Advice on Dual-Deployment from a Single Compartment?

I am building a large (12.75" x 10') rocket with a group of my students. The rocket should weigh in at around 40-45 pounds loaded, maybe a little less. This is the first rocket of this size we have built, the largest previous being a 4" all-fiberglass rocket.

The rocket will be a 4.9x upscale of the Binder Design Dragonfly and will have elliptical fins with a 10.85" root chord and a 23" span. Because of these long fins and a rocky launch site I want to set it down gently. This is definitely going to be a low-and-slow rocket, but even so it will be going up 5000'+ on a full L or higher on a small M. All this makes me think that we definitely want dual parachute deployment. In order to keep things lightweight and simple, I would like to deploy both parachutes from the main compartment.

It seems that a fairly "standard" recovery sequence might go something like this:

Rocket separates at apogee with a 12-pound rated parachute attached to the nosecone (nosecone should weigh just a bit under 12 pounds).
Nosecone is attached to a deployment bag (containing main) which is attached to airframe via a Tender Descender or similar release mechanism.
Release mechanism is triggered at main altitude and nosecone pulls out deployment bag.
Parachute is pulled out and nosecone and airframe recover separately.

One worry I have (someone brought this to my attention) is that the large compartment (12.625" x ~22") will allow the deployment bag to move around too much while falling. It was suggested that I use a 6" or 7.5" stuffer tube to hold the parachute - I am reluctant to do this because of the weight. Should I be concerned? How might I mitigate this?

I would love to hear people's opinions of the "best" way to go about recovering a rocket of this size and shape! What would you do?

You could use either a cable cutter or the Ratt Works ARRD for your setup.
You can use a centering ring to support the chute.
Another option would to create a gas piston, that way you only have to pressurize the internal space of the piston.
Say a 3" tube or so???


JD

Another option is a chute cannon, a smaller tube inside the airframe with the chute inside and ejection charge.

A stuffer tube doesn't have to be heavy fiberglass. PML tubing would work.

Gerald

Thanks for the replies!

JDcluster said:

You could use either a cable cutter or the Ratt Works ARRD for your setup.
You can use a centering ring to support the chute.
Another option would to create a gas piston, that way you only have to pressurize the internal space of the piston.
Say a 3" tube or so???


JD

Click to expand...

I am leaning towards using a reefed chute with a cable-cutter to release it. The ARRD looks really sweet, but a bit more complex than the cable-cutters.

rharshberger said:

Another option is a chute cannon, a smaller tube inside the airframe with the chute inside and ejection charge.

Click to expand...

I've thought about something like that, but I am hoping to keep the airframe as simple as possible.

G_T said:

A stuffer tube doesn't have to be heavy fiberglass. PML tubing would work.

Gerald

Click to expand...

True, but even the kraft phenolic PML 7.5" tube weighs almost as much per foot as the 12.75" cf body tube.

For something that large and not wanting to pressurize the entire thing I would use a Z-Pard. Basically a double sleeved aluminum piston powered by BP. You can try and locate the free article that was in Rockets magazine by Rick V. I've used a version that uses a two pieces of nested tubing. With just over 1 gram of 4FFFF I've been able to toss a regular brick 30' in the air when testing. It always shears the pins in the 5.5" rocket I fly it in.

Edit: I found the free sample of Rockets Magazine where you can see how it is built.

https://www.rocketsmagazine.com/RocketsMagazine/Issue0007/sample.pdf

Edward

RocketFeller said:

Thanks for the replies!


I am leaning towards using a reefed chute with a cable-cutter to release it. The ARRD looks really sweet, but a bit more complex than the cable-cutters.


I've thought about something like that, but I am hoping to keep the airframe as simple as possible.


True, but even the kraft phenolic PML 7.5" tube weighs almost as much per foot as the 12.75" cf body tube.

Click to expand...

I'm thinking your rocket doesn't weigh enough to need 7.5"... 6" should do it. But yes, it does add some weight. What it should add is reliability compared to having a chute rattling around in a big area. Personally I don't like any rocket which, when packed, provides any potential movement of the recovery system. I want it to come out the way I put it in. Then I know what it's going to do.

Gerald

One thing about the drogue and nose cone. I'm using a similar set on my 6" x 12' rocket. The pilot chute on the nose cone is size for about 10-12 ft/sec decent rate and the main is sized for about 17 ft/sec. That ensures the pilot stays above the main tube and has enough difference in decent rate to pull the d-bag with the main out of the payload tube.

The thing I did that has worked very well is a small 150 lb Dacron 100 ft. line between the top of the main chute and the inside of the d-bag. This keeps the pilot chute, nose cone, and d-bag attached to the main rocket, but because the pilot has a lower decent rate it stays well above the main, but they both land together. Probably at about 16.5 ft/sec decent rate.

Check out the thread on Jolly Logic Chute Release.

In this case I would use a Tender Descender. Gene of Fruity Chutes does this with his extremely wide Thunderdome rocket, and I am sure he would be happy to advise you as well as sell some of the parts!

https://www.fruitychutes.com/contact_us.htm