Payload Construction Considerations & Techniques

1. Payload Construction Considerations & Techniques Ballooning Unit, Lecture 2 Payload Construction

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 Payload Construction

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 Payload Construction

4. Rough Payload Dimensions Payload Construction

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 Payload Construction

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 Payload Construction

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 Payload Construction

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 Payload Construction

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 Payload Construction

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 (Elmer’s) glue appears to work fine • Electrical contact with taped down, roughened copper foil strips • Al foil surface also good for thermal properties Payload Construction

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 Payload Construction

12. 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 Payload Construction

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) Payload Construction

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

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 Payload Construction

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 Payload Construction

17. 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 Payload Construction