Transcript Payload Construction Considerations & Techniques

Payload Construction
Considerations & Techniques
Ballooning Unit, Lecture 2
LSU 05/11/2004
Payload Construction
1
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
LSU 05/11/2004
Payload Construction
2
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
LSU 05/11/2004
Payload Construction
3
Rough Payload Dimensions
LSU 05/11/2004
Payload Construction
4
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
LSU 05/11/2004
Payload Construction
5
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
LSU 05/11/2004
Payload Construction
6
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
LSU 05/11/2004
Payload Construction
7
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
LSU 05/11/2004
Payload Construction
8
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
LSU 05/11/2004
Payload Construction
9
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
LSU 05/11/2004
Payload Construction
10
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 nonporous 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
LSU 05/11/2004
Payload Construction
11
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
LSU 05/11/2004
Payload Construction
12
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)
LSU 05/11/2004
Payload Construction
13
Steps for Mitered Corner
Step
1
Step
2
Step
3
LSU 05/11/2004
Figures from “Foamcore Payload
Construction by Mike Manes.
Available on EOSS website
Payload Construction
14
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
LSU 05/11/2004
Payload Construction
15
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 hotglued 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
LSU 05/11/2004
Payload Construction
16
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 05/11/2004
Payload Construction
17