Monday, May 25, 2009

Opinions needed

I'm planning to go for my High Power certification at NERRF on August 14th, with a backup date of the 16th in case it rains. I've got the rocket, a Madcow Mozzie, and the reload hardware, a Doctor Rockets 29/180 system. However, I need both advice and help.

First, the rocket itself. It's a very high quality kit, and it's built more like a normal high-power rocket than a model rocket or even a mid-power rocket. It's got thick-walled body and motor tubes, thick kevlar shock cord, and a big, heavy screweye set solidly into the nose cone. It's survived two flights, including a complete parachute failure with no damage. However, after that crash, I'm worried about possible internal damage to the fin joints, and they're just 1/8" thick (good-quality 5-ply, though) and extend over 7 inches from the trailing part of the glue joint to the tip, although only about 5" from the body. I think they should be able to survive 450 mph without damage, but I'd like to hear your opinions. (At least 2 of my readers are / were certified, so I know you're out there)

Second, the stability. The Mozzie is a short, fat rocket - stability hell. It's got the huge swept fins, which really help, but I'm still a bit worried. The 29/180 motor is very long, almost 8" including the forward closure, and almost half of it is actually forward of the CP. The chute and protector and screweye add some weight, so I think I'll be fine. I'll run a CP sim just to be sure. Also, it's been exceptionably stable, even overstable, on D12s at under 50 mph, and it'll be going almost twice that by the time it even clears the rod on an H128W-M. A bit of speed makes up for sub-caliber stability. (I apologize for that awful pun).

Update: CATO, the rocketry club I belong to, has a new field that may be able to handle small HPR motors just half an hour from where I live. This would save me a trip to New York and such.

6 comments:

Dick said...

The fins are kind long and could flutter, but....

IF the fins seem to still be firmly attached under a slight attempt to fled them.

AND

You double check the stability margin vs. the recommended motors and add any extra weight.

AND is were mine.

THEN I'd give it a go.

The 29/180 H's are baby H's after all.

It should really get up there!

Dick said...

er...that was flex them, not fled them...

The EGE said...

Well... there aren't any recommended H motors for the Mozzie. The largest that Madcow recommends is the G38 (1500') and the largest that Apogee recommends are the G79 (2500') and the G104 (2000') The 29/180 just barely fits, and I left the motor mount an inch longer than recommended.

The fins seem fine, though, so at this point I think I'll go for it. It's not ludicrously expensive, even for me, and it's well worth the risk.

The EGE said...

Interestingly, the altitudes that Madcow gives are nearly identical to my own from my RASP program, while the Apogee / Rocksim values are about 30% higher. Odd...

Sascha Grant said...

I've heard that RockSim are very generous with the altitude prediction. I'd run a few sims with different programs (there's even some online that are good). You should get an idea of min/max and you can average it from there :)

Good luck with your cert! The Mozzie will scream!!

Dick said...

You commented about the mmt length. Unless you added a Questes-style motor block (as I did on a few early mid-power builds), then the length is not an issue even with the longer hardware.

Unless you spend a huge amount of time tweaking RockSim, then (as Sascha mentioned), it is generally optimistic. I usually go a tad on the low end when I drill my delays, which also helps account for weathercocking. I don't try to fiddle with wind in my sims even though Rsim will let you.