The use of composite materials is older then the space race itself which started way back in 1956 when the Russians launched ‘Sputnik’. ‘Bakelite’ components were used in research rockets before that time. However, when we discuss composites in space, we are going far beyond carbon fiber and such organic matrix composites. Spacecraft (and vehicles such as the Mars Lander) also use MMC – metal-matrix composites. This latter group are not alloys in the conventional sense – they are composites in the sense that the individual metals are discrete in the matrix – heterogeneous as opposed to homogeneous. There are ceramic-based composites under development too, so the field of composites in space is vast and growing.
Space is very demanding, with zero pressure (gas voids in a composite could explode – ‘outgassing’) and huge thermal cycling in earth orbit requiring very low thermal expansion of materials. In earth orbit, temperatures may range through 250 degrees centigrade as a craft moves from the sun into the earth’s shadow.
To those needs, add in high exposure to radiation and re-entry temperatures of up to 1500 degrees centigrade. Of course, if a craft is going beyind the earth to drop through the atmosphere of a very cold planet then even lower temperatures have to be expected. Ultraviolet radiation increases aging rates, as does atomic oxygen.
Another requirement is stiffness – measured as the natural frequency of a structure (almost like the ringing of a bell). Typically, these natural frequencies have to be as low as 80 Hz to prevent breakup during liftoff.
Needless to say, composites materials in space have intense requirements.
Advantages of Composites in Space
Besides being able to satisfy the unique requirements of space, there are other major benefits. The combination of high strength and low weight makes composites attractive for use in space because of the high cost of hoisting payload into orbit. Every pound of mass costs anywhere from $8,000 for low earth orbit, to almost $100,000 when geostationary orbits are targeted (24,000 miles out).
With organic-matrix composites such as carbon fiber, the ability to build up monococque structures using 3 dimensional weaving (‘spinning’) offers tremendously high strength:weight ratios with combined with structural design efficiency.
With carbon fiber and to eliminate the risk of voids in a structure, pressure impregnation is necessary.
Testing requirements are very stringent and complex, and production scale construction is some way off, though prefabricated joints (‘Pi’) joints are now being produced in volume using spinning.
There are currently upwards of a dozen different composites in use in space, each with specific properties for its purpose.
A high percentage of all spacecraft, are built with composites – the return on reducing mass is exponential because of the fuel factor and supporting systems.
Conventional type composites are typically used on:
- Bus structures
- Solar panels
- Tubes and trusses
Antennae use metal matrix composites and regular composites too, in combination. ‘Self Lubricating’ composites based on PTFE and glass fiber are now being used for ball-bearing cages.
‘Rollable’ carbon fiber structures are being developed – long booms can be ‘rolled up’ for transfer into orbit, but when unrolled they lock into shape.
Future of Composites in Space
The range of composites for space applications continues to grow dramatically.
‘Hot structures’ such as nozzles and thrust steering vanes have traditionally been the province of metals, but now ceramic matrix composites are being developed. These promise to deliver the requirements for such components but with valuable mass savings.
The use of composites is directly proportional to the amount of ‘space’ activity; although the US has reined in its efforts following the end of the Cold War, the emergence of China and India as space powers maintains the level of research into new composites – both the binders and the reinforcements.
There are exciting prospects of special composites being ‘grown’ by chemical deposition in a zero gravity environment, providing significantly improved strength:weight ratios and three-dimensional mechanical properties as yet unobtainable on earth.
New production techniques are being researched for composite component production. ‘Pultrusion’ produces finished composite parts as a continuous stream. It is highly automated, much like metal extrusion and will reduce the cost of ‘volume’ components significantly.
So, space continues to drive technology – especially in composites – as it has done since the days of Chuck Yeager and the ‘X’ Rocket program which led directly to space flight.