Help with CP & CG ...

[Steve Shannon]

I've spent some time thinking about what to use as the recovery arrangement for this rocket. Watching NHRA drag racing and seeing them using parachutes that deploy at 300 mph, I'm confident this rocket can be recovered safely.

I think I would try what I call an "over the top" parachute that utilizes 300# braided kevlar for the parachute lines and rip stop nylon for the chute material. The lines go up and over the rip stop nylon parachute and thus the chute will not be ripped away from the lines when the chute blossoms at high speed.

I did something similar on the 2nd flight of my F-79. On the first flight, using regular parachute lines that attach to the chute, the chute ripped away from the rocket and the rocket came in ballistic. The 2nd chute with the over the top lines didn't shred.

In regard to attaching the chute to the rocket, I would create a bulkhead and attach an eyebolt to that bulkhead in the 3D printed nose cone section, attach a swivel to the eyebolt and then attach the parachute to that swivel, again using 300# braided Kevlar. For the body tube / fin can sub assembly I would secure the recovery device using 300# braided Kevlar attached to the motor mount, and then attach an elastic shock chord to the Kevlar, then attaches that elastic shock chord to the parachute chords.


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Here's some photo's of the over the top chute I made using 100# Kevlar and a plastic chute.

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That’s how many (I would say all but I saw one shred yesterday that wasn’t - every shroud line was gone) parachutes made for high power rockets are made. Rocketman (not meant as an endorsement - just a reflection of my experience) makes their Mach One drogue chute with over the top shroud lines and Kevlar gores which is designed to survive high velocity deployments. The problem is that it is probably too large for this application. But I bet any of the chute makers would be willing to build a custom chute. Controlling deployment would also help. Giant Leap had a slider that fit on the shroud lines of their parachutes which keeps the chutes reefed initially and then slides down the shroud lines to allow the chute to open completely.
Of course the other end of the issue is how to anchor such a chute so that it doesn’t rip the rocket apart when it deploys. Rear deployment like a drag racer would help. Incorporating a rear facing chute cannon inside the ring fin assembly, between the ring and the motor mount might be possible.

 

[Fifinella]

Lakeroadster: Thank you! I will look into it.

One of the issues with stability in flight is the rocket to pivot around the CG as we all know and having the CP basically behind it. This has to do with the air pushing against the flat surfaces aera of the body in 2D.
In the design I have shown the top nose cone area will over ride the fin area thus not making the rocket recovery stability in flight if pushed beyond its vertical axis.
I did download OpenRocket and tried a few configurations. In trying to keep a scale like look and having two diameter features a distance apart to act as centering guides down the tube it comes down to making a set of fins that POP-OUT when the rocket leaves the tube. This is the only way I see to keep the CG and CP where they need to be and keep the overall weight down of the complete rocket.

THANKS FOR ALL THE HELP AND GREAT SUGGESTIONS!

KAP

 

[lakeroadster]

Lakeroadster: Thank you! I will look into it.

One of the issues with stability in flight is the rocket to pivot around the CG as we all know and having the CP basically behind it. This has to do with the air pushing against the flat surfaces aera of the body in 2D.
In the design I have shown the top nose cone area will over ride the fin area thus not making the rocket recovery stability in flight if pushed beyond its vertical axis.
I did download OpenRocket and tried a few configurations. In trying to keep a scale like look and having two diameter features a distance apart to act as centering guides down the tube it comes down to making a set of fins that POP-OUT when the rocket leaves the tube. This is the only way I see to keep the CG and CP where they need to be and keep the overall weight down of the complete rocket.

THANKS FOR ALL THE HELP AND GREAT SUGGESTIONS!

KAP

You might want to check out @BigMacDaddy 's M1147 Tank Amp Round thread.

 

[BigMacDaddy]

I wonder if it might be possible to stabilize the initial design using GDS... Basically end the body tube right where the ring fin starts and move motor forward so that air is pulled in through front of ring tail.

Couple of forums and examples of GDS --
https://www.rocketryforum.com/threa...ilization-streamlined-finless-rockets.169672/https://www.rocketryforum.com/threa...t-utilizing-gas-dynamic-stabilization.170055/
If you are going to fold fins out I recommend that you fold them front to back so that drag keeps them open and you do not need to fight against their weight during launch. I did not make the fins open on my model but it should be pretty easy to wrap a few rubber bands around the fins and the motor where it sticks back behind the fin mount to make them open.

If you use the weight as you describe it, you are putting more weight in the back of the rocket (makes stabilizing the rocket even more difficult) -- plus as that weight moves it will move the CG backwards (dynamic stability is another complexity you do not need).

 

[lakeroadster]

I wonder if it might be possible to stabilize the initial design using GDS... Basically end the body tube right where the ring fin starts and move motor forward so that air is pulled in through front of ring tail.

I doubt it, not without adding considerable nose weight.

 

[Fifinella]

Playing with OpenRocket and looking at keeping the CG & CP were they belong this is the new design. Uses a D to E motor.

Fins are held close within the bazooka tube, they are torsion spring deployed once they leave the tube. 3D printing the fin support ring and the nose cone with a paper tube. Will show the completed model when all the parts are done.

Thanks again everyone for the help.

 

[dhbarr]

That should do fairly well. Getting some folgore vibes.