Rocket motor design problem?

Question

where do you start when designing a solid propellant based rocket motor (kno3/sucrose) ?

it seems that the nozzle is the key part but the design of that is dependant on the chamber pressure, mass flow rate, etc.

but how do you calculate that without a working nozzle and surely if you changed the nozzle throat CSA to optimise performance then the chamber pressure will change also affecting the whole kit and kaboodle (technical terms:).

are there a series of steps in designing a rocket motor ?

(i've been all over richard nakka's website, brilliant info but none the wiser)

cheers

Answer 1

You start with your requirements.  That's how all engineering problems start.

You want to make it to orbit?  From where?  What sort of pollution are you willing to endure?  What are your cost goals? What diameter fairing do you need? Without these requirements, you can't begin.

If you're fuel is kno3/sucrose, you've already decided on environmental impact, and probably cost.

For example, if you need a sounding rocket to make it from the ground to 150,000 feet, that's different from a balloon launched rocket that goes from 100,000 feet to 250,000 feet.  For one thing, nozzle design changes if you're optimizing for higher atmospheric back pressure.

Answer 2

You could use some CFD software if you know the burn rate of the propellant and the resultant pressure.  Usually the best way is to make several small prototypes.

Answer 3

A test rocket is usually a good idea.

Since it is a rocket made from scratch, and uses unpredictable fuel,

theres really no way to calculate its exact perfect pressure OF the motor itself, except the fact you need the "pounds of thrust" to be greater than your exact "pounds" of the total weight.

I'd make a test rocket, and if it sucks, modify the nozzle.

An opening too big can burn your fuel to quick, or just explode.

An opening too small will not allow it to leave the ground, build up pressure, and explode.

You need to find a good center ground when it comes to nozzle size.

remember:

NASA's flights didn't work to begin with:

http://www.youtube.com/watch?v=13qeX98tA...