Need a second opinion on 4H HPR

Hi TRF,

I tried posting this in the scratch build section, but no responses so I decided to ask here. In a few days, our 4H group is putting together our dual deploy HPR with motor BU which the kids designed themselves (see attached). I weighed all the components and I put them in OpenRocket as overrides. I was just wondering a few things:

1) The rocket stability is around 4 with a CTI J600RL. Would you recommend weights to lower the stability a bit or fly it as is?

2) Will the fins be big enough to maintain stability in the flight? They are over 2 caliber in length, but they look a bit small when I dry fit them. I personally go with big fins on my builds and I want to make sure.

These kids have been working their butts off for the last month and we have 3 build weeks before our launch so any input is greatly appreciated. I really want them to succeed! Thanks!

View attachment 4HRocketNew.ork

If the stability margin is four and the day is not very windy, push the button!

I downloaded OR --again-- so I could take a look at the file. If your flying field will support 5-6K, I'd say mash the button. Fins appear of appropriate size to me --not disproportionally small, nor disproportionately large like most Estes-- and it's long enough for good stability with the recovery and electronics --I'm seeing 4.64C with the motor. I certainly wouldn't add AFT weight to decrease that. Let it ride...

Get video!


Later!

--Coop

Since the rocket is 38mm MMT, I would have went with a 36" lower tube and 24" upper to move the av-bay and CG aft some which would reduce the stability calibers without changing the length of the rocket, but that's water under the bridge at this point.

As Coop says, don't add aft weight.

The rocket will tend to windcock into the wind, which isn't all bad, especially with DD. When it windcocks you get a horizontal speed even at apogee and popping the chute right at apogee becomes even more important with motor deploy. Using DD with a drogue greatly reduces many of those issues and problems the horizontal speed adds.

If field and wind conditions are such that you can handle the 6,200 ft altitude of the J600, push the button!

BTW, when you configure the Motor Mount, the ID should be 38mm instead of the OD. That way you can use the Motor Mount for the motor and configuration instead of having to use the lower body tube. It doesn't really affect the simulations, but....

Thanks for the replies! I'll keep everyone updated on the outcome with video!. I gave the kids the task with designing everything and they were really adamant about the length even when I mentioned the length is a bit too long on the one tube lol. Thanks for double checking my sanity

We'll be sanding, gluing and doing ebay stuff tomorrow so I can't wait to see what they end up with. Thanks again!

What kind of speed are you looking at? Even in transonic speeds, the CP can shift rearward some.

Does anyone know if it's possible to add more black powder to a CTI 6XL reload? The motor eject will only produce 7.43 PSI on ejection which probably won't be enough to separate the tubes. We will be using the motor eject as a backup, but we need it to be a workable backup . Any help is appreciated!

Tube Length: 48in
Dia: 3"
Chute size: 12in
Shock chord 30in

Yes--easily. Simply add the desired amount on top of the paper cap, and cover with tape. Same processes for 24-54mm.


Later!

--Coop

Thanks! I was thinking about that, but I wasn't sure if it would light with the cap between the original motor charge and my new amount.

When I saw the thread title I thought you were doing a 4 motor cluster with H motors.

Lol that would be a project for them to do another day :wink: