Here is the initial design of the rocket I was talking about in another thread.

It's a four 18mm engine cluster booster with a single 2mm engine sustainer. Rocksim says it's stable (0.93 marginal) with engines installed.

It's strange that RS says what the deployment velocity is. Also it looks like I'd need a reaaaaaly long launch rod to get it up into the air properly. Maybe something I'm doing is causing the numbers to be off.

Not sure what to call it, so suggestions are welcome.

Comments/suggestions welcome.

Thanks!

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two stage.rkt (121.8 KB, 43 views)

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Dale Marshall, San Antonio Texas
NAR #90990
Semroc SAM #083

BAR Since 2008
Enjoying model rocketry since 1977.
The Spotted Dragon
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You need to run the simulations in single-stage mode first. The sustainer is too large for those 24mm engines to handle by itself, and a quad-18 booster isn't going to make it fly any better.

More underware-killing and entertaining, perhaps, as Wallyum would say...

That sustainer doesn't start becoming a viable design until you force-feed some "F" class power into it, and that eliminates what I think you're trying to achieve.

Start with a working sustainer first, then boost it. It'll work better.

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Craig McGraw

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I'll have to look at it when I get home. I don't know why a 24mm engine wouldn't lift it, it's not all that complicated. Just a basic 4FNC on a BT-70 base. It's just a little longer than the Estes Rock-it which flies great on a single 24mm engine. Strange.

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Dale Marshall, San Antonio Texas
NAR #90990
Semroc SAM #083

BAR Since 2008
Enjoying model rocketry since 1977.
The Spotted Dragon
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Following Craig's message, you have 93g tied up in G10 fins on the sustainer. That's roughly 40% of the sustainer's mass just in those fins. There's another 93g in the booster. Your rocket is lifting nearly a half pound just in fins. You could probably get by with G10 fins half that thick (and half the mass), or maybe consider 1/8" balsa @ 13g per stage.

Your idea of clustering has merit. It increases the lifting capability and can get the rocket off the rod with sufficient speed. But right now, it's kinda heavy.

I built a very similar rocket to this several years ago: a 2-stage, BT-70 model rocket. Mine crawled off the rod with the single D12-0 grunting to get it airborne. Then crashed when the sustainer didn't light. It was ugly. (And one of my rare staging failures.)

Anyway, I rebuilt the rocket with a clustered booster, and it works great. In your case, one simplification to consider is using a cluster arrangement which has its center motor aligned with the sustainer motor. That will simplifiy construction - you won't need to funnel the boosters. You will want to include hooks for a burn string to hold it on the pad in case the center motor does not light.

One other thing is to use enough outboard motors on the booster such that if one doesn't light, you still have enough thrust and impulse for stable flight as it leaves the rod.

http://www.doug79.com/apogeedx/

Doug

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Following Doug's observations:

I just ran a single-stage simulation with a C11-3, changing out those G-10 fins with 3/32" balsa, and got a much better set of numbers...

How does a Dv of 11 FPS sound?

It also reached 220' altitude, and only needed a 48" launch rod. The new weight of the sustainer with a C11-3 is 7.1 oz.

I'm sure you can find other ways to reduce the mass. One set of runs I tried dropped the number of fins to 3 instead of 4. I got at least one run with a Dv of 10 FPS, with an improvement in altitude of 10'.

Try using the "Custom Fins" option instead of the basic trapezoidal styles. The shape of the fins can change the aerodynamics dramatically. There are some shapes that greatly improve the stability margin while not requiring as much total mass (also reduces the drag by reducing the surface area).

Try these numbers for plot points:

1. 4......0
2. 3.5...3
3. 2......3.625
4. 0......0

One quickie note - the diameter of your NC is off by 0.03". The diameter should be 2.217", not 2.417". This is also affecting the performance numbers.

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Yea, fins... it figures. I didn't want to take the time to draw my own so I just chose some from the menu that looked basic. Forgot to check all of the specs on the fins. Grr...

I need four fins on the booster at least because it's a Sunward canted four-engine mount. So I'll likely have to go with four on the sustainer as well simply for aesthetics.

I'll play with the fins over the weekend to see what kind of numbers I can get.

I'm trying to use up parts on a crashed scratch-build so that's where the requirements for the rocket are coming from Should be interesting, though, once it's built.

Thanks!

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Dale Marshall, San Antonio Texas
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Semroc SAM #083

BAR Since 2008
Enjoying model rocketry since 1977.
The Spotted Dragon
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Circumstances understood...

So, continuing with this theme, I switched out the booster fins with a duplicate of the new sustainer fins, using four on both stages. The best (and only recommended) combination of engines appears to be (4) B6-0's and a single C11-5. Altitude is reaching an average of 650', with a Dv between 19 FPS and 41 FPS.

By reducing the scale of the sustainer fins to 80% of the booster fins, but keeping the thickness at 3/32" (0.94") I was able to increase the altitude by 40' (up to 690') while raising the low-end Dv to 18 FPS. This narrowed the Dv range to between 18 FPS and 40 FPS. Deployment occurs in the DESCENT of the coast phase.

See attached RKT file.

Something these versions don't cover is the focus transition that needs to be added at the top of the booster to make sure the ejection particles find their way into the nozzle of the sustainer engine. It shouldn't add that much mass, but it will affect the overall performance in the simulation numbers. Just be aware of it.

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two stage [Mk II].rkt (140.2 KB, 36 views)

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Craig McGraw

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Playing around with it, I put in new sustainer fins and used 1/8" balsa on both stages. With four C6's staging to an E9-6, I got it 1810' with a Dv of 4'/s. (I had to remove the streamer from the booster as it was fubar'ing my sims...)

Doug

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Attached Files

two-stage-feyd.rkt (124.5 KB, 37 views)

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I've been wondering whether the streamer in the booster is even necessary to slow its descent. I haven't looked at the weight of the booster all by itself with the new fins others have recommended. I'll see what it looks like tonight or over the weekend.

Thanks, all, for the help!

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Dale Marshall, San Antonio Texas
NAR #90990
Semroc SAM #083

BAR Since 2008
Enjoying model rocketry since 1977.
The Spotted Dragon
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-

If the booster is unstable, it will tumble, which will create quite a bit of drag. You can check its stability by deleting the upper stage and looking only at the booster in Rocksim. (Be sure to save the file before deleting.) I'm not sure why Rocksim doesn't include that feature without needing to delete.

Doug

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