Drogue + Main chute descent rate?

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spooked

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I was wondering how could you calculate the the descent rate for the main chute while accounting for the drogue chute. I’ve find some calculators online but none show the equations. I’ve read that the drogue should have little effect but I would still like to see some calculations.
 
I was wondering how could you calculate the the descent rate for the main chute while accounting for the drogue chute. I’ve find some calculators online but none show the equations. I’ve read that the drogue should have little effect but I would still like to see some calculations.
In all my years of flying dual deployment I’ve found exactly that - the drogue has almost no impact on the overall descent rate once the main opens. But I’m talking about drogues that have a descent rate of 60-90fps. It seems to me that chute descent rate calculations are really more of general estimate due to the fact the there are so many other draggy items, and especially dependent on how the fin can comes down.

But it will be interesting to see if there is some way to account for all that.

Tony
 
Cd is probably not additive, meaning that the part of the descending object has the highest Cd determines the descent rate for the whole object.

But a real scientist can correct me on that.
 
Couple screen shots from some flights last weekend. Orange rocket is a 25 lbs tube fin rocket and the Yellow rocket is a 10 pound 3" Darkstar. The chutes on the orange rocket: 84" Iris Compact Main and a 24" Recon Drogue. Yellow rocket: 72" Iris Compact main and an 18" Iris drogue.

In both cases, it is pretty clear that neither drogue is providing any lift. They are inflated in the cross-wind only. Not sure there is anything to calculate here, unless you want to calculate their effect on lateral drift.

1636571540041.png

1636571510060.png
 
Cd is probably not additive, meaning that the part of the descending object has the highest Cd determines the descent rate for the whole object.

But a real scientist can correct me on that.

Speaking as an engineer, this is pretty much right. Unless your drogue is pretty big compared to your main parachute, the effect of the drogue is negligible.
 
I was wondering how could you calculate the the descent rate for the main chute while accounting for the drogue chute. I’ve find some calculators online but none show the equations. I’ve read that the drogue should have little effect but I would still like to see some calculations.

The formula you're looking for is the one for terminal velocity

1636573634812.png
https://en.wikipedia.org/wiki/Terminal_velocity

Cd is probably not additive, meaning that the part of the descending object has the highest Cd determines the descent rate for the whole object.
You can't add Cd, but you can add A*Cd.
So changing the above formula for two chutes changes the denominator term from rho*A*Cd into rho*(A1Cd2 + A2Cd2). If both chutes have the same Cd this can be simplified to rho*(A1+A2)* Cd

Because of the square root, the descent speed doesn't scale linearly. If you increase the surface by, let's say 10%, the descent speed gets only reduced by about 5%.
sqrt(1/1.1) = 0.95
This is hardly noticeable.

A higher fidelity calculation would also take the drag of the airframe sections and shock cords into account. Under drogue, this provides some significant contribution. Note how rockets often tumble somewhat chaotically under drogue but calm down after the main gets deployed. This is because the often "chaotic" rocket body stops contributing a relevant amount of drag as soon as the main is out.

Reinhard
 
You can't add Cd, but you can add A*Cd.

Came here to say the same.

One other important thing, is where the chutes are relative to each other. The above equation assumes they are full independent. If one chute is above, below, or even very near the other, then that probably impacts the effective Cd.
 
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