RockSim Simulations

[GrossApproximator]

According to spherachutes' weight chart, a 30 inch parachute should work in one of my rockets. However, when I build the rocket in RockSim, the parachute (after I've converted the parachute "size" into a diameter measurement and entered the spherachute Cd), is too small. Which source should I believe? Erring on the side of caution is a good thing, but if I put a big motor in this rocket, I don't want it to drift too far (It's single deployment only). Perhaps I should put a big 'chute in the rocket and only launch it on calm days?

 

[cjl]

If you used 15" as the diameter for a 30" spherachute in rocksim (as you indicated in another thread), you're doing it wrong. For a spherachute, you want to use a Cd around 1.2-1.4, and the diameter is exactly what is advertised. So, for a 30" spherachute, the diameter is 30".

Oh, and (also going by memory from what you've said in another thread) if your rocket weighs ~30oz, a 30" is fine.

 

[GrossApproximator]

If you used 15" as the diameter for a 30" spherachute in rocksim (as you indicated in another thread), you're doing it wrong. For a spherachute, you want to use a Cd around 1.2-1.4, and the diameter is exactly what is advertised. So, for a 30" spherachute, the diameter is 30".

Oh, and (also going by memory from what you've said in another thread) if your rocket weighs ~30oz, a 30" is fine.

Actually, I entered a 20 inch diameter in RockSim (or something like that). But whatever, I guess the idea of correcting the advertised size to a diameter is wrong. Reasons must be buried in the math that I never even bothered to look at . . . :cyclops:
The rocket should weigh 30 ounces while descending--after the propellant burns, of course--but that's VERY optomistic. It'll probably weigh significantly more after I get through with it. Epoxy+Me=Heavy I'd like to get the rocket completely built and weigh it before I purchase a chute, but I'll never get this thing done before the spherachutes sale ends.

 

[cjl]

Don't bother correcting the diameter. Adjustments should be made with the Cd. Here are some Cd values for various chute shapes:

Rocket Rage: 2.2ish
Hemisphere/Elliptical: 1.2-1.5
Conical: 1.0
Flat: 0.75

 

[GrossApproximator]

Okay. It makes remarkable sense when one actually looks at the math. :bang:

 

[ScrapDaddy]

You know, you could always consider duel deployment

 

[GrossApproximator]

You know, you could always consider duel deployment

I have considered it. But, I'm not altimiter-proficient, I don't even own an altimiter (yet), and my current design (which I don't really want to change) has only 1 compartment for parachutes.

 

[Heisenberg]

Ok I'll bite, why wouldn't you correct the Spherachute advertised size to the diameter other than your answer will not match Spherachute's size vs weight chart?
According to their web site, Spherachute's size is not the diameter but rather 1/2 the circumference of the chute. So a 48 inch spherachute has a diameter of about 30.5 inches. Most of the descent calculators I have seen ask for a diameter and that includes Rocksim. So using the calculated diameter and adjusting Cd to 1.5 it doesnt come close to spherachute's size vs weight. So what gives, why or what makes Spherachutes different? Any parachute scientists out there?

 

[Heisenberg]

I think I have figured this out. As it turns out in the world of parachute science, The Drag Coefficient, Cd, can be based on the Surface Area of the parachute or the Projected Area (cross section or pi *r^2) of the parachute. Either method is correct but you need to know which area the Cd is based on in order to calculate the correct velocity. Surface Area and Projected Area can be significantly different for the same parachute. This all gets very confusing when vendors don't give you this information and descent calculators don't specify Surface Area or Projected Area.

In the case of a Spherachute, their web page states that a 48" chute and a 6 lb rocket will descend at 20 ft/sec. If you plug 48" and 6lbs into a descent calculator you wont get 20 fps without manipulating the Cd. Looking into this further, Spherachute is a true hemispherical chute and the sizing is based on 1/2 the circumference. So knowing this you can then calculate the surface area. So for a 48" chute the surface area is something like 1466.77 sq in. If you substitute this back into the velocity formula and solve for Cd using 6 lbs and 20 fps, you get a Cd of 1.24 which is well within specs for a hemispherical chute. So bottom line it appears Spherachute uses Surface area in their Cd calculation.

It's interesting to note that Fruity Chutes is an ellipsoid parachute so calculating the surface area of an ellipsoid correctly requires more information than just the diameter so I wasn't able to test this method on it. However, they show you how to enter the information in Rocsim to get the correct results so i guess it doesn't matter.

 

[MarkII]

How would that work for something like a SkyAngle?

MK

 

[Heisenberg]

SkyAngle is a bit more interesting. From their website.. Cd calculation is based on reference surface area of the circular cap only for comparison purposes. Using total surface area as listed will decrease Cd value. So SkyAngle only uses a portion of the surface area when calculating their Cd. One really needs to understand what a vendor is basing their Cd on in order to make most descent calculators match what the vendor lists.