Multi-stage advice

I'm working on my first HP multi-stage, and could use some advise. It will be a 3" rocket, about 9 feet fall. ~3 foot booster with 54mm motor mount, most likely with a K and its own chute, then a dual deploy sustainer with a 54mm J or K. I'm hoping for a test launch at some METRA launch, and then a high altitude launch at LDRS next summer.

To couple the booster to sustainer I'm using about a 6" long piece of coupler tubing, with a bulk head on the bottom, and a very small ring of actual body tubing in the center. The bulk head will be attached to the recovery harness of the booster. The lower centering ring of the sustainer is sufficiently far up the body tube that the coupler fits up into the sustainer ~2" and bottoms out firmly on the centering ring. The timer to ignite the sustainer will be in the sustainer electronics bay, with a small SS tube getting the igniter wires down to the motor through the centering rings. The electronics bay includes an altimeter for dual deploy recovery, and a GPS with RF telemetry to find it!

My question is, what is the best way to insure good clean separation? A natural drag separation would be nice. Can I rely on this? If this doesn't happen, is it sufficient to allow the lighting of the second stage force off the coupler (which has a sealed bulk head in it)? In this case I should set the delay time on the booster eject charge sufficiently longer than the delay time on the timer electronics which will ignite the sustainer? Therefore the sustainer ignites first, then the eject charge in the booster.

Or, should I set the eject charge shorter on the booster, therefore forcing a separation prior to ignition of the sustainer? Or might this risk distorting the flight path of the sustainer prior to ignition?

Any thoughts on a test flight? I was thinking to fly it first with everything connected, except leave out the igniter in the sustainer. This would check the separation, and give me a feeling for stability, without risking a 2nd stage ignite in case it doesn't fly straight, or something else goes wrong.

Any comments/thoughts/suggestions are more than welcome.

Thanks - Adrian

You describe a very interesting and challenging flight.

Definitely separate and fire the sustainer BEFORE deploying booster recovery. Otherwise booster recovery deployment will destabilize rocket and likely be burned by sustainer ignition.

A key is assuring that there is not much flex at the booster to sustainer connection. Booster mating to sustainer only 2" may not provide good stiffness, unless part fit is very precise and mating surfaces are hard, potentially destabilizing flexing is a risk.

I would suggest using shear pins to retain coupler "cap" on booster and also both separation points (main and drogue) on sustainer. This would increase confidence that separations will only occur when ejection charges are fired. Proposed flight has four separation points, the last three separations should all be shear pinned.

Would also suggest using flight computer(s) instead of simple altimeter(s):

Then a minimum altitude condition could be set as a condition of sustainer ignition. This would add a big safety factor, greatly reducing the possibility of sustainer firing sideways or downwards.

Could also be used to fire an additional ejection charge to force stage separation.

Featherweight altimeters has a couple low-cost highly capable flight computers for under $160.

Sustainer firing can execute stage separation, but two downsides:
1) Top of booster will suffer scorching.
2) Depending on sustainer motor, there may be ignition sensitivity/failure due to the pressure variation caused by rapidly executing separation simultaneously with pressurizing sustainer motor.

AMX/Cesaroni motors are great motors overall and particularly for staging sustainers due to fast and easy ignition.

I would fly sustainer all by itself as first test.

Suggested event sequence for all up staged flight:
1) At booster motor burnout + 1 or 2 sec, fire stage separation ejection charge
2) +1 or 2 sec later, IF flight computer determines that acceptable altitude has been achieved, sustainer motor is ignited
3) +1 or 2 seconds after that, eject booster recovery

A huge flying field and working up a detailed flight simulation are also essential for a flight like this.

Good luck.

Thanks Boris as always for your input.

Last year, your suggestion to put an RF tracker in my mach attempt, and to launch it early in the weekend, to give me the rest of the weekend to find it, paid off! It took me the full day on Sat to find it, but it was recovered eventually.

Regards - Adrian