Payload Construction Considerations

1
Payload Construction Considerations Techniques

• Ballooning Unit, Lecture 2

2
Payload Constraints

• Federal Aviation Authority (FAA) places
  constraints on what can be flown on a balloon
  without a flight waiver
• Maximum weight of 5.4 kg (12 lbs.)
• Density less than 13.2 g / cm2 (3 oz / in2 )
• Weight of balloon vehicle is 2.5 kg
• Leaves about 2.5 kg for student payloads
• We will fly about 5 payloads, so each payload
  will be constrained to 500 grams
• Size is limited to a cube volume with 15 to 20 cm
  sides

3
Vehicle Interface

• Only a mechanical interface between payload
  flight vehicle
• Two flight string cords are separated by 17 cm
• Both strings must pass through payload with a
  break

• Use a thin walled tube (straw) secured to payload
  structure

• Payload secured vertically on flight string with
  spring clips

4
Rough Payload Dimensions
5
Construction Materials - Metal

• Advantages of using metal like Aluminium
• Strong and relatively light weight
• Comes in many different forms sizes
• Easily machined and bolted together
• Relatively inexpensive
• Disadvantages of using metal
• Heavy (15 cm cube with 1 mm sheet Al would weigh
  360 grams)
• Excellent thermal conductor

6
Construction Materials - Composites

• Composites include materials such as G-10,
  Fiberglass, Carbon fiber extrusions, NOMEX
  honeycomb sandwich
• Advantages of using composites
• Extremely strong and light weight
• Comes in sheets and extruded shapes
• Relatively easy to cut
• Excellent thermal insulator

• Disadvantages of using composites
• Can be very expensive
• Can be difficult to form and glue

7
Construction Materials - Foamcore

• Foamcore (or mattboard) is a composite material
• Core of closed-cell, high-density polystyrene
  foam
• Cladding is thin poster board
• Advantages of foamcore
• Inexpensive and readily available
• Strong and rigid
• Lightweight (15 cm cube with ¼ thick walls
  weighs 116 g)
• Thermal insulator
• Easy to cut and glue
• Disadvantages of foamcore
• Available as sheet stock only
• Not as strong as other composite materials or
  metals

8
Foamcore Properties

• Trade name is Fome-Cor and Gatorfoam
• Fome-Cor website site http//www.gatorfoam.com/
• Fome-Cor thickness from 1/8 to 3/8 and sheet
  size from 20 x 30 to 48 x 120
• Gatorfoam thickness from 3/16 to 2 and sheet
  size of 4 x 8 or 5 x 10
• Gatorfoam uses a resin impregnated wood-fiber
  veneer as the cladding and is more durable than
  Fome-Cor
• Similar product sold in hobby and art supply
  stores and sometimes referred to as mattboard
• 2 x 4 sheet of white ¼ mattboard cost 5
• ¼ thick material weighs 0.065 g / cm2

9
Foamcore Handling

• Recommended document from Edge of Space Sciences
  (EOSS)
• Foamcore Payload Construction by Mike Manes
• Document contents include
• Surface and moisture treatments
• Adhesives
• Foamcore joinery
• Cutting Holes Bends Curved Shapes Cones
• Hardware attachment
• Summarize contents over next slides

10
Surface Treatments

• Mattboard will tolerate moderate amounts of water
  without damage
• Light coat of acrylic spray paint to improve
  resistance
• Loud orange color for visibility
• Use desiccant packs to reduce condensation
• Packs from pharmacy bulk drug shipments or
  consumer products
• EMI shielding by gluing Al foil to outside
  surface
• White (Elmers) glue appears to work fine
• Electrical contact with taped down, roughened
  copper foil strips
• Al foil surface also good for thermal properties

11
Adhesives

• Silicone RTV sealant is adhesive of choice for
  critical joints
• High adhesion and resilience even at low
  temperatures
• Long curing time and out gassing of acetic acid
  vapor
• Epoxy is very strong but can become brittle in
  extreme cold

• Cyanoacrylate for quick bonding of non-porous
  surfaces which mate closely
• 3M Kapton tape (space tape) is strong, bonds to
  nearly everything but is very expensive
• Low-temperature, hot-melt glue for joints
• More rigid than RTV, but no cold embrittlement
• Hot-glued joints stronger than the foamcore
• Glue cools slowly, so have a moment for alignment
• Reaches full strength in about a minute
• High-temp, hot-melt tends to melt the foam

12
Cutting Foamcore

• Materials needed for construction
• Modeling knife and a good supply of new, sharp
  blades
• Machinists square, metal straightedge (12,
  36), spring clamps
• Cutting surface (large cardboard or hardboard)
  and flat work table
• Hot-melt, low temperature glue gun and glue
• Keep knife blade sharp
• Replace blade after 3 5 feet of cutting
• Clearing mark using square and straightedge where
  to cut
• Cut in three passes
• Hold knife blade against straightedge and cut
  upper surface on 1st pass
• Dont move straightedge and on 2nd pass cut
  through foam layer
• On 3rd pass cut through bottom surface

13
Holes and Bends

• Mark center and diameter of hole on both sides of
  foamcore
• Use a straight pin pushed perpendicular to the
  board to transfer the hole center
• Use knife to cut perpendicular to board through
  cladding on both sides, then finish up by cutting
  through foam
• For a mitered bend cut a groove of width W
  through the top clad and the foam to depth D
• Put hot-glue in groove and bend
• For a bend of angle ? the groove width is W
  2(T-P)tan(?/2)

14
Steps for Mitered Corner
Step 1
Step 2
Step 3
Figures from Foamcore Payload Construction by
Mike Manes. Available on EOSS website
15
Other Construction Tips

• Can also make curved and conical shapes using
  same groove and folding technique (See document
  for formulae)
• Minimize number of glue joints
• Put glue joints where stress is minimized
• Reinforce glued joints with tape, glued strips or
  triangular webs
• Joints to box floor should be folded or
  reinforced
• Heavy components (e.g. batteries) should be
  distributed near floor of box and supports
  reinforced
• Might use a double box where interior box houses
  temperature sensitive components and is separated
  from the other box by loose, high R insulation
• Access to interior is probably best through top
  where stress are less

16
Interior Attachments

• Electronic card guides can be formed from strips
  of mattboard glued to the interior walls
• T-nuts or ordinary nuts taped or glued to the
  mattboard can be used for machine screw mounting
• Sheet metal or deck screws can be used for
  low-stress, openable joint if screw passes
  through a double layer of cladding
• Connectors / electrical feedthroughs / switches
  need to be hot-glued to snug-fitting holes in the
  wall
• Flight vehicle string interface tubes should
  penetrate box top and bottom and be securely
  glued to joint corners

17
References

• http//www.eoss.org/onlinepubs/construction/foamco
  re/foamcore.htm - Foamcore Payload Construction
  by Mike Manes, available on EOSS website, 2nd
  Edition, 3/28/01
• http//www.gatorfoam.com/ - Website of Fomecor
  and Gator board products