Gap staging questions

I'm investigating building a semi-scale, 1.6" sized model of the Nike-Nike Smoke that we'd discussed elsewhere. Basically, from what I can gather, the rocket takes the Nike Smoke that we all know & love and adds a second Nike booster to turn it into a 2-stage rocket for high altitude use. Of course, it would be easy to simply build a long rocket that was detailed to look like a 2-stage model, but flies single stage only (like my Estes Terrier-Sandhawk). But I'd really like to make it a proper two-stage. That means gap staging. For the design I've come up with, there's a little less than 10" from the front of the booster motor case(s) to the nozzle of the sustainer motor.

Basically, what I'd like to do is cluster three 18mm motors in a BT-60 tube, two of which will duct their blow-through up to the sustainer by way of near full length stuffer tubes, and the third will carry a delay and deploy a parachute after the booster is falling away. The sustainer will also have an 18mm mount; anything bigger that far forward requires serious nose weight to balance. My questions are thus: 1) is 10" too far to allow reliable ignition of a gap-staged sustainer motor? And 2) if I leave the space between the three booster motor mount tubes open all the way back, does that provide sufficient venting for the air in the forward booster to escape? Air flow would be up the stuffer tubes, into the chamber just below the nozzle, and then back out between the tubes to the rear. Or, should it have vent ports near the top of the booster to simplify the airflow?

10" is fine, just make sure and put venting ports near the top of each stuffer tube being used to guide the gasses forward or you will get pressure seperation when the motors burn through.
I would also put a flame resistant cap on your parachutes stuffer tube if you decide to vent between the mmts, I would do that anyways as protection from the sustainer flame, balsa bulkheads work great.

I've read that up to 12" is OK for gap staging, and that some have managed to go even further.

Another option would be electrical ignition of the sustainer. However you want to make sure that the rocket is within acceptable limits for the ignition, or allow for a fail safe ejection and deployment of the laundry.

rharshberger said:

10" is fine, just make sure and put venting ports near the top of each stuffer tube being used to guide the gasses forward or you will get pressure seperation when the motors burn through.
I would also put a flame resistant cap on your parachutes stuffer tube if you decide to vent between the mmts, I would do that anyways as protection from the sustainer flame, balsa bulkheads work great.

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Well, I guess the question is if venting out the back between the MM tubes will cut it. If it goes out the back or out the side, will it make a difference as long as it can get out? Does slipstream around the body tube versus draft/vacuum at the rear have bearing on evacuating the upper booster? I guess I can always assume rear evacuation, and if it appears to still allow pressure separation (blow test), then ports can be added.

K'Tesh said:

Another option would be electrical ignition of the sustainer. However you want to make sure that the rocket is within acceptable limits for the ignition, or allow for a fail safe ejection and deployment of the laundry.

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I want to keep this simple - if I can't do it strictly with motor ignition/deploy, then it isn't worth doing to me. This won't really be big enough to practically put electronics onboard, nor do I want the weight. And, I have no experience with that sort of thing. Although, if someone wanted to kit bash two PSII Nike Smokes into a 3" version of the same, I'd imagine that would be the only practical way to pull it off...

Usually with gap staging you have a good connection between the upper stage motor and the stuffer tube from the booster. That's why the vents in the stuffer tube are needed. Since you want to do a cluster in the booster, you probably won't need vents in the stuffer tubes since they most likely won't be directly connected to the upper stage motor. You'd still need a vent to the outside, and a hole or two in the centering rings would do that.

If you are planning to do some sort of weird offset transition to mate the two stuffer tubes to the upper stage motor, then think about a vent in that single tube right below the upper stage motor. You'd have to worry about your chute fouling on that transition though.

Maybe use something in the upper stage like the old K-27 Honest John kit, where there is an inverted cone that connects a recessed motor to the body tube to help direct the hot gas and particles to the motor? ( https://spacemodeling.org/jimz/estes/k-27.pdf )

kj

I was thinking of using some sort of shroud to funnel the gasses from the two motor tubes to the sustainer nozzle. But I hadn't quite worked out what it would look like, because as you said, it would be intersected by the third tube containing the parachute. In order to minimize the stage gap, I actually have the sustainer motor mount extending out of the body tube, into the top of the booster a 1/2", so the funnel cone can't be on the sustainer. It would have to be set into the coupler at the top of the booster. I can probably make a cone and simply trim it so the chute tube goes through it like a chimney. I'm trying to make it so that the sustainer can be flown as a stand-alone Nike Smoke, with everything related to two-stage operation as part of the booster. So, the stage coupler and the funnel should be part of the booster.