Presentation Format Guidelines for the Aerospace Corporation

Smart Propellant (SSC10-X-1)

Siegfried W. Janson The Aerospace Corporation

Physical Science Laboratories August, 2010

Standard Rocket 0.2-to-50 km/s exhaust stream

Propellant

Say goodbye to the ejected propellant

Specific Impulse = Isp = thrust / (go * mass flow rate)

Specific Impulse = directed exhaust velocity / go

Propellant mass fractions (propellant mass / initial “wet” spacecraft mass) increase as required velocity changes (increments) increase.

2 Standard Rocket: Power and Propellant Energy Density

Power per Newton of thrust is proportional to specific impulse. Propellant energy density is proportional to specific impulse squared. 3 Smart Propellant: Eject Mass That Returns for Re-Use

• Host ejects small spacecraft – Quantized ejection velocities required – Electric, pneumatic, or mechanical ejection

Orbital Smart Propellant Rocket • Spacecraft eventually return Mechanics Mass Driver – Intelligent use of orbital mechanics – Spacecraft fine-tune their trajectories

Propellant • Host recaptures ejected spacecraft Returning – Centimeter precision required Mass – Propellant mass is recycled 0.001-to-50 km/s ejected velocity – Additional impulse at recapture – Incoming propellant kinetic energy can be stored for later re-ejection

Smart propellant is recycled and can be used again and again. Reusable space systems are enabled. 4 Orbit Rephasing Using Smart Propellant Orbit Overlap Spacecraft Flight Direction

Earth Earth

Smart Propellant Circular Original Spacecraft Orbit Orbit

T = 0 T ~ 35 minutes When ejected into the flight direction, smart propellant goes into a higher, longer period orbit. The spacecraft goes into a lower, shorter period orbit.

5 Orbit Rephasing: Ejection • Initial orbit - 700-km circular • Reference frame rotates with CM • Smart propellant - Ejected at 100-m/s - 1% of total mass

Spacecraft initially Propellant initially moves backward, then moves forward, then continues forward. continues backward.

The trick is to make spacecraft and propellant meet at zero radial displacement elsewhere in the orbit. Note difference in spatial scales.

6 Orbit Rephasing: Rotating Reference Frame

Smart Spacecraft Orbit 1 Orbit 2 Propellant Spacecraft Spacecraft Orbit 3 ∆θ θf

Orbit 4 Orbit N Smart Propellant A. Eject smart propellant B. Wait for propellant C. Recapture propellant to return

If propellant was initially ejected in the forward flight direction, it returns from the anti-flight direction. For anti-flight ejection, propellant mass returns from the flight direction. This re-circularizes the spacecraft orbit. Mass, energy, and momentum are returned to the spacecraft.

If the propellant was ejected at the correct velocity, spacecraft and propellant meet at zero radial displacement elsewhere in the orbit.

7 Ejection Velocities for LEO Rephasing

N = 10 N = 10 N = 11 N = 11 N = 12 N = 12

N = 15 N = 15

Phase Change = 3.6o Phase Change = 36o

N = Number of smart propellant orbits before recapture N+1 = Number of spacecraft orbits before recapture

A 0.1-m/s error in ejection results in a 640-m miss for N=10. A 0.1-m/s error in ejection results in a 605-m miss for N=40. Specific ejection velocities must be used in order to recapture the smart propellant.

8 Smart Propellant as a High-Altitude Probe

Smart propellant can probe high altitudes and return to the host spacecraft for downloading data.

9 Landing and Take-Off from the Moon

VL = lander velocity VSP = smart propellant velocity Vo = orbit velocity

VL = Vo VL = 0 VL = 0, HL = 0 VL = 0 VL = Vo VSP VSP Lander

Moon Moon Moon Moon Moon

Elliptical Smart Initial Circular Propellant Orbit Orbit

A. Initial State B. Ejection C. Landing D. Post-Liftoff E. Post-Impact

Just above the lunar surface, Vo = 1681-m/s and VSP < 2377-m/s

Smart propellant provides the capability to go from low Lunar orbit (LLO) to the surface, and back into LLO with almost no propellant expenditure.

10 Landing and Take-Off from the Moon: Mass Fractions

Smart propellant velocity (VSP) must not exceed the local escape velocity. This limits smart propellant mass fractions to > 71%. 11 Propellant Mass Fractions for Landing and Take-Off from

Airless Bodies Using Space Storable (311-s Isp) Thrusters

Body Surface Surface Prop. Mass Prop. Mass Prop. Mass Orbit Escape Fraction Fraction Fraction Velocity Velocity 1 Landing 2 Landings 6 Landings Phobos 7.3-m/s 10.3-m/s 0.48% 0.95% 2.83%

6-Hebe 91-m/s 130-m/s 5.8% 11.4% 30.4% Green: Smart 2-Pallas 220-m/s 311-m/s 13.4% 25.1% 57.9% propellant 4-Vesta 248-m/s 351-m/s 15.0% 27.8% 62.4% beats out conventional 1-Ceres 359-m/s 508-m/s 21.0% 37.6% 75.7% propulsion Europa 1430-m/s 2020-m/s 60.9% 84.7% 99.6%

Moon 1681-m/s 2377-m/s 66.8% 89.0% 99.9%

Callisto 1730-m/s 2440-m/s 67.9% 89.6% 99.9%

Mercury 3000-m/s 4250-m/s 86.0% 98.1% 99.9993%

Smart propellant makes sense (propellant mass fractions > 71%) for landing on Mercury, and more than one landing on Europa, the Moon, and Callisto.

12 Apoapsis Reflection Maneuver

VS = spacecraft velocity Ve = escape velocity

V > V V > V Vs = Vo s o s o

VSP < Ve VSP < Ve

Primary Body

Initial Circular New Elliptical Final Elliptical Orbit Spacecraft Spacecraft Orbit Impulse Orbit Elliptical Smart Propellant Orbit VSP << Ve VSP << Ve B. Ejection C. Pre-Apoasis D. Post-Apoapsis E. Pre-Impact A. Initial State Thrusting Thrusting

The apoapsis reflection maneuver requires reversing the flight direction of smart propellant near apoapsis where flight velocity is low. Orbit raising

13 requires net energy input; orbit-lowering generates energy. Apoapsis Reflection in Low Lunar Orbit: Reflection ∆V

The apolune reflection maneuver requires ejection speeds up to 4050-m/s. Smart propellant ∆V for return can be on-the-order-of 100-m/s. 14 Apoapsis Reflection in Low Lunar Orbit: Orbit Raising

The apolune reflection maneuver requires quantized ejection speeds in order to return the smart propellant to the host spacecraft.

15 Accelerator/Decelerator Lengths and Accelerations

Springs

Electromagnetic Accelerators

Mechanical springs can be used for low ejection velocities, electromagnetic accelerators for high velocities. Accelerations will range from ~100 to

16 ~10,000-g’s. Basic Smart Propellant Design • Spherical spacecraft - Reduced attitude control requirements during recapture • 5-cm to 50-cm diameter - Minimum size to fit required systems; e.g., propulsion and GPS - Maximum size limited by energy storage (e.g., 0.5 MJ/kg @ 1000-m/s velocity) • Required systems: - Propulsion for correcting ejection errors and fine-tuning trajectories - Attitude control for the propulsion system - Fine position and velocity determination over most of trajectory (GPS receiver) (10-meter position and 0.01-m/s velocity accuracy at ~1-Hz rate) - Relative position and velocity determination for recapture (1-cm position and 1-cm/s velocity accuracy at up to 10-km range) - Communications to receive host spacecraft orbital element updates - On-board computer with accurate trajectory calculation software

• Mass-production for long-term applications requiring thousands of kg’s

Many of these systems, except propulsion and relative position determination, have been demonstrated in a CubeSat.

17 Conclusions • Smart propellant can reduce propulsion mass – Propellant mass is re-used multiple times • Potential applications have been identified – Satellite rephasing (possible within a decade) – Lunar, or other airless moon, landing shuttle (possible within 2 decades) – Orbit raising and lowering (possible within 2 decades) • Satellite rephasing is a good first application – Ejection and recapture velocities can be below 200-m/s – GPS receivers can provide trajectory position and velocity determination • Long-term applications could require thousands of small spacecraft - A lunar shuttle like the ~15,000-kg mass Lunar Excursion Module would require more than 10,000-kg of smart propellant • More work needs to be done - Impact of atmospheric drag and higher-order geopotential terms on smart propellant ∆V budget needs to be calculated - Specific accelerator technologies need to be analyzed - Rendezvous (recapture) sensors need to be developed

….. ….. I thank The Aerospace Corporation’s Independent Research and Development program for funding this work.