Scratch - Fat Boy (7 x 18mm Motor Eater) {Modification}

Scratch Fat Boy (7 x 18mm Motor Eater) Modification

Scratch - Fat Boy (7 x 18mm Motor Eater) {Modification}

Contributed by Dick Stafford

Manufacturer:	Scratch
Style:	Cluster

(MODS) Fat Boy x 7

Brief:
This Fat Boy flies on seven 18mm motors.

Modifications:
Here is what I changed:

The base of the cone was cut off
The stock centering rings were not used
Additional 18mm motor tubes were added

Construction:
Parts list:

Estes Fat Boy kit
18mm tubing for the 6 additional motor tubes
~3’ of ¼" elastic
~5 “ of ¼" tubular Kevlar
First Fire igniter tube
Lead shot
2-part foam (Giant Leap or equivalent)
Epoxy and wood glue
Elmer's Fill ’n’ Finish
Krylon X-Metals purple and red Monokote

(MODS) Fat Boy x 7

I'll start with the business end. The center mount is held in place by the fin set. I dry-fit the stock centering rings and then removed them. Next, I added 6 more motor tubes at the fin-body tube joints. These were placed there vs around the center tube because I wanted more separation between them. I'll be igniting the cluster with quick match and the extra room can be useful. I also extended these 1/2" below the body tube so I can wrap tape around the motor tube/motor boundary. This hurts stability a bit (i.e., will make the rocket heavier), but the coast time will still be too long for some motor configurations. The top section of the motor mount area was plugged with wood glue-soaked tissue and I then backfilled it with 2-part foam.

I used long pieces of 1/4" elastic (from the "use what I got" rocket building philosophy) for separate nose cone and body shock tethers. The body tether is routed through the body tube at the top of the 1/4" lug. It is epoxied to the body tube and the lug is epoxied on top of that. I cut the shoulder off the cone and embedded a Kevlar loop and lead shot (~6 oz) in 2-part foam. About 1/3 of the cone is available for a second parachute.

The rocket was assembled with wood glue except for the lug. There is no fin reinforcement since the body should come down slowly under its own chute.

(MODS) Fat Boy x 7

Finishing:
I covered the fins with Monokote. Unfortunately, my skills have deteriorated from their already low level so they aren't very smooth. The body and cone were painted with Krylon X-Metals primer. The body was left with this already pretty layer and the cone was painted with my remaining X-Metals purple. The first time I used this paint, the purple clouded badly, however, I'm happy to report that despite the high summer humidity, it came out great.

(MODS) Fat Boy x 7

Flight:
I prepped the rocket with two Fat Boy chutes. The nose cone's chute was packed in the body and the body chute was packed in the cone, so they'd tend to pull each other out.

The unloaded rocket weighs 12oz and will have 1 caliber stability when loaded with C6's. RockSim says:

Seven C6-7's will push it to 1488' and the optimal delay is 9.6 seconds. The motor delay is a bit short!
One C6-7 and 6 B6's will go to 691' and the optimal delay is 6.8 sec. A much better match!
One C6-5 and 6 A8's will go to 316' and the optimal delay is 4.9 sec. Also a good match.
Three C6-7's and 4 B6's will got 953 feet with an optimal delay of 7.9 sec. Also OK.

The Motor Eater flew nicely on a C6-5 and six B6-4s all lit with quick match. The boost was a bit slow, but man, did it generate some smoke! Next time it will get a full load of C6-7s.

Summary:
This Fat Boy is a good way to get rid of those extra 18mm motors. I might have been able to get away with a bit less nose weight, but this model is mostly about the smoke and fire rather than performance. If I want performance, I’ll fly my 29mm version.