Motor Burn Question

Two completely identical motors.
Two rockets, identical except for weight.
Care taken to position weight to match CP and CG.
First one is a light weight rocket with a thrust to weight ratio of 15:1.
Second one is a heavier rocket with a thrust to weight ratio of 3:1.

Will the motor in the heavier rocket (3:1) burn faster?
Will it burn at a higher pressure and have an increased chance of overpressure?
My thought experiments tell me that it will on all counts.

Greg

How do you come to that conclusion?

It sounds like you're thinking that the extra weight would make the motor "work harder"?

If that were so, then test stands wouldn't work. I.E. a test stand is essentially an infinite mass (compared to the motor's thrust), therefore it would require infinitely more work, therefore according to your assumptions, the motor would always overpressurize and have a very brief duration (i.e. explode!).

Chamber pressure is based on the decomposition of the fuel and oxidiser, the area of the burn vs. the throat area of the nozzle.
Thrust is based on the chamber pressure, ambient pressure at nozzle exit, combustion product mass, and the size and shape of the nozzle (which determines the speed at which the combustion products are ejected, and whether they reach the outside pressure at the nozzle exit). Or to put it another way, thrust is determined by how much stuff is being ejected at what speed.

The mass of the airframe has nothing to do with any of it.

Now, obviously the results of the thrust on each airframe will indeed be different. The lighter one will go higher and faster during the burn, but once it burns out, it will have less momentum, since it has less mass, than the heavier airframe, which will go slower and burn out at a lower altitude, but will have some momentum due to its mass. But it does not affect the motor itself.

Motors are stupid.

They don't know what rocket you put them in.

If you use the exact same motor, it will deliver the same time/thrust performance wherever you use it.

In cases of extreme acceleration, the motors can have a higher than usual burn rate. However, 99.999% of flights will never see that magnitude of acceleration. For our purposes the motors burn at the same rate no matter what rocket they are in.

Todd Harrison

Cool.
Thanks,

Greg