Dual parachutes - parachute interaction.

[blackwing1]

Hello,

I'm doing TARC and since there's a time range, optimizing the parachute size is extremely important. However, based on calculations, my rocket needs a 21 inch chute to hit that range. The problem is that all the parachutes I find around that size are way too heavy because the cloth that they employ is way too thick.

So instead, I've decided to use two parachutes. One of them will be 15". I have yet to decide on the other size, but I think it should be from 12-15 inches in diameter.

The problem is that I have no idea how to calculate how the parachutes interact, so I was wondering if there was anyone out there with extensive dual-chute experience who could shed some light on the topic.

Does the drag just depend on the net area? As in, two 15" would equal to a 21" due to the area calculated from radius? Or is it more complicated due to the aerodynamics caused by interactions with the parachute slip stream caused by another parachute?

I've also heard that the main reason people use dual chutes is redundancy? Is that true? To clarify, I'm thinking of attaching one parachute to the fin section and another one to either the shock cord or the sections with the payload bay and nose cone.

Thanks for your help.

[troj]

It's more complicated than just matching the surface area. I don't have the book in front of me, so going from memory, but if I remember right, each canopy generates roughly 2/3 the drag it normally would.

[RocketManDan]

also a lot has to do with the positions of the chutes. You make want to research Apogee's website or even contact them and they might be able to help you? Tim is very knowledgeable.

[CarVac]

The parachutes angle away from each other quite dramatically, so their effective area is reduced a lot. This is mitigated by using looooooooong shock cords above the split, but that adds weight and volume.

[1tree]

There are a couple of factors which say to me that there needs to be a better way:
1) Two parachutes with the same total area will not slow an item as much as a single parachutes.
2) Two parachutes (or the same material) with the same total area will always weigh more than one parachute.

Not sure where you would find them, but Aerotech lists a 22 inch parachute in their catalog. The reviews on their rockets often mention how light their parachutes are. Also you might consider the Dynastar parachute. They are available from here:
http://www.apogeerockets.com/parachu...d-Power_chutes
This is sold as a 32 inch plastic parachute designed for mid power rockets. You could probably cut it down to the size you need.

[1tree]

First, I have no connection to any source I have posted. But I did come across another source to consider in your search for a parachute:
http://topflightrecoveryllc.homestead.com/thinmil.html
They have several lightweight parachutes in the size range you are looking for. And again, I would not easily give up having a single parachute. You can also find nylon parachute flares that are thin enough to be translucent. That option would leave you with a need to modify it so it was small enough. But the cost isn't too high. (About $15 for a 60 inch chute.)

[SMR]

Hello,

I'm doing TARC and since there's a time range, optimizing the parachute size is extremely important. However, based on calculations, my rocket needs a 21 inch chute to hit that range. The problem is that all the parachutes I find around that size are way too heavy because the cloth that they employ is way too thick.

Not sure where you would find them, but Aerotech lists a 22 inch parachute in their catalog. The reviews on their rockets often mention how light their parachutes are. Also you might consider the Dynastar parachute. They are available from here: http://www.apogeerockets.com/parachu...d-Power_chutes This is sold as a 32 inch plastic parachute designed for mid power rockets. You could probably cut it down to the size you need.

A couple quick things come to mind...

The yellow parachute that come with AeroTech kits (Aerreaux, Mustang, etc...), is very light - by far the lightest of the nylon parachutes I have used. Available here... http://www.aerotech-rocketry.com/products.aspx You can also adjust the descent rate with an appropriately sized spill hole.

I agree with 1tree - Cutting down one of those giant plastic parachutes from apogee, or making your own, is another good option.

If weight is an issue, you will always be better off with a single parachute than multiples. Less chance of entanglement during deployment. The chutes tip away from each other, so they are individually not as efficient as either would be alone, although the two together are still more drag than a single of the same size.