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Prepping the CR4 at MTMA #2

This page details the construction of CR4, a 4-inch diameter rocket outfitted with a 35 mm camera, and includes photos taken during construction as well as photos of the completed rocket. Clicking a thumbnail will download a larger image. While the Estes AstroCam provides an efficient and inexpensive means of rocket-based aerial photography, I wanted to obtain higher-quality photographs and more than one frame per flight. The AstroCam uses 110 film, the smallest format currently available. Enlargements made from 110 negatives are very "grainy". The CR4 was to be designed to use 35 mm film, in an auto-wind camera, yielding a better negative and multiple shots during ascent and descent.

The 35 mm camera has two notable disadvantages, size and weight. A larger diameter body tube would be necessary, as would larger motors. A quick check of camera, body tube, nosecone, and accessory rocket component weights led my to believe that F engines would be needed. While more expensive than Estes C or even D engines, at least high power certification and a low-explosives user's permit (LEUP) would not be required.

After some head scratching, rough measurements and calculations, the following design outline was formulated:

Objective:
capture numerous high-quality pictures during a single flight (up & down)
Constraints:
use existing consumer photographic equipment
low to moderate initial cost
low per-flight cost
Initial Thoughts:
digital too expensive but allow change to it in future
35 mm format best film-based (ignoring large format)
APS film too small but wide pictures would be cool
"point & shoot" units smallest and least expensive
camera must have built-in winder
camera must have "hot-wireable" shutter release
camera should have low f-stop and large lens
Design Criteria:
wRASP: http://www.wrasp.com/wrasp.html
VCP: http://v-serv.com/vcp/
4" diameter (3.97") to fit most less expensive "point & shoot" 35 mm format cameras
nose cone attached to one body tube section to form payload bay
two-piece recovery
side-facing camera in payload bay
launch-activated circuitry to start continuous picture taking
harness recovery for payload bay, orienting camera in a "lens down" position
possible on-board timer for flash-bulb ejection as a backup to engine-based ejection charge
2+ caliber stability
fineness ratio (slenderness) 10 to 15 (12.2 optimum?); therefore, 39.7", 48.434", 59.55" would be possible lengths
minimum fin area to limit weathercocking
rocket as light as possible to allow for weight of camera
internal baffle to eliminate wadding (this conflicts with flash-bulb w/o removable baffle)
convenient to use increments of 17" (tubing on hand at the time)
shock absorbent materials around camera (especially between camera and NC)

Additional information was obtained from Steve Roberson's article in the March/April 1992 issue of American Spacemodeling, and from Robert Nee's Rocket Photography Web Page describing the construction of AYUCR, a High Power 35 mm Camera Rocket.

Description of CR4 as constructed:
Two 17" sections of old NCR BT-39 (3.97" OD).
LOC 3.9" nose cone (12-5/8" tall) with bottom removed to within 1" of shoulder.
Four basswood TTWTTMMT fins: 4" root, 2" leading edge sweep, 2" leading edge, 4" exposed semi-span.
Two plywood centering rings.
29 mm by 6" MMT projecting approximately 3/4 inch from rear centering ring (CR even with bottom of BT).
Kaplow Klips using #6x32 tee nuts, brass finish steel "molding hooks", and #6x32x5/8" Allen head cap screws.
Epoxy used on all joints at fins, centering rings, MMT, and fin to BT fillets.
Two coats of primer and two coats of Painters Touch Sun Yellow Gloss paint (see photo at top).
One coat of Painters Touch Gloss Black paint was later applied over the yellow on the nose cone, one side of each fin, and some accent stripes on the payload bay (see photo at bottom).
An autowinding, sidelooking, 35 mm camera is mounted in the payload bay (upper 17" section of BT-39 plus nose cone).
The original camera was a stripped (exterior case removed) Minolta Freedom AF 35 with the shutter button contacts "shorted", and a slide switch soldered to the lens cover door sensor contacts. When turned on it took pictures at a rate of about 1 frame every 2 seconds.
The original camera was ruined when the payload bay landed in a wet ditch at the end of the first flight.
The current camera is a Polaroid 3000AF. Although manufactured by Polaroid, it uses 35 mm film.
This camera has external timer & relay circuitry connected to the shutter button contacts, and is currently set up to take pictures at a rate of about 1 frame per second.
The external circuitry includes a photo-transistor, triggered by a hand-held spot light just prior to launch.
The camera, external circuitry, 9 volt battery, and foam padding are housed in a tube coupler that is held in place in the payload bay by screws through the body tube and coupler from the outside.
A nylon rope harness on the payload bay is arranged to allow this section to descend horizontally, with the camera facing down, under a 28" rip-stop nylon chute.
A nylon rope shock cord and 24" rip-stop nylon chute are attached to a length of braided Kevlar epoxied to the inside of the booster section body tube wall.

The rip-stop nylon parachutes were constructed based on details provided in an article by William J. Orvis in the LUNAR Handbook.

Here are some pictures taken during various stages of construction:
 

Nose cone, 17" section of body tube, tube coupler, slotted lower 17" section of body tube, and fin can.

The Minolta Freedom AF 35 camera with exterior housing removed, shutter button soldered closed, wires soldered to lens cover door contacts, and flash circuitry covered with electrical tape. The last item is the most critical as a high voltage shock from the flash capacitor is quite painful.

 

Fin can installed in booster, and braided Kevlar shock cord anchor epoxied to body tube wall.

Close up of inside of payload coupler showing split BT-5 used as tunnel for nylon rope recovery harness. Made from shortened tube coupler with two plys of 1/8" balsa (glued with grain oriented perpendicular).

Here are some pictures taken upon completion of construction, but prior to painting:
 

Aft end with single-use motor installed. Masking tape wraps used as thrust ring, Kaplow Klips used for motor retention.

The Booster and Payload separate for recovery. The Payload is fitted with a nylon rope harness to allow descent in a horizontal orientation with the camera aiming downward.

 

The upper portion of the harness is passed down the exterior of the Payload bay and enters the Booster body through a "tunnel" groove in the Payload coupler. The lower portion of the harness is attached to the inside of the Payload body tube just above the "tunnel" groove and also passes through it.

Close up of the "tunnel" groove in the Payload coupler. Notch in body tube allows for tight, square fit where the upper portion of the harness passes through.

 

The nylon rope harness is fashioned to allow the Payload to descend horizontally with the camera facing downward.

Previously, resilient foam was used to cushion the camera. The foam fit tight in the Payload body tube and was held from sliding by friction only. Stranded fishing line was wrapped around the foam to hold its shape and allow for easy "pull handle" extraction. This has all been replaced with a mounting inside a tube coupler

 

Here is a video still of a launch of CR4 showing the final paint scheme. Taken at MTMA #8 on June 11, 2000. Motor was an Aerotech H97J-S reload in a Dr. Rockets 29/240 RMS case. This was my Level 1 certification flight.

Some of the photographs taken with the CR4 have been scanned. Thumbnails can be viewed at CR4 Photo Album. Low-resolution images are available for all images thumbnailed. Medium-, and high-resolution images are available for some of those images.

This page has been visited  times since May 25, 1999.

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