Author Topic: Rocket engine questions  (Read 913 times)

Offline plasma

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Rocket engine questions
« on: February 27, 2021, 05:27:55 AM »
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

Offline Twospoons

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Re: Rocket engine questions
« Reply #1 on: February 27, 2021, 05:53:55 AM »

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.

Offline plasma

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Re: Rocket engine questions
« Reply #2 on: February 27, 2021, 08:03:49 AM »
Just so I understand you and vice verse, how so?

Offline TMaxElectronics

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Re: Rocket engine questions
« Reply #3 on: February 27, 2021, 07:59:26 PM »
Quote
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.

Offline plasma

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Re: Rocket engine questions
« Reply #4 on: February 27, 2021, 09:11:08 PM »
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.

Offline Twospoons

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Re: Rocket engine questions
« Reply #5 on: February 27, 2021, 10:41:00 PM »
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.

Offline plasma

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Re: Rocket engine questions
« Reply #6 on: February 28, 2021, 05:00:13 AM »
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
« Last Edit: February 28, 2021, 05:18:09 AM by plasma »

Offline MRMILSTAR

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Re: Rocket engine questions
« Reply #7 on: February 28, 2021, 05:19:23 AM »
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/NO2.
« Last Edit: March 06, 2021, 05:38:51 AM by MRMILSTAR »
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Online klugesmith

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Re: Rocket engine questions
« Reply #8 on: February 28, 2021, 06:04:35 AM »
>>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?
« Last Edit: February 28, 2021, 06:17:53 AM by klugesmith »

Offline rikkitikkitavi

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Re: Rocket engine questions
« Reply #9 on: February 28, 2021, 09:07:56 AM »
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.
A man can not have too many variacs

Offline plasma

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Re: Rocket engine questions
« Reply #10 on: March 05, 2021, 10:17:39 PM »
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.

Online klugesmith

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Re: Rocket engine questions
« Reply #11 on: March 06, 2021, 04:24:44 AM »
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 !
« Last Edit: March 06, 2021, 04:44:20 AM by klugesmith »

Offline Twospoons

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Re: Rocket engine questions
« Reply #12 on: March 06, 2021, 04:50:01 AM »
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.

Online klugesmith

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Re: Rocket engine questions
« Reply #13 on: March 06, 2021, 06:03:40 AM »
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.

Offline plasma

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Re: Rocket engine questions
« Reply #14 on: March 06, 2021, 06:20:59 AM »
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.
« Last Edit: March 06, 2021, 08:20:07 AM by plasma »

Offline plasma

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Re: Rocket engine questions
« Reply #15 on: March 13, 2021, 09:33:50 PM »
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.

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Re: Rocket engine questions
« Reply #15 on: March 13, 2021, 09:33:50 PM »

 


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