Performance Characteristics of Laser Ablative Thrusters for Small Satellites

IEPC-2005-017

Presented at the 29 ~ International Electric Propulsion Conference, Princeton Universi04, October 31 November 4, 2005

Hirokazu Tahara*, Eiichiro Koizumi + and Shin-ichiro Kitagawa + Graduate School of Engineering Science, Osaka Universi04, Toyonaka, Osaka 560-8531, Japan

Abstract: In a laser ablative thnkster, laser is irradiated to some solid propellant; it is ablated, and then produced small powders and/or gas panicles with high energy are expanded resulting in thrust generaliolt In this study, a Q-switeh Nd:YAG laser with a wavelength of 1064 nm and an output energy of 0.65 J was irradiated to polymer propellants to examine peffonmnce characteristics of laser ablative thrusters for small satellites. Impulse bit and mass loss were lmaStkred. As polylner propellants, PTFE, PTFE(cafool~ 101nass%), PTFE(calbol~ 15mass%), POM, POM(calbol~ 20mass%), PE and PVC were selected. The perfonmnce characteristics m~qinly depended on specific weigl-~ and calbon concenlralion of polylner propellam PTFE(calbolz 10mass%) and fOM(calbolz 20mass%) were preferable propellants for high performance although with PTFE(cal~lZ 10mass%) laser should be irradiated to its new surface for evely shot In laser irradialion with PTFE divergent nozzles, there existed an op~nmln nozzle geometly for improvement of perfonmnce characteristics. In a case with a nozzle half angle of 15 deg and a length of 3 mm, the momelmma coupling coefficient and the specific impulse reacl~1112 ~NS/J and 300 sec, reSlXXztively.

I. Introduction The laser thrtkster is one of atlractive ncar-ftmkre thrtkstels. Laser transmission from laser bases in space and/or on ground to

will be utilized, and the transmitted laser energy is converted to propulsive enelgy. 1 Generally, laser-s~ported plasma, laser-induced detonation and laser ablation etc will be used for the energy exchange from laser to propulsioll As shown in Fig. 1, a laser ablative thrtkster is the simplest in laser propulsion from the viewpoint of structure and system2 Therefore, laser ablative thnksters with low energy (low power) are expected for small and micro satellites, in which long~uance transmission of laser will not be needed; that is, laser systems are onboard satellites) -7

In a laser ablative thnkster, laser is inadiated to some solid propellant; it is ablated, and then produced small powders and/or gas particles with high energy are expanded resulting in thnkst gencraliolt Performance characteristics for laser ablative thrtksters strongly depelxl on laser wavelength and energy, and propellant species and shape. 7 Because the ablation and thrust generation processes are vely complicated, the physical phenolnem and their dependence on thnkster performance are unclea~.

In the present study, a Q-switch Nd:YAG laser with a wavelength of 1064 nm and an output energy of 0.65 J is irradiated to solid polymer propellants in order to examine performance characteristics of laser ablative thrusters. Impulse bit and mass loss are measured, and molnenmm coupling efficiency, thrtkst efficiency and specific impulse are evaluated. As polymer propellants, PTFE, PTFE(calbol~ 10mass% ), PTFE(calbon: 15Imss%), FOM, FOM(calbol~ 20mass% ), PE and PVC are selecte& Influences of polymer propenies and divergent nozzle geolnetly on thnkst peffonmnce are mainly investigated.

* ~ t e Professor, Department of Mechanical Science and Bioengineering, 1-3, Machikaneyama, E-mail: tahara@}ne.es.osaka-u.acjp, AIAA Member.

+ Graduate Student, Department of Mechanical Science and Bioengineering, 1-3, Machikaneyan~

1 The 29 th International Electric Propulsion Conference, Princeton University,

October 31 November 4, 2005

L~ser ]

m

t~aia Pro~nant (a) Laser irradiation.

I I I I I I I I

&Nat ion p l u m e ]

( \

fli31id 1~pellant (b) Laser ablation and thms~ generation.

~iele

Propellant

[

Space satellite

"x

Laser beam

(c) Laser ablative thruster with simplest supply system of sofid propellant.

Roller

Propellant

Nozzle

Space satellite .k Laser beam

(d) Laser ablative thmsler with miler supply system of sofid propellant and divergem nozzle.

Figure 1. Laser ablation and thms~ generation processes and concept of laser ablation thms~er~

II. Experimental Apparatus Figure 2 shows the experimental system for impulse bit measurement of laser ablative thnksters. The thnkqer body is installed in

a stainless vacuum tank 1 m in diameter x 1.2 m long. The main vacuum pump is an off-free Rkrbo-molecular pump with a high pumping speed of 5 m3/s. The tank pressule is kept some 10-3 Pa during all expemnents. Impulse bits are measaned by a pendulum

2 th The 29 International Electric Propulsion Conference, Princeton University,

October 31 November 4, 2005

{Llu.~..iz .w i l ]~ i~ r

I

II I Izn.sa~ h e a a l

Figure 2. Impulse bit measurement system for laser ablative thrusters.

i , i • i • i • i i

10o

1flo

- 5

- 1 0 0 ~ Ii]l][ / , , , , , p ,

0 6 12 18

Ttme, ~

(a) Vibration signal of pendulum just after impulse.

25

E =L~0

e x

y--0. 35x

/-I

, I , I , I , I , I

0 I0 20 30 40 frO

Impulse bit, ~Ns (c) Calibration line of displacement vs impulse bit.

i ] 2 0 4 0 6 f l I ~ l l

Time, see (b) Damping signal of pendulum just after damper-on.

Figure 3. Typical vibration signals of pendulum and calibration line for impulse bit measurement.

3 th The 29 International Electric Propulsion Conference, Princeton University,

October 31 November 4, 2005

(a) PTFE

(b) POM

(e) PTFE(carbon: 10mass% )

(f) PTFE(carbon: 15mass% )

(c) PE (g) POM(carbon: 20mass%)

(d) PVC

Figure 4. Features of cylindrical polymers for laser ablative thms~er~

4 th The 29 International Electric Propulsion Conference, Princeton University,

October 31 November 4, 2005

t"l

/ \

/

t

It

I

i

|

I

!

I

I

be w

at

0, degree x, mm

15 3,4.5,6

30 3, 6

Figure 5. Features of cylindrical polymers with diver,gem nozzles for laser ablative thms~ers.

metho4 The thnaster is m o ~ on the pendulunt Thependulumrotatesaroundfulcmmsoftwoknifeedgeswithoutflicfion_ As shown in Fig.3, the displacement of the pendulum is detected by an eddy-cunent-type gap sensor (non-contacting micro-displacement meter), whose resolution is about +0.5 grit The sensitiveness of penduham is variable in a range by changing positions of the fulcrums and some weigl-~s. The sensitiveness is adjusted to 0.70 bun/buNs in this experiment. The alnplilnde of the nechanical noise, as shown in Fig.3(b), is reduced by an eleclromagnefic dnmper to about ___0.5~ml The callSrafion of impulse, asshownin Fig.3(c),isCalTiedoutbyacollisionofaballtothependulum The ball made oflmd, withaweigl-aof65mg, is l-~ng by a string with a length of 300 mm and a diameter of 0.064 mm An adhesive tape is stuck in the area including the colliding point so that the collision is inelastic. The upper end of the string is connected to the suppolling stand at the point lear the rotational axis of the pendulum The difference of balancing positions between before and after colliding is not obselved because the colliding point is an'anged on the perpendicular centerline of the pendulum and because the ball is much ligt~er than the pendulum The calibration is CalTied out in the almospheric environment just before closing the tank

Laser is introduced into the vacuum tank and is focused onto the target point of the thruster through a lens with a focal length of 200 mm A Q-switch Nd:YAG laser with a wavelength of 1064 ~ an oulput energy of 0.65 J and a pulse width of 6 nsec is usect The number and intelval of shot are 1-40 shots and 20-30 sec, respectively.

The laser ablative thnkster bodies used for this study ale shown in Fig.4. The thnkstel~ are cylindrical polylmr blocks 12 mm in diameterx30mminlenglk Alaserbeamisdirectlyirradiatedontothecylinderfrontplateofthepolymerbody itself, and it is ablated resulting in thrust genelatiorL As polymer propellants, I~E(-[CF2CF2]n-), l~calbOl~ 10Imss%), PTFE(calbor~ 15Imss%), POM(-[OCH2]~-), POM(carbon: 20Imss%), PE(-[CH2CH2]~-) and PVC(-[CHC1CH2]~-) ale selected. The spedfic weights of PTFE, POM, PE, PVC, PTFE(CalbOlE 10mass%) and PTFE(caltx)n: 15mass%) are 2.18, 1.41, 0.94, 1.4-1.45, 2.08 and 2.04, respectively. The melting tempemtmes ofPTFE, POM, PE and PVC ale 327, 165, 110-141 and 217 °C, respectively. The thermal conductivifies of PTFE, POM, PE, PVC, PTFE(caltx)n: 10mass% ) and PTFE(cafl~IE 15n~s%) are 0.25, 0.28, 0.25-0.34, 0.165, 0.45 and 0.465 W/mI~ respectively. The specific heats ofPTFE, POM, PE and PVC are 1.0, 1.1-1.3, 0.53 and 0.84-1.26 kJ/kgI~ resIxxztively. As shown in Fig.5, a cylindrical polymer body with a divergent nozzle is also prepared. The cylindrical bodies made of PTFE have divergent nozzle parts of axial lengths of 3, 4.5 and 6 mm with half angles of 15 and 30 deg. Laser is focused onto the upstream end surface on the cenlral axis of the cylindricalbody, i.e., at the nozzle throat A direct-reading balance, whose accuracy is 0. ling, is used to ~ the mass of solid propellank i.e., the mass of the cylindrical thruster body itself. The mass loss per shot is calculated by averaging the total mass loss after 1040 shots.

From measured impulse bit and nass loss, lnolnelmam to energy coupling coefficient, thn~t efficiency and s I x ~ c impulse are evaluatecL AccolNngly, we examine influences of polymer propellant propellies (specific wei#d, ca~oon nass concentration etc) and of with/without divergent nozzles and nozzle geometry on performance characteliSfics for laser ablative thnksters.

5 th The 29 International Electric Propulsion Conference, Princeton University,

October 31 November 4, 2005

IlL Results and Discussion

A. Influences of PolymerPropellant Species

100

80

Z• 6o

}4o 20

I I I I I

a POM [] PTFE o PE v PVC

] i

0 2

[ ] o [ ]

[ ] v o [ ]

v

o g ~ 8 ° g o

I ~ I ~ I ~ I

4 6 8 10

Shot number

Figure 6. Impulse bit vs shot number characteristics with polymer propellants of PTFE, POM, PE and PVC.

Figure 6 shows the impulse bit charactelisfics dependent on shot mmber with polymer propellants of PTFE, I<)M, PE and PVC, in which laser is irradiated up to 10 shots to the focal point on the cylinder front plate of the polymer. The impulse bit for PTFE is the highest of all polylmrs at all shot lmmbers. Because the ~ c weight ofgIFE isthe highest of all polymers and the differences in thermal conductivity and ~ c heat are small a high impulse bit is expected to be produced using a polymer with a relatively large ~ c weight Howevel; with PTFE an increase in shot number decreases the impulse bit Because the morphology of the laser-irradiated PTFE surface, as shown in Fig.7, conlinues to intensively change to a porous-like surface with imnea~g shot number of laser ilmdialion, the absorption efficiel~y of laser energy to solid propellant is considered to be decreasing. Lots of large particles ablated are also suslx~ted to be exhausted with a vely low velocity, and they would not be conmbuted to impulse generaliol~

Figure 8 shows the impulse bit vs shot lmmber characteristics with pure POM and POM(calt~l~ 20nass%), i.e., including calt~ncolN~onent or not Figure 9 shows the impulse bit and nass loss dependent on calbon coneentralion with FIFE-base polymers of pure PTFE, giFE(calbOl~ 10mass%) and PTFF4caltx)r~ 15mass%), in which the impulses of the second shot are represente4 From Fig.8, the impulse bit with POM(ca~n: 20mass%) is found to be higher than that with pure POM at all shot numbers. Also, boththe impulses of pure t ~ M and t~M(calbOl~ 201nass%) hardly depend on shot lmmber because their surface morphologies hardly change during

Side view

Bot~omview

Topview

Solid

(a) Blus~rafion of microsaructure.

(b) Top view.

(c) Bottom view.

(d) Side view.

Figure 7. Features of laser-ablated spot on PTFE surface after 10 shots.

6 th The 29 International Electric Propulsion Conference, Princeton University,

October 31 November 4, 2005

I00

8o

: t 40

20

I I I I

l~OM(carbozt, 20mass). POM

I I I

8 I ~ I ~ I ~ I

0 2 4 6 10

Shot number

Figure 8. Impulse bit vs shot number characteristics with polymer propellants of pure P O M and POM(carbon: 20mass% ).

I I I I

. . . . -A . . . . Impulse bit" 70 . . . . -v . . . . ~ a ~ Io~s

60 . . . . . . . {'" " '7

5 0 , , ' " ' " J " "" " ' t 4 0 " ~

40 O

20

I0 J0

0 , I , I , I 0 5 I 0 15 Carbon concentration, mass%

Figure 9. Impulse bit and mass loss dependent on carbon concentration with PTFE-base polymers of pure PTFE, PTFE(carbon: 10mass% ) and PTFE(carbon: 15mass% ).

. .~ IOO

80 4 J

~ 40

t_b

20

I ~ ' I I ' I ' I

" POM(c arboa,2Oma~s%) t ~ z P O M

t \ = PTFI~ • I~TFE (c arb oa, 10mas~/o)

t \ x PTFN(c arbott, 15massO/o) t \ - Pg

~ 11 =~/o t, 1 \

T] "" I

~ ~1 =2%

~ 1 6 0

~ 120 C I

• - ' 80 g- o f .3

4O

0

o

'1 '\ . . . . . A P CIIVI( c~bo m,20mass%) t \ ± POM

PTFE l \ -~ P'rrmZ~'ooato,-=e/o)

\ × PTFFgc~on.t%tmss%) - PE

\ \ v PVC

\ 1~ ", \ t "

\

.,. ,,..q=lO% .,,...,

' Z " , , ..[ " ~ - ~ ' ' " ,...~ -.-- "q=5%

, I , I , I , I , i I i I i I i I i 0 100 200 300 400 500 I00 200 J00 400 500

S p e c i f i c impulse, so: Specific impulse, s e c

(a) With 10-shots average impulse bit. (b) With highest impulse bit of first or second shot

Figure 10. Momentum coupling coefficient vs specific impulse characteristics using 10-shots average impulse bit and highest impulse bit of f'wst or second shot with all polymer propellants.

repetitive laser ilradialiol~ On the other hand, in cases with PTFE-base polymers, the impulse bit, as shown in Fig.9, has a peak at a ca~oon mass concelmation of 10 %. This is b~ause the mass loss per shot intensively ~ s with c a i n concelmation resulting in vev/ low exhaust average velocity. Accordingly, there exists an optimum cmbon concelmation with some polymer propellant,

with PTFE bases, for high impulse bit generaliorL Figure 10 shows the molnel~m coupling coefficient vs ~ c impulse charactelistics using 10-shots average impulse bit and

the highest impulse bit of the first or second shot with all polymer propellants, in which the thn~t efficiency is represented with dashed lines. The momentum coupling coefficient ofPTFE(cmbol~ 101nass%) is the highest of all polymers although the SlX~C impulse is relalively low. As mentioned above, this is because of large ~ c weight and c a i n inclnsiol~ The highest specific impulse is achieved withPOM(ca~oon: 20mass%). Because PE has the lowest ~ c weight, the ~ c impulse is relatively high In cases

7 th The 29 International Electric Propulsion Conference, Princeton University,

October 31 November 4, 2005

with PTFE-base polymels, because the impulse bits decrease with increasing shot lmmbe~; the perfonmnce characteristics, as colnlmred with Figs.10(a) and 10(b), are deteriorated. Howevo; with other polymers of FOM, POM(carbon: 20mass% ), PE and PVC the performances hardly vary with shot lmmber In this expelment, the maximum momelmam coupling coefficient of 100 ~NS/J arid tile nlaximum thrast efficiency of 8 % are achieved at a ~ c impulse of 170 sec for the second shot with PTFE(cafool~ 10nnss%); the maximum molmnmm coupling coefficient of 42 ~NS/J and the nlaximum thrast efficiency of 5 % are achieved at a ~ c impulse of 300 sec for 10-shots average with POM(c~bon: 20mass%).

Accordingly, all results lead to the conclusion that the perfomnnce of a laser ablalive thruster mainly depelds on ~ c weight and carbon concelmation of polymer propellant In this study, PTFE(carbon: 10nnss%) and POM(cafoon: 20mass%) are preferable propellants for high perfonmnce although with PTFE(carbon: 10mass%) laser should be irradiated to its new surface for evely shot

B. Influences of Divergent Nozzle

~ I60

I20 Q

O I I I

Wt~ho~t D.O~ ' r ] e

Dirergeat-aozzte half angte ..... A .... 15degree ..... o .... 30degree

- . . . . . . . . . . . . . . . . . . ?

i I i I i I i I i :3 4.5 6 Nozzle length, mm

Figure 11. Momentum coupling coefficient vs divergent nozzle length characterislics with half angles of 15 and 30 deg at second shot

Figure 11 shows the m o m e l ~ n coupling coefficient vs divergent nozzle length charactedslics with half angles of 15 and 30 deg at the second shot in which the coefficient without nozzle shown in Fig.10(b) is presented. Figure 12 shows the photographs of exhaust pltmes with a half angle of 15 deg depelxlent on nozzle length including without nozzle. The momelmam coupling coefficient with a nozzle half angle of 15 deg is higher than that with 30 deg at all nozzle lengths. The momelmam coupling coefficient with a half angle of 15 deg and a length of 3 mm is roughly 1.4 times higher than that without nozzle although the coefficients with a half angle of 15 deg and lengths of 4.5 and 6 mm are lower than that without nozzle. This is exlx~ted because ablated particles lose their high momentum by collision with the nozzle wall with nozzle lengths of 4.5 and 6 ~ although the exlnust plume with a nozzle length of 3 ram, as shown in Fig.12(b), is e ~ e d smoothly. Accoldingly, there exists an opthrnnn nozzle geometly for improvement ofpeffonnar~ characteristics. Inthis experiment, the lraximum momelmam coupling coefficient of 112 NNs/J and the maximum specific impulse of 300 sec are achieved with a

(a) Without divergem nozzle.

N,:,=

(b) Nozzle length: 3 ml~

I

(c) Nozzle length: 4.5 mm.

(d) Nozlle length: 6 mm.

Figure 12. Photographs of exhaust plumes with half angle of 15 deg dependent on nozzle length including without nozele.

8 th The 29 International Electric Propulsion Conference, Princeton University,

October 31 November 4, 2005

nozzle half angle of 15 deg and a lengthof3 ml~

IV.. Conclusions A Q-switch Nd:YAG laser with a wavelength of 1064 nm, an oulput energy of 0.65 J and a pulse width of 6 nsec was irradiated to

solid polymer propellants in order to examine perfonmnce characte "nslics of laser ablative thrusters for small satellites. Impulse bit and mass loss were lmautr~ As polymer propellan~ PTFE, PTFE(calborr 101ross%), PTFE(calbon: 15mass%), POiVL POM(calbon: 20mass%), PE and PVC were selected. In this study, influences of polymer properties and divergent nozzle geolretry on thrust performance were investigated. The peffonnanee characteristics mainly depended on ~ c weight and calbon COlr, entrafion of polymer propellal~ PTFE(calborr 10mass%) and FOM(calborr 20mass%) were preferable propellants for high perfonmnce although withPTFE(calbon: 10mass%) laser should be inadiated to its new surface for every shot In laser madialion to flat surfaces of the polymers, the maximum momentum coupling coefficient of 100 ~NS/J and the lI~XilIRlm thrust efficieuey of 8 % were achieved at a ~ c impulse of 170 sec for the second shot with PTFE(ca~ol~ 10mass%); the n~,dmum molnel~m coupling coefficient of 42 NNs/J and the maximum thrust efficieuey of 5 % were achieved at a ~ c impulse of 300 sec for 30-shots average with FOM(ca~l~ 20nass%). In laser irradialion with PTFE divergent nozzles, there existed an opfinmln nozzle geometry for improvement of peffonnar~ charactelislics. In a case with a nozzle half angle of 15 deg and a length of 3 mm, the lnomelmam coupling coefficient and the ~ c impulse reached 112 NNs/J and 300 sec, reswctively.

References 1 • 7 , • • , , Kantrowltz, A., Propulsion to Olblt by Ground-Based Lasers, Aeronaut. Astronaut., Vol. 10, pp.74-76, 1972. 2Pakhomov, A.V.," Ablative Laser Propulsion: An Old Coneept Revisited,"A]AA Journal, Vol.38, pp.725-727, 1999. 3phipps, C.R," Diode Laser-Driven Microthmsters: A New ~ for Micropropulsion," A/AA Journal, Vol. 40, pp.310-318, 2002. 4 Phipps, C.R.," Laser-Driven Micro-Rocket" J.Appl. Phyx, Vol.77, pp.193-201, 2003. 5 Phipps, C.R, and Luke, J.," Laser Ablation of Organic Coatings as a Basic for Micropropulsion," Thin Solid Films', Vols.453-454, pp.573-583, 2004. 6 Phipps, C.R, and Luke, J.," Micropropulsion Using a Laser Ablation Jet," Journal of Propdsion and Power, Vol.20, pp.1000-1011, 2004. 7 Koizumi, E., Tahara, H., and Yoshikawa, T..," Effects of Nozzle Geometry on Perfonmnce Charactelislics of Laser Ablative Thnksters," 24 ~ International Symposium on Space Technology and Science, Miyazald, Paper No.ISTS 2004-b-39, 2004.

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