Monday, January 31, 2011

Review: D-Fusion Bomb

Denny's serves(-ed) kid's drinks in this cool retro-styled rocket cup. I whined until our waitress gave me one.

I originally wanted to make this fly on 29mm motors without added fins. The dimensions of the base made me change to 24mm. I then designed a RockSIM model without added fins and it looked like it would be stable. It wasn't. So, I back-pedaled again and added a set of fins. This review will only describe the successful, finned version.



Construction:

Parts used were:

* One Denny's D-Fusion drink cup
* One Firstfire igniter tube
* 2.125" mailing tube x 4.5"
* One 24mm to 2" fiber centering ring
* 24mm motor tube - one 3" piece and one 3.25" piece
* Spent 24mm case to connect the motor tubes (you could use one piece of 24mm tube and skip this item).
* 3/32" plywood
* Steel fishing leader
* Elastic strap
* 4" of tubular Kevlar®
* Lead shot
* 2 small sheet metal screws

I prepared the drink cup by grinding the base to fit a 24mm motor tube. I also ground the threads off the cap and bottle neck so that the cap would slip on and off easily.

I found that 2.125" OD mailing tube fit nicely in the top of the bottle. Furthermore, this tube also fit in the inner neck of the cap. I cut a 4.25" piece of tubing to serve as a parachute tube and to keep the clear body from getting ugly from ejection gasses. It extends above the neck on the body and mates with the cap/cone.

I installed the short piece of 24mm tubing in the base using a single fiber ring. A steel fishing leader was screwed to the wall of the tube just above the ring. I wrapped the tube with a layer of blue laser contact paper and installed it in the body.

The nose weight was determined from the RockSim model. The weight is a slurry of lead shot and Gorilla polyurethane glue. A Kevlar® loop is embedded in the slurry and a screw through the tip of the cone ensures the weight stays in place. The weight proved inadequate for the fin-less version and was retained as is when the fin unit was added.

The fin unit is another piece of 24mm tubing with a 2" piece of a spent casing glued in. The fins are 3/32" ply with a 4" root, tip and span. They are swept slightly backwards and appear over sized. They were designed iteratively in RockSim.

I ground a trough in the wall of the body to accommodate a piece of a Firstfire tube. I wanted to fly this away from the crowd so a 1/4" lug seemed appropriate. This lug was attached with CA. It is aligned with one fin and a corresponding hole was bored in that fin.

The cone is attached to the steel leader with a foot long piece of elastic.

Finishing:
One good think about converting plastic things to rockets is that they often don't need much finishing. I painted the fin can with Testor's yellow spray paint and added squares of the laser contact paper to match the body. The lug was painted dark blue.

Flight:
Although the flight of the fin less version appears in the flight logs below, I'll only describe the successful flight of the finned version.

I decided to use an E15-4 SU motor since there is no positive motor retention. I stuffed the body with a ton of dog barf and attached an 18" nylon 'chute. The nose was a bit loose so I wrapped the neck of the body with masking tape until it was snug. The motor was given a tape thrust ring and was friction-fit. It weather cocked a bit but had a nice flight anyway. It landed near the pads, which usually is a good thing!

Summary:
Even with the fin unit, the rocket looks pretty cool and flies nicely. This is one of the 2-3 rockets I've built where RockSim was wrong. It may have been stable with a higher impulse motor and less wind but I elected not to test this theory. If you got one to fly without added fins, PLEASE comment (and write a review)!