Boba Fett Fresh Out of the Sarloc Pit

[hermanjc]

Leveraging what I learned from my Mandalorian Rocket build, I have made a very similar Boba Fett, again using a 3D printable action figure model purchased online and modified in Fusion 360 for my intent. I updated the ring fin I designed as well, and this time printed it out of PETG (hoping it will withstand the summer sun better than Mando's PLA version). Main construction is done and starting to paint.

 

[mjennings]

You know what this means Mando drag races!

 

[Grog6]

Heh. This reminds me of the battle a friend and I staged when we decided we were too old for GI Joes, lol. It involved a dam across a creek, a gross of m-80's, assorted bottle rockets, and firecrackers, and a gallon of gasoline. I think we were 12.
An hour later, the creek was out, we were out of fireworks, and there were no recognizable GI Joes left.

 

[hermanjc]

More paint progress. Just need to finish the body tube and jetpack and these two bounty hunters will be ready for spring.

 

[Five]

“This is the way”

 

[hermanjc]

Paint is done. Need to make him a parachute now. I'm thinking of ordering some yellow ripstop and adding the Mythosaur to it with a sharpie.

 

[hermanjc]

Boba took his maiden this weekend as part of the CMASS 2022 season opening. His trajectory was good for the first 3/4 of the burn phase but then he tipped to one side, with the altimeter onboard topping out at about 125'. Still clean ejection and good chute for a safe landing.




View attachment PXL_20220423_154901684.mp4

 

[boomtube-mk2]

These rockets are beyond my .

 

[Five]

Boba took his maiden this weekend as part of the CMASS 2022 season opening. His trajectory was good for the first 3/4 of the burn phase but then he tipped to one side, with the altimeter onboard topping out at about 125'. Still clean ejection and good chute for a safe landing.

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Nice!!!

 

[boomtube-mk2]

What do you think caused that right-angle turn?
Chang in the center of gravity at burn out?
Wind shear?
Attack of the Clones?

 

[hermanjc]

What do you think caused that right-angle turn?
Chang in the center of gravity at burn out?
Wind shear?
Attack of the Clones?

Not entirely sure. I would say weathercocking, but the flag in the video is pretty still and it wasn't all that much higher up than it. He is about the same dimensions/weight as my Mandalorian rocket that launched a few minutes later and flew nice and straight.

Boba will fly again and we'll see if it's the same result.

 

[mjennings]

Perhaps he saw a bounty down range?

 

[lakeroadster]

That's awesome. Thanks for sharing.

The ring fin is big, but I wonder if the right turn has anything to do with Gas Dynamic Stabilization, and thus when the motor thrust stops, stabilization is reduced?

Can you post a photo of the rocket with the motor installed? Just curious how much motor overhang there is.

 

[hermanjc]

That's awesome. Thanks for sharing.

The ring fin is big, but I wonder if the right turn has anything to do with Gas Dynamic Stabilization, and thus when the motor thrust stops, stabilization is reduced?

Can you post a photo of the rocket with the motor installed? Just curious how much motor overhang there is.

The ring fin on Boba is about a 1/2" larger in diameter than that of my Mandalorian rocket which flies quite straight. With that said, they have similar mass (Mando is 261g without motor, Boba is 256g without motor), but I did find that the C.G. of Boba is about 1/2" further aft without motor.



The motor on Boba is flush with the bottom of the ring fin, but another "negative" when compared with Mando is that it is about 1/8"-3/16" further aft than Mando's motor. Boba's CG with motor is 5/8" further aft than Mando.



Might try some swing tests just to see how he behaves on a string.

 

[lakeroadster]

I'll bet it's just due to the asymmetrical configuration of the figurine.

Under thrust, when the rocket is being pushed upward, it's not an issue, but once the thrust of the motor stops the rocket is pushed off course. Might be the angle of the head, might be the feet, the jet pack?

This would be a neat rocket to analyze in a wind tunnel.

The swing test might give you some good data. Since you can rotate the figurine on the string and look at multiple axes (axises).

 

[BABAR]

Boba took his maiden this weekend '.

wouldn’t that be his “Mister”?
congrats on a nice design, brilliant execution and paint job, and a successful flight!

 

[BABAR]

Enlarging the ring likely explains the CG shift, but I suspect this is more than balanced by the increase surface area of both the pylon fins and especially the ring fin. Ring fins are I think especially efficient since they are so far off center, as opposed to perpendicular fins where the effectiveness is zero at the attachment point/fillet and increases to maximum at the tip. For ring fins, the whole RING is at the tip. i don’t think anything gives you more bang for your buck regarding CP change than a ring fin, at the cost of more DRAG. So they aren’t great for altitude or speed performance, but they are difficult to beat for Odd Rocs where you just want it to fly straight, reach a respectable altitude, and recover safely.

any breakage or other damage or downsides you experienced?

 

[Rob Campbell]

Sounds like a hip-hop album, Boba Fett: Right Outta Sarloc.

 

[cbwho]

Are you selling rough Mando kits? This would be an awesome build.

 

[hermanjc]

Enlarging the ring likely explains the CG shift, but I suspect this is more than balanced by the increase surface area of both the pylon fins and especially the ring fin. Ring fins are I think especially efficient since they are so far off center, as opposed to perpendicular fins where the effectiveness is zero at the attachment point/fillet and increases to maximum at the tip. For ring fins, the whole RING is at the tip. i don’t think anything gives you more bang for your buck regarding CP change than a ring fin, at the cost of more DRAG. So they aren’t great for altitude or speed performance, but they are difficult to beat for Odd Rocs where you just want it to fly straight, reach a respectable altitude, and recover safely.

any breakage or other damage or downsides you experienced?

No breakage that I can tell. I actually increased the parachute size for Mando too because he was landing rough last year. Sized up to a 22" octagon chute in each.

 

[Grant_Edwards]

[...] perpendicular fins where the effectiveness is zero at the attachment point/fillet and increases to maximum at the tip.

Can somebody explain the aerodynamics behind that? I would think that once you're outside the boundary layer next to the body-tube and into "clean air" the distance would be irrelevant.

 

[BABAR]

Can somebody explain the aerodynamics behind that? I would think that once you're outside the boundary layer next to the body-tube and into "clean air" the distance would be irrelevant.

I may be wrong but I think it is a simple leverage issue.

Remember the “plane” of the ring fin is perpendicular to nearest Adjacent body tube at every given point.

therefore whenever the rocket angle of attack exceeds 0 degrees, the further off axis the fin the greater the correcting force.

 

[BABAR]

No breakage that I can tell. I actually increased the parachute size for Mando too because he was landing rough last year. Sized up to a 22" octagon chute in each.

Wow, 22” chute!?! You are a master chute packer. One of my on concerns with the original Mandalorian was that recovery space was going to be at a premium. Do these rockets use baffles?

 

[Grant_Edwards]

I may be wrong but I think it is a simple leverage issue.

Remember the “plane” of the ring fin is perpendicular to nearest Adjacent body tube at every given point.

True, but I don't see the relevance. What matters is the angle of the fin with relation to airflow, and the distance from the axis about which you want to turn the rocket.

therefore whenever the rocket angle of attack exceeds 0 degrees, the further off axis the fin the greater the correcting force.

Also true, but the root of a perpendicular fin and the tip of a perpendicular fin are both the same distance from the pitch and yaw axes. The tip is further from the roll axis, and is therefore more effective at rolling the rocket — but as the tip and fin are the same distance from the pitch and yaw axes, they have equal "righting moment" to correcting the heading to match the course (ignoring the reduction in airflow at the root due to turbulence and boundary layer effects).

 

[hermanjc]

Soooooo..... This happened


Immediately after leaving the rail Boba arced over backwards. I think I figured out what happened to suddenly chang his trajectory during his first flight. There was a very noticeable "buzzzzzz" during flight 2, which I think was indicative of a layer failure in the ring fin itself. This probably started to happen mid flight on the first go, and was catastrophic for the second. The chute deployed a millisecond before impact. I'll have to do a 100% inspection, but the damage seems to mostly be to the ring fin.

He will (most likely) fly again.

 

[mjennings]

Hmm. Delamination failure. That does make sense.

 

[BABAR]

True, but I don't see the relevance. What matters is the angle of the fin with relation to airflow, and the distance from the axis about which you want to turn the rocket.

Also true, but the root of a perpendicular fin and the tip of a perpendicular fin are both the same distance from the pitch and yaw axes. The tip is further from the roll axis, and is therefore more effective at rolling the rocket — but as the tip and fin are the same distance from the pitch and yaw axes, they have equal "righting moment" to correcting the heading to match the course (ignoring the reduction in airflow at the root due to turbulence and boundary layer effects).

Concur that for fins centered on the longitudinal axis, the tips are more effective than the roots because of the ”clear air.”. Not so for off center fins, defined as fins which are oriented PARALLEL to the longitudinal axis but don’t run through the axis.

a good example is the Jayhawk Drone, here a pic from Apogee site

https://www.apogeerockets.com/Rocket-Kits/Skill-Level-5-Model-Rocket-Kits/Jayhawk-AQM-37C
the vertical fins OFF-axis are much smaller than the horizontal ON-axis fins, but their effectiveness is magnified precisely because they ARE off-axis, and proportionately to how far off axis they are.

The Galaxy Taxi is another model that uses this principle

from
https://bellevillehobby.com/product/custom-rockets-galaxy-taxi-model-rocket/

 

[hermanjc]

Boba's got a new tail. I went back to (papered) balsa. I had to use a Dremel and oscillating saw to eat away the PETG material still epoxied in the fin slots around the motor. Then I was able to slowly push the too thick balsa so that it compressed a little.

Without paint this is about half the mass of the PETG ring (30ish grams vs 60ish).

Might be back on the rail as soon as this Saturday.

 

[hermanjc]

Swing test confirmed Boba is stable in the two asymmetric planes. Now just need to paint the box tail, reprint & paint a couple jetpack details that broke off, and apply some new decals to the fin base.

 

[hermanjc]

Boba's back from the dead (and 34g lighter than before his time in the pit... Which is a little different than the show)