Solar Sail Mission Applications and Future Advancement

1. The Second International Symposium On Solar SailingThe New York City College of Technology of the City University of New York, Brooklyn, New York, U.S.A. & Colin McInnes Solar Sail Mission Applications and Future Advancement Malcolm Macdonald

2. Introduction Photons perturb spacecraft by conservation of momentum Solar sailing uses the perturbation to reduce propellant mass Momentum carried by individual photons is extremely small • Requires large reflector to provide a useful momentum transfer Malcolm Macdonald

3. Romanticism Absence of reaction mass makes solar sailing romantic • Romanticism ≠ Realism Proponents have traditionallyseen solar sailing as atechnical nirvana • i.e. the complete solution Difficulty in advancing low TRL concepts often underestimated Malcolm Macdonald Worm Hole Space Art

4. Solar Sail Mission Catalogue Diverse range of mission applications have been proposed Must identify the concepts which are truly enabled • Or, significantly enhanced Enables development of anapplication-pull technologydevelopment roadmap Malcolm Macdonald

5. Solar Sail Mission Catalogue Mission catalogue considers wide range of mission concepts • Allows definition of key characteristics of enabled/enhanced missions Critical missions act as facilitators to later missions Application-pull technology development roadmap is thus established Malcolm Macdonald

6. Mission Categories • Planet-Centred...and other Short Orbit Period Applications • Highly Non-Keplerian Orbits • Inner Solar System Rendezvous • Outer Solar System Rendezvous • Outer Solar System Flyby • Solar Missions • Beyond Neptune Malcolm Macdonald

7. Planet-Centred...and other Short Orbit Period Applications Trajectory design largely restricted to escape manoeuvres • Or, relatively simplistic orbit manoeuvring such as lunar fly-by’s Significant technology demands on the solar sail • Optimal energy gain requires sail be rotated 180 degrees once per orbit and then rapidly reset • Other simplisticorbit manoeuvresrequire similarlyagile sailtechnology Malcolm Macdonald

8. Planet-Centred...and other Short Orbit Period Applications Requirement for an agile sail is a significant disadvantage Two applications identified which don’t require an agile sail • GeoSail and Mercury Sun-Synchronous Orbiter • Use a fixed attitude to independently vary a single orbit parameter creating a non-inertial orbit Malcolm Macdonald

9. Highly Non-Keplerian Orbits Requires small, continuous acceleration in a fixed direction Displace the spacecraft to artificial equilibrium point • A location some distance from a natural libration point Malcolm Macdonald

10. Highly Non-Keplerian Orbits Continuous thrusting lends well to solar sailing romanticism • Two primary applications have been proposed • Polesitter and Geostorm • Use fixed attitude sail to provide continuous thrust Malcolm Macdonald

11. The view from a Polesitter... Approximate UK-DMC FOV Malcolm Macdonald

12. The view from a Polesitter... Approximate Landsat-7 Enhanced Thematic Mapper Plus FOV Malcolm Macdonald

13. The view from a Polesitter... Approximate Deep Space Climate Observatory Scripps-EPIC FOV Malcolm Macdonald

14. Inner Solar System Rendezvous Sample return to the inner planets discussed extensively • Perceived as high-energy and therefore good for solar sailing Low-Thrust rendezvous requires v∞ ≈ 0 at target body Transfer is thus significantly increased • True for bodies which are “easy” to get to, i.e. Mars, Venus • Once captured into a bound orbit typically require an agile sail Malcolm Macdonald

15. Inner Solar System Rendezvous Mars Sample Return • “grab & go” optimal for solar sailing • 5 – 6 year mission v’s ~2 years for equivalent chemical mission Venus Sample Return • Similar to MRS but with increased launch mass sensitivity • 1000 m2 sail for sample return leg offers potential launch cost saving Mercury and Small Body Sample Return • Truly high-energy transfer trajectories • Only low-thrust propulsion systems offer viable mission concepts • Solar sailing offering potential benefits • Small Body missions do not typically require an agile sail Malcolm Macdonald

16. Outer Solar System Rendezvous Low-Thrust rendezvous requires low v∞ at target body • Now even more difficult to “slow-down” with a solar sail • Can use gravity assists at large moons to capture Following capture, all orbit manoeuvres are slow Consider Europa, • Deep inside Jupiter gravity-well • Long duration • Deep inside Jupiter’s intense radiation belts • Significant shielding required Malcolm Macdonald

17. Outer Solar System Flyby Removes requirement for low v∞ at target body • Consider a Jupiter trajectory Malcolm Macdonald

18. Outer Solar System Flyby Jupiter atmospheric probe mission was considered • Chemical propulsion was concluded to still be superior • Due to mass and number of probes required sail was very large • As target moves further from Sun, solar sail propulsion becomes increasingly beneficial • Leading to a peak in benefits for missions beyond Neptune Malcolm Macdonald

19. Solar Missions Ulysses used Jupiter gravity assist to pass over solar poles • Orbit is highly elliptical; pole revisit time of approximately 6 years ESA’s Cosmic Visions mission concept Solar Orbiter • Maximum inclination of order 35 deg using SEP Mid-term sail could deliver spacecraft to solar polar orbit in ~5yrs • SPO is an example of type of high-energy, inner-solar system mission which is enabled by solar sail propulsion Malcolm Macdonald

20. Beyond Neptune Significant benefit to missions beyond Neptune • For either a Kuiper Belt or Interstellar Heliopause mission • Destinations beyond the Heliopause are challenging for solar sailing alone Malcolm Macdonald

21. Key Characteristics Reducing launch mass does not directly reduce mission cost • Launch cost is only reduced if the reduced launch mass allows a smaller launch vehicle to be used • Saving 10 – 20 M€ launch costs is 2 – 4 % total cost reduction • Is that a good cost/risk ratio for the project? • Reduction must be a significant percentage of mission total Can sub-divide all solar sail missions into two classes • Class One • Solar sail is used to reach a high-energy target, after which the sailis jettisoned by the spacecraft • Class Two • Uses continuous thrust to maintain an otherwise unsustainable observation outpost Malcolm Macdonald

22. Key Characteristics Class Two Class One Malcolm Macdonald

23. Key Characteristics Malcolm Macdonald

24. Key Missions GeoSail • Earth-centred, non-inertial orbit • ~40 m square sail, at an assembly loading of ~35 g m-2 • To provide heritage to later missions, the design is required to be more demanding than considered in isolation Solar Polar Orbiter • Close solar mission, rapid polar revist • ~150 m square sail, at an assembly loading of ~8 g m-2 • Sail slew rate of 10 deg per day required Interstellar Heliopause Probe • 200 AU in ~15 – 25 years • ~150+ m disc sail, at an assembly loading of ~2 g m-2 Malcolm Macdonald

25. Application Pull... The culmination of any technology roadmapmust be enabled by previous milestones • IHP requires a sail architecture withlow assembly loading Malcolm Macdonald

26. ...Technology Development Route IKAROS Design point@ 200 m2 & 75 gm-2 Current applications are clustered about the mid to far term Malcolm Macdonald

27. Future Advancement Roadmap Current applications are clustered about the mid to far term • To much risk in attempting to directly jump to, say, SPO Initial flight tests must provide confidence in the technology and a clear path towards some enabling capability • JAXA sounding rocket deployments an excellent example of this • Risk was spread across several tests and led to IKAROS Malcolm Macdonald

28. Future Advancement Roadmap Requirement exists to backfill the roadmap • Either develop new mission concepts, or • Re-engineer the mission concepts and the vision of the future of solar sailing • Removing the gap between near and mid-term applications Malcolm Macdonald

29. Advancement Degree of Difficulty Consider the concept of Advancement Degree of Difficulty • AD2 categorises risk, from 0 – 100 % • Consider system level engineering risk of solar sailing • The programmatic risk of an advanced technology demonstrator is found to be, at best, acceptable • And, dual development approaches should be pursued to increase confidence • The AD2 of solar sailing must be reduced Malcolm Macdonald

30. Future Advancement Roadmap Romanticism ≠ Realism • Spacecraft engineering realism seeks to evolve technology Consider solar sail propulsion as a spectrum of advancement Now Primary propulsion Secondary propulsion Perturbation Requires reaction mass to counteract Attitude control systems Traditional solar sail vision Malcolm Macdonald

31. Future Advancement Roadmap Solar sailing for attitude control is well established • Used on many GEO spacecraft – often called a “trim tab” • Used by Mariner 10 and MESSENGER spacecraft • Used by Hayabusa Malcolm Macdonald

32. Future Advancement Roadmap Solar sailing is a mature technology • Programmatic risk in advanced solar sailing can be reduced by hybridising the propulsion Mariner 10 & MESSENGER both used a small kite • No reason why other inner solar system missions would not similarly benefit • Missions could be primarily SEP, with a secondary sail • Can incrementally balance and then switch this • AD2 is thus significantly reduced Malcolm Macdonald

33. Future Advancement Roadmap Hybridisation of solar sail mission with SEP can significantly enhance the mission Malcolm Macdonald

34. Future Advancement Roadmap Hybridisation of solar sail mission with SEP can significantly enhance the mission • Consider a hybrid sail/SEP Geostorm variant • A 45-m square sail, at an assembly loading of ~45 g m-2 • Storm warning time is doubled! Malcolm Macdonald

35. Future Advancement Roadmap • May not needsails >50 – 100 m IKAROS Design point@ 200 m2 & 75 gm-2 Malcolm Macdonald