Low-thrust trajectory design ASEN5050 Astrodynamics Jon Herman

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

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

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

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

Specific impulse

Rocket equation LEO/GTO to GEO SMART-1 Dawn

Why is a higher I SP not always better? (Elvik, 2004)

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)

Trajectory design

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!

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

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.

Strengths Fast Robust Flexible Optimizes trajectory & spacecraft!

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

Real world applications

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

Dawn

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)

SMART-1 (ESA, 1999)

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

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

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)

Commercial GEO satellites (Bostian et al., 2000)

Commercial GEO satellites

(Byers&Dankanich, 2008)

Outlook

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?)

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

Questions?