InflateSail is primarily intended as a satellite de-orbiting device. Its purpose is to provide a viable means of removing satellites from orbit once the satellites have reached the end of their service lives.
The InflateSail is an inflatable drag sail. It was one of three specific, useful, robust and innovative large space deployable technologies developed as part of the Large Deployable Technologies for Space project (DEPLOYTECH). The other technologies developed during the project were a deployable solar array of dimensions 1 x 5 metre and solar sail booms of maximum length 14 metres.
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 three unit (3U) CubeSat (satellite platform/nanosatellite) when stowed. The standard 3U structure size is 10x10x34 cm. Find out more about sails in space.
Folding space sails
Inflatable structures make an attractive option for space applications because they can be lightweight due to their incredibly thin sail membranes, can be stowed in a relatively small volume, and can be expanded to fill a relatively large volume. To efficiently package sails, engineers have drawn inspiration from a variety of sources, including nature and the art form of origami. Find out more about folding solar sails.
Why it is needed
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 atmospheric entry; burning up in the process. The slow process of momentum loss, leading to atmospheric 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. It 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.
- Fernandez, J.; Viquerat, A.; Lappas, V. & Daton-Lovett, A., Bistable Over the Whole Length (BOWL) CFRP Booms for Solar Sails, 3rd International Symposium on Solar Sailing, 11-13th June, Glasgow, Scotland, 2013
- Johnson, L.; Russell, T.; Young, R. & Heaton,A., Reduction of Martian Sample Return Mission Launch Mass with Solar Sail Propulsion, 3rd International Symposium on Solar Sailing. 11-13th June, Glasgow, Scotland., 2013
- Lappas, V.; Fernandez, J.; Visagie, L.; Stohlman, O.; Viquerat, A.; Prassinos, G.; Theodorou, T. & Schenk, M., Demonstrator Flight Missions at the Surrey Space Centre involving Gossamer Sails, 3rd International Symposium on Solar Sailing. 11-13th June, Glasgow, Scotland., 2013
- Russell, T., DEPLOYTECH: Mars sample return concept study delta results, National Aeronautics and Space Administration, Technical Report, 2012
- Schenk, M.; Kerr, S.; Smyth, A. & Guest, S., Inflatable Cylinders for Deployable Space Structures, Proceedings of the First Conference Transformables 2013, 18-20th September 2013, Seville, Spain., 2013
- Schenk, M.; Viquerat, A.; Seffen, K. & Guest, S., SUBMITTED: Review of Infatable Booms for Deployable Space Structures: Packing and Rigidization, ASME Journal of Spacecraft and Rockets, 2013
- Secheli, G. & McKenzie, G., ACCEPTED: Design and evaluation of inflatable structural concepts for aerodynamic drag augmentation, 64th International Astronautical Congress. 23-27th September, Beijing, China., 2013
- Straubel, M.; Zander, M. & Hühne, C., Design and Sizing of the GOSSAMER Boom Deployment Concept, 3rd International Symposium on Solar Sailing. 11-13th June, Glasgow,Scotland., 2013
- Viquerat, A., DeployTech: Large deployable technologies for space, 2nd FP7 Space Research Conference. Larnaca, Cyprus, November, 2012
- Viquerat, A.; Schenk, M. & Lappas, V., DEPLOYTECH: nano-satellite testbeds for gossamer technologies, 54th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, 2013.
- June 2013: M16 progress meeting at Braunschweig
- February 2013: M12 progress meeting at the Athena Space Programmes Unit
- November 2012: FP7 Space Conference held in Larnaca, Cyprus. Download the DEPLOYTECH presentation
- September 2012: M6 progress meeting held at the Surrey Space Centre
- September 2012: InflateSail design meeting held at the Surrey Space Centre
- July 2012: Publication of EC Framework 7 4th Call projects. Download the DEPLOYTECH section
- February 2012: DEPLOYTECH kick-off meeting held at the Surrey Space Centre
- December 2011: Approval of grant for DEPLOYTECH project.
Large Deployable Technologies for Space project
The main concern of the Large Deployable Technologies for Space (DEPLOYTECH) project, was to increase the technology readiness level (TRL) of the three deployable space structure concepts in order to address the need for greater competence in manufacturing and testing capabilities in Europe.
The technologies that were developed and flight qualified as part of the project were:
- InflateSail, an inflatable drag sail
- A deployable solar array of dimensions 1m x 5m (using bistable reeled composite booms for deployment)
- A type of deployable solar sail boom that was designed by DLR.
The solar sail boom was made of carbon fibre reinforced plastic (CFRP), and consisted of two co-bonded half shells to give a closed cross section.
The three concepts involved in this project were further developments of existing technologies, rather than technologies that had to be developed from scratch. The effort needed to raise the TRL of deployable CFRP booms, inflatable structures and deployable solar arrays was considered to be achievable and realistic. The applicability of these concepts was also in line with the current demand for space deployable structures and each concept fulfils an existing and growing need.
The availability of upcoming flight opportunities added an exciting dimension to the project. In order to raise the TRL's to maximum level, demonstration of operation in space was a necessity. The timing of these concepts and the targeted flight opportunities were ideally placed to achieve this.
The DEPLOYTECH project involved eight European partner institutions, and was assisted by NASA Marshall Space Flight Center (as an external consulting partner). The project was funded by the European Commission through the Seventh Framework Programme (FP7).
As part of their involvement in DeployTech, NASA Marshall Space Flight Center has compiled an interactive document (PDF) detailing many of the large deployable technologies created to date.
One of the boom designs under consideration for the DEPLOYTECH project is the Conical Boom concept. In their deployed state, these booms are thin-walled, slightly tapered cylinders. The taper allows the cylinder to be folded inside-out repeatedly without difficulty. The internal folding greatly shortens the boom, allowing it to be stored in a small space. When inflation gas is introduced into the base of the folded boom, skin friction causes the boom to deploy in a controlled manner.
Previous inflatable structure based missions
Inflatable space structures have been of interest in the United States at least since the early 1960's. The number of space missions featuring inflatable structural components is small, despite a large quantity of research having been performed in support of inflatable space structures as a concept.
The early spherical inflatable satellites were experiments in satellite triangulation and passive communication. Their large, metal-coated surfaces were reflective to a broad range of frequencies, and were often clearly visible in the night sky. These early missions illustrate the types of applications for which inflatables are well suited: large, lightweight, and with low structural loads.
The significant challenges involved in producing a reliably deployable inflatable structure limit the range of practical geometries which can be employed. This is reflected in the types of structures which have been launched to date. Specifically, deployed shapes are usually limited to spheres and cylinders (a notable exception being the lenticular inflatable reflector of L'Garde's Inflatable Antenna Experiment).
Just like a regular balloon, the inflation gas used to drive inflatable structure deployment eventually escapes. When it does, the structure will lose its rigidity unless its skin is rigidizable. Some of the inflatables launched to date have been rigidizable, while others have not. A great many methods of rigidization have been proposed, but only two have actually flown: stretched metal laminates, and Sub-Tg (glass transition) resins.
Metal laminate skins consist of very thin layers of metal foil (usually aluminium) bonded to similarly thin layers of polymer. Once a metal laminate inflatable structure has been deployed, it is slightly over-inflated, causing the metal in the skin to strain harden and the surface to become very smooth. The resulting structure remains rigid after the escape of the inflation gas. Missions making use of stretched metal laminate rigidisation include Echo II and Explorer IX and XIX.
Glass transition resins are used in combination with high tenacity fibres to produce a composite which is flexible above a certain temperature, but rigid below its glass transition point. Inflatable skins made of composites like these are heated before deployment, and allowed to cool in space once deployment is complete, resulting in a rigid structure. Missions making use of glass transition resins include the Cibola Flight Experiment, and the RIGEX shuttle experiment.