The InflateSail 3U CubeSat of SSC (Surrey Space Centre) at the University of Surrey, UK – is one of the technology demonstrators of the QB50 mission. The InflateSail nanosatellite is equipped with a 1 m long inflatable boom and 10 m2 deorbiting sail. InflateSail’s primary goal is to demonstrate the effectiveness of using a drag sail in LEO to increase the rate at which satellites lose altitude and reenter the Earth’s atmosphere. A system like this could be fitted before launch to any satellite bound for LEO, and then used at the end of the satellite’s life to remove it from orbit, and prevent the satellite from being involved in any potentially disastrous debris-creating collisions in the future. 1) 2)
The InflateSail project is being funded by two European Commission FP7 (Framework Program Seven) projects: DEPLOYTECH and QB50.
SSC is leading a team with DLR, Airbus Defence and Space (formerly Astrium SAS), SSTL , RolaTube, Cambridge University, TNO, CCGSS and others — to develop a low cost deorbiting demonstration mission. An improved, inflatable based sail called “InflateSail” is being developed at the University of Surrey with Astrium as the industrial partner as a generic, scalable deorbiting solution for any size/mass of satellite/launch vehicle upper stage and is proposed to be flight-tested as a CubeSat in-orbit demonstrator in the QB50 program. While orbiting in LEO the sail will use drag to decelerate the spacecraft, thus decreasing its altitude and its orbital velocity.
The objective of DEPLOYTECH is to develop 3 specific, useful, robust and innovative large space deployable technologies:
• 3.3 m diameter inflatable sail – Inflatesail – Surrey/Astrium
• 1m x 5 m deployable solar array – RolaTube/Surrey
The aim is to develop these technologies from a current TRL (Technology Readiness Level) of 2-3 to 6-8 within the 3 years of the proposed DEPLOYTECH project.
Figure 1: Illustration of the InflateSail 3U CubeSat with 1 m long inflatable boom and 10 m2 deorbiting sail (image credit: SSC)
The InflateSail nanosatellite is a drag deorbiting demonstration mission, built on a 3U CubeSat platform. In its launch configuration, approximately 1U is dedicated to the bus electronics (EPS, OBC, ADCS, and communications) and the remaining 2U is divided between the inflatable mast and the deorbiting sail payloads.
A single deployable panel opens at one end of the satellite, and the inflation of the boom pushes out the sail deployment mechanism to position it away from the body of the satellite. This “jack-in-the-box” deployment method avoids some of the complexity of a multi-panel opening design, and results in a satellite with solar cells facing in multiple directions. However, this approach requires a more complicated internal structure consisting of very smooth inwards-facing walls, and a linear guide system to allow the top of the inflatable to move inside the satellite structure without twisting or rotating.
The 10 m2 sail deployment mechanism is derived from the system described in Fernandez et al. 5), while the inflatable mast was developed specifically for InflateSail.
Inflatable mast subsystem:
The inflatable mast consists of a cylindrical boom with two end fittings, an inflation system, and facilities to feed wires through the mast to the sail deployment system. An exploded view of the subsystem is shown in Figure 2 and a break-down of all components is presented in Figure 3.
Figure 2: Exploded view of the inflatable boom subsystem in its stowed configuration (image credit: SSC)
• Cylindrical boom: The inflatable cylindrical boom is constructed from an aluminum-polymer laminate , with an internal Mylar bladder to improve airtightness. The 1 m long boom is folded using an origami pattern, and fully stowed occupies merely 63 mm in height (Figure 4).
• End fittings: The end fittings ensure an airtight connection to the boom, and provide a feed-through for the wiring. The bottom end fitting also contains an inlet (2x) for the inflation gas, a pressure sensor, and a solenoid valve. The normally-open valve vents air during launch ascent, and provides controlled venting of the inflation gas after the boom deployment. The pressure sensor monitors the internal boom pressure during inflation.
The end fittings seal the internal bladder and the aluminum-laminate skin separately. This approach was found to improve airtightness, and minimizes loads on the internal bladder. The internal bladder is sealed using two concentric O-rings, and the laminate is held in place by wrapping it around a PTFE (Polytetrafluorethen,Teflon) ring which is compressed using the end plates.
• Inflation system: The N2 inflation gas is stored using CGG (Cool Gas Generators). A dual CGG setup is used for redundancy, and provides the option of topping up the internal pressure in case of unexpected leaks.
Figure 4: The inflatable cylindrical mast in its stowed (left) and fully deployed (right) configuration (image credit: SSC)
The inflatable booms are constructed of a thin aluminum-polymer laminate: the aluminum layers provide structural stiffness after deployment, while the polymer layer increases toughness.
Strain rigidization: The creases introduced during the folding process will affect the mechanical properties of the boom once it is deployed. The residual creases can significantly reduce the effective material stiffness by deforming as a plastic hinge, and the boom strength will decrease due to imperfections which trigger local buckling. In order to recover the stiffness and strength of the inflatable boom, the creases will therefore be removed using strain rigidization. This is achieved by increasing the pressure in the cylinder until the aluminum layer in the skin deforms plastically, thereby permanently flattening the creases (Figure 5).
Figure 5: Residual creases after depressurization from different inflation pressures (10–70 kPa), image credit: SSC
Mechanical properties: The laminate material used for the EM tests consists of three layers: two 14.5 µm aluminum layers sandwich 16 µm of Mylar and adhesive. The thickness of the layers was measured from a micrograph of the cross-section. The mechanical properties of the laminate material directly affect the performance of the inflatable boom in its deployed state. The elastic modulus determines the boom stiffness (although any residual creases will significantly lower the effective modulus and the flexural rigidity affects the boom strength as it buckles locally under compression or bending loads.
Measuring the mechanical properties of the thin laminate films proved to be challenging. The tests were performed using a standard Instron tensile testing machine, and an effective elastic modulus of 15–25 GPa was found for the laminate. This implies a Young’s modulus of 20–35 GPa for the aluminum layers, which is significantly lower than the expected value of 70 GPa. What is more, literature suggest that some thin aluminum foils may exhibit unusually low elastic moduli. The source of the low measured elastic modulus is being investigated, and until the issue has been resolved, a standard value of 70 GPa is assumed for the analysis. The yield stress is more reliably found, and was estimated to be approximately 50 MPa for the laminate material.
The flexural rigidity was measured using the heavy elastica method. By measuring the deflection of a cantilevered strip of laminate under gravity, the flexural rigidity can be estimated. The aluminum-polymer laminate was found to have a flexural rigidity of 5.6 x 10-4 Nm. This allowed the Young’s modulus of the aluminum foil layers to be estimated at 68 GPa, supporting the hypothesis of unidentified error sources in the tensile tests.
Boom folding method:
Several methods for compactly folding inflatable cylindrical booms have been developed. For the InflateSail project the inflatable boom will be folded using an origami pattern. Here the developable cylindrical surface is formed into flat facets separated by fold lines; the facets pop through into the cylindrical form at the end of inflation. A primary reason for the selection of an origami pattern for packing the inflatable boom is the open cross-section in the stowed configuration. This allows for rapid dispersal of the inflation gas from the CGG, and enables fast deployment of the boom. Moreover, origami booms have been shown to combine compact stowage with straight-line deployment.
Fold pattern selection: The family of origami patterns selected for folding the inflatable boom is shown in Figure 6, and is fully defined by its geometric parameters n = 5, j1 = 67º, H=R = 0:67 and R = 90 mm. The repeating layer is therefore 60.3 mm in height, fixing the boom length to integer multiples of this value. In its fully stowed configuration the diameter of the circumscribed circle is 90 mm, and the open cross-section is at least 35 mm in diameter.
Figure 6: The fold pattern selected for the inflatable boom (left), with the cross-sectional view of its fully folded configuration (right), image credit: SSC
The geometric richness of the origami patterns enabled various design trade-offs. The primary selection criterion was the minimization of material deformation during deployment, which was shown to be linked to straightness of the boom deployment. The deployment strains are characterized by the number of circumferential folds n and the reverse fold angle j1. A combination was selected which minimizes material deformation for the initial deployment phase, while providing a second undeformed configuration at approximately 70%deployment.
Decreasing the bay height of the pattern (H/R) reduces the tip rotation during deployment, but will also result in a greater number of folds in the boom membrane. In turn, this increases the packaged height and might impact the mechanical properties of the deployed boom by leaving more residual creases in the boom membrane. In the selected pattern, only four folds meet at each vertex, helping to minimize the risk of pin-hole punctures due to the high local curvatures at the vertices (the inflatable bladder was later added to the design, to ensure airtightness of the inflatable boom). In its stowed configuration, the boom is circumscribed by a 90 mm diameter circle, preventing the folded boom from protruding beyond the end fittings, and the large internal space (≥ 35 mm inscribed circle) accommodates the solenoid valve used to vent the inflation gas after deployment.
Manufacturing process: The origami folding of the inflatable boom is challenging; the limited boom diameter precludes manually supporting the material from the inside, and the aluminum-polymer laminate does not allow reversal of folds. A novel manufacturing method (Figure 7) was therefore developed:
I) First, the cylindrical boom is assembled. The bladder and aluminum-laminate are cut from flat sheets and wrapped around a cylindrical mandrel before being sealed using transfer tape. The end fittings are then attached, taking care to ensure accurate alignment of the fitting so the boom deploys straight from the CubeSat.
II) A stiff plastic sheet is scored with the desired fold pattern, and manually folded so the creases become flexible. The folded plastic sheet is then flattened, wrapped around the pressurized boom, and held in place using masking tape.
III) The pressure inside the cylindrical boom is lowered by opening the solenoid valve, and the pressure is automatically maintained at a set value. The pressure should be high enough to press the laminate material flush against the plastic mandrel, but low enough to allow the boom to be gradually folded, using the plastic mandrel as a guide.
IV) After folding the boom the plastic sheet is removed and the boom compressed is to its stowed dimensions (Figure 8).
Figure 7: Manufacturing and folding process: I) assembly of cylindrical boom, II) the plastic master sheet wrapped around a pressurized boom, III) gradual folding of boom while maintaining a low internal pressure, IV) removal of mandrel to reveal the origami-folded boom (image credit: SSC)
Figure 8: A 105 cm long boom folded to~6 cm (the allowed vertical height for the folded boom in InflateSail), image credit: SSC
In laboratory tests inflation of the deployable mast has been achieved using compressed air. However, the flight model of InflateSail will be inflated using a CGG (Cool Gas Generator) developed by TNO and CGGSS (CGG Safety and Systems BV), both located in The Netherlands. The Cool Gas Generator is an innovative way of storing gas by chemically binding it in a solid propellant. After ignition a self-sustained reaction passes through the grain and releases the gas at ambient temperature. The remainder of the propellant is left behind in the CGG.
Two types of Cool Gas Generator have been tested in space. Four CGGs, each producing 40 liter of nitrogen at 1 bar and 273 K, have been on board the ESA PROBA- 2 satellite since 2009, and eight miniaturized CGGs, each producing 0.15 liter of nitrogen at 1 bar and 273 K, have been flying on board the Delfi-n3XT (Delfi Triple CubeSat NeXt) mission of TU Delft since November 2013. To date, two generators on board of each satellite have been fired with excellent results.
The CGG, designed for InflateSail, is of a completely new design, and produces 3.2 normal liter of nitrogen gas (3.9 gram±5%). One of the main goals has been to avoid a pyrotechnic classification. To this end the CGG is equipped with an innovative resistance wire igniter, developed by TNO. Another innovation is the use of stainless steel as a construction material, instead of titanium (used for the other space qualified CGGs). Stainless steel is easier to machine and has lower material cost, but is also slightly heavier than titanium. Furthermore, the CGG is designed to be modular: its length can be adjusted to decrease or increase the amount of gas produced, without changing the ignition system or the aft part of the CGG with the gas exit. Due to these unproven innovative features, two CGGs are mounted on board InflateSail for redundancy reasons.
The CGG itself is cylindrical with a diameter of 16 mm and a length of 90 mm ,including flying leads (Figure 9). The igniter is mounted on the top, while the gas outlet is at the bottom. The CGGs will be mounted in the satellite by a clamp band. A ring on the circumference of the CGG can be used to fix the CGG inside the clamp band. After the ignition signal is given, the igniter will be powered and after a few seconds the CGG will start releasing gas. The CGG propellant is isolated from the outside atmosphere by means of a breaking foil, which ruptures when sufficient pressure is built up. The burning profile is such that 90% of the gas will be released in about 6 seconds, with 99% within 60 seconds after activation. The InflateSail boom is inflated directly from the CGG, and no further gas flow control is implemented.
The rapid release of inflation gas was an important design driver for the design of the inflatable boom, and the selection of the origami folding method to stow the boom.
Figure 9: Datasheet for the InflateSail CGG (image credit: CGGSS)
To verify the functionality of the InflateSail design, a dimensionally accurate engineering model of the satellite was constructed, and numerous deployment tests have been performed: both in ambient conditions (Figure 10) and in vacuum (Figure 11). These tests were also used to verify the repeatability of the folding process, the airtightness of the deployed boom, the manner of the deployment (twisting or rotation once outside the body of the satellite), and the effect of the seam on the straightness of the deployed boom.
Figure 10: Deployment test of inflatable boom with gravity compensation. The buoyancy of the balloon was adjusted to counterbalance the weight of the top fitting (image credit: SCC, CGGSS)
Figure 11: Deployment test of inflatable boom in vacuum chamber (image credit: SCC, CGGSS)
Feasibility of design: Functional tests have confirmed that it is possible to construct and fold aluminum-polymer laminate booms of 1 m length and compress them to a height of approximately 6 cm (Figure 8) including end fittings. The boom has never been observed to fail to exit the satellite body and deploy to its full length. The walls of the satellite and two linear guides constrain the deploying boom to push almost straight out. The inflation pressure required to push the top of the inflatable out of the satellite has been observed to be relatively small (≤5 kPa). Any slight hindrance to deployment is quickly overcome by the resulting build-up of pressure in the boom. Consider that, at a final inflation pressure of 55 kPa, the inflation gas exerts a force of 350 N on the end fittings.
The InflateSail satellite will consist of a mix of COTS (Commercial-Off-The-Shelf) and custom components. The majority of the electronic components selected are COTS. This minimizes risk, and allowed the focus of development to remain on the payload. The 3U CubeSat is divided into two main sections: approximately 1U is reserved for the avionics/ electronics stack, leaving 2U for the inflatable and sail payloads. 6)
Figure 12: Illustration of the InflateSail nanosatellite prior to sail deployment (image credit: SSC)
Table 1: InflateSail main subsystems and components
Table 2: InflateSail main parameters
Launch: A launch of the QB50 constellation is planned for Q1 2016. The VKI (Von Karman Institute) signed a launch contract for the launch of the 50 QB50 satellites with ACS (Alcantara Cyclone Space). This joint Brazilian/Ukrainian company plans to use the Cyclone (Tsyklon)-4 launch vehicle from the Brazilian Alcantara launch site. The rocket is designed by the Yuzhnoye Design Bureau and manufactured by Yuzhmash, both based in Dnepropetrovsk, Ukraine. 7) 8) 9)
The goal of the QB50 mission is to demonstrate the launch of a network of 50 CubeSats/nanosatellites built by University Teams all over the world as the primary payload on a low cost vehicle to perform first-class science in the largely unexplored lower atmosphere.
Orbit: The majority of the QB50 CubeSats will be placed into a near-circular orbit, altitude = 380 km, inclination = 98º. A smaller group of satellites (including InflateSail) will be deployed into an elliptical orbit of 700 km x 380 km.
Before deploying its inflatable mast and drag sail, InflateSail will be in an orbit in which it could be assumed to remain for several months, or even years, before reentering the atmosphere. Once InflateSail has deployed its inflatable mast and thin polymer drag sail, it will rapidly lose kinetic energy, and reenter the Earth’s atmosphere, causing the satellite to be destroyed. During the reentry process, the support structure of the drag sail will buckle and collapse. The sail will cease to slow the satellite down, and the structure will enter the upper atmosphere with sufficient kinetic energy to ensure disintegration.
Figure 13: The InflateSail nanosatellite in the deployed configuration (image credit: SSC)
The drag sail functions by imparting the satellite’s momentum to the small number of atoms and molecules present in LEO (~100-1000 km altitude) over a long period of time. The number of particles present in the Earth’s upper atmosphere and in LEO fluctuates over time, and is dependent on levels of solar and geomagnetic energy. The results of simulations of InflateSail’s deorbiting process, calculated using the extremes of solar and geomagnetic activity levels, are shown in Figures 14 and 15.
Figure 14: Deorbit time simulations at extreme values of atmospheric density (high density model), image credit: SSC
Figure 15: Deorbit time simulations at extreme values of atmospheric density (low density model), image credit: SSC
1) A. Viquerat, M. Schenk, B. Sanders, V. Lappas, “Inflatable Rigidizable Mast for End-of-Life Deorbiting System,” Proceedings of the13th European Conference on Spacecraft Structures, Materials & Environmental Testing (SSMET), Braunschweig, Germany, April 1-4, 2014, ESA SP-727, URL: http://www.markschenk.com/research/files/SSMET2014-InflateSail.pdf
2) “Space Vehicle Control,” Surrey Space Center, URL: http://www.surrey.ac.uk/ssc/research/space_vehicle_control/deploytech/mission/
3) Martin Hillebrandt, Sebastian Meyer, Martin Zander, Marco Straubel, Christian Hühne, “The boom design of the de-orbit sail satellite,” Proceedings of the13th European Conference on Spacecraft Structures, Materials & Environmental Testing (SSMET), Braunschweig, Germany, April 1-4, 2014, ESA SP-727
4) Sebastian Meyer, Martin Hillebrandt, Marco Straubel, Christian H¨uhne, “Design of the de-orbit sail boom deployment unit,” Proceedings of the13th European Conference on Spacecraft Structures, Materials & Environmental Testing (SSMET), Braunschweig, Germany, April 1-4, 2014, ESA SP-727
5) J.M. Fernandez, M. Schenk, G. Prassinos, V.J. Lappas, S.O. Erb, “Deployment mechanisms of a gossamer satellite deorbiter,” Proceedings of the 15th European Space Mechanisms and Tribology Symposium 2013 (ESMATS 2013), Noordwijk, The Netherlands, URL: http://www.esmats.eu/esmatspapers/pastpapers/pdfs/2013/fernandez.pdf
6) Andrew Viquerat, “InflateSail Specifications Document,” SSC (Surrey Space Centre), September 19, 2014
7) Dan Oltrogge, “QB50 Proposed Deployment ConOps,”Proceedings of the 11th Annual CubeSat Developers’ Workshop - The Edge of Exploration,” San Luis Obispo, CA, USA, April 23-25, 2014, URL: http://www.cubesat.org/images/cubesat/presentations/Developers-Workshop2014/Oltrogge_QB50_Deployment.pdf
9) C. Bernal, “QB50 Deloyment System,” VKI, Brussels, Belgium, January 28, 2013, URL: https://www.qb50.eu/download/workshop/workshop5th/4-5thQBWS29Jan2013-Launch_loads_and_deployment.pdf
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (email@example.com).