Safe descent rate for fiberglass rocket?

Hi all,

I just finished building my first FG rocket, a Formula 75. It was a lot of firsts for me and a good intro to FG, and I plan to fly it on an I175-11 this Saturday. However, I'm a bit concerned about picking a safe descent rate with the parachutes I have.

The rocket is 61 oz without a motor, and I have a 36" and a 50" chute. Rocksim shows a descent rate of 26 ft/s on the 36" chute and 19 ft/s on the 50" chute. I fly my Mini Magg (64 oz) with a 50" chute and I would lean towards the 50" for the Formula 75 as well, but if the fiberglass airframe can handle the faster landing speed I may try the 36" chute to reduce drift. I'll be flying off a dirt field, so not a hard concrete or lakebed landing. Any suggestions?

Thanks,
SC

Wayco said:

I have a RW X-Celerator that weighs about 12 lbs. and flies with a Recon 50. Many successful flights that my RRC3 tells me descend at 30 fps. It has a tailcone and the fins sweep forward, so nothing hits before the tailcone. If your 4 lb. rocket is landing on a metal motor retainer, I wouldn't worry about hurting it. RW kits are pretty stout, and even with the added weight of a burned out motor, yours will be much lighter than mine.

Click to expand...

The fins do sweep back, but the retainer should hit first. Thanks for the info about the descent rates, I'll probably try the 36" chute then.

I've flown a Formula 75 many times on a 36" with no problems.

jmight said:

I've flown a Formula 75 many times on a 36" with no problems.

Click to expand...

Thanks!

A descent rate of 30 fps at impact is about the equivalent of dropping it onto the ground from fifteen feet. If you have a 12-ft step ladder, climb up to the second to highest step, hold the rocket straight out at arms length, and think about how you would feel about letting it go. You built the rocket, so only you know how well it is constructed. How good are your fillets? How much and what kind of epoxy did you use?

Holding it in your hand, fifteen feet above the ground, if you feel like you can drop it with no worries, then go with the 36". Otherwise, better go with the fifty.

Bat-mite said:

A descent rate of 30 fps at impact is about the equivalent of dropping it onto the ground from fifteen feet. If you have a 12-ft step ladder, climb up to the second to highest step, hold the rocket straight out at arms length, and think about how you would feel about letting it go. You built the rocket, so only you know how well it is constructed. How good are your fillets? How much and what kind of epoxy did you use?

Holding it in your hand, fifteen feet above the ground, if you feel like you can drop it with no worries, then go with the 36". Otherwise, better go with the fifty.

Click to expand...

It's built with lots of Rocketpoxy and heavy fillets, and since I'm landing on dirt/grass, I think I'm going to try the 36".

SCrocketfan said:

It's built with lots of Rocketpoxy and heavy fillets, and since I'm landing on dirt/grass, I think I'm going to try the 36".

Click to expand...

You should be fine with that chute!

timbucktoo said:

You should be fine with that chute!

Click to expand...

Guess I'll give it a try and see how it goes. 26 ft/s seems fine with a FG rocket landing on dirt, but it is much faster than the descent rates on my cardboard rockets. Then again, the FG should handle that velocity fine like you said.

I had the formula 75 land on a 18" chute and a tangled main (bad deployment and altimeter not working properly) and suffered no damage. It could probably survive a lawndart.

For what it's worth, my experience is that fiberglass is pretty resilient. I've dropped a Rocketry Warehouse Tomahawk sideways from about 2000 ft with a parachute that failed to deploy on hard cracked dirt and barely made a dent in the fin.

Check with the club where you launch to see if they have a max decent rate.
I doubt 26 f/s would be an issue but best to know in advance.

M