Spherachutes

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AfterBurners said:
I couldn't find the descent rate chart but I'll look for it. I would think it would higher?
Click to expand...

Also remember Julie measures her parachute in a different way she measures the size over the top of the canopy. 1/2 circumference. This means the actual opening of the canopy in diameter is quite a bit smaller. For example my parachute which is measured as a 120" chute is actually 76" in diameter.
 

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Me Likey!!!
When I buy my 1st aftermarket chute...It will be from Spherachutes...
Can't argue with that!!!
Nice!!!
 
John Williams said:
Also remember Julie measures her parachute in a different way she measures the size over the top of the canopy. 1/2 circumference. This means the actual opening of the canopy in diameter is quite a bit smaller. For example my parachute which is measured as a 120" chute is actually 76" in diameter.
Click to expand...

Correct. The nominal Cd is not the same as the actual Cd, and that’s true for almost every brand of chute, not just Spherachutes. She picked an odd measurement system that makes the nominal chute size unusually big for the actual chute size, so it means her nominal Cd has to be unusually low. If she had picked diameter of the opening instead, then she could say her say her Cd was something like 1.2 instead of .75. All the chute makers are using different systems to calculate their “reference diameter” and that means it’s really hard to compare chutes from different makers. A 36” Fruity Chute is not the same size as a 36” Spherachute or a 36” Top Flight. You would expect them all to have a different Cd due to differences in design, but it’s also due to differences in how they are measured.
 
Also keep in mind that Cd is merely a fudge factor to make the force equation work. Listing Cd without the reference area is meaningless. In fact, the product CdA is a more complete description of the performance.
 
Buckeye said:
Also keep in mind that Cd is merely a fudge factor to make the force equation work. Listing Cd without the reference area is meaningless. In fact, the product CdA is a more complete description of the performance.
Click to expand...

The best information to have for picking chutes is a graph of descent rate versus payload weight. The Cd and reference sizes are all so screwy across the chute industry.
 
ThirstyBarbarian said:
Correct. The nominal Cd is not the same as the actual Cd, and that’s true for almost every brand of chute, not just Spherachutes. She picked an odd measurement system that makes the nominal chute size unusually big for the actual chute size, so it means her nominal Cd has to be unusually low. If she had picked diameter of the opening instead, then she could say her say her Cd was something like 1.2 instead of .75. All the chute makers are using different systems to calculate their “reference diameter” and that means it’s really hard to compare chutes from different makers. A 36” Fruity Chute is not the same size as a 36” Spherachute or a 36” Top Flight. You would expect them all to have a different Cd due to differences in design, but it’s also due to differences in how they are measured.
Click to expand...

Wouldn't her chutes still have a higher CD that a flat sheet parachute? Most flat type chutes come in at .750?
 
There is a problem with thinking that a chute with a higher quoted Cd than another is actually “better”. The Cd isn’t enough to tell you the drag. The calculation also needs to use the size of the chute along with the Cd to determine the drag, and most rocketry sim programs ask you to enter a diameter to use in the calculation. There’s no standardization for how to measure the diameter, so chute manufacturers are able to do it however they want and then just pick a Cd to go with the diameter they quoted.

For example, I saw a chute marketed as the most efficient chute in the galaxy with a Cd of over 3.0, which isn’t even possible. But supposedly the descent rate sims work because the manufacturer picked a ridiculously small diameter to quote for the chute size. I’m sure that manufacturer thought it would be a good marketing ploy to say they had the highest Cd in the industry and just arbitrarily picked a smaller chute size to quote to make it all work out.

That’s a deceptive marketing ploy, but the reality is that even if you weren’t trying to fool anybody, there would still be judgement calls to make. Say you have a flat hexagonal chute. What is the “right” way to measure it? Do you measure from one corner to the opposite corner? Or from one flat side to the opposite flat side? Those are 2 different numbers. Pick the larger number and a smaller Cd? Or the larger number and a smaller Cd? Or just fudge it all for marketing purposes? It could be 36” corner to corner, 34” flat to flat, or maybe just 30” across the opening when it’s actually deployed and pulled into a curved shape by the shroud lines. Should that be called a 36”, 34”, or 30” chute? The Cd will go higher with each smaller size quoted for the exact same chute.
 
I use a lot of Spherachutes, more than any other manufacturer. That being said, you will need a size or 2 larger than other brands of chutes. The opening size is shown on the details page for each chute where you select the colors. My 54mm Deuce has a 8' chute with 1/2 blue and 1/2 yellow to match the rocket's color scheme. 47 flights and counting :)
 
AfterBurners said:
It does make it difficult to input data. I figure if kit came with a flat 36" chute than a size smaller of hers would still work well or same would be even better?
Click to expand...

Yes, it makes it a complete pain in the ass. The Fruity Chutes company has a descent rate calculator that supposedly allows you to compare different chutes from different manufacturers using each manufacture’s quoted size or model number. You pick the brand of each chute and put in the manufacturer’s size, and then fruity chutes uses the geometry of that kind of chute to estimate a diameter and Cd and compare the chutes.

Here’s the link: https://fruitychutes.com/help_for_parachutes/parachute-descent-rate-calculator.htm.
 
ThirstyBarbarian said:
The best information to have for picking chutes is a graph of descent rate versus payload weight. The Cd and reference sizes are all so screwy across the chute industry.
Click to expand...

It doesn't matter what area or length squared term is used, as long as it is provided with the stated Cd. I assume the manufacturers do the proper force and velocity measurements and then compute Cd from the data. She can use 1.39528901 square nanometers as the reference area for every chute. Doesn't matter. CdA is the quantity of interest to determine the aerodynamic force to carry the load.

Yes, a descent table is best for the purchaser trying to pick a chute. I tried to back-calculate the A used in each row in the descent table, and the values are not as consistent as I would expect, especially at 10,000 ft. The area used is roughly a circle with diameter of the given chute size. Her calcs are a bit off, or maybe there is round-off on the recommended weight ranges.

As for the sim programs, most use a circle of the given diameter as the reference area. When I compare to my flight data, I adjust either Cd or diameter (or both) to match the descent speed. I don't care what the individual values are, as long as their product works out correctly.
 
The standard Spherachutes are nice and a great value, but they are bit bulky for my needs. Maybe I will try the Ultralights next time.

I like min diameter, fiberglass rockets flown with big motors. This results in a very large mass to volume ratio, so I want chutes that pack down small.
 
AfterBurners said:
I couldn't find the descent rate chart but I'll look for it. I would think it would higher?
Click to expand...

https://www.spherachutes.com/decent-rate-chart

As noted above if you use their stated sizes and a Cd of 0.75 a calculator will give you the right descent rate. Cd depends on the reference area you are using and no two chute companies seem to use the same reference area. Spherachutes is on one extreme where the chutes might seem a little small for their stated size but they work very well at the stated rates.
 

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