Low-thrust trajectory design

1. Low-thrust trajectory design ASEN5050 Astrodynamics Jon Herman

2. Overview • Low-thrust basics • Trajectory design tools • Real world examples • Outlook

3. Low-thrust • Electric propulsion • Solar electric propulsion (SEP) • Nuclear electric propulsion (NEP) • SEP is mature technology, NEP not exactly • Solar sails • Comparatively immature technology • Performance currently low • All very similar from trajectory design stand point

4. Electric Propulsion • Chemical propulsion • Up to ~17 000 000 N • About 4 000 000 000 sheets of paper • Engine runs for minutes • Electric Propulsion • About 0.2 Newton • About 4 sheets of paper • Engine runs for months-years • 10 times as efficient

5. Hall thrusters Exhaust velocity: 10 – 80 km/s Conservation of momentum (University of Tokyo, 2007)

6. Specific impulse Specific impulse: Rocket equation:

7. Rocket equation Dawn SMART-1 LEO/GTO to GEO

8. Why is a higher ISP not always better? (Elvik, 2004)

9. Implications for optimal trajectories • The optimal transfer properly balances • Specific impulse • Spacecraft power • Mission ΔV • Unique optimum for every mission • ΔV no longer a defining parameter!(arguably: ΔV no longer a limiting parameter)

10. Trajectory design

11. Trajectory example • What is difficult about low-thrust? • Trajectory is “continuously” changing • No analytical solutions • Optimal thrust solution only partially intuitive • Specialized, computationally intensive tools required!

12. Example Method Fly by, probe release, etc...(discontinuous state) Backward integration • JPL’s MALTO • Mission Analysis Low Thrust Optimization • Originally: CL-SEP (CATO-Like Solar Electric Propulsion) Match Points Forward integration Small impulsive burns Source: Sims et al., 2006

13. MALTO-type tools • Optimize... • Trajectory • Subject to whatever desired trajectory contraints • Specific impulse (Isp) • Spacecraft power supply • Using solar power • Using constant power (nuclear) • Possible: solar sail size, etc.

14. Strengths • Fast • Robust • Flexible • Optimizes trajectory & spacecraft!

15. Weaknesses • Ideal for simple (two-body) dynamics • Limited to low revolutions (~8 revs) • No problem for interplanetary trajectories • ~Worthless for Earth departures/planetary arrivals

16. Real world applications

17. Dawn (NASA) • Dawn ( 2007 – Present day) • Most powerful Electric Propulsion mission to date • Visiting the giant asteroids Vesta and Ceres

18. Dawn

19. SMART-1 (ESA) • Launched in 2003 to GTO • Transfer to polar lunar orbit • Only Earth ‘escape’ with low-thrust • Propellant Mass / Initial Mass: 23% (18% demonstrated later)

20. SMART-1 (ESA, 1999)

21. Hayabusa (JAXA) • First asteroid sample return (launched 2003) • 4 Ion engines at launch • 1 & two half ion engines upon return

22. Hayabusa end-of-life operation Engine 1 Engine 2 (University of Tokyo, 2007)

23. AEHF-1 (USAF) • GEO communications satellite, launched 2010 • Stuck in transfer orbit (due to propellant line clog) • Mission saved by on-board Hall thrusters (Garza, 2013)

24. Commercial GEO satellites (Bostian et al., 2000)

25. Commercial GEO satellites

26. Commercial GEO satellites (Byers&Dankanich, 2008)

27. Outlook

28. Electric propulsion developments • Boeing • Four GEO satellites, 2 tons each • Capable of launching two-at-a-time on vehicles as small as Falcon9 • Private endeavor • ESA/SES/OHB • Public-Private partnership • One “small-to-medium” GEO satellite • Possibly the second generation spacecraft of the Galileo constellation • NASA • 30kW SEP stage demonstrator (asteroid retrieval?)

29. Conclusion • Electric propulsion rapidly maturing into a common primary propulsion system • This enables entirely new missions concepts, as well as reducing cost of more typical missions • Very capable trajectory design tools exist, but not all desired capability is available or widespread

30. Questions?