3" Fiberglass airframe with extremely heavy payload.

So here's the story, we're in a competition to implement active roll control, and test it in a real rocket. At the suggestion of our NASA mentor, we're using cold-gas thrusters.

The implementation of that is still a rat's nest of unanswered questions, and pressing deadlines, so please for the love of god, don't go there...

Every section of the rocket has a very specific (very heavy) payload, and space for each has been minimized. Some payload items include:

1. 30.5ci 3000 psi HPA cylinder 1kg
2. high-flow pressure regulator 0.25kg
3. Pneumatic solenoids (x3) 0.75kg (total)
4. lots of plumbing hardware (weight included in solenoid total)

I've created the most accurate OR file I can possibly make at this juncture. The cg can be easily adjusted through the shifting of several payload items, so that's not an issue. The design was created to be flexible as components choices could change (components have been finalized btw).

The fin can is 30in long and is joined via a 9" coupler to the nozzle bay, which is joined to the forward payload tube by a 6" coupler. The 9" coupler is joined to the nozzle bay using epoxy, the 6" coupler is joined to the forward payload section using epoxy as well. The NC, Forward payload bay, and nozzle bay are secured together using machine screws. This is a single split recovery with a jolly logic dual deployment. All tubing is MadCow standard thickness fiberglass. Again, this is NOT thin-wall FG. All screw holes are backed by a threaded nut to provide compression and avoid relying solely on the threading of the fiberglass. In the diagram below, the payload items from left to right are:
1. thruster control avionics
2. HPA cylinder
3. Solenoids and regulator package.
4. the lower-most coupler houses a commercial tracking and recovery package.

The rocket is 7.2 kg before motors, and just over 78in tall. Our chosen motor is the Aerotech J800T. This is the largest motor the competition will allow us to fly.

Stability margin during flight is predicted to remain above one caliber throughout the flight. The heavy payload items have pushed the CG far forward, and have minimized the required size of our fins.

I have never attempted to loft THIS MUCH WEIGHT in a 3" package. Here's my question... given the diagram below, are there any red flags that you believe place the mission at risk? (Aside from the nature of the payload, and assuming that all payload items are properly secured against forces due to boost and recovery).

You think it'll fold in half? The damned thing weighs 15+lbs dry.

I wouldn't worry about the rocket folding up, or somehow the FG not being up to the job. 15 pounds really isn't that much weight.

Given, however, the amount of weight that's far forward, have you considered drag separation where the rocket breaks for the recovery gear? Might want to shear pin there- but make sure you ground test the charge first.

Also, personally, I'm not the biggest fan of "little fins", although I don't have any hard engineering data to back up my preferences on that

3” thick wall FG? You could pour lead in the NC and payload bay and it won’t fold up.

7.2 Kg on a J800 isn't going to go very high... you'll be lucky to get more than a few thousand feet out of it. But you didn't specify your altitude requirements, so maybe that's OK. It's going to want to weathercock into the least amount of wind, so you should plan on flying as early in the morning as possible if you want it going straight up.

Might want to shear pin there- but make sure you ground test the charge first.

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Absolutely, I forgot to mention those.

Weather cocking is assured, sadley. And we're only trying to get close to 4k feet. I've given us a really generous fudge factor for weight so that we can have lots of room to balance, so it may end up lighter than i'm expecting.

Part of what I'm worried about is the multiple sections of tubing. Just felt like a lot of failure points. I felt really good when I was designing it, but part of being old enough to know better is running things past other people just to be sure. lol

Probably tool late in the design phase, but I've worked for two years with a student group that uses grid fins. They have much less weather cocking than similar planar fins from flights. On two flights in higher winds the rocket translated with the wind more than weather cocked into it.

Edward

Nice project. Good luck with it.

Should not have any problems with buckling if the couplers are a nice fit and not just slopping about. This also assumes the bonded side of the coupler has sufficient surface area and is bonded correctly. That part is difficult to stuff up.

If you have your simulations right then the fins will be fine. Remember too that the rocket will likely end up heavier than your initial expectations :wink:. If you are deliberately going for very small fins you may want to leave the fincan build till after all the CGT stuff is built. That part of the build has probably the highest risk of mass-creep. That way if the fins need a bit more area it is easy to add.

A longer than standard launch rail may be of use in getting you off the ground with less weathercocking. Your sims will tell the story for that as well.

Have fun

djs said:

I wouldn't worry about the rocket folding up, or somehow the FG not being up to the job. 15 pounds really isn't that much weight.

Given, however, the amount of weight that's far forward, have you considered drag separation where the rocket breaks for the recovery gear? Might want to shear pin there- but make sure you ground test the charge first.

Also, personally, I'm not the biggest fan of "little fins", although I don't have any hard engineering data to back up my preferences on that

Click to expand...

You beat me to the separation issue. Not just linearly either- think radially from the thrusters firing and causing the section to spin.


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I would suggest flying the rocket without the stabilization system first- maybe with just dummy weights in place of the equipment there. That will give you some experience as to how the rocket performs without risking all the expensive stuff

I think your fins are short enough you may see a bit of coning. I agree that running it passive would be a pretty good idea.

dhbarr said:

I think your fins are short enough you may see a bit of coning. I agree that running it passive would be a pretty good idea.

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Was just thinking- the gas jet system may cause issues for the small fins to see "clean air" to work in... Nathan- your thoughts?