Parachute Dynamics Reading

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LamdinRockets
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Parachute Dynamics Reading

Post by LamdinRockets »

I am making a rocket as part of a project in my degree as such all design parameters have to be justified. Leading from that does anyone have any knowledge of reading material for the design and implementation of a parachute. I.e. its size, shape and material used

I have made a similar post for rocket fins so if you can help with that please post in the other topic

Thanks for any help

Apologies for posting so many topics but I am really in the squeaky bum time of my project.


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anachronist
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Re: Parachute Dynamics Reading

Post by anachronist »

Yes, there's some good information available. I already answered your other post about buying a parachute, and I recommended you make one instead, pointing you at a tutorial for this.

One other thing you need is a way to calculate how big your parachute should be. This requires two inputs: the mass of your rocket, and your desired descent rate.

I've made a Google spreadsheet with the parachute sizing calculation, based on information I got from a NASA page. You can download it and use it. You'll find it and other resources on my page here: http://www.nablu.com/p/water-rocketry.html

-Alex


LamdinRockets
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Re: Parachute Dynamics Reading

Post by LamdinRockets »

Unfortunately due to the academic nature I'm going to have to do the calculations myself. However I've found useful information in Fluid Dynamic Drag by Hoerner


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anachronist
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Re: Parachute Dynamics Reading

Post by anachronist »

The calculations are pretty trivial in this case. It's just the standard equation for drag of an object, using the drag coefficient of a parachute (0.75). The spreadsheet I referenced in the previous post has all the calculations and includes a link to the NASA page I used.


LamdinRockets
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Re: Parachute Dynamics Reading

Post by LamdinRockets »

I've used S = (2w)/(Rho*Cd*v^2) to calculate the required area and therefore diameter of the parachute, however in Fluid Dynamic Drag it states the inflated diameter will be approximately 2/3rds of the laid out diameter , does this mean I need to divide the calculated diameter by 2/3rds. Additionally in Fluid Dynamic Drag it states the minimum drag of hemispherical parachute as 1 (1-1.4) not 0.75. Perhaps one is based upon the laid out area and the other the inflated. Thoughts?

Also you have stated a cd 0.75 and used it in the spreadsheet however the NASA links states a Cd of 1.75

Thanks for all the help


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anachronist
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Re: Parachute Dynamics Reading

Post by anachronist »

You're right, it's 1.75. A parachute that you make from a flat sheet doesn't form a hemisphere, so it isn't correct to use that. The actual cross sectional area will be somewhere in between a hemisphere and a flat sheet. Assuming the area is a flat sheet, you need a lower drag coefficient to compensate. So I used 0.75, which increased the necessary diameter of my parachute. For a 2-liter bottle rocket I ended up with a 9 inch diameter parachute, which seemed pretty small to me. But after I tested it, I found that it worked fine. The class D rules require that the rocket descend at LESS than 10 m/s, not exactly 10 m/s.

I had a more complicated spreadsheet that accounted for the curvature of the parachute to get cross sectional area but I realized that this was more complicated than needed.

I made some minor modifications to the spreadsheet, and corrected the drag coefficient.


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