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Rocksim drogue and main

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timothyterpsalot

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Does rocksim take into account after deploying a main that there will be drag from the drogue as well? it doesn't look like it takes it into account but I wanted to hear if others had noticed/had problems with it. Thanks!
 

talkin Monkey

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Actually, I'm pretty sure it does take the drogue into consideration.

An easy way to find out would be to reverse your chute sizes. Make your drogue larger than than the main in the sim. The rocket *should* fall very slow and then a bit slower when the tiny main deploys.

If it drops like a rock, then I guess I'm wrong.
 

timothyterpsalot

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When I entered the values for my drogue and main, rocksim showed the rocket falling at 55 f/s, when I made the drogue smaller, the rocket fell slower due to less weight. That made me think it doesn't account for it...
 

talkin Monkey

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When I entered the values for my drogue and main, rocksim showed the rocket falling at 55 f/s, when I made the drogue smaller, the rocket fell slower due to less weight. That made me think it doesn't account for it...
Hmmm, You've got my curiosity going now, I'll run a couple sims and post 'em when I'm done in the yard. :)
 

talkin Monkey

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Tough to type with my foot in my mouth but I'll do do my best. As you said timothyterpsalot, something a little wonky here. (RS8)

Here's a fairly heavy example where chute weight shouldn't effect the sim too much.

I called up my current L2 build with 24" drogue at apogee and 60" main at 700'. I cranked up the wind a bit (20~30 mph) for better 2D definition.

-Sim 1- (24" drogue with a 60" main)

75 fps under drogue then 28 fps when main opens...a nice happy sim...

-Sim 2- (60" drogue with a 24" main)

29 fps under drogue then 28 fps when main opens...almost believable...

-Sim 3- (60" drogue with a 60" main)

Same results...29 and 28 fps...NOT very believable...


Note that Sim 2 and Sim 3 both landed the rocket about 6K' down wind. Not very realistic if your counting on the software as a reality check for recovery distance.

Sorry to chime in so obnoxiously, just shamefully proving myself wrong. :monkey:

View attachment The Black Spark of Insanity.rkt

View attachment The Black Spark of Insanity test.rkt

View attachment The Black Spark of Insanity test 2.rkt
 
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timothyterpsalot

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yeah, after plotting a graph with two 72 inch chutes I see that it doesn't account for both of them, only one at a time. Does anyone know a way around this? I suppose adding the drogue diameter to the main diameter and doing a weight override could be one solution. Any other ideas?
And why wouldn't Apogee have corrected this?
 

GiachiG aka Typico

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that's very interesting

i have the same problem with my project... the 120" main isn't enought for the weight i'm getting from last finishing operation, and the 60" drogue isn't considered on descending

it's possible that rsim doesn't take account of it because in many cases the rocket came down in two or more pieces(depending on the recovery layout obviously ehehehe)

i think add the diameter to the main chute isn't very acurate
maybe handling weight as data can be more reliable... i mean, if you know your 42" can handle a determinate weight, try to subtract that weight to the total
it changes altitude in simulation of course but once the main is open you probably will find the real descending speed

i will try with this method... the 19th of this mont I have my big launch so once downloaded altimeters data, i can check if it's true or a totally wrong tough

tell me what you think about that...
 

timothyterpsalot

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The only problem that I see wih subtracting the weight (if I am understanding your method correctly) is that, the weight you are subtracting would be the weight the drogue would be subtracting at full speed. With the main deployed the drogue won't be quite as effective.
 

clreynolds

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The only problem that I see wih subtracting the weight (if I am understanding your method correctly) is that, the weight you are subtracting would be the weight the drogue would be subtracting at full speed. With the main deployed the drogue won't be quite as effective.
I think he means to add a second drouge chute, the first/original one set to deploy at apogee, and a second set to deploy with the main, but having zero mass. So the result is that the mass of one drouge is accounted for, not two. But the chute would appear effective after both events. You could also decrease the size of the second "ghost" chute to simulate a decrease in effectiveness.
 

troj

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The drogue's effect is typically negligible, or you need to reduce the size of your drogue.

In terms of your main not being large enough, if that's the case, then use something like a Tether to hold the drogue to the main part of the rocket. When the altimeter fires the Tether, let it pull a bagged main up and out of the airframe, then strip the bag up and off of the main, and let the drogue bring down part of the rocket, while the main does the rest.

Trying to use two parachutes at the same time, especially a drogue and main, is begging for an entanglement. It may not happen the first time, it may not happen the fifth, but eventually it will, and you won't like the results.

Play the odds -- two parachutes dancing around together is a recipe for disaster when rigged up that way.

-Kevin
 

GiachiG aka Typico

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The only problem that I see wih subtracting the weight (if I am understanding your method correctly) is that, the weight you are subtracting would be the weight the drogue would be subtracting at full speed. With the main deployed the drogue won't be quite as effective.
you understood perfectly what I meant

troj says what we(kinda) already knew(I'm not intendet to offend you of course.. my low-grade english may make confusion ;):D;):D)
it's possible that rsim doesn't take account of it because in many cases the rocket came down in two or more pieces(depending on the recovery layout obviously ehehehe)
we(almost me) are talking about find the right method to predict descend speed with rsim with dual deployment... and in particular, if the drogue isn't exactly a drogue but is a fair big chute

I use a 120" main and a 60" drogue... of course the drogue isn't very performing because it's over the main, but, it will lower the descending speed of course, the question is.. how much??? and the related second question is, how can I simulate that with rsim to obtain an affordable result??
 

hardinlw

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If you want to add the effect of the drogue to the main by changing the diameter of the main, you must end up with the equivalent area. Adding diameters will over-correct. The formula to use would be

EquivDia = SquareRoot(MainDia^2 + DrogueDia^2)

where the ^2 means to square those numbers.

Typically, the drogue is so small compared to the main that the correction is not much. With drogueless recovery, there is no drogue chute at all and the rocket simply separates into to pieces, neither of which is stable so it falls with the two sections basically horizontal instead of nose down. I don't think Rocksim handles this at all unless it is by putting in an equivalent zero-mass drogue. Once the main pops, however, the pieces end up in a vertical orientation and their drag is so small as to be negligible.

I guess the bottom line is that ignoring the drogue once the main is out works more often than not and since Tim had to set Rocksim up to work without additional work for the most common situations, that is how it should be done. For situations where that assumption is invalid, we do work-arounds like have been suggested in this thread.
 
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