the electrical energy is converted into microwave energy and is beamed silently and safely to an earth receiver or rectenna. The earth located rectenna, as shown in Figure 1, reconverts the mircowave energy into electrical energy which is then connected to the ground electrical grid for our use. Each Solar Power Satellite is located at a 22,200 mile geosynchronous altitude. At this altitude it takes exactly 24 hours for the satellite to circle the earth and since the earth rotates once every 24 hours the Solar Power Satellite is constantly overhead. This makes it possible, therefore, for us to receive electrical power from the Solar Power Satellite on a nearly continuous basis. The only time electrical energy is not available from the Solar Power Satellite during the year, is during the relatively small period of time when it is shadowed by the earth from the sun's rays. This period, which amounts to less than 1% of the total operational time, occurs near local midnight when electrical requirements will normally be minimal.

Each Solar Power Satellite can generate between 5 million and 10 million kilowatts of electrical energy, enough to supply the needs of most states in our nation and even to meet the

needs of an area as populous as Washington, D.C.

This system, with its many attractive features including

potentially small environmental impact, nevertheless requires

many years of development.

Much work must be done in many

critical technology areas before it can become a reality. Interestingly enough, however, the technical challenges for the Solar Power Satellite are primarily in the engineering areas; scientific breakthroughs do not have to be made in order for this system to be made operational.

ITS STATUS

Grumman has been involved in studies of the solar power system since 1970 along with increasing numbers of other private companies. NASA in-house and funded work have strengthened the study results. The independent ERDA Task Group on Satellite Power Stations stated that "Both economic and net energy compatibility for SPS with future inexhaustible systems (e.g. fusion) were deemed possible if R&D targets could be met."

Early experimental work on some of the critical elements of Solar Power Satellite has also been undertaken over the past seven years. For instance, experiments have been conducted ground-to-ground to show the feasibility of receiving substantial amounts of electrical power by the microwave beam and the needed efficiencies for the system have been roughly verified. Solar array technology is progressing at a heartening

rate and the very lightweight solar arrays needed for the

Solar Power Satellite appear to be within our grasp in the next

10 to 15 years. Techniques for developing the capability for fabricating and assembling these large space structures are beginning to be developed by imaginative engineers.

Although these isolated efforts are encouraging, it is

clear that the work is fragmentary and inadequate.

It does not have the scope or the sense of urgency which should be assigned to one of the possible solutions to our long range energy deficiency. Even when completed, the current efforts

will not bring us substantially closer to the point where commitment to Solar Power Satellite development can be justified.

WHAT IS NEEDED NOW IS A FOCAL POINT FOR TECHNOLOGY

A far more vigorous effort is needed now to provide the firm experimental data that can justify further development steps for the Solar Power Satellite. A coordinated ground program followed by Shuttle sortie missions and leading to a Power Technology Module, PTM, program should be undertaken so that the necessary experimental work will be obtained in a timely low cost fashion.

A clear and early goal such as that provided by the Power Technology Module which could be operational by 1984 will pull diverse and fragmented efforts together toward a common goal. The total system can be deployed in only a single Shuttle flight. Once operational, the Power Technology Module can answer in a year's time the critical SPS technology questions.

The Power Technology Module concept is illustrated in Figure 2. As can be seen, there are two solar arrays which absorb solar energy and provide between 25 and 35 kilowatts of average electrical power. The electrical power is

conditioned and stored in batteries in a pressurized container located between the two arrays. This container could be an advanced Spacelab module. Radiators for rejecting excess heat released by the storage batteries and other electrical equipment are shown above the pressurized container.

Some of the key features of the PTM system are illustrated in Figure 3. The solar arrays differ from conventional arrays used in present day satellites in that they have been fabricated using lightweight manufacturing techniques and have been unrolled like window shades during the assembly process. From the results of its assembly and construction in space, valuable Large Space structure lessons are learned even before the system becomes