LSU 06/04/2007Payload Construction1 Payload Construction Considerations & Techniques Ballooning...

LSU 06/04/2007 Payload Construction 1

Payload Construction Considerations & Techniques

Ballooning Unit, Lecture 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


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


Rough Payload Dimensions


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


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


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


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


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


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 (Elmer’s) glue appears to work fine– Electrical contact with taped down, roughened copper foil

strips

• Al foil surface also good for thermal properties


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


Cutting Foamcore

• Materials needed for construction– Modeling knife and a good supply of new, sharp blades

– Machinist’s 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

– Don’t move straightedge and on 2nd pass cut through foam layer

– On 3rd pass cut through bottom surface


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)


Steps for Mitered Corner

Step 1

Step 2

Step 3

Figures from “Foamcore Payload Construction by Mike Manes.

Available on EOSS website


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


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


References

• http://www.eoss.org/onlinepubs/construction/foamcore/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

LSU 06/04/2007Payload Construction1 Payload Construction Considerations & Techniques Ballooning...

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