EUROSPACE ESA Contract N° 9390/91/F POWERSAT STUDY Executive Summary A Pragmatic Economic Assessment of Future Powersat Operational Concepts & Prospects for an Inexpensive/Near-Term Powersat Demonstration Programme March 1992 EUROSPACE 16, bis avenue Bosquet 75007 PARIS, France Tei.: +33(1)45 55 83 53 Fax: +33 (1)45 51 99 23 Telex: 210 716
EUROSPACE ESA Contract N° 9390/91/F POWERSAT STUDY Executive Summary A Pragmatic Economic Assessment of Future Powersat Operational Concepts & Prospects for an Inexpensive/Near-Term Powersat Demonstration Programme EUROSPACE 16, bis avenue Bosquet 75007 PARIS, France Tel.: +33(1)45 55 83 53 Fax: +33 (1)45 51 99 23 Telex: 210 716 March 1992
EUROSPACE POWERSAT STUDY EXECUTIVE SUMMARY Overview of the Study This Executive Summary summarises the results presented in the Final Report of the Eurospace Powersat Study for ESA (Contract N° 9390/91/F). The Report is split into two major parts: PART I is a general discussion of the economics and potential applications of Powersats within future space infrastructure programmes. The example of using Powersats to augment Space Station Freedom is presented. PART II is a discussion of the prospects for a Powersat Demonstration Programme, and reference concepts for an initial and later advanced demonstrator are outlined. The majority of the discussion focuses on a relatively inexpensive/near-term initial demonstrator using the Ariane 4/ASAP micro-satellite launch capability. An assessment of related activities in other countries is also provided, and possible opportunities for international cooperation are highlighted. The principal observations of the study are summarised below: PART 1: The Economics of Powersats • The vision of constructing large Solar Power Satellites (SPS) to beam clean energy for utilisation on Earth has been recognised as a potentially important application of space technology. In pursuing development of SPSs, a number of intermediate steps are likely to be necessary. One such step involves use of a “central power station” in space for economically supplying power to various user spacecraft. This is the Powersat concept.
• Moreover, as the space-faring nations deploy a diverse and cost-effective in- orbit infrastructure, Powersat-type systems are likely to be mandatory. Growth of the in-orbit infrastructure will be enabled by the availability of economically-supplied power - a situation analogous to the power stations that enabled the industrial revolution on Earth. • As current space operations are very modest, it is has been argued that the cost of developing Powersats cannot be rationalised. The example of communications satellites was studied for clarity, and it was concluded that the possible savings realised from using a Powersat would be overwhelmed by the high development and operational cost of the Powersat itself. Simply put, the cost of a power subsystem (including launch) is relatively modest when amortised over the life-time of each individual communications satellite. • However, Powersats appear economic when they enable a significant recurring cost saving over the life-time of a particular programme. In this sense, the “one-off’ savings generated from not having to build or launch a full-sized power subsystem are of less importance. • Large space stations on the scale of the International Space Station (Freedom) or a later proposed European Manned Space Infrastructure (EMSI), could provide a suitable niche market opportunity in the relatively near-future for a simple Powersat system. • The specific example of Freedom is particularly interesting. Doubling the power of Freedom using integral solar arrays approximately doubles the aerodynamic drag. As a result, this doubles the amount of propellant that must be launched every year to keep the station in orbit. For Freedom’s planned 56 kW Assembly Complete configuration with three solar array wings, about 10 tonnes of propellant, including the container, will need to be
launched each year. (This propellant load changes roughly proportional to the number of solar array wings.) • The study used Freedom data as a baseline for assessing the practicality of Powersats as its nominal support requirements are already very well defined. The objective here is not to propose that Freedom itself should use a Powersat. Rather, Freedom data is used to provide a “credible” gauge of the likely impacts Powersats would have on any future low-Earth orbit space stations. In this respect, similar conclusions could be drawn for an EMSI space station of comparable size and supported by the proposed Ariane V Transfer Vehicle. • For the purposes of the economic analysis, instead of adding two additional arrays to double the user power level of a Freedom-c/a^ station (i.e. from 37.5 kW to 75 kW), a notional Powersat system - whether in low or high Earth orbit - is used to supply this additional 37.5 kW of user power. (Figure 1) As a result, the station would benefit from a doubling in power without having to launch any additional propellant, except for a small amount to overcome the drag of the rectenna. (Important note: For funding reasons and Shuttle constraints, the actual Freedom station will be expanded to only the 56 kW, three solar array wing configuration.) • Two possible Powersat configurations are envisaged, designated the Microwave and Laser Solutions. - In the Microwave Solution, a single Powersat would continuously trail a Freedom-class station within 5-20 km. (Figure 2) As a result, the annual station-keeping propellant requirement for the station would increase only slightly due to the addition of a small rectenna on the station.
Alternative Power Upgrades for Freedom-Class Space Stations (Figure 1)
The Microwave Solution (Figure 2)
- In the Laser Solution, three operational Powersats would be located in a higher orbit (7,500 km) and provide power to Freedom during eclipse passages. (Figure 3) As a result, both station-keeping propellant and battery mass savings would be realisable. Battery savings result because they would not be cycled as frequently, thereby lengthening their lifetime. • Based on existing technology, it appears that only a microwave design can be used for a low-Earth orbit Powersat because its drag coefficient would be much less than an equivalent laser-based design. (This assumes power is generated with photovoltaic cells.) Typically, the global efficiency of microwave-based designs is on the order of 25%, whereas for laser-based designs it is about 1-3%. Likewise, it appears that only lasers can be used from high-Earth orbits because their very tight beams allow the power reconversion equipment on the station to be kept relatively small. This situation would change with the anticipated improvements in laser efficiency. • These two solutions have inherent advantages and disadvantages. - The Microwave Solution appears technically feasible with existing technology. However, as the single Powersat would need to fly in close formation with the space station - decaying and re-boosting at the same time and rate - it could probably only serve one user at a time. - The Laser Solution, by contrast, has greater potential because it could service several users simultaneously. However, this potential is tempered by the fact that such Powersats would require a significant investment in laser systems and reconversion technology. • The Powersats in both these solutions would be launched as fully integrated spacecraft - analogous to large communications satellites. Ariane 5, for
The Laser Solution (Figure 3)
example, would be a suitable launch vehicle. The Powersats would be unsupportable because the capability does not yet exist to routinely and cost- effectively support, upgrade and maintain spacecraft in orbit. • The study provides a first-order assessment of the economic viability of a Powersat system. Although it is not possible to accurately calculate the cost of a Powersat, it is possible to calculate with reasonable accuracy the financial savings from not having to launch the additional propellant (and battery mass) otherwise required by a Freedom-class station if the conventional alternative of adding solar arrays was followed. Hence, in the simplest sense, if some of these user-savings can be used as the revenues to pay for the full cost of a Powersat, such a Powersat would be deemed “economic.” • It was determined that over a period of 10 years the maximum total revenues available to Powersat operators would be in the region of 2 to 4 billion AUs for a Shuttle-supported space station on the scale of Freedom. If the proposed Ariane 5 Transfer Vehicle was used to support such a station, the maximum revenues would be around 1.5-3 billion AUs. • For the Shuttle-supported space station, the estimated revenues might be considered somewhat high as annual Shuttle costs are effectively independent of launch rates. However, other issues must be considered. For example, because the Shuttle is limited to about 8 missions per year, the maximum power level to which a station can be expanded with its own solar arrays would be obviously constrained. In the case of Freedom, expanding the power level beyond the current 56 kW level could be precluded simply because the Shuttle would not be able to launch the additional station-keeping propellant, batteries, and so on. Similar conclusions are likely for an Ariane V-supported space station.
• Hence, Powersats might be a convenient way to enable power expansion of Freedom-class space stations supported by the current generation of launchers. In this sense, the estimated revenues should be viewed as “opportunity cost savings” equivalent to adding 50-75% of one Shuttle or Ariane V mission per year, for example. PART II: Prospects for a Powersat Demonstration Programme • Given the future potential of Powersats and SPS systems, together with the high level of interest in this subject in other countries, it is considered worthwhile to implement a modest technology demonstration programme culminating with an inexpensive flight experiment performed in the 1997/98 timeframe. This initial demonstrator would provide information that can only be achieved through experimentation in space. Equally importantly, it would act as a catalyst for creating interest in Powersat applications and spur development of the advanced demonstrator. A possible time-line for the Powersat demonstrator programme is shown in Figure 4. • The initial demonstrator is not an end in itself, but the first step toward more advanced and costly activities later. Therefore, every effort should be encouraged to minimise the cost of the demonstrator and maintain a compressed schedule. Management innovation may be critical in this regard. • However, the fundamental driver to an inexpensive/near-term demonstrator are the combined effects of launch costs and launch opportunities. Launch systems that are expensive, fly infrequently, and are historically susceptible to significant delays are incompatible with efforts to stay within low costceilings. • Many launch and platform options for the initial demonstrator were reviewed, ranging from sounding rockets and small launchers to Eureca and
Powersat Time-line (Figure 4)
Spacelab. It was concluded that by far the best option (within Europe and the U.S.) for the initial demonstrator should be the use of the Ariane 4/ASAP micro-satellite launch system. Shuttle launch options for the initial demonstrator all fall within the start of the Space Station Freedom assembly sequence and, therefore, will be highly susceptible to significant delays. • The initial demonstrator reference concept is for microwave experimentation only. It uses five or six positions on a single ASAP ring, enabling an equivalent demonstrator mass of up to 200 kg. (Figures 5a & b) The concept hinges around the use of a small tether system needed to maintain an inflatable, 12.5 m diameter rectenna pointed at the transmission system remaining secured to ASAP. Travelling wave tubes (12 Ghz) are used for the microwave source, and power is supplied from a high-rate lithium battery. Depending on whether a fixed or inflatable reflector is used, it is estimated that around 500W can be transmitted between 0.25 and 1.5 km at 60% transmission efficiency. • Total cost of the initial demonstrator is estimated to be in the region of about 10 million AUs spread over 4 or 5 years. • The initial demonstrator would require use of some unproven technologies, such as tethers and inflatable structures. Therefore, the ASAP initial demonstrator offers a rare opportunity to act as a test-bed for new technology - but only for technology critical to the success of the demonstrator’s mission. It is because of the high cost and minimal number of opportunities to access space that such technologies have limited or no spaceflight heritage. • The requirements for a microwave and laser demonstrator are very different.
Concept for the Powersat ASAP Demonstrator (Option 1) (Figure 5a)
ASAP Demonstrator Mission Overview (Figure 5b)
- For microwaves, the most critical aspect is being able to control a microwave beam distorted by non-linear space plasma interactions in low Earth orbit. As a result, this mandates a microwave space experiment at high power levels. - For lasers, plasma interactions are not a concern, hence, low power levels can be used. Precise tracking and beam pointing over enormous distances is the primary requirement for an initial laser experiment. However, many of these requirements are enveloped by the current SILEX intraspace communications experiment planned for launch on Artemis. As a result, it could prove preferable not to embark on a laser ASAP demonstrator (although more detailed analysis would be required) but, instead, to focus on the development of a high-efficiency/light- weight/high-power laser system in preparation for later advanced demonstrator activities. • Throughout the course of the launcher and platform evaluation, it was deemed that the German Astro-SPAS platform would be the most suitable option for an advanced demonstrator programme with a 2002-2005 launch date. Although dependent on the Shuttle and restricted to a mission length of about one week, it would be significantly less expensive than building a dedicated spacecraft and launching it on Ariane 5 or another expendable launcher. In addition, as current US Powersat efforts seem to be moving in the direction of a Shuttle-launched free-flyer option, Astro-SPAS could be offered as a suitable platform for an international cooperative or participatory programme. • The Astro-SPAS advanced demonstrator would be for either a microwave or laser experiment. The microwave demonstrator would essentially be a subscale version of an operational Powersat, demonstrating orbital control concepts for close formation orbiting. (Figure 6) A full-scale rectenna would
Microwave Advanced Demonstrator (Figure 6) Laser Advanced Demonstrator (Figure 7)
also be used and the transmitted power level would be 2-5 kW. The laser demonstrator would test a subscale version of a laser system for a future operational Powersat with power levels on the order of 1-2 kW, for example. (Figure 7) In addition, the laser demonstrator would facilitate the first space-to-ground experiments. Precise definition and configuration of the Astro-SPAS mission would be based on the results of the ASAP mission and progress of the ground laser development work. Options for Cooperation and Participation with other Countries • Current US, Japanese and CIS activities in this area are far more advanced than those in Europe. However, there is strong overlap between the proposed experiments in these countries with those discussed in this study. Because future Powersat and SPS systems will inevitably require international users beyond Europe, it would be prudent to explore cooperation and participatory programmes at an early stage. While Europe could undertake all of the proposed activities, it might be more advantageous to conduct them within an internationally coordinated framework. • Cooperation or participation would also help develop a constituency of support, especially if efforts were focussed on one or two specific experiments. The ASAP demonstrator could act as such a focus, and it already has received some interest from U.S. organisations. • The activities of the Commonwealth of Independent States are considered to be more extensive than in Europe, although relatively little information is available. Therefore, additional Powersat activities should include an assessment of the former Soviet capabilities to avoid possible loss of their experience. The cost of liberating this information, through a joint study effort, might be small to Europe and produce disproportional benefits.
Acknowledgement of Support Eurospace would like to acknowledge a number of European and US organisations for their support during the course of this Powersat study. In Europe • Thomson Tubes Electroniques, • Thomson CSF-SDC, • AEA Technology, Culham Laboratory, • ETCA, SAFT, • Oerlikon-Contraves, • Technology Detail, DASA/MBB-ERNO, • DASA/MBB-Space Communications & Propulsion Division, • DASA-Telefunken Systems Technik, • MAN Technologic, ONERA, ESTEC, • Association International Arsat, • Surrey Satellite Technology, • Kayser-Threde, • Laserdot, • Sylarec, • Aerospatiale, • Arianespace, • Swedish Space Corporation, DLR, • Alenia, • Electronika.
In the United States • The Center for Space Power, Texas A&M, • The Electromagnetics & Microwave Laboratory, Texas A&M, • The Space Studies Institute, • ExtraTerrestrial Materials, Inc., • University of Alabama in Huntsville, • International MicroSpace, • NASA Goddard Space Flight Center. Specific contributions and comments from many of these organisations are provided in the Appendix of the Powersat Final Report.
digitized by the Space Studies Institute. Founded by Gerard K. O'Nell
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