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bkuker / motorsim

Solid rocket motor simulation
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P2 = P3? #1

Open 1337-chris opened 5 years ago

1337-chris commented 5 years ago

https://github.com/bkuker/motorsim/blob/75fd4952424c719e9d5a1c459b6174a374cf9ff9/src/com/billkuker/rocketry/motorsim/Burn.java#L297

Can you always assume p2 = p3 like this? Isn't that only when the nozzle is optimally expanded?

bkuker commented 5 years ago

That is a very good question.... About code I wrote more than a decade ago so it is taking a little remembering. In my own usage I tended to design a motor, test it, and then fudge the nozzle efficiency up or down a little... If this is a bug I think at the scales I actually used and tested this software that the expansion ratio being wrong gets lost in the noise.

I see places where the difference between these values is calculated and used, so I think you are right.

If you want to compare the code to Richard Nakka's SRM.XLS excel spreadsheet, which I based it on, and find the right thing to do, that'd be great! His work was the brains behind this code, I just translated it as best I could.

1337-chris commented 5 years ago

Thanks for the quick and complete response Bill!

It looks like if you assume p2=p3 in all cases, this equation produces the same thrust irrespective of the exit area. If I had a large exit cone or none at all it would claim the same thrust. That's why I'm at a conflict with it. Mass exit rate also only takes into account the throat area. So it would be quite the same.. haha.

Edit: Props to you for actually testing it!!