Centurion Project - Letter from Marc Goldstein and his Successful Flights

Apparently, I shouldn't have assumed there might be problems with Marc Goldstein's Interchangeable Engine Mount Design.
Marc sent me a note through The Rocketry Forum:

Hi Chris!

I read with interest your concern about pressure loss due to the centering ring holes. The thought had occurred to me also, when I built my first BT60 bird with this design (it was a Der Big Red Max, so had an 18" BT60 to be pressurized, with no stuffer tube assist). I was quite nervous the first time I flew it... It was probably on a C6-3 (might have been a B6-2). In any event, even with a single 18 mm Estes motor, it deployed just fine. I think the pressure wave is quite strong, and the leaks are not enough to prevent adequate pressurization in the instant of ejection. Sort of like vent holes in gap staged rockets don't prevent the booster from popping off.

Since then I've had probably close to two dozen flights on the BT60 rockets (2 DRM, 2 DBRM, a Screamer upscale, 2 Vagabonds...) with the interchangeable mounts, mostly using either a single 24 mm motor or 3x18 mm motors. None of these has a stuffer tube. And I've never had a deployment failure likely attributable to low ejection pressure.

Even so, a solution could be a small strip of tape over the open holes on the lower centering ring, and similar bolstering of the seal if using a 3x18 mount.

Also, you could use a wider centering ring in the body tube at the top (which acts as a stop or thrust ring for the motor mount)... the wider centering ring would cover holes in the upper ring of the interchangeable mount. You could even put some sort of gasket material on there if you wanted to reduce airflow, I suppose.

Good luck with your investigation of the issue. I'm sure you'll come up with an inventive solution to alleviate the concern. Keep me posted. I'm interested in anything you learn!

Marc

Thanks Marc - I stand corrected!