Launch Report - June 28, 2008

Launch Report - June 28, 2008 - McGregor, Texas

Saturday morning found me trekking south down Interstate 35 with fellow DARS member Sam Barone, bound for McGregor, Texas and the club's launch there.

My goal for the day was to get my Hawks Hobby Cherokee 3D into the air along with my upscale Ranger.

Right: Here's a great shot Bill Gee got of the bird climbing up the rod. The Aerotech G67 Redline motor has an impressive flame in the photo, and is even more brilliant in person.

Sam was particiapting in the model rocket contest while I was preparing the Cherk for its maiden flight. Even though I had spent much time Friday working out final details, I still found myself taking over two hours to get this thing ready to fly.

Seems like everything had to be done twice. The motor instructions had the wrong callout for the aft o-ring resulting in some delay getting that resolved. Then I found the altimeter channels were wired backwards (probably would have been better off if I'd left them that way as it turns out). After scrounging up some ejection charge powder, and getting the rocket assembled, it was nearly 3 o'clock.

The kit's intended propulsion is a trio of 24mm D motors, but when I got the kit and realized it had a thick walled tube, I realized it could handle much higher power. I decided to cut the tube in two and add an electronics bay to provide dual deployment recovery.

The idea is that at apogee, the altimeter fires a charge that separates the rocket into two tethered sections, then at 500', the altimeter fires a second charge that puts the chute out. After the first charge, the rocket falls fairly fast, but not ballistically fast, as it would if still in one piece. The parachute is only out for the last 500' of descent, so it can't drift far, and the rocketeer has a shorter walk.

Photo courtesy Bill Gee

Even the launch took extra time. The Firstfire ignitor that came with the motor spit itself out at ignition so I scrounged a Twiggy. That did the trick. Bill Gee got this awesome liftoff sequence.
Photos courtesy Bill Gee

The rocket had a nice climb to about 800 feet, and arced into the wind. The airframe soon separated and the rocket was falling as expected. Another puff of smoke appeared and was judged to be the motor ejection charge going off (intended as a backup to the altimeter charge).

Then things got interesting. When the next ejection should have occurred to deploy the chute, nothing happened. The rocket continued down with the fin can slowly spinning in a horizontal attitude and the forward section tumbling unstably.

It hit the ground just beyond the range head landing in some low weeds.

The nosecone was beat up some with a deep gouge near the tip, and it was still in the airframe, but cocked off axis some.

The tail section lucked out. It hit in a soft spot and had no significant damage. Some scuff marks on one fin and some damage to the forward launch lug. Lastly, I noticed a compression crack in the forward airframe, so I knew it would need replacing.

Walking back to my awning, my first thoughts were that the shear pins were too much for the forward ejection charge which was supposed to deploy the chute. There was damage to the airframe that was consistent with shear pins tearing the hole instead of being sheared. I was beating myself up thinking I should have done more testing before flying it. I had done at least two tests. One showed that the soft balsa wasn't stout enough to shear the pins. The nosecone base was fortifed with some epoxy and another test was run which proved successful.

Nevertheless, I was mad at myself for not having been successful, for not getting the laundry out, for wrecking my pristine Cherk on its first flight.

As I was tearing the rocket apart, I realized the nosecone damage did not come from the ground but from hitting the aft airframe section in the air. And the the nosecone was knocked cockeyed by that blow and not by the ground. The two airframe halves collided after separation with the result that the nosecone was jammed hard into the forward section and that the charge could not overcome this.

Lessons learned: The nosecone shoulder length is only about a half caliber. It's really not long enough to resist cocking / jamming in the airframe. I intend to make a replacement from cedar. I had about 55g of noseweight in the balsa one, so a cedar one won't really add any weight. But it will have a longer shoulder and also be a little more dent resistant.

The tether between the rocket sections was about 8' long, and had a big tape loop to dissipate the separation forces. Not sure if recoil was indeed a factor, but I will lengthen the tether and use a longer tape loop in hopes of avoiding this in the future.

This morning, I placed an order with Semroc for a replacement tube and picked up a few other tubes as well. Looking forward to trying this again.

Doug - June 29, 2008