Sizing of Hemispherical Chutes vs. Flat

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ThirstyBarbarian

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I have a Warlock that weighs about 8 lbs with a spent motor, and it came with a flat LOC chute described as a 60" chute, which is probably closer to 58". I have not flown an altimeter on this rocket, so I don't know the exact descent rate, but it looks like it lands hard, and it tends to swing a lot during descent, and I have had to fix the fins a few times. I want to get a better chute for it, and have been looking at Spherachutes.

How do I compare the two different kinds of chutes, and what it the standard way of measuring a hemispherical chute?

Below is an image from the Sphearchutes website that shows the measurement as going "over the top" -- 1/2 the circumference of the sphere. Is that standard? The reason I ask is that I've seen some discussions of hemispherical chutes that say things like "A 24-inch hemispherical chute is equal to a 36-inch flat chute because the surface area is the same." But that can be true measured this way. It seems like it could only be true if the measurement were across the opening. Is there a standard method?

parachutemeasure.jpg




I'm interested in knowing how the measurement is typically done, but I'm even MORE interested in knowing what size Spherachute I should pick to have a softer landing than the 60" flat chute I am using. I'm leaning toward a 72" Spherachute, and have emailed some questions to Spherachutes, but I was wondering what others on TRF thought.

Thanks!
 
I have a Warlock that weighs about 8 lbs with a spent motor, and it came with a flat LOC chute described as a 60" chute, which is probably closer to 58". I have not flown an altimeter on this rocket, so I don't know the exact descent rate, but it looks like it lands hard, and it tends to swing a lot during descent, and I have had to fix the fins a few times. I want to get a better chute for it, and have been looking at Spherachutes.

How do I compare the two different kinds of chutes, and what it the standard way of measuring a hemispherical chute?

Below is an image from the Sphearchutes website that shows the measurement as going "over the top" -- 1/2 the circumference of the sphere. Is that standard? The reason I ask is that I've seen some discussions of hemispherical chutes that say things like "A 24-inch hemispherical chute is equal to a 36-inch flat chute because the surface area is the same." But that can be true measured this way. It seems like it could only be true if the measurement were across the opening. Is there a standard method?

parachutemeasure.jpg




I'm interested in knowing how the measurement is typically done, but I'm even MORE interested in knowing what size Spherachute I should pick to have a softer landing than the 60" flat chute I am using. I'm leaning toward a 72" Spherachute, and have emailed some questions to Spherachutes, but I was wondering what others on TRF thought.

Thanks!

I've heard similar things about spherical chutes, not sure what the exact numbers are. I have a 12" (horizontal diameter) hemispherical chute that I use on many of my rockets that usually would use an 18"- I think it's a tiny bit small but not too different. Mostly I use it because it's nylon rather than plastic, so it opens faster and plays better with the JLCR.
 
some time ago I did a study of Hemi Chutes to flat and X-form; I haven't got the data handy but recall the volumn of an 30" hemi as more then double that of a 30" flat chute. found the chart:
Hope this pdf transfers OK

Thanks for this. The pdf came through fine.

This document uses the diameter of the chute as the "size" for both a hemispherical and a flat chute for comparison purposes. The "size" of the hemispherical is measured across the opening, not over the top like in the sphereachutes graphic. That is what I think is causing me problems with finding consistent info comparing flat chutes to hemispherical. If you are measuring across, then it seems obvious to me that a hemisphere of a given diameter has more area than a circle of the same area. Likewise, if you measure "over the top" then it also seems obvious that a circle of a given diameter has more surface area than a hemisphere whose half circumference is equal to the diameter of the circle.

Basically, when we say the "size" of the hemispherical chute is 12", is that the diameter or half the circumference? It makes a really big difference! (Pi over 2) So if I am getting a 72" chute using Sphereachutes' measurement system, it is 72" over the top, but it is only 45" across. Is that right?
 
What you are interested in is comparing the area of the open chute looking straight up a the descending chute.

With a spherical chute, the circumference is pi * diameter. if the Sphearchute is 72" then the circumference is 144" Take 144/pi and you get 45.8"' diameter. The area of the cross section of the chute would be (22.9)(22.9)(pi) or 1647.5 sq inches. A 60" flat chute would have an area of 30*30*pi or about 2827 sq inches.

That would tell me that the 60" flat should drop your rocket much slower than the 72" Sphearchute. You would need about a 95" Sphearchute to be equivalent to the flat.

Now factor in the distortion of the chute shape when the shroud lines pull in the chute, and the faster decent when the chute is tilted to the side and spilling air as the rocket swings under it. Since both those conditions seem to be more prevalent with a flat chute then the Sphearchute, the decent rates might be similar, but I would still think the 60" flat would be slower.

This is a perfect example of how each manufacture measures their chutes and decent rates differently and you really can't compare them oranges to oranges.
 
What you are interested in is comparing the area of the open chute looking straight up a the descending chute.

With a spherical chute, the circumference is pi * diameter. if the Sphearchute is 72" then the circumference is 144" Take 144/pi and you get 45.8"' diameter. The area of the cross section of the chute would be (22.9)(22.9)(pi) or 1647.5 sq inches. A 60" flat chute would have an area of 30*30*pi or about 2827 sq inches.

That would tell me that the 60" flat should drop your rocket much slower than the 72" Sphearchute. You would need about a 95" Sphearchute to be equivalent to the flat.

Now factor in the distortion of the chute shape when the shroud lines pull in the chute, and the faster decent when the chute is tilted to the side and spilling air as the rocket swings under it. Since both those conditions seem to be more prevalent with a flat chute then the Sphearchute, the decent rates might be similar, but I would still think the 60" flat would be slower.

This is a perfect example of how each manufacture measures their chutes and decent rates differently and you really can't compare them oranges to oranges.

It is complicated, and like you said, there are factors, like the actual shape of the flat chute and the size of its opening on descent, that are hard to know.
 
The way Spherachutes measures the chute, a hemispherical chute has about the same shape, projected area and hence drag as a flat chute with the shroud lines loaded, but the hemi is spilling less air from the lip of the chute and so should be somewhat more efficient. I haven't found this to be a huge differences in practice, but the Spherachute is easier to pack.
 
Eric, check out the Fruity Chutes web site. There’s a descent rate calculator that will give you all the information you need to make an informed decision.

Bear in mind that a hemispherical chute usually has a better Cd than a parasheet. This allows you to use a “smaller” chute and get the same descent rate.

Fruity Chutes will cost you dearly but they are worth every penny. And the owner is a LUNAR member as you probably know.
 
Eric, check out the Fruity Chutes web site. There’s a descent rate calculator that will give you all the information you need to make an informed decision.

Bear in mind that a hemispherical chute usually has a better Cd than a parasheet. This allows you to use a “smaller” chute and get the same descent rate.

Fruity Chutes will cost you dearly but they are worth every penny. And the owner is a LUNAR member as you probably know.

Perfect! This descent rate calculator is exactly what I needed. It’s great how they account for the different brands’ measuring systems and geometries. Now I just need to get my head around paying as much for the parachute as I did for the rocket...
 
I am using a 52" Skyangle but mine weighs 10.7 lbs. It's been rebuilt several times & wrapped in glass & T2T as well.
 
I am using a 52" Skyangle but mine weighs 10.7 lbs. It's been rebuilt several times & wrapped in glass & T2T as well.

Unfortunately, I can see how this rocket might end up that way — rebuilt, wrapped in glass, and tip to tip.

I really like the Warlock, because of its light weight for its size. But that means the fins are a little bit vulnerable, I think. Unfortunately, solving that with glass just adds to the weight. I’m hoping the bigger chute will help. I don’t put this rocket up very high, so a slow descent rate should not be a huge problem. And I have a chute release I could use if I wanted to.
 
Unfortunately, I can see how this rocket might end up that way — rebuilt, wrapped in glass, and tip to tip.

I really like the Warlock, because of its light weight for its size. But that means the fins are a little bit vulnerable, I think. Unfortunately, solving that with glass just adds to the weight. I’m hoping the bigger chute will help. I don’t put this rocket up very high, so a slow descent rate should not be a huge problem. And I have a chute release I could use if I wanted to.

Have you considered making your own? All you need is a basic sewing machine and know how to sew a straight and zigzag stitch. (Learning to thread the machine will probably be harder then using it) A cheap soldering iron with a flattened tip to use as a hot knife to cut the nylon and you are all set. You can make it any size you want and use an elliptical design which is very efficient and packs about the smallest.

I made my own 9 ft. diameter, 24 gore, chute for my L3 rocket for about $100 in materials and had enough left over to make the 30" drogue and 60" pilot chutes too. It did take 40 hours of sewing time, but that's "hobby" time so it doesn't count.

The advantage of "rolling your own" isn't so much the cost savings as much as you get exactly what you need or want for your rocket.
 
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