This paper presents a new architecture option for low Earth orbit (LEO) Space-based Solar Power (SBSP) using wireless power transmission (WPT) and a space power relay (SPR) for Earth and Space energy. The goal is to determine the technical viability of this new space power concept to provide energy to the Earth and to a variety of space architectures and determine the possible reductions in mass to orbit.

A Sun-synchronous orbit (SS-O) is a special case of the polar orbit. Like a polar orbit, the satellite travels from the north to the south poles as the Earth turns below it. The orbital plane of a sun-synchronous orbit must also precess (rotate) approximately one degree each day, eastward, to keep pace with the Earth's revolution around the sun. Sun-synchronous orbits are typically low Earth orbits (LEO) with altitudes of 550 to 850 km. There is a special kind of sun-synchronous orbit called a dawn-to-dusk orbit. In a dawn-to-dusk orbit, the satellite trails the Earth's shadow. When the sun shines on one side of the Earth, it casts a shadow on the opposite side of the Earth. Because the satellite never moves into this shadow, the sun's light is always on it. Since the satellite is close to the shadow, the part of the Earth the satellite is directly above is always at sunset or sunrise. This allows the satellite to always have its solar panels in the sun. However, there is a problem with using a SS-O PowerSat. This problem is the Earth’s rotation under the satellite. Because the Earth rotates under the satellite the beam time to a ground rectenna would be very short. To overcome this limitation we turn to the Space Power Relay (SPR) to complete the solution. The proposed solution is to place low mass Reflector Satellites in a 2,000km Equatorial Medium Earth Orbit (EMEO) and use the reflector satellite to reflect the power transmitted from the PowerSat to the rectenna on the Earth’s surface. In the SPR concept, the satellites act as waveguides, and do not perform conversion to DC, therefore it is very efficient. Placing just a few reflector satellites in EMEO to reflect the energy to rectenna on the Earth’s surface is potentially a low cost solution since the reflectors can be small, low mass inflatable structures. We will call this new concept the Space Grid.

Radarsat is an example of a satellite in a low sun-synchronous orbit. More specifically; as it circles the globe from Pole-to-Pole, Radarsat orbits' at an altitude of 798 kilometers above Earth's surface and at an angle of inclination of 98.6 degrees relative to the Equator. Because of its trajectory; Radarsat can rely on its dawn-to-dusk orbit to keep its solar panels facing the sun and therefore rely on constantly generated energy from the sun.

Placing a Space-based Solar Power satellite in SS-O can be very beneficial as it would allow the satellite to produce and use constant power. By using these dawn-to-dusk orbiting power-satellites, these could produce the same power levels of power as those of SolarSats located at geostationary orbits, as it was originally proposed by Peter Glazer in 1969 and which is still discussed by many in the space community. Moreover, by locating a SolarSats in the SS-O (550 to 850 km) it would be much closer to Earth than a SolarSats on GEO (31,000km), therefore allowing for the use of much smaller transmitters and rectenna and so consequently reducing the amount of mass to be transported into the operational orbit.

A comparison of SSP concepts dating back to the 1980s shows the mass problem related to SSP satellites in. The transmitter mass problem for GEO PowerSats can be seen below.

NASA 1980 Option 1: 1x Concentration, 16% efficient PV, 5GW, Mass 51,000,000kg, Transmitter 13,000,000kg, Power 38,000,000kg

NASA 1980 Option 2: 2x Concentration, 20% efficient PV, 5GW, Mass 34,000,000kg, Transmitter 13,000,000kg, Power 21,000,000kg

ISC 1990: 4x Concentration, 20% efficient PV, 5GW, Mass 23,500,000kg, Transmitter 13,000,000kg Power 10,500,000kg

ISC 2010: 4x Concentration, 40% efficient PV, 5GW, Mass 18,250,000kg, Transmitter 13,000,000kg, Power 5,250,000kg

Notice above that even when you double the efficiency of the power system by doubling the solar concentration or doubling the PV efficiency or both that the transmitter mass is still the same - 13 million kilograms for a 5GW transmitter.

To move the Integrated Symmetrical Concentrator (ISC) SSP design version with a total mass of 18,250,000kg into GEO would take 1,825,000kg of ion propellant (ion propellant mass 10% of payload mass). At $5,000 per kg to place this propellant into orbit the cost would be $9,125,000,000. This is just for the propellant for the LEO to GEO trip and does not count the SSP satellite launch costs.

Let's look at this another way. Let's remove the solar power system completely and just launch the transmitter, which would have a mass as indicated above of about 13,000,000kg. To launch this mass into LEO, at $5,000 per kg, would cost $65,000,000,000. Now you still need to launch the power system. Using the ISC 2010 at 4x Concentration and 40% efficiency the estimated mass would be 5,250,000kg x $5,000 per kg for launch is $26,250,000,000. Clearly, launch costs need to be reduced and there are two ways to do this. First, you can reduce the cost of the launch vehicle and this has been talked about constantly since 1969. The second option is to reduced the mass placed into orbit which is actually much more achievable because it can be accomplished by simply moving the PowerSat much closer.

Several concepts have been proposed in the past for low-earth-orbit (LEO) PowerSat beaming to Earth to alleviate the launch cost problem. However, there is a problem with using a SS-O PowerSat. This problem is the Earth’s rotation under the satellite. Because the Earth rotates under the satellite the beam time to a ground rectenna would be very short. To overcome this limitation we turn to the Space Power Relay (SPR) also referred to as the Space Power Grid (SPG) to complete the solution. In the SPR concept, the satellites act as waveguides, and do not perform conversion to DC. We will call this new concept the Space Grid. The proposed solution, which is a new invention, is to place low mass Reflector Satellites in an Equatorial Medium Earth Orbit (EMEO) at approximately 4,000km and use the reflector satellite to reflect the power transmitted from the PowerSat to the rectenna on the Earth’s surface. Placing just a few reflector satellites in EMEO to reflect the energy to rectenna on the Earth’s surface is potentially a low cost solution since the reflectors can be small, low mass inflatable structures.

The Space Grid relies on the use of two separate constellations of satellites. The SS-O SolarSat constellation has access to constant sunlight and is used to produce the power. The Equatorial ReflectorSat constellation is used to distribute the power to the rectenna on the Earth’s surface. Another advantage of SS-O for PowerSat stationing is that they can always be facing the sun side of the Earth. This means that all of the power transmitters can be located on the day side where power is need most. A GEO PowerSat will follow the ground transmitter day and night and does not have the flexibility of the SS-O PowerSat without adding space reflectors. Adding reflectors to a GEO SolarSat would defeat the primary reason for considering GEO. The SS-O PowerSat can provide the same or more benefits as a GEO SolarSat and do so with substantial mass and cost savings. Therefore, there is no longer any reason to consider GEO for SSP SolarSat stationing.

The proposed system would use eight SSP SolarSats and ten ReflectorSats. The reason for ten reflectors is related to the view time of each reflector over a rectenna below, which is about eighteen minutes. From the SS-Orbit each PowerSat can view most of the reflectors and chose which one to beam to using electronic beam steering. The proposed constellation can provide power to eight ground stations.

It has been know since at least 1980 that placing SolarSats in LEO would reduce satellite transmitter mass. The problems with stationing SolarSats in LEO are the limitation on satellite utilization, which would be approximately 60% and beam time to the ground rectenna because the satellite is moving so quickly around the Earth. Very low orbiting constellations of SolarSats would only have 200 – 300 seconds of beam time.

The period of an orbit is;

T = 2 * pi * SQRT( a^3 / mu)

mu = 398,600.4418 km3/s2

T = 90 minutes = 5,400 seconds

Rearranging the equation

a = mu (T/(2 * pi)^2)^(1/3) obtains an altitude of 286.36 km - and the utilization is precisely 59.37%

Moving the SolarSats to 4,000km would get you about eighteen minutes of beam time. By placing approximately ten reflectors in a 4,000km orbit we can achieve constant power as shown below.

Time over target 18 minutes

175.32 sec orbit / 18 minutes of beam time = 9.74 reflectors (rounded to 10)

Another problem with LEO SolarSats is the Earth’s Oceans. For example, the distance from Indonesia to the coast of Colombia and Peru across the Pacific Ocean is 19,800 kilometers (12,300 mi). This will result in a low utilization rate for the reflector satellite constellation. However, since the SolarSats can select from a number of reflectors they can select those reflectors not over an ocean. In comparison with proposed alternatives, the Space Grid approach has clear potential to enable radical improvement in terms of higher performance, lower cost, less mass, higher reliability, improved safety, and ease of manufacturing. For an SBSP system operating in SS-O and incorporating an SPG in equatorial orbit, the addition of the space-based microwave reflector has to be taken into consideration. The much smaller transmission distances means smaller transmitters and smaller rectenna which increases the economic viability verse concepts based in GEO. The reflector satellites are very low mass and therefore the deployment of these satellites would not be overly expensive. Also, the reflectors can be two-dimensional, i.e., flat inflatable structures making them cheap to build.

The development of space-based WPT is important not just for space energy on Earth but also for the exploration and development of space. There are numerous potential uses for WPT in space, including powering ion drive spaceships, MagSails and providing power to planetary bases, factories and colonies. The Space Grid is a revolutionary concept that can improve the capabilities and lower the cost of other government and commercial space activities by providing low mass and therefore low cost solutions to space power.

Power generation is one of the crucial elements of space vehicles and of future infrastructures on planets and moons. The increased demand for power faces many constraints, in particular the sizing of the power generation system. The SPS Space Grid system is a candidate solution to deliver power to space vehicles or to elements on planetary surfaces. Beaming energy to spacecrafts could lower spacecraft mass and improve mission-economic potential. It promises a significant reduction in the cost of space transportation.

Reusable in-space transportation systems must be capable of both high fuel efficiency and high utilization of capacity, or economic costs will remain unacceptably high. Solar electric propulsion (SEP) systems can provide high fuel efficiency but only at the cost of low thrust and transit times that are not compatible with crewed missions. The major contribution of beamed power to the development of space is its unique ability to transfer energy across long distances and across large differences in gravitational potential, making possible such developments in space as the Solar Power Satellite. This technology can also be applied to other space projects such as a space tug in low-Earth orbit propelled by electric thrusters whose power is supplied by a microwave beam originating from SSO SolarSats.

WPT and it relation to space may be thought of as extending our two dimensional power transmission networks on the Earth (or other planets) into space or power collected in space is beamed back to the Earth (or other planets). Such a system could be used for a wide variety of applications. One such application would be providing large amounts of power for an electric space tug needed for an in-space transportation system. Electric propulsion has long been recognized for its benefits if there were a suitable energy source for the large amounts of power required by electric thrusters. Conventional prime power sources in space are massive relative to electric thrusters and must be accelerated along with the less massive parts of the vehicle. Further, they are expensive and costly to transport into space. In contrast, beamed microwave power uses the prime power sources on the Earth’s surface and therefore has a very low mass relative to other potential prime power sources in space, including chemical, nuclear and solar electric. The combination of WPT and electric thruster technology would make it possible to replace conventional chemical rocket propulsion for missions beyond low-Earth orbit with enormous economic and safety benefits.

The only prime source of energy in space is solar. All other sources of energy, fuel cells, batteries, nuclear, and even the arrays that capture the sun’s energy have to be transported to space across punishing gravitational barriers.

An interesting potential application would be to power a Magnet Sail (MagSail) space tug. The magsail would use the energy to power its superconducting or livewire magnet. The magnet would accelerate the magsail using a perigee boost off the Earth’s South Pole.

WPT using microwave power represents a technological breakthrough because the mass of the rectenna on the space tug is about equal to the mass of the ion thrusters, as contrasted to twenty to thirty times as much mass for nuclear or photovoltaic propulsion. As a result of the very low specific mass of the rectenna and its power supply, the space tug can have unprecedented accelerations for an electric propelled vehicle.

The space grid approach integrates the issues of global warming and energy demand with the technologies for space-based solar power and space power relay. Combined these two technologies offer a potential solution to an energy hungry planet. When you consider that there are currently plans to build 50 new coal-fired electrical plants across Europe, dozens of new coal burning plants in China and several dozen new coal and natural gas burning plants across the US, the ability to generate clean energy in space and transfer to the Earth can play a major role in reducing Global Warming by reducing or eliminating the need for new CO2 producing power plants. There are over 49000 electric power plants in the world, generating a total of 2812 GW. Power needs during emergencies, such as the ones in Japan and New Orleans might be better met by transporting lightweight deployable rectenna to the area. These rectenna are simple in function yet they can provide access to large amount of energy directed it to by the Space Grid.

The development of an economically viable SBSP system is critical to the Earth’s future. WPT technology is also critical to supporting sustainable private and government space ventures, including space lift, space exploration and space development. This is an enabling technology that would greatly expand the need for space lift capability from small reusable launch vehicle for space maintenance to large expendable launch vehicles for deploying GW class SBSP satellites into orbit.

The development of a Space Grid using Sun-synchronous orbiting SBSP Satellites with Equatorial orbiting Reflector Satellites appears to offer all of the advantages of GEO SBSP Satellites, including constant power production for base-load energy. Additionally, it appears to offer many advantages over GEO stationing including, no LEO-GEO transportation, ease of maintenance due to closer positioning, much less mass to orbit due to transmitter. While operationally the concept is more complex than GEO stationing, this is overshadowed by the huge potential mass and therefore cost savings. The basic problem being addressed here is the high initial cost of solar power satellite ("SPS") systems. The initial cost is high because the size is high; the size is high because the minimum aperture of the transmitter (and the receiver) is high, and the minimum aperture size is proportional to the distance the power is beamed. So decreasing the distance beamed means that the system can be made smaller, hence cheaper.