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.

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