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  #1  
Old 12-08-2015, 09:45 AM
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Default Mega Red Max RocSim

Background: I'd like to use the Mega Red Max for a school demo rocket, but the smallest recommended motor (F50-6T) puts it over 500 ft. That almost guarantees that I won't be able to get it down inside the small playground. Therefore, I'm looking at clustering to get about 1/2 the altitude and still get similar or better peak thrust for liftoff safety.

The Estes/AT F60-6T specs:

76.83 N-s total impulse
79.50 N peak thrust
53.73 N average thrust
1.43 second burn time
6.05 second measured delay

3 D12-3 cluster

50.52 N-s total impulse (66%)
89.19 N peak thrust (112%)
30.63 N average thrust (76%)
1.65 second burn time
2.39 second measured delay

As you can see, I'm looking at 112% peak thrust compared to the single composite motor while total impulse is only 66%. That should theoretically get me around 1/2 the altitude or a little less and still get a decent velocity off the rod. However, I'm worried about the ramp-up time and quick drop-off of the D12 thrust compared to the near instant and consistent peak thrust of the F50. There's not much time spent at max thrust.

Could somebody with RocSim model these two motor configurations for me? Maybe model a 4 motor D12-3 cluster too? If the sim shows any promise at all, I plan to do a test flight with the cluster config this spring to compare RocSim results and to make sure it will work in the real world before flying it in public.
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Old 12-20-2015, 12:47 PM
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Have you considered the F42-4T? Much faster getting up to thrust than 3xD12, and good sustaining thrust. Anyhow, doing quick sims with SmartLaunch, on my MDRM (36oz), using a 5' Rail (I use a 6' rail, but subtract 1 ft for the offset of the top rail button):

F50 - 6.8:1 Thrust-to-weight, 28.5MPH off the rod, 391 ft, 3.8s delay
F42 - 6.4:1 Thrust-to-weight, 26.9MPH off the rod, 282 ft, 3.1s delay
D12x3 - 7.9:1 Thrust-to-weight, 25.2MPH off the rod, 203 ft, 2.6s delay

I'm not sure how exactly SmartLaunch calculates thrust-to-weight, it seems optimistic with the D12, given the short thrust peak.
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Old 12-20-2015, 01:19 PM
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As a CTI motor user my first thought would be the single-grain F59, probably drilled to a 3s delay on the MDRM. 57 Newton seconds total impulse. Max thrust about 71N, achieved in about 0.05 seconds. A whole lot simpler than clustering D12s...albeit a bit more expensive especially if you don't have any 29mm CTI hardware yet.

Added ....

I just looked at my MDRM in Smart Launch with that motor and it is predicted to go 340 feet - maybe too high for your purposes.

The F36 Smoky Sam looks better - drilled to 2s delay. About the same peak thrust as quickly as the F59 but about half that after the initial spike. Smart Launch predicts 195 feet with a burnout-to-apogee time of 2.8s....so 2s would cut it short or going to 4 would provide a little recovery suspense .

My MDRM weighs about 31.5 ounces sans motor, so that's what I used in the app....
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Old 12-20-2015, 10:29 PM
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Thanks, PaulK, you've given me what I'm looking for.
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Old 12-21-2015, 01:05 AM
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I'd adapt down to 24mm and use a F39-6T.
This is only a 50 n-sec F and the delay is nowhere near 6 seconds.
Check NAR S&T regarding the delay. It tests out at 4 seconds, not 6 and the motor has enough kick to get it moving.
Probably would only hit about 350' max.
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Old 12-21-2015, 08:29 AM
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Quote:
Originally Posted by ghrocketman
I'd adapt down to 24mm and use a F39-6T.
This is only a 50 n-sec F and the delay is nowhere near 6 seconds.
Check NAR S&T regarding the delay. It tests out at 4 seconds, not 6 and the motor has enough kick to get it moving.
Probably would only hit about 350' max.

350' is too much for the field. I'm looking for a 150-200 ft flight, fairly close to Estes Saturn V altitudes. Triple D12-3 is even pushing the field's limits. We fly Generic E2X rockets on A8-5's with streamers instead of 12" chutes on this particular field.

I'm not asking for motor selection advice. I'm asking for someone to run a sim on the configurations I've indicated. Part of the fun is that it's a cluster. Stuffing a single APCP motor in it is too easy.
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