My L3 Project...A 7.5" LOC Goblin...AMA

[DAllen]

Hey I remember that rocket!

LDRS 30:

 

[rharshberger]

I worked with Barry (former owner) on modifying a Bruiser EXL which I originally purchased from Ken Herrick at Al's Hobby Shop in Elmhurst, IL. Barry cut new fins based on my Rocksim modifications.

Looks like the lower fins are slightly smaller than the upper set, would you mind sharing the Rocksim file, I might want to build a smaller scale version as its a nice looking rocket.

 

[Speaknoevil]

Looks like the lower fins are slightly smaller than the upper set, would you mind sharing the Rocksim file, I might want to build a smaller scale version as its a nice looking rocket.

Yes, smaller trailing fins. I'll look for Rocksim.

EDIT: Actually looks like the Rocksim is still "live" back on an early post in the original build thread here: https://www.rocketryforum.com/threads/level-3-build-diary.8020/#post-66258.

 

[JLebow]

Rolling shutter doesn't change the motion - it just doesn't capture all motion at the same time.

The excursions you see are very real.

The excursions in the image are "real", and a good portion of the displacement is likely the fins. From the amplitude of the waves in the previously straight road, a significant portion of the excursion in the image is also bending and deflections in the camera mount. The frequency of the fin flutter, the camera structure, and "sampling rate" of the rolling shutter all superimpose in the image you see. You have quite a puzzle to try and back out which structure is shaking and in what amplitude. If the resonant frequency of any of the fluttering parts is more than 15 Hz (half the frame rate of the camera, which I assume to be 30 Hz), then there will also be alias artifacts that can give some pretty interesting mode shapes in the image. This can include what I observed, that a few frames, between very wavy frames, show the fins to be straight, when they might not be static - just captured at the same phase of periodic motion, each time the next scanned line in the frame captures part of the image.

I don't know that any of this changes the conclusion that the fins shook until they failed.

DAllen, good luck with the rebuild. Thanks for posting what happened, even though it sucked.

 

[rharshberger]

Yes, smaller trailing fins. I'll look for Rocksim.

EDIT: Actually looks like the Rocksim is still "live" back on an early post in the original build thread here: https://www.rocketryforum.com/threads/level-3-build-diary.8020/#post-66258.

Thanks Tim.

 

[MetricRocketeer]

Hi everyone,

Could anyone please post the RockSim file for a LOC Goblin 7.5.

The file posted on the LOC/Precision page is definitely not for a Goblin.

https://locprecision.com/product/goblin75/
Thank you.

Stanley

 

[thzero]

Looked correct to me.

 

[MetricRocketeer]

Hi thzero,

Thank you very much. This is very helpful.

Stanley

 

[MetricRocketeer]

Looked correct to me.

Hi thzero,

I have now had time to begin looking in detail at the RockSim file that you posted. I really appreciate your having done this.

Everything seemed correct, except for this -- the motor-mount tube fit a 132 mm motor. But the LOC Goblin 7.5 is supposed to fly on a 54 mm motor.

I understand, of course, that I could choose to load smaller-diameter motors. But the fact the motor-mount tube was so drastically oversized has caused me to wonder what is going on.

Do you, or any other rocketeers, have any thoughts about this.

Thank you.

Stanley

 

[DAllen]

What you are looking at is the central tube for the LOC Precision MMAS System. This is basically a motor mount adapter system. I have three adapters, 54mm, 75mm and 98mm. IIRC the 7.5" Goblin kit comes with one of them and you get to pick which one. Works quite well IMHO.

 

[Tobor]

Ooops, nm.

 

[MetricRocketeer]

IIRC the 7.5" Goblin kit comes with one of them and you get to pick which one. Works quite well IMHO.

Hi DAllen,

OK. Excellent.

Just one last question, please. What does IIRC stand for?

Thank you.

Stanley

 

[jd2cylman]

If I recall correctly

 

[MetricRocketeer]

If I recall correctly

OK. Thanks.

 

[Zeke Johnson]

Yes, it's mine. There are a couple videos of other Goblins, the owner is mentioned in the description.

.
I watched a few more of the Goblin videos. Looks like HED.

Zeke