Right size for parachute.

I've built my own design and just need to select a parachute for it, but it's larger than anything I've built so far so I don't know what size would be right. How do you know what size parachute to use with your rocket? I'm sure there's an equation or ratio for it.

It is a function of weight of the rocket and the style of the chute. Most of the chute companies have charts on their web page for their chutes. For just a regular, round flat chute, I use Top Flight's chart https://topflightrecoveryllc.homestead.com/descent_rates.html

In design, I typically have a gross approximation of what I'm going to need --say, "Around an 84"," and weigh-in once I've got her built, primered, and painted, and adjust the Rocksim file as necessary. Once at that point, I see if the idea chute will work, or if I'm going to have to go a size bigger or smaller (the latter doesn't happen often, but one or twice, it has).

How much recovery weight are we talking about? That's vital in determining 'chute size --(fin shape and location are part of it, too...)

Later!

--Coop

darthgriffin said:

I've built my own design and just need to select a parachute for it, but it's larger than anything I've built so far so I don't know what size would be right. How do you know what size parachute to use with your rocket? I'm sure there's an equation or ratio for it.

Click to expand...

Darth, one can almost never go wrong with using 3 - 3.5 s.f. (canopy surface area) per pound of *recovered* weight (i.e. liftoff weight minus propellant weight). 3 s.f. will give you something on the order of 20 FPS (feet per second) and 3.5 will approach something like 15 FPS of descent rate. This also is useful in preliminary design of the rocket (to establish total liftoff weight for motor selection). There are plenty of items that this 'rule of thumb' either ignores or simplifies - including variability of CD (with different canopy designs), air temperature (i.e. density) and launch site elevation (among others) - but, again, if you never went any deeper than that 3.0 - 3.5 figure, I could almost guarantee that you'd never have to cut your flying short due to damage from some anomalous recovery.

It's also a good 'reality check' of other methods -- if 'their' answer falls outside that range (by very much), I'd check *their* numbers. However, to satisfy yourself, take a typical case (say 10lbs) and 'run the numbers' with all these other online calculators, etc. I think you'll find the above 'shortcut' is pretty close.

-- john.