Rocket engine questions

[plasma]

Hi I know this is a electrical forum but the people here seem to branch out.
I'm designing a liquid/gas rocket and was wondering if a combustion chamber is need, is larger area than a throat, is there any pros or con's?
I'm thinking about building a protype from mild steel, and was wondering if it gets to hot the oxygen will react with the steel and turn it into a molten pool, does that happen at quite low temperature with high O2 levels?
Thanks all

[Twospoons]

I'm designing a liquid/gas rocket and was wondering if a combustion chamber is need, is larger area than a throat, is there any pros or con's?

This question tells me you have done little to no research, and are a long, long way off making any sort of prototype.

[plasma]

Just so I understand you and vice verse, how so?

[TMaxElectronics]

This question tells me you have done little to no research

I agree. Attempting to build a rocket without having at least read enough to have heard about nozzle theory is like building a tesla coil without knowing what an inductor is.

If you are really interested in building one you should go and look for some info on the basic principles of rocketry and rocket engines. I do not recommend attempting this until you can answer these questions yourself.
Not because I'm trying to gate-keep here at all but because rocket engines can easily cause serious damage and have killed people before.

[plasma]

I've done some more reading and the velocity comes down to the difference in pressure from the chamber to the nozzle exit pressure.
So far I've got 150psi max, which I was hoping to sustain through the throat, if the mass flow rate was high enough, couldn't I make the combustion chamber cross section area close to the throat.

Its going to be a small engine maybe 14kg thrust.

[Twospoons]

couldn't I make the combustion chamber cross section area close to the throat.

Do you know what the flame speed of your chosen fuel/oxidiser is? Have you calculated the gas velocity in the throat? Have you calculated your combustion chamber temperature?
The answers to these questions will let you know the answer to the first question.

You will want to go and find a piece of software called "Propep". I don't have a link - use your search-fu.

[plasma]

Thanks I searched and richard-nikata is a site I havnt seen in twenty years.
It says the software doesn't work nowadays, would a energy of combustion chart be accurate enough, assuming complete combustion?
I've got a lot more reading by the looks of it.

Methane is 55.5MJ/kg with 700J specific heat ,would 25grams of methane reach 2000K

[MRMILSTAR]

If I wanted to pursue something as technically sophisticated as a liquid-fueled rocket engine the first thing that I would do is to get a good text book on the subject and understand it cover-to-cover. Just looking at web sites and forums isn't going to cut it. We amateurs should stick with the relatively simple solid-fuel rocket motor or at most a hybrid rocket motor such as solid-fuel/NOS.

[klugesmith]

>>Methane is 55.5MJ/kg with 700J specific heat ,would 25grams of methane reach 2000K

Learning how to answer that yourself is one of the prerequisites. Why do you think it's called rocket science?

First you have to define the question more completely. We are all too far away to read your mind.
One could pass methane through a resistively heated tube and have gas emerge at 2000 K at atmospheric pressure.

Suppose we guess that you mean "combustion products temperature of 2000 K".
At what place in the compressible flow?
What is the oxidizer, and in what phase to begin with, at what temperature?
J is not a unit of measurement for specific heat; you need a material quantity like mol or g in the unit.
What is the specific heat of the reaction product mixture?
Why does the temperature depend on the number of grams of methane?

[rikkitikkitavi]

Thanks I searched and richard-nikata is a site I havnt seen in twenty years.
It says the software doesn't work nowadays, would a energy of combustion chart be accurate enough, assuming complete combustion?
I've got a lot more reading by the looks of it.

Methane is 55.5MJ/kg with 700J specific heat ,would 25grams of methane reach 2000K

Whether och wether not you fuel-oxidizer mix reaches a certain temperature is all about heat loss  through various ways. A small butane torch has difficult to heat copper to the melting point even if the hottest point is 1500K.

[plasma]

Thanks everyone for the pointer's. Its going to be compressed air at 120psi with LPG, latter deseil.
I've got 20mm2 for the throat and sticking to a standard multiple of 4 taking it to 80mm2 for the chamber. I'm assuming a temperature of 973K. The air flow rate is 2Litres/sec 0.0012kg but expanded up to three off the shelve compressors making 6L/s.
I have a sparkplug continue fired. The volume of heated gas is 0.33m3/kg, velocity 297m3/s with 180m/s in the throat. A standard multiple of 15 for the expansion part.

[klugesmith]

Cool! We await your measurements of engine thrust.   How about thrust with compressed air only, then with the fire going?

Are your compressor and engine airflow ratings based on volume at standard pressure or volume at 120 psig?  Could reduce compressor cost and size by using storage tank for intermittent engine operation.   How 'bout very high pressure tank (e.g. for SCUBA, or welding oxygen) with high-flow pressure regulator?  Can always scale down your engine. Could characterize converging-diverging nozzle thrust with just compressed air.  Nozzle area ratio greater than chamber:ambient pressure ratio (with some gamma-dependent exponent) will produce a wasteful overexpanded flow.

How will you manage the flow of LPG into rocket engine? 
LPG tank pressure at room temperature is barely higher than your nominal chamber pressure.
As you consume gas, tank temperature and pressure will drop.  We see tanks frosting up in people's DIY jet-engine videos, and in my old crucible furnace setup. Beware of backflow of compressed air into a cold fuel tank !

[Twospoons]

If you have calculated 180m/s velocity in the throat, then you have designed a gas burner, not a rocket, and the exhaust gas will decelerate in the expanding section and generate very little thrust.
Throat velocity needs to reach Mach 1 (local speed of sound, based on throat pressure/temperature/density) for the expansion to accelerate to supersonic speeds.

But you should know this already if you've done your homework.

[klugesmith]

Agreed, the velocity figure is too low.   With plain air at room temperature, I think you can get Mach 1 flows with only 30 psig (2 bar). Higher chamber pressure generates more mass flow 'cause higher density at the throat.  Ideally the pressure in expanded flow, at big end of nozzle, matches ambient pressure.

[plasma]

The compressor has a free flow rate at 85psi, the 120psi it cuts out at.
Didn't think about maybe pulse it, I expect after the airs flowing and I induced LPG there will be more temparay thrust rather than steady state.
Oxygen tanks wouldn't have the run time compared to air breathing, but it would be a possible step.
Wednesday will be getting the steel for the chamber and throat,but still working out how to do the plumbing



Mach number = 180m/s / √(1.3k*345R*973K) =0.272
At like just said more flow rate then that increases.

Thanks everyone for the pointer's. Its going to be compressed air at 120psi with LPG, latter deseil.
I've got 20mm2 for the throat and sticking to a standard multiple of 4 taking it to 80mm2 for the chamber. I'm assuming a temperature of 973K. The air flow rate is 2Litres/sec 0.0012kg but expanded up to three off the shelve compressors making 6L/s.
I have a sparkplug continue fired. The volume of heated gas is 0.33m3/kg, velocity 297m3/s with 180m/s in the throat. A standard multiple of 15 for the expansion part.

[plasma]

Kludgesmith what value would you use to pulse the inflows?
I was thinking the value on the exhaust of a internal combustion engine, I ask a mechanic but he doesn't have a name for that.

[nabzim]

I'm designing a liquid/gas rocket
I'm thinking about building a protype from mild steel

So... These sentences are exactly what we would read on a forum, just prior to reading news headlines the next day:

"Amateur rocket enthusiast tragically dies in explosion in garage"

Seriously, Mr. hobbyist-builder-guy (plasma): Don't go there. Don't do it. You clearly don't understand the dangers involved with building rocket engines, let alone a damn liquid engine. Just because you're good at making other things and proficient in welding, is no excuse to try and build a goddamned liquid propellant rocket engine.  You cannot do that successfully without years and years and years of science and engineering education and background, which you clearly don't possess since you're asking this in an electronics forum... A liquid-propellant engine is a BOMB, get it? it's literally a continuously-exploding, self-detonating BOMB; that's how it provides the literally-explosive-force required to accelerate huge massive objects against Earth's gravity.

If you were TRULY interested in rocketry, you would join a local model-rocketry club (either Tripoli or NAR if you're American) and start small. Then you eventually move up in engine size, until you want to earn your high-power rocketry certifications, and then you can eventually, build and launch up to size O rocket motors, after which, you'll need FAA clearance to launch anything bigger.
Everything up to that point, is still SOLID-PROPELLANT... Because Liquid is STILL, TOO DAMN COMPLEX for it to be a viable solution to most people. There are even legitimate orbital rockets which use solid propellant engines...
So please, pull your head out of your ass, and think about your safety....
Just accept that it's something you should NOT attempt, no matter how deliciously-badass it looks and sounds...

Sorry about the rant and lecturing you... but, believe it or not, PEOPLE HAVE DIED, trying to do exactly what you said you're trying to do.
You're welcome for saving your life. 'nuff said...

[Patrick]

Hybrid is where I would start if your going gaseous or liquid. You have half the plumbing and the oxidizer is the easier half. Then you can 3D print and pour a grain of HTPB and a powder fuel with your welding valve can meter out the oxidizer.

https://www.rocketmotorparts.com/Hydroxyl-terminated_Polybutadiene_Resin_wCAO-5_Antioxidant___HX-752_Bonding_Agent_1_Gallon/p1577809_20282764.aspx

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[MRMILSTAR]

From the look of the exhaust, I think that all he has succeeded in building is a blow torch. I don't see any thrust to speak of.

I love how he is running these very flammable experiments in his attic with all that exposed wood nearby. 

The Mythbusters built a working hybrid rocket using NOS on one their programs. Perhaps he can find that program and see how they did it.

[plasma]

@nazbim
You are right, I wanted to stay away from solid fuel, because of the impression.
@patrick
It was one of those projects that faded out, doing some computer work at present instead.