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Laser Space PropulsionOverview

Claude Phipps, Ph. D., managing partnerJames Luke, partner

Photonic Associates, LLC, Santa Fe, NMWesley Helgeson, senior research associate

NMT/IERA, Albuquerque, NM

XIV Advanced Laser Technologies ConferenceBrasov, Romania

September 12, 2006

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Perspective• I am now of a certain age where “practical” strictly means “what I’ll

see in my lifetime.”• So, I’m not going to talk about some gee-whiz laser propulsion

possibilities that I avidly discussed 15-20 years ago » Launching 10 tonnes into low Earth orbit with a 4GW laser» Deflecting asteroids bent on destroying life on Earth using a multi-GW laser and a

10-km diameter mirror on the moon

• But I am still quite excited about» Assembling space stations from 10 to 20-kg launches of components at 1% of

current costs per kg» Micro-, mini- and macro-thrusters» Several other promising concepts in laser space propulsion

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Outline• Introduction to Photonic Associates, LLC

• Terminology and Theory

• Advantages of Laser Space Propulsion (LSP)

• Taxonomy» Pure Photon Propulsion

» Propulsion by Laser Gas Detonation

» Laser Jet Engines

» Propulsion by Laser Expulsion of Liquids

» Laser-Electric Hybrids

» Laser Heat Exchangers

» Propulsion by Laser Ablation

• Perspective and Conclusions

Terminology for LSPHere are the most important parameters:

1) Momentum coupling coefficient Cm=J/W=mvE/W = F/P

2) Specific ablation energy Q* = W/m

3) Exhaust velocity vE = CmQ*

4) Specific impulse Isp = J/(mgo) = vE/go

5) Mass usage rate

6) Ablation efficiency AB = WE/W = mvE2/(2W) = CmvE/2

7) Energy conservation CmvE = CmIsp*go = (2/)AB

where = <vx2>/(<vx>2) ≥ 1 is a parameter1 that is often

(The CmvE product = 2.0 when AB = = 1, but can’t be larger unless AB >1)

≈1

[1See Phipps & Michaelis, Laser and Particle Beams, 12(1), 23-54 (1994)]

m =P / Q *

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Conservation of EnergyI would like to make this point very clear. Take a “drift Maxwellian”:

8)

9)

10)

11)

f(vx,vy,vz) = CxCyCz {exp – β[(vx – u)2 + vy2 + vz2]}

< vx > = dvx

– ∞

∞vx f(vx) = Cx π / β u = u

ψ = <vx2>(<vx>)2

= {u2 + kT

mE

u2 } ≥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 jets 2 can give M≈2 and = 1.15, and we can take ≈ 1.

[2See Kelly and Dreyfus, Nucl. Inst. Meth. B32, 341 (1988)

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Terminology, cont’dHere are some ancillary relationships among LSP parameters:

12) Thrust efficiency T = eoAB

13) Fuel lifetime AB = go2MIsp

2/(2PAB)

• Severe penalty paid for Isp = 10s (Yabe)

» Lots of thrust, but 10,000 times less AB than if Isp =1000s

14) Optimum coupling fluence opt = 480 0.5 MJ/m2

15) Surface absorber vacuum model3:

16) Volume absorber vacuum model4: Cm2 = (2t/)(T - d/ - p/ - ln/)

[In Eqs. 12 & 13, A is mean atomic number, Z is mean ionic charge state, = A/2[Z2(Z+1)]1/3, = /th, th is thrust threshold fluence, d and p are fluences required for dissociation

and plasma formation, is mass density, t is target thickness and is incident fluence]

Isp = 442

A 1/8

Ψ9/16

(I λ τ )1/4

3 Isp is just a matter of intensity! See: Phipps et al. J. Appl. Phys., 64, 1083 (1988)4 Parameters defined in J. Prop. & Power, 20 no.6, 1000-1011 (2004)

Advantages of LSP1) Lower costs with laser launching. Reducing the cost of getting to space by 2 orders of

magnitude ($100/kg vs. $10,000/kg today) will change our relationship to space travel.

Today’s LEO launch costs

Launch System Minimum

Cost (k$/kg)

Rockot 10

Shuttle 12

Athena 2 12

Taurus 20

ISS, commercial 22

Pegasus XL 24

Long March CZ-2C 30

Athena 41

Greater than the price of gold!

But it need not be so! [Myrabo Lightcraft flight, White Sands]

Photo: Courtesy Leik Myrabo

Lower costs, cont’d

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.

Connection between the charts: 3.3USD/MJ of laser light delivered at 5 flights per day. Is that reasonable5? Compare cost of wallplug energy on the ground (0.03USD/MJ).

[5See Phipps & Michaelis, Laser and Particle Beams, 12(1), 23-54 (1994)]

Advantages of LSP2) Lower Dead Mass

Do not have to raise turbines, pumps, tanks, exhaust nozzles, etc., along with the payload

3) Variable Exhaust Velocity (crucial!)

• Accomplished by varying intensity on target (, As)

• Uchida6 showed that varying vE during a LEO to GEO transfer mission gave a factor-of-3 lower energy cost than conducting the same mission with a constant vEopt.

» vE was varied by a factor of 300 according to vE/vEo = C(t/to)0.5

» Special case of fact that, when vE = v(t), theoretical maximum efficiency of momentum transfer to flyer can be

obtained, because exhaust stream velocity is zero in the starting reference frame in which v is measured .

6Uchida, 1st International Symposium on Beamed Energy Propulsion, Huntsville, AL, 5-7 November 2002, AIP Conference Proceedings 664 214-222 (2002)

Advantages of LSP4) Enabling Otherwise Impossible Missions: A recent “BAA”* set the following

graduate-level problem:

*See http://www.fbo.gov/spg/USAF/AFMC/AFFTC/BAA%2D06%2D02%2DPKTB/listing.html

• Prime power: 1kW

• Spacecraft total mass: 180kg

• Engine mass including fuel: 80kg

• Initial orbit: 500km altitude circular

• Complete any of 4 missions:1. Rephase s/c 180 degrees in 12 hours

2. Raise s/c from 500km to 1500km in 2 days, return in 30 days

3. Crank orbital plane 15 degrees in 90 days

4. Drop s/c to 300km, fight ram pressure for a year with 50W prime power, return in 30 days

• These requirements were set to be impossible with current technology

• But, a 20-kg laser propulsion engine using a 470W diode-pumped glass fiber laser can accomplish all the tasks, according to our calculations.

Taxonomy of LSP1) Pure Photon Propulsion

• Earliest LSP concept, before lasers were demonstrated8

• Cm is only 2/c = 6.7N/GW (for total reflection)

• Isp is as large as it can be, 31Ms

• But: to accelerate 1 tonne at go, requires 1.5TW laser power

• Möckel9 did not shrink from this, envisioning a 1km diameter xray laser beam with 1Å wavelength impinging on a 1km diameter sail to propel a spacecraft to Centauri in 10 years.

• At present, only two practical applications exist:» Bae’s concept10 for intracavity photon thrusters to precisely position nanosats

» Photon sails (but here the sun or another star is the photon source)

8 Sänger, J. Spacecraft, in Probleme der Weltraumforschung (IV. Internationaler Astronautischer Kongress, Zurich, 1953) Biel-Bienne, Laubscher, p.32 (1955)

9 Möckel, J. Spacecraft and Rockets 9, no. 12, pp. 942-4 (1972)10 Bae, Final Report, NIAC Phase I Program 07605-003-041, April 30, 2006, NASA Institute for Advanced Concepts, 75 Fifth St. NW, Suite 318,

Atlanta GA 30308, USA

LSP Taxonomy2) Propulsion by Laser Gas Detonation• The air-breathing Myrabo Lightcraft11,12 would, in principle, require no ablation

fuel other than ambient air, in the atmosphere.

11Myrabo, AIAA/SAE/ASME 18th Joint Propulsion Conference, Cleveland, OH (1982)12Myrabo, Proc. 1987 SDIO Workshop on Laser Propulsion, J. T. Kare, ed., LLNL CONF-8710452,

Lawrence Livermore National Laboratory, Livermore, CA 94550, pp. 173-209 (1987)

• Biparabolic design: laser light coming from below forms a ring focus under the rim, propels craft via successive detonations in air.

• Outside atmosphere, the device would use solid ablatants located in the rim.

• Flown to 71m in spin-stabilized flight, driven by a repetitively-pulsed, 10kW CO2 laser.

• Cm ranged from about 250N/W for air to 900N/W for solid propellant.

• Materials problems are challenging• Apollonov laser jet engine: efficiency and materials survival

advantages (later)

Photo: Courtesy Leik Myrabo

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Results of the flight experiments with AeroSpace Laser Propulsion Engine (ASLPE) model

Dr. Yuri Rezunkov, NIIKI OEP, Russia

Engineering characteristics of the model:

Cm = 25 dyne/W (air-breathing mode), and 45 dyne/W (laser-chemical

propulsion)Model mass – 150 g

Model flight characteristics:Laser power – 5 kWMaximal acceleration – 4-5 m/s2

Maximal velocity – 3.5 m/sThrust (max) – 1.5 NFlight duration – 3 s

16Rachuk et al., AIP Conference Proceedings 830, pp. 48-57 (2006)

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LSP Taxonomy3) Laser Jet Engines• Apollonov has proposed and measured the

performance of a model version17 of a laser driven jet engine which can simultaneously achieve Cm = 4.17mN/W and vE = 2,520m/s.

• These parameters give CmvE = 5.25, an unusual result which is 2.5X larger than permitted by Eq. 7.

• However, factors of 2 are not a big problem• When verified, this is an extremely important

result for nonenergetic ablation fuel

17Apollonov, et al., Quantum Electronics 36, pp. 673-83 (2006)

• Device is based on the resonance merging of shock waves generated by an optical pulsed discharge to form a quasi-stationary wave, and has obvious advantages in thrust conversion efficiency and lifetime.

LSP Taxonomy4) Propulsion by Laser Expulsion of Liquids

• In 2002, Yabe18,19 proposed a laser-powered microairplane

» For, e.g., collecting climate data or observing volcanic eruptions

• Engine uses shock generated on a laser-irradiated absorber at the back of a liquid container to expel droplets of liquid

• Result: largest Cm values ever observed, but Isp = 10s (for some applications, OK)

» Measured Cm’s up to 5mN/W (simulations gave up to 70mN/W)

18Yabe et al., Appl. Phys. Letters, 80, pp. 4318-20 (2002)19Yabe et al. Appl. Phys. A77, pp. 243-249 (2003)

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LSP Taxonomy, cont’d5) Laser Heat Exchangers

• Kare has proposed20,21 the HX thruster» Essentially a laser-heated boiler (simulations give Isp = 600s with 1000C exhaust)

» Achieves very low specific mass (0.001kg/kW), but assumes a 100MW laser

» Water injection at low altitude to increase thrust, H2 above atmosphere

» Launch mass M = 5400kg, m = 180kg delivered to LEO, m/M = 0.033

• Rather22 has proposed a similar concept, in which a Shuttle H2 tank with M = 30Mg could be propelled to GEO in 45 days

» Would use a 10MW laser to heat 4Mg H2 achieving Isp = 1500s

» Could be used to build a manned GEO station or LEO-GEO shuttle

6) Laser-Electric Hybrids

• Horisawa and coworkers23 have built and tested a hybrid laser-electric thruster (next slide)

20Kare, J. Prop. and Power, 11, pp. 535-543 (1995)21Kare, AIP Conference Proceedings 664, pp. 442-453 (2002)22Rather, AIP Conference Proceedings 664, pp. 37-48 (2002)23Horisawa et al., AIP Conference Proceedings 830, pp. 213-223 (2006)

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Research Activities at Horisawa Lab. at Tokai University : Laser-Electric Hybrid Acceleration Propulsion System

Coaxial L-EM Hybrid Thruster

Anode

Cathode

j

F

B

Cathode

Anode

Length ： 22mm Channel length ：3mm

Thruster head

Cu-Anode: 6.0mm

A ｌ 2O3 –Insulator: 5.0mm

C-Cathode & Propellant: 4.0mm

Ibit vs Charge Energy

Isp vs Charge Energy

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Taxonomy of LSP7) Propulsion by Laser Ablation

• Kantrowitz24 suggested the first practical approach to LSP, in which a laser is used to heat a solid propellant surface to generate a vapor or plasma jet that provides the thrust.

» Cm is as much as 8 orders of magnitude larger

» Watts (rather than GW) of laser power can now do useful tasks

• Propellant may be inert, or exothermic.

• Laser may be remote (e.g., groundbased), or onboard.» Invariably, these should be pulsed rather than CW

— Allows achievement of high Isp when necessary

— Broadens the range in which Isp and Cm can be varied

— Facilitates clearing plasma from the optical path between pulses

24Kantrowitz, Astronaut Aeronaut. 9, no. 3, pp. 34-35 (1971)

The msLPT• Lippert group at PSI25: target photochemistry• Market: attitude & position control for satellites• Patents: we own the patents on this application

• Total mass 0.5kg• Thrust 0.1 – 10mN, 20W max input• Thrust efficiency = 133% (chemical input)• Isp = 200s, thrust-optimized• Minimum impulse bit = 100nN-s• Uses GAP:C or GAP:dye fuels• Likely to be 1st laser ablation propulsion

application to fly in space

25Lippert, et al., Appl. Surf. Sci., 186, 14-23 (2002)

The nsLPT• Isp is just a matter of intensity (Eq. 15). 7ks has been seen in other work26.• The nsLPT, operating at I = 0.25PW/m2, is the basis of our 3,000-s Isp claim• Research device, not as completely developed as the msLPT• Benchtop and microchip lasers produced data

» Fluence ~ 1MJ/m2, = 4ns

Gold-coated, 2.5-inch IBM hard drive disk

(4400 rpm)

25nN precision, 1.25 nN/rad26Phipps, & Michaelis, Laser and Particle Beams, 12 (1), 23-54 (1994)

Vacuum Torsion Balance Thrust Gage

The two LPT’s can be combined• Previously, the only way to obtain the range 3ks < Isp < 200s in one vehicle

was to use separate chemical and electrical thrusters

• A scaled-up macro-LPT with a fiber laser target driver can use the combined operating principles of the two LPT’s

3000s at 1MJ/m2, 4ns 137s at 0.25MJ/m2, 1msAll Isp’s based on measured mass loss

nsLPT msLPT

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Requirements for macro-LPT’s• Thrust of order 1N

• Electrical power of order 1kW

• 200 < Isp < 3000 seconds

» Based on what we’ve already demonstrated in solid fuels

• Diode-pumped fiber lasers will accommodate pulsewidth range

» ns- and ms-pulse modes in the same laser

• Liquid fuel

» Eliminate moving parts, except for a small pump

• Mass budget 20kg (thanks to fiber lasers!)

kW Pavg pulsed fiber lasers• First critical ingredient for the success of this project

• Yb-doped fiber amplifier MOPA will access 10ns, 1MJ/m2 operating point

• Based on very large mode area cores ~ 70m diameter Photonic crystal fibers (“holey fibers”) permit propagating single transverse modes in a structure whose core size is 50 - 100

wavelengths Reported at HPLA6: more capability than we need27

3MW peak in a 70-m core fiber with 200m OD (we only need 300kW)

M2 = 1.1 in a 30W Pavg oscillator-amplifier with 900ps pulses

Fiber length: few m

eo can be 45%

27di Teodoro, paper 6261-1N, SPIE 6261 (2006)

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Liquid Fuels• Second key ingredient

• The only reasonable format for supplying kg quantities of ablation fuel

• We can illuminate fuel in a way that protects the optics

• Candidates:» Raw GAP [m/m = 0.02% in 4 hours at 2 torr]

» Polymerized GAP [dissolves completely in ionic liquids (methyl-trioctylammonium-trifluoroacetate)]

— Ionic liquids have “immeasurably low” vapor pressure and melting points ~ – 80C— Already feature, and are used in, electrospray thrusters

• Then add laser absorbing dye» In a tank,does not have to be protected from UV

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Anticipated motor performanceLPT Motor Parameters

Motor Pin(electrical) 1.2kW

Motor Mass 20kg

Fuel Mass 60kg

No. of Fiber Lasers 14

Fuel Type Energetic Liquid Polymer

High Isp mode Low Isp mode

Isp 3,200s 180s

Thrust 10N t 28o mN 1mN to 1.09N

Electrical/optica l Efficiency 39% 39%

Mass Us age Rate 0.90mg/s 620mg/s

Lifetime 2.1 years 1.1 days

Cm 61N/W 2.32mN/W

Thrust Efficiency 37% 80%

Lifetime Impulse 1880kN-s 106kN-s

v for 180kg spacecraft 12.7km/s 715 /ms

Fiber Lasers:

Total Optica l Power 468W 468W

Pavg (optica)l , EA 33W 33W

Ppeak (optica)l , EA 334kW 334W

Pulse Duration 10ns 1ms

Pulse Energy, EA 3.3mJ 330mJ

Pulse Repetition Rate 10kHz 100Hz

Perspective and ConclusionsNear term (say, 5 years): • Accomplishing near-Earth missions using laser-powered macrothrusters is

an exciting new application of laser ablation propulsion» This can be done with lightweight, high-power diode-pumped fiber lasers and liquid ablative

fuel based on the PSI fuel design» Unmatched thrust efficiency derives from exothermic laser ablation fuels» Competitive technologies have greater mass/thrust and lower thrust density

• Using pure photon propulsion to provide very small impulse bits needed to position nanosatelites with nm precision could be the first useful application of this technology

• Laser jet engines look exciting. Could avoid materials problems in Myrabo designs and increase coupling efficiency and lifetime.

» Duplication of results will improve acceptance.

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Perspective and ConclusionsMedium term (say, 10 years):• Propelling a lightcraft from Earth surface to proof-of-concept altitude

(10km) should be supported» However, infrastructure has to be in place, at the same place, and this is costly

— Large beam director with adaptive optics— Zoom optics— Laser guidestars— Tracking and illuminating lasers— Repetitively-pulsed thrust laser with at least 100kW average power

• Also, it will be relatively easier than Earth launch to send samples into low Mars orbit from its surface in conjunction with MSR

» Surface atmosphere is similar to Earth’s at 20km, as regards drag» Mars’ surface gravity is considerably weaker (38%)

• Hybrid laser-electric thruster has great promise if efficiency can be increased

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Perspective and ConclusionsFar term (say, fifteen years):

• Possible to routinely launch 10-20kg nanosatellites, or parts of a larger satellite, into LEO, as a far cheaper way to carry mass to orbit

• With 5 launches/day from one site, 3 tonnes can be placed in LEO each month

» Space hardware can be assembled rapidly on orbit

» Permits launching completed space vehicles into interplanetary trajectories at very low cost

• Achieving this goal will entirely change our relationship to space and space travel

• Whether this is done depends on priority decisions of government scientific and engineering programs (and we know how wise they are).

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The msLPT• Uses ms-duration diode laser pulses• Thomas Lippert group21 at Paul Scherrer Institut played

a major role in its development» Suggested the use of energetic ablatants» Perfected the chemistry of GAP and other materials

to make them suitable for coatings» Made a series of sample films for us

1ms, 9MJ/m2 result with1XDL laser diode

Highlighted:all 1ms, 0.25MJ/m2

Representative performance of various ablating layer compositions

Ablatant Absorber Cm (N/W) Isp (s) AB (%)Chemical

Energy (kJ/kg)PVC 5% nanocarbon 60 750 20 Endotherm

GLYN 2% nanocarbon 1280 116 73 2661GAP Epolin 2057 IR dye 1300 200 125 2500GAP 1% nanocarbon 3000 160 235 2500PVN 5% nanocarbon 116 2890 164 4941

Laser input 500kJ/kg21Lippert, et al., Appl. Surf. Sci., 186, 14-23

(2002)

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ms- and ns-Laser Plasma Thrusters

An LPT is a device which uses self-contained lasers to create thrust via laser ablation of a specially-prepared solid or liquid fuel

(Note: macro-LPT will not use T-mode)

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kW Pavg pulsed fiber lasers• A tentative kW Pavg laser design (courtesy Aculight Corp.)

» Endcaps avoid fiber damage

• We anticipate needing N≈14

msLPT commercial laser diodes• MTBF as operated: 200,000 years.

» Lasers will not limit device life