1. A Review of Laser Ablation Propulsion
Claude Phipps1, Willy Bohn2, Thomas Lippert3, Akihiro Sasoh4, Wolfgang Schall5 and John Sinko6
1Photonic Associates LLC, 200A Ojo de la Vaca Road, Santa Fe, New Mexico USA 87508
Phone/Fax: ,
2Bohn Laser Consult, Weinberg Weg 43, Stuttgart, Germany
3Paul Scherrer Institut, CH5232 Villigen PSI, Switzerland
4Department of Aerospace Engineering, Nagoya University, Chikusa-ku, Nagoya, Japan
5DLR Institute of Technical Physics, Stuttgart, Germany (retired)
6Micro-Nano GCOE, Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya, Japan
Advanced Laser Technologies 2009
Antalya, Turkey
September 30, 2009

2. Contents Benefits of laser ablation propulsion (LAP)
Scope of this review
History starting with pure photon propulsion
Pulsed laser ablation propulsion
Operating range
Vapor and plasma regime theory
Applications
Laser plasma thruster (LPT)
Laser-driven in-tube accelerator (LITA)
Liquid-fueled laser-plasma engine
Lightcraft
Laser space debris mitigation (ORION)
Direct launch to low earth orbit
Promise for the future

3. Benefits of LAP
1) Lower costs with laser launching. Today’s cost of launching one kg into low Earth orbit (LEO) is equivalent to the cost of gold.
Greater than the price of gold!
But it need not be so! [Myrabo Lightcraft flight, White Sands]
Today’s LEO launch costs
Launch System
Minimum Cost (k$/kg)
Rockot 10
Shuttle 12
Athena 2
Taurus 20
ISS, commercial 22
Pegasus XL 24
Long March CZ-2C 30
Athena 41
Photo: Courtesy Leik Myrabo

4. Benefits of LAP 2) Lower Dead Mass
Do not have to fly turbines, pumps, tanks, exhaust nozzles, etc., along with the payload
3) Variable Exhaust Velocity (crucial!)
From chemical rockets up to and surpassing that of ion engines
Accomplished by varying intensity on target (t, As)
Permits maximum efficiency flights1,2 in which exhaust and flight velocity are matched, leaving exhaust particles with zero momentum
1C. W. Larson, F. B. Mead, Jr. And S. D. Knecht, “Benefit of constant momentum propulsion for large v Missions – applications in laser propulsion,” paper AIAA , 42d Aerospace Sciences Meeting, Reno, 5-8 January 2004
2Uchida, 1st International Symposium on Beamed Energy Propulsion, Huntsville, AL, 5-7 November 2002, AIP Conference Proceedings (2002)

5. Benefits of LAP 4) High thrust density 5) High thrust to mass ratio
30kN/m2 demonstrated in the PALLC minithruster3
5) High thrust to mass ratio
15kN/kg demonstrated in Russian ASLPE engine4
6) High thrust efficiency
125% expected for kW laser thruster5
This is possible due to exothermic fuels
Not a trivial distinction for spacecraft
3 C. R. Phipps, J. R. Luke, W. Helgeson and R. Johnson, AIP Conference Proceedings 830, (2006)
4 Yu. Rezunkov, A. Safronov, A. Ageichik, M. Egorov, V. Stepanov, V. Rachuk, V. Guterman, A. Ivanov, S. Rebrov and A. Golikov, AIP Conference Proceedings 830, 3-13 (2006)
5 C. R. Phipps, J. R. Luke and W. Helgeson, AIP Conference Proceedings 997, (2008)

6. Scope Propulsion by laser ablation Primarily, applications
Less emphasis on:
Pure photon propulsion, except for historical context
Inertial confinement fusion except as a reference point
Fundamental plasma physics theory
Coulomb explosions, LASNEX modeling, etc
Photo courtesy Yuri Rezunkov (time exposure of flight in lab)

7. History in a Nutshell Tsander Rezunkov ASLPE
Fridrich Tsander, 1924: : Pure photon propulsion
But Cm = thrust / laser watt = 2/c = 6.7 mN/MW
Wolfgang Möckel 1972: Basic theory of laser driven rockets
Arthur Kantrowitz6 1972: Laser ablation propulsion (LAP)
Cm ≈ 100N/MW to 10kN/MW due to plume acceleration
Leik Myrabo 20017: Flight to 72m altitude in New Mexico desert
Rezunkov 20064: 2N thrust demonstrated
Tsander
6 A. Kantrowitz, Astronautics and Aeronautics 10 (5), (1972)
7 L. N. Myrabo, paper AIAA , 37th AIAA/ASME/ SAE/ASEE Joint Propulsion Conference, 8-11 July 2001, Salt Lake City, UT (2001)
Rezunkov ASLPE

8. Pulsed LAP Terminology
Here are the most important parameters:
1) Momentum coupling coefficient Cm=I / W=mvE/W = F/P
2) Specific ablation energy Q* = W/m
3) Exhaust velocity vE = CmQ*
4) Specific impulse Isp = I /(mgo) = vE/go
5) Mass usage rate
6) Ablation efficiency hAB = WE/W = myvE2/(2W) = yCmvE/2
7) Energy conservation
CmvE = CmIsp*go = (2/y)hAB
where y = <vx2>/(<vx>2) ≥ 1 is a parameter8 that is often ≈ 1
(The CmvE product = 2.0 when hAB = y = 1, but can’t be larger unless hAB >1)
[8Phipps & Michaelis, Laser and Particle Beams, 12(1), (1994)]

9. Operating Range From water cannons nearly to photon propulsion!
[References below can be found in the JPP review paper]
From water cannons nearly to photon propulsion!

10. Terminology, cont’d i = 2ne/(no + ne + ni)
Some ancillary relationships among LSP parameters:
8) Thrust efficiency hT = heohAB
9) Fuel lifetime tAB = go2MIsp2/(2PhAB)
[Severe penalty paid for Isp = 10s as in water cannons]
Lots of thrust, but 10,000 times less tAB than if Isp =1000s
10) Optimum coupling fluence Fopt = 480 t0.5 MJ/m2
11) Ionization fraction where (Saha equation):
Opt. Coupling Fluence vs. t
i = 2ne/(no + ne + ni)

11. Theory 12) Plasma regime model9: 13) Vapor regime model10:
[In Eq. 12, A is mean atomic mass, Z is mean ionic charge state, Y = A/2[Z2(Z+1)]1/3.
In Eq. 13, x = F/Fo, Fo = thrust fluence threshold,T = transmissivity from laser to surface, a = ablation layer absorption coefficient, r = target solid density and F = incident fluence]
 Plasma model was not meant to be valid as Z  0, Y  , Vapor model was not meant to treat the plasma state. Problem: how do we make the transition between the two models?
9 Isp is just a matter of intensity! See: Phipps et al. J. Appl. Phys., 64, 1083 (1988)
10 New results: J. Sinko and C. Phipps, Appl. Phys. Lett., accepted for publication (2009)

12. Solution to the problem
We use Cm = [i pp +(1-i) pv]/I = i Cmp + (1-i) Cmv
 Vapor
Plasma

13. Laser Plasma Thruster (Note: macro-LPT will not need T-mode)
ms thruster (10mN, 250s)
ns thruster (50mN, 3660s)
See Phipps & Luke, reference 3.

14. LITA Laser in-tube Accelerator concepts of Sasoh11
11 A. Sasoh, S. Suzuki and A. Matsuda, Journal of Propulsion and Power, accepted for publication (2009).

15. Liquid-fueled Laser Engine
3-kW, 6.5-N engine design driven by 18x100-W fiber lasers5
See Phipps, Luke and Helgeson, reference 5.

16. Lightcraft
Myrabo Lightcraft12 would, in principle, require no ablation fuel other than ambient air, in the atmosphere.
Biparabolic design: laser light coming from below forms a ring focus under rim, propels craft via successive detonations in air.
Outside atmosphere, the device would use solid ablatants located in rim.
Flown to 72m in spin-stabilized flight, driven by a repetitively-pulsed, 10kW CO2 laser.
Cm ranged from about 250N/MW for air to 900N/MW for Delrin solid propellant.
Materials problems are challenging
Rezunkov ASLPE engine4
Uses 6kW rep-pulse CO2 laser
Wire-guided flight in laboratory
Generates 2N thrust
Photo: Courtesy Leik Myrabo
12Myrabo, AIAA/SAE/ASME 18th Joint Propulsion Conference, Cleveland, OH (1982)

17. ORION
Ground-based system causes ablation jet on near-Earth space debris targets, eventually lowering perigee until re-entry occurs
13C. Phipps, AIP Conference Proceedings 318, (1994)

18. Direct Launch to LEO
Connection between the charts: 3.3USD/MJ of laser light delivered at 5 flights per day. Is that reasonable14? Compare cost of wallplug energy on the ground (0.03USD/MJ).
[14See Phipps & Michaelis, Laser and Particle Beams, 12(1), (1994)]
Above: theoretical predictions for flight in vacuum. Laser launching facilitates frequent launches, diluting recurrent and sunk costs.
Above: (•) flight simulation results for 1-m diameter craft laser-launched from ho = 30km in air compared to vacuum predictions at left.

19. Promise for the Future Timeframe Technology Problems to be Solved
1-2 years
Spaceflights for Laser Plasma Thruster
ORION system
100k$ funding
100M$ funding
2-10 years
Lightcraft flights through atmosphere to LEO
Ablation of Lightcraft material
5-10 years
5kg payloads to LEO
LEO to GEO transfer vehicles
kW, N-thrust liquid-fuel engines
Building MW-class RP lasers & launch vehicles
15-20 years
Launch to LEO with tonne payloads
Initial investment (multi-B$)

21. The Parameter y
I would like to make this point very clear. Take a “drift Maxwellian”: 1) 2) 3) 4) ≥1
If M = u/cs = 1, and cs = (kT/mE)1/2 with  = cp/cv =5/3, we have  = 1.60
Comment: forward peaking of most free, high-intensity laser ablation jets1 can give M≈2 and  = 1.15, and we can take  ≈ 1.
[1See Kelly and Dreyfus, Nucl. Inst. Meth. B32, 341 (1988)