Do Two Smaller Chutes = One Larger Chute?

[jmmome]

Question is this: I'm trying to land an 18 pound rocket at 10fps so as not to snap the deployed legs. Using an online descent calculator, I found that I need a single chute of a gigantic diameter to achieve a 10fps descent..

If I take the same square inches of parachute and divide it by two, I would need two 132" diameter chutes, which is manageable for my chute manufacturer.

Is the square inches of material, as long as the construction of the chutes remains the same, the single factor I should be looking at? Does one big chute equal two smaller chutes, as long as the square inches of chute material are equal, and the construction method is the same?

Thanks!!

 

[MClark]

With multiple chutes they are usually at an angle due to airflow pushing them apart. Making less effective.
so you need more area than a single chute.

 

[neil_w]

I don't know whether the calculation works exactly like that, but shouldn't your chute manufacturer be able to answer a question like that?

Nice wall of rockets BTW!

 

[Banzai88]

There's several Peak of Flight articles that have the math. It works. Pay special attention to how you fold and pack and stack the burritos. It'll take you a few launches to find out what's most reliable in your rockets.

The basic concept is that the shape of the canopy WILL effectively change since you don't get 100% of the available surface area working on descent from each canopy unless you get the math just right on shroud length, riser length, and effective rigging length.

I know of, and have used, 2 methods to set parachute size and harness/riser length:

Method 1:
Clusters and the math on risers (it is an article on small rockets, but the math is the exact same)
Clusters in a sim like OR and RS (article written for RS, but the same principles apply in OR)

Basic answer is step down one (rounded up) or two (rounded down) sizes, use two, and have about +25% of the shroud line length extra between each chute and the tie point between the two so that they don't interfere with each other. (Ex. if you usually use a 36" with 40" shroud lines, it'll take 2x30" PLUS about 10" extra line to the central harness tie point on each one to get approximately the same descent rate).

If you get the 'effective rigging length' too short, they WILL interfere with each other to the point of being almost unpredictable in performance. Oscillations and/or uncontrolled swaying are symptoms of this. To fix it, increase riser length from the tie point to the chute attachment point. Or re-do the math!

The math is scalable, so it will answer your question for size and line length, no matter how many canopies you want to use. Even if you want to get complex, like two sets of two, for four total canopies! The most I've done has been 3 sets of 2, for a total of 6 canopies. Really gets folks' attention when the canopy flower inflates!


Method 2:
Most use this diagram instead of the math (which is quick and easy, but not optimal, for line length. Simply step down one canopy size class and double up the number of parachutes. With 3 or more, this method does not work well, so you need Method 1 and the math):

 

[QBrandt]

From the perspective of simplified fluid mechanics, yes. Reason being, the terminal velocity equation:

Considering the two cases (descent on a single large parachute and descent under two smaller parachutes of equivalent area):
The mass (m) shouldn't differ much (couple of extra shroud lines, maybe).
Still falling on Earth, so g and rho (density of air) don't change.
Coefficient of Drag is more complicated, as it is shape dependent. But generally speaking, this shouldn't change too drastically.

This does assume that the multiple chutes are oriented exactly as they would be if they were alone. Because they will angle somewhat, the "effective area" will be smaller. (Imagine looking straight up at them on descent, they won't look like perfect circles, they will be squashed a little bit). So strictly speaking, the dual parachute setup will have a slightly higher descent rate.

For this project, go for 3 chutes. With only 2, the descent could be unstable. The rocket would likely rock back and forth, potentially landing with a lot of horizontal velocity, or at a high angle. Either of those might snap your landing legs. (If you want to see this physically: Hang a small weight in the middle of a string, hold both ends and swing it. Tie another length of string to it, hold all 3 and swing.)

 

[tfish]

it takes some playing around and test flights to dial things in. I wanted 3 smaller chutes in a 98mm min dia..because of room restrictions. I end up with Three 74" home made flat sheet parachutes. recovery weight was 19 pounds...decent from apogee was 92 ft sec drogueless, Main (s) set for 2500 slowed to 22 ft sce.




Tony

 

[jmmome]

Wow- thanks to all for the comments.

I assume my manufacturer will know the techniques and the math, but i certainly don't. That's why I wanted to come to the forum experts and get a quick tutorial. There is much more to it than I would have ever guessed- thanK you all!!

 

[Cape Byron]

Not wanting to derail the conversation, but that DC-X is awesome.

 

[jmmome]

Thank you! Sadly, I cut it up and threw it away. Was never going to fly it again, and it was taking up floor space needed for my newest project.
November launch of "Marvin Martian Jr." at Michiana Rocketry's Mayhem event.

Two large experimental rockets to launch at the event- what could go wrong?

 

[Banzai88]

Wow- thanks to all for the comments.

I assume my manufacturer will know the techniques and the math, but i certainly don't. That's why I wanted to come to the forum experts and get a quick tutorial. There is much more to it than I would have ever guessed- thanK you all!!

When I went searching for the information, I contacted all of them. Only 2 knew how to do it (sorta) and only 1 knew the math (he's the one that put me on the PoF newsletters for the simple math). Sadly, he's not active here much anymore.

 

[warnerr]

Question is this: I'm trying to land an 18 pound rocket at 10fps so as not to snap the deployed legs. Using an online descent calculator, I found that I need a single chute of a gigantic diameter to achieve a 10fps descent..

If I take the same square inches of parachute and divide it by two, I would need two 132" diameter chutes, which is manageable for my chute manufacturer.

Is the square inches of material, as long as the construction of the chutes remains the same, the single factor I should be looking at? Does one big chute equal two smaller chutes, as long as the square inches of chute material are equal, and the construction method is the same?

Thanks!!

Not all chutes are szme. I checked fruitychutes at 18# for aprox 11fps and sized to IFC-96-S - 96 in diameter Iris Ultra Compact with Spectra lines

Question is this: I'm trying to land an 18 pound rocket at 10fps so as not to snap the deployed legs. Using an online descent calculator, I found that I need a single chute of a gigantic diameter to achieve a 10fps descent..

If I take the same square inches of parachute and divide it by two, I would

Question is this: I'm trying to land an 18 pound rocket at 10fps so as not to snap the deployed legs. Using an online descent calculator, I found that I need a single chute of a gigantic diameter to achieve a 10fps descent..

If I take the same square inches of parachute and divide it by two, I would need two 132" diameter chutes, which is manageable for my chute manufacturer.

Is the square inches of material, as long as the construction of the chutes remains the same, the single factor I should be looking at? Does one big chute equal two smaller chutes, as long as the square inches of chute material are equal, and the construction method is the same?

Thanks!!

Not all chutes are same: fruity is small and compacts well and has great cd: 18# at 11fps i get a IFC-96-S - 96 in diameter Iris Ultra Compact with Spectra lines as being in ball park. go on there site and try some of the calcs for there chutes.

 

[Louis Wu]

Use retro-thrusters

 

[jmmome]

Use retro-thrusters

I actually am using dual retro thrusters- 8 second burn @ 1/2 pound each- but they are more for show.