Trans. JSASS Aerospace Tech. Japan Vol. 8, No. ists27, pp. To_4_25-To_4_31, 2010 Topics

First Solar Power Sail Demonstration by IKAROS

1) 1) 1) 1) 1) By Osamu MORI , Hirotaka SAWADA , Ryu FUNASE , Mutsuko MORIMOTO , Tatsuya ENDO , 1) 1) 1) 1) Takayuki YAMAMOTO , Yuichi TSUDA , Yasuhiro KAWAKATSU , Jun’ichiro KAWAGUCHI , 2) 3) Yasuyuki MIYAZAKI , Yoji SHIRASAWA and IKAROS Demonstration Team and Solar Sail Working Group

1)JAXA Space Exploration Center, Japan Aerospace Exploration Agency, Sagamihara, Japan 2)Department of Aerospace Engineering, Nihon University, Funabashi, Japan 3)Department of Aeronautics and Astronautics, The University of Tokyo, Tokyo, Japan (Received August 18th, 2009)

The Japan Aerospace Exploration Agency (JAXA) will make the world’s first solar power sail craft demonstration of photon propulsion and thin film solar power generation during its interplanetary cruise by IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun). The spacecraft deploys and spans a membrane of 20 meters in diameter taking the advantage of the spin centrifugal force. The spacecraft weighs approximately 310kg, launched together with the agency’s Venus Climate Orbiter, AKATSUKI in May 2010. This will be the first actual solar sail flying an interplanetary voyage.

Key Words: Solar Power Sail, Membrane, Deployment System

1. Introduction the large membrane, the power generation by the thin film solar cells, the acceleration verification and the navigation The Japan Aerospace Exploration Agency (JAXA) will technology acquisition by the solar sail. The proposal was make the world’s first solar power sail craft demonstration of endorsed in July 2007 and is now ready for the fabrication photon propulsion and thin film solar power generation during toward the launch in May 2010 together with the Venus its interplanetary cruise by IKAROS (Interplanetary Kite-craft Climate Orbiter, AKATSUKI on H-IIA. Accelerated by Radiation Of the Sun) as shown in Fig. 1. The Solar sail missions have been studied in the world 3,4), spacecraft deploys and spans a membrane of 20 meters in however, they are not achieved yet. IKAROS will demonstrate diameter taking the advantage of the spin centrifugal force. not only the solar sail but also the solar power sail. The solar The spacecraft weighs approximately 310kg, launched power sails can save fuel by the solar sail and supply together with the agency’s Venus Climate Orbiter, sufficient electricity from the thin film solar cells. JAXA will AKATSUKI in May 2010. Both spacecraft are boosted by the lead the future solar system exploration by solar power sails. H-IIA vehicle directly into their cruise orbit bound for Venus. The thin film solar cells attached to the sail are lighter than This demonstrator attempts to deploy thin film solar cells on solar cells normally used for spacecrafts, and easier to handle. the membrane, in order to evaluate its thermal control Our missions will lead to lower cost in the solar cells market, property and anti-radiation performance in the real operational whose growth is a key factor for global warming prevention. field. The sail spacecraft steers its orientation in time-to-time Those low-cost solar cells are also the foundation of future to demonstrate photon acceleration in accordance with the solar power satellite systems. guidance strategy in cruise flight to Venus. This will be the This paper introduces the extended solar power sail and first actual solar sail flying an interplanetary voyage. IKAROS missions and presents the description of IKAROS JAXA has studied an extended solar power sail craft toward spacecraft. the outer solar system via hybrid electric photon propulsion 1) since 2001. It uses fuel-efficient ion engines along with the solar sail, flying to Jupiter and Trojan asteroids as shown in Thin film 20m solar cell Fig. 2. It generates electricity using the thin film solar cells on the sail. It is defined as an engineering technology demonstrator, similar to the HAYABUSA (MUSES-C) that is Membrane currently flying back to the earth as the world’s first sample-return attempt 2). The mission proposal passed the Mission Definition Review in 2005 and now is eligible to go into the pre-project phase (Phase-A). Using spin centrifugal force However a stepping stone to the boost of the technology readiness level toward the extended solar power sail craft is required. The project team applied first for the small technology demonstrator mission, IKAROS as a Front- Loading of new key technical issues of the extended solar power sail craft. The mission has four parts: the deployment of Fig. 1. IKAROS mission.

Copyright© 2010 by the Japan Society for Aeronautical and Space Sciences and ISTS. All rights reserved.

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remarkable. This will reveal the fundamental questions as to the extraordinary young stars observed only in deep IR region. Furthermore, this single spacecraft carries a Jovian orbiter and an atmospheric reentry probe, both of which will constitute a spacious and simultaneous magnetosphere measurement at the Jovian polar region, via a formation flight. This will be the first attempt ever in the long solar planetary exploration

history. Fig. 2. Extended solar power sail mission. JAXA has been seriously investigating the spacecraft

development and the mission proposal passed the Mission 2. Extended Solar Power Sail Mission to Jupiter and Definition Review in 2005. It now puts a technology Trojan Asteroids demonstrator in 2010.

The extended solar power sail craft which is shown in Fig. 3 uses the world’s first hybrid photon / ion propulsions taking the advantage of thin film photo-voltaic technology. The mission has very new multi-purposes: First of all, the mission aims at the exploration of the Trojan asteroids for the first time in the world. It is the first spacecraft to the Jupiter’s distance powered only by solar cells as shown in Fig. 4. Utilizing the power surplus available at the Earth distance, the spacecraft is supposed to drive its ultra-high specific impulse ion engines aboard with the combination of the Earth gravity assist. The intended specific impulse will be 10,000 seconds, almost as 3.3 times efficient as existing Fig. 3. Extended solar power sail craft system. contemporary ion engines. Here are summarized major characteristics of the extended solar power sail mission; It is the 1. World’s First Solar Powered Jovian Explorer, 2. World’s First Combined Jovian Orbiter / Flyby Mission, Jupiter 3. World’s Highest Performance Ion Engines, 2nd Swing-by 4. World’s First Photon/Electric Hybrid Sail Propulsion, Spacecraft 5. World’s First Background Emission Mapping, 6. World’s First Access to Trojan Asteroids, 7. World’s First Formation Flight in Jovian Magnetosphere. Sun Earth The proposed extended solar power sail craft is innovative Launch as it carries the following new technology demonstrations. 1. A Large Membrane Space Structure including Deployment strategy, 2. A Hybrid Propulsion using both Photon and Ultra High Fig. 4. Extended solar power sail mission to Jupiter. Performance Ion Engines,

3. Thin Film Solar Cells, 3. Small Solar Power Sail Demonstrator (IKAROS) 4. Reaction Control at very Low Temperature,

5. An Integrated Propulsion/Power System using Fuel Cells. The small solar power sail demonstrator, IKAROS is a 6. Formation Flight in Jovian system, stepping stone to the boost of the technology readiness level 7. Electric Delta-VEGA (Delta-V Earth Gravity Assist) toward the extended solar power sail craft. The spacecraft technique for outer solar system, spans a square membrane sail whose tip-to-tip length is 20 8. Ultra Stable Oscillator for 1-way range or VLBI (Very meters long, and weighs about 310kg. As its name infers, the Long Baseline Interferometry) orbit determination, sail spanned includes thin film solar cells which occupy 9. Ka-band Communication for Interplanetary Missions, approximately 5% of the total area. It is now ready for the 10. Radiation-Resistant Technology for Jovian Orbiter, fabrication toward the launch in May 2010 together with the It is full of new technologies requisite for future solar system Venus Climate Orbiter, AKATSUKI on H-IIA. voyages. The success criteria of the IKAROS mission are Not only the technology demonstration, in addition to the summarized as follows; Trojan asteroid exploration, there are more new innovative (1) Deployment of Large Membrane Sail scientific purposes carried by this spacecraft. A background -Deployment and Expansion of a Large Membrane in space emission mapping excluding ecliptic dust cloud, which is using similar mechanical device and procedures to those in the cleared beyond four AU distance from the Sun, is particularly Extended Solar Power Sail craft.

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-Obtaining a number of data indicating the expansion status of The demonstration flight assumes an operation scenario the membrane. described in the following figure, Fig. 6. (2) Generating Electricity by Thin Film Solar Cells In IKAROS mission, not only the deployment of the sail, -Demonstrating Solar Power from Thin Film Solar Cells. but also steering capability shall be demonstrated. The attitude -Obtaining a number of data to evaluate the efficiency of Thin requirement mainly attributes to radio communication Film Solar Cells on the membrane. constraints for LGA (Low Gain Antenna), while the attitude These two items belong to the Minimum Success Criteria to capability strongly relies on the Sun angle as shown in Fig. 7. achieve in several weeks. The flight period to Venus is only half a year, during which (3) Demonstrating Photon Propulsion IKAROS demonstrator is supposed to perform the above -Verification and Comparison of Reflectance with Diffuse & mentioned flight sequences. Fig. 8 shows a steering plan. The Specular Property. -Measurement of overall Reflectance with a rigorous relation horizontal axis shows the flight time, and the vertical axis examination of the temperature and surface status. shows the sun angle θ1. The sun angle is restricted to be in the (4) Demonstrating Guidance, Navigation Control Skills for red zone in order to generate power and demonstrate photon

Solar Sail Propulsion propulsion. The contour line shows the earth angle θ2. In the -Navigation / Orbit Determination under continuous and small white zone, communication interference with the membrane is acceleration. occurred and IKAROS needs to be controlled automatically. -Acceleration Direction Control by Steering via appropriate attitude control means. The steering plan minimizes the interference period. The solar These latter two items belong to the Full Success Criteria to radiation acceleration is about 100m/s in half a year. achieve in six months. Reorientation and deployment operation are conducted with IKAROS is stowed inside a PAF (Payload Attach Fitting) auxiliary solar cells atop the hub bus portion and RCS of the H-IIA vehicle when it launches a Venus Climate (Reaction Control System) thrusters, and they are not totally Orbiter, AKATSUKI in 2010 as shown in Fig. 5. dependent on the thin film solar cells which are part of the

mission to be demonstrated.

Venus

Rocket Fairing Full success (in six months)

Minimum success (in several weeks) Earth

AKATSUKI 5) Guidance, navigation and control by solar sail

4) Photon propulsion by solar sail PAF900M IKAROS 1) H-IIA Launch 3) Membrane deployment experiment Sun-pointing 2) Radio telemetry ON Initial operation check Spinning-down (1rpm) Piggyback 4 Piggyback 1-3 Spin separation (5rpm) Spinning-up (25rpm) Solar power generation by thin film solar cells Raising Adapter Fig. 6. Nominal operation sequence of IKAROS. PAF1194M

Spin direction Sun angle θ1 (Z axis)

Earth angle θ2 θ1 LGA1 θ2 (+Z axis)

Membrane

LGA2 (1) When stowed (2) Piggyback 1-3 (3) AKATSUKI Communication interference Separation Separation (-Z axis) with membrane ±30 deg

Condition for power supply and acceleration |θ1|<45 deg

(using LGA1) |θ2|< 60 deg Condition for communication θ (using LGA2) 120<| 2|<180 deg (4) PAF900M (5) IKAROS (6) Piggyback 4 Fig. 7. Conditions of sun and earth angles. Separation Separation Separation Fig. 5. Launch and separation of IKAROS.

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A half year becomes lighter and stronger and can decrease the outgassing. Earth angle (contour line) Aluminum is vapor deposited on front side of the membrane in order to reflect sunlight more efficiently. In addition, the Using LGA2 (green zone) membrane is reinforced in such a way as to prevent it from +45deg splitting all the way if it's ripped. If the membrane is torn, its performance will decline slightly, but it can still continue its Communication Interference space travels. with membrane (white zone) Sun angleSun [deg] Ꮕ䈚ᷰ䈚 20 m -45deg 20m ᆫSteering൓೙ᓮ䊂䊋 䉟device䉴 Trapezoid petal Using LGA1 ⭯⤑ᄥ㓁㔚ᳰ (blue zone) Thin film Solar cells

Fight time [day] PVDFDust䉻 䉴counter䊃䉦䉡䊮䉺 (back side) Fig. 8. Steering plan. Tether

Harness䊁䉱䊷 4. IKAROS Spacecraft Description 䊑Bridge䊥䉾䉳 IKAROS spacecraft is a spinner and its shape is cylindrical. It carries a drum around which a membrane is wound to be వ┵䊙䉴 unfolded via a special mechanics aboard. Whole spacecraft Tip mass view is presented below. Fig. 10. Sail shape and equipment layout. Where sail membrane is wound is Mission portion that is independent of the bus hub portion as shown in Fig. 9. The spacecraft design started from fall of 2007, just 2.5 years prior to the launch and the design process intentionally adopted No-EM (Engineering Model) development strategy taking the advantage of relatively sufficient mass margin indicated from the launch vehicle. Besides, the existing hardware surplus as well as reproduction of existing design is fully exploited to Fig. 11. Trapezoid petal. shorten the development period.

This paper introduces the major development component: sail and deployment system.

SAP/Upper deck

Mission module Membrane Thrust tube Middle deck Lower deck Vehicle I/F ring Polyimide2 Fig. 9. IKAROS spacecraft configuration.

Polyimide1 4.1. Sail The sail shape and equipment layout are shown in Fig. 10.

Membrane: The shape of the membrane is a square whose diagonal distance is 20m. It consists of four trapezoid petals as Polyimide1 Polyimide2 APICAL-AH7.5 ISAS-TPI (Thermoplastic Polyimide) shown in Fig. 11. The direction of folded lines is Material PMDA/4,4'ODA (Kaneka) A-ODPA/4,4'-ODA (ISAS) perpendicular to the direction of the centrifugal force. It is made of two kinds of polyimide resin whose thickness is only 7.5μm. Both of them have a high tolerance for space Chemical formula environment requirement. Polyimide1 is an existing film and it needs to be connected to each other by adhesive. On the Elasticity 3.8 GPa 3.2 GPa Breaking strength 263 Mpa 132 Mpa other hand, polyimide2 is thermoplastic polyimide which is Breaking elongation 74% 90% developed and manufactured by JAXA newly for the extended Thickness 7.5-8.5 μ m 7.5-8.5 μ m 2 2 solar power sail mission as shown in Fig. 12. It can be fused Area 154.28 m (88.9% of the membrane) 19.35 m (11.1% of the membrane) Weight 1.643 kg 0.206 kg with each other by heater control without adhesive. If the Al deposition 80 nm 80 nm< membrane is made of thermoplastic polyimide, the membrane Fig. 12. Membrane material.

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as well as shape maintenance during the cruise phase. This deployment method can be realized with simpler and lighter mechanisms than conventional mast or boom types as it does not require rigid structural elements. This method can be applied to large solar sails with a diameter of 50 to 100m. Polyimide2 (a-ODPA-PI) The proposed deployment method of the sail is shown in Fig. 12. Membrane material (continued). Fig. 15. It consists of two stages. In the folded configuration, each petal is line-shaped and rolled up around the main body.

In the first stage, the rolled petals are extracted like a Yo-Yo Thin film solar cells: a-Si (amorphous silicon) solar cells are despinner, and form a cross shape. The shape is maintained by attached to certain areas of the membrane. They generate stoppers. In the second stage, the stoppers are released and almost 300W. The area ratio is 5%. The multilayer of the film each petal expands to form a square shape. If the first stage of prevents solar cells from curling because of symmetric the deployment is performed dynamically, each petal will be mechanism as shown in Fig. 13. It also shields solar cells from twisted around the main body just after the deployment. radiation. Therefore, the first stage of the deployment needs to be Steering device: Variable reflectance elements are loaded near performed statically. On the other hand, the second stage of the tips of the membrane. They can be switch diffuse and the deployment can be performed dynamically as shown in specular reflection by power ON and OFF to control the sun Fig. 16. angle as shown in Fig. 14. The deployment sequence is defined as follows: Dust counter: PVDF (PolyVinylidene DiFluoride) film is 1) Separation from rocket with slow spin (5rpm) attached on the back side as dust counter. It is optional 2) Spin down using RCS (5rpm -> 2rpm) equipment. 3) Tip mass separation Tip mass: A 0.5kg weight is attached to each tip of the 4) Spin up using RCS (2rpm -> 25rpm) membrane. It supports the deployment of the membrane. 5) First stage of the deployment (25rpm -> 6rpm) Tether and Harness: The membrane is connected to the main 6) Second stage of the deployment (6rpm -> 2.5rpm) body by tethers and harnesses mechanically and electrically. 7) Spin down using RCS (2.5rpm -> 1rpm)

8) Control of spin direction and rate The spin rate is decreased in the first and second stages of the deployment, because the inertial momentum of the sail is increased.

First stage deployment Folded configuration 7.5(Polyimide2) 30~40 (adhesive) 25(a-Si) 30~40 (adhesive) 25(a-Si) Four petals are rolled up Tip masses are released. Rolled petals are extracted 30~40(adhesive) and tip masses are fixed. like a Yo-Yo despinner. 25(Polyimide1) Cross shape Second stage deployment Sum 170~200 [μm] Fig. 13. Thin film solar cells (a-Si).

Shape is maintained Four petals expand to Stoppers are released. (Sun light) by stoppers form a square shape Incident ray Reflected light Incident ray Reflected light Fig. 15. Deployment method.

Power ON Power OFF (Diffuse reflection) (Specular reflection) First stage deployment Second stage deployment (Torque) F2 F1 (statically) (dynamically) Solar pressure F2 < F1

Fig. 14. Steering device. (1) (2) (3)

4.2. Deployment system Several kinds of deployment methods 5,6) have been Mechanism of fix and investigated in the world, and JAXA has studied the spinning Mechanism of relative 7,8) release of tip masses Mechanism of release type . The membrane is deployed, and kept flat, by its rotation of stoppers of stoppers spinning motion. The centrifugal force is used for the Fig. 16. Deployment mechanism. membrane deployment in the initial sequence after the launch

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Fig. 17 shows the mechanism to wind the sail onto deployment system uniformly. To verify that the membrane can be deployed safely, ground experiments and numerical simulations are performed. The ground experiments shown in Fig. 18 confirm that the membrane is deployed smoothly by the deployment mechanism. The multi-particle model and the finite element method model 9) shown in Fig. 19 are used mainly to predict the deployment dynamics. The multi-particle model is suitable for macro dynamics analysis and used for comprehensive analysis. On the other hand, the finite element method model is suitable for macro and micro dynamics (stress, crease) analysis and used for sensitive analysis to validate and define (a) Pictures of first stage of the deployment motion the range in application of the multi-particle model. by multi-particle model

(b) Pictures of second stage of the deployment motion by multi-particle model

Fig. 17. Winding mechanism. (c) Expansion state by finite element method model Fig. 19. Numerical simulations.

5. Conclusion

In this paper, it is reported that the Japan Aerospace Exploration Agency (JAXA) will launch the world’s first solar power sail craft, IKAROS in 2010 together with its Venus Climate Orbiter, AKATSUKI. It demonstrates both photon propulsion and thin film solar power generation during its (a) The first stage of the deployment interplanetary cruise. This demonstrator attempts to deploy thin film solar cells on the membrane, in order to evaluate its thermal control property and anti-radiation performance in the real operational field. The sail spacecraft steers its orientation in time-to-time to demonstrate photon acceleration in accordance with the guidance strategy. This paper also introduces the extended solar power sail mission and its relationship with the IKAROS mission. The IKAROS flight hardware currently undergoes a (b) The second stage of the deployment. fabrication process. This will be the first actual solar sail Fig. 18. Ground experiments. flying an interplanetary voyage.

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References

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