The availability of megawatt laser systems in the next century will make laser launch systems from ground to orbit feasible and useful. Systems studies indicate launch capabilities of 1 ton payload per gigawatt laser power. Recent research in ground to orbit laser propulsion has emphasized laser supported detonation wave thrusters driven by repetitively pulsed infrared lasers. In this propulsion concept each laser repetition cycle consists of two pulses. A lower energy first pulse is used to vaporize a small amount of solid propellant and then after a brief expansion period, a second and higher energy laser pulse is used to drive a detonation wave through the expanded vapor. The results are reported of numerical studies comparing the detonation wave properties of various candidate propellants, and the simulation of thruster performance under realistic conditions. Experimental measurements designed to test the theoretical predictions are also presented. Measurements are discussed of radiance and opacity in absorption waves, and mass loss and momentum transfer. These data are interpreted in terms of specific impulse and energy conversion efficiency

Bailey, A.

Gauthier, M.

Gelb, A.

Goldey, C.

Lo, E.

Resendes, D.

Rollins, C. J.

Rosen, D.

Weyl, G.

English

NASA Technical Reports Server

IN91-22152
I_,,-,o...., DR_I_,_._ IAb_CB _BiCL_S YO_ CU_i'iNUOU5 ACCESS TO SPACE
C.J. Rollins, A. Bailey, h. Gelb, M. Gauthier, C. Goldey
E. Lo, D. Resendes, D. Rosen, and G. Weyl
Physical Sciences Inc.
Andover, HA 01810
ABSTRACT
The availability of megawatt laser systems in the next century w_ll
make laser launch systems from ground to orbit feasible and useful.
Systems studies indicate launch Lapabilities of I ton payload per giga-
watt laser power. This corresponds to 20 Kg payloads with 20 megawatt
lasers. A launch repetition frequency of one payload every I0 min would
allow delivery of parts for assembly of large space structures, and
would allow resupply of space mtations on a continuous basis. Recent
research in ground-to-orbit laser ropulsion has emphasized laser
supported detonation wave thrusters driven by repetitively pulsed
infrared lasers. In this propulsion concept each laser repetition cycle
consists of two pulses. A lower ene_ • first pulse is used to vaporize
a small amount of solid propellant ant _hen after a brief expansion
period of a few microseconds, a second and higher energy laser pulse is
used to drive a detonation warp through the expanded vapor. Tempera-
tures of order i0,000 K are achieved. During a several millisecond
intercycle delay, expansion of the hot vapor converts thermal energy to
directed kinetic energy. High specific impulses of "600 to 800s are
achievable at energy conversion efficiencies of "20 to 40 percent. The
physics of such thrusters has been explored both theoretically and in
the laboratory. Ve report here the results of numerical studies com-
paring the detonation wave properties of variouc candidate propellants,
and the simulation of thruster perfor[:ance under realistic conditions.
Experimental measurements designed to test the theoretical predictions
are also presented. We discuss measurements of radiance and opacity in
absorption waves, and mass loss and momentum transfer. These data are
interpreted in terms of specific impulse and energy conversion effi-
ciency. Directions for future research are indicated.
193
1991012826-193
https://ntrs.nasa.gov/search.jsp?R=19910012839 2020-03-19T18:56:09+00:00Z
The availability of megawatt laser systems in the next cent,_ry will
make laser launch systems £Lu,,iground to orbit feasible and USeful.
Systems studies indicate launch capabilities of I ton payload per giga-
watt laser power. This corresponds to 20 Kg payloads with 20 MW lasers.
A launch repetition frequency of une payload every I0 mln would allow
aelivery of parts for assembly of large space structures, and would
allow resuppiy of space stations on a continuous basis (Figure l).
(
J
Figu=e 1. Laser propelled ground-to-orbit launch vehicle
Recent work has been conducted under the SDIO Laser Propulsion
Program to demonstrate feasibility of the concept through fundamental
research, with an eye toward future applications. In anothez symposium
paper, Jordin _are (ref. 1) reviews the status of pulsed laser
propulsion and identifies future applications. In this paper, we focus
on the current hasic research which forms the foundation for future
advances. We emphasize here a research effort oriented toward the earth
194
1.q.qln199F _1aA
ito low-earth orbit problem vhtch offers the most immediate challenge,
and promises the most imm_diate rewards, but it shov!d be kept in m!_d
that simple and _traightforward extensions of this effort can e_sily be
brought to frustion in support of inter-orbit and inter-planetary
mission opportunities.
Recent research in ground-to-orbit laser propulsion has emphasized
laser supported detonation wave thrusters driven by repetitively pulsed
infrared laser. The detonation rave thruster concept was introduced by
Pirrt and Veiss (ref. 2). The current york has centered on a double
pulse version invented by Reilly (ref. 3), and reintroduced by
Kantrowitz (ref. 4) in simplified form for use in ground-to-orbit
launches of small payloads. In this propulsion concept each laser
repetition cycle consists of tvo pulses. A lover energy first pulse is
used to vaporize a small amount of solid propellant and then after a
brief expansion period o£ a fev microseconds, a second and higher energy
laser pulse is used to drive a detonation rave through the expanded
vapor. Temperatures of order 10,000 K are achieved. During a several
milllsecond intercycle delay, expansion of the hot vapor converts
thermal energy to directed kiiietic energy. High specific impulses of
- 6o0 to 800s are achievable at energy conversion efficiencles of
- to 40 perce,,t. Figure 2 illustrates the thruster concept during
ea_ of ti:etwo laser pulses of a thrust cycle.
f PAYLOAD _
_IilFPD_c ELI_ T j..____'_T R_T'ON
" 91S;.' ' ,' .....
_"""' LOW FLUX _ HIGH FLUX -'-"
LASER PULSE LASER PULSE
B_959
Figure 2. Laser-sustalned-detonatlon wave rocket
The physics of such thrusters has been explored both theoretically
and in the laboratory. Ve report here the results of numerical studies
comparing the detonation rave properties of various candidate
propellants, and the simulation of thruster performance under realistic
conditions. Experimental measurements designed to test the theoretical
predictions are also presented.
195
199J 012826-195
Detonation rave properties
In a detonation wave thruster, several requirements must be made on
the propellant properties to insure proper operation. Among the
important considerations are; moderate heat of vaporization, short
optical absorption depth to the laser beam, low ionization threshold in
the gas phase, and low avorage atomic weight. These requirements
combine to ensure that there is a proper amount of mass delivered during
laser irradiation, minimal mass flow between pulse pairs, and uniform
processing of the gas during the high irradiance detonation wave portion
of the thruster cycle. To achieve this last condition, it is necessary
that a detonation wave can be formed quickly in the high irradiance
portion of the thruster cycle, and that the wave must be thin compared
to the gas slug which has been produced by the low irradiance portion of
the cycle. Failure on the first point would result in considerable mass
being ejected from the thruster at low specific impulse, while failure
on the second point causes non-uniform processing of the gas, with
consequent low energy conversion efficiency.
In order to address the differences between candidate propellants
with respect to detonation wave properties, we have constructed a
computer code which simulates such waves under asssmed conditions of
steady state and local thermodynamic equilibrium. The details of the
code have been published elsewhere (ref. 5). For the present paper, we
have chosen two specific cases to illustrate the utility of the method.
Figures 3a and 3b show the density versus distance profiles of two
[,.,,,,0,_w_,1
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o,, /°-°._'°_' 1
012 1
.0_0
0.03%
_-'-_-illl/l/,ltl. I
0 010 ._ _ _ _ 060
Figure 3a. Absorption wave for water propellant seeded with Li at
indicated percentages, density versus position
196
Q
1991012826-196
00060_ I _ I '' I I I ! I _---r--'_
k J i
000_ 1.5._ • 106(w/_ _1 !
Po• i OOE- 04_¢_
.00045 0 =853E c_s
000'¢0 --
.00035 -
.00030
,00025
•_ _
.ooo15L l I I I L 1 I I I I
o .ocz .oo4 .o06 008 olo
WAVE THICKNE_ (_) E_
Figure 3b. Absorption wave for lithium hydride, density
versus position
candidate propellants, water and lithium hydride, under conditions of
comparable detonation wave velocity, which in turn implies comparable
specific impulse. In these figures, the laser is incident from the
left. Two factors are of particular importance in the comparison. The
first is that at compareble specific impulse, considerably higher
Irradlance is required to operate uslnE water (ice) as the propellant.
This is because of the corresponding higher heats of vaporization,
dissociation, and ionization of ice, as compared to lithium hydride.
The second important feature is that in order to achieve the required
thin detonation wave structure (< i00 um is desirable for a - _ cm gas
slug), it is necessary to add substantlal atom fractions of a low
ionization potential material to the water. On-going research in this
area includes incorporation of non-equilibrlum properties, exploration
of other possible propellants, and assessment of the unsteady portions
of the process.
Reactive flow modelin E
Once the detonation wave processing is complete_ the hot gas expand3
away from the rocket. Both electron-ion and atn_-atom recombination
take place in this expansion phase of the thrust cycle. In order to
adequately describe the non-equilibrium aspects of these recombination
processes, we have constructed a hydrocode which includes finite rate
kinetics for the important reactions. The details of this code have
197
.J
.
1991012826-197
been presented elsewhere (ref. 6). For the present context_ we have
chosen to emphasize a dependence on parameter variations for a
particular propellant.
A variety of different cases was simul_ted. To most clearly
identify the effects of different parameters, a base case was chosen for
which the effect of perturbations were exami:_ed. This base case was
stoichiometrically COH2, that is one mole of carbon to one mole of
oxygen to two moles of hydrogen. A mixture of this molar concentration
is produced by the breakdown of a polyacelal resin. A 2 cm thick gas
slug with a density of 2 x 10-3 grams/cm 3 and a temperature of 15,000 K
was used, where the slug was initially at rest w_th respect to the base
of the rocket and allowed to expand one dimension_lly into vacuum. This
cmse produced an impulse of 730s with a coupling of 36 dyne-s/Joule and
an energy efficiency of 33 percent (neglecting radiation losses).
The results (see Table i) clearly demonstrate the advantages of a
dense gas slug over a more diffuse one. At low densities the
recombination rate drops precipitously, substantially reducing the
impulse. If a lower mass removal rate per pulse is needed, it is
preferable to reduce the slug thickness rather than lower its density.
If this drives one To an excessively thin slab which deposits its
Table i. Gas expansion simulation results
w E
CO I Impulse/Gas Recombina- Energy Effi-
Temper- Mesh Slug tion Density IImpulse (dyne-s/ ciency
Fuel ature Points (cm) (%) (g/cm3) I (s) J) (%)
COH2 15,000 40 2 47 2 x 10-3 730 36 33
COH2 15,000 80 2 54 2 x 10-3 760 38 36
COH 2 15,000 40 0.2 33 2 x 10-3 650 34 29
COH 2 I0,000 40 2 33**74 2 x 10-3 620 42 _"J_
COH 2 15,000 40 2 0 (forced) 2 x 10-3 550 27 18
COH 4 15,000 40 2 ,." 2 x 10-3 840 33 34
COH 2 15,000; 40 2 16 2 x10 -4 640 32 25
COH 2 15,000 40x3012 x 50 44 2 x 10-3 610 31 24
198
I
1991012826-198
impulse over two brief an interval, then propellants for which
recombination is unimportant should be considered. Going to lower
temperatures improves the impulse per unit energy, but, as expected, the
total impulse suffers.
On-going research seeks to model a wide variety of propellants and
to incorporate the effects of specific projectile geometries.
Experimental work
In con3unction with the theory and modeling activities, an experi-
mental effort has been conducted to verify the validity of the con-
clusions. In particular, measurements have been made of the effective
heat of mass removal, and the delivered impulse for various propellant
materials, along with various optical and radiometric measurements. To
illustrate some of the ongoing work, we offer an example from experi-
ments designed to visualize the detonation wave as it processes a slab
of vaporized propellant.
In order to visualize these phenomena in our experiments, we have
used a technique to form temporally and spatially resolved images of the
plasma radiance, and opacity to its own radiance. The technique is a
two-dimensional, temporally resolved generalization of a method devel-
oped by Goncharov (ref. 7). The key to the technique involves
positioning on the side of the plasma opposite the observation site an
imaging lens and a concave reflecting mirror whose surface has been
divided into z fine pattern of alternating reflective and non-reflective
(opaque) stL_ps. With appropriately chosen and positioned imaging
lenses, the radiance distribution recorded by the observation camera
consists of a striped pattern with the amplitude of adjacent stripes
being proportional to, respectively, the local plasma radiance and the
plasma radiance amplified by its self-transmission through the plasma.
By comparing the relative intensities of adjacent stripes and knowing
the effective transmission of the retro-reflecting optical components,
one then finds the spatial profile of plasma transmission.
Detonation waves were observed in soda lime glass vapor (Figure 4).
The figure consists, in principal, of eight exposures proceeding raster
fashion from the lower left to the upper right. The first two frames
precede the introduction of the high intensity laser pulse and are
consequently black.
A lower intensity laser pulse was used about 6 US before the main
pulse to vaporize some of the glass target. The vapor from this pulse
occupies a zone extending about a centimeter from the target surface at
the time when the high intensity pulse is introduced. This second pulse
199
)
1991012826-199
iv _...............__ ... -_
I
J
SURFACE SURFACE
2 ,I, 4 ,1. 6 8
1 T3 T5 7
SURFACE SURFACE
I_smm _1
V-345
Figure 4. Self-transmission framing photo for double pulse
experiments on glass
ignites a laser supported detonation wave wave and d:les it at speeds
approaching 106 em/s. the wave is clearly visible i,. the third frame at
a position several millimeters off the target surface. In the inter-
vening microsecond between the third and fourth frames, the wave would
be expected to propagate to the edge qf the initial pulse vapor dis-
tribution, where the rarefied gas density would no longer support an
opaque wave. The fourth frame confirms this expectation, showing that
laser light is once again being admltted to the surface. By the fifth
frame, the TEA laser pulse has considerably weakened and is able to
support only a comparatively tenuous plasma near the tarKet surface. No
exposure at all is detectable in the subsequent frames.
Future Research
While the current paper has focused on some particular examples to
demon3trate the character of current research, simple extension of the
research will lend themselves to a vlde variety of future laser
propulsion applications. In particular, there may be advantages in
exploring lower specific impulse pro2ellants for a multistage ground-to-
orbit concept, or to make use of radio frequency free-electron laser
waveforms. High specific impulse systems and nozzled thrusters may lend
themselves more to in space missions. In any event, transition from the
current research phase to 21st century actual systems appears
straightforward.
200
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i*_ - ._ ..... ..........-_ ............. ......
!
I_I_P.ENCES !
I. "Ground-to-Orb!t '.... "...._- "L_vpu,_ion Advanced Applications,
J.T. Kare, these proceedings.
2. Pirri, A.N. and Weiss, R.F., "Laser Propulsion," AIAA Reprint
72-719, Fifth Fluid and Plasma Dynamics Conference (1972).
3. Douglas-Hamilton, D.H., Kantrowitz, A., and Reilly, D.A., "Laser
Assisted Propulsion Research," Progress in Astronautics and
L Aeronautics, Vol. 61, K.W. Billmann, ed., AIAA, New York, 271
(1978).
4. Kantrowitz, A., "Laser Propulsion to Earth Orbit," Workshop on
Laser Propulsion, Vol. 2, J.T.Kare, Ed. (LLNL, Livermore, CA,
1987).
i
5. Rollins, C.J., Bailey, A., Gelb, A., Resendes, D., and Veyl, G.,
"Evaluation of Laser Supported Detonation Wave Thrusters for
Ground-to-Orbit Launch," AIAA 89-1914, AIAA 20th Fluid Dynamics,
Plasma Dynamics and Lasers Conference, Buffalo, NY, June 1989.
6. Rollins, C., Bailey, A., Gelb, A., F_sen, D., Weyl, G. and Wu, Y.,
"Issues in Laser Propulsion," PSI SR-334, and Proceedings 1987 SDIO
Workshop on Laser Propulsion (to be published), Lawrence Livermore,
1987.
7. Goncharov, et al., Soy. J. Quant. Electron., Vol. 3, No. i (1973).
201
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Laser driven launch vehicles for continuous access to space

Laser driven launch vehicles for continuous access to space

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