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.


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.

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.

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!

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)!