Select Committee on Innovation, Universities, Science and Skills Written Evidence

Memorandum 35

Submission from Bristol Spaceplanes Limited


  A very important action the United Kingdom can take to address both our energy security and climate change problems is to join our friends in chartering Space Solar Power System (SSPS) with a view to building an experimental system. An SSPS would use very large satellites—several kilometres across to capture the sun's energy in high GeoSynchronous Orbit (GSO) and cleanly beam it down using wireless power transfer (WPT) to rectennas on the ground; directly into the electric power grids of contracting utilities. SSP has the potential to provide virtually unlimited clean power. The power reaching the Earth from the Sun is about ten thousand times greater than the power consumed by the world's population. High space transportion costs have been a major obstacle, but there are now realistic prospects for large reductions in the next decade or two. There are still several issues to be resolved, but a programme of study and experiment is now well worthwhile.

  Solar energy is converted to direct current by solar, or photovoltaic, cells. That direct current then powers microwave generators which feed a highly directive satellite-borne antenna, which beams the energy to the Earth. There a rectifying antenna (rectenna) converts the energy to direct current. After processing, this is fed to the power grid.

  The first practical WPT demonstration was done by Bill Brown of Raytheon in 1975. Rectennas would be kilometers across, however crops or other farming could be done under these, just as now done under electric power lines. Maxwell's equations governing power transmission argue strongly for a large scale solution, which has been impractical to undertake to date. Many past and current studies and demonstrations of SSP concepts have been done.

  A typical SSP satellite—with a solar panel area of about 10 km2, a transmitting antenna of about two kilometres in diameter, and a rectenna about four kilometres in diameter—may yield an electric power of several Gigawatts. Critical aspects motivating SSP research are:

    (i)  low attenuation of the microwave transmission beam by the Earth's atmosphere;

    (ii)  24-hour energy availability, except around midnight during the equinoxes;

    (iii)  very carbon dioxide emission per unit of power generated; similar to dams;

    (iv)  potential availability of many Terawatts of clean energy for a billion years into the future; and

    (v)  zero fuel costs. (Except for station keeping).

  The Pentagon's National Security Space Office (NSSO) is now objectively exploring SSP for its potential contributions to tactical, operational, and strategic energy security in addition to space security. These studies include exploring the political, scientific, technical, logistical, and commercial feasibility of SSP.

  We recommend UK fund an SSP economic impact and environmental analysis study.

1.  Low cost access to space

  The first requirement before an SSPS could be considered is greatly reduced launch costs. Current commercial space access prices are far in excess of what known SSPS concepts could afford. Graphing what they are and what they would be at higher flight rates, we see the curve below, however. The red dots below are Elon Musk, SpaceX, $1300/lb and below that, Sandia National Laboratories projects $20/lb[52] SpaceX' Demonstration Flight 2 Flight Review Update (PDF version) has been cleared by DARPA and is approved for public release. Two flights are scheduled by year end. The key to SSP is being financially able to charter a company able to financially negotiate the path to those much higher flight rates—the same market SSP provides. This is what Sunsat Corp offers.

  The key is to move space transportation into the private sector. Many businesses and settlements will one day thrive in space; we just have to provide a market that will incentivize low-cost space transportation. Groups such as the Space Solar Power Workshop are recommending that Congress charter a space solar power corporation, to build power satellites, just as they chartered Comsat in 1962, to build communication satellites. This is the simplest and fastest way to throw open the doors to space development, while providing clean baseload power to the planet.

2.  Microwave power transfer

  An SSP satellite would consist of a solar energy collector, to convert solar energy into dc electric power; a dc-to-microwave converter; and a large antenna array to beam the microwave power to a rectenna (rectifying antenna) on the ground.

  For transmitting the power to the ground, frequency bands around 5.8 or 2.45 GHz have been proposed, which are within the microwave radio windows of the atmosphere. The antenna array to transmit the energy to the ground would require a diameter of about 1—2 km at 2.45 GHz, and its beam direction would be electronically controlled and locked to an accuracy of significantly better than 300 m, corresponding to 0.0005 degree (for a geostationary orbit of the satellite).

  In addition to the orbiting SSP satellite, a ground-based power receiving site, the rectenna,—a device to receive and rectify the microwave power beam—has to be constructed to convert the beamed energy back to dc electric power. The size of the rectenna site on the ground depends on the microwave frequency used and the transmitting antenna's aperture. A typical rectenna site would have a diameter of two to three kilometres for a transmitting antenna of km2. This is frequency dependent, however.

  The rectenna (located on the Earth) receives the microwave power from the SPS and converts it to dc electricity. The rectenna is composed of an RF antenna, a low-pass filter, and a rectifier. It is a purely passive system, apart from a low-power pilot beam to maintain assured beam lock. A low-pass filter is necessary to suppress the microwave radiation that is generated by nonlinearities in the rectifier. Most rectifiers use Schottky diodes. Various rectenna schemes have been proposed, and the maximum conversion efficiencies anticipated so far are 91.4% at 2.45 GHz and 82% at 5.8 GHz. However, the actual rectenna efficiency will also depend on various other factors, such as the microwave input power intensity and the load impedance.

  The rectenna array, with a typical radius of approximately 2 km, is an important element of the radio technology for which high efficiency is essential. The peak microwave power flux density at the rectenna site would then be 300 W/m2, if a Gaussian power profile of the transmitted beam was assumed. The beam intensity pattern would be non-uniform, with a higher intensity in the centre of the rectenna and a lower intensity at its periphery. For human safety requirements, the permissible microwave power level has been set to 10 W/m2 in most countries and the SPS power flux density would be constructed to satisfy this requirement at the periphery of the rectenna.

  After suitable power conditioning, the electric output of the rectenna is delivered to the power network.

  Besides microwave power transmission very recently also laser power transmission has been suggested. In such a scenario highly concentrated solar radiation would be injected into the laser medium (direct solar pumping) and transmitted to Earth. On the ground the laser light would be converted to electricity by photovoltaic cells. Such a system would be fundamentally different from a "classical" microwave power transmission: In space there would be the light concentration system and lasers instead of a photovoltaic cell array and the transmitting antenna, and on the ground there would be a photovoltaic cell array instead of a rectenna. Other differences from power density to rectenna/receiver characteristics would be quite different, if laser were to become available or preferred by a customer contract.

3.  Space photovoltaics

  The key elements in the dc power generation for the SPS system are solar cells. Thin-membrane (amorphous) silicon solar cells are expected to be the most suitable today, because of their good performance for a given weight (W/kg), although their conversion efficiency is lower than the figures for crystalline cells. But progress beyond 2000 Watts/Kg in several companies and new technologies continues.

  EMCORE Corporation, for example, announced last month that its PhotoVoltaics Division attained a record solar conversion efficiency of 31% for an new class of advanced multi-junction solar cells optimized for space applications. The new solar cell, the Inverted Metamorphic (IMM) design, is one fifteenth as thick as conventional multi-junction solar cell.

  Developed with the Vehicle Systems Directorate of US Air Force Research Laboratory, the cell will enable extremely lightweight, high-efficiency, and flexible solar arrays to power next generation satellites. EMCORE's investment in technology innovation will enable the introduction of concentrator solar cell products with conversion efficiency of 40% as a part of planned high-volume product roadmap.

  David Danzilio, Vice President and General Manager of EMCORE's PhotoVoltaics Division stated, "The successful demonstration of this new class IMM cell represents the most significant improvement in terms of watts/kg and $/watts in the past decade, which will enable never before envisioned space power applications. Our industry leading scientists and engineers continue to refine and optimize our terrestrial concentrator products and production capabilities to meet our customers' needs and enable CPV systems to achieve the lowest cost of power."[53]

4.  Political/economic planning

  There is no question SSP can be built; the question is how to build it economically—as a private company would. An engineer has been defined as someone who can build for a dime what any fool can build for a dollar. When America has faced such seemingly insurmountable problems as SSP before, often a public/private corporation has been chartered—a cooperation between government and individuals. In 1862 the Transcontinental Railroad Act, which spanned North America with rail, was enacted by Congress.

  The process to create a congressionally chartered corporation, the SunSat Act, is well understood. This was the same legislative tool used to create Comsat in 1962, 100 years after the Transcontinental Railroad. An SSP system is no less a challenge than Comsat or the Transcontinental Railroad were in their day and would also seem to dictate a public/private corporation to reduce those risks via compensating appropriate rewards.

  The only successful path to build SSP, is a congressionally chartered corporation, we call it SunSat Corporation. The purpose in this paper is to explore SunSat Corp's forest as we look at the trees ahead of us. We want to understand the new and complex business process which we must cultivate and drive. Draft Sunsat Legislation has been placed on the web at

5.  Telerobotics

  On 16 June 16 2007, Boeing's Orbital Express system, validated telerobotic and autonomous spacecraft servicing capabilities, performing a fully-autonomous "fly-around and capture" of a client spacecraft. During the five-hour test, the ASTRO (Autonomous Space Transport Robotic Operations) spacecraft used its onboard cameras and video guidance system to separate from, circle and re-mate with Ball Aerospace's NextSat spacecraft. The test primarily used passive sensors with no active exchange of relative navigation information or involvement by ground controllers.

  "Positioned in orbit 60 meters above NextSat, ASTRO followed an imaginary line called the "Rbar," extending from Earth's center to a satellite and beyond, to capture the spacecraft. Rbar is the approach direction needed to effectively service a satellite without interfering with its cameras or antennas.

  ASTRO and NextSat began Scenario 5-1 in the Mated Nominal mode. At the predicted time, ASTRO's autonomous systems separated it from NextSat to a range of up to 120 meters. ASTRO then circled NextSat using its sensor systems to continuously track NextSat during the fly-around. If sensor inputs had deviated outside established limits, an autonomous safing action would have repositioned the spacecraft to a safe location. It did this successfully in mid-May when Orbital Express experienced a computer sensor anomaly. The system's autonomous safing feature maneuvered the spacecraft to a safe location until the team could re-mate them. The team has resolved that anomaly.

  After completing the fly-around, ASTRO maintained its relative position with NextSat at 120 meters for 17 minutes then maneuvered above NextSat to perform a corridor approach to within centimeters of the client spacecraft. The capture mechanism grappled NextSat and performed a soft berth, drawing NextSat and ASTRO together.

  During the next major unmated operation (Scenario 7-1), ASTRO will depart NextSat to a range of four kilometers before approaching the client spacecraft and performing a free-fly capture using its robotic arm.[54]

  Carnegie Mellon's Skyworker, a robot designed for assembling immense projects in space, in particular SSP satellites, can be reviewed at

  An award winning film showing Skyworker in action is also available for viewing.

  NASA's Space Telerobotics Office was closed in 1997, but useful resources remain there:—page/telerobotics.shtm

6.  Satellite Control and Programme Interfaces

  All modern SPS concepts rely on robotic assembly and maintenance systems rather than on human astronauts. Suitable orbit transfer vehicles may need to be developed to transport very large structures from lower to higher orbits. Solar electric propulsion orbital transfer vehicles have been suggested for this purpose. Some corresponding prototype propulsion systems, like a magneto-plasmadynamic thruster, a Hall thruster, and a microwave discharge ion engine have been tested ([1], section

  Other key issues of SPS technology are subsystem lifetime, especially photovoltaics, and maintenance. The limited lifetime of solar cells has already been mentioned, but a long-term radiation hazard also exists for any solid-state device on the SPS, such as, for instance, dc-to microwave converters.

  Both effects can in principle deform the structure and change its attitude. In particular, the radiation pressure exerts a force which is continuously changing in direction with respect to the line joining the satellite and the rectenna. This may pose serious problems concerning the control of the orbit and the orientation of the RF beam. The amplitude of this force is of the order of 100 N for a solar cell area of 10 km2 (2 * solar radiation power flux * 10 km2/velocity of light).

    Regarding maintenance, the present-day experiences for low-Earth orbits with the Hubble space telescope and the International Space Station indicate that maintaining and servicing a much larger system in a much higher orbit may be very difficult and much more expensive than for low Earth orbits. A completely new approach to space maintenance may be required to maintain large assets at geostationary orbit. Currently, progressive replacement is the only viable option. An active defense against "small" incident meteorites could also be valuable.

7.  Alternative energy overview

  Very active discussions concerning global oil peak production dates are in progress. We find the most current and authoritative research as of this date, predicts that global oil production to peak during the 2008 to 2018 timeframe. While we will never "run out" of petroleum; it will simply become too expensive to burn in most cars and trucks. To quote from that study, "In a worst-case scenario, global oil production may reach its peak in 2008, before starting to decline. In a best-case scenario, this peak would not be reached until 2018. These estimates were made in a Swedish study by Fredrik Robelius, whose doctoral dissertation estimates future oil production". and Fields.html

  Also very recently, the most current and authoritative research predicts global coal production to peak around 2025.—"Peak coal by 2025 say researchers", initiated by a German member of Parliament. Authors were Dr Werner Zittel and Jrg Schindler. and

On the Terawatt scale of interest, Biofuels are also not the answer (from EnergyPulse Weekly):

  Peak Soil: Why Cellulosic ethanol and other Biofuels are Not Sustainable and a Threat to America's National Security—Part I By Alice Friedemann, Freelance Journalist—two more parts also linkable from there.

  Briefly summarized below, we find no other baseload energy source as clean, safe or reliable considering the MASSIVE energy quantities we require.


Fossil Fuel
Decades remaining
Fuel very limited
Wind Power
No, intermittent
Ground Solar
No, intermittent
No; drought; complex scheduling
Very limited quantities—competes directly with food production.

8.  Conclusions

  The main conclusion from work done so far on Space Solar Power is that it has potential for providing virtually unlimited clean power but that much research work remains to be done to establish practicability. The emerging space tourism industry offers the prospect of the economies of scale needed to drive down the cost of transport to orbit to levels where experimental SSP satellites can be afforded. The time is therefore right for HMG to start to fund a programme of research into SSP.

9.  Some SSP Links

  1.  URSI Space Solar Power White Paper, report, and appendices at—papers.htm A major focus is on Wireless Power Transfer.

  2.  The Space Solar Power Workshop website at .

  3.  International Telecommunication Union, Question ITU-R 210-1/1 on "Wireless power transmission", 2006,

  4.  L Summerer, Solar Power from Space—European Strategy in the Light of Global Sustainable Development, ESA SPS Programme Plan 2003/2005, GS03.L36, July 2003,—SPS—ProgrammePlan2—06.pdf

  5.  Space Frontier Foundation/National Security Space Office (NSSO) Public discussion area:

  6.  "Pentagon Considering Study on Space-Based Solar Power" By Jeremy Singer, April 11, 2007,—tech—wed.html

July 2007

52   Space Sunshade Might Be Feasible, 3 November 3, 2006, Back

53   EMCORE Announces Significant Performance Advancements of Multi-Junction High-Efficiency Solar Cells for Space and Terrestrial Applications, Back

54   The Boeing Orbital Express, 27 June 2007, Other Orbital Express news releases at Back

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