Gaining Performance from Cesaroni Vmax Motors

Robert Steinke of SpeedUp provides an interesting update on estimating the performance of the Cesaroni Vmax motors, to find that they are probably under expanded.

For the Moment of Inertia rocket I’m going to be using a Cesaroni Vmax motor.  In particular, the 29mm 3-grain H410.  410 Newtons is a lot for this size motor: 42 kgf, or 92 lbf.  It makes you wonder, how do they get that much thrust out of such a small motor?  It has to have either a high chamber pressure, a large throat, or both.

Chamber pressure came out to 870 psia, and sure enough, the nozzle is properly expanded for one and a third atmospheres.

The place where I launch is at 5000 ft elevation, about 0.83 atmospheres ambient pressure.  Proper expansion would be a nozzle area ratio of 8.8 instead of 6.25.  That would mean extending the exit diameter from 0.75″ to 0.89″.  It should get 1.6% more thrust, or 417 instead of 410 Newtons.

Continue to read the full update here.

SpeedUp Update

Robert Steinke of SpeedUp has posted a new update, building a Mach breaker rocket

I’m starting on an extra little side project. I’m going to build a Mach-breaker rocket using the Cesaroni 29mm H410 Vmax motor. Here’s a picture of the basic layout.

I imagine the fist question I’m going to get is, “Why did you choose tube fins? Tube fins are high drag. Don’t you need low drag for a Mach-breaker?”

Well…it depends.

Read the full update here.

Worth a read for those interested in the physics of it all.