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Liquid Fuelled Rocket Engine

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British Reaction Research:
Hello Bill and PK,

Many thanks for taking the time to read and post.

Bill - My plan was to inject the silicon oil into the fuel flow after the cooling circuit, for the reasons you point out. Something similar happens if fuels like kerosine get too hot in cooling channels. The kerosene cracks and produces an insulating substance that can also block flow channels.

PK - Thanks for the wise words. I'm aware that 0.8m is low for an L* for nitrous. I'm just playing with figures at the minute. If I go to, say, 1.25m L* then the chamber length goes to 300mm. No problem, and the diameter is about 140mm so neither a pencil nor a pancake.

I take your point about nitrous being reluctant to part with its oxygen. One idea I had was to have a catalyst bed before the chamber. Thus would the Nitrous be decomposed to hot nitrogen and oxygen. This would be hot enough to auto ignite the fuel too.

The snags are three-fold. The first two are less intractable than the last. A part of the cooling problem is moved elsewhere. The catalyst unit needs to be preheated initially to get the reaction going. Suitable catalyst materials are very expensive indeed.

Lastly, there is an error in my heat transfer calcs. I used chamber stagnation temperature for the adiabatic wall temp in the nozzle. It should be about 0.9 times chamber temperature at that location. So not a huge deal, just makes the calcs more conservative.

Carl.

British Reaction Research:
Hi,

I've been working on some more heat transfer stuff, this time for the chamber. I'll post it once I've finished,

Carl.

British Reaction Research:
Still working on the chamber calculations. Apparently these turbulent flow in pipe type analogies for rocket chamber and nozzle flow tend to overestimate for the nozzle and underestimate for the chamber. This apparently is to do with the high pressure gradient in the nozzle increasing convective transfer.

British Reaction Research:
I'm going to be posting the heat transfer calculations for the chamber in the next few days.

Earlier I said I made an error in the nozzle calcs. I stated that the adiabatic wall temperature should have been 0.9 times the chamber temperature. That is not the case. The recovery factor at the nozzle is 1, so the two temperatures are the same at the nozzle location.

Recovery factor in the chamber is about 0.9.

PK:
It's important to remember that a rocket motor is not like a boiler or a stressed beam.
There have been so many boilers and beams designed that the mathematical models have been carefully honed over the last hundred years with thousands of published documents available describing said models.

In comparison, very few small rocket motors have been designed. There are maybe 50 meaningful papers/books describing aspects of them. 

What I'm trying to say is: "Don't get too hung up on chasing the micron fairies of your design. You'll learn more from the first three motors that you destroy than you will from a spreadsheet, no matter how long you spend staring at it."

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