Two chutes

I've got the huge MRS Andromeda. It's going to weigh about 17 oz without chute. It is extremely fragile, and the recovery area is @ 5000+ feet. I have effectively unlimited recovery area with no trees. The rocket is a treasure, and I want to give it every chance possible for a safe recovery.

It comes with one 30" circular (non-hemispherical) chute. I have a second, identical chute in my "spares" box. I also have a ~38in X-form chute. These are all nylon. I have 12in of BT60 tube to cram it in. The descent rate with one 30" chute looks marginal.

I am tempted to use *both* 30in chutes. (a) how do I prevent them from getting tangled or interfering with each other? (b) how do I attach them to the shock cord? (c) how can I calculate the descent rate (in rocksim or emrr calculators)?

Would anybody advise an alternative, such as only the X-form, or only one circular chute?

Thanks

Geof

I'd personally go with a 48" chute. Knowing where you fly I'd say you are better off with that instead of doing two 30" chutes. I always make mine descend between 5-10 fps and oversize my chutes. If you do use the two chutes I'd generally say that the cumulative area is SQRT(2)/2. I'd say you'd be within a Gelfand on that. Each is only about 70% as effective. I've used 3 chutes (120" ones) on a project and it descended at about the right rate to satisfy SQRT(2)/2.

Edward

This is a decent rate calculator I have used. If you use 2 chutes you need to know the weight of each piece and figure out what size chute you need from there.


https://www.rocketreviews.com/tool_descent_rate.shtml

geof said:

I am tempted to use *both* 30in chutes. (a) how do I prevent them from getting tangled or interfering with each other? (b) how do I attach them to the shock cord? (c) how can I calculate the descent rate (in rocksim or emrr calculators)?

Click to expand...

Geof,

I've used 2 and 3 chutes with success. I prefer to have a single "extender" cord from nose cone to chutes, and have each of them connected to the cord. This gets them away from possible tangles with the nose cone. I also wrap my chutes with the shrouds inside the bundle, further reducing the possibility of tangles. Use a long shock cord to spread everything out while chutes are opening.

Does the "sqrt(2)/2" reference mean that
30 & 30 = 60*sqrt(2)/2 ?

I'll try the emrr calculator when I'm back on a Windows machine. I had always thought that the multiple chute option there was for sequential, not simultaneous, chutes.

G

Yes, I've found that if you sum the diameter of the chutes and multiply by SQRT(2)/2 that is about the diameter they get. I though the calculator on EMRR was for simultaneous chutes, not sequential.

Edward

I second the suggestion to go with a single larger chute. 48" might be tight, depending on brand and material. The 48 I use packs VERY loosely (in my 4" rocket:rotflol

Check with CJL about Rocket Rage parachutes. If they make one small enough, I believe they are very efficient and pack small.

I look forward to seeing this one. Are you saving it for MHM? I'll be there with a big yellow/black rocket and I-to-6xF airstarts!

Ken

Top Flight has some thin-mill 'chutes that pack tightly. But, if you have room for two 30" 'chutes, one 48" 'chute should fit easily.

Having said that, my Aerotech Astrobee-D uses two 'chutes - one attached to the top half of the rocket, the other to the bottom half. I've flown it probably a dozen times without any problems with the 'chutes tangling.

Originally, I would put a piece of masking tape across the two shock cords to ensure that the two parachutes would get pulled out of the body tubes. One time, the tape didn't tear or let go and the two parts of the rocket came down together. But both 'chutes deplyed okay and didn't tangle.

Now, I just skip the tape idea and pack each 'chute into a bundle with the shroud lines wrapped around. Then I put the rocket together with the bottom 'chute in the top tube and the top 'chute in the bottom tube. You'd think this would be a recipe for tangling the parachutes, but it's never happened.


-- Roger