2.6kg (5.7lb) rocket: two small parachute or a big one?

[Neway]

On monday I'm going to launch my Public missiles Intruder wich is strongly modified (it is heavier than a normal Intruder).
It weighs 2.6kg (5.7lb) and I have two choices:
1) Use the original 36" PML parachute as main plus a 18" PML drogue
2) Use no drogue and a 54" PML parachute that comes from an Eclipse.

What is better in your opinion?

Thanks a lot

[Rex R]

a 54" should land it at just under 20fps, you might be better off using the 18" + the 54".

[blackjack2564]

a 54" should land it at just under 20fps, you might be better off using the 18" + the 54".

....Ditto..

Just in case you were thinking it..... No the drogue and the 36in won't give you the same descent as just the 54.

[Neway]

Ok, thanks.
Another small question: is rocksim (I have the old v.7) descent speed quite accurate? Can I trust in it?

Thanks another time

[Zeus-cat]

If you are using dual deploy I would go with the 54" main. However, I think you would be OK using the 36" chute. Several times I have launched a 2 kg rocket on an 18" drogue and a 36" main. The rocket landed softly on the two chutes. It also landed safely once when the main 36" failed to deploy and the rocket landed only on the 18" drogue. The descent speed was much higher than I would have liked on just the drogue, but the rocket was fine. It landed in a plowed farm field.

[Neway]

Yes I'm using a dual deploy burrito technique. I have a lot of experience with it and I also made a little article for the italian rocketry forum:
http://www.rocketryitalia.com/viewto...dd43221b3e8d3d

Finally, after your advices I'm going to use 18" + 54".
Thank to all.

[darisd]

Ok, thanks.
Another small question: is rocksim (I have the old v.7) descent speed quite accurate? Can I trust in it?

It shouldn't matter what version you have, because the math on the back end is the same: as long as the Cd value is correct you can trust it. I think 0.7 is the default value in rocksim, whereas a spherical chute should be in the 1.1-1.5 range. I stick with 1.2 myself, aim for a low range nominal value. I have had good results so far.

[Neway]

Thanks darisd!
I have an idea! I can calculate the real Cd from my altimeter data! When descent speed is constant, this means that drag is equal to m*g, and so Cd=2*m*g/(ro*V^2*S) where m is the rocket mass, g the gravity acceleration, ro the air density, V the descent speed (from altimeter data) and S the reference area (in this case it is the parachute area).

[Neway]

Altimeter data are not very accurate and show some oscillations, but descent speed is clearly from 7 and 6 m/s, rocket mass was 2.3kg (without propellant), so Cd is between 0.79 and 1.08.

[Rokitman88]

I also have an Intruder with extra nose weght and it weighs 60 oz without motor. I use a single 48" and have had no problems. Just dont forget on windy days you might wish for just the 36" or chase it for a couple of miles.

[Neway]

Yes, but my intruder weights 79oz without motor and the motor is an I class sugar motor that weights other 25oz (10oz of propellant).