Water Rocket Forum

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.

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

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

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.

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

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.