Type

Fast Boy (FB) is a six motor cluster kit bash of an Estes Fat Boy.

Design

The goal of this project is, simply, a high performance, long burn (E motor) stubby rocket. Why? Cause I've never built a stubby rocket.

I decided on a six motor configuration in a stock dimension Fat Boy for this project; 3x24mm + 3x18mm. The many possible cluster motor combinations require a way to adjust the model's Center of Gravity (CG) and the possibilty of flying it on APCP motors requires a stronger than stock build.

Construction

Motor mounts

An alternating series of 110mm long BT-50 and BT-20 motor mount tubes nestle around the inside of the BT-80 airframe. Each motor tube has a thrust ring for the corresponding Estes motor length. This leaves a short section of motor mount tube extending ahead of the thrust rings. Unused motor tubes (for a 3x24 or 3x18 launch) are plugged by short sections of epoxy/tissue filled expended motor cases inserted forward of the thrust rings. The motor mount pressure seal is accomplished with epoxy and balsa scrap/heavy cardboard gap fillers.

Motor retention is accomplished by a washer which overlaps all six motor casings. A bolt is embedded in the central space between the motor mount tubes and the washer and a nut is threaded on to hold the motors in against ejection pressure.

The motor tube assembly is positioned so that the fins extend through the slotted airframe and lie in the valley along the motor tube contacts. The root edge of the fins are filleted to the motor tubes and to the airframe body tube making a very strong fin mount.

A  - Fender washer nosecone weights on bolt embedded in epoxy. Bolt is removeable to change weight for different cluster combinations.
B  - Unused motor mount tubes are plugged with discs (cut motor sections filled with tissue-epoxy) forward of the thrust rings.
C  - Its hard to see, but the airframe interior forward of the motor mounts is lined with a split section of body tube for multiple ejection charge protection.

Nosecone

To provide for adjustable nose weight, and give some additional room in this short body tube, the bottom of the nosecone shoulder was cut off and a nut and washer were epoxied into the tip of the nosecone. A removeable eyebolt screws into the nut and provides the forward recovery mount. (A liberal spray of PAM on the the bolt threads acts as a release agent while epoxying the assembly in.) Nose weight (fender washers) is changed simply by removing the eyebolt, adding or removing washers, and replacing the bolt.

Airframe

The airframe was glassed with a single layer of 5oz cloth and Z-Poxy and the weave was filled with Fill'n'Finish. The original balsa fins were replaced with basswood and were laminated with bond paper to help prevent breaking a fin on landing.

Recovery

A kevlar cord loop is epoxy filleted to motor tube junctions. About eight feet of 3/8" elastic attaches to a 24" nylon chute and NC. Dog barf (cellulose insulation material) protects the recovery system from ejection heat.

Notes

Unfamiliar with short, fat rockets, I searched the Web for reviews and articles on the Fat Boy before beginning this project. In general, the stock Fat Boy is a popular and reliable model rocket. However, I ran into more than the usual number of flight logs which reported "arcing" and even horizontal flights. My first inclination was to assume weather cocking as the cause of these erratic flights.

A Rule-of-Thumb for most rocket designs is that stable flight is achieved with one caliber of stability margin. Over stable rockets tend to weather cock and I assumed the Fat Boy might be slightly overstable. Suprisingly, I found that my Fat Boy had a stability margin of 0.7 when put in a stock condition. I didn't think a margin like that would have led to the number of reported arcing flights I had encountered. Perhaps there is something about short, fat, rocket flight dynamics that I am unfamiliar with.

Excessive tip off could also cause the arcing flights. Related to weather cocking, excessive tip off can be caused by a combination of low velocity off the launch rod and crosswinds. Running some sims, it was apparent that that the recommended motors in a Fat Boy met the usual minimum safe launch velocities.

At this point, with the usual suspects eliminated, flight dynamics get complicated. A CP shift due to AoA may be a contributor as a short, fat rocket may shift the CP aft. However, the solution to weather cocking, excessive tip off, and CP shifts is the same; higher velocity off the rod. AoA results from a vector sum of the winds operating on the airframe. The larger the relative wind velocity, the lower the AoA for a given crosswind.

Rocksim simulations do show significant horizontal motion when a Fat Boy is launched in crosswinds using a standard 36 inch launch rod which results in a typical launch velocity of about 13 m/s. Horizontal displacement can be close to 75% of the altitude achieved. Increasing the launch velocity to 17 m/s in the same crosswind (8 mph) reduces the horizontal displacement at apogee to about 40% of altitude. Increasing the velocity to 20 m/s reduces horizontal motion to about 15% of the altitude achieved. These simulation results are to be taken with a grain of salt as several variables were changed as I investigated this issue; launch rod length, motor thrust, total impulse, etc. But it shouldn't surprise anyone that a higher velocity off the rod would result in a more vertical flight path.

The point of all this? I "designed" this cluster mod in Rocksim and set the nose weight to give over 1 caliber static stabilty margin. It required over 150 gms of added noseweight (beyond the weight of the eyebolt mount) to get over 1 caliber. Thinking this to be excessive, I posted some questions to r.m.r and the resulting discussions informed me that short, fat rockets are "more stable" and 1 caliber stability margins may be excessive. Using the stock margin of 0.7 caliber, the nose weight required now is only 50gms; a more intuitively reasonable amount. And with over 20 m/s off the rod with a 3x18 cluster launch, I am confident the Fast Boy will exhibit a fairly vertical flight profile.

Flight Update

The first flight of Fast Boy, also called Stumpy, was on a 3x24 cluster of D12-5 motors. The flight required 150 grams of nose weight to keep the CG in the same position as a stock Fat Boy with a C6-3 motor. The total flight mass of the rocket was right at 450 grams, a fairly heavy modroc. The flight was nominal in light crosswinds with only a very slight tail waggle during ascent. The was no indication of the horizontal flight profiles I had encountered on the Web.

Fast Boy will have even more mass during a full, 6 motor E9/C6 launch and much of that will be nose weight. This short of a cluster rocket is at the point of diminishing returns as regards mass and motors, at least with the stock airframe dimensions (fin size and planform).

Ejection issues

I routinely coat the inside of paper body tubes with CA glue to increase abrasion resistance. Fast Boy had about 1 1/2" of the forward internal airframe coated with CA. The inside layer of the CA soaked BT-80 paper tubing was found to be delaminated and appeared to be scorched after the flight. The delaminated paper was brittle and dark brown in color. This looks like an ejection heat issue with the CA soaked paper. CA essentially polymerizes into acrylic plastic as it hardens. Some interaction with the adhesive in the body tube resulted in the CA soaked (acrylic) layer lifting off the underlying layers after it was heated by the three ejection charges.

Perhaps the acrylic layer melted and loosened the bond with the underlying layers. The BT-80 which was not coated with CA had no delaminations and appeared fine after the flight. In any case, if this tube had not been glassed, it would have, effectively, been destroyed by the flight. I would caution others to consider heat effects and the possible delamination of CA soaked body tubes in the presence of multiple BP ejection charges.