P1: Nitrous at Balls in '08

Airframe Design Notes

Last Update, 6/21/07 by Stephen Daniel

Design Goals:

  • 4" light-weight super-sonic capable airframe.
  • Minimize total height without crowding anything too much.
  • Built from a total of 8' of 4" tubing.
  • Budget is $300.

Airframe Construction

Fin Can:

  • 8" long section of CF tube.
  • 3 fins, G10 glass with CF reinforcing.
  • My plan is to buy the fin cores from PML. The cores will be glued to the can, and then lapped with CF.
  • Shape and size of the fins is still TBD. Much thought still needed.
    • Drag on the fin leading edges is a major determinant of the rocket's performance.
    • This rocket has a length to diameter ratio of 116" to 4" or 29:1.
    • Most rockets are designed under the assumption that if the center of pressure of the rocket is 1 body-diameter behind the center of gravity, the rocket will be stable. This paper shows that tall skinny rockets need more than this. If the rocket's angle of attack deviates from zero the center of pressure will shift forward, perhaps substantially.
  • One lap will go 2/3rds of the way to the fin tip, and the second 1/3rd of the way.
  • Bottom of the can sits on the thrust ring of the motor.

Lower Airframe Tube (includes positive motor retention):

  • 20" long section of tube.
  • Mates to the top of the motor using a custom machined coupler.
  • Top of motor has a 1/4" threaded hole. Airframe has a bulkhead that is anchored to the airframe and bolted to the top of the motor by an eyebolt. Needs to be room between the motor's forward closure and the airframe's bulkhead for the motor plumbing.
  • Tie-in to the uppers is through this eyebolt.
  • The 20" airframe us used as follows:
    • Top 5.5" of the airframe holds half of the payload bay (coupler).
    • Next 2" hold the payload eyebolt, ejection charges, and recovery wadding.
    • Next 2" holds shock cord and streamer. The eye of the bolt that holds the motor is in this section.
    • Next 0.5" is the bulkhead.
    • Next 9" holds plumbing.
    • Bottom inch or so mates to the engine coupler (design TBD).
  • Note that the eyebolt has a long shank, such that the eye is above all the plumbing.
  • There are two pipes that come up from the motor. One is dipped to the bottom, one connects to the top.
  • The 9 " of plumbing include a pressure relief valve and a solenoid valve (dump) QD fittings, etc. Details belong to the motor design.
  • I'm unclear on how the wires to the solenoid dump valve. One could be the motor casing. How do we route the other to the bottom of the airframe? I suppose we could attach to the disconnect clip to the side of the airframe. Yuck. The solenoid must work or we will have trouble dumping nitrous.
  • The airframe connects to the payload bay / coupler tube via a friction fit. Simulation is needed to verify, but I believe this joint is under compressive load at all times. Regardless we should figure out the maximum load on this fit so we ensure the rocket doesn't separate accidentally. However, I don't want to make this fitting more work than it needs to be.
  • There needs to be a vent port below the internal bulkhead, and a vent hole through the internal bulkhead. Need to ensure leaking nitrous is vented, as is atmospheric pressure air.
  • Rail buttons: one on the fin can, one near the center of gravity. Probably we will mount this on the coupler between the motor and the lower airframe tube.
  • Questions:
    • What size rail buttons? Or dual mounts like the OB-II?
    • Is 9" enough to hold the plumbing? Plumbing must stay clear of the 1/4" eyebolt's shank.
    • Is 1/4" the right diameter for the bolt? This bolt takes the ejection shock of of the entire motor, plumbing, and lower airframe. OTOH a 3/8" bolt is heavy and large.
    • Where do the vent/dump valve's wires run?
    • Need detailed design of plumbing connections from the motor design.

Payload Bay:

  • 10" long section of CF reinforced coupler tube.
  • Bulkheads fit on either end, held on by mated eyebolts through the center
  • Electronics are on a floating shelf, bonded to the threaded coupler that connects the mated bolts. The shelf is prevented from rotating by loosely riding rails on the sides of the couple. The shelf and rail design is identical to the OB-II payload bay, but the shelf is a little longer.
  • The outside of each bulkhead has a staple that holds the eyebolt's locking cable-tie. A pair of two-wire cables come out through sealed holes and connect to e-matches through connectors. Connectors TBD, but something that is inexpensive and protected from ejection gases.
  • The payload bay is assembled outside of the rocket. The electronics are loaded first, then the eyebolts are placed, and finally the charges are connected.
  • As discussed above, the payload bay connects to the lower airframe via a simple friction fit. However, it must be hard attached to the upper airframe. The ejection charge above the payload bay must shear the pins and eject the parachutes. It must not eject the payload bay from the bottom of the tube.
  • The upper section of airframe has a wooden ring permanently glued in. This is a 3/8" thick bulkhead with a 2-3/8" hole drilled in it. Four 10-32 tee-nuts are mounted radially around the ring. Matching holes have been drilled in the payload bulkheads. The prepped payload bay is inserted into the upper airframe. Then four 12" long 10-32 bolts go through the entire length of the payload bay and anchor the bay to the wooden ring in the upper ai frame.
  • In order to make placing these bolts relatively easy, 4 long aluminum tubes are mounted to the interior surface of the payload bay. The bolts run loosely through these tubes. Two of these tubes serve as the payload shelf rails.
  • Power control is by screw switches, accessed through holes drilled through the upper airframe and the payload bay wall. The screws are recessed enough to provide pressure ports for the bay.

Upper Airframe Tube:

  • This is a 28" long section of CF tube.
  • It holds whatever is needed to mount the payload bay (see discussion above).
  • Space within this tube is used as follows:
    • The top 4" hold the nose cone shoulder.
    • The next 14.5" hold the parachutes and rigging. May include a Walston or other RDF transmitter.
    • The next 4" hold recovery wadding and charges.
    • The next 5.5" hold the top of the payload bay.
  • This is about 4.5" more than the OB-II has for parachutes.
  • Question:
    • Do we need 3 chutes? My current plan is for 3.

Parachutes:

  • I've been thinking about using a cluster of 3 of Aerocon's 60" artillery parachutes. They are quite inexpensive. The Orange Bird II lands on two of these at a decent rate of about 27 feet per minute. P1 will have a higher decent mass and will land in thinner air. These are available for $12 each and are by far the least expensive option.
  • Clustered chutes are less efficient and require proportionally more rigging. An alternative is a single parachute in the 8' to 10' range. Cost would be around $150 or so. My biggest question is whether the parachute will pack nicely into a 4" airframe.
  • We may have to decide on and acquire parachutes before we can finalize the length of the upper airframe section.

Nose Cone:

  • Start with a 5:1 tangent Ogive fiberglass nose cone from Performance Rocketry. I bought tow of these for about $70 from Ken (Performance Hobbies).
  • Near the bottom of the shoulder on the inside is a wooden collar, glued in. A bulkhead with permanent eyebolt screws into this collar to provide the attach point. Preferred mechanism is to have T-nuts glued into the collar and use machine screws to anchor the bulkhead.
  • A wooden payload bay consisting of a permanently mounted box will be built into the nose cone. A wooden shelf slides into the bay. GPS and telemetry radios will be built onto this shelf.
  • The nose cone is held in by shear-pins, driven through the upper airframe, the nose cone shoulder, and into the wooden collar. There are 2 shear-pins, made of #4 nylon machine screws.

This entire package is close to 10' tall.

Rocksim models coming soon.