On the way to creating a system of distant power supply for space vehicles

Solar Energy Vol. 56, No. 1, pp. 97-109, 1996

Pergamon

0038-092X (95)00085-2

Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-092X/96 $15.00 + 0.00

O N THE WAY TO CREATING A SYSTEM O F DISTANT P O W E R SUPPLY FOR SPACE VEHICLES V . F . P R I S N I A K O V , * S. F. L Y A G U S H I N , * I. N. S T A T S E N K O * and V. I. D R A N O V S K Y * * *Dniepropetrovsk State University, 13 prov. Naukovy, Dniepropetrovsk, 320625, Ukraine and **Design Enterprise "Pivdenne", 3 Kryvorizka Str., Dniepropetrovsk, 320059, Ukraine Abstract--Using power resources of the outer space and its industrialization have become an urgent task in the development of present day civilization. Solar energy is the most suitable basis for power supply for the majority of projects in the near-Earth space. Passing on to the large-scale space-based systems of power supply requires putting into life demonstration experiments in which power transmission by electromagnetic emission is supposed. In the paper the expedience of developing an autonomous power module which provides the possibility of space programs with great power consumption and wireless transmission system running-up is proved. Ukraine can design a solar power satellite of 10-20 kW power on the basis of the research satellite AUOS-SM. Two variants of design solutions for such satellite with solar arrays of great area are presented. Power transmission to the space vehicle-consumer can be conducted by cable as well as in a wireless way. The possibilities of placing microwave and laser energy transmission systems on board the satellite are analysed. It is shown that a power supply system for space vehicles with transmission distance of thousands km can be designed basing on modern lasers. Some experiments making use of great electric power generated by the plant of the satellite under consideration are proposed. The running-up of electric thrusters which are necessary for orientation and distancing is of great interest. Wireless power transmission may be carried on to a small satellite equipped with electric thrusters.

Using power from space on the Earth can be realized in different ways: illumination of cities, regions of farming and civil engineering work, polar regions of the Earth with orbital reflectors based on thin-film mirrors (the first such experiment was carried out by Russian c o s m o n a u t s in February 1993), increase of biomass production on the Earth t h r o u g h light day prolongation, rising energy p r o d u c t i o n by o n - g r o u n d solar power plants due to additional illumination. The most universal way of obtaining solar power from space is the transmission of converted energy from a solar power station t h r o u g h electromagnetic radiation passing easily t h r o u g h the atmosphere, namely microwave or laser emission. Thus, the inexhaustible space power source becomes accessible without restrictions imposed by o n - g r o u n d conditions, i.e. dependence u p o n season and meteorological factors. In outer space solving the problems of Sun tracking and power flux concentration is simplified. The idea of space power stations has won wide public recognition after the pioneer paper by Glaser (1968). P o w e r c o n s u m p t i o n in space grows year by year. Prisniakov (1991) has proposed the formula describing the growth of the m a x i m u m power of space power plants:

1. SOLAR ENERGETICS IN SPACE: PLACE AND PROSPECTS

C o u r a g e o u s plans for space solar power stations are based on the real achievements in designing space vehicle power systems. The development of space technology faced with the problem of energy supply from the very beginning and here using solar power has become the main direction in contradistinction to o n - g r o u n d power industry. Russian scientist Konstantin Tsiolkovsky (1911) was the first who proclaimed the necessity of mastering space power resources. N o w a d a y s , when traditional energetics is confronted with insuperable difficulties and causes the threat of the global ecological catastrophe, when nuclear energetics has turned into a source of new dangers, it becomes clear that the only radical way to the solution of the mankind's global energetic problems consists in going over from two-dimensional industry on the Earth surface to three-dimensional industry. This means partial transfer of power production as well as harmful production with great power c o n s u m p t i o n to the near-Earth space. K v a s n i k o v et al. (1989) believe that in 4 0 - 5 0 yr the space solar energy part in the global energetic balance will make up 26%, including half of electric power c o n s u m p t i o n (except illumination) and high-potential heat for industry.

N = 10 ~/°, where z = t 97

1968 (yr), 0 = 11 yr.

( 1)

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V.F. Prisniakovet

Space vehicles are divided into information, technological and transport ones. Information satellites solve the problems of communication, navigation, geodetic and meteorological supply, the Earth surface observation. The development of information space vehicles goes by way of improving special eqaipment, enhancing its reliability and service life. Their terms of active functioning and working orbit heights increase. This results in the growth of power expense connected with the arrangement of the satellites in space and their operation, causes the increase of electric power consumed on board the space vehicles. Nevertheless, power consumption for the majority of information satellites is not great and will not exceed several tens of kilowatts of electric power in future. The exception is space vehicles intended for explorations of natural resources and environment with the help of radar apparatus and for direct broadcasting and television. Their on-board power consumption will gain hundreds of kilowatts. Space technological system construction and operation will demand considerable energy outlay. Their levels of power consumption will exceed that of information systems by 2-3 orders on average. The development of information and technological space systems essentially depends upon the efficiency of their transport supply. In prospects the mean-year scale of load transportation (including transportation to high orbits) will reach tens and hundreds of thousands of tons. This will require new space transportation systems among which those based on electric propulsion with high specific pulse will occupy the significant place. An energy source for electric propulsion may be placed both on board the transport vehicle and beyond its borders--on the Earth or in space. Power transmission from an external source to the vehicle should be conducted by the directed flux of electromagnetic emission. As objects put into space become more complicated, space systems for assembling and constructing are supposed to be created. Robotic systems with power consumption of tens and hundreds of kilowatts order will be widely used for assembling and repairing largescale space objects. Quantitative characteristics of power consumption levels at different stages of space industrialization are presented in Table 1. A power consumption level, differences in maximum and minimum power, requirements to the power source service life and performance

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stability mainly determine the choice of space vehicle energy installation type. Each type of power plants has its rational region of application. These regions are presented in Fig. 1 developing the scheme proposed by Gilzin (1965) in the coordinates "electric power outputoperation time". According to present-day estimates, power values for which nuclear plants will be expedient economically are more than 25 kW at service life not less than 5 yr. The problem of prolonging operation time has not been solved yet neither for Soviet thermoemission plants "Topaz", nor for Japanese hybrid converters HYDECS with nuclear power sources. The service life of solar plants with photovoltaic converters is determined by the rate of their degradation and can equal to several years increasing when concentrators are used. Such solar power plants must inevitably have great radiation-receiving area, this causes essential difficulties in the case of necessity to provide power more than 100 kW. Solar power plants with dynamic energy conversion possess high radiation resistance and greater efficiency, this allows to decrease the concentrator area and to plan their application at power levels up to 1 MW. According to the prognosis by scientists from the Research Institute of Thermal Processes (Moscow) and Dniepropetrovsk State University, on-board electric power demands for the whole spectrum of tasks in the near-Earth space will not exceed 1 MW up to 2015. That is why the main types of space vehicle power plants in the nearest 20 yr will be solar ones with photovoltaic conversion (and chemical storage) and with turboengine conversion (and heat storage). These estimates are valid for the solar constant value corresponding to the distance between the Sun and our planet. For Martian mission and flights to more remote planets using radioisotope power sources (for small power) and nuclear ones (for great power consumption) is necessary. One can see that solar power is the most important direction in energetics both from the point of view of the global civilization problems and from that of the development of space industry and scientific research. 2. SPS D E M O N S T R A T I O N PROJECTS AS A NECESSARY STAGE OF SPACE POWER DEVELOPMENT

The absence of principal scientific-technological problems which have not been solved up to now is the attractive feature of space power station creation projects differing them from

Power supply for space vehicles

99

Table 1. Power consumption levels at different stages of space industrialization Stage 1 2

3 4

Main consumers and kinds of activities in space Modern information satellites and orbital stations. Radar apparatus of information satellites, transmitting apparatus of communication satellites for direct broadcasting. Experimental production of new materials. Interorbital transport vehicles with electric propulsion. Industrial material production. Assembling large-scale structures. Interorbital transportation using external power sources. Placing harmful industry in space. Using the bowels of Moon and asteroids. Space construction. Transportation from Earth to low orbits based on space power sources.

III ii

1000

ni

" 100 C "~ Z

10 C 1

0.1 0.01

0.1

1

10

100

1000

"r (days) Fig. 1. Rational applications for various types of space power plants: A--chemical accumulators; B--electrochemical generators; Cl--solar photovoltaic; C2--solar turboengine; D--gas-turbine plants with chemical fuel; E-radioisotopic with thermoelectric converter; F--radioisotopic with thermoelectrochemicalconverter; N--nuclear with dynamic and static converters, NI and N2--1imits for thermoemission and thermoelectric plants. fusion energetics. But the work on putting largescale space power projects into life has not been begun yet because there was no country able to finance such a significant undertaking. Indeed, solar power satellite construction could not be of great importance for the military purposes, so this program did not receive an impetus under the conditions of the confrontation of two blocs. At the same time enormous primary expenses are necessary for economic efficiency of space based power supply system. Notice that rapid growth of power consumption is necessary for economical progress of developing countries which have no possibility of creating space solar power stations. In this situation the experimentation in the field of space power systems is important not only for testing the technical principles of their functioning but also for popularizing the idea of space power, its efficiency and safety, providing public opinion support. It is necessary that wide public in the industrial countries possessing mighty rocket-space potential become aware SE 5G: J.-H

Power consumption level (kW) 1-10 102-103

103-104 105-106

of the fact that financing SPS projects means financing peaceful and wealthy future of all the countries because the present resources and environment possibilities, according to M a y u r (1992), can provide the modern American standard of living only for 500 million of people. There are two main directions of experimental works on space solar power station problems, namely, experiments on wireless power transmission on the Earth and solar power satellite (SPS) demonstration projects. The leading space powers have projects of space stations of small power which are capable of energy transmission to the Earth. In all the cases when a microwave beam is used for this purpose the problem of the acceptable (both from economic and technical points of view) size of the power transmission system space segment should be solved. The point is that the transmission coefficient between transmitting and receiving antennas is determined by diffraction, and independently from the value of transmitted power rather large antenna apertures are needed at great distances. An American project developed by "Ad Astra" company (Santa Fe, N.M.) is calculated for 100 k W power. The total satellite mass is 10.5 tons and can be put into a low Earth orbit (LEO) by one "Proton" launcher. The further elevation to the geostationary orbit (GSO) is supposed to be exercised by electric propulsion. The dimensions of the solar array is estimated as 2 0 x 2 5 m. Leonard (1991) proposes to use transmitting antenna of 100 m in diameter and receiving antenna (terrestrial segment) of 1 km in diameter. This proves to be possible due to application of emission with frequency 100 G H z (approx. 40 times greater than in the reference D O E / N A S A system). A Russian project (Central Research Institute for MachineBuilding (TsNIIMash), Kaliningrad, Moscow region) contemplates to put a satellite with the mass of 70 tons having the solar power plant of

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250 kW power with accumulators into the low circle orbit, this enables Mozjorine et al. ( 1 9 9 1 ) to propose applying phased antenna array of 30 m in diameter if the standard frequency 2.45 G H z is used. In Japan the Institute of Space and Astronautical Science has designed the project of SPS 2000 of 10 M W power which is a triangle prism with length 800 m and side width 100 m equipped with a phased antenna grid of 100 m in diameter. According to calculations by Nagatomo and Kiyohiko (1991), the effective operation of such a system at the usual for SPS projects frequency is possible if the SPS is in the equatorial orbit of 1000 km height and the net of rectenna stations with 3 km apertures is available in the Earth's equatorial zone. Each station receives power from the SPS for 200 s during one revolution due to the beam turn to 30 ° by antenna array. The original idea of compact SPS was proposed by Pospi~il et al. (1993): solar cells are placed not in planar structure, but on the surface of a parabolic cylinder serving as an antenna of large size for power transmission to the Earth through microwave beam. Richarz and Spies (1991) develop the German global solar energy concept (GSEK) supposing the connection between on-ground and space solar energetics, i.e. building large-scale solar power stations on the Earth and then providing their day-and-night additional illumination from space. Hence the German program is oriented to the laser power transmission. The demonstration project of SPS with 1MW power is proposed, this power satellite is based on a thin-film solar array and solid-state laser with diode pumping. Such satellite's mass is 20 tons and it can be put into low orbit by one "Ariane 5" launch and then placed into GSO through using electric propulsion. The natural attention of SPS demonstration project designers is attracted by systems of distant power supply for space vehicles. They allow to work out power transmission systems at moderate distances between transmitter and receiver (this is of great importance for microwave transmission) and do not deal with the restrictions on frequencies imposed by the conditions of propagation in the atmosphere. Let us mention the Japanese project of a power supplying satellite (PSS) designed by Matsumoto et al. (1991). The satellite in question is deployed to great dimensions in the circle orbit of 500 km height. It has the form of a disk with diameter 40 m, incidentally planar solar

array generates 100 kW of electric power. The structure has three layers: solar cells, semiconductor power amplifiers and transmitting phased antenna grid formed by microstrip antennas. The spatial coincidence of solar array and antenna one allows to decrease the system mass. The wavelength 1.26 cm (approx. 10 times less than in the reference DOE/NASA system) is used. Then effective microwave power transmission to the distance of hundred km proves to be possible. A space vehicle--consumer put into low elliptic orbit of 200-800 km height hypothetically obtains energy at fixed PSS solar array orientation towards the Sun due to the changes of mutual position of two vehicles. Power receiving rectennas are placed on the spherical balloon in order to provide the process at arbitrary beam direction. In our opinion, the PSS project indicates the limits in distances of microwave power transmission in power supply systems for space vehicles. Toussaint (1991), Prisniakov and Lyagushin (1994a) have shown the limited prospects of using radio frequency emission for this purpose and the necessity of proceeding to the laser power transmission channel. Duchet et al. (1991) developed the project of laser power supply network in space. They consider a solidstate laser with solar pumping to be the most promising variant of laser power transmission technology. Such generating systems were researched by Brauch et al. (1991). 3. UKRAINIAN EXPERIENCE IN THE FIELD OF SPACE POWER PLANTS. HOW CAN IT BE USED? Our country having the powerful space industry may not keep aloof the international activities in space power sphere. Dniepropetrovsk enterprises have designed and manufactured the reliable and economical ecologically clean "Zenit" launcher capable of putting into LEO the pay load of 20 tons (Konyukhov, 1993). In Design Enterprise "Pivdenne" the satellites of the famous series "Cosmos" and "Intercosmos" were designed. The power systems of these research satellites were based on planar solar arrays and chemical accumulators as a secondary source for powering on-board devices in eclipse parts of the orbit and at peak loading. The evolution of on-board power systems of space vehicles designed by "Pivdenne" enterprise is presented in Table 2. It is expedient to commend the following

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Table 2. Performance of power plants of space vehicles designed by "Pivdenne" enterprise Space vehicle

Yearof launching

Operation time (days)

Electric power (W)

Solararray area (m2)

Method of orientation

Cosmos-1 Cosmos-6 AUOS-Earth Ocean Oreol-3 AUOS-SM Ocean-O

1962 1962 1976 1979 1981 1994 1995

60 60 180 180 2000 365 365

3-5 30 250 400 200 850 1500

-4 12 7 12 18 32

-Non-oriented Non-oriented Non-oriented Non-oriented Towards the Sun One-axis

achievements in the elaboration of on-board power systems: • increasing the solar array operation time up to 5 yr; • using c a r b o n - c a r b o n composite frame as a bearing structure which has improved the specific mass characteristics by 1.5 times; • designing an electromagnetically pure photovoltaic array; • using the unfolding mechanisms for putting solar array into operation position based on the material with memory effect. M a n y advances were applied in the design of the satellite "Oreol-3" of the Soviet-French project "Arcad". For the last years rather powerful solar energy plants were used at our space vehicles--up to 1500 W on board the "OceanO" satellite. The efficiency of solar emission power conversion into the direct current form came to 12-15%. Ukraine has some successes in silicon solar cell technology. Semiconductor Physics Institute of the National Academy of Science has elaborated thin-film photovoltaic converters on silicon carbide capable of working at very high temperatures (about 1000 K) and passivated solar cells with efficiency 17-18%. Kiev Polytechnical Institute proposes solar cells of inverse type for space application with surface layer of Si3N4. At the Experimental Designers' Office "Foton" of Dniepropetrovsk State University photovoltaic converters with tunnel contacts characterized by high radiation resistance are produced, the problems of the technique of imposing protecting layers on solar cells are investigated, solar arrays with different concentrators providing from 3 to 50 suns are designed. Such solar arrays need rather exact orientation towards the Sun, but possess a number of virtues: increased radiation stability, greater efficiency due to the increase of the coefficient of gathering inequilibrium carriers by p - n boundary at greater fluxes of light and the diminished area of expensive semiconductor structures. Two experimental panels (dimen-

sions 575 x 570mm) with concentrators of focline and focone types shown in Fig. 2 have been installed on board the space vehicle A U O S - S M (automatical universal orbital station-solar modification) launched on 2 March 1994, for testing under natural conditions. The tests have confirmed their capacity for work and high performance at satellite disorientation up to five angular degrees. The report by Prisniakov et al. (1994a) at the 45th International Astronautical Congress contains more precise information about this experiment. Thermodynamic heat power converters based on closed cycles of Stirling, Brayton and Rankine are the subject of wide theoretical and experimental research by scientists of Dniepropetrovsk University (see Prisniakov, 1993). The operation of turboalternators with solar power source was studied, a system "concentrator-heat receiver" was optimized. The project of a space solar power plant operating by the Brayton cycle with power 3.5 k W and efficiency up to 25% has been designed by Prisniakov et al. (1991). Its using in near-Earth orbits supposes the availability of heat storage for operation in eclipse periods. New interesting results in the theory of phase change materials and heat storage design were the basis for this elaboration. Under the conditions of restricted economic possibilities of Ukraine it is necessary to put

(a)

(b)

Fig. 2. Fragments of experimental solar panels with focline (a) and focone (b) concentrators.

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forward the small-scale space power program of using our achievements in space technology. In our opinion, the creation of a solar power satellite providing the basis of a system for distant power supply in space may be such a program. Energy obtaining in space and power transmission between satellites can be expedient at much less power levels than projects for delivering energy to on-ground consumers. Solar power plants of space vehicles may have longer term of operation than their apparatus for mission purpose. The plant power may be sufficient for simultaneous supplying several vehicles. Thus, autonomous power module making use of solar energy flux seems to be a necessary element for ecologically clean industrialization of space. Up to now the power capacity of space vehicles and orbital stations is a limiting factor. A power module for space programs with great power consumption has not been designed yet. Ukraine can solve this problem. A good result may be attained if a space vehicle being in operation is used as a basic model. Taking into account the specific vehicle task we think that this role can be played by the satellite AUOS-SM possessing the system of precise orientation towards the Sun. The satellite was designed by Design Enterprise "Pivdenne" in the 1980s. 4. DESIGN SOLUTIONS FOR SOLAR P O W E R SATELLITE O N THE BASIS O F

AUOS-SM The basic space vehicle AUOS-SM is intended for designing specialized satellites on its basis with the purpose of supplying complex research of the Sun, other scientific observations as well as carrying out model experiments under space conditions with apparatus which will be used independently in future. Main characteristics of the basic space vehicle: • mass: 1576+50 kg • circle orbit height: 500 km • orbit inclination: 82.5 or 73.5 ° • active operation time: not less than 1 yr • precision of the longitudinal axis orientation towards the Sun: not worse than 10 angular minutes • angular velocities of stabilization in the stationary mode: not more than 0.005°/s • feeding voltage of devices: 24-34 V • temperature conditions in the hermocontainer: from 0 to 40°C

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• launcher: "Cyclone". Composition of the basic space vehicle: • apparatus of command-program trajectory radioline • apparatus of telemetering system • power supply system • thermal control system • antenna-feeder devices • commutation, feeding and control units • on-board cable network • structure. Design-composition scheme of the basic space vehicle supposes its division into the permanent part (unified platform) and the variable part. The permanent part includes: • service apparatus truss • hermocontainer • units of solar array panels. The variable part includes an external truss united by design with an inner one. The main zone for placing special apparatus is the external truss. The possibilities for placing devices inside the hermocontainer are restricted by volumes free from electronic units of basic part apparatus. The general view of the basic vehicle equipped with experimental devices is shown in Fig. 3. Characteristic features of structure and operation of the AUOS-SM vehicle allow to adapt it into the power satellite with minimum additional "polishing-up". We think that the keystage level of on-board power consumption for space vehicles will be 10-20 kW in the nearest future. Orbit programs with great power consumption demand raising to this level from the level of 1-2 kW. This increase of energy plant power is provided at AUOS-SM mainly by enlarging the solar array panels area. The 1st variant of a power satellite presented in Fig. 4 supposes placing solar panels around the tubular part of the hermocontainer in the transport state. The solar array is assembled in two symmetrical units, each of them consists of two sections with 48 single panels. The panels has a rectangular form with dimensions 2400 x 230 x 8 mm. In the transport state they are folded in an "accordion" structure in the zone where the permanent AUOS-SM panels were placed. If single panels are manufactured in the form of three-layer plastic honey-comb shell, the free volume under the launcher fairing is economized greatly. Using the rolling hinges is the peculiarity of contiguous panel coupling. This joining allows to turn panels relatively to the neighbouring ones without increase of their

Power supply for space vehicles

103

Solararraypanels

Externaltruss

/•J

/r~ "-"~

Service apparatus truss

Hermocontainer

Unit of solararraypanels

Fig. 3. Space vehicle AUOS-SM in KI modification.

construction height. The design of rolling hinges permits to duplicate the drive of unfolding and to fix the panels in the unfolded state additionally. All unit sections are fastened by the root panel to the turning bar which takes the position perpendicular to the vehicle axis after the unit liberation. A section deployment is provided by the special device including the drive of a sliding rod, the rod itself and the traverse bar fastened to the free panel of the section. The drive is installed on the cantilever of the turning bar. The total area of the SPS solar array in the 1st variant makes up 95 m 2. Its mass does not exceed 370kg while taking into account the unfolding mechanism, structure elements and a cable network. In the 2nd variant of the energetic space vehicle shown in Fig. 5 the power plant occupies the same free volume around the tubular part of the hermocontainer, but unlike the 1st variant solar array panels are fastened to four bars situated crosswise. The longitudinal axes of bars in the folded and fixed state are parallel to the

longitudinal axis of the basic space vehicle. So the photovoltaic converter panels are organized into four units each including two sections disposed on the turning bars symmetrically. A section consists of six single panels coupled in turn to the section girder and contiguous panels in such a way that arranging in the transport state and successive unfolding are allowed. The process of unfolding occurs in the following way. The bar with the panel unit is turned to 90 ° in respect to the transport state. Then the packets joined through the root panels of the sections are unfolded in regard to the longitudinal girder axis (to the angle 22.5°). The next step: the packets are unfolded in the direction given by the hinges of two contiguous root panels coupled by the smaller sides (to the angle 180°), after this the end panels are unfolded successively (each to the angle 180°), these panels having been coupled along the longer sides. The total solar array area is 63.4 m 2 for the 2nd variant, the array mass is 243 kg. Other arrangements of the solar array of great area at AUOS-SM may also be proposed. We

104

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~

~

Externaltruss

Serviceapparatustruss

l

j/ "~Unitof solararraypanels

~,x:- Turningbar

Solararraypanels

\

Hermocontainer

~in~gl pane

~

Rolling

Fig.4. The 1st variantof SPS:schemeof solararraydeployment. research the ways of arranging large solar panels with concentrators under the launcher fairing. It should be pointed out that if solar arrays of enlarged area are installed on board the A U O S - S M satellite the additional elaboration is necessary for a number of basic vehicle systems such as power supply system, on-board

cable network, system of orientation control and stabilization. Regulative devices, an energy storage and cable network should be calculated for greater power. The increased inertia moments requires the solution of orientation problems. At the same time the power satellite does not need so precise orientation towards the Sun as the research one.

Power supply for space vehicles

Externaltruss

105

f ¢,/~N ~/// Unitofsola~rraypanels

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]

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Fig. 5. The 2nd variant of SPS: scheme of solar array deployment. 5. M E T H O D S O F P O W E R TRANSMISSION T O A SPACE VEHICLE-CONSUMER

Special attention should be paid to the problems of arranging the apparatus for power transmission to the space consumer on board the power satellite. This power transmission may be conducted at direct electric contact between the vehicles due to using a cable system. Cables can be placed in the pay load zone between the hermocontainer and the launcher fairing in the transport state. The difficult problems of the cable transmission, i.e. electric contact setting, distancing and mutual orientation of our SPS and the vehicle-consumer, avoiding the cable twisting and getting tangled, can be solved if electric thrusters and computer control

system are used. Probably, at distances less than 1 km cable transmission is preferable. Naturally, in this case bringing the vehicles together is necessary in advance. If the microwave channel of power transmission is used, antenna systems of dimensions larger than that of SPS are required for effective powering at distances greater than 1 km. So this type of power transmission also supposes bringing the space objects together. That is why microwave systems in reference to the vehicle under consideration is of interest as a basis for demonstration experiments on the whole. The antenna system in the operation state should not darken the solar array. So the room near the hermocontainer bottom is the best place for

106

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the antenna device on board the AUOS-SM. During launching time the central part of this room is occupied by the cylindrical support being the main forced element. We can use this place for the antenna system and other experimental apparatus only under the condition that a tubular support will occupy the peripheral part of the space under the hermocontainer. If such rather complicated changes in the vehicle design are done, a deployed reflecting antenna of 1.5 m diameter or another device with greater dimensions in the folded state can be placed there. Then the microwave generator may be put in the pay load zone between the hermocontainer and the fairing. The solution of the problems of laying the waveguide between the generator and antenna emitter and of cooling the microwave generator requires additional design elaborations. Very likely that the simplest variant is unfolding antenna fastened to the telescopic rod in the pay load zone. Incidentally additional radiators can be placed under the hermocontainer. Modern lasers provide the efficiency of power conversion in the optically active medium up to 70-80%, this enables us to consider them as an element of power transmission systems. Prisniakov and Lyagushin (1994b) analysed the possibility of placing such laser systems on board the power satellite of 10-20 kW power based on AUOS-SM. Two promising laser technologies were considered. 5.1. Metal vapour lasers The active medium is formed by metal vapours (Cu, Pb, Ca, Sr, Mn, Au, etc.). The laser is an electric gas-discharge tube from the designer's point of view. The most attractive feature of such lasers is the high efficiency of the conversion of electric power brought up to the tube into the power of laser light--up to 35%. At the same time their operation is connected with certain difficulties, namely, the high temperature in the t u b e - - m o r e than 1500 K, the necessity of generating high voltage pulses. So their present day efficiency is less than theoretical limit by 1.5 orders. But we believe that the losses can be decreased considerably. On board the solar power satellite oriented towards the Sun with good precision the problem of working medium heating can be solved by the concentrating system of parabolic focline type, modern electronics can minimize energy expenses in the pulse generation system. Thus the conversion efficiency from direct current

into the laser emission form can make up 0.3 for metal vapour lasers. At on-board energy plant power equalling to 20 kW the laser power 6 kW can be produced by the working medium of 1.5 m length and 10 cm diameter. We evaluate the laser mass as 200 kg and the mass of the feeding unit as 50 kg. On the external truss of the AUOS-SM vehicle 450 kg of experimental devices can be arranged. So such a laser can be placed on the hermocontainer of the basic vehicle. Two hundred kilograms seems to be an admissible mass for the laser orientation system. The radiators for heat rejection may be installed near the hermocontainer bottom. The aperture of a receiving photovoltaic array equalling to 2-3 m is enough for effective power transmission at the distance of 1000 km if we increase the aperture of the exit light beam to 0.5 m by a telescope system. Taking the efficiency of monochromatic emission conversion into the direct current form as 70% we obtain 4 k W of power at vehicleconsumer. 5.2. Solid-state lasers with solar pumping The lasers are based on the property of crystals of sapphire-titanium (Al:O3 with Ti admixture) and alumoyttrium garnets alloyed with Cr (black YAG) or Nd to convert the solar radiation energy into the coherent directed emission. The generation takes place in the continuous mode. The solar emission is concentrated on the active crystal contained in the resonance cavity, the generated emission is brought out through the semi-transparent cavity mirror and then passes the telescope system in order to decrease the beam dispersion. Nowadays the conversion efficiency is about 10% but the theoretical limit equals to 90%, everything depends upon the crystal quality. For space vehicle power supply a generating monocrystal of 1 0 - 1 5 m m length and 10mm diameter is enough. The necessity of high solar radiation concentration demands very precise orientation of the concentrating system towards the Sun. If it is placed on the vehicle of AUOS-SM type, the concentrator must be fastened rigidly to the vehicle body. Hence the exit emission has the fixed direction, and this is inconvenient for us. We may propose to install a light guide at the laser outlet. A telescope system for decreasing the beam dispersion should be installed at the external end of the light guide. Such a scheme for power up to l0 kW can be realized at the

Power supply for space vehicles present level of technology. The previous estimates for the aperture of the receiving system are valid. The mass and dimensions of the laser, radiators and orientation system are not a limiting factor. The problem of the large concentrator deployment in space and its sufficient rigidity have to be studied additionally. 6. PLANS OF USING POWER SATELLITE: DEMONSTRATION EXPERIMENT WITH POWER TRANSMISSION TO THE SMALL SATELLITE The wide program of experiments making use of the solar power satellite possibilities can be proposed. At first the considerable amount of energy (in the space technology scale) produced by our SPS can be used on board the satellite itself. We see the prospects of designing a powertechnological module on the SPS basis where experiments on material science and material production using high-temperature technology will be carried on. There is a task for the solar power satellite which has taken shape completely and does not demand a new experimental equipment. This task, namely, the electric thruster running-up under the natural conditions is very urgent for Ukraine. Belan et al. (1994) presented the information about electric propulsion elaborations in Ukraine during the last 30 yr. At Dniepropetrovsk State University (DSU) the powerful electric space propulsion plants providing the thrust of 1-2 N have been designed, this allows to solve the cruise tasks. The problem of the plant service life is the crucial one. On-ground testing electric thrusters requires the availability of complex equipment. Surely, their testing under natural conditions will promote the advances in the increase of electric propulsion service life. Measuring devices for determining thruster performance have been designed by the scientists of DSU. The working thrusters must be placed in diametrically opposite position in reference to the satellite centre of mass and synchronized in order they do not disturb the orbit and orientation of the satellite. The problems of co-ordination of the functioning thrusters have been investigated at DSU. Certainly, we can use the electric propulsion thrust for obtaining the positive result. Prisniakov et al. (1994b) have shown that an effective system for putting space vehicles into high orbits can be designed basing on the solar

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power plant of 20-30 kW power and electric propulsion plant. However, the creation of such a power-propulsion complex is an independent designers' problem. Our SPS can be used as a space tug without great changes in design if it is equipped with a rope system. But the solar power satellite under consideration is of the greatest interest for us as a basis for the system of distant power supply for space vehicles. As we have pointed out, experimentation on wireless power transmission plays an important role in the international SPS activities, and first of all the microwave transmission is treated in view of the large-scale prospects. In the U.S.A. the experiment on microwave power transmission to the distance of about 10 m was carried out in the cargo bay of "Shuttle". In Japan scientists deal with this problem actively, there airplane and helicopter models powered by microwave beams have been designed, the system of power transmission from the first rocket stage to the second one after their division was elaborated. Shimokura et al. (1994) analysed the expedience of microwave power supply for islands. Notice that these experiments perceived as technical achievements though they use the well known principles. Experiments under space conditions are of the greatest importance, their value is accounted for the devices functioning in vacuum upon the space radiation influence. In American experiments a rather great distance between the emitter and the receiver was provided by the great dimensions of the "Shuttle" spacecraft. The basic AUOS-SM has the distance between the opposite borders of the solar array of about 12 m, and we have analysed the possibility of placing microwave devices on the solar panel ends. But this variant of the experimental system should be declared poor because the design solution for it causes considerable difficulties and the microwave emission would be a source of hinders for radio devices in such a scheme. If transmitting and receiving apparatus are placed in suitable points in the pay load zone or near the hermocontainer bottom, we are not able to put into life the graphic example of power transmission within the limits of one vehicle. The problem solution consists in assembling receiving devices on board the filial satellite divided from our SPS. Surely, the problems of maintaining a stable distance between the power satellite and the filial one and of using the received energy in a rational way should be solved. We propose to install electric thrusters

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Fig. 6. Experiment with microwave power transmission to small satellite equipped with electric thrusters.

on b o a r d the small satellite, these thrusters providing its orientation and distancing. Such an experiment (see Fig. 6) would unite the most interesting features of the foreign experience, i.e. operation in space and distant powering the flying vehicle. Testing such a system would give the experimental facts for investigating the methods of power transmission from SPS to a vehicle-consumer. In general, small satellites are widely used for different purposes in m o d e r n space research. The Design Enterprise "Pivdenne" possesses the great experience of creating small satellites delivered by D r a n o v s k y and K o n y u k h o v (1994). Designing the satellite for the suggested experiment m a y be realized on the basis of Ukrainian technologies. Rectennas manufactured in K h a r k i v can provide electric power 0.5 k W per square meter. If the small satellite is placed in the pay load zone, its rectenna cross section can readily make up 2 m 2, the corresponding a m o u n t of energy is sufficient for electric thrusters functioning. The idea of installing electric thrusters on b o a r d the small satellite has arisen in connection with experiments planned by Sanz et al. (1993) for liquid bridge investigations, there electric thrusters are necessary as a source of microaccelerations. The precise thruster control which is the most non-trivial problem here

should be provided by the o n - b o a r d c o m p u t e r of our SPS and the telemetering information channel to the small satellite. All the small satellite systems have to be designed proceeding from the experimental task proposed.

REFERENCES

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Power supply for space vehicles Matsumoto H., Kaya N., Kinai S., Fujiwara T. and Kochiyarea J. A feasibility study of power supplying satellite (PSS). SPS '91, pp. 381-385 (1991). Mayur R. Energy crisis and SPS for Third World future. Syrup. SPS RIO '92 Space Power Systems and Environment in the 21st Century, Rio de Janeiro, pp. 29-36 (1992). Mozjorine Y. A., Senkevich V. P., Koval A. D. and Narimanov E. A. Small-scale space power station: feasibility and usage prospects. SPS '91, p. 99 (1991). Nagatomo N. and Kiyohiko I. An evolutionary satellite power system for integration demonstration in developing nations. SPS '91, pp. 356-363 (1991). Pospifil M., Pospigilova L. and Ko~andrle M. Compact space power station state of art (1993). Prisniakov V. F. SPS interest and studies in U.S.S.R. SPS '91, pp. 36-44 (1991). Prisniakov V. F. Thermodynamic solar energy. Solar Energy and Space, World Solar Summit, Paris, pp. 35-45 (1993). Prisniakov V. F. and Lyagushin S. F. Comparative description of microwave and laser channels for power transmission in a power supply system for space vehicles. 29th Microwave Power Syrup., Chicago (1994). Prisniakov V. F. and Lyagushin S. F. Possibilities of laser power transmission in an energy supply system for space vehicles (1994).

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Prisniakov V. F., Statsenko I. N., Kondratyev A. 1., Markov V. L., Petrov B. E. and Gabrinets V. A. Space power Brayton system with solar heat input. Research of working processes of high temperatures latent heat storage system. SPS '91, pp. 465-470 (1991). Prisniakov V. F., Lyagushin S. F., Statsenko I. N. and Dranovsky V. I. Design solutions for a solar power satellite of small power (1994). Prisniakov V. F., Lyagushin S. F., Statsenko I. N., Konyukhov S. N. and Gorbulin V. P. Power-propulsion complex for putting space vehicles into high orbits (1994). Richarz H. P. and Spies J. The global solar energy concept, "1 MW Demonstration Mission". SPS '91, pp. 373-374 (1991). Sanz A., Lopez-Diez J., Espino J. L., Marco-Gomez V. and Hernandez L. UPM/LB Sat: a small, scientific, educational satellite. 7th Ann. AIAA/Utah State Univ. Conf. Small Satellites (1993). Shimokura N., Sonoi Y., Kaya N. and Matsumoto H. Pointto-point microwave power transmission experiment (1994). Toussaint M. Solar transmission in space: an enabler technology. SPS '91, pp. 285-295 (1991). Tsiolkovsky K. E. Investigation of world spaces by reactive vehicles. Vestnik vozdukhoplavaniya (The Aeronautics Record), No. 18-22 and 2-9 (1911) (in Russian).