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Originally Posted by aeppel_cpm
That's a very nifty flight. Thanks, George.
My main thought about putting forward weight in the booster was so that I could tell the RSO that all the parts are stable - since I don't have the software to sim them together. I'm happy to leave weight for the core payload.
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A RSO worthy of being an RSO should not be expecting for all parts to be stable separately, but whether it is stable for launch and up until the boosters separate. Take for example a Boost Glider with a pop-pod. The Pop-pod is totally unstable without the glider.
But I do see the problem of trying to prove anything. My rule of thumb would be, whatever size you need for the four fins to be for the core, and you CAN get Rocksim to simulate the core by itself, use the same size fins, two at + and - 45 degrees to the span, on each of the side pods (since you said you wanted 2 fins per pod). I think that should err on the side of being a bit over-stable.
Otherwise, if I was doing this and wanted to be sure, i'd make up a very crude stability test boilerplate out of 18mm tubes and a 24mm payload section, to prove it was stable, whatever the fin layout. That is what i did for a model of the X-20 Titan-II (24mm boilerplate for 2.6" final model), because Rocksim ran away and hid in under the house when I tried to get it to simulate something that aerodynamically complicated. I ended up using the old fashioned Cardboard cut-out method to find the worst-case CP and chose the desired CG location accordingly, but had to do that small boilerplate test to see if it really was stable, which it was.
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I'm planning on upper and lower counter facing lugs/posts. Much like the Sentra SRB. Nose cone pop lifting the nose from the forward posts and drag pulling the body away.
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Yeah, that can be good. IIRC Bob Biedron used a similar method for his FAI scale Ariane models, to separate the two small solid boosters.
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And I'm planning for C11/E9 for early flights, working up to D12/G33. I really want to try a long burn in the core. With an Eggtimer TRS payload  . I'm planning on providing attachment points for a burn string, though the CTIs seem to light readily enough. I'm leaving myself an option to air start the boosters.
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Well, as I said, mixing BP engines and composites for a cluster can be very tricky. I have had mixed results, and some disasters. On a previous shuttle boilerplate, I kept trying ot use a composite F in the ET (as I did for nearly al the boilerplates including the final version), with 13mm A3 engines in the SRB's. The A3's were not going ot produce much thrust, just help provide extra flight points and also to provide a "simple" time delay and ejection for the SRB's. IT NEVER WORKED! Time after time, either the left SRB A3 did not ignite, or the right SRB did not ignite. And once, neither SRB ignited…. but the F took it all up fine. So on all those flight,s when the SRB's got sepped, the SRb's fell to the ground without any chute ejection or deployment.
Actually, before any of those flights, there was a notable disaster…. both the A3's lit, hooray….. but the F Chuffed and took a second or so to come up to thrust (it was an F14 BlackJack). Well, the two A3's were able to lift the model up the rod about a foot, then it fell back down, one SRB hit the pad hard, and broke part of the aft SRB attachment. Then the F came up to full thrust, and it took off, with one SRB lost, being pulled along near the nose but flopping around at the back, causing the model to buck all over the sky then crash (The SRB's had fins that the whole rocket needed to be securely attached to fly properly).
But I look back fondly at that boilerplate and all those failures, as showing me not to screw around with engines in the shuttle SRB's anymore, just have one engine in the ET and make it the model work reliably. So for the SRB's to get their chutes out, I had to use an onboard timer in each (ended up using a modified wind-up toy as a short timer, started at SRB sep, to activate a rubber-band driven system to kick the chutes out).
Anyway, you would really need a great reliable method to get D12's and a composite G33 to ignite together well.
Air starting the boosters…..that reminds me one of the schemes I was trying with that shuttle boilerplate with the outer A3's, after the A3's both lit but the F was late…… was I rigged up an automated ignition box to the pad, so when the F motor was ignited, and moved the rocket upwards 1 inch, the automated ignition box sent current to set off the flashbulb ignitors in the SRB A3 motors. I allowed for about 18-24" of wire slack for the system to ignite the flashbulbs, and the extension wires were free to slide out from the box as the A3's might not ignite and release the flashbulb ignitors until it was several feet into the air. It's on of those things that in theory should have worked great.
But anyway, since you mentioned air-start of the boosters, there is something that a lot of people did not realize about the Titan-III and IV. When it took off, only the solids were lit. The two liquid engines in the Titan core were air-started long after liftoff, but before SRB sep. So if you wanted a realistic launch, and want to do an air-start, then air-stating the core would be the way. However, you'd want to have some sort of safeguard so if the air-start failed to ignite the ore, it would not plummet to the ground after the SRB's separated. So that is one plus to doing an air-start of the boosters, could be more practical to rig it so if the air-start failed to ignite the boosters, they would stay on and ride down on the core. Indeed the method you described, they might stay attached, depends on the ejection charge kick forces of the core.
OK, as I think on this model some more, the Estes 1/73 scale kit, you would be making it way too difficult and overkill to have a G33 in the core even by itself, never mind any clusters for even more n-sec. I think it would fly great on a C11 or D12 in each booster, and one E9 in the center core. And that would be keeping it all black powder, avoiding the headaches of clustered BP mixed with Composites. With that much engine mass in the back, you'd definitely need to add some weight to the nose unless you used crazy-big fins.
Also, for a clustered rocket like this, with the engines that far apart, being extra nose-heavy is a plus. Won't veer off to one side as easily as with a more aft CG, due to minor (inevitable) mismatches in the thrust levels of the side engines. A long launch rod or rail would also help lot, like 6 feet of rod or 6 to 8 feet of rail. If you can do a rail, mention it and i'll describe a very good rail lug to use that slides very easy, unlike too many rail buttons that have binding issues.
- George Gassaway