Design

Guidelines

The design guidelines for Arnor are set by the Hedonic Treadmill project

• The rocket design cannot be based off:
  • Past or present hobby rocket kits (no clones or upscales/downscales)
  • Past, present, or future sounding rockets, missiles, or other military, commercial, or scientific launch vehicles
• The rocket design will be tailored for its missions. Arnor will be designed for:
  • Subsonic speeds
    • The nose cone is a 3:1 Ogive with a phenolic tip. The tip will be slightly rounded to make the nosecone into a roughly elliptical shape
    • Three fins is lower drag than four and there's no need to worry about the trans-mach CP shift
    • The fins will have a true NACA airfoil profile to lower drag and improve force/lift when making aerodynamic corrections
  • G–I motors
    • The motor mount will be designed around an Aerotech 38/480 reload casing
    • Initial flights will be on unrestricted, Hazmat-free, reloadable motors: Aerotech G79W (29/120) ALTERNATIVE: Loki Research G80 (38/120)
    • Certification flight: Aerotech H128W (29/180)
    • Full send: Aerotech I59WN (38/480) (dual-deploy configuration) >7 second burn to >7000 feet
  • Flight as both a Class 1 and a Class 2 rocket
    • The maximum liftoff weight of a Class 1 rocket is 1500g; the recommended thrust to weight ratio is 5:1.
      • An Aerotech G79 has an initial thrust of 92N so it can lift a 1880g rocket at a 5:1 ratio (that number comes from converting N to lb [force], dividing by 5, then converting lb to g—I’m sure there’s a better way to do that calculation)
      • A Loki Research G80 has an initial thrust of 112N so it can lift a 2280g rocket at a 5:1 thrust to weight ratio
  • Single & dual deployment configurations
    • The estimated liftoff weight for the dual deploy configuration is too high to achieve the 35 MPH target airspeed when leaving the launch rail when flying with unrestricted motors (≤80 N average thrust)
    • The booster/fin can is a zipperless design so the single deployment configuration will use a piston to ensure the recovery gear is properly expelled from the airframe
    • In both cases the avionics bay will be “pinned” to the airframe, Jarvis-style, using the heads of socket-hed screws but instead of plywood bulkhead backing they will be anchored with PEM nuts
  • Testing avionics payloads (and LiPo batteries) of increasing complexity towards redundant dual deployment
    • Configurations:
      • Single-deploy with EggTimer Apogee
      • Single or Dual deploy with redundant AltusMetrum EasyMinis
      • Single or Dual deploy with redundant EggTimer Quarks
      • OPTIONAL: Some other configuration of available electroncics:
        • Eggtimer Apogee (1 available)
        • Eggtimer Quark (2 available)
        • AltrusMetrum Easy Mini (2 available)
        • EggTimer Wifi Switch (2 available)
    • The goal is to have WiFi arming for all avionics payloads. All wireless systems will have independent, externally-accessible screw switches (likely FingerTech) for a safety cutoff
    • LiPo batteries will be used throughout but more research on connector, capacity, and care is needed
    • Terminal block design is TBD due to tight tolerances in the avionics bay
  • Testing ejection charge designs
    • Centerfuge tubes for convenience on simpler avionics payloads
    • Jarvis-style elongated charge canisters e.g. ¼” × 5.75” paper straw lined charge in a high-temperature G10 tube
  • Testing composite reinforcement techniques
    • Fillets thickend with Colloidal silica
    • Multi-layer fiberglass & carbon fiber reinforcement across fin span with stepped layer sizes and alternating weave orientations
    • Fiberglass laminated plywood bulkheads pressed under mylar sheeting for a smooth finish
  • Avoiding metal parts in the recovery harnesses, recovery hardpoints, and the avionics bay
    • Favor soft links instead of traditional quick links
    • Favor anchoring Kevlar lines with loops around internal tubes instead of metal anchoring points
    • Favor PEM-style nuts and socket-head screws for joining airframe sections together
  • Testing recovery system designs
    • Using Möbius-Brummel locking splice (finger trap) loops for line termination
    • Using a heavy-weight drogue chute but a mid or light-weight main chute
    • Using Nomex sleeving over Kevlar chord anchored with marine heat shrink tubing for sections of the recovery harness that will be exposed to repeated ejection charge blasts
    • Using high working-weight swivels at key points to avoid tangling the harness:
      • Booster to harness
      • Harness to drogue
      • Harness to main
    • Using #2 nylon screws as shear pins at airframe breaks

Fins

This rocket has three fins. The core of the fins will be laminated balsa stock. This balsa-ply core will be shaped into a Prandtl-D tip NACA airfoil profile before being reinforced with 1/64” plywood, 3oz fiberglass, and 6oz carbon fiber. The fin stock will be tapered radially to maintain a proportional airfoil across the entire span of the fin to keep the Reynolds number low (see Apogee Technical Publication #16 et. al.).

Shaping a complex airfoil by hand is difficult so a few steps will be taken to make the results more consistent. Each fin will be assembled from two halves. Each half will be tapered radially using a custom extra-long sanding block and beveling jig made from Makerbeam based on the work of Bart Hennin published in Peak of Flight #271. The fin shape was chosen so that the thickest part of the airfoil will be perpendicular to the root edge which makes it easier to form the aft bevel of the airfoil with the tapering jig. The fore bevel of the airframe will still have to be shaped by hand.

Once the fin cores are shaped, a strip of 1/64” plywood will be bonded to the outer surface of the trailing edge of the airfoil to increase toughness. The two halves of each fin core will then be bonded together. The through-the-wall fin tab will be made slightly oversized so that it can be sanded to conform to the surface of the motor mount tube.

The fins will be bonded directly to the motor mount. Once they have been filleted epoxy thickened with colloidal silica a 4-layer tip-to-tip composite layup will be applied: 3 layers of 3 oz fiberglass with a top layer of 6 oz plain weave carbon fiber.

Avionics Bay

Ejection Charges

The ejection charge design is based on an article by Jim Jarvis that was originally published in Rockets Magazine in June 2011 and further refined in subsequent years both by Jim and by the Princeton Space Shot team. The charge canister dimensions used in the first revision of the Princeton Space Shot launch vehicle was 1/4” by 6” (page 21). I’ll be shortening that slightly to 5.75” to match a standard size of paper straw that will act as a disposable liner.

Help Me Choose

The cocktail-size straws have an OD .248” and are 5.75” long. They should nest nicely into this high-temperature FR4 tubing:

McMaster-Carr

Additional notes:

Power

From the EggTimer Quark User’s Manual: