POWERSAT STUDY Final Report EUROSPACE ESA Contract N° 9390/91/F 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
Digitized by the Space Studies Institute. Gerard K. O'Neill, Founder
EUROSPACE ESA Contract N° 9390/91/F POWERSAT STUDY Final Report 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 7«?nA'7 DADT« Pronto
Contents List EXECUTIVE SUMMARY The Economics of Powersat Prospects for a Powersat Technology Demonstrator Programme Acknowledgement of Support PART I : THE ECONOMICS OF POWERSATS 1. Introduction : Space Power - Understanding the Problems 2. Finding the First Application 2.1 The Economics of Powersats 2.2 The Niche Market Approach 3. Example 1 : Communications Satellites 4. Example 2 : Space Stations and Low Earth Orbit Platforms 4.1 Meeting a Recurring Operational Need 4.1.1 Example of Space Station Freedom 4.1.2 Columbus Free-Flyer 4.2 Two Possible Options for LEO Space Stations 4.2.1 Option 1: The Co-orbiting Microwave Solution 4.2.2 Option 2: The High-Orbit Laser Solution 5. Economic Analysis of Powersats for Space Stations 5.1 Analysis Approach 5.2 launch Costs 5.3 Economic Analysis Technique 5.4 Assumptions 5.5 Results 5.6 Discussion & Summary 6. Principal Conclusions 7. References & Footnotes
PART II : PROSPECTS FOR A POWERSAT DEMONSTRATION PROGRAMME 1. Introduction : Pragmatic Rationale for a Demonstrator 2. Minimising Cost & Schedule 3. Overview of Launch Opportunities 3.1 Ariane 4/ASAP 3.2 Columbus Precursor Missions 3.2.1 Eureca Mission Opportunities 3.2.2 Spacelab E-l 3.3 Space Shuttle Launch of Small Payloads 3.3.1 Get-Away Special Canister (GAS Can) 3.3.2 GAS Complex Autonomous Payloads 3.3.3 Hitchhikers G&M 3.4 Other Possible Launch Opportunities 3.4.1 Astro-SPAS 3.4.2 SPARTAN 3.4.3 CIS Launch Opportunities 3.4.4 Small Launch Vehicles 3.4.5 MASER & MAXUS Sounding Rockets 3.5 Space Access Discussion 4. Derivation of Powersat Demonstrator Requirements 4.1 Technical Issues: Lasers or Microwaves 4.2 Operational Considerations 4.3 Technology Test-Bed 4.4 Space-to-Ground Demonstrations 4.5 Guideline Requirements 4.5.1 Initial Demonstrator 4.5.2 Advanced Demonstrator 5. Reference Options for a European Powersat Demonstrator Programme 5.1 The ASAP Reference Concept 5.1.1 Concept Evolution
5.1.2 Concept Overview 5.1.3 Specific Technology Issues & Rationale 5.1.4 Preliminary Mass Budget 5.1.5 Programmatics & Schedule 5.1.6 Cost 5.1.7 Laser Discussion 5.2 . The Astro-SPAS Reference Concept 5.2.1 Concept Evolution 5.2.2 Microwave Advanced Demonstrator 5.2.3 Laser Advanced Demonstrator 5.2.4 Programmatics & Schedule Discussion 6. Other Powersat Options 6.1 Columbus Precursor Missions 6.1.1 Eureca-3 6.1.2 Spacelab E-l 6.2 Space Shuttle Launch of Small Payloads 6.2.1 Use of GAS CAP for a Shuttle-Based Experiment 6.2.2 Deployment of Transmitter & Receiver from GAS 6.2.3 Use of Hitchhiker G & M 6.3 Maser & Maxus Sounding Rockets 7. International Cooperation Overview 7.1 With the USA 7.2 With Japan 7.3 With the Commonwealth of Independent States 8. Principal Conclusions & Recommendations for a Powersat Demonstrator Programme 8.1 Conclusions 8.2 Recommendations 9. References & Footnotes APPENDIX : INDUSTRY CONTRIBUTIONS & COMMENTS
EXECUTIVE SUMMARY This Final Report provides the findings 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-based infrastructure programmes. PART II is a discussion of the requirements for a Powersat Demonstration Programme, and reference concepts for an initial and later advanced demonstrator are defined and summarised in Figure 1. The principal conclusions and observations are as follows: The Economics of Powersats • If Europe and the other space faring nations intend to develop a wide-spread, diverse and cost-effective in-orbit infrastructure, then Powersat-type systems are considered mandatory prerequisites. They are also important intermediate steps in the development of the closely related Solar Power Satellite concept for terrestrial power supply. • No existing operational space programme could justify the high development cost of a Powersat. This is primarily because the actual financial savings amortised over the satellite’s lifetime are relatively modest, even if most of the power subsystem could be deleted. • Space Stations, like the proposed Freedom, could provide a suitable first niche market opportunity for a relatively simple power-augmenting Powersat. This is because the Powersat would enable significant savings in the amount of propellant and, potentially, batteries that must be launched every single year.
(Figure 1) Powersat Timeline
• Two possible Powersat designs are envisaged, designated the Microwave and Laser Solutions. The Microwave Solution would continuously trail a space station within 5-20 km. The Laser Solution would be located in a higher orbit and provide power to a station during eclipse passages every orbit. • The inability of the Space Shuttle to be launched more than 6-8 times per may preclude Freedom’s expansion to higher power levels simply because of the increased logistics requirements it would demand. Therefore, Powersats may make power expansion of Freedom practical while providing an opportunity cost savings equivalent to about 50-75% of one Shuttle mission per year. (e.g. around 2,000-4,000 MAU over 10 years) Prospects for Powersat Demonstration Programme • Powersats are not presently recognised as a high-priority for the European space agenda. Therefore, it is not considered pragmatic to embark immediately on a major Powersat technology and space demonstration programme. • Given the future potential of Powersat and other Solar Power Satellite systems, and the higher level of interest in this subject in other countries, it is considered worthwhile to implement a modest effort culminating with an inexpensive flight experiment performed in the 1997/98 timeframe. This initial demonstrator would provide information that can only be achieved through space flight. Equally importantly, it would act as a catalyst for creating interest in Powersat applications and spurring development of more advanced demonstration activities.
• 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 is critical in this regard. • The fundamental driver to an inexpensive/near-term demonstrator is, however, the combination of launch costs and launch opportunities. Launch systems that are historically susceptible to significant delays are incompatible with efforts to stay within low cost ceilings. • Many launch and platform options for the initial demonstrator were reviewed, ranging from sounding rockets and small launchers to Eureca and Spacelab. It was concluded that by far the best option for the initial demonstrator should be the use of the Ariane 4/ASAP microsatellite launch system. Shuttle launch options 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, and uses five or six positions on a single ASAP ring, enabling an equivalent demonstrator mass of up to 200 kg. The concept hinges around the use of a small tether system needed to maintain an inflatable 12.5 m rectenna pointed at the transmission system remaining secured to ASAP. Travelling wave tubes 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. • The requirements for a microwave and laser demonstrator are very different. For microwaves, the most critical aspect is being able to control a
microwave distorted by non-linear space plasma interactions in LEO. As a result, this mandates a microwave space experiment at high power levels. For lasers, plasma interactions are not a concern, therefore low power levels can be used. Precise tracking and beam pointing over enormous distances is the primary requirement. However, these requirements are enveloped by the current SILEX intra-space communications experiment planned for launch on Artemis. Therefore, a laser ASAP is not considered worthwhile to propose. Instead, initial laser efforts should be put into development work for a high-efficiency/light-weight/high-power laser system for the advanced demonstrator activities. • Throughout the course of the launcher and platform evaluation, it was deemed that the German Astro-SPAS would be the most suitable option for an advanced demonstrator programme with a 2002-2005 launch date. Even though it is dependent on the Shuttle and is restricted to a mission of about one week, it would probably be significantly less expensive than building a dedicated spacecraft and launching it on Ariane 5. 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 international cooperative programme. • The Astro-SPAS Demonstrator itself would be for either a microwave or laser experiment. The microwave demonstrator would be essentially a subscale version of an operational Powersat, demonstrating orbital control concepts for close formation orbiting. A full-scale rectenna would also be used and the transmitted power level would be 2-5 kW. The laser demonstrator would test out a subscale version of a laser system for a future operational Powersat, up to 1-2 kW of power, for example. In addition, the laser demonstrator would facilitate the first space-to-ground experiments.
• Current US, Japanese and CIS activities 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 Report. Because future Powersat and SPS system 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. • The activities of the Commonwealth of Independent States are considered extensive. Therefore, any immediate future Powersat activities must include an assessment of the former Soviets capabilities to avoid the possibility of information being lost altogether. 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, Culhain 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, • Lasersdot, • 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 these organisations are providing the Appendix.
PART I THE ECONOMICS OF POWERSATS
1. INTRODUCTION: SPACE POWER - UNDERSTANDING THE PROBLEMS Every single spacecraft launched since October 4. 1957 has had no other choice but to carry with it a fully integrated power subsystem able to independently generate, regulate, store and distribute electrical energy. The most widely-used power subsystem involves solar photovoltaic cells in combination with batteries. When the spacecraft is in view of the Sun. the solar arrays provide both electrical power for proper functioning and keep the batteries fully charged. When the spacecraft is in Earth’s shadow, the batteries can be used to maintain the required power without interruption. The only alternative to the solar array/battery combination employed “operationally” is to use the thermal energy given off by the decay of radioactive materials. The US and. to a greater extent, the former USSR are the only nations to have developed nuclear power systems, although Europe used a US-developed Radioisotope Thermoelectric Generator (RTG) to power Ulysses. This situation starkly contrasts with terrestrial activities where most new facilities obtain electrical power supplied remotely from national or international grid systems. A dedicated power station is not built to support each new building, although integral emergency back-up power supply systems are normally installed. On Earth, there are a number of facilities which, like spacecraft, require their own integral power systems. However, these are generally confined to either small-scale activities or remotely located systems (e.g. the Antarctic research base). There are a number of concepts for deploying a “space grid system” in Earth orbit. [1] Simplistically, these concepts generally involve large spacecraft dedicated entirely to providing electrical power to a range of in-space users. Within the context of this study, this is known as the “Powersat” concept
(Figure 1-1). However, at the present time no organization is seriously pursuing the deployment of a Powersat. although long-term plans have been formulated. This situation is exemplified by the fact that the only known space-based power transmission and reception experiment ever flown was on a 7 minute-long sounding rocket flight in 1984. [2] The current major space “infrastructure" programs. Mir. Freedom and the Columbus Free-Flyer, could be considered self-contained Powersats in the sense that they provide power to experiments located within the pressurised modules or on the external structure. In other words, space station experiments do not need to be launched with their own power source, but can be plugged into the generic station facilities. If centrally supplied power is so wide-spread on Earth, why is space apparently so different? There are many technical and economic answers to this question. Perhaps the most profound relates to the high cost and limited volume of space activities today. Terrestrial power stations are economic because the costs are spread over an extremely large customer base. In space today there are few. if any. suitable candidates for space power users, except for perhaps Mir and long-duration Shuttle missions. [3] The launch of Columbus and Freedom might provide the first opportunity for a Powersat. However, the high economic cost of supporting the Powersat could place a heavy burden on these two facilities. This does not necessarily mean that a Powersat would be uneconomic for these limited applications, as will be discussed at length in Section 4. The limited number of potential Powersat users raises important questions about the economics of such systems. Other problems relate to the actual technical and operational practicalities of relying on an external power source for space missions. These issues are summarised below and in Figure 1- 2.
Possible Range of Powersat Impacts on a Typical Spacecraft Power Subsystem (Figure 1-1)
Sample of Current Payload Characteristics (Figure 1-2)
• the vast majority of spacecraft require relatively small power levels, typically on the order of 2 to 5 kW for communications, science, and Earth applications spacecraft. Space Shuttle/Spacelab microgravity missions require considerably higher power levels of about 12 kW, but such missions occur only once or twice every year, and each has a duration of about 7-10 days. (i.e. average annual power is less than 0.5 kW) • spacecraft orbit the Earth at different altitudes and inclinations, from low Earth orbit at 400 km and 28.5°, to sun synchronous orbits at 900 km and 98.5°. all the way to geosynchronous orbits at 36.000 km and 0° inclination. • spacecraft are separated by enormous and continuously changing distances relative to other spacecraft. Depending on the orbit parameters, these distances can range from hundreds to tens of thousands of kilometres. • many spacecraft operate in a dynamic fashion relative to the Earth. Astronomy spacecraft must have the freedom to re-orient themselves to acquire particular targets, and Earth observation spacecraft are Earth pointing, meaning they rotate once every orbit. Therefore, the impacts of having to rely on a central Powersat might be seen as a system which could unnecessarily: • Constrain the operational flexibility of the spacecraft in terms of its orbital motion. • Force spacecraft to stay within close proximity to the central power station, • Add complexity to the spacecraft that must already be equipped with an integral power generation and storage system for initial operations, and covering periods of outage.
At a cursory glance, it might be convenient to conclude that the need for a Powersat is limited or more trouble than it is worth. Further, delays and complications with today s existing and comparatively "simple" programs such as Space Station Freedom, the Columbus Free-Flyer and others, might reinforce this conclusion. Even though these large spacecraft might be considered as potential users of Powersats, in this light one conclusion might be that it would be inappropriate and premature to begin contemplating the use of Powersats until experience in operating and supporting large space elements had been accumulated for some time. However, a negative attitude to space power ignores the future development of space. If the space-faring nations eventually hope to develop a significant and diverse space-based infrastructure, beyond the comparatively small first steps of Columbus. Freedom and Mir. then the rules by which the value of space power is judged will change. Indeed, the wide-scale exploration and future industrialisation of space will almost certainly make the need, for centrally-supplied space power a mandatory prerequisite. In the more distant future, the potential for transmission of power from space to Earth will necessitate a variety of intermediate steps which could include the transmission of power between spacecraft in orbit. Even in the immediate future, there could be rational (i.e. economic) justifications for Powersats if the right mix of technology and applications is sought - the first Powersat does not necessarily need to be a large and cumbersome piece of machinery, as are Earth-based power stations. Further, the current practise by all spacecraft of carrying their own integrated power subsystem is purely because there has been no other alternative. How might the economics and operational aspects of a user’s spacecraft change given the guaranteed availability of power in space? As a paper by the U.S. Center for Space Power for the 41stIAF Congress explains, [4]
...the problem remains the old chicken and egg question as to why design a satellite to receive broadcast power wlien no power utility satellite exists and is anyway questionable, or why design, develop and deploy a power utility satellite for winch no market exists. Overall, a negative attitude to the potential of centrally-provided power through a Powersat is considered destructive. At the very minimum, developing future space infrastructure concepts, such as the proposed European Manned Space Infrastructure (EMSI), that take into consideration the possible application of Powersats might provide answers to other problems that might not otherwise be contemplated. For example, could locating a large percentage of a space station s power supply on a relatively robust Powersat minimise the complexity of the station’s operations and support requirements? For as long as future options for large-scale space activities remain open, Powersat concepts should be given as much trade-off consideration as any other form of space power approach. The objective of this study is to address these issues in their proper context. 2. FINDING THE FIRST APPLICATION 2.1 The Economics of Powersats Powersats might be considered ‘economic’ provided they have the capacity to realise: Cost Savings User power cost savings might be possible if the Powersat can be operated in a manner whereby the price users are charged is less than the cost of users providing their own power integrally. The price charged must enable the required level of cost
recovery over a particular time-scale. New Mission Options The unique characteristics and functions of certain Powersat configurations could enable new mission opportunities previously considered impractical. In this case the Powersat would be developed based on a potential market that would emerge directly as a function of the availability of higher power levels. Serendipity Benefits Powersats have the potential to lead to the stimulation of new technological capabilities which can be exploited elsewhere, either in space or on the ground. The prospect of Powersats becoming economically viable is encumbered by the expected high cost of their development and operations. Like all space systems. Powersats will be unavoidably expensive, especially if they are to service multiple users simultaneously. This situation is exasperated by the fact that Powersats are without precedent and they will be large by necessity. Consequently, the high risk/large size characteristics will make Powersats susceptible to significant cost increases. In this light, the potential for achieving cost savings could be overwhelmed by the enormous burden of recouping the inherent high initial investment for developing Powersats. The very limited number of possible existing users might make such an economic case difficult to rationalise. But in the future, as indicated in Section 1 above, the eventual exploration and exploitation of space will necessitate high power demands, and almost certainly mandate the use of centrally supplied power for wide-scale activities which might include, for example.
* Multiple, low-Earth orbit space stations and transportation nodes. • Space-based industrial processing facilities. • Lunar bases for He3 extraction. • Electric transfer vehicles. • Asteroid mining. * Geosynchronously orbiting antenna platforms, • Arrays of large Earth observation platforms, • etc., etc. It might, however, be an enormous “leap-of-faith” to attempt to justify the development of Powersats based on this future of wide-scale space industrialisation, particularly in light of today’s relatively humble space activities. Under present circumstances, there is little evidence that such a future is going to emerge in the next few decades. This is fundamentally because of the current methods of accessing space. Specifically, the high cost and limited number of opportunities to transport payloads to and from space each year simply precludes the wide-scale utilisation and exploration of space, much in the same w'ay the industrial revolution on Earth would have been precluded without the development of railway and canal transportation infrastructures. The development of Powersats is unlikely to provide sufficient stimulus to encourage enough new mission options to justify their high cost. An economically-driven Powersat strategy dependent on new users emerging is unlikely to be sufficient to warrant the high cost of developing Powersats in the first place. With regard to serendipity benefits or spinoffs, this is another area where economic advantages could be obtained. Powersats might enable the development of light-weight/high-efficiency/high-power laser systems that could find uses in some terrestrial manufacturing applications. Likewise, light- weight/high-efficiency power transfer and storage systems could similarly find
application in other economic sectors, such as in electric transportation. However, the extent of these benefits is uncertain and they are unpredictable - perhaps even more so than the emergence of new space missions - and. generally speaking, such benefits can only be accrued "globally" as this is the nature of spinoffs. Furthermore, a Powersat strategy which makes technology choices based partially on the promise of those technologies having the highest spin-off benefits, conflicts strongly with the central reasons for pursuing Powersats in the first place: to provide a means of delivering power more economically than any other way. To minimise life-cycle costs, the choice of technology used should be driven entirely by the means of meeting an economically and operationally effective solution. Unless governments are willing to fully fund the enabling technology effort, then serendipity benefits are insufficient for the rationale to develop Powersats. 2.2 The Niche Market Approach Powersats could have an important role in the future of space development. However, this does not necessarily mean that they must wait until such activities arise if an economic rationale can be made now. Essentially, the future prospects for the development of Powersats would be considerably enhanced if a first niche market application can be found that would completely support its economic case. The first Powersat would, as a result, need to be a very simple system, optimised to service one or a few users. In this sense, the Pow ersat would be more like a power-augmenting capability than a true central pow'er station. This first Powersat w'ould provide initial experience in constructing and operating such systems. It would also provide leverage into other user areas by demonstrating the practicality of using space transmitted power. Starting small
also allows the application of Powersats to evolve with increased demand as it occurs, analogous with the development of Earth-based power stations during the 20th Century. They may also provide an important intermediate step toward the ultimate goal of building Solar Power Satellites, as will be discussed later. The following sections will discuss the economics and operational aspects of two applications that might be considered as potential niche market candidates. The first is a brief overview of the use of Powersats to support communications satellites in order to highlight in more detail some of the fundamental problems discussed above. The reasons why communications satellites would make a weak niche market are presented. The second is a rather more thorough discussion of the more promising niche market application of space stations and future manned space infrastructure elements. A notional power-augmenting satellite is discussed as a reference system. Discussing these two very different niche market possibilities provides a fuller and more quantitative understanding of Powersats.
3. EXAMPLE 1: COMMUNICATIONS SATELLITES The current generation of communications satellites might at first glance be considered an appropriate market for Powersats. given that comsats are by far the most common type of satellite launched today. Unlike all other satellites, comsats seem to have the advantage of being very similar, many in number and. importantly, located in essentially fixed positions relative to each other and the Earth. This is in contrast to Earth observation spacecraft which orbit the Earth in high inclination low Earth orbits or scientific satellites which have very specialised needs, as discussed in Section 1. It is possible to envisage a scenario (Figure 3-1) where a number of large Powersats are positioned in among the nearly 100 communications satellites in geosynchronous orbit, the majority of which (around 75%) are grouped over the US. the Atlantic Ocean and Europe. At the very minimum, three Powersats would be required, one being a back-up, to ensure total coverage and optimise the maximum distances over which the beam must travel. The comsats themselves could, for example, be equipped with a rectenna secured to one side of the satellite, and the other side would retain a solar array wing, albeit of much shorter length. (Figure 3-2) Assuming a very optimistic scenario in which 75 satellites are equipped with rectennas and could use the Powersats, a simple assessment of the economics is provided in Figure 3-3.
Conceptual Comsat Powersat System (for discussion purpose only) Figure 3-1
Comsat With Rectenna (schematic) (Figure 3-2)
Figure 3-3: Revenues of Comsats using Powersats
Figure 3-3 indicates that the total revenues due to these savings available to the Powersat operator from all 75 comsats being served would be little more than 600 MAU. More fundamentally, this revenue would be spread out over 10 years because of each comsat s life expectancy. Hence, the total annual revenues would be around 60 MAU. Although these revenues might be considered significant, they must be compared with the cost of establishing and operating the Powersat constellation. If it is assumed that each Powersat costs no more to build than a typical large comsat - clearly a very, very optimistic assumption, as each would produce around 100 kW in transmitted power to 30-40 simultaneous users - and require a dedicated Ariane 4 launch, then the total cost to establish a Powersat constellation on station and to operate it for 10 years would be approximately as shown in Figure 3-4. Figure 3-4 : Cost to Build, Launch & Operate Povversats for the Comsat Applications Without going any further with the analysis, it seems clear that the economic case for Powersats supplying power to all comsats is currently not
realistic even in the very, very optimistic scenario described above. The cost to build a Powersat that can simultaneously supply power to 30 or 40 comsats positioned several thousands of kilometres away - if. indeed, it is technically feasible at all - would, in fact, turnout to be much greater than the cost to build one comsat. Although it is impossible to estimate accurately, a cost in the region of "hundreds of millions of AUs” is probably realistic. This example also brings up other important issues, particularly those relating to spacecraft missions and launch costs. While the launch costs in Figure 3-3 were calculated on the basis of cost per kilogramme, this can be misleading. Most launch vehicles do not charge precisely on the basis of how much the satellite weighs, but rather what mass range it falls into. For example, a dedicated Ariane 44L vehicle can carry up to 4.3 tonnes into GTO, but it costs a discrete amount more than an Ariane 44LP booster that can carry 3.7 tonnes. Further, the cost per kilogramme of an Ariane 44LP is more than an Ariane 44L. Hence, it costs more per kilogramme to launch a smaller satellite than a larger one, minimising the benefits of saving relatively small amounts of mass. The same distinction is true between an Atlas 2AS and an Atlas 2A. Thus, although a satellite which uses a Powersat might weigh 280 kg less than a satellite that doesn’t use a Powersat, the launch costs might still be about the same. In some cases, such as with a dual launch or where a 280 kg reduction makes the difference between two different types of launchers, there might be a launch cost savings, but these are unlikely to be as large as suggested by Figure 3-3. Further, this saving is effectively spread over the 10+ years lifetime of the comsat. If the launch cost savings shown in Figure 3-3 could be realised, this would lead to an amortised savings of 560,000 AU per year. (i.e. [280 kg x 20.000 AL/kg]10 years). By the same token, the cost to the user of not needing to be equipped to carry a full-sized solar array, batteries and other
power subsystem components, is relatively small when amortised over the satellite's lifetime. Based on the earlier example, this averages at around 250.000 AU per year. As a result, the average cost saving to the user is less than 1 MAL’ per year, and more realistically it is probably nearer 0.5 MAU. If satellite operators were suddenly given the opportunity to save 280 kg in launch costs, most would probably choose to load more propellant into the satellite to increase its lifetime and revenue-generating capacity. Under these circumstances, the total launch cost savings of 420 MAU might be considered as very optimistic. A final problem is that while there are many comsats, each only needs a relatively modest amount of power. Over the next 10-20 years, this power is unlikely to increase dramatically for a number of reasons, including, for example, the production of higher efficiency travelling wave tubes (TWTs), solid state power amplifiers (SSPAs) and other payload and bus subsystem components. Also, the larger the power requirements, the larger the spacecraft will have to be to effectively radiate the waste heat Limits imposed by current launchers for dedicated satellites of up to 3 to 4 tonnes will ensure that the average power demands will not grow significantly. Neither will the total demand for comsats grow very much, because there is only a limited number of orbital positions in GEO for comsats. In summary, it seems clear that communications satellites would not present a suitable initial niche market for the first Powersat system, as the economic arguments simply do not support it. The same conclusion is equally applicable to all other one-shot, non-recoverable spacecraft such as scientific satellites, Earth observation platforms and weather satellites. This situation may change in the future if large telecommunications platforms are built that require "hundreds of kilowatts of power." However, no such plans exist presently.
4. EXAMPLE 2 : SPACE STATIONS 4.1 Meeting a Recurring Operational Need The type of communications satellites launched today are clearly not suitable niche market candidates for the first Powersat system because the power requirements and savings per user are very small. Obviously, potential Powersat developers need to look for a niche market that involves users who consume large quantities of power. However, more fundamentally, the most suitable market will be one where the annual recurring cost to support a particular user’s power system is large. For a comsat that uses a Powersat, the cost savings occur only once per spacecraft purely as a result of not having to launch a large solar array, battery, or other components. Like most spacecraft, comsats are launched as fully self-contained units. If comsats fail in GEO there is currently no real possibility for retrieval, repair and re-launch. For the same reasons, comsats must work without the possibility of a continuous supply of propellant and other consumables to keep them functioning. This is not the same for large low Earth orbit platforms and space stations. Here, the one-off cost savings from not having to build and launch a large power subsystem are of less importance than the annual recurring cost savings from not having to support this large power subsystem. Again, as discussed in Section 2.1, it is important to emphasise that even in the '‘Powersat era." all spacecraft will need a power subsystem of some description in the event of extended outage periods. In some cases, this integral power subsystem will need to be very substantial to maintain the health status of large space platforms. These arguments will become clearer in later examples. Analogies with the terrestrial supply of power are appropriate here. Potentially, every home or business could have its own electrical power generator systems. However, the recurring costs of having to purchase and
store the fuel, as well as periodically service the generator, are likely to be significantly greater and less convenient than receiving energy from a centrally supplied power station. The above terrestrial analogy might be considered obvious. However, in the first three and a half decades on orbit the only facility that has been deliberately expanded and supported in space on a continuous basis over many years is the Mir Space Station. This is a very important point, considering that Europe will not obtain any similar experience until the first servicing mission to the Columbus Free-Flyer in 2004. The high cost and limited opportunities to access and return from space severely prohibits such activities. 4.1.1 Example of Space Station Freedom The deployment of Freedom sometime in the next 10 years will provide the West's first experience with the problems of supporting a very large, high- power system in space. Freedom's current logistics support planning is an instructive guide in understanding the impacts on a large space facility that must provide its own large solar panels for power. The principal impacts are felt in two key areas: Starion-Keeping Propellant Freedom will be located in an approximately 400 km low Earth orbit where it is in easy reach of the Space Shuttle, but where atmospheric drag remains significant. Unfortunately, this high drag conflicts strongly with the desire to increase the power capability of the station: the larger the solar arrays, the higher the drag. The current version of Freedom (Figure 4.1-1) at the Permanently Manned Capability (PMC) will have two solar array wings capable of producing 37.5 kW of user power. These wings represent a total surface area of the solar array of about 1,300 m2. Originally, the plan was to
Space Station Freedom Figure 4.1-1
then add two more solar array wings in order to increase the user power to 75 kW for the Assembly Complete (AC) configuration. Now, however, budget cutbacks have forced a revised plan where just one array wing will be added, enabling a 56 kW user power capability at AC. Nevertheless, the consequence of using such large solar arrays to generate power at this low altitude is that the drag will be high. As a result, propellant must be ferried to Freedom every year to keep re-boosting the station and avoid it falling back into the atmosphere. Current NASA planning indicates that for the 56 kW configuration nearly 8 tonnes of propellant will have to be launched every single year. [5] To this must be added the mass of the propellant container of about 2 tonnes. [6] Thus, to keep Freedom in orbit for one year requires launching around 10 tonnes dedicated entirely to meeting the station-keeping requirement. At the 37.5 kW capability, this mass is about 8 tonnes, and at 75 kW it would be around 12 tonnes if Freedom was ever upgraded to this level of power. It is important to note that Freedom cannot be placed in a higher Earth orbit (i.e. 500-600 km) where the drag is much less because the Shuttle's payload capability falls off rapidly at higher altitudes. One way to counter this would be to launch the Shuttle more often to make up the performance shortfall. However, limitations on the Shuttle's flight rate to around 6-8 flights per year simply preclude this possibility. Battery life As Freedom is in a low altitude and inclination orbit, 30 to 40 minutes of each 90 minute orbit will be spent in the Earth’s shadow. This means that batteries (NiH^ must be used to maintain a constant level of power. When Freedom is in sunlight, the solar arrays are used to both power the station and charge the batteries. When in shadow, the batteries are discharged. As a
consequence, the batteries are cycled between being charged and discharged nearly 6.000 times per year. Thus, after several years, the batteries will have to be completely replaced because of limitations on cell cycle life. The total mass to be replaced every 5.5 years is about 5 tonnes in the 57 kW AC configuration, and about 3 tonnes in the 37.5 kW PMC configuration.[7] Two possible Powersat options are analysed in the following subsections. The scenario takes Freedom in its permanently manned 37.5 kW configuration as a starting point. However, instead of adding an additional pair of solar arrays to upgrade the station's power to the original goal of 75 kW, an alternative is proposed using Powersats. This is shown schematically in Figure 4.1-2. Using Freedom as the example is considered a reasonable approach to take because the specific quantities (i.e. logistics upload, power requirements etc.) are very well defined. In this sense, it is not proposed that Freedom itself should use a Powersat - although it could - but rather that its defined configuration provides '‘hard’’ numbers that can be analysed with a certain degree of precision. It is important to note that the user power referred to above does not include the power needed by Freedom for operational purposes. The operating power is a little more than half the user power level (i.e. about 60%). Hence, the Powersat concepts defined must be sized to deliver that much more power in addition to the 37.5 kW of user power. 4.1.2 Columbus Free-Flyer Before discussing Freedom, it is worth briefly discussing the Columbus Free-Flyer. It is very important to note that Freedom is designed from the very beginning to eventually support the 75kW user power configuration. Therefore,
Power Upgrade Alternatives For Freedom (Figure 4.1-2)
thermal dissipation and power distribution problems associated with handling power transmitted to the station by a Powersat are considered small. In this sense. Freedom's overall configuration is transparent to where the power comes from. By comparison, the Columbus Free-Flyer is currently optimised for one maximum power level. Hence, for it to be augmented to receive higher levels of power from a Powersat. significant modifications to the Free-Flyer's power and thermal subsystems would be necessary. In addition, special interfaces for the attachment of the rectenna would need to be provided, unlike with Freedom where the rectenna can be attached to the same interfaces that otherwise would be used by a solar array wing. Importantly, such modifications would need to be made to the Free-Flyer long before its planned launch in 2003 because the maximum Hermes flight rate of 1-2 per year, coupled with a payload capability of 1-3 tonnes, precludes any possibility for modifying the Free-Flyer when on orbit. The Columbus Free- Flyer is not a good niche market candidate as it is presently configured and supported. 4.2 Two Powersat Options for Space Stations 4.2.1 Option 1: The Co-Orbiting Microwave Solution Reference Concept Configuration By far the simplest approach for the first Powersat would seem to be in the role of augmenting the power of a single, major user such as Space Station Freedom. This would be achieved by having a Powersat positioned within 5-20 kilometres behind (-V bar) the station. The Powersat would then beam power by microwave radiation to a rectenna located on one end of the boom, as shown in Figure 4.2-1.
The Microwave Solution (Figure 42-1)
For the purposes of this exercise, an attempt was made to configure such a Powersat into a realistic design. The purpose was not to propose an optimal technical or economic solution, but rather to demonstrate what could be practical with more or less existing technology and systems. The overall configuration is shown in Figures 4.2-2 & -3. The main driver for this Powersat configuration was for it to be launchable on one Ariane 5. and operate autonomously for 5-10 years before being replaced. In was not considered practical to incorporate a servicing capability into Powersat simply because Hermes’ limited pay load and flight rates mean it is simply incapable of performing such servicing missions. Further, high launch costs might make a servicing mission as expensive as launching a new Powersat in the first place. The Shuttle could potentially perform the servicing function. However, all of its yearly missions are expected to be tied up in meeting Freedom’s support needs. Beyond these two vehicles, the only other possibility is to develop a variant of the proposed Ariane V Transfer Vehicle (see Section 5) that is capable of performing an automatic rendezvous and docking with the Powersat. However, even here servicing operations would need to be limited to propellant loading only. Any other activities, such as replacing failed units, would probably be more expensive than launching a new Powersat because the telerobotic and other systems would be discarded. For all of these reasons, it was decided that the most suitable Powersat configuration would be one that is similar in configuration to today’s highly integrated communications satellites, although considerably larger. Approximately 200-300, 250 W space-qualified TWT amplifiers (12 GHz) are arranged at 200 mm intervals along the space-facing radiator walls. These tubes w'ere chosen over the use of one or more large klystron or gyrotron microwave generator sources for a number of reasons. Most notably, a suitable klystron or gyrotron generator does not exist in Europe for space applications. The TWTs,
Conceptual Powersat Deployed View (Figure 4.2-2)
Conceptual Powersat Launch Configuration (Figure 42-3)
which are basically identical to those of a direct broadcast satellite, exist and are qualified for a decade of space operations. These types of TWTs are normally also qualified to be repeatedly switched on/off 20.000-30.000 times, equivalent to 5 years on orbit. Although the use of TWTs might lead to a heavier design that is more expensive to launch, this would be off-set by the cost of not having to develop and qualify a microwave generator source. Another advantage of the TWT solution is that it enables a graceful degradation of the Powersat’s capability as each tube failure would eliminate only 250 W7 of power. This degradation would be forestalled by the incorporation of a number of redundant tubes. Groups of TWTs can also be linked, via appropriate phase-shifters, to a feed horn array which would, therefore, be able to electronically control the shape and pointing of the beam. Alternatively, an active subreflector could be used. Power is provided by two 80 m long by 8 m wide solar array wings similar to the Space Station Freedom arrays, alsos similar to the arrays proposed for the SOLA experiment described in the previous Eurospace Powersat study.[8] This array size would provide between 120-150 kW of total power. For propulsion, a high-performance bipropellant system would be used. This systems would be needed to reboost the Powersat when Freedom performs a similar reboost manoeuvre. In addition, a continuously operating ion propulsion system is also necessary to ensure the Powersat always tracks closely behind Freedom as the orbital pair spiral in toward Earth. A deployable mesh-type reflector is configured for the Powersat, and it is sized at around 15 m. Alternatively, a phased-array system could be used, although it would be more difficult to package the required large aperture. With regard to the rectenna, this could potentially use an inflatable torus-type
structure across which is a diaphragm containing the rectenna micro-strips needed to rectify the microwave power (see PART II). Potentially, the rectenna could be attached to the same electrical and mechanical interfaces that otherwise would be used if a second pair of solar array wings were added. This arrangement ensures that the power provided by the Powersat is effectively “transparent,” in the sense that Freedom’s power bus would not know whether the power was coming from the arrays or from the Powersat. This option is dubbed the Microwave Solution because using microwaves are considered the only practical solution for a co-orbiting Powersat at an altitude of 400 km. The principal reason is that microwave systems offer far higher overall efficiencies than possible with lasers. Typically, a global efficiency as measured by the output from the rectenna could be as high as 25- 35%. meaning the size of the solar array on the Powersat must be about 3-4 times the size of an array on the station generating the same power. Although this is a significant increase in area, it is small by contrast to that needed for a laser Powersat where global efficiencies are measured in the “few” percent region. For example, at a relatively optimistic global efficiency of 5%, this would mean the solar array would need to be 20 times larger than for an equivalent integral station array, and 5-7 times larger than for the microwave solution. Such a large surface area would obviously present a very high drag. Other problems are associated with the technology availability of a high- powered laser system, and converting the high-density laser energy into electrical power. Advantages and Disadvantages of the Microwave Solution Quantifying the benefits to Freedom of a Powersat is, of course, difficult to do with any accurate precision given all the operational and technological unknowns. However, the advantages to a space station the size of Freedom are primarily associated with the reduction in the amount of
propellant that must be launched to the station annually. Specifically, the advantages for Freedom and the Powersat are as follow: For Freedom • The station-keeping propellant needs remain roughly the same even though the power level has been doubled. • The station remains small and. therefore, easier to control, and • Maintenance activities will only be slightly increased due to the need to inspect and repair the rectenna as necessary. For the Powersat • Existing microwave technology can be used, ♦ Only one Powersat is needed, and • It can be built in a heavy and robust fashion. The principal disadvantages, assuming the Powersat uses solar arrays for power generation, are as follow: For Freedom • Battery mass would still have to be doubled as before in order to maintain the required power level during an eclipse, • Batteries will have to be replaced as frequently as before, • There are safety hazards associated with having another large spacecraft within Freedom’s command and control zone. For the Powersat • The ballistic coefficient is likely to be very large, necessitating a
continuously operating propulsion system for station-keeping with respect to Freedom. • Controlling the positioning of the Powersat with respect to Freedom will be a complex operational problem. Understanding the Drag Problem If the Powersat uses solar arrays to generate power, then these arrays will need to be significantly larger than those that would be required to augment Freedom directly due to efficiency losses of the microwave generator, and the transmission and reception process. As a result, the ballistic coefficient (proportional to the exposed Area/Mass) of the Powersat will be much larger than the increase in drag coefficient of Freedom with the 75 kW solar array capability. Therefore, the effects of atmospheric drag on the Powersat would be much more pronounced, requiring practically continuous station-keeping manoeuvres to keep up with Freedom and stay within power-beaming range. How can there be a net savings in the total amount of propellant mass used by Freedom and the Powersat, especially if the size of the Powersat’s solar arrays are three times the size of those otherwise needed to augment Freedom? Adding another pair of solar arrays to Freedom would approximately double its ballistic coefficient, leading to a doubling of the drag and, very approximately, a doubling in the propellant requirements. Critically, however, the Powersat w ould be as much 15-20 times less massive than Freedom, i.e. 15-20 tonnes for the Powersat versus 300 tonnes for Freedom. As a result, the Powersat’s propellant needs would be somew'hat less than the increase Freedom would otherwise need, even though its drag is far higher. Although the Powersat’s solar array size would be three times that for an equivalent Freedom capability, the specific amount of propellant mass needed for station-keeping would be less because the Powersat is so much lighter. How much the propellant could be reduced would need to be the subject of detailed analysis. (Figure 4.2-4)
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