Analysis of the Laser Propelled Lightcraft Vehicle

Analysis of the Laser Propelled
Lightcraft Vehicle
Douglas Feikema
Glenn Research Center, Cleveland, Ohio
Analysis of the Laser Propelled
Lightcraft Vehicle
NASA/TM—2000-210240
June 2000
AIAA–2000–2348
The NASA STI Program Office . . . in Profile
Since its founding, NASA has been dedicated to
the advancement of aeronautics and space
science. The NASA Scientific and Technical
Information (STI) Program Office plays a key part
in helping NASA maintain this important role.
The NASA STI Program Office is operated by
Langley Research Center, the Lead Center for
NASA’s scientific and technical information. The
NASA STI Program Office provides access to the
NASA STI Database, the largest collection of
aeronautical and space science STI in the world.
The Program Office is also NASA’s institutional
mechanism for disseminating the results of its
research and development activities. These results
are published by NASA in the NASA STI Report
Series, which includes the following report types:
• TECHNICAL PUBLICATION. Reports of
completed research or a major significant
phase of research that present the results of
NASA programs and include extensive data
or theoretical analysis. Includes compilations
of significant scientific and technical data and
information deemed to be of continuing
reference value. NASA’s counterpart of peerreviewed
formal professional papers but
has less stringent limitations on manuscript
length and extent of graphic presentations.
• TECHNICAL MEMORANDUM. Scientific
and technical findings that are preliminary or
of specialized interest, e.g., quick release
reports, working papers, and bibliographies
that contain minimal annotation. Does not
contain extensive analysis.
• CONTRACTOR REPORT. Scientific and
technical findings by NASA-sponsored
contractors and grantees.
• CONFERENCE PUBLICATION. Collected
papers from scientific and technical
conferences, symposia, seminars, or other
meetings sponsored or cosponsored by
NASA.
• SPECIAL PUBLICATION. Scientific,
technical, or historical information from
NASA programs, projects, and missions,
often concerned with subjects having
substantial public interest.
• TECHNICAL TRANSLATION. Englishlanguage
translations of foreign scientific
and technical material pertinent to NASA’s
mission.
Specialized services that complement the STI
Program Office’s diverse offerings include
creating custom thesauri, building customized
data bases, organizing and publishing research
results . . . even providing videos.
For more information about the NASA STI
Program Office, see the following:
• Access the NASA STI Program Home Page
at http://www.sti.nasa.gov
• E-mail your question via the Internet to
help@sti.nasa.gov
• Fax your question to the NASA Access
Help Desk at (301) 621-0134
• Telephone the NASA Access Help Desk at
(301) 621-0390
• Write to:
NASA Access Help Desk
NASA Center for AeroSpace Information
7121 Standard Drive
Hanover, MD 21076
June 2000
National Aeronautics and
Space Administration
Glenn Research Center
Prepared for the
31st Plasmadynamics and Lasers Conference
sponsored by the American Institute of Aeronautics and Astronautics
Denver, Colorado, June 19–22, 2000
Douglas Feikema
Glenn Research Center, Cleveland, Ohio
Analysis of the Laser Propelled
Lightcraft Vehicle
NASA/TM—2000-210240 AIAA–2000–2348
Acknowledgments
The author gratefully acknowledges the financial support of NASA MSFC and the American Society of Engineering
Education (ASEE) under the 1998 and 1999 summer faculty fellowship program. The author also gratefully
acknowledges the assistance of Mr. Kevin Buch and Mr. Sandy Kirchendal during the summer of 1999.
Available from
NASA Center for Aerospace Information
7121 Standard Drive
Hanover, MD 21076
Price Code: A03
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22100
Price Code: A03
This report is a formal draft or working
paper, intended to solicit comments and
ideas from a technical peer group.
NASA/TM—2000-210240 1
American Institute of Aeronautics and Astronautics
AIAA–2000–2348
ANALYSIS OF THE LASER PROPELLED LIGHTCRAFT VEHICLE
Douglas Feikema
National Aeronautics and Space Administration
Glenn Research Center
Cleveland, Ohio 44135
ABSTRACT
Advanced propulsion research and technology
require launch and space flight technologies, which can
drastically reduce mission costs. Laser propulsion is a
concept in which energy of a thrust producing reaction
mass is supplied via beamed energy from an off-board
power source. A variety of laser/beamed energy concepts
were theoretically and experimentally investigated since
the early 1970’s. During the 1980’s the Strategic
Defense Initiative (SDI) research lead to the invention of
the Laser Lightcraft concept. Based upon the Laser
Lightcraft concept, the U.S. Air Force and NASA have
jointly set out to develop technologies required for
launching small payloads into Low Earth Orbit (LEO)
for a cost of $1.0M or $1000/lb to $100/lb. The near
term objectives are to demonstrate technologies and
capabilities essential for a future earth to orbit launch
capability. Laser propulsion offers the advantages of
both high thrust and good specific impulse, Isp, in
excess of 1000 s. Other advantages are the simplicity
and reliability of the engine because of few moving
parts, simpler propellant feed system, and high specific
impulse. Major limitations of this approach are the laser
power available, absorption and distortion of the pulsed
laser beam through the atmosphere, and coupling laser
power into thrust throughout the flight envelope. The
objective of this paper is to assist efforts towards
optimizing the performance of the laser engine. In order
to accomplish this goal (1) defocusing of the primary
optic was investigated using optical ray tracing and (2),
time dependent calculations were conducted of the
optically induced blast wave to predict pressure and
temperature in the vicinity of the cowl. Defocusing of
the primary parabolic reflector causes blurring and
Copyright © 2000 by the American Institute of Aeronautics and
Astronautics, Inc. No copyright is asserted in the United States under
Title 17, U.S. Code. The U.S. Government has a royalty-free license
to exercise all rights under the copyright claimed herein for
Governmental Purposes. All other rights are reserved by the
copyright owner.
reduction in the intensity of the laser ignition site on
the cowl. However, because of the caustic effect of raytracing
optics the laser radiation still forms a welldefined
ignition line on the cowl. The blast wave calculations
show reasonable agreement with previously
published calculations and recent detailed CFD
computations.
NOMENCLATURE
C average plasma velocity
D* injector diameter
E laser energy deposited
h enthalpy
I laser intensity
ISP specific impulse
L plasma length
M molecular weight
P gas pressure
q heat addition from laser
r blast wave radius
RU universal gas constant
T gas temperature
t time
u* sonic gas injection velocity
V blast wave velocity
Z gas compressibility factor
Greek
ρ gas density
η percentage of laser energy absorbed
γ ratio of specific heats
Subscripts
CJ Chapman-Jouget Condition
LSD laser supported detonation
o initial or ambient state
p laser pulse
REF reference condition
2D two-dimensional
NASA/TM—2000-210240 2
American Institute of Aeronautics and Astronautics
INTRODUCTION
Advanced propulsion concepts for the 21st century
include the use of beamed energy such as a ground
based laser to provide thrust to a vehicle propelled by a
laser-induced blast wave.1,2 Jointly, the U.S. Air Force
and NASA have set out to develop technologies and
concepts required for launching small payloads into
Low Earth Orbit (LEO) for a total cost of $1.5M or
$1000/lb to $100/lb. This concept is shown
schematically in Fig. 1. Among the technology options
available, one potential high pay off approach is using a
high power pulsed laser as an off board energy source.3
Laser propulsion offers the advantages of both high
thrust and theoretically infinite specific impulse, Isp, at
altitudes less 20 km during the first stage and high
thrust and specific impulse in excess of 1000 s during
the second stage above 20 km. Other advantages are the
simplicity and reliability of the engine because of few
moving parts, simpler propellant feed system, and high
specific impulse. Major limitations of this approach are
the laser power available,4 absorption and distortion of
the pulsed laser beam through the atmosphere,5 and
coupling laser power into thrust throughout the flight
envelope.6
Successful operation of a pulsejet engine relies on
synchronous interplay of complex transient phenomena
and the introduction of new vehicle design concepts,
which integrate the total mission requirements.6-9,19 The
mission requirements include blast wave formation and
propagation, pulsed laser heating, inlet air
replenishment in air-breathing mode, first stage, and
propellant injection in rocket mode, second stage.
The concept of Laser Propulsion originates to at
least 1972 and is accredited to Kantrowitz.3 During the
1960’s after the invention of the laser, scientists
investigated the basic phenomenon of laser-induced
breakdown of gases and plasma ignition which forms
the fundamental basis of pulsed laser propulsion.12-15
During the remainder of the 1970’s, much attention was
directed towards conceptual design and basic research of
using beamed energy from a ground based laser to
assess the possibility and feasibility of laser energy for
rocket propulsion.16 Several of these studies involved
laboratory scale experimentation for proof of concept.
One of the most promising of these concepts at this
time appears to be the repetitively pulsed (RP) laser
concept.1,10-11 Typically, a laser pulse is fired into the
rear of a parabolic reflecting engine/nozzle where the
high-intensity radiation is sufficient to cause an
electrical breakdown of a small propellant volume
creating a high temperature, rapidly expanding plasma.
This plasma, which closely resembles a supersonic
blast wave, rapidly exits a supersonic nozzle to create
thrust.
Two methods for obtaining thrust using the RP
concept from laser radiation have been considered. The
first involves rapid vaporization of a solid propellant
with a first pulse followed by a second pulse, which
forms plasma at the surface, and the formation of a laser
supported detonation (LSD) wave. A second approach
involves focusing the laser intensity into a gas near a
solid surface in order to break down the gas and induce
a detonation wave transferring momentum to the
surface. Previous research has shown that ablation and
pitting of the surfaces occur which can strongly reduce
the efficiency of the laser absorption process. Raizer12
(1977) noted for high temperature air where
I = 105 MW/cm2, ρo = 1.3×10–3 g/cm2, and γ = 1.33
that VLSD = 133 km/s and Tequilibrium = 910,000 K.
These values were in reasonable agreement with
experiment, VLSD = 110 km/s and T = 700,000 K.
Therefore, future advances in pulsed detonation laser
propulsion will require mitigating the adverse effects of
material degradation in the regions of intense laser
radiation, which induces high plasma temperatures 12 in
the vicinity of the surface.
Pirri8 et al. (1974) reported on experiments
utilizing a solid and a gas propellant. They concluded
that the best way to achieve a high specific impulse is
to add laser energy directly to gas rather than to first
vaporize a solid propellant. In order to further improve
performance of the laser engine, a mixture of low
molecular weight propellant and a small percentage of
an easily ionizable seed (i.e., H2 or He plus Li or Cs) as
a seed to the gaseous propellant may provide
improvements. Such improvements would enable
shorter duration laser pulses of higher power, which
would enable high thrust times and higher specific
impulse.
In the past several studies have been conducted to
investigate various effects which include threshold
breakdown laser intensities above various surfaces in
air, the effect of air pressure and density on threshold
energies, and the vaporization of various solid
materials. Maher13 et al. (1973) reported that the
ignition thresholds for laser supported detonation waves
for selected materials were determined in air at one
atmosphere pressure. The type of solid reflecting
material has an important effect. For example, for
aluminum the measured threshold laser intensity is
59 J/cm2 and fused silica it is 310 J/cm2. Therefore,
careful selection of the solid reflecting surface requires
careful consideration.
Hettche15 et al. concluded that the impulse coupled
to the solid surface is controlled by the interaction
between the laser energy and air breakdown products at
the surface. They also noted from the pressure
measurements generated by laser fluxes of 1×108 to
3×108 W/cm2 the peak pressure generated varied from
~20 to 220 atmospheres. However, the measured
pressure relaxes rapidly behind the expanding plasma
front. Coupling of the pressure to the target surface after
the initial reaction is dependent on the complex
interaction between the opacity of the dispersing plasma
and the incident beam intensity.
NASA/TM—2000-210240 3
American Institute of Aeronautics and Astronautics
Currently, laser propelled Lightcraft vehicles10,11 are
being successfully flown in a series of experiments
conducted at the High Energy Laser Systems Test
Facility (HELSTF), White Sands Missile Range, New
Mexico. The axisymmetric Lightcraft vehicles are
propelled by airbreathing, pulsed-detonation engines
(PDE) with an infinite specific impulse. Spin-stabilized,
free-flight launches have been conducted and were
limited to altitudes approaching 30 m. High speed
Schlieren and shadow-graph pictures have shown that
the laser-induced shock wave wraps around the outer
perimeter of the engine’s shroud. Also, testing has
shown the shroud or cowl structure fails after 100 laser
firings into the thrust cavity.
The grand vision19 is to integrate the Lightcraft
concept into an earth-to-orbit vehicle. This concept
includes launching a 2-meter diameter Lightcraft vehicle
with initial mass of 100 kg into low earth orbit using a
100 MW laser. The Lightcraft vehicle operates in airbreathing
mode to about 20 km, then transitions into
rocket mode using an onboard inert propellant to a
vertical ascent of 100 km. At 100 km the vehicle turns
over and acceleration down range occurs at 5 to
6 degrees from horizontal. Burnout would occur at
~1000 km after ~15 mins from launch with an estimated
payload of 150 kg in orbit.
A cost estimate for a smaller system,4 however,
using a 20 MW laser, 20 kg orbital payload, and
150 kg of propellant would cost $500M to establish the
laser facility, telescope, adaptive optics, tracking, power
generating plant and the structural facility. The 20 MW
– 20 kg system was noted to be the smallest size that
would be cost effective. Larger systems gain linearly in
payload size versus laser power. It is perhaps best to
begin development of a lower power, low payload
system first in order to develop and demonstrate
technology to provide proof of concept before a
(i.e., 1 GW – 1000 kg) system is implemented.
THEORETICAL DESCRIPTION OF THE LASER
SUPPORTED DETONATION (LSD) WAVE
The basic phenomenon occurring inside the nozzle
is the ionization of gas, plasma ignition, and blast wave
propagation.12 During the 1960’s scientists
demonstrated that a focused laser beam from a high
power laser beam in stagnant gas produced electrical
breakdown and the propagation of a blast or detonation
wave.13-15 When the intense laser radiation was focused
next to a solid surface, impulse and momentum
exchange was observed and quantified.
The inverse bremsstrahlung effect (IB)12,17 is the
prime process assisting to achieve the very high plasma
temperatures (104 to 105 K) necessary for laser engine
operation and creation of high thrust. Initial ionization
of the plasma takes place in the gas as a result of
focusing the laser beam to higher laser intensities than
the threshold value, which generates free electrons.
When the plasma is ignited its unobstructed length18
can be estimated by:
L=2VLSDtp (1)
L is the initial length of the plasma, VLSD is the plasma
detonation velocity, and tp is the laser pulse duration.
In this plasma zone the laser beam energy is absorbed
into the gas and L is proportional to the electron mean
free path. The ionization process is faster and therefore
shorter than a single laser pulse. During the remainder
of the laser pulse free electrons are accelerated until the
laser pulse ends. The gas temperature rises, the plasma
length increases, which leads to expansion of the hot
plasma region. After the laser pulse terminates, a blast
wave front exists because of the high energy absorbed in
the plasma, causing further ionization of the cold gas
layers. Raizer17 noted that the majority of the laser
energy is already absorbed into the plasma rather than
in the gas, because the plasma ignited about
9 ns after firing the laser whereas the laser pulse
duration was 30 ns. Because L increases with
temperature and time, the location where high-speed
electrons come into contact with the cold gas front is
eventually shifted away from the focal point. When the
propagation distance of the plasma exceeds the outer
boundaries of the cold gas in the nozzle cavity, the
wave front simply evaporates into the vacuum outside
the gas boundaries.
The initial absorption wave or Laser Supported
Detonation (LSD) wave velocity12 is:
V
I
LSD
o
= −

 

 
2 2 1 2
1
3
(γ ) ()
ρ
where the maximum heat absorbed or energy release by
the wave is:
q I
oVLSD
= ρ (3)
and I is the laser intensity (W/m2), ρo is the initial gas
density, and γ is the ratio of specific heats. The
conservation of mass and energy applied across the
initial LSD wave can be combined to give:17
h
V
q h
LSD VLSD
0
2
0
2 2
2 2
4 + + = +

 

 
η ρ
ρ
( )
where h is the enthalpy and η is the percentage of laser
energy absorbed into the gas. Reilly et. al.21 note that
this conversion efficiency is typically 80 to 90 percent.
The pressure generated directly behind the Laser
Supported Detonation (LSD) wave front corresponds to
the upper Chapman-Jouget point17 and is given by:
NASA/TM—2000-210240 4
American Institute of Aeronautics and Astronautics
P
V
CJ
= LSD
+
ρ
γ 0
2
1
5
( )
( )
By applying the method of characteristics combined
with Raizer’s equations, the blast wave expansion
process can be described. For the case when a flat
surface or plate is placed at the focal point normal to the
axis of the laser beam the maximum pressure on the
surface of the plate is given by:19
P
V
LSD
=  + LSD
 

 
+
γ −
γ
ρ
γ
γ
1 γ
2 1
6
2
1 0
2
( )
( )
The self-similar solutions for the set of differential
equations for axisymmetric and isentropic conditions
were analyzed by Sedov.20 These solutions are used to
determine flow and thermodynamic variables scaled to a
reference point corresponding to a cylindrical blast wave
when the blast wave has evolved into a cylindrical
shape which corresponds to plasma ignition adjacent to
a flat plate. The unpowered Sedov scaling law’s used
for the present set of conditions for pressure behind the
blast wave and the radius of the wave are:
P
P
t
t
r
r
t
REF REF REF tREF
=


=


−1 12
; (7)
/
The reference conditions19 PREF is PLSD, tREF is t2D
21 or
the time for the blast wave to evolve into a completely
cylindrical expansion, and rREF corresponds to the radius
of the wave when wave can be considered completely
cylindrical, rLSD. Shortly after the laser pulse is
completed, a rarefaction wave moves in from the edges
of the blast wave front, according to the method of
characteristics this wave travels at an average plasma
velocity of CLSD
C
V P
LSD
= LSD=
2
8
γ
ρ
( )
When this rarefaction fan first arrives at the centerline of
the surface the blast wave geometry is considered to be
completely cylindrical such that:21
t
r
C D
LSD
LSD
2 = (9)
A simplistic thermodynamic relation for the plasma
expansion is assumed using an equation of state
incorporating the compressibility factor, Z, such that:19
T
C
Z R
M
LSD
u
=
2
10
γ
( )
where M is the gas molecular weight.
RESULTS AND DISCUSSION
Design A pproaches S uggested b y S pecific Impulse
Simmons and Pirri6 derived an analytic expression
for the specific impulse, Isp, of a repetitively pulsed laser
ignited thruster. The payload mass can be maximized
by maximizing the specific impulse:
I
u E
M t t D sp
p s
≈
−
( *) ( )
( ) *
( )
/ /
/ /
1 2 1 2
1 4 1 2 11
where u* is the gas sonic injection velocity in the
thrust cavity, E is the total laser energy deposited, D*
is the nozzle throat diameter, tp is the time between
pulses, M is the propellant molecular weight, and ts is
the ignition time of the blast wave. As noted by this
expression the payload mass fraction can be improved
by (1) Pulsing the laser at a high rate preferably close to
the same rate as the ignition time (i.e. < 1 μsec),
(2) Increasing the total laser energy deposited, (3) Using
low molecular weight propellants, (4) Injecting hot gas
instead of a cold gas during the second stage since u* is
higher, and (5) Injecting propellant through a small
throat diameter. They also noted an important difference
between this expression and that of a chemical rocket.
The specific impulse for a pulsed laser system favors a
low molecular weight propellant but not by as large a
margin as in a chemical rocket where Isp ~ (M)–1/2.
Therefore, the possibility of using heavier propellants
than in chemical systems opens the door for some
revolutionary new opportunities.
Optical R ay T racing: E ffect o f O ff-Axis D efocusing o f
Primary O ptic
One aspect in assessing the performance of the
lightcraft pulsed laser engine involves studying the
effects of defocusing the laser radiation caused by
misalignment of the laser beam with respect to the
primary optic. These effects will affect flight stability
and performance of the laser engine and are more likely
to occur as the distance between the vehicle and the
ground based laser increase because of the atmospheric
effects on the laser radiation.5
An optical ray-tracing program called code V
developed by Optical Research Associates, Inc. was
used to analyze the effectiveness of the lightcraft
parabolic reflector to focus laser light at a laser
wavelength of 10.6 μm with off axis focusing. Figure 2
shows a cross section of one optical set-up under
perfectly focused conditions. The optical system
consists of (1) the point source at 10 miles behind or to
the left of the lightcraft, (2) the aperture or diameter of
the vehicle, 4.4 in. (3) the parabolic reflector with a
focal distance of ~0.5 in. and (4) the cowl or image
surface. Figures 2 and 3 show the optical system
perfectly focused with traced rays in two and threedimensional
perspectives respectively. Only rays of
NASA/TM—2000-210240 5
American Institute of Aeronautics and Astronautics
light originating from the source, which enter through
the aperture or ring in Fig. 3, are reflected off the
100 percent reflective parabolic surface of revolution. All
rays terminate on the image plane or cowl, which is
shown in Fig. 3 as a bent, rectangular strip wrapped
around the axisymmetric parabolic reflector. In this case
the laser beam and the lightcraft are perfectly aligned.
The parabolic reflector is similar in size and shape to
the actual flight test vehicle recently conducted.10,11
Figure 4 shows the parabola used to construct the
primary optic surface for the optical ray-tracing program
results reported here. The equation for this relation in
units of inches is given by:
z(y)=1.9504y−0.4564y2 (12)
The intensity of laser radiation on the top half
perimeter of the cowl, 180°, is shown in Fig. 5 in
arbitrary units. This cowl surface has been flattened out
and the intensity of the laser radiation in arbitrary units
has been plotted as a function of height on this surface.
For the case of perfect alignment, the intensity is
constant along the hemisphere or length of the cowl and
a well-defined ignition line is observed. Figures 6 and 7
show the effect of rotating the lightcraft 5 degrees about
the x-axis. The intensity of the focused laser energy on
the cowl is shown in Fig. 7. The intensity of the
radiation is a factor of 4.5 less than the perfectly focused
case and nearly constant along the ignition line. It is
observed, however, that the ignition line is now curved
rather than being straight. Figure 8 shows the effect of
rotating the lightcraft 5 degrees about the y-axis. The
peak intensity in the center, which corresponds to the
top of the cowl in Fig. 3, is reduced by a factor of
1.4 from the intensity level shown in Fig. 5.
Significant blurring is observed with a high intensity
on the top of the cowl and a line of decreasing intensity
moving in either direction. Figure 9 shows the effect of
rotating the lightcraft both 5 degrees about the x-axis
and the y-axis. The intensity is blurred somewhat and
is reduced in magnitude by a factor of 4.8. The spatial
deviation of the focused laser intensity is observed to
have a 0.5-in. variation in the y-axis and a well-defined
ignition line is observed. This effect is known as the
caustic effect in ray tracing optics whereby the rays tend
to pile up onto a local region on the image plane. This
is a positive observation of this study, namely that a
well-defined ignition line of focused laser radiation is
observed even when the primary optic is defocused.
This implies that laser ignition will still occur under
defocusing conditions but since the laser intensity
varies along the perimeter of the cowl, the local
thermodynamic conditions will also vary along the
perimeter. This will induce multidimensional effects
associated with the gas expansion process.
Blast W ave C alculations o f the E xpanding P lasma
An analysis and computation using MATHCAD
and the NASA Glenn Chemical Equilibrium Code,
CEA, 1999 were used to determine pressure and
temperatures histories in the vicinity of the cowl. This
analysis is described in detail in Refs. 19, 20, and 21
and the preceding section. In the calculation reported
herein the following parameters were used in the model:
γ = 1.2, ρ0 = 1.2 kg/m3, M = 28.97, Compressibility
factor Z = 2.1, Laser pulse duration = 1×10–6 sec, size
of plasma at end of pulse |rLSD = 5 mm, focused laser
intensity I = 5×1011 Watts/m2. Prior to the beginning of
the laser pulse the air was assumed to be at room
temperature and pressure. The surface pressure history
on the cowl in the region of the focused laser radiation
is shown in Fig. 10. The peak surface pressure builds
up during the laser pulse to a peak pressure of about
250 atmospheres. The pressure decays very rapidly
at t–1 as the plasma expands, however, and
reaches atmospheric pressure after 350 μs or
250 nondimensional time units. The blast wave travels
a distance of 7.9 cm when the blast wave has
completely expanded.
In order to more accurately predict the initial state
of the plasma and the temperature and composition of
the gas, the NASA Glenn CEA code, 1999, was
utilized. The maximum heat absorbed by the gas due to
the laser flux is given by Eq. (3) to be 58.7 MJ/kg.
Initially the peak pressure directly behind the wave as
predicted by Eq. (5) is 271.8 ATM. Next, a T-P
calculation using the NASA CEA 1999 code with a
pressure of 271.8 ATM and an assigned temperature
was completed for atmospheric air. The temperature and
composition of the gas were determined by iteratively
solving Eq. (4) with η = 0.86 and the NASA CEA
code. A temperature of 17,400 K and pressure of 271.8
ATM gave good agreement between the computed
enthalpies (7.0×107 J/kg) using Eq. (4) and the NASA
CEA code assuming 86 percent of the laser energy is
absorbed by the gas. The mole fractions of the highly
ionized and dissociated gas as computed by the code are
shown in table I.
In order to model the temperature decay of the
expanding plasma a simple thermodynamic model is
used to predict the gas temperature using Eq. (10). For
a compressibility factor of Z = 2.1, M = 28.97, and
γ = 1.2 a temperature of 17,400 K is computed. Figure
11 shows the gas temperature decay (t-1) behind the
blast wave for a laser intensity of 5×1011 W/m2. Initially
the temperature in the vicinity of the focused laser beam
reaches a maximum of 17,400 Kelvin. The temperature
is observed to decay rapidly to about 1800 Kelvin in
10 t2D or 14 μs after the laser pulse.
Figure 12 shows the radius of the cylindrically
propagating LSD Wave from initiation of the wave at
t2D of 1.41×10–6 seconds from a radius of 5 mm to a
NASA/TM—2000-210240 6
American Institute of Aeronautics and Astronautics
final radius of 79 mm at 3.5×10–4 seconds at which
point in time the blast wave has dissipated to one
atmosphere of pressure. Figure 13 shows the blast
wave velocity decay from initially 7100 m/s
corresponding to a Mach number of 20. The wave
velocity is observed to decay rapidly at t–1/2.
Figure 14 shows the effect of ambient gas density or
altitude on the peak gas pressure for the conditions
described above for a laser intensity of 5×1011 W/m2. At
an altitude of 20 km where the density of the air is
~0.01 kg/m3 the peak surface pressure on the cowl has
decreased from 250 ATM at sea level to 50 ATM at
20 km.
Since the focused laser intensity is such an
important parameter in generating thrust and heating of
the gas, the effect of laser intensity on the peak surface
pressure and the peak equilibrium gas temperature is
shown in Figs. 15 and 16. In Fig. 15 it is shown that
the peak pressure can theoretically attain 10,000 ATM
at 1×1014 W/m2. Also, from Fig. 16 the theoretical peak
gas temperatures approaches 106 °K when the laser
intensity is 2×1014 W/m2.
CONCLUSION
1. Defocusing of the primary parabolic reflector causes
blurring and reduction in the intensity of the laser
radiation at the ignition site on the cowl. However,
because of the caustic effect of ray-tracing optics the
laser radiation still forms a well-defined ignition
line on the cowl. Under off axis focusing of 5°, the
ignition line becomes curved and reduced in
intensity by up to an order of magnitude in
intensity.
2. The blast wave calculations show reasonable
agreement with the values given in Ref. 19 and
recent CFD calculations.22 The pressure and
temperature decay rapidly and the decay process is
completed in 350 μs. The effected radius measured
from the ignition site is 7.9 cm.
REFERENCES
1. Glumb, R.J. and Krier, H.,” Concepts and Status of
Laser-Supported Rocket Propulsion”, J. Spacecraft,
Vol. 21, No. 1, pp. 70–79, 1984.
2. Birkan, M.A.,” Laser Propulsion: Research Status
and Needs”, Journal of Propulsion and Power,
Vol. 8, No. 2, pp. 354–360, March–April 1992.
3. Kantrowitz, A.,” Propulsion to Orbit by Ground
Based Lasers”, Astronautics and Aeronautics, Vol.
10, No. 5, pp. 74–76, May 1972.
4. Kare, J.T.,” Pulsed Laser Propulsion for Low Cost,
High Volume Launch to Orbit”, Space Power
Journal, Vol. 9, No. 1, pp. 6775, 1990.
5. Volkovitsky, O.A., Sedunov, Y.S., and Semenov,
L.P., P ropagation o f Intensive L aser R adiation in
Clouds , American Institute of Aeronautics and
Astronautics, 1992.
6. Simmons, G.A. and Pirri, A.N.,” The Fluid
Mechanics of Pulsed Laser Propulsion”, AIAA
Journal, Vol. 15, No. 6, pp. 835–842, June 1977.
7. Nebolsine, P.E., Pirri, A.N. Goela, J.S., and
Simmons, G.A.,” Pulsed Laser Propulsion”, AIAA
Journal, Vol. 19, No. 1, pp. 127–128, Jan. 1981.
8. Pirri, A.N., Monsler, M.J., and Nebolsine, P.E.,”
Propulsion by Absorption of Laser Radiation”,
AIAA Journal, Vol. 12, No. 9, pp. 1254–1261,
Sept. 1974.
9. Pirri, A.N.,” Analytic Solutions for Laser-Supported
Combustion Wave Ignition above Surfaces”, AIAA
Journal, Vol. 15, No. 1, pp. 83–91, Jan. 1977.
10. Mead Jr., F.B., Myrabo, L.K., and Messitt,
D.G.,”Flight and Ground Tests of a Laser-Boosted
Vehicle, 34th AIAA/ASME/SAE/ASEE Joint
Propulsion Conference and Exhibit, Cleveland, OH,
AIAA 98–3735, July 13–15, 1998.
11. Myrabo, L.N. and Messitt, D.G., and Mead Jr.,
F.B.,” Ground and Flight Tests of a Laser Propelled
Vehicle”, 36th Aerospace Sciences Meeting and
Exhibit, Reno, Nevada, AIAA 98–1001, Jan. 12–15,
1998.
12. Raizer, I.P., L aser-Induced D ischarge P henomen a ,
Plenum Publishing Corporation, New York, NY,
pp. 189–268, 1977.
13. Maher, W.E., Hall, R.B., and Johnson, R.R.,”
Experimental Study of Ignition and Propagation of
LSD Waves”, Journal of Applied Physics, Vol. 45,
pp. 2138–2145, May 1974.
14. Pirri, A.N., Schlier, R., and Northam, D.,”
Momentum Transfer and Plasma Formation above a
Surface with a High-Power CO2 Laser”, Applied
Physics Letters, pp. 79–81, August 1972.
15. Hettche, L.R., Schriempf, J.T., and Stegman,
R.L.,”Impulse Reaction Resulting from the in-air
Irradiation of Aluminum by a Pulsed CO2 Laser”,
J. Appl. Physics, Vol. 44, No. 9, pp. 4079–4085,
Sept. 1973.
16. Caveny, L.H., Orbit Raising and Maneuvering
Propulsion: Research Status and Needs, Vol. 89,
Progress in Astronautics and Aeronautics, AIAA,
New York, NY, 1984.
17. Raizer, I.P.,” Breakdown and Heating of Gases
Under the Influence of a Laser Beam”, Soviet Physics
Uspekhi, Vol. 8, No.5, pp. 650–673, March–April
1966.
18. Brandstein, A. and Levy, Y.,” Laser Propulsion
System for Space Vehicles”, Journal of Propulsion
and Power, Vol. 14, No. 2, pp. 261–269,
March–April 1998.
NASA/TM—2000-210240 7
American Institute of Aeronautics and Astronautics
19. Myrabo, L.N., “Transatmospheric Laser
Propulsion: Lightcraft Technology Demonstrator”,
Final Technical Report, SDIO Laser Propulsion
Program, Contract No. 2073803, pp. 117–142,
1989.
20. Sedov, L.L., S imilarity a nd D imensional M ethods
in M echanic s , Fourth Russian Ed., Academic Press,
New York, NY, 1959
21. Reilly, J.P., Ballantyne, A., Woodroffe, J.A.,
“Modeling of Momentum Transfer to a Surface by
Laser-Supported Absorption Waves,” AIAA Journal,
Vol. 17, No. 10, p. 1098–1105, 1979.
22. Personal Communication.
TABLE 1—Initial State of the Laser Ignited Air Plasma as
Computed by NASA CEA, 1999, Equilibrium Air Mole
Fractions at 271.8 ATM and 17,400 K.
*e- 1.0830-1 *N+ 9.1863-2
*Ar 3.3503-3 *N- 7.4395-4
*Ar+ 8.5684-4 NCO 3.782-11
*C 1.0079-4 *NO 3.6288-4
*C+ 4.1572-5 *NO+ 1.8947-4
*C- 1.8139-7 NO2 1.0435-8
*CN 4.5023-7 NO2- 1.307-11
*CN+ 1.0521-7 NO3 1.380-14
CN- 7.833-10 *N2 1.5659-3
CNN 4.311-11 *N2+ 5.6956-4
*CO 1.5655-7 N2- 2.1197-6
*CO+ 4.9732-8 NCN 9.153-11
*CO2 8.008-12 N2O 2.7163-8
*CO2+ 6.324-12 N2O+ 4.1142-8
*C2 3.579-11 N3 4.7608-8
*C2+ 2.597-11 *O 1.7189-1
C2- 1.130-13 *O+ 1.5759-2
CCN 2.694-14 *O- 2.4471-4
CNC 3.885-14 *O2 1.5923-5
C2O 1.534-14 *O2+ 8.6215-6
*N 6.0414-1 O2- 3.8408-8
O3 4.5254-9
Figure 1.—The Laser Lightcraft Vehicle Propulsion Concept.
NASA/TM—2000-210240 8
American Institute of Aeronautics and Astronautics
Figure 2.—Cross Sectional view of one Code V Optical System
of Lightcraft Parabolic Reflector showing Ray Tracing
for a perfectly focused laser beam.
Figure 3.—Code V Optical System of Lightcraft Parabolic Reflector
showing Ray Tracing for a perfectly focused laser beam.
NASA/TM—2000-210240 9
American Institute of Aeronautics and Astronautics
Figure 4.—Plot of Parabola used for Primary Focusing Optic.
Figure 5.—Laser Intensity in Arbitrary units along upper half, 180°,
of Cowl of the focused laser radiation for Figure 3. Near Constant Intensity.
Spatial Dimensions of Focused Projected Area (Black):
X: 5 inches, Y: 0.25 inches.
Figure 6.—Code V Optical System of Lightcraft Parabolic Reflector
showing Ray Tracing for a Rotation of 5 degrees about X-axis.
z(y)
y
0 0.55 1.1 1.65 2.2
0
0.5
1
1.5
2
2.5
Parabola for Primary Optic
Distance from Central Axis (inches)
Distance (inches)
NASA/TM—2000-210240 10
American Institute of Aeronautics and Astronautics
Figure 7.—Five Degree Rotation about the x-axis only. Laser Intensity in arbitrary units of the
Focused laser radiation on the Cowl. Near Constant Intensity. Spatial Dimensions: X: 5 inches,
Y: 0.25 inches. Peak intensity decreased by a factor of 4.5 from perfectly focused case,
well formed ignition line but curved. The intensity is near constant along the arc.
Figure 8.—Five Degree rotation about the y-axis only. Laser Intensity in arbitrary units of the
Focused laser radiation on the Cowl. Peak Intensity decreased by a factor of 1.4 in the
center from perfectly focused case. Spatial Dimensions: X: 5 inches, Y: 0.25 inches.
Figure 9.—Five Degree rotation about both the x and y axes. Laser Intensity in arbitrary units of the
Focused laser radiation on the Cowl. Peak intensity decreased by a factor of 4.8 from
perfectly focused case, well formed ignition line but curved.
Spatial Dimensions: X: 5 inches, Y: 0.5 inches.
NASA/TM—2000-210240 11
American Institute of Aeronautics and Astronautics
Figure 10.—Surface Pressure History of Blast Wave For Laser Intensity of 5×1011 W/m2.
Figure 11.—Equilibrium Temperature behind Shock Wave Decay
as a function of time for Laser Intensity of
f
5×1011 W/m2.
P(t)
t
t2D
1 10 100
0
50
100
150
200
250
Surface Pressure History of LSD Wave
Non-Dimensional Time
Surface Pressure (ATM)
T(t)
t
t2D
1 10 100
0
1800
3600
5400
7200
9000
1.08 10
4
1.26 10
4
1.44 10
4
1.62 10
4
1.8 10
4 Temperature Behind Wave
Non-Dimensional Time
Temperature (Kelvin)
NASA/TM—2000-210240 12
American Institute of Aeronautics and Astronautics
Figure 12.—Shock Wave Radius as a function of time for
Laser Intensity of 5×1011 W/m2.
Figure 13.—Shock Wave Velocity as a function of time for
Laser Intensity of 5×1011 W/m2.
VS(t)
t
t2D
1 10 100 1 10
3
0
750
1500
2250
3000
3750
4500
5250
6000
6750
7500
Velocity of LSD Wave
Non-Dimensional Time
Blast Wave Velocity (m/s)
radius(t)
t
t2D
1 10 100 1 10
3
0
0.008
0.016
0.024
0.032
0.04
0.048
0.056
0.064
0.072
0.08
LSD Wave Radius Versus Time
Non-Dimensional Time
LSD Wave Radius (Meters)
NASA/TM—2000-210240 13
American Institute of Aeronautics and Astronautics
Figure 14.—Effect of Gas Density or Altitude on the Peak Surface Pressure
for a Laser Intensity of 5×1011 W/m2.
Figure 15.—Effect of Focused Laser Intensity on Peak Surface Pressure,
ρ0 = 1.2 kg/m3.
PLSDATM (ρ)
ρ
0 0.5 1 1.5 2
0
50
100
150
200
250
300
Effect of Gas Density on Peak Pressure
Gas Density (kg/m3)
Peak Gas Pressure (ATM)
PLSDATM (I)
I
1 10
9
1 10
10
1 10
11
1 10
12
1 10
13
1 10
14
1 10
15
1
10
100
1 10
3
1 10
4
1 10
5
Effect of Laser Focused Intensity
Laser Intensity (Watts/m2)
Peak Surface Pressure (ATM)
NASA/TM—2000-210240 14
American Institute of Aeronautics and Astronautics
Figure 16.—Effect of Focused Laser Intensity on Peak Equilibrium Gas Temperature,
ρ0 = 1.2 kg/m3.
TLSD(I)
I
1 10
9
1 10
10
1 10
11
1 10
12
1 10
13
1 10
14
1 10
15
100
1 10
3
1 10
4
1 10
5
1 10
6
1 10
7 Effect of Laser Intensity on Peak Temp.
Laser Intensity (W/m2)
Peak Equilibrium Plasma Temperature (K)
This publication is available from the NASA Center for AeroSpace Information, (301) 621–0390.
REPORT DOCUMENTATION PAGE
2. REPORT DATE
19. SECURITY CLASSIFICATION
OF ABSTRACT
18. SECURITY CLASSIFICATION
OF THIS PAGE
Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources,
gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this
collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson
Davis Highway, Suite 1204, Arlington, VA 22202-4302, and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188), Washington, DC 20503.
NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89)
Prescribed by ANSI Std. Z39-18
298-102
Form Approved
OMB No. 0704-0188
12b. DISTRIBUTION CODE
8. PERFORMING ORGANIZATION
REPORT NUMBER
5. FUNDING NUMBERS
3. REPORT TYPE AND DATES COVERED
4. TITLE AND SUBTITLE
6. AUTHOR(S)
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)
11. SUPPLEMENTARY NOTES
12a. DISTRIBUTION/AVAILABILITY STATEMENT
13. ABSTRACT (Maximum 200 words)
14. SUBJECT TERMS
17. SECURITY CLASSIFICATION
OF REPORT
16. PRICE CODE
15. NUMBER OF PAGES
20. LIMITATION OF ABSTRACT
Unclassified Unclassified
Technical Memorandum
Unclassified
National Aeronautics and Space Administration
John H. Glenn Research Center at Lewis Field
Cleveland, Ohio 44135–3191
1. AGENCY USE ONLY (Leave blank)
10. SPONSORING/MONITORING
AGENCY REPORT NUMBER
9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)
National Aeronautics and Space Administration
Washington, DC 20546–0001
June 2000
NASA TM—2000-210240
E–12359
WU–101–42–0B–00
20
A03
Analysis of the Laser Propelled Lightcraft Vehicle
Douglas Feikema
Propulsion; Future flight
Unclassified - Unlimited
Subject Categories: 20, 15, and 36 Distribution: Nonstandard
Prepared for the 31st Plasmadynamics and Lasers Conference sponsored by the American Institute of Aeronautics and
Astronautics, Denver, Colorado, June 19–22, 2000. Responsible person, Douglas Feikema, organization code 6711,
(216) 433–5707.
Advanced propulsion research and technology require launch and space flight technologies, which can drastically reduce mission costs. Laser
propulsion is a concept in which energy of a thrust producing reaction mass is supplied via beamed energy from an off-board power source. A variety of
laser/beamed energy concepts were theoretically and experimentally investigated since the early 1970’s. During the 1980’s the Strategic Defense
Initiative (SDI) research lead to the invention of the Laser Lightcraft concept. Based upon the Laser Lightcraft concept, the U.S. Air Force and NASA
have jointly set out to develop technologies required for launching small payloads into Low Earth Orbit (LEO) for a cost of $1.0M or $1000/lb to
$100/lb. The near term objectives are to demonstrate technologies and capabilities essential for a future earth to orbit launch capability. Laser propulsion
offers the advantages of both high thrust and good specific impulse, Isp, in excess of 1000 s. Other advantages are the simplicity and reliability of
the engine because of few moving parts, simpler propellant feed system, and high specific impulse. Major limitations of this approach are the laser
power available, absorption and distortion of the pulsed laser beam through the atmosphere, and coupling laser power into thrust throughout the flight
envelope. The objective of this paper is to assist efforts towards optimizing the performance of the laser engine. In order to accomplish this goal
(1) defocusing of the primary optic was investigated using optical ray tracing and (2), time dependent calculations were conducted of the optically
induced blast wave to predict pressure and temperature in the vicinity of the cowl. Defocusing of the primary parabolic reflector causes blurring and
reduction in the intensity of the laser ignition site on the cowl. However, because of the caustic effect of ray-tracing optics the laser radiation still forms
a well-defined ignition line on the cowl. The blast wave calculations show reasonable agreement with previously published calculations and recent
detailed CFD computations.