Two stage (InterStage) Design?

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Two stage (InterStage) Design?

Postby joeman » Sun Apr 22, 2018 9:31 pm

I'm still trying to figure out what my next project(s)...but while I've been thinking about this, I decided to investigate how one might 'mate' a Booster (MD) and a Sustainer (MD). I've concentrated on the Mechanical aspects, rather than the electrical(ignition) aspects.

I've done up a few designs and played around with a few 3-D Prints... to get the ideas flowing. I've produced a little video of what I've done.

I'm interested to know what people think.

Has anyone else seen other ways of mating MD Booster/Sustainer?


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Re: Two stage (InterStage) Design?

Postby SpaceManMat » Mon Apr 23, 2018 10:09 am

I’ve not built HPR 2 stage, but the usual method is to have an adapter (usually aluminium) at the end of the booster that the end of the sustainer slides into. The case you use of course cannot have a flange on it. Separation is usually done in 3 ways. 1. Fire in the hold - just light the booster motor. 2. Ejection charge, followed by a coast to optimise altitude, then light the sustainer. 3. Drag separation, coast, then light the sustainer.

My biggest worry with your design that the booster will see lots of asymmetrical drag at the nose when the adapter opens and this will send your booster tumbling.
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Re: Two stage (InterStage) Design?

Postby drew » Mon Apr 23, 2018 6:19 pm

Joeman, tbh, I think you're really overthinking this one... Not to say there's anything wrong with your design per se, just that it's probably overtly complex for what you're attempting.

I've had 4 MD staging attempts across three stacks that were all failures but from a stage separation perspective all but the first operated nominally. As in the stages separated cleanly and were fine from a "sustainer still pointing up" perspective post stage separation. Here are some links of the last 3 attempts.


I'm currently working on a 54mm to 38mm ISC, a post on that can be found here.

As long as you're staging to a smaller diameter sustainer drag separation is your friend. This will/should occur immediately after booster motor burnout. If you'd like to make it more active have a look at Team AeroPac's 100k project; they used an ISC with a piston (similar to PML's preferred way of deployment) to actively push separation.

If I were you I'd invest in some Carolina Composite Rocketry HEI hardware. I've found "bottom lighting" a sustainer motor from the ISC somewhat challenging and problematic. Not to say it can't be done (looking at you PK and Strud) but for me I'm not that keen on it.

If you have any more questions (and I'm sure you do!) please fire away.
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Re: Two stage (InterStage) Design?

Postby joeman » Sat May 05, 2018 5:30 pm

Hi Matt & Drew,
Matt: Yes, I thought the asymmetry of might be an issue and thought I'd do some analysis on likely forces...but now I think I'll steer clear of this idea. It was a good exercise in thinking about a problem...and investigating it. Perhaps this idea will find some use somewhere else.

Yes, I probably am overthinking it a little. I noted the motor case cap would cause issues and thought I might be able to take advantage of it and use it to help retain the sustainer during Booster flight . Then one need to allow it to split, rather than slide. But yes...lots of moving parts, lots more things that can go wrong... greater risks. Need to manage the risks.

I read up on another post and some people talk about increasing the diameter of the Air-frame below the fins, so that the retainer cap isn't an issue any more. I've spent a few hours doing some simulations to see what loss of speed/altitude is with this sort of solution. It isn't a "massive" hit.... so perhaps this is the compromise that needs to be made.

RE: Drag Separation.
After having a bit more of a think, I appreciate/understand what you mean your comment:-

As long as you're staging to a smaller diameter sustainer drag separation is your friend.

The difference in diameters leads to a significant drag ratio....which makes drag separation force significant. Makes good sense.

That being said, I do like the idea of an "Active" system, some ejection charge to initiate the separation as opposed to Drag Separation.
I could use shear pins to keep the two together... between the ISC and the Sustainer. And rivets from Booster to ISC.

I love it!
Yes, I was thinking about snaking some "ribbon?" wire, really thin down... like the wire used in some Electronics for displays. Obviously would need to handle the currents....Hadn't done much there. Had thought about HEI, but didn't dwell on it. I was just thinking about how to keep everything in the "right" place ...ready for Sustainer ignition.

Back to designing the next rocket... Bound to be many questions!

L1 - Callisto (H133) - 11-Jan-2016
L2 - March Fly (1633K940) - 18-Mar-2018 - RSO

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