Lighting the sustainer from the booster

[mbeels]

I've been reading through many of the 2-stage threads here, and the overwhelming preference seems to be for lighting the sustainer from the sustainer. I can see the advantages:

• Separation charge to get booster clear of sustainer before ignition
• Sustainer still lights even if booster prematurely drag separates

But many things get simpler (especially for a MD sustainer) if the booster does the sustainer ignition. I can see these disadvantages:

• Sustainer motor potentially roasts the booster
• Sustainer won't light if the booster prematurely separates

Are there any other reasons to avoid lighting the sustainer from the booster? It sure makes a MD arrangement much simpler.

 

[Charles_McG]

My 1/5 scale Apache wouldn’t drag separate, and I didn’t start with a separation charge. I like the CTI motors for the sustainer, but in this case, they blew the fincan apart on the Apache. Specifically, popping a panel between the fins out. This was a BT55 based sustainer with TTW fins. It did it more than once, and I’m pretty sure it was the starter pellet overpressurizing the aft space between the motor and the body, where the booster coupler slides in.

With a separation charge (or drag separation) the sustainer is in free air. Or even slightly negative pressure from the base drag.

 

[mbeels]

This was a BT55 based sustainer with TTW fins. It did it more than once, and I’m pretty sure it was the starter pellet overpressurizing the aft space between the motor and the body, where the booster coupler slides in.

So its not just potential for roasting part of the booster, but actually causing significant damage. Were those 3D printed parts that got popped out?

 

[Charles_McG]

No. That model was/is all paper tube and papered balsa fins.

 

[mbeels]

Alright, thanks for the responses, Charles.

 

[BABAR]

I've been reading through many of the 2-stage threads here, and the overwhelming preference seems to be for lighting the sustainer from the sustainer. I can see the advantages:

• Separation charge to get booster clear of sustainer before ignition
• Sustainer still lights even if booster prematurely drag separates

But many things get simpler (especially for a MD sustainer) if the booster does the sustainer ignition. I can see these disadvantages:

• Sustainer motor potentially roasts the booster
• Sustainer won't light if the booster prematurely separates

Are there any other reasons to avoid lighting the sustainer from the booster? It sure makes a MD arrangement much simpler.

Depending on rocket design and your goals, going with booster ignition may cost you a bit of altitude. If you have the velocity to spare, dumping the booster (with its extra drag, which is commonly the case with multistage rockets to make up for the extra tail mass of at least two motors) with either a charge or drag separation may allow the sustainer to coast a bit before lighting up the sustainer motor. If more altitude isn't needed or wanted (I suspect this may be the case for you, but ya never know), this becomes a non-issue. I wish there was a non-electronic way to do it with black powder motors.

Haven't done it (all my stagers have been black powder), but I am thinking that if you have much altitude and need to go dual deploy, you will need a set of electronics on both the sustainer AND the booster (I count a Jolly Logic Chute Release as electronic.) So you will have more money invested in the rocket, which can be a bummer if fecal turbine interaction occurs.

 

[Rocketclar]

I, too, tried to rely on drag separation. Most of the time I did not get it. I did paint the inside of the ISC with a high temp paint and packed the ISC with dog barf and wadding and that minimized damage when the sustainer lit. However, a small separation charge works great. I used (I think Charles' technique) of packing BP around one of the red caps of a Firewire and taping it off. I light the upper stage from the sustainer since I didn't want to deal with wires between the two and always firing into the ISC.

 

[Charles_McG]

I -have- considered doing ignition from the prior stage interstage for space considerations. I have all the parts for a Nike-Nike-Cajun. I'd put a different payload/nosecone on top of my Apache, and *poof*, I've got a Cajun. And I've already done a Nike-Nike Smoke.

But at ~1/5 scale, My Apache/Cajun is BT55, and I'm tired of trying to wedge electronics into such a small space. An eggfinder mini is going to be bad enough. So Plan B would be to light it from below. A Proton can handle separation+ignition+upper ignition+drouge+main - and still have a channel left over.

 

[JimJarvis50]

If you want, you can connect the separation charge ematch and the igniter ematch together and fire both from the booster. You would need to arrange for a breakwire to the igniter and for the igniter to stay with the sustainer upon separation. This would at least get the sustainer away from the booster when the motor lights.

My preference is to light both of these things from above, but MD does present challanges.

Jim

 

[Tractionengines]

What's wrong with a conduit external to the body tube for MD. Even with 2 pairs of e-match wires, it can be very small diameter tube.. This passes your separation charge and sustained ignition around the motor.
The altitude penalty for the protrusion can't be very much. For high speed contour the leading edge with J-B Weld for any localized heating.

 

[SolarYellow]

What about just using a small ejection charge (due to tiny volume to be pressurized) and short delay on the booster motor to get separation? Just time the sustainer ignition to be after the ejection. With the cap over the nozzle holding the ignitor in a composite motor, there should be no worry about partial/improper starting of the sustainer motor.

 

[mbeels]

I -have- considered doing ignition from the prior stage interstage for space considerations. I have all the parts for a Nike-Nike-Cajun. I'd put a different payload/nosecone on top of my Apache, and *poof*, I've got a Cajun. And I've already done a Nike-Nike Smoke.

That's part of my motivation, I'm contemplating a 29mm MD sustainer and space becomes a real issue.

If you want, you can connect the separation charge ematch and the igniter ematch together and fire both from the booster. You would need to arrange for a breakwire to the igniter and for the igniter to stay with the sustainer upon separation. This would at least get the sustainer away from the booster when the motor lights.

My preference is to light both of these things from above, but MD does present challanges.

Jim

That's an interesting idea that I hadn't considered. I assume you'd need a relatively fast acting igniter for this to work? Either an ematch + BP (ala CTI), or dipped ("enhanced") ematch in an Aerotech motor?

What's wrong with a conduit external to the body tube for MD. Even with 2 pairs of e-match wires, it can be very small diameter tube.. This passes your separation charge and sustained ignition around the motor.
The altitude penalty for the protrusion can't be very much. For high speed contour the leading edge with J-B Weld for any localized heating.

Yeah, that's probably the superior option, it seems almost everyone does it this way. I was hoping to save some space in the sustainer and possibly simplify the wiring slightly, so considering alternatives. But maybe that is just the best way to go.

 

[Charles_McG]

Yeah, that's probably the superior option, it seems almost everyone does it this way. I was hoping to save some space in the sustainer and possibly simplify the wiring slightly, so considering alternatives. But maybe that is just the best way to go.

I've used ghost wire (flat conductor spear cable/tape) with pigtails soldered to each end to connect to ematch and altimeter. My application was internal - but still to save space. Outside, with a strip of Al tape, or even fiberglass, would be virtually invisible. The conductors themselves would fit alongside the motor in the motor mount, but I think the plastic coating is too thick for that.