THE directions are simple. 鈥淧ut tab A into slot B. Place assembled piece into
slots E and F, making sure not to cover tab B or its counterpart, tab G.鈥� Just
the sort of instructions you might expect to follow when putting a model
aeroplane together鈥攂ut soon they could just as easily come from the assembly
manual for a new type of strong, light spacecraft. Later this year, the first
satellite built along these lines will be launched from the US, and by the end
of the century, telecommunications satellites could be mass-produced this way.
It is an idea that could revolutionise thinking about building ultralight
spacecraft cheaply and quickly.
All spacecraft have to be made as light as possible. Any unnecessary weight
has to be paid for by reducing the payload鈥攚hether it is a scientific
experiment, remote sensing equipment or telecommunications relays. In the past,
the frame that supports the payload has typically been built from bolted
aluminium struts. Aluminium is strong, relatively easy to cut and cheap. Above
all, it is light, allowing payloads to take up the highest possible proportion
of a spacecraft鈥檚 weight.
But aluminium has its limits, and for some time satellite builders have been
investigating carbon composite materials that are even lighter but just as
strong. One difficulty is that carbon composites are difficult to mould into the
complex shapes needed to build a satellite, and this has made them more than
twice as expensive to use than aluminium. But now a Californian company called
Composite Optics (COI), which has already had experience building carbon
composite parts for the Hubble Space Telescope and NASA鈥檚 Advanced X-Ray
Astrophysics Facility (AXAF), has developed a way of building satellites from
flat, cut-out pieces of carbon composite that snap together like a child鈥檚 toy.
COI鈥檚 technique vastly simplifies the way satellites are built, reducing manu
facturing and assembly time and so cutting costs enough to offset the expense of
the new materials.
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Carbon composites are built up from layers of carbon fibres laid down at
right angles to each other, like the grain in successive sheets of plywood. The
layers are cemented together with epoxy resin and the resulting material, known
as graphite epoxy, is extremely strong and light. For example, graphite epoxy
composites the weight of aluminium have the strength of steel, says Stephen
Knox, who heads the project to build the first all-composite satellite at Los
Alamos National Laboratory in New Mexico. These properties have already been put
to good use building tennis rackets, the bodies of racing cars and aircraft
fuselages. But in spacecraft, graphite epoxy has been something of a rarity,
mainly because it has proved difficult to produce complex moulded parts that are
free from defects. Moulding, testing and assembling the graphite epoxy
components that would be needed for an entire spacecraft takes at least 30
weeks, according to Gary Krumweide, development manager at COI in San Diego.
For an aluminium spacecraft, the job takes 16 weeks.
With COI鈥檚 snap-together technique, which it calls Short Notice Accelerated
Production Satellite or SNAPSAT, the whole process can be completed in 10 weeks,
says Krumweide. In this system, the spacecraft frame is designed in such a way
that it can be built from components cut out from flat sheets of graphite epoxy.
Each piece has tabs and recesses to fix it to its neighbours. To assemble the
spacecraft, engineers simply glue the components together to form a structure
that is up to 40 per cent lighter than an equivalent aluminium model, and just
as strong. This translates into extra payload capacity or lower launch
costs鈥攅ach kilogram shaved off the weight could save up to $20 000 in launch
costs. The graphite epoxy sheets can be prepared in advance, ready to be cut to
shape the moment the design is finalised.
Master blaster
To carve the snap-together components from the sheets, COI uses a water jet
carrying tiny granite particles that blast into the material at a speed of
around Mach 2. 鈥淭he water jet can cut through six inches of steel like butter,鈥�
says Gary Tremblay, the product manager for COI鈥檚 spacecraft structures. The
machine operates at ten times the rate of conventional tools such as laser
cutters. 鈥淭he water jet reduces to two or three hours, a task that normally
requires two or three days,鈥� Tremblay claims.
The jet can also be aimed more precisely than conventional tools. This allows
the components to be more closely spaced on the sheet, thereby reducing waste.
And by stacking a number of sheets, the jet can carve out the components for
several identical satellites at the same time, opening the way to mass
production. With these techniques the SNAPSAT concept has the potential to
reduce the cost of fabricating spacecraft by a factor of ten, Krumweide
says.
As well as lightness and relatively low cost, graphite epoxy has another big
advantage as a material for fabricating satellites: it hardly expands at all as
the temperature increases. The expansion coefficient of the composite is about
one-hundredth that of aluminium, which is a priceless advantage for astronomers
who want to build space-based telescopes in which the components have to be
positioned precisely. For example, when NASA was designing AXAF, it asked COI to
make the 9-metre graphite epoxy tube to hold the four pairs of mirrors that will
focus the X-rays at a precise angle. The instrument is now under construction
and will be launched in 1998.
The first SNAPSAT craft, due to be launched this year, is the Fast On-orbit
Recording of Transient Events satellite, otherwise known as FORT脡, being built
for the US Department of Energy as part of the American effort to spot other
nations鈥� clandestine nuclear tests. FORT脡 is designed to detect the
electromagnetic pulse from a nuclear blast, says Knox. The SNAPSAT frame for
FORT脡 weighs only 40 kilograms; an equivalent aluminium frame would weigh 50 per
cent more. 鈥淭he weight saving maximises the amount of payload that I can put
into orbit,鈥� explains Knox. And despite the expense of graphite epoxy, COI鈥檚
manufacturing techniques kept the price of the satellite competitive. The cost
of FORT脡鈥檚 structure was $160 000 and the estimated cost of an aluminium
structure was $133 000. 鈥淎lthough the cost of graphite is more than aluminium,
an overall saving can be made due to quicker fabrication times,鈥� says
Knox.
FORT脡 will test technologies designed to pick out the electromagnetic signals
produced by a nuclear explosion from the incessant background of TV and radio
broadcasts that fills the airwaves. By looking for light flashes as well as
radio frequency pulses, FORT脡 should be able to distinguish natural lightning
flashes from the telltale pulse of a nuclear device. The data gathered by the
satellite will also be released to scientists studying the distribution of
lightning around the globe. FORT脡鈥檚 sensors can only cover part of the globe at
any time, but if the mission is successful, similar devices may be built into
the next generation of global positioning satellites to provide a
round-the-clock global watch for nuclear tests.
Late launch
Despite the speedy SNAPSAT construction technique used for FORT脡, the
satellite鈥檚 lift off has been delayed. It was originally scheduled to be
launched this month aboard a Pegasus rocket released from the underside of an
aircraft flying at high altitude. But following two failures last year, Pegasus
launches have been delayed indefinitely so Knox has booked a place for FORT脡
aboard a private launch vehicle operated by the aerospace company Lockheed
Martin. The launcher鈥檚 first flight test last August was a failure too, but a
second test is due shortly. If all goes well, FORT脡 should be in orbit by the
end of this year.
Other SNAPSAT space vehi cles are also coming off the drawing board. The US
Air Force has chosen the technique to build MightySat-1, a military satellite
due to be flown on the space shuttle next year. NASA鈥檚 Goddard Space Flight
Center in Maryland is also considering the technique for an infrared satellite
due to be launched in 1998. The SNAPSAT technique would be particularly well
suited to mass production of identical satellites. Robotic manufacturing
techniques, like those used in the car industry, could help cut costs, says
Krumweide.
The rewards could be huge. Satellite manufacturers are casting hopeful
glances towards plans by several tele communications companies to launch
constellations of satellites to provide global coverage for mobile phone
systems. Each orbiting constellation could comprise dozens of satellites, and
there will be a constant demand for replacements. Krumweide and his colleagues
at COI have high hopes that snap-together craft could prove ideal for the
task.
Flyweight: FORT脡鈥檚 frame weighs only 40 kilograms. The flat, cut-out pieces of
carbon composite material are designed to minimise waste
