P1: Nitrous at Balls in '08

Airframe Construction Notes

Altimeter Bay

Last Update, 6/17/07 by Stephen Daniel

Design Goals:

  • Easy to build, easy to prep, modest cost
  • Holds a pair of commercial altimeters, independent batteries, independent power switches
  • Prevents exposure of the altimeters to the ejection charges.


  • The altimeter bay will be a refinement of the Orange Bird II design. It will be built into a coupler tube. Above the coupler will be the airframe section holding the parachutes (deployed at low altitude). Below the couple will be the streamer (deployed at apogee).
  • The altimeter bay uses forged eyebolts running through the center to take the load and shock of the recovery hardware. These eyebolts are the hard attach points for the recovery rigging.
  • Design details:
    • 10" length of PML 4" coupler tube. The interior of the tube is reinforeced with a single layer of carbon fiber.
    • 2 3/8" bulkheads made of 7-ply birch plywood
    • A 4" forged bolt, a steel 1/4" threaded coupler, and a 6" forged bolt form the center core. These screw together to hold the bulkheads on to the end of the coupler tube.
    • The 1/4" threaded couple is built into the middle of a thick shelf that slides into the coupler tube. Each surface of the shelf provides space to mount an altimeter, its batteries, power switch, etc.
    • The edges of the shelf have slots that mate to rails glued to the inside of the couple tube. This prevents the shelf from spining and makes screwing in the eyebolts easier.
    • The outside surface of each bulkhead has a 1/4" fence staple permanently mounted. After the eyebolt is fixed a cable-tie connects the eye to this staple. This prevents the bolt from becoming unscrewed, even if the rocket spins badly while descending.
    • The outside surface also contains a 4-terminal screw-block for connecting ejection charges. Permanent wires lead from these through a sealed hole in the bulkhead and are connected to the altimeters.
    • The altimeter is fixed to the forward airframe by a set of 4, 12" long 10/32 machine screws. These run through the entire length of the altimeter bay and anchor to a ring glued into the forward airframe.
    • Each screw is guided through the altimeter by a tube, glued to the interior wall of the coupler.
    • Two of these tubes form the rails that the altimeter mounting shelf slides on.
  • Note: this payload bay will hold the Orange Bird II's altimeter shelf. My plan is to build a new shelf for P1, but being able to use the OB-II shelf is convenient.

Construction Notes

  • Parts:
    • 10" of PML 4" coupler tube, reinforced with CF. This tube was built previously as part of the OB-II build-out.
    • 4 bulkheads, 3.9" by 3/8" 7-ply birch. These can be purchased as a customer order from PML. I made mine in a friend's wood shop.
    • Metal tubing, 4 pieces, 10" long. I am using 9/32" x .014" aluminum tube because it was the cheapest thing my local hardware store had that comfortable fits a 10/32 machine screw. Anything metal about this size will work fine. The tubing is spaced out from the interior wall of the payload bay using small balsa strips. This allows the holes to be far enough from the edge to ensure structural strength of the supporting rings and bulkheads.
    • Forged eye bolts. I am using McMaster-Carr bolts, 1 each of part 3016T16 and 3016T18.
    • 12" long 10/32 machine screws. These are McMaster-Carr part 99117A780 (qty 4)
    • 10/32 T-Nuts. I'm using McMaster-Carr part 90975A015.
  • Drilling the bulkheads
    • The bulkheads are drilled with 4 holes 7/32" dia for the mounting screws. These are equally spaced around the edge, each is 3/8" on center from the edge. A drill template ensures consistency in the face of measurement errors.
    • Each bulkhead as a 5/16" center hole for the 1/4" eye-bolt to go through.
    • The mounting ring has peripheral holes at the same spots as the bulkheads, but they are 1/4" holes for the tee-nuts.
    • The mounting ring is built by cutting a 2-3/8" hole in the center of a bulkhead blank.