Large deployable structures are a critical part of a number of space structures and systems, such as large reflectors, Earth observation antennas, radiators, sun shields and solar arrays. The original InflateSail proposal was for a large drag sail based on a gas generator system, and ultra thin inflatable booms for structural support. The intention was to build a drag sail that would fit in a 3 unit (3U) CubeSat (satellite platform/nanosatellite) when stowed. The standard 3U structure size is 10x10x34 cm.
For more information on space sails, please visit the "sails in space" section on our CubeSail webpage.
Balloons in space
Inflatable structures make an attractive option for space applications because they can be lightweight, can be stowed in a relatively small volume, and can be expanded to fill a relatively large volume. The load-bearing capacity of inflatable structures is not typically particularly high, but this is often of little consequence in the space environment where the forces acting on a spacecraft are much smaller than those experienced in day-to-day life on Earth.
Inflatesail is primarily intended as a satellite de-orbiting device. Its purpose is to provide a viable means of removing satellites from LEO-MEO once the satellites have reached the end of their service lives. Satellites orbiting in the thermosphere experience tiny amounts of drag as they collide with the small numbers of atmospheric molecules present at very high altitudes. The number of atmospheric molecules present in the thermosphere varies greatly under the influence of temperature and solar activity fluctuations. Over time, satellites impart enough energy to the molecules they encounter to cause them to re-enter; burning up in the process. The slow process of momentum loss, leading to re-entry, can take many years. Decommissioned satellites in orbit can pose a threat to both manned and unmanned spacecraft.
By increasing the projected frontal area of the satellite, Inflatesail will accelerate the process of momentum transfer from the satellite to the thin atmosphere, dramatically reducing the time a decommissioned spacecraft spends in orbit.
Inflatesail will consist of a series of inflatable booms, which act as a supporting truss for a very thin sail, or membrane.
The source of inflation gas for Inflatesail will be small devices called Cool Gas Generators, provided by TNO and CGG Technologies. The gas (most likely Nitrogen) is stored in a solid state, before being released on cue in an uncontrolled decomposition which delivers a specific quantity of gas.
Eventually, the gas used to inflate the structure will escape. Reasons for loss of pressure include small pinholes introduced into the structure's membrane during folding and storage, and impacts with small pieces of debris and micro-meteors. Once internal pressure has been lost, it is important that the structure maintain its shape in order that it may continue to serve its purpose.
Some proposed methods for performing rigidisation in space are:
- Mechanical rigidisation: strain-hardening/yielding of aluminium-polymer laminates
- Physical rigidisation: shape memory alloys, solvent evaporation outgassing
- Chemical rigidisation: thermal/UV curing, gas catalysed polymerisation
Of these methods, only mechanical rigidisation has been demonstrated in space, and so appears to be the most promising option for Inflatesail. The process of material yield rigidisation involves inflating a polymer-metal laminate such that its ductile metal component is strained just beyond the point of plastic deformation. This removes imperfections in the previously folded membrane, and increases structural rigidity once internal pressure has been lost.