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Craig McGraw

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I know you've been having a 13mm challenge, but here is a big one. Over 4' tall and 24mm powered.

I'm working on the EMRR challenge. #5 is to design a rocket using a simulation program and then build and fly it. Here is my design.

Some things I've had to work around in Rocksim.
The mid and bottom fins (extending into landing points) are connected with 1/4" dowel.
The launch lugs will be on the ring fin and on the upper tube fins (I will need a spacer for the upper one).

This is my first real attempt at a Rocksim design. It has been doing weird things. It always shows I have margin - sometimes even over-stable. But when I do the flight profile, sometimes it is stable and other times it is not (once I just exited without saving and re-started with no changes and got different results). When it has shown to be unstable for the flight sim, I've removed a part of the design. It then flies stable. I then add the original part back, and it is still stable. I've also noted that when I make a design change, the 2D view would not immediately update. If I switched to the 3D (or rear) and then back to 2D the view would then be updated. Could this be part of my stable/unstable issue - when Rocksim updates?

Anyway, please look over and let me know what you think.
Thanks
Les

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EMRR 2008.rkt (237.6 KB, 84 views)

Les,

Sounds like a program instability issue to me. Have you tried uninstalling the program, then re-installing, to see if that might clear things up?

A registry cleaner program might also help. Spoken from experience...

I'll look the design over and post back here...

Thanks!

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Craig McGraw

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Well, someone was bound to do it sooner or later. I guess it was my turn

Ladies and Gentlemen.....

(drum roll please)

I give you........




The Goony Flare. An 18mm two stage Goonybird Payloader.

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goonyflare.rkt (58.8 KB, 79 views)

Ahh, the sweet sounds of an asylum inmate going goony!

And an appropriate subject, too! The RockSim image looks nice, BTW...

This should bring the magic number to 18...

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Craig McGraw

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Why thank you! I appreciate that. Coming from you, that's high praise.

I do think the orange on the ring really completes the look.

Now, maybe I can talk Phred into making some decals for it? Not sure which name would sound better though.

I took a look at your RockSim file. Nice design overall, it really captures the goony philosophy. I do need to draw your attention to the "Margin" value on the 2D line drawing. "Margin" here represents the stability, or "calibre" (caliber), of the model. When you load up the model with a C6-0/C6-3 combination [which is the right combination for this bird] your stability gets whacked. Notice in your file that the margin falls below the "magic number" of 1.00 when you add the motors. As shown in your file, the rocket will cartwheel and go everywhere else but up.

Or, more graphically, this means it'll go "butt-up"...

It was a simple fix to get this number back into our "greater than 1.00" ballpark. The model just needed 1/4 oz of ballast at the inside tip of the nose cone. A couple of tiny fishing sinkers epoxied into the plastic nose cone should be enough.

I added a launch lug to the model, placing it at the CG. It does contribute to the drag value, but not enough to kill the performance.

I also 'tweaked' the model in the ringtail area by setting the "Pylon Count" to zero, instead of using the '3' you left in place. This removes those extra 'fins' from the simulation (which add a measure of drag, no matter how thin they are). When you want to use custom fins with a ringtail, set this value to '0', set the pylon thickness to 0.063" (1/16") so the program doesn't bark at you, and draw in your fins as you like.

I'm attaching the 'repaired' file below. Again, welcome to the goony side of life!

Addendum: And now, here's the real value in using RockSim. Finding out the model doesn't have enough OOMPH to get off the launch rod. After posting the above changes, I failed to notice the flight performance section of the full-stack simulation.

Launch guide data:
Launch guide length: 36.0000 In.
Velocity at launch guide departure: 36.0432 ft/s
The launch guide was cleared at : 0.249 Seconds
User specified minimum velocity for stable flight: 43.9993 ft/s
Minimum velocity for stable flight reached at: 74.0976 In.

This is a show-stopper issue. It took a full quarter-second to reach the top of the 36" rod, and still isn't going fast enough to be stable in free-flight. See that last (bold) line? You need an 80" launch rod before the model reaches a safe flight velocity.

The problem is that C6-0 booster motor. It just doesn't have the thrust spike needed to kick that model hard enough to get airborne properly. This is also one of the reasons you'll hear us talking about the elusive "B14" and "C5" motors, which would more than likely have done the job.

You might need to change the design to a 24mm mount in the booster, and use a D12-0 instead.

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goonyflare.rkt (61.5 KB, 74 views)

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Craig McGraw

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Thank you Craig.

I did notice the margin went down to around .87 with the motors installed. I was going to add a little weight, but it was too close to the time when John McCain was going to speak when I posted the file. Never got to it.

Also, thanks for the tip about the pylons. I'll remember that for future designs.

Now.... if we get the rain up here that is being called for, maybe I can get it built up this weekend

24mm to 18mm is even better More like the original design.

Excellent idea. I'll get right on it!

Here's the up-rated Goony Flare, using a D12-0 booster and a C6-3 sustainer. The flight data looks better, too.

Launch guide data:
Launch guide length: 48.0000 In.
Velocity at launch guide departure: 55.0935 ft/s
The launch guide was cleared at : 0.257 Seconds
User specified minimum velocity for stable flight: 43.9993 ft/s
Minimum velocity for stable flight reached at: 36.0546 In.

If we ever get those nice cored 18mm motors again, maybe this rocket can go back to an 18mm booster. For now, though, this seems to be the only solution.

Addendum: Sorry I jumped ahead of you, but I found this to be an interesting design and wanted to see for myself what would happen.

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goonyflare.rkt (53.2 KB, 81 views)

__________________
Craig McGraw

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BARs helping BARs

SAM 0044
AMA 352635