[AR] Re: Math and specifics of hybrid nitrous.
- From: Norman Yarvin <yarvin@xxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Wed, 13 Jan 2021 21:14:20 -0500
On Wed, Jan 13, 2021 at 10:58:15AM -0600, Jack Hanna wrote:
So, how to math?
I know NASA had some site that has a very in depth calculator for some sort
of either ISP / thrust / fuel or something, but it's broken as far as I can
tell and I don't know of any alternatives.
I think the usual answer is not to do any sort of complicated math:
that's more for optimizing things than merely getting them to work.
And for a hybrid in particular, optimizing things is hard: you're
dealing with combustion from a solid state, which involves the surface
first turning into gas and then burning. It does so at different
rates in different places on the fuel, so the fuel changes shape as it
erodes, which affects gas flows, which further affects the erosion
pattern. This means either (1) complicated simulations, or (2) just
not trying to calculate it and going by cut-and-try and/or experience.
Of course you still need some basic math: it wouldn't do to have a
huge nozzle and tiny nitrous supply plumbing. Still, a lot of people
just bolt an engine together and give it a try, using relative sizes
that have worked before.
One set of formulas that can be of use for sizing things are the
choked flow equations:
https://en.wikipedia.org/wiki/Choked_flow
(This is not to imply that that page is a good source; it's not. For
instance if you look at the equation for the mass flow rate ṁ, it uses
a fudge factor Cd; the next equation tells you how to calculate
Cd... starting from ṁ! Probably the right course is just to omit the
fudge factor, accepting that the results will be approximate.)
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