Longerons

Deckplates:
small
large
parachute

Requirements:

Fit inside payload tube
Provide mounting surface for electronics
Withstand 20g in high vibration environment
Have mounting structure for parachute recovery system
Withstand shock from parachute
opening

Description:

The payload frame consists of two main components: the longerons and the deck plates.
The longerons will consist of ½” x ½” (1.27 cm x 1.27 cm) square rods that bend in at the top to clear the sides of the nose cone. Fastened to the inner side of each longeron will be ½” x ¼” (1.27 cm x 0.64 cm) strips. These strips will support the weight of the deck plates. They will be fastened with 2 button-head screws per strip. This eliminates stress risers that would occur if the longerons were solid pieces with notches cut out of them. This also greatly increases the flexibility of the deck plate locations.
The deck plates will be ¼” (0.64 cm) plates that are machined to fit the interior diameter of the payload tube. They should fit as close as possible while still being able to slide freely within the tube. (It should be noted that the tube is not expected to be an exact diameter and machining of the deck plates should not be done until the payload tube is in hand and can be measured accurately). The deck plates will have two ½” x ½” (1.27 cm x 1.27 cm) notches located 180 degrees apart. These will accommodate the longerons and allow the deck plate to extend to the payload tube.Located 90 degrees from the notches the deck plates will be “trimmed” back ½” (1.27 cm). This will allow the plates to clear the other two longerons and still be installed and removed while three of four longerons are assembled. This will allow easy removal of the deck plates while working on the electronics during integration. The deck plates will have four 2” (5.1 cm) holes drilled in them for wiring throughholes and to reduce weight.

Materials

The deck plates and longerons can be machined out of either 6061 or 7075 aluminum alloy. Simple compression calculations from Mechanics of Materials by F.P Beer and E. R. Johnston for the longerons have been done.

Compression of columns under static load for 6061 aluminum:

Buckling Load = 20.2 - 0.126(L/R)Ksi

where L = length of column, and r = radius of column

The 6” section of longeron between screws would have compression strength of 4294 lbs (1951.8 kg). This is far in excess of the 120 lb (54.5 kg) maximum load expected on an individual longeron. A similar equation modified for 2014 aluminum yields a compression strength of 6295 lbs (2861.3 kg). (2014 has a tensile strength similar to that of 7075 aluminum). The deck plates do not carry enough load for strength to be an issue and as designed they do not weigh enough to justify going to a thinner deck plate. Interfaces The longerons will rest directly on top of the radex joint’s upper ring. This will allow the weight of the payload to transfer directly to the motor instead of to the payload tube. The longerons will be attached to the payload tube at least every 6” with flat-head screws to prevent deflection under load. Although the top deck plate is inside the nose cone, there is no contact between the payload frame and the nose cone.