something.

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I found the following through google:

http://www.cientificosaficionados.com/libros/cohetes.pdf

Here they use gaseous nitrogen to pressurize the fuel tank, there is a diagram on page 41. They use gaseous oxygen instead of liquid, but I imagine you could use gaseous nitrogen to pressurize a LOX tank as well. Further google searching tells me that Helium is also a viable gas. Basically any inert gas should work.

I REALLY hope you know what you are doing. I can't emphasize safety enough. Do you actually plan to launch a rocket with this engine, or will it never leave a test stand?

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well, we have ambitions for the engine! :P

Of course we have figured out all the theoretical stuff we need. The alloy for the nose cone and combustion chamber will be made from 6061 T-6 Aluminum alloy.

We will be prepared for the tests and thats why we need a steady pressure for the fuel to flow in the chamber without combustion instabilities occuring. We know rockets tend to explode (thanks KSP) and thats why we have noticed the fire department for safety instructions. It will be a rrelatively small engine with no more than 400N thrust.

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Right, at 14 bar, kerosene, and 400N, I wouldn't even bother with the bell. Get the nozzle diameter for desired thrust at your desired pressure and just regulate flow rate with tank pressure. Should be a simple enough build and you won't have to worry about overpressure. Though, it can still resonate and explode, so don't let your guard down. As far as maintaining tank pressure, you are looking at a flow rate upwards of 130g/s. That's 100g/s (~90ml/s) of LOX and this will soak up about 20kJ of heat if you let it boil. So heating tanks is probably a no go. You'll have to pressurize them with an inert gas instead, as someone has suggested earlier in the thread.

Now, for pressurizing tanks, nitrogen liquefies before oxygen does, so it's not going to work in the LOX tank. You are pretty much limited to a light nobble gas. Helium's expensive, but you should be able to get away with something like neon, which is cheaper. Kerosene tank, of course, can be pressurized with nitrogen gas, but it might be easier to use the same gas for both. The only tricky part is maintaining the desired pressure in tanks. The best way to go about it is having a small, high pressure tank with the inert gas and use a regulator to reduce it to 14 bar, or whatever. First stage regulator for Scuba will typically give you something in the 5-10 bar range. You might be able to adjust one of these to go up to 15 bar. A used regulator can be bought for very reasonable prices on-line. I've seen entire octo sets for under $40. I'd never recommend buying something like that for diving, but for an amateur rocket engine it's still probably going to end up the safest part of your rocket.

Edit: You are way past melting point for 6061 T-6. You are looking at nearly 3,000°C under optimal mixing ratio. This particular alloy will melt at around 600°C. You should consider other materials.

Edited by K^2
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Right, at 14 bar, kerosene, and 400N, I wouldn't even bother with the bell. Get the nozzle diameter for desired thrust at your desired pressure and just regulate flow rate with tank pressure. Should be a simple enough build and you won't have to worry about overpressure. Though, it can still resonate and explode, so don't let your guard down. As far as maintaining tank pressure, you are looking at a flow rate upwards of 130g/s. That's 100g/s (~90ml/s) of LOX and this will soak up about 20kJ of heat if you let it boil. So heating tanks is probably a no go. You'll have to pressurize them with an inert gas instead, as someone has suggested earlier in the thread.

Now, for pressurizing tanks, nitrogen liquefies before oxygen does, so it's not going to work in the LOX tank. You are pretty much limited to a light nobble gas. Helium's expensive, but you should be able to get away with something like neon, which is cheaper. Kerosene tank, of course, can be pressurized with nitrogen gas, but it might be easier to use the same gas for both. The only tricky part is maintaining the desired pressure in tanks. The best way to go about it is having a small, high pressure tank with the inert gas and use a regulator to reduce it to 14 bar, or whatever. First stage regulator for Scuba will typically give you something in the 5-10 bar range. You might be able to adjust one of these to go up to 15 bar. A used regulator can be bought for very reasonable prices on-line. I've seen entire octo sets for under $40. I'd never recommend buying something like that for diving, but for an amateur rocket engine it's still probably going to end up the safest part of your rocket.

Edit: You are way past melting point for 6061 T-6. You are looking at nearly 3,000°C under optimal mixing ratio. This particular alloy will melt at around 600°C. You should consider other materials.

Hmmm...Well.. our problem is the right container for the fuel that can hold up to 15 bar... I'll look it up. I've figured out the flow to be around 130 ml/s of fuel. giving a 1.5 ratio of mixture I think it would give off some decent results..

Seriously?? 600 degrees??? lol.. I only looked it up for the pressure limits which seem to be pretty good.. what do you think will hold up to that temperatures?? and for a reasonable price of course. I live in greece. :sticktongue:

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Yeah, aluminum is pretty easy to melt. That's why the airplane's frame is built of aluminum, but engines aren't. Alloys are always easier to melt than any component as well, so that makes it even worse. Personally, I'd go with steel combustion chamber and a carbon nozzle. (You can buy a carbon crucible and grind it to the right shape. You can find these for under $10 around here.) Failing that, make everything out of steel. It will still burn through, but hopefully you'll have time to run the thing.

Where are you getting the 1.5 ratio from? Are you by any chance using the molar ratio here? Kerosene is pretty close to a (-CH2-)n, which does burn with oxygen at 2:3 ratio. But that means the mass ratio is going to be something like 14:48. You need almost 3x the LOX by weight as you do kerosene. LOX is a bit denser, though. So by volume, you are looking at something closer to 18:42. Still more than 2x the difference.

I might be wrong here, though. The above numbers come from stoichiometry. It is possible that it is not optimal for a rocket engine. (Though, I can't think why it would be so.) Do you have a source for the 1.5 ratio? Other ratios will burn cooler, but it will also give you a lower ISP. In other words, you'll waste more fuel to get the same thrust. Might not be a big deal, though, and depending on materials you settle for, it might be worth it to run mixture a bit fuel rich just to avoid burning through the combustion chamber.

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what do you think will hold up to that temperatures?? and for a reasonable price of course. I live in greece. :sticktongue:

Probably no materials available will be able to withstand those temperatures without an adequate cooling system (I'm hoping this is part of your design). The best bang for your buck is probably mild steel as K^2 mentioned. It's dirt cheap and plenty strong.

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im just reading this and I just want to do something like that myself.

Its a shame none of my friends is a rocket engineering enthusiast like me :(

I just hope the best of your rocket and whether it comes out successful or not, i would love some footage.

best of luck

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On the fuel ratio you want to run fuel rich. This will lead better actual isp in atmosphere and will lower your burn temperature. Not sure what the "right" ratio should be for normal kerosene, RP-1 american rocket grade kerosene is normally used at 3.0 mass ratio.

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Not sure what the "right" ratio should be for normal kerosene, RP-1 american rocket grade kerosene is normally used at 3.0 mass ratio.

It should be very close. RP-1 will give you a slightly better ISP, but it makes very little difference for fuel mixture. Though, I'd definitely recommend seeing if you can get some Jet-A. It's not quite RP-1, but it's better than your typical technical uses kerosene.

3:1 LOX to fuel is just slightly fuel rich, so it sounds about right. One more reason to run slightly fuel rich is because it will help prevent the metal in the combustion chamber from burning away. If you run oxidizer rich, that will be a serious problem. If you run balanced, it can still be a bit of a problem, because you'll never have perfect mixing.

P.S. This thread is also making me think about building a small rocket engine. I'd probably go smaller, though.

Edited by K^2
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So, for every 100ml of fuel you suggest we should go 300ml oxidizer? sound a bit of an overkill to me. I thought 200ml would be a reasonable amount. the LOX would be at extremely high pressures and thus expand when at the combustion chamber. :)

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So, for every 100ml of fuel you suggest we should go 300ml oxidizer? sound a bit of an overkill to me. I thought 200ml would be a reasonable amount. the LOX would be at extremely high pressures and thus expand when at the combustion chamber. :)

The ratios they've given aren't volume ratios, they're mass ratios. The expansion of the oxygen isn't going to create more oxygen atoms for the reaction. Using a mass ratio, you can find a volume ratio.

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Right. And because LOX is denser than kerosene, it does actually work out to about 210ml of LOX for 100ml of kerosene to get the desired 3:1 ratio.

Controlling such flow ratios, on the other hand, is not going to be exactly easy. These fluids will also have different viscosity, so getting flow rate from given pressure differential is not going to be exactly trivial. I wonder if there is some feedback mechanism you could design that would automatically ensure fuel-rich mixture... Electronic control is always an option, but you'd need some rather interesting sensors to figure out exactly what's going on with the mixture. There are flow rate sensors on the market, but they are typically designed for completely different purposes. At the end of the day, you are probably looking at a trial and error approach.

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Have you tried looking at copenhagen suborbital's work in the area? They try to make their entire process open source, and have already produced a number of quite large, workable engines. The design documents for the large TM65 engine haven't apparently been published yet, but the report on the smaller 'spectra' on there looks like the kind of thing you'd want to be looking at.

In terms of pressurisation they use fuel pumps rather than pressurised tanks; a modified ex-fire-engine pump for TM-65, and a V2-style peroxide turbopump for their larger engine in development.

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Kryten, you wouldn't use pumps for a 400N engine. You should take a look at how the anti-aircraft missiles have been designed. They have roughly the same design parameters. Relatively small, high thrust-to-weight ratio. The liquid fuel ones have always used pressurized pumps to keep the rocket light and less likely to fail.

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Seriously?? 600 degrees??? lol.. I only looked it up for the pressure limits which seem to be pretty good.. what do you think will hold up to that temperatures?? and for a reasonable price of course. I live in greece. :sticktongue:

Use stainless steel (V2A) and a pintle injector with Kerosene in the center, as this will result in a fuel rich environment at the chamber wall and cool it. Here's a nice paper that describes that injector type (nothing too specific though)

Also, if you don't have any experience with rocket engines or LOX, I have to strongly advise you to be very, very careful!

Liquid oxygen can be very dangerous, as it burns with basically everything and builds up enormous pressure if you don't employ proper cooling measures!

A very good alternative is laughing gas (N2O) as oxidizer. At room temperture, it is liquid and at a pressure of about 40-50bar. Therefore you wouldn't need a pump as the vapor pressure alone is enough to feed it to an engine with 20 bar chamber pressure.

Edited by philly_idle
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NOX is not a bad alternative, yes. But availability varies by country. In the States, it's relatively easy to get from car mod shops, as it's used for the nitrous boost. In some places, however, it is outlawed for that very same reason.

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NOX is not a bad alternative, yes. But availability varies by country. In the States, it's relatively easy to get from car mod shops, as it's used for the nitrous boost. In some places, however, it is outlawed for that very same reason.

I don't know about greece, but in western Europe it's readily available. Also, whipped cream chargers contain N2O...just use a 100 or so of them :wink:

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