[ibanezmatt13]

Hi all,

I'm working on designing a bipropellant rocket motor using isopropyl alcohol (propanol-1) as the fuel, and o2 gas as the oxidizer. The reaction for this combustion is shown here:

C3H7OH(l) + 9/2O2(g) → 3CO2(g) + 4H2O(l)

The problem I have is preventing me from continuing with the design, and the problem is that I need to work out the specific heat ratio of the mixture of the two and the combustion temperature...

http://www.ausetute.com.au/heatcomb.html This site shows that the combustion temperature for exactly this reaction is 2021KJ/mol. The problem is that, I need the combustion temperature in Kelvin, and the mass of propellant is unknown, so I can't get away from this KJ/mol unfortunately.

So the problem, for this reaction as above, for an unspecified mass I need to work out the specific heat ratio K, and the combustion temperature in Kelvin.

Kind regards
Matt

[Zyklonb]

using isopropyl alcohol (propanol-1) as the fuel, and o2 gas as the oxidizer.

How are you going to get all that oxygen in the motor?
You need the fuel and the oxidizer in the motor, at the same time in order for it to work.
For this reason, oxygen gas is never used as an oxidizer. Try > 70% H₂O₂, although liquid fuel rockets are way harder to build, as the oxidizer must be stored in a differant compartment than the fuel.

This site shows that the combustion temperature for exactly this reaction is 2021KJ/mol.

That's not the combustion temperature, rather it's the "Heat of Combustion" and shows how much energy each mol gives when fully oxidized - two very different things.

The problem is that, I need the combustion temperature in Kelvin, and the mass of propellant is unknown

What reason can't you know the mass of the propellant?
Just pick an arbitrary number, and them multiply it to whatever you need later, to fill your rocket.
I strongly recommend that you don't use either propanol-1 as the fuel or any other liquid fuel, solid based rocket fuels are the only suitable fuels for a beginner with less than several hundred dollars to spend. And again, oxygen will not work at all, unless it's liquid - which I somehow doubt you are capable of producing or using.

[Enthalpy]

Sorry for the late answer...

Gaseuos oxygen can be stored in a bottle under pressure. This will let a rocket engine push. It demands a heavy oxygen tank, so the rocket is unefficient - the reason for liquid oxygen.

Do NOT use concentrated hydrogen peroxide. This is really dangerous stuff. Search for
"Field Handling of Hydrogen Peroxide"
detonation, self-ignition - that's nothing for amateurs, and professionals suppress it everywhere they can.

Instead of an alcohol, which is a bad fuel, and especially propanol, which is very volatile and flammable and makes a badly visible flame, use Diesel oil. Not as flammable, better fuel, brilliant flame.

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You reaction is wrong. Combustion with oxygen never makes CO₂, but a mix with CO. At best 50-50 with 200bar, but much more CO at a few bar. This equilibrium is difficult to predict by hand and also means that the best mix ratio is fuel-richer than your estimate.

Do as everyone: use a software to predict the combustion. Two are reasonably easy:

RPA http://www.propulsion-analysis.com/downloads.htm

CPropepShell http://users.cybercity.dk/~dko7904/cpropepshell/cpropepshell.htm less obvious to use

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The flame will be too hot for a metal. Either let the engine work for a time short enough, or cover the walls with an ablative protection (not easy) or run the fuel through cooling channels in the walls (gets difficult).

Very few rocket engines are uncooled. Very small thrusters use hydrazine decomposition (brutal poison, do not) in a ceramic chamber. The others, usually solid ones, work for short and have ablative protection.