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PK:

--- Quote from: British Reaction Research on August 16, 2017, 02:03:16 PM ---I don't fully understand though. Why would I not get enough flow?
--- End quote ---

Apologies if this is (as we say in my country) telling you how to suck eggs, but a simple explanation will probably be useful for the non rocket nerds  of this non rocketry forum.

In the simplest sense, a rocket motor cooled by one of it's propellants (usually the fuel because hot oxidiser is nasty) flows that propellant over the outer walls of the chamber at a rate and velocity sufficient to
a: Extract enough heat from the chamber walls to prevent them from melting,
b: Not raise the temperature of the propellant past it's boiling point at the local pressure.

IPA has a fairly low boiling point at the 300-400psi you are going to see inside your cooling channels (I'm assuming that you'll pressurise the fuel tanks to 600+PSI), so you need to flow a lot of it to remove enough heat without getting local boiling and hence hotspots. This is one of the reasons kerosene is a popular fuel. You do this by increasing the velocity of the fuel in the cooling jacket, which increases the pressure drop which drops the boiling point..... welcome to rocket design.

This isn't a too much of a problem until you consider that nitrous oxide doesn't have a lot of oxygen in it, so you don't need a lot of fuel (by weight) to run near a stoichiometric point OTTOMH it's about 6:1.
Again this doesn't sound so bad, because less fuel == less burning == less heat right?  The kicker is that the decomposition of N2O is reasonably exothermic, so you are getting heat from a process that doesn't require fuel flow. That extra heat generally means that you'll struggle to get enough fuel flow to cool the chamber without boiling.

All biprops also use boundary layer cooling, which might get you out of trouble as much by increasing the fuel flow rate as by direct shielding of the walls.



Now this is a lay mans description. There is, of course, maths to work all this stuff out.


--- Quote ---
 And are you saying you built a Nox cooled motor?

--- End quote ---

Well, it was a NOX cooled motor for about 4 seconds, then it was a self digesting fireball, then a bucket of molten shrapnel.

I've built a radiation cooled NOX biprop that worked, but it was done before we figured out how to do reliable ignition so it was a pig to start...
I'll have a pic somewhere, but it was based on this:



PK

British Reaction Research:
Hello Paul,

This is just a quick reply to acknowledge your highly knowledgeable and informative post.

I'm glad to say that I have my head around (most) of the maths involved in motor design. No problem with the egg suck approach. I'm British so I know the saying and appreciate the style. Simplest explanations are the best and often reveal far more that the complex involved ones.

I'll post again with more info soon. I'm going to start a separate thread for my project. Meantime you can look at my blog, British Reaction Research.

Thanks again,

Carl.

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