AIAA 2002-2152

A LASER-ABLATION-BASED MICRO-ROCKETClaude R. Phipps*

Photonic Associates200A Ojo de la Vaca Road

Santa Fe NM 87508Phone/fax: 505-466-3877Email: crphipps@aol.com

James R. Luke† and Glen G. McDuff‡

NMT/Institute for Engineering Research and Applications901 University Blvd. SE

Albuquerque, NM 87106-4339

Thomas Lippert§

Paul Scherrer Institut5232 Villigen PSI

Switzerland

* Member, AIAA, President† Member, AIAA, Research Assistant Professor‡ Senior Research Staff Member§ Research Group LeaderCopyright © 2002 by Photonic Associates. Published by the American Institute of Aeronautics and Astronautics,Inc., with permission.

Abstract: We describe the creation and testing of amicro-rocket for pointing small satellites based on laserablation of a specially-designed target strip by acommercial diode laser. We describe the design of theengine, and discuss its performance. Specific impulseup to 1000 seconds has been obtained, together withlaser momentum coupling coefficient of 6 dyn/W. Wediscuss the basic physics of the laser-target interaction,and the materials science involved in creating thespecialized target. The target is a tape with a transparentlayer through which the laser passes before ablating thesecond layer, which is absorbing. The technology isextensible to larger devices.

1. Introduction

Unknown as a field until 30 years ago1, laser ablationpropulsion is now progressing rapidly. One of the latestinnovations is the micro laser plasma thruster (µLPT).The micro-Laser Plasma Thruster (µLPT) is a sub-kgmicropropulsion option which is intended for attitudecontrol and stationkeeping on microsatellite platforms.It takes advantage of the recent commercial availabilityof 4-W multimode diode lasers with sufficientbrightness to produce a repetitively-pulsed orcontinuous plasma jet on a surface in vacuum. A lens

focuses the laser diode output on the ablation target,producing a miniature jet that provides the thrust.Single impulse dynamic range is nearly 5 orders ofmagnitude, and the minimum impulse bit is 1 nano N-sin a 100µs pulse. The diode is a low-voltage devicewith electrical efficiency in excess of 50% 2. The µLPTis an alternative technology to the micro pulsed plasmathruster (µPPT) for micro- and nano-satellitemicrothrusters.

Transparent Substrate

Ablatant

t2 ≈ 100µmt1 ≈ 60µm

“Transmission Mode”(Analogous to LIFT)

Pro: Optics Protected Improved Geometry

Con: Lower Isp, Cm

“Reflection Mode”Pro: Higher Isp, CmCon: Optics Require Shield

Awkward Geometry

100µm

1µm

Emitters

4W Diode

4W Diode

e.g., acetate film

JET

Figure 1. Defining R- and T-mode

33rd Plasmadynamics and Lasers Conference20-23 May 2002, Maui, Hawaii

AIAA 2002-2152

Copyright © 2002 by the author(s). Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

2. Operating Principles

The principle of operation is shown in Figure 1.Usually, the device is operated in Transmission mode(T-mode) to protect optics from contamination by theablation jet. In this mode, a lens focuses the laser diodeoutput on a 25-µm diameter spot on the transparent sideof a specially-prepared fuel tape. Passing through theacetate substrate without damaging it, the beam heatsan absorbing coating on the opposite side of the tape tohigh temperature, producing a miniature ablation jet.Part of the acetate substrate is also ablated. The µLPT isexpected to show good performance in system specificimpulse Issp, defined as 3:

Issp = Isp(1+k) (1)

where k = Mdry/Mfuel , (2)

because k ~ 4 for this thruster. Dry mass consists of the

package, a laser diode, two tiny lenses, an electronicsboard and two tiny motors to move the fuel tape pastthe laser diode focus and change tracks. The µLPTrequires no neutralizers, heaters, high voltage supply,high voltage switches, magnetic fields, nozzle, gas,tanks, or valves. It is also free of mysterious small-scalephysics. Most importantly, nothing erodes duringoperation except the ablation fuel. The µLPT canoperate pulsed or CW, and power density on target isoptically variable in an instant, so Isp can be adjusted“on the fly” to match mission requirements. Figure 2shows the "preprototype" thruster, which will not bespace qualified, but was developed to test the basiccomponents of the prototype. A small gearmotor drivesthe tape longitudinally at speeds up to 12 mm/s, and astepper motor moves the laser transversely to illuminateone of 254 tracks. Figure 3 shows the µLPT jet duringnormal operation.

Materials we have explored for the transparent substrateare cellulose acetate, PET and Kapton™ polyimideresin. For the ablatant, over 120 materials have beenstudied, but the best standard material is blackpolyvinylchloride. Many of these target materials wereso-called "designer materials" created especially for thisapplication 4-6. The µ LPT is expected to produce120µN thrust, Cm up to 120 µN/W and Isp up to 650 s,with less than 20W electrical power input2. Lifetimeimpulse of the prototype unit, which will contain 0.2 kgof ablator fuel, is expected to be 180 N-s 7. Forperformance of the Figure 2 preprototype, see Table 2.

3. Pulsed Laser-Surface Interactions

Pulsed lasers offer a much richer parameter space inwhich to work than CW lasers. The latter have beenwell treated elsewhere8. For pulsed lasers, themomentum coupling coefficient Cm is defined as theratio of target momentum m∆v produced to incidentlaser pulse energy W during the ejection of laser-ablated material (the photoablation process). Forcontinuous lasers, it is the ratio of thrust F to incidentpower P:

(3)

In the ablation process, Q* joules of laser light (theasterisk is customary notation: Q* is not a complexnumber) are consumed to ablate each gram of targetmaterial:

(4)

Figure 2. Preprototype µLPT weighs 0.85 kg (gage isused for focus setup, is not part of device)

Figure 3. The µLPT in operation Q * =

W

∆m

Cm =

m∆vW

=FP

For the sake of discussion, we will consider amonoenergetic exhaust stream with velocity vE.Momentum conservation requires

m∆v = ∆mvE , (5)

so the product of Cm and Q* is the effective exhaustvelocity vE of the ablation stream, independent of theefficiency with which laser energy is absorbed. Thiscan be seen by writing:

(6)

If for example, a significant amount of the incidentenergy is absorbed as heat in the target substrate ratherthan producing material ejection, Q* will be higher andCm will be proportionately lower, giving the samevelocity in the end.

While it is understood that real exhaust streams havevelocity distributions, we have shown [9] that themonoenergetic stream approximation will not introducelarge errors [<v2>/<v>2≈1.15] for laser-producedplasmas, and the principal points we want to make willbe easier to understand using that assumption.

The specific impulse Isp is simply related to thevelocity vE by the acceleration of gravity:

CmQ* = vE = gIsp (7)

Energy conservation prevents Cm and Q* from beingarbitrary. Increasing one decreases the other. UsingEqs. (4) and (7), energy conservation requires thatseveral constant product relationships exist:

2ηAB= ∆mvE2/W =Cm2Q* =gCmIsp =CmvE. (8)

In Eq. (8), we introduce the ablation efficiencyparameter, ηAB ≤1, the efficiency with which laserenergy W is converted into exhaust kinetic energy.Choosing combinations of Cm and vE that exceed 2violates physics, since ηAB must be less than 1.

Since the maximum specific impulse of ordinarychemical rockets is about 500s, limited by thetemperatures available in chemical reactions, exitvelocity vE > 5km/s (Isp > 5000s) is accessible only bylaser ablation, where temperatures can be many times10,000K, or some other non-chemical process such asion drives. Specific impulse Isp up to 8000s has beenmeasured with pulsed lasers9.

Ablation efficiency can approach 100%, as direct

measurements with other types of lasers on cellulosenitrate in vacuum verify9, but a value of 50% or evenless is likely. The impact of ηAB<1 is that the Cmvalue deduced from a given vE may be less than themaximum permitted by conservation of energy. Exitvelocity vE is the fundamental quantity.

The rate of mass usage is g/s (9)

where P is laser optical power. When considering Cmand Q* as design variables it must be kept in mind thatthe ablator lifetime increases with Q* and decreasesvery rapidly with increasing Cm:

. (10)

For this reason, in laser propulsion applications,increasing Cm to get more thrust via the relationship

F = PCm (11)

from a given laser entails a serious penalty for ablatorlifetime, because τAB ∝ 1/Cm

2 from Eq. (10).

The vacuum coupling coefficient Cm is in the range10 – 100 µN/W for standard surface-absorbingmaterials10.

Note that, from Eq. (8), Cm*Isp <= 2/g = 0.204. Inmeasurements with energetic target materials, productsCm*Isp = 0.18 have been obtained, which is 90% of0.204 9.

In the laboratory, Cm and Q* are relatively easyquantities to measure, and their product convenientlygives vE which is a more difficult quantity to measure,requiring, e.g., a laser-induced fluorescence setup ortime-resolved shadowgraphy. Typical values of thecoefficient Cm taken from the literature are shown inTable 1.

Table 1. Summary of Cm literature

Ref. Target Laser λλλλ,ττττMax. Cm(µN/W)

10 Simple passive(front illuminated)

various 100

11 Energetic absor-ber pyroxylin

(front illuminated)

10.6 µm,2µs

950

12,13

Confined passiveabsorber

1.06 µm,85ns

4920

14 Confined passiveabsorber

1.06µm,3ns

7000

CmQ* =

(N– s)(J)(J)(kg)

=(kg)(m)(kg)(s)

= m/s

m =P

Q *

τ AB = M/m =MQ *

P=

2 η ABM

P Cm2

4. Preprototype Thruster Performance

Table 2 gives the main parameters of the preprototype.

Table 2. Preprototype Specifications

Item Value

Weight with fuel 850 g

Tape dimensions 50.5 x 2.54cm

Backing thickness 100µm

Ablative coating thickness 60µm

Laser power (average, peak)¶ 2 W, 10W

Laser target illumination area 500 µm2

Tape speed 10 mm/s

Pulse duration 4 ms

Pulse repetition frequency 50 Hz

Track width 100µm

Tracks 254

Tape lifetime 3.6 hours

Coupling coefficient Cm 60 µN/W

Force output (predicted)§ 120 µN

Q* 11 kJ/g

Isp 550 s

Minimum impulse bit 0.6 µN-s †§: using Cm and Pavg, ¶: when fiber-coupled JDSU diode lasers areinstalled (now underway), †: at 1ms pulsewidth

With the full 2W average power of the repetitively-pulsed laser now being installed (10W peak), the thrustpredicted in Table 2 should easily be obtained.

Figure 5 shows an example of a target tapephotomicrograph after a thruster test. The bottom of thetrack is slightly smaller than 100µm, permitting us tostack tracks at a density of 100 tracks/cm or 250tracks/inch.

Torsion Pendulum Thrust Stand

We measure thrust in vacuum with a torsion balance2

[Figure 6] which can measure 50µN with 10%accuracy. Its force response is 5mN/rad, and deflectionrate is 5mN/rad. Resonant frequency is 0.03 Hz.

The thrust stand was calibrated using a pair of magneticcoils. A large field coil is attached to the support frame,and a small armature coil is attached to the pendulum,which is oriented perpendicular to the field coil. Byapplying current to the coils, a small well-definedtorque can be applied to the pendulum and its responsemeasured. The field coil produces a magnetic fieldmuch larger than the local horizontal component of theEarth's field.

Control and Communications

Thruster control is divided into three major sections:laser power control, motor speed control, andcommunications and diagnostics.

An optical data link transmits commands to, and datafrom the unit when it is under vacuum. Power issupplied to the thruster through contacts immersed inmercury [Figure 4].

Figure 4. Detail of Figure 6 showing mercury cupcontactor beneath small mirror.

Figure 5. Photomicrograph of continuous target tapeburn.

This provides electrical connections that are free ofstatic friction, and the small viscous forces due to thecontacts moving in the liquid mercury serve to dampthe pendulum oscillations.

The heart of the controller is a Texas InstrumentMSP430 microcontroller. The MSP430 was chosen forits ultra low power, only 7 mW at full computing speedand only 5µW in standby. With only a 105 mm2

footprint in its plastic quad flatpack (PQF) package anda mass of only 1.2 grams (unmounted) the MPS430 isideal for lightweight, low power applications. In/outfunctions of the MPS430 include six pulsewidth-modulated channels for motor and laser control, aneight channel 12 bit analog-to-digital converter, twouniversal asynchronous receiver transmitter channelsfor communications and non-volatile memory, andseveral discrete I/O ports. Total mass for themicrocontroller and ancillary electronics is estimated at2.2 grams.

The laser current, pulse width, and frequency are set bythe user and input to the microprocessor through theuser interface software. The laser can be operated inpulsed or CW mode. The tape speed and trackseparation are also user selectable. Control software forthe thruster development was written to expedite

development and data acquisition. The embedded codewas written in C and compiled with the TexasInstruments compiler supplied with the microcontroller.The user interface program was written in NationalInstruments LabView.

Figure 6. The thruster and electronics mounted on the force test stand. Both units aresuspended from steel fiber attached to middle of crossbar just behind small mirror.

Figure 7. Illustrating plume rotation with CW operation

Plume Steering Requires Pulsed Operation

A totally unexpected result of our work was thediscovery that continuous (CW) operation always steersthe plume unacceptably in the direction of tape travel. Itwas observed that with the laser operating CW, theexhaust plume was not perpendicular to the target tape,but that the center of the plume was deflected by about45 degrees in the direction of the tape motion. Thisplume steering is apparently due to the shape that formsat the edge of the fuel as it is ablated [Figure 7].

The solution was repetitive-pulse operation. During ams-duration pulse, tape motion is negligible and theplume is well-defined [Figure 3] and perpendicular tothe tape.

Our electronics package was designed to work in eithermode, and our diode lasers are capable of peak powerup to 200% of their average power rating in pulses withduration of order 1ms.

This change brought with it three benefits: operation incloser contact with the bulk of our experimental data(which is pulsed data), operation at higher peak powerfor better Isp, and low duty factor operation of our laser

diodes, which is better for heat dissipation. It alsobrought a difficulty: the low duty factor requires us touse more diodes for the same average power and thrust.The requirement for pulsed power will have only aminor impact on the weight of the microthruster unit.

Preliminary Results

Figure 8 shows our first measurements of thrust in lowpower tests. These results meet Air Force requirementsfor TechSat21-type missions. Much larger Isp will beseen in full power tests now being implemented withnew diodes which will achieve a 3-fold increase inoptical power. The four JDSU 6380-A fiber-coupled,920-nm diodes will deliver 10W peak power to thetarget. Specific impulse as high as 1000 seconds hasbeen seen in optimized single-shot static tests of thesame target material.

Future Work

Very encouraging results have recently been obtainedwith optimally designed, proprietary ablatants4-6. These

results guarantee that we will be able to exceed the Cmvalues quoted in Tables 2 and 3, while maintaining Ispin excess of 500s [Figure 9].

4. The Prototype Thruster

When tests with the preprototype are complete, we havedesigned and plan to build a space qualifiable prototypethruster containing 80m (0.4 kg) of target fuel tape.Table 3 lists the prototype's operating specifications.Existing data supports the achievement of eachspecification. In particular, with our new targetmaterial, the 120µN thrust specification should besubstantially exceeded.

Figure 8. First measurements of thrust, thrust-to-powerratio and specific impulse for the preprototype in low-power tests.

Figure 9. Recent static test data using proprietary targetmaterial is very encouraging for future work. Curve fitsare to Isp (dash-dot) and Cm (dash). Maximum values ofparameters: Cm = 521µ N/w; Is p =546 s andCm*Isp=0.169, 83% of passive target maximum given byEq. (8).

5. Summary

A new type of microthruster has been developed whichis an alternative to the µPPT for spacecraftmicrothrusters. The µLPT is one of the first practicalapplications of laser ablation propulsion. Whencomplete, its performance will exceed all Air Forcerequirements for TechSat21-type microthrusters. Theserequirements include 4 axis thrust, 0-75 µN thrust peraxis, 100 N-s impulse per axis, 320N-s total impulse,2mN-s minimum impulse bit, less than 20W electricalpower input and less than 1kg system mass.

Table 3. Prototype Specifications

Dimensions 14x12x7 cm

Wet mass 0.95 kg

Dry mass 0.55 kg

Tape length 80 m

Tape velocity 0.75 cm/s

Time per track 2.96 hrs

Total lifetime 753 hrs

Lifetime impulse 325 N-s

Q* 11 kJ/kg

Cm 60 µN/W

Laser average power 2 W

Specific impulse 550 s

Thrust 120 µN

Operating temperature 0 - 80C

Acknowledgment

This work was completed with support from AFOSRcontract F49620-00-C-0005

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