How deep can you place the motor?

[Gunstar]

I'm considering a scratch build where the bottom 6 inches is LOC 4" tube, and placing the motor so it ends 4 inches or so before the end of the body tube to keep the CG forward without adding weight. I'd line the inside with foil as a precaution. Would that be OK or too risky?

 

[GlenP]

General rule of thumb for avoiding Krushnic effect is a depth of 1 diameter, so you are right on the edge and should be okay. But if you are concerned, you might add some vent holes to allow external air to flow into that recessed portion. That would also help cool it down a bit.

Also, take care to launch with the model raised above the blast plate or you may get a Bernoulli lock.

 

[jqavins]

General rule of thumb for avoiding Krushnic effect is a depth of 1 diameter, so you are right on the edge and should be okay. But if you are concerned, you might add some vent holes to allow external air to flow into that recessed portion. That would also help cool it down a bit.

Adding to that, the air coming in from the sides not only keeps things cool, it also (from what I've read, not from experience) kills the Krushnic effect, so recessing the motor deeper is OK. (But I'm sure there is some other limit to that.)

As an aside, I've long wondered... This got too long, so I'm starting a new thread.