Burnrate solid rocket engine?

[HattoriHanzo]

I am doing som calculations on burn rates. I have a problem.

Equation for burnrate: r=aP^n

R.Nakka uses a=pressure coefficient and n=pressure exponent

You can see for yourself here:

http://members.aol.com/riccnakk/bntest.html#Temperature

or

pdf: http://www.geocities.com/exodus_trinity ... s_burn.pdf

But G.Sutton states a=temperature coefficient and n=burning rate or combustion index

I cant make it right when using Nakkas values. I dont understand how a coefficient can be expressed in mm/s?

[MJD]

Where does Sutton say that? What edition do you have? There are typos in some earlier editions.

a is the burn rate pressure coefficient, or "prefactor".

n is the burn rate pressure exponent.

burn rate = a x Pc ^ n is correct.

Sigma p is burn rate sensitivity with respect to temperature. When the propellant is cooled or heated from the reference temperature, a will change in proportion to e^ sigma p x delta t .

Mike D.

[HattoriHanzo]

On page 428 in seventh edition.

"a is an empirical constant influenced by ambient grain temperature"

And on the same page "also a is known as the temperature coefficient"

[MJD]

So it does. Don't let that confuse you. "a" certainly is influenced by grain temperature as I mentioned in my first post. Normal reference temperature is 70F. "a" is adjusted from that reference value using the burn rate temperature sensitivity - sigma p - if known, for calculating performance at different grain temperatures. If you are characterizing a propellant you have developed, you should try to fire it at a consistent grain temperature, so why not use 70F or as close to it as you can? Because of the direct influence of temperature on "a", some use the term temperature coefficient. You can call it whatever you want, it is still the same prefactor and has the same significance.

When we are characterizing new propellants, we condition the grains in an environmental chamber. If the grain temperature varies from 70F at the time of firing for whatever reason, the variance will be small. We then use our best guess at temperature sensitivity to extrapolate a at standard temperature, and verify with later tests if refinement is needed. Fortunately, for propellants of similar composition and similar pressure exponents, the approximated sensitivity is usually close enough to get the ballistics nailed down pretty tightly. If the propellant is to be used in an application where operating temperatures vary greatly, we spend more effort at nailing down temp sensitivity. If the operating temp regime is narrower, we worry less. The particular application also defines how critically we need to analyze temp sensitivity, i.e. if acceleration is close to lower limits at 70F, we have a potential problem and need to look very closely at lower temper performance.

MJD

[HattoriHanzo]

Thank you for the clarification!