Flyin Jenni

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Flyin Jenni

Brief

Flyin Jenni is named after my daughter, Jennifer, and the Russel Keaton comic strip. She is a scratch built, 4FNC rocket I will use for my NAR/TRA Level 1 high power rocketry certification flight.

The design goals are simple and dictated by my local NAR section club launch waiver (non-waivered) meaning she has to fly under total rocket and motor propellant weight restrictions.

She is a baffled, rear separation, anti-zipper design with airframe sections of fiberglassed BT-80 body tubes. Currently, there are three sections:

Motor/Fin Can - There are two motor sections under construction, both based upon a single length (14") of BT-80. One has a 4x24mm motor mount, the other a single 29mm mount for the H128 I plan to fly for cert. Both have ejection baffles and rear recovery system mounts built into bulkheads in the tube and coupler.

Body - A 28" section of joined BT-80 tubes. The parachute(s) and shock cords are housed in this part of the airframe.

Payload/NC - A single length of BT-80 (14") which provides for nose weight and forward recovery system mounting.


Flyin Jenni
Airframe 2.6" diameter (BT-80), one wrap of 5oz fiberglass.
OAL 60"
Weight Anticipated : 8N (See note below)
Recovery Baffled, rear separation, anti-zipper design with motor deployed 30"-36" flat octagonal nylon chute.
Motors 4x24mm D/E cluster or single 29mm F/G/H

Note: The unit of weight (force) in the SI system is the Newton (N), NOT the kilogram. A kilogram of mass weighs 9.8N, 10N for estimates.

For Estes BP cluster flights, a 4xD12 (F48 equiv) launch will generate about 50N of average thrust. For a 5:1 thrust-to-weight ratio, this gives a 10N MLOW. Estes recommends a MLOW of about, sigh, 250gm (2.5N) per long delay D or E motor, also implying a 10N MLOW. Up to 2N of this weight could be the motors (4xD12). That leaves 8N for the rocket itself. So far (80% complete, no recovery system installed), the weight is 5.2N (or 520gm). I seem to be in the ballpark, but the weight IS increasing rapidly.

Another anticipated flight is a 2xD12 and 2xE9 motor combo. This will give higher total impulse than a 4xD12 launch, 100N-S vs 80N-S, but lower average thrust, 42N vs 48N. This is not a real concern as the max thrust of Estes BP motors are significantly higher than the average thrust values and will give the rocket a good initial boost.

Composite motors will have to be selected on average thrust ratings alone. An H128 should make Flyin Jenni really, um, fly!

The main concern is a cluster ignition failure:

3xD12 => 7.5N MLOW. Not bad.
2xD12 => 5N MLOW. Worse.
1xD12 => 2.5 N MLOW. Bad.

Motor Mount/Fin Can - 4x24

The 4x24 mount is to make good on a pledge I made to another club member to drag race his 4x24 LOC Viper-IV rockets.

Fin can without coupler. The offset baffle bulkheads are visible. The intrastage coupler will be mounted above the top bulkhead.

(Click on image for full-size picture. Use "BACK" button to return.)

The mount uses CTTR (Completely Through The Rocket) fins, 4 BT-50 motor mount tubes, a pressure bulkhead, two ejection gas baffle bulkheads, and a load bearing bulkhead in the tube coupler for the recovery system mount. The body tube was glassed with a single layer of 5oz cloth and four 3/16" x 3 1/2" fin slots were cut.

I fashioned the fins from two 10.6" x 3" pieces of 3/16" aircraft plywood. The pieces were slotted halfway through at their midpoints and joined to form an "X" shaped assembly; each arm of the "X" forms one 4" x 3" rectangular fin. The BT-50 motor mount tubes are epoxied around the center of the "X" and the MMT/fin assembly slides into four fin slots cut in the airframe. The thickness of the fin material and the diameter of the motor mount tubes equal the inside diameter of the BT-80 and makes for a self-centering fit. Motor retention is accomplished by a bolt and washer which fit a threaded T-nut epoxied to the aft section of the ply fins between the motor tubes.

(Click on image for full-size picture. Use "BACK" button to return.)

The motor mount tubes are 9" long and have thrust rings at the 6" point. The inside of the forward section of each motor tube is painted with thin epoxy to protect them from hot ejection gasses. Cut motor casings provide spacers for D or E (or longer) Estes motors. Ejection gas travels through holes in the pressure bulkhead and into a reinforced airframe section containing baffle bulkheads and the intrastage coupler. Because up to four ejection charges may fire per flight, the airframe above the motor tubes is lined with a section of split BT-80 to improve heat resistance. The baffle bulkheads are epoxy covered balsa disks with sections cut off along a 2" chord. They are mounted to prevent burning particles from traveling in a straight line to the forward coupler bulkhead.

A CA soaked and stiffened BT-80 coupler was epoxied in and couples the fin can to the center airframe body section. A 1/8" reinforced ply bulkhead is recessed 1/2" below the forward coupler lip beneath a retaining ring made of split BT-80 and the entire bulkhead is glassed into the coupler. The ply bulkhead contains a U-bolt recovery system mount point and is drilled to allow ejection gases to pass forward and separate the fin can from the center airframe section for rear separation recovery.

Center Airframe Section

The current center section is a 28" length of joined BT-80 sections glassed with a single layer of 5oz cloth. Currently, a recessed ply bulkhead 12" from the aft end provides the forward recovery system mount and allows ejection pressurization of the airframe for rear separation recovery. The center section receives the couplers from the fin can and payload sections, or the shoulder of the nosecone for non-payload flights.

The rocket is being modified for modular components and a new center section is being constructed. The new section will accomodate a pressure bulkhead with removeable port covers to allow either rear separation with the port cover in place, or with it removed, ejection gasses will pass forward for standard recovery deployment. Dual hard points on this bulkhead can be used as either a forward/rear recovery mount, or as a piston retaining cable mount.

Payload Section

The payload section is a 14" length of glassed BT-80. The aft end contains the intrastage coupler with a pressure bulkhead and recovery mount. The forward end accepts the nosecone. The 14" length was selected to allow space for electronic recovery testing. The plan is to arm a circuit when the payload section ejects from the airframe and recover it separate from the booster.

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Last modified: 4/17/2004