Four technologies were evaluated to determine their effect on Satellite Power System concepts. Two of these technologies, solid-state power amplifiers and magnetrons, are replacements for the Klystrons used for dc to RF conversion on the satellite. A third technology, laser power transmission, transmits the energy at laser frequencies rather than microwave frequencies. The fourth technology, multibandgap solar cells, has the promise of significantly increased solar to dc conversion efficienty as compared to the reference-concept silicon and gallium arsenide solar cells. The design characteristics of concepts resulting from application of these technologies are summarized

Hanley, G.

Woodcock, G. R.

English

NASA Technical Reports Server

EMERGI~G SPS CONCEPTS 
by G.M. Hanley, Rockwell International 
G.R. Woodcock, The Boeing Company ~ NS2' ~ - - - 22G83 
Four new technologies have recently been evaluated to determine their effect on Satellite Power 
System (SPS) concepts. Two ef these technologies, solid-state power amplifiers and magnetrons, are 
replacements for the klystrons used for dc to RF conversion on the satellite. A third technology, laser 
power transmission, transmits the energy at laser frequencies rather than microwave frequencies. The 
fourth technology, multibandgap solar cells, has the promise of significantly increased solar to dc 
conversion efficiency as compared to the reference-concept silicon and gallium arsenide solar cells. 
This paper summarizes the design characteristics of concepts resulting from application of these 
technologies. 
One of the solid-state microwave concepts, shown in Figure 1, has a configuration similar to the 
reference concept, although this concept has two end-mounted antennas rather than one. Solar energy 
is collected in the same manner as it is on the reference satellite and is conducted to the attached 
microwave antenna using a modified power distribution system. Because of a lower operating 
temperature' requirement for the solid-state amplifiers (compared to klystrons), larger antennas are 
needed that produce less power at the utility interface than the reference concept (2.6 GW per 
antenna compared to 5.0 GW). Although the concept shown in Figure 1 uses a concentrated GaAs 
solar array, a nonconcentrated silicon solar array also could be employed and would have a similar 
collector area. The antenna configurations for this concept are quite different from the reference 
klystron design because of the large number of solid-state power amplifiers needed to provide the RF 
power compared to klystrons (5-10 watts per amplifier for solid state compared to 50-70 kilowatts for 
klystrons). The major problem, however, is distribution of power from the solar array to each of the 
power amplifiers. Feed voltages for the solid-state amplifiers is ten volts compared to voltages for the 
klystron that vary from 8,000 volts to 40,000 volts. One approach proposed by Rockwell to alleviate 
this problem employs a combination of series/parallel strings of power amplifiers to achieve higher 
module voltages along with two-step dc/dc power conversion on the antenna to transform from high 
voltage from the solar array (40,000 volts) to the string voltage (640 volts). The resulting system 
requires advanced technology dc/dc converters with masses of 0.27 kg/kW. An alternate approach 
proposed by Boeing utilizes parallel/series strings up to very high voltages (e.g., 5500 volts) with direct 
power from the array. 
A solid-state sandwich concept, shown in Figure 2, overcomes the power distribution problems of 
the previously described solid-state concept by putting the microwave antenna and power amplifiers 
directly behind the solar array in a "sandwich" configuration. Since the antenna must constantly be 
pointed at a receiving site on the ground, the sun must be reflected onto the array with a two-reflector 
1350 M DrAM 
2 PLCS 
Figure 1. Solid -State End-Mounted Antenna Satellite Concept 
95 
https://ntrs.nasa.gov/search.jsp?R=19820014809 2020-03-21T09:20:52+00:00Z
system. The secondary reflector is fixed to the sandwich array, while the primary reflectors point at 
the sun continuously. Although the reflector area is large, the reflector mass is low since it is 
fabricated from 1/2 mil aluminized kapton. The concept as illustrated in Figure 2 has two microwave 
antennas, each of which transmits 1.2 GW of power to the utility interface on the ground. This 
concept is not practical with silicon solar cells because it is necessary to have concentrated sunlight 
(5.2 suns in this concept) on the solar cells to provide adequate power density to the transmitters. 
Either GaAs or multibandgap solar arrays ar~ feasible under these conditions. 
The sandwich panel concept designed by Rockwell is shown in Figure 3. The solar array is 
bonded to a fiber honeycomb core. The back of the solar array serves as a ground plane for the power 
amplifier drive distribution system located midway through the honeycomb core. Another ground 
plane is located on the other side of the sandwich. This ground plane serves both the amplifier drive 
system and the transmitter. The honeycomb core assembly is attached to a truss structure that 
supports the antenna and power amplifiers. The power amplifiers are shown mounted at the front of 
the truss structure to beryllium oxide wafers that dissipate waste heat from the amplifiers. A single 
amplifier drives each dipole. The total mass per unit area of the sandwich is only 1.68 kg/m2 
Magnetrons appear to have significant advantages compared to klystrons, including increased 
lifetime (up to 30 years compared to ten years for klystrons between replacements), a single rather 
than multiple operating 'voltages (thus eliminating dc/dc conversion), a simpler waste heat rejection 
system (use conduction/radiation of heat rather than heat pipes and radiators), and possible a higher 
dc/RF conversion efficiency (e.g., 90 percent compared to 85 percent). The resulting design concept is 
shown in Figure 4. Although the satellite appears very similar to the klystron reference concept, there 
are some significant differences. Power at the utility interface is 5.6 GW compared to 5.0 GW for the 
reference concept. The mass is only 26.7 million kg compared to 31.6 million kg for the reference 
concept. 
Table 1 compares some of the most significant characteristics of the above described concepts. As 
indicated, the magnetron concept has much lower specific mass compared to the reference concept, 
the solid-state end-mounted antenna concept has higher specific mass, and the solid-state sandwich 
concept, using multi bandgap solar cells, has a specific mass that is similar to the reference concept 
using multibandgap solar cells. The multibandgap solar cells significantly reduce specific mass. They 
are particularly effective on the sandwich concept in this regard. 
Laser power transmission concepts have not yet been defined to the level of detail of microwavr 
concepts. The Boeing Company is currently conducting initial studies of this concept. Three 
approaches are being considered: electric discharge, solar pumped, and free-electron lasers. 
Figure 2. Solid-State Sandwich Satellite Concept 
96 
Figure 3. Solid-State Sandwich Design 
Figure 4. 
ORiQ1NAl PAG~ 1S 
OF POeR QUALITY 
Table 1. Concept Comparisons 
REFERENCE HAGNETRON SOLI D~STATE SOLI D~SiATE 
CONCEPT CONCEPT END~HOUNTED CONCEPT SANDWICH CONCEPT 
TYPE OF SOLAR ARRAY GaAs HBG GaAs HBG GaAs HBG GaAs HBG 
EFFECTIVE CONCENTRA- I. 83 I. 83 I. 83 1. 83 1.83 I. 83 5.2 5.2 
TlON RATIO 
MAXIMUM TRANSMITTED 21 21 28 28 6.6 6.6 0.70 1.1 
POWER DENSITY (kW/m 2 ) 
ANTENNA POWER TAPER 10 10 10 10 10 10 0 0 
(dB) 
TRANSMITTING ANTENNA 1.0 1.0 0.92 0.92 1.4 1.4 1.8 1.6 
APERTURE (km) 
RECTENNA BORESIGHT 10.0 10.0 11.0 11.0 7.5 7.5 4.8 5.4 
DIAMETER (km) 
POWER AT UT Il. INTER- 5.0 5.0 5.6 5.6 5.2 5.2 2.4 3.0 
FACE/SATELL ITE (GW) 
POWER AT UT Il. INTER- 5.0 5.0 5.6 5.6 2.6 2.6 1.2 1.5 
FACE/ANTENNA (GW) 
-5ATELL ITE MASS 31.6 26.0 26.7 21.5 40.0 35.6 20.5 16.4 
(10 kg) 
SATELLITE SPECIFIC 6.2 5. I 4.8 3.8 7.7 6.8 8.5 5.40 
HASS (kg/kW) 
Electric discharge lasers require electric power to drive a high-voltage discharge that pumps the 
laser medium to an excited discharge state and to circulate the lasant through a cooling loop to remove 
waste heat. For this type of system, a solar array may be employed to produce the power. This type of 
system is extremely inefficient, resulting in a large solar array and large radiators. The result is a 
system mass and cost that is not competitive with microwave power transmission systems. 
Direct solar-pumped lasers also are inefficient because of the narrow lasant spectral band and the 
broad spectral characteristics of solar energy. For this reason, an indirect solar-pumped approach is 
used to achieve more compatible spectral characteristics. Solar energy is focused by reflectors into a 
cavity collector (Figure 5). A temperature is achieved in this cavity that releases thermal radiation in 
the spectral region that excites the lasant. Efficiencies of this system are considerably improved. 
The final laser system, the free-electron laser, is shown in Figure 6. In this concept, an electron 
beam is formed (using a klystron as the electron source, which is accelerated in an R F accelerating 
cavity) that produces laser frequency energy upon passing through a magnetic field that causes lateral 
electron movement. The beam is directed to mirror assemblies on each end of the satellite that form a 
laser beam which is directed to a receiving station on the earth. The solar array provides the energy 
that powers the system. The system on the ground for conversion of laser to electrical energy uses 
optical diodes that are analogous to the microwave rectenna. Conversion efficiencies are similar to 
the rectenna system. This system appears to provide the highest efficiency and lowest mass of all laser 
systems studied. 
Figure 7 compares the specific masses of the laser concepts and the reference silicon solar array 
concept that uses klystrons for dc/RF microwave conversion. Current estimates made by the Boeing 
Company indicate that the lowest mass laser concept (free-electronic laser) is about twice the specific 
mass of the reference concept. 
Additional effort remains to be accomplished to evaluate and compare these concepts. Even 
lower mass and cost solid-state antennas need to be developed because of the importance of antenna 
mass on the cost of these concepts. Device development also must proceed to ensure that the 
requirements can be met. Magnetron concepts appear to have the best combination of characteristics, 
but development is needed to determine whether predicted lifetime and efficiency goals can be 
obtained. Because of the obvious advantages of multibandgap solar arrays in improving system 
97 
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~'.RI"TlnNS .. aoUl "'OIlIlN"l 
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Figure 5. Indirectly Optically Pumped Laser SPS 
General Arrangement 
Figure 6. 1-GW Single-Pass Free Electron Laser SPS 
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i 
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~ IiFF'CUNT,OftT'CAlIIIIECTEHI\iA, 
RECEPlII)NAS$l.IliIIfDfOllllloll,StIllS 
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Figure 7. Comparison of Laser Options and 
Reference SPS Concept 
efficiency, research leading to cells with the desired high efficiencies with little increase in cost and 
mass is needed. Additional laser systems studies are needed to determine approaches that may lead to 
reduced mass and cost to make them more competitive with the microwave SPS concepts. In addition, 
because of the problems related to penetrating heavy cloud layers, total power system integration 
studies are needed to determine the degree to which a laser system might penetrate the utility 
network. 
98 


Emerging SPS Concepts

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