I'm interested in this thread as I'm trying to work out separating boosters on an R-7 or an Angara.

I'll echo Bill's question: Why to the boosters need to be individually stable? Is there a scenario where they are under boost without the core?

How about three or four fins just on the core? For instance, what if you took the Gemini-Titan or Titan II Estes kit with the clear plastic fin unit and added the boosters?

Maybe we could have more info on your projected profile? A10s in the boosters with a B6 or C6 core? How were you going to separate the boosters ?

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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

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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.

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.

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.

So far I'm just putting together parts. The big Estes TARC pack. Some parts from sandman.

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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.


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.


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

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I'll dig up the video and possibly a photo when I get time, but was thinking it was a finless design. I recall wanting to talk to them to find out how they made it stable, but I never did. If there were fins, it was core only and the boosters were powered, separated and recovered. Vince Hugele was doing the launch duties and called it a heads up flight. Another heads up flight was a modified Aerotech Astrobee-D from Tim Pickens. I think your sun seeker had flown enough that he just brought it to everyone's attention how it was tilted away from the sun on the pad and didn't call it a heads up flight. (I may be combining two RCC's, but I think all these flew at the same one.) IIRC, it was the debut of HARA's new launch control trailer. I guess this would merit a different thread to clear way for aeppel_cpm's progress on his project.

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I certainly start with the BP motors.

If I ever get that far, for air starting, or maybe I should say rod-starting, the boosters, I was playing with the idea of putting a 23A size 12V battery in each vector control pod. And have the motion of the rocket up the rod/rail pull a tethered insulating pin to complete the circuit and fire the igniter. From your description, it sounds like I might be able to do the same trick with long leads, rather than on board power.

And I launch with clubs that have rails. WOOSH and TWA.

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Having the pods be stable is a bug not a feature. Design it for what you actually want to do and any good RSO will approve it. Or go misfire alley and be your own RSO. I use one fin on my pods as did CMR with much smaller, lower power pods.

Jerry

Jerry, isn't a bug mostly in the sense that it would end up on the conservative side? Over-stable?

I've got a .25" rod, and I could (and have) test fly here at the orchard. I also like the idea of a downscale prototype.

For personal/business reasons, it's usually easier for me to schedule launches with the clubs, rather than flying on my own.

Flying in the orchard does have one benefit - it's guaranteed that the ladders on hand will reach the tops of all the trees

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If you wanted to have actual "flyaway" pods, that thrust off the rocket then deploy separately, I can see the logic of some sort of stability, although even then I would opt for something like cones that act as the fins and even attenuate them.

For "pop pods" you want oversize booster fins as the first choice, but if a nearly scale appearance is your goal, given the long boosters I can see the logic in adding weight to the pod cones. Either way the parts you are starting with look great. Try to figure a way to get an E6 or an F10 in the center!

Jerry

I like the idea of POP on a downscale, so I'm gathering the parts for it, too b

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