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Originally Posted by tbzep
George Gassaway has been online here today and I'm sure will drop back in soon. He will know something about your project if nobody else has direct experience with it. If he hasn't done it himself, he's seen it done in competition at various levels. There was a finless Titan 3 or 34E at one of the Rocket City Classics in Huntsville several years ago. At the very least, he's probably familiar with that one. Compared to a finless Titan, one with fins should be a cakewalk.
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Wow, that model was so long ago. I saw it fly but don't know what they did mechanically to get the outer boosters to separate. Finless..... I od not recall that. Not saying you are wrong, just that if it was finless I do not know how they made it stable (short of a massive amount of noseweight). Unless you mean the core had fins but the outer boosters did not have any fins.
Stability-wise, what aeppel_com describes sounds good. Center core with four fins, and outer blotters with two fins which I presume will be at 45 degrees to the side-by-side axis.
Well, do not add nosweight to the boosters. If the rocket needs any noseweight, put it into the nose of the center core. It does not matter for the outer boosters to be stable by themselves, it matters for the whole rocket to be stable. Indeed once the boosters separate, if they are not stable they will tumble and that'll be realistic.
edit - I did realize one reason to have the outer boosters stable by themselves. If the attachment method is a bit weak, or allows for any wobbling of the attached boosters when on the core. They could try to wobble one way or another way due to aerodynamic instability and possibly affect the flght path. Or to allow for wobbling/flutter so bad that the attachments could fail. But that would tend to be an issue mainly with a really sloppy attachment system, or crazy-fast velocity.
Where it might have a problem would be if the outer boosters did not separate at the same time. If one sepped early, then the remaining core and one side booster would have unequal stability, and likely would get into a yaw/roll couple. Probably not go end-over-end unstable, but could be a wild "bucking around" till the other booster came off. Now if it was just a matter of say 1/4 second, probably not go off course badly, just not the smooth realistic kind of sep you'd want.
On my shuttle model, I worked up a latching system to separate the SRB's. A Servo in the ET intertank retracted bellcrank arms to allow the front ends of the SRB's to be released and pushed away horizontally, and attachments at the rear of the SRB's allowed them to pivot outwards about 20 degrees before separating. Info on this page:
http://georgesrockets.com/GRP/Scale...odeldetails.htm
But that's complicated to get into.
I can recall from the early 1970's, I think in Model Rocketry Magazine, a photo of a Titan-III model that was the original Estes Gemini-Titan modified with side boosters added (perhaps a MOL model). Key thing is that the outer boosters were powered by B14 engines, way more thrust to weight ratio in those outer boosters than the center core. It was built so that the outer boosters were attached in a way so they slid off backwards very easily. So, when the B14's burned out, the center core was still thrusting and accelerated away , as the boosters fell off. A very simple way to go. But, risky, because the cluster had to ignite properly or the core could take off without the boosters, or one booster could ignite late and the rest would have taken off and left that booster on the pad as the rest of the rocket spun crazily due to uneven thrust. And in those days cluster ignition was a lot harder to do. IIRC, it was mentioned as something that flew once….. I'd have put more faith into the concept if it had been reported as being flown say 10 times without any cluster issues.
To do something like that today where the only thing holding the boosters on during launch are their thrust, it would call for something like a C11 or D12 in the outer boosters and a D12 or E9 in the core (or two C6's in the core).
But if it used another method for the boosters to be attached, where the thrust level was not critical, then the side boosters could be B6's, B4's, C6's, whatever. In any case, stick to all black powder motors, mixing BP with composite is a real headache for achieving reliable simultaneous ignition.
If the "new" Quest Q2G2 ignitors are as reliable as the originals, use those for the clusters. I have not used any of the new Q2G2's so I have no idea if there is a difference. I do know there is enough of a difference with the new Solar Ignitors to cause a bit higher misfire rate than before (flammable gray epoxy heads instead of the black pyrogen).
FWIW - youtube video link below of one of my shuttle boilerplate test fights, with SRB's separating. Each SRB had one fin on it, at 45 degrees down relative to the wingspan. Onboard flight computer commanded the SRB's to release 1/2 second after burnout detected.
- George Gassaway
https://www.youtube.com/watch?v=OQbt2VoIuZo