Payload Construction

This was the sixth 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 - Parachutes.

1. Before Construction

Design on paper first and then cut material

Drawings are less expensive than wasting material

Pencil lines are easy to erase

Drawings often reveal design flaws before construction

See the web site for diagrams of several DD payload bays

Collect necessary parts to form a payload “kit”

Build or acquire bulkheads, coupler and tubes

Build up smallest sub-assemblies, combine into finished product

2. Bulkheads

Bulkheads are necessary to provide distinct chambers for electronics, deployment charges, parachutes, etc.

Commercial bulkheads are available from many rocketry vendor sources

Come in a variety of sizes and thicknesses

For tubes

For couplers

Custom bulkheads can be cut out from plywood, G10, composite board or other stock

Layout bulkhead pattern on sheet stock

Indicate exact positions, hole sizes, etc.

Helpful tools for laying out patterns include:

Compass

Protractor

Straight-edge

Helpful to have a tube samples to determine exact size

Vendor catalogs also a source for tube ID/OD data

Tools that can cut out circular patterns include:

Scroll saw

Drill press with circle cutter attachment

Hole saws

Note: Select hole saws based on the tubing size, not motor size:

Motor tube	Hole saw size
18mm	19mm / ¾ “
24mm	25mm / 1 “
29mm	30mm / 1 3/16 “
38mm	41mm / 1 5/8 “
54mm	57mm / 2/1/4 “

3. Ejection Charges

Commercial ejection charge holders are available from many rocketry vendor sources

Generally consist of a small tube to hold the BP plus a way to seal the tube

Most commercial units are designed for use with e-matches

Custom ejection charge canisters can be fashioned from paper, plastic or small tubes

See the ROL InfoCentral article on Ejection Charges:

http://www.info-central.org/index.cgi?recovery

Charge cannons can be used in conjunction with ejection charge canisters:

Contain the initial explosion

Direct the expanding gasses

Constructed from short sections of small tubing (24mm, 29mm, etc.)

4. Ejection Baffles

Effective way to remove heat from ejection gasses

Most use re-direction of gasses to remove burning particles

Can also be accomplished with steel mesh, ChoreBoy, etc.

Optional part payload assembly

Can achieve same effect with ejection blanket or wadding

Not necessary if piston is used

See Part 3 for construction examples

5. Arming Switches

Switches provide flexibility and safety

Can be very useful if properly installed

Can be affected by G-forces

Switch placement

Switches that are accessible on the pad allow:

Last minute arming of electronics before launch

Conservation of battery power

External switches may be affected by flight or landing events

Types of switches

Slide switches, toggle switches and phono jacks are susceptible to G-forces 

Orient switch at 90 degrees to direction of flight

Push-button or key switches usually the best choice

6. Wiring & Connections

Major components to be considered:

Altimeter wiring harness

Proprietary, based on specific altimeter designs

Usually provided with the altimeter

Wiring to switches

May be part of altimeter harness

Usually soldered in place

May stay with rocket

Wiring to connect ejection charges

Terminal block

Wire wrap / Screw terminal

In-line lugs

Connections through bulkheads

Electronics must be isolated from charges

Various techniques can be used

Pass-through hole

Allows wire runs to pass through bulkhead

Sealed with epoxy or putty

Pass-through conduit

Brass or other metal tube through bulkhead

Wiring runs inside tube

Bolt through bulkhead

Bolt head on altimeter side

Nut on charge side

Typical usage

Permanent connection on altimeter side

Temporary connection on charge side

Wiring runs should be as short as possible

Minimize mess and tangles

Minimize voltage drop

7. Static Ports

Static ports are necessary for barometric altimeters

Not used for accelerometer-based altimeters

Port sizing is critical

Too small will result in “post-apogee” deployment

Too large will result in “pre-apogee” deployment

Rule of thumb is 1/4-inch diameter hole for each 100 cubic inches of volume

Ports should be:

Smooth and free of burrs, etc.

Not downstream of external airframe joints, fins, transitions, etc.

If multiple ports are used, they should not be directly opposite each other

For more information, visit Vern’s Rocketry site:

http://www.vernk.com/AltimeterPortSizing.htm

Use woodworking bit to drill static port(s)

Brad-point bits

Forstner bits

If possible, position static port over internal arming switch

Use dowel or small screwdriver to arm electronics

8. Recovery Attachment Points

Payload section must provide recovery system attachment points

Typically accomplished with U-bolts installed on fore and aft payload bulkheads

Eyebolts used to secure electronics bays may also provide recovery attachment point(s).

Eyebolts as attachment points OK for light rockets (< 10 lbs)

U-bolts recommended for rockets over 10 lbs

9. Installation for Flight

Well-designed electronics bays will be removable and must be installed into the rocket and secured for flight

May be secured into payload section with bolt(s) and nut(s)

May use pass-through eyebolt or threaded rod

May be “trapped” between fore and aft removable bulkheads

Removable bulkheads typically used to facilitate payload access

All-thread or eyebolts can be used to secure the bulkhead(s)

Electronics should always be powered down (if possible) before installation into the rocket

Use switches whenever possible