ABSTRACT
Solar thermal propulsion is a form of space craft propulsion. Space
craft propulsion is used to change the velocity of space craft and artificial
satellites. There are many methods for space craft propulsion. Each method has
draw backs and advantages, and space craft propulsion is an active area of
research. Solar thermal propulsion conceived in 1956 by Kraft Echrike. Solar
thermal propulsion is an excellent choice because it requires only one
propellant gas and combines moderate thrust with moderate propellant
efficiency. Solar thermal propulsion effectively bridges the performance gap
between chemicals and electric propulsion by potentially offering higher
specific impulse (800 to 1000 seconds) than chemical propulsion (300 to 500
seconds). Typically hydrogen is used as the propellant due to its low molecular
weight corresponding to a high specific impulse.
A solar thermal rocket has to
carry only the means of capturing solar energy such as concentrators and
mirrors. Instead of converging solar energy to electric power as like a
photovoltaic system, a solar thermal propulsion system uses the solar energy
directly as heat. The heated propellant is fed through a conventional rocket
nozzle to produce thrust. The engine thrust is directly related to the surface
area of the solar collector and to the local intensity of the solar radiation.
INTRODUCTION
Solar
thermal propulsion is a form of space craft propulsion. Space craft propulsion
is used to change the velocity of space craft and artificial satellites. There
are many methods for space craft propulsion. Each method has draw backs and
advantages, and space craft propulsion is an active area of research. Solar
thermal propulsion conceived in 1956 by Kraft Echrike. Solar thermal propulsion
is an excellent choice because it requires only one propellant gas and combines
moderate thrust with moderate propellant efficiency. Solar thermal propulsion
effectively bridges the performance gap between chemicals and electric
propulsion by potentially offering higher specific impulse (800 to 1000
seconds) than chemical propulsion (300 to 500 seconds). Typically hydrogen is
used as the propellant due to its low molecular weight corresponding to a high
specific impulse.
A
solar thermal rocket has to carry only the means of capturing solar energy such
as concentrators and mirrors. Instead of converging solar energy to electric
power as like a photovoltaic system, a solar thermal propulsion system uses the
solar energy directly as heat. The heated propellant is fed through a
conventional rocket nozzle to produce thrust. The engine thrust is directly
related to the surface area of the solar collector and to the local intensity
of the solar radiation.
BASIC PRINCIPLE
The
propulsion system of a solar thermal powered space craft consist of three basic
elements.
1.
Concentrator
2.
Thruster/Absorber
3.
Propellant system
Concentrator
focuses and directs incident solar radiation to an absorber/thruster which
receives solar energy, heats and expands propellant (hydrogen) to produce
thrust. A propellant system which stores cryogenic propellant extended periods and
passively feeds it to the thruster/absorber.
The basic principle
of solar thermal propulsion is to utilize the solar light to heat up a
propellant and providing thrust by expanding the resulting hot gas through a
conventional rocket nozzle. Therefore, the light is collected by parabolic
reflectors and focused into a black-body cavity. Inside the cavity the high
temperatures in the focal area are radiated to its walls where the heat is
absorbed and transferred to the propellant flowing around the cavity. The
propellant heats up to temperatures above 2000 K and is expanded through the
nozzle, thereby generating the thrust. The best propulsive performance can be
achieved with hydrogen (lowest molar mass) preferably stored in the liquid
phase.
SOLAR CONCENTRATORS:
Solar concentrators
for use in space have received growing attention in the past few years in view
of their many potential applications. Among those, perhaps the most important
ones are space power generation and solar thermal propulsion. In the former,
the concentrator is used to focus solar radiation on a conversion device, e.g,
a photovoltaic array or the high temperature and of a dynamic engine; in the
latter, concentrated solar radiation is used to heat a low molecular weight
gas, thereby providing thrust to a solar rocket.
In this propulsion scheme,
solar energy is reflected by the large parabolic reflectors towards the rocket
body, where hydrogen fuel is heated to a very high temperature and exhausted
through a nozzle. Another application of space borne solar concentrators is for
power generation. Future mission in space will require abundant power for use
on satellites. While conventional photovoltaic have been used in the past and
provide a reliable source of power, they do have several drawbacks. Their low
efficiencies make it necessary to use large areas of cells, requiring extendible
hard structures for support. These large structures make for a complex
deployment scheme as well as a high system weight. Another drawback is that the
large area required for the low efficiency cells will create significant drag
for satellites, especially in low earth orbit. Solar dynamic power systems
[SDPS] offer a viable alternative to photovoltaic, with lower system weight and
drag area. These power systems typically consist of large parabolic reflectors
that focus solar radiation into a receiver where high intensity heat is
collected. This heat is then used to generate mechanical power using a Brayton,
Rankine, or Stirling cycle engine. The lower
system weight and area is mainly due to the higher efficiency of dynamic power
systems; for a given area of collector surface more energy is generated with
the dynamic power system than with photovoltaic.
A solar concentrator
uses lenses called Fresnel lenses, which take a large area of sunlight and
directs it towards a specific spot by bending the rays of light and focusing
them. Fresnel lenses uses like a dart board, with concentric rings of prisms
around a lens that’s a magnifying glass. All these features let them focus
scattered light from the sun in to a tight beam. Solar concentrators put one of
these lenses on top of every solar cell. This makes much focused light come to
e ach solar cell, making the cells vastly more efficient.
Two concentrator designs,
rigid or inflatable were originally being evaluated under two different
contracts. However, these two different programs have since been merged, with
the inflatable concentrator design taking lead as the primary technology. An
inflatable solar concentrator offers significant advantages in comparison to
state-of-the-art rigid panel concentrators, including low weight, low stowage
volume, and simple gas deployment.
TORUS AND SUPPORT STRUCTURE:
The reflector is
mounted on the torus and support structure such that the mirror focuses solar
radiation into the receiver to the solar energy absorber. An inflatable torus
and support structure can be fabricated with kevlar-weave teflon laminate
materials. Upon deployment, the torus and support structure would have nickel
carbonyl introduce. Solar radiation exposure heats the inflatable, causing
pyrolitic deposition of nickel metal on the inside of the inflatable,
rigidizing it to produce load-heaving capacity, high-rigidity and
high-pointing-accuracy.
GIMBALING RECEIVER ASSEMBLY:
The gimbaling
receiver-assembly is made up of the receiver housing, the reflector mounting
ring rotation systems, and the rotation system that mates from the receiver
housing to the spacecraft. The receiver mechanically points the reflectors to
maintain solar energy focus on the solar energy absorber.
SOLAR ENERGY ABSORBER
The solar energy
absorber produces superheated hydrogen with the heat from the absorption of focused
solar energy. Small capillary metal-matrix heat transfer elements may be useful
in the construction of solar energy absorbers. In the operation of a solar
thermal engine, the absorber configuration as a heat exchanger. Transport of
high intensity solar flux from the concentrator to the solar receiver via
optical fiber cable the solar receiver core is made of graphite cylinder
because of high solar absorbtivity [.7-.9] ,excellent thermal mechanical
stability and ease of fabrication The gas was injected tangentially in to the
graphite cylinder and flows out through the molybdenum tube. The graphite core
is surrounded by the molybdenum radiation shields. Achievement of high temperature
via radiative heat transfer.
POINTING AND NAVIGATION SYSTEM
In order for the
reflectors to remain focused on the solar energy absorber at all times, the
navigation and sun sensing and pointing systems must be integrated in
real-time. Upon change in attitude to the sun the receiver mechanism will make
suitable adjustments to maintain solar radiation pointing accuracy
INFATABLE CONCENTRATORS
Each
solar thermal propulsion vehicle will have two pre-molded, inflatable solar
concentrators made almost entirely of a new polyimide material developed by the
NASA Langley Research
Center Hampton , Virginia
Inflatable space
systems invariably require less packaged volume, are lower in weight and
cheaper through both development and production phases than competing
mechanically erected systems. The potentially harmful effects of the space
environment, including that of micrometeoroids, are much less than originally anticipated
since large inflatable concentrators require very low inflation pressure; gas
lost through leaks can be easily replaced from a small supply of reserve gas.
Inflatable deploy and function very well in space, where the absence of gravity creates extremely low loads.
High surface accuracy is obtained due to the constant force provided by the
inflatant. The ultimate system will require two reflectors, each having an
elliptical rim with a 40m major axis, to provide 40 lbs of thrust to the two
engines of the rocket. Under the present project, a one-fourth scale, 9*7m off
axis concentrator has been under development as a pilot for the full scale
flight unit. The reflector component consists of a reflective membrane made of
specially designed gores and a geometrically identical transparent canopy. The
two forms together an inflatable lens like structure which, upon inflation,
assumes the accurate paraboloidal shape. This inflatable structure is supported
along its rim by a strong, bending-resistant torus.
SOLAR THERMAL PROPULSION
CONCEPTS
Two system level approaches
for STP are currently being explored. Direct gain approach and thermal storage
concept. That determines the amount of rotation required from the concentrator
pointing mechanism.
DIRECT GAIN CONCEPT
In the direct gain
concept the concentrator continuously tracks the sun during the burn while the
space craft remain pointed along the desired orbital trajectory. This requires
that the concentrator be able to rotate up to 180 degrees while the space craft
rolls 180 degrees. The direct gain concept will eventually require that the concentrator
be mounted on a turn-table capable of the large deflections. The absorber
configuration is a windowless heat exchanger having a delivered specific impulse
of 800-960 seconds. Volumetric absorber concepts can potentially provide
performance levels approaches 1100 seconds.
THERMAL STORAGE CONCEPT
The second design approach involves
the incorporation of a thermal storage medium in which solar energy is required
and stored during the coast period of the orbit and when a propulsive burn is
required, propellant flows through the thermal storage medium to provide
thrust. The storage of solar energy enables a higher thrust than the direct
gain concept with smaller concentrators. For efficient operation, the burns of
this engine concept should be performed in the eclipse portion of the orbit.
This greatly simplifies the sun tracking and thrust orientation compared with
the direct gain concept since the system does not have to be "on sun"
during the burn. In the current design concept, which uses rhenium coated
graphite as the thermal storage medium, a delivered specific impulse of 700 to
900 sec is predicted dependent on the thermal storage temperature. Once the
vehicle is in orbit, the concept can also provide on orbit power using the
concentrators and thermionic elements to generate electricity. To achieve the
desired long life for the power system, the concept typically incorporates a
rigid concentrator.
METHODS FOR HEATING PROPELLANT
There are two
methods for heating the propellant. They are direct method and indirect method.
DIRECT METHOD
In the direct
method the propellant flow through sandy material within the heat exchange
cavity. We put holes in the pipes or walls of the indirect heat exchanger so
that the gas flows directly into the heat cavity, which requires a window, as
pictured below: Direct solar radiation absorption (steam goes into windowed
heating chamber In the direct concept, the cylindrical heating chamber rotates
so that the centrifugal force keeps the sand, or "seeds", along the
chamber wall, which is porous to let the gas in. The seeds are chosen for
stability at high temperature and heat transfer properties. (Tantalum carbide
and hafnium carbide are popular.)Heat transfer is more efficient in the direct
concept, i.e., it's more compact, but clouding of the window or eventual
leakage around and other seals are serious concerns. The rotating chamber is
considerably more complex
IN DIRECT METHOD
Indirect solar
radiation has the propellant flow through only pipes or passages in the wall of
a windowless heating cavity as shown below. Then this gas passes through a
nozzle.
WORKING OF SOLAR THERMAL SPACE
CRAFT
The concentrator
and the absorber/thruster are optically coupled with the absorber located at
the concentrator focus. Due to large size inflated concentrators and non rigid
support structure, the optically coupled concentrator absorber configuration
can be sensitive to structural deformations caused by concentrator sub system
rotation or acceleration. The optical wave guide transmission line is the key
component to integrate the concentrator system with the solar thermal receiver.
The cable inlet interfaces with the concentrator system and the outlet
interfaces with the solar thermal absorber. The propellant was injected
tangentially in to the graphite core, which contain channels for heating the
propellant Hydrogen is expanded and produce thrust.
SOLAR THERMAL PROPULSION FOR A SMALL SPACE CRAFT
The
Boeing Company is developing an innovative solar thermal propulsion system for
application to small solar thermal propulsion system for application to small
space craft with funding support by the Air Force Research Laboratory. In this
system, as schematically presented in Fig.7, solar radiation is collected by
the concentrator which transfers the concentrated solar radiation to the
optical waveguide transmission line consisting of low-loss optical fibers. The
optical waveguide cable transmits the high intensity solar radiation to the thermal
receiver for efficient, high performance thrust generation. Part of the solar
radiation can be switched to attitude control thruster as necessary. The
features of the proposed system are:
l. Highly concentrated solar radiation (I03 suns) can be
transmitted via flexible optical waveguide transmission line to the thruster’s
absorber cavity;
2. The flexible optical
waveguide linkage de-couples the thruster from the concentrator to provide
freedom from the constraints imposed on previous solar propulsion system
designs;
3. The configuration of the
solar receiver can be optimized for efficient heat transfer with minimal
re-radiation loss;
4. Aiming and tracking for
the concentrator become significantly easier by moving the termination of the optical
fiber cable to follow the focal point of the primary concentrator
5. High intensity solar
radiation can be switched to different receivers to deploy several them1a1
thrusters as necessary.
The
experimental facility consists of two solar tracking units each with two 50 cm
parabolic concentrators. The two concentrators are mounted on a rotating frame
to track the sun. The optical fiber cable placed at the focal point of
the concentrator transmits the concentrated solar radiation to the solar
receiver located at the center of facility. The optical fiber cable (4 m long)
consists of’37 fused silica fibers (1.2-mm dia). The four optical fiber cables
deliver about 200 W of solar power into the receiver.
The solar receiver is located at the center with four optical fiber cables connecting it to
four concentrators. The configuration of this experimental setup simulates the solar thermal propulsion system described in Fig.8.
The solar receiver is located at the center with four optical fiber cables connecting it to
four concentrators. The configuration of this experimental setup simulates the solar thermal propulsion system described in Fig.8.
The
hardware components that we developed in this program include: optical
waveguide transmission line; interface optical components; and the solar
thermal receiver.
Optical waveguide transmission line
The
optical waveguide transmission line is the key component to integrate the
concentrator system with the solar thermal receiver. The cable inlet interfaces
with the concentrator system and the cable outlet interfaces with the solar
thermal receiver. The cable inlet design we used in this program is based on
our heritage: the quartz secondary concentrator collecting the solar radiation
and injecting it to the optical fibers. Figure 9 shows the inlet portion of the
four optical fiber cables used for this program. All four cables are 4 m long
and each consists of 37 high numerical apertures. The fiber has an excellent
off-axis transmission up to 25 degrees. The design of the cable outlet was
developed for optimum interface with the high temperature solar receiver. A
photo of the fiber cable outlet is given in Fig. 10. The 37 optical fibers
transfer the solar radiation to the 10 mm quartz rod. The quartz rod, by the
principle of total internal reflection, transfers the solar radiation to the
thermal receiver. The tip of the quartz rod is placed close h the receiver high
temperature heat exchanger in order to deliver the solar power directly to the
receiver.
Solar receiver
One
of the important objectives of this program was to demonstrate the basic solar
receiver heat transfer mechanisms:
·
Transport of high intensity
solar flux from the concentrator to the solar receiver
via optical fiber cable;
·
Efficient delivery of high
intensity solar flux to the solar receiver heating element;
·
Achievement of high temperature
via radiative heat transfer; and .
·
Viability of optical
components.
A schematic of the solar
thermal receiver is given in Fig. 11.
The
solar receiver core is made of graphite cylinder (diameter = 1.75 cm; height =
2.54 cm), because of (i) high solar absorptivity (a= 0.7-0.9), (ii) excellent
thermal-mechanical stability, and (iii) ease of fabrication. The gas was
injected tangentia1ly into the graphite cylinder and flows out through the
molybdenum tube. The graphite core is surrounded by the molybdenum radiation shields.
Solar power (200 W) was delivered to the graphite core by four quartz rods
(dia. = I cm).
The
solar receiver housing with four optical fiber cables is shown in Fig.11. The
construction of this housing was similar to the materials processing experiment
conducted in the previous NASA Program. The propellant gas flows from the
bottom of the housing, flows through the heat exchanger, and flows out of the
housing.
BENEFITS OF SOLAR THERMAL
PROPULSION
§ High efficiency at potentially low cost
§ Higher payload fraction than chemical
§ Solar derived electric power
§ Concentrator & high-gain antenna or aero assist system
§ Higher Isp (> 700 s) than chemical options (300 -500 s)
§ Higher thrust-to-weight ratios than electric systems
§ Space solar power
§ Synthetic Aperture radar
§ Sunshield for space telescopes
§ High temperature materials
LIMITATIONS OF SOLAR THERMAL
PROPULSION
- It would not be very useful where places of intensity of sunlight is low
- This propulsion system generates relatively low thrust necessitating 20-30 days to travel from LEO TO geo
- Difficulty of ground level testing
CONCLUSION
In the distant future, low cost
propulsion will be needed for interplanetary travel and unmanned exploration.
NASA forces solar thermal propulsion as a way to boost future payloads from a
low earth orbit to a geosynchronous earth or high orbit. For more distant
travel, a solar thermal engine using this propulsion would acts like a simple,
efficient tugboat in space. Solar thermal propulsion systems would be less
expensive, much simpler and more efficient than today’s rocket engines. A large
liquid hydrogen tank with a innovative feed system was tested at Marshall to
simulate a 30 day solar thermal mission. Data gathered from the tests would
have applications for missions to the moon and mars, as well as boosting
payloads to higher orbits. Solar absorber, thruster, and inflated concentrator
technology development have continued to be advanced under Air force research
laboratory [AFRL] over the last 2 years. Small scale hardware has been designed
and fabricated AFRL for ground level evaluation. Therefore solar thermal
propulsion can be literally defined as the future of space explorations
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