Booster Construction

This was the third part in our continuing project to get club members up to speed on building a High Power Rocket.  If you missed the meeting here is the basic outline of what was discussed.  Don't miss next month - Dual Depolyment Payload.

1. Fin Construction

Material chosen should be strong enough for the intended application

As fin span increases, material strength and stiffness becomes more important

For typical rocket fins with span less than 5 inches, good material choices are:

• 1/16" G10
• 1/8" aircraft plywood
• 1/8" composite board

If span exceeds 5 inches, better choices are:

• 3/32" G10
• 3/16" aircraft plywood
• ¼" composite board

For custom fins, cut out a paper template to transfer the fin outline to your fin stock

Don’t forget the fin tab

With plywood, orient wood grain with leading edge

Cut out fin using your choice of saw and sand to fit

2. Airfoiling Fins

Edge treatment can reduce drag and improve altitude:

• For sub-sonic fins:
• Rounded leading edges
• Tapered trailing edges
• For supersonic fins
• Tapered leading and trailing edges

With plywood or G10, edges can be sanded into shape

With composite board, edge material must be added:

• Dowel rod
• Solid balsa or basswood wedges
• Built-up plywood wedges

3. Centering Rings

Good centering rings can be purchased from many rocketry dealers. See resource listing.

• Inexpensive
• Typically high quality

Materials

• 3/16" ply a good choice for Level 1 and Level 2 projects
• G10 rings a better choice for Level 3 or "extreme" projects

Centering rings can also be cut out from plywood stock or other materials

• Use a "commercial" ring as a pattern if available
• Create your own pattern with a compass
• Airframe tube or coupler tube can also be used to mark outer circle
• Motor tube can be used to mark inner circle
• Center hole can be cutout with a hole-saw or drill press using a circle cutter
• Outer circle can be cutout with a scroll saw, band saw or drill press using a circle cutter
• Sand to fit

4. Motor Mount Construction

Best approach is to build an integral motor mount / fin unit assembly

With this technique, both fin alignment and ultimate strength of fin unit are determined at motor mount construction time.

Mark motor tube

• Use a fin-marking template to insure even spacing around the tube
  • VCP will print out custom template for any size tube / fin combination
• Set motor tube upright on template and mark each fin location
• Extend fin lines using aluminum angle

Epoxy centering rings into place

• Aft centering ring should be immediately behind fin tab
  • Move ring forward from "stock" position if necessary
• For maximum strength, place 2nd centering ring immediately forward of fin tab
  • Fin tab is ultimately bonded on three sides instead of one side
• Locate forward centering ring at forward end of motor tube
  • Ejection baffle could impact exact position (see below)
• Make sure each centering ring is "square" with the motor tube
  • Use masking tape or paper "cuffs" if necessary
• Add fillets on each side of centering rings for added strength

Epoxy fins to motor mount

• Use a small amount of 5-minute epoxy or CA to "tack" the fins into position
• Make sure fins are:
  • Straight along the fin lines on the motor tube
  • Projecting straight away from the centerline of the tube
  • Touching the forward and aft centering rings
• Apply epoxy fillets on each side of each fin / tube joint, including:
  • Root edge along motor tube
  • Forward and aft tab edges along centering rings
  • Use 15-minute epoxy or long cure epoxy such as West Systems for fillets
• Add additional reinforcement if desired
  • Root-to-root glass or carbon across motor tube
  • Tip-to-tip glass or carbon across motor tube
• Fin channels can also be used to secure fin tabs if desired
  • Along the motor tube, between centering rings
  • Added to each centering ring where fin tab touches ring

5. Motor Retainers

Should be part of motor mount / fin unit design

• Generally built into aft centering ring BEFORE ring is epoxied to motor tube
• Generally consists of an attachment point and an external clip
• Redundancy is good

Attachment points can be:

• Blind T-nuts epoxied into aft ring from forward side
• Machine screws epoxied into aft ring from forward side
• Machine screw (less head) or all-thread epoxied along motor tube under aft ring
• Threaded ring epoxied to motor tube (SlimLine/Aeropac)

Motor clips can be:

• Window screen clips
• Electrical conduit clips
• Homemade, fashioned from heavy wire or sheet brass
• Nut and washer (for all-thread)

6. Ejection Baffle

Optional part of motor mount / fin unit assembly

• Can achieve same effect with ejection blanket
• Can be left out entirely if motor ejection will NEVER be used

Typical baffle can be installed inside a tube coupler

• Positioned behind forward bulkhead and ahead of forward centering ring
• Forward bulkhead and recovery attachment point become part of the baffle assembly
• See attached examples

7. Rail Guide Attachment

Optional part of motor mount / fin unit assembly

• Not necessary if surface mount rail guides or launch lugs are used

For lighter rockets (< 5 lbs) rail guides can be attached via holes drilled into airframe

• Sometimes possible to drill into centering rings for added strength

For heavier rockets, consider adding internal backing material where rail guide attaches to airframe. Typical examples:

• Centering ring "wedges"
• Notched centering rings with 1 x 1 wood stringer
• Note: Record position of internal supports

8. Recovery System Attachment

Typically attached to the forward bulkhead

• Single U-bolt or eyebolt for small diameter rockets
• Multiple U-bolts for larger rockets
• May be integrated with ejection baffle

On heavier rockets, forward bulkhead should be reinforced

• Laminated with glass
• Sandwiched with G10 bulkhead

9. Cutting Tubes

First step is to mark tube correctly

• Wrap tube using paper template
• Mark all the way around

Paper tubes can be cut with a hobby knife or box knife

• Use light pressure on knife
• Rotate tube under knife until cut

Phenolic tubes or any type of glassed tubes are difficult to cut with a hobby knife

• Use a razor saw or power saw of your choice

Sand end of tube for a "finished" edge

10. Slotting Tubes

With pre-slotted tubes

• Use an aluminum angle to draw straight lines from the existing slots to the end of the tube
• Extend the slots using a razor saw or power saw of your choice

With un-slotted tubes, marking fin slots correctly is critical

• Best approach is to:
  • Slide pre-built motor mount/fin unit into airframe until fins contact the tube
  • Lay steel ruler against side of fin
  • Mark tube where ruler touches tube
  • Repeat with ruler on opposite side of the fin
  • Use aluminum angle to extend the lines up the tube
• Resulting marks agree with:
  • Fin placement around the tube
  • Fin orientation relative to centerline of tube
• Marking and cutting the tube without reference to the actual fin unit can result in useless fin slots

Slots can be cut out using a variety of tools

• Razor saw
  • Use steel straight-edge to get cut started
• Router or Power Saw
  • Requires construction of appropriate jig
  • See November 2003 issue of HPR

11. Booster Assembly

Slide completed motor mount / fin unit into slotted airframe

• Can be joined "dry" and epoxy added later
• Can be joined "wet" with epoxy in place at forward bulkhead location
  • Use at least 15-minute epoxy to avoid premature adhesion
• Epoxy rear edges of tube to rear centering ring
  • Add short section of coupler behind aft ring if desired for extra strength
• Add bead of epoxy on top of forward bulkhead
  • Drinking straw and dowel rod "syringe" will dispense epoxy accurately
• Add short section of coupler on top of forward bulkhead if desired for extra strength

12. Fin Fillets

Add epoxy fillets to both sides of each fin / airframe joint

• Decreases overall aerodynamic drag
• Adds some strength

Support booster assembly horizontally 

• Work on one fillet at a time

For "clean" fillets use masking tape

• Protects both fin surface and airframe tube from unwanted epoxy
• Provides a clean straight edge

Apply initial (minimal) fillets to seal fin / airframe joint

• Use 5-minute thick epoxy

Apply final fillets after initial fillets have cured

• Use 5-minute epoxy on small fillets
• Use 15-minute epoxy on larger fillets to allow more work time
• Smooth fillets with (gloved) finger dipped in alcohol

Remove masking tape before epoxy sets up!

13. Launch Lug / Rail Guide Attachment

Scribe straight line parallel to airframe centerline between fins

• Use aluminum angle or string

For launch lugs:

• Attach one lug at CG or 
• Attach 2 lugs equal distance fore / aft of CG
• Tack on with 5-minute epoxy or CA
• Add fillets to each side just like fins

For rail guides:

• Position guides at aft bulkhead and CG
• Longer / heavier rockets need a third guide near payload section
• External (conformal) guides may be 
  • Epoxied in place
  • Provided 3M adhesive OK for small rockets
• Drill holes for standard guides and attach with screws
  • If backing material was installed, check notes before drilling!

Resources

Circle cutters

• Sears part no. 00925293000, $19.99. Heavy duty, does a good job
• Harbor Freight part no #44499-ovga. This has a micro adjustment screw. With this one you can make small adjustments with greater precision.

Good sources for Rocket Kits, Parts and Motors:

Impulse Aerospace:

Motors, Aerotech kits, tubular nylon, fiberglass, composite board
http://www.homestead.com/impulseaerospace/

Giant Leap Rocketry:

Motors, parts
http://www.giantleaprocketry.com/

Magnum Rockets, Hobbies and More:

Motors, parts, kits
http://www.magnumrockets.com/

LOC/Precision

Kits, parts, chutes
http://www.locprecision.com/index.html

Public Missiles, Ltd

Kits, parts, chutes
http://www.publicmissiles.com/

Binder Design

Kits, parts, chutes
http://binderdesign.com/

Performance Rocketry

Custom nose cones, custom tubes,G10 rings and plates
http://www.performancerocketry.com/

Copyright © 2003, Tulsa Rocketry
Page last modified Thursday, August 25, 2005