Spacefaring America

24 - Space solar power and America's energy future (Part 5)

Mike Snead — Sat, 24 May 2008 16:52:01 GMT

The Space-Based Solar Power study undertaken with the sponsorship of the U.S. Department of Defense's National Security Space Office has continued to elicit significant interest. As noted in Part 1 of this series, the fundamental premise of this study was that the United States should lead the world in making use of space solar power as a primary means for achieving an energy-assured future based on renewable energy sources.

Needless to say, the validity of the study's premise is not yet established. Significant technical, economic, and logistical issues related to beaming energy from space to the Earth still need to be addressed. Yet, the attractiveness of renewable, high-quality electrical energy delivered from space to terrestrial electrical power grids remains high.

Is strictly an economic comparison of space solar power vs. terrestrial alternatives still valid?

Some argue that while the needed technologies for building and operating space solar power systems may be resolvable, such space systems are unlikely to ever be economically competitive with terrestrial renewable and non-renewable alternatives. This argument, of course, presumes that terrestrial renewable and non-renewable energy sources will be sufficient in capacity and acceptable in terms of environmental impact to meet the world's energy needs. If this presumption is true, then the economics of space solar power vs. terrestrial alternatives likely will become the basis of a go/no-go decision. Looking at the flip side of this argument, however, a different decision criterion emerges.

In Part 1 and Part 2 of this series, I developed the following list of the most important to least important desirable energy outcomes as part of a discussion of developing a rational U.S. energy policy.

Energy sufficiency and energy availability
Energy assuredness and energy affordability
Energy acceptability
Energy economic opportunity

If terrestrial renewable and non-renewable energy sources are sufficient to meet future energy needs, then energy policy can turn from ensuring "energy sufficiency and energy availability" to ensuring domestic "energy assuredness and energy affordability." (See Part 1 for definitions.) However, if future needs cannot be met by terrestrial sources, then energy policy must remain focused on resolving this shortage. In this case, space solar power becomes a necessity. Comparative economic costs of space solar power vs. terrestrial energy sources, under these circumstances, are primarily useful in balancing investment in space solar power vs. terrestrial sources and for preparing the world's economy to provide the needed economic resources.

Whether building space solar power systems is an economic decision or an essential need hinges on whether terrestrial energy sources can meet the world's future needs. Currently, non-renewable hydrocarbon (NRH) fuels provide most of the world's energy—about 85%. Despite escalating NRH prices and apparent production limits, many still believe that these current primary world energy resources are sufficient to sustain world needs beyond any required planning horizon. Discussions of a post-NRH world are dismissed as being irrelevant based on the assumption that new discoveries and/or recovery technology improvements will prevent these resources from being exhausted within any time period of concern. Advocacy of space solar power, therefore, must start with assessing the future of NRH supplies.

What are the known and projected world reserves of non-renewable hydrocarbon energy supplies of oil, coal, and natural gas?

Table 1: World Non-renewable Hydrocarbon Proved Recoverable Reserves

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Table 2: World Non-renewable Hydrocarbon All Recoverable Reserves

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These two tables reproduce information contained in the World Energy Council's 2007 Survey of Energy Resources. The following notes explain how these tables were complied.

1. Proved recoverable reserves are what experts believe "can be recovered in the future under present and expected local economic conditions with existing available technology." Table 1 shows these values for conventional oil, coal, and natural gas.

2. The reserves for the two forms of unconventional oil (natural bitumen and extra-heavy oil), included in Table 1, are not proved reserves. Rather, as discussed in the cited reference, these "reserves or probably reserves, generally with no further distinction, are quantities that are anticipated to be technically (but not necessarily commercially) recoverable from known accumulations.” As with most NRH fuels, the expected recoverable reserves of these two sources of oil are only a modest percentage of the known accumulations which, in some sources, is placed in the trillions of barrels for these two sources. (See the note below regarding oil shale in Table 2.)

3. Table 2 lists the additional recoverable reserves generally defined as the "resource additional to the proved amount in place that is of foreseeable economic interest. Speculative amounts are not included." With the inclusion of these additional reserves, the total shown in Table 2 is an estimate of the total remaining NRH reserves that are recoverable at production levels needed to meet world needs. In this simple depletion model, when these reserves are exhausted, other energy sources will be needed.

4. Recovery of oil from oil shale is comparable to the recovery of the bitumen in that both are either mined or extracted using in situ heating to liquefy trapped hydrocarbons. The cited reference states: "Total world resources of shale oil are conservatively estimated at 2.8 trillion barrels." However, unlike, natural bitumen or extra-heavy oil, the cited reference does not include estimates of useful reserves. The total 2.8 trillion barrels of oil from oil shale is included in Table 2, not to reflect the actual expected proved recoverable reserves, but to provide a massive additional source of NRH fuels that can be used to assess what impact this may have on world energy consumption. Essentially, the inclusion of the full amount of the oil shale covers uncertainties in the additional recoverable reserves of natural bitumen, extra-heavy oil, oil from oil shale, as well as increased production resulting from improved recovery methods for conventional oil, coal, and natural gas.

5. In their 1983 book, The Future of Oil, Odell and Rosing reach the conclusion that 5 trillion barrels is the likely amount of recoverable conventional and unconventional oil. The conventional and unconventional oil reserves, including oil shale, in the two tables total about 5 trillion barrels providing one indication that the total for oil used in this simple analysis is of the right magnitude.

What are the units of energy used?

A Q-BTU is a quadrillion (one billion million) British Thermal Units (BTU). It is used as the standard unit of measure by the U.S. government for comparing various energy sources. (See Part 4 for additional information on this topic.) The M-BTU, used later, is one million BTUs. It is a measure of per capita energy consumption. There are one billion M-BTUs in a Q-BTU. The advantage of using these two units is that they enable extremely large magnitudes of national and personal energy consumption to be described with numeric values that are easily comprehended.

Why are all of the NRH reserves combined into one total?

As a heat source and as a raw material for the chemicals industry, modern technology has made oil, coal, and natural gas interchangeable. Coal can be transformed into either a gas product to replace natural gas or a transportation fuel to replace oil. One major oil company is touting the conversion of natural gas into a liquid transportation fuel. As the reserves of oil are depleted, with a corresponding increase in price of the remaining oil, natural gas and coal production will likely increase to replace the dwindling supply. For the simple reserve depletion model used in this analysis, combining all reserves in one total and comparing this against the world's total demand for NRH fuels was appropriate.

What will be the world's demand for NRH fuels throughout the rest of the century?

Figure 1: U.S. and Non-U.S. Population Model Used

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The starting point for projecting the world's demand for NRH fuels is to model the world's population growth through the end of this century. Figure 1 shows both the U.S. and non-U.S. world population based on U.S. Census Bureau data to 2050 extrapolated to 2100. (Note: The Census Bureau projections through 2050 show a declining birth rate compared to the death rate. In the simple population model used, births equal deaths about 2080 for the non-U.S. population. In the U.S. however, the U.S. population is still growing because of the larger families and second and third generation births of the large influx of aliens in the last decade and the coming decades.)

Figure 2: U.S. and Non-U.S. Per Capita Energy Needs (M-BTU)

[Larger copy of the above chart]

The U.S. data is based on U.S. Energy Information Administration (EIA) data for 2004 and projected 2030 national energy consumption. Using the Figure 1 population model for the U.S., the per capita energy needs were computed for 2004 and 2030. These two data points then were used to extrapolate usage to 2100 using the same rate of change as projected from 2004 to 2030. (Note: A decline in per capita U.S. energy consumption through 2030 is projected by the EIA. This rate of decline was extended to 2100 where per capita consumption would be about 91% of current usage.)

The non-U.S. per capita energy consumption was based on international energy consumption data also reported by the U.S. EIA. The total world consumption was reduced by the U.S. consumption. The resulting value was divided by the non-U.S. population to arrive at an estimate of the non-U.S. per capita energy needs. As with the U.S. per capita energy needs, the trends from 2004 to 2030 were extrapolated to 2100.

In this model of non-U.S. per capita energy usage, by 2100 the non-U.S. per capita energy needs would be approximately 50% of the U.S. per capita energy needs. Currently, Japan and Western Europe per capita energy consumption is about 50% of that of the U.S. Hence, reaching a 50% value in 2100 represents a target where the per capita energy consumption and standard of living, outside of the U.S., would be comparable to that of Japan and Western Europe. The transformation of the world to a western standard of living would then be essentially complete.

Figure 3: Annual Future Energy Needs (Q-BTU)

[Larger copy of the above chart]

The U.S. and non-U.S. world per capita energy needs can be applied to the U.S. and non-U.S. populations to project the annual future U.S. and non-U.S. energy needs. Summing these two values yields the world's total annual future energy need. This is shown in Figure 3, above. By 2100, the U.S. percentage of world energy use would fall from about 25% today to about 10%. (Note: If the rapid industrialization of China and India continues, the annual non-U.S. energy demand would be expected to increase more rapidly in the coming decades than shown in the above figure.)

In 2005, the world used about 455 Q-BTU. By 2100, this need will grow to about 1,500 Q-BTU, based on population growth and standard of living improvement. Hence, in 2100, the world will require about 3.4 X of the 2005 level of energy production.

Figure 4: World Total and Non-renewable Hydrocarbon Energy Needs (Q-BTU)

[Larger copy of the above chart]

From U.S. EIA and similar projections of future world energy use, the world is "still" expected to meet about 85% of its energy needs through 2030 with NRH fuels. Figure 4 extends this percentage through 2100 to predict that, should NRH fuels still be readily available and affordable, about 1,300 Q-BTU of NRH fuels will be needed. This is about 3.4X the NRH fuels consumed in 2005.

Figure 5: World Cumulative NRH Future Energy Needs vs. Reserves (Q-BTU)

[Larger copy of the above chart]

Using the annual world NRH fuels needs, shown in Figure 4, a simple resource utilization analysis can provide a ballpark estimate of when the NRH fuels may be exhausted. In Figure 5, the cumulative world need for NRH fuels is plotted. The projected NRH fuel reserves are then drawn down by this cumulative world need until the reserves are exhausted.

As seen in Figure 5, the current proved recoverable NRH reserves will be exhausted about 2060. This leaves an end-of-century NRH deficit of about 45,000 Q-BTU, which is equivalent to over 100 years of current NRH consumption.

For the "best case" using all proved and additional recoverable reserves—including the 3 trillion barrels of oil from oil shale—these reserves will be exhausted about 2084; slightly more than one generation after the first case. The end-of-century NRH deficit would be about 20,500 Q-BTU, which is equivalent to about 50 years of current NRH consumption.

What improved understanding does this model provide?

1. Growth in world per capita energy needs, spurred by expectations of an improving standard of living as a sign of political and economic success, will dramatically increase needed energy supplies throughout this century.

2. An energy planning horizon of 20-30 years, when non-renewable hydrocarbon fuels provide the vast majority of the world's energy, does not adequately anticipate nor respond to the world's energy situation this century.

3. By the end of the century, the world will need to fully replace nearly all production of non-renewable hydrocarbons with renewable or nuclear alternatives with a total world energy production capacity of 3-4 times that of today.

4. Undertaking a transformation from primarily non-renewable to primarily renewable/nuclear energy production is a necessity, if this model's projections of NRH reserves are reasonably accurate.

5. The stage is now set to compare space solar power against terrestrial renewable and nuclear energy sources, not in terms of economics, but in terms of the magnitude of the energy needed to meet world needs.

Closing caveat:

This simple model did not look at the potential to make use of the frozen methane hydrates that are discussed as a future non-renewable hydrocarbon supply. Neither the technology needed to exploit these reserves on an industrial scale nor the environmental issues associated with such energy production are sufficiently understood to incorporate this into energy planning.

23 - AstroPolitics essay - "Spacefaring Logistics Infrastructure: The Foundation of a Spacefaring America" by Mike Snead

Mike Snead — Tue, 01 Apr 2008 11:58:07 GMT

AstroPolitics: The International Journal of Space Politics and Policy (Vol 6, No. 1; Janurary-April 2008) recently published my essay "Spacefaring Logistics. The entire paper can be downloaded, for personal use, from the spacefaring resources page on http://mikesnead.net.

Introduction

Surprisingly, almost two generations after Apollo 11, neither America nor any other country is yet a true spacefaring nation. None possess the spacefaring capabilities needed for humans to routinely and safely access space and operate in space. This status stands in sharp contrast to the general recognition that, as highlighted in the United States National Space Policy, America will significantly benefit from becoming a true spacefaring nation with full freedom of action in space. What’s missing is the spacefaring logistics infrastructure that forms the core of America’s spacefaring dream. This dream of Americans, as spacefarers, being able to safely and routinely access and work in space remains the benchmark for assessing American progress in space.

This paper focuses on how this American spacefaring dream can now start to be realized. Specifically, this paper focuses on how America can undertake the transformation from an aging space exploring nation to a vibrant spacefaring nation by, first, focusing America’s aerospace industries on building and operating an integrated spacefaring logistics infrastructure and, then, using the newly acquired mastery of human space operations to enable the emergence of a new generation of commercial space enterprises. This combination of new spacefaring infrastructure, new industrial mastery of spacefaring operations, and new commercial space enterprises will take America into a new era of the space age where America is truly spacefaring.

Table of Contents

Introduction
Birth and decline of the American spacefaring dream
The renewed importance of America becoming spacefaring
Logistics infrastructure's role in establishing mastery of operations in new frontiers
Crossing the threshold to true spacefaring operations
Spacefaring logistics infrastructure implementation objectives
Conclusion

Mike Snead

22 - Assessing the Practicality of Scramjet-Powered, Single-Stage Aerospaceplanes

Mike Snead — Mon, 31 Mar 2008 14:26:39 GMT

The following is an essay published in The Space Review on March 31, 2008. This essay responds to a earlier Space Review essay by Eric Hedman.

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Assessing the Practicality of Scramjet-Powered, Single-Stage Aerospaceplanes

By Mike Snead

Clearly, the United States needs improved space access with “aircraft-like” safety and operability. In his Space Review article, “Space launch evolution and revolution” (The Space Review, December 10, 2007), Eric Hedman advocates focusing American space access development efforts on scramjet-powered, fully-reusable aerospaceplanes. He states: “The Holy Grail of low per passenger cost launcher concepts keeps coming back to scramjets.”

Scramjet-powered space access was first investigated, in the United States, in the late 1950’s and early 1960’s as part of the Department of Defense’s first aerospaceplane studies. These studies started what has now been nearly a half century of research into the design and performance modeling of scramjet engines and scramjet-powered flight systems.

From a top-level system design and operability perspective, a single-stage aerospaceplane is certainly, as Hedman highlights, the Holy Grail. As the United States struggles in its transformation into a true spacefaring nation, there is a critical need to identify a practical strategy to meet the need for aircraft-like, fully-reusable space access. Should scramjet-powered aerospaceplanes be placed front and center in the pursuit of the needed improved space access capabilities or is another approach the more practical approach at this time? As with any quest, success lies with selecting the more readily implementable path. This article summarizes some of the challenges involved in the scramjet-powered aerospaceplane path.

Scramjet fundamentals

All airbreathing jet engines have four primary segments – inlet, compressor, combustor, and nozzle. What is unique about the scramjet is that the airflow velocity through the combustor is designed to exceed the local speed of sound. “Scramjet” is an abbreviation for “supersonic combustion ramjet.”

In turbojets and ramjets, the velocity in the combustor is kept below the local speed of sound to enable efficient fuel injection, mixing, and combustion. As the flight velocity increases, useful energy in the incoming air is lost—converted to excessive heat that must be actively cooled—as the air flow is decelerated in the inlet and compressor to local subsonic speeds for entry into the combustor. This subsonic combustor speed limit effectively places an upper bound on aircraft’s flight velocities of around Mach 3-5 for pure turbojets and around Mach 6-7 for pure ramjets.

The scramjet reduces this loss by intentionally keeping the local flow speeds through the engine supersonic. If the scramjet engine and vehicle can be designed to withstand the flight external and internal temperatures and pressures, and if fuel combustion can be maintained, then very high airbreathing flights speeds, perhaps as high as Mach 15, may be achievable.

Research efforts to advance basic scramjet propulsion technologies, first begun in the 1960’s, continue. In 2004, for example, the 1,400 kg, 4.7 m-long, scramjet-powered, X-43A achieved an airbreathing speed of Mach 9.6 for about 10 seconds. A large rocket was used to boost the X-43A, following its release from the B-52 carrier aircraft, to the hypersonic test conditions needed to perform the scramjet engine’s tests.

Space access design considerations

For typical space launch systems using liquid-fueled rocket engines, both the fuel and oxidizer must be carried internally. The Space Shuttle does this with its large External Tank. Through the addition of airbreathing propulsion, some of the oxidizer is instead provided by the atmosphere. Ideally, the use of airbreathing as well as rocket propulsion enables the mass of the oxidizer and oxidizer tanks to be reduced. This approach, it is anticipated, will yield an aerospaceplane design that has a reduced takeoff gross weight when compared with the pure rocket baseline design.

The apparent advantage of using scramjets for aerospaceplanes is that the maximum flight velocity, where air still provides the needed oxidizer, can be high. Theoretically, a significant reduction in the propellant mass fraction can be achieved. The airbreathing “Holy Grail” Hedman notes is that such a reduction may enable a “closed design” for a single-stage aerospaceplane to be achieved. As described in the following, this path has many challenges that have yet to be successfully addressed. It is worth noting that closed designs of pure rocket-powered, single-stage aerospaceplanes, capable of achieving aircraft-like safety and operability using current technologies, have also not been proposed.

Single-stage space access system mass fractions

To better understand the design challenges of achieving a closed single-stage aerospaceplane design, an understanding of the concept of mass fractions is needed.

A single-stage aerospaceplane’s takeoff mass can be divided into two categories—propellant mass fraction and empty mass fraction—usually expressed as a percentage of the takeoff mass. The sum of the two fractions, by definition, is 100 percent. As one increases, the other must decrease.

As the name implies, the propellant mass fraction is the percentage of the aerospaceplane’s takeoff mass that is propellant—fuel and oxidizer. The propellant mass fraction required to achieve orbit is mathematically calculated from the change in velocity needed to achieve orbit, the losses due to drag and trajectory that must be accounted for, and the propulsion subsystem’s net efficiency in converting the fuel’s energy into increasing vehicle velocity.

The net propulsion efficiency is the net thrust produced per unit mass of propellant consumed. This is the familiar “specific impulse” term that is used, along with the thrust, to describe the performance of a rocket engine. It can also be used for airbreathing engines. By using oxygen from the air, instead of from onboard tanks, airbreathing engines ideally increase the net propulsion efficiency by decreasing the mass of propellant consumed each second to yield the same thrust. This has the benefit of reducing the theoretical propellant mass fraction.

For a rocket-powered, single-stage aerospaceplane, the theoretical propellant fraction to achieve orbit will range from about 88-95 percent. This means that 88-95 kg of every 100 kg of takeoff mass must be propellant if orbit is to be achieved. Rocket-powered aerospaceplanes are essentially large flying propellant tanks.

Ideally, when a very efficient airbreathing-rocket engine combination is used, the theoretical propellant fraction reduces to about 60-70 percent. As a result, the empty mass fraction for the scramjet solution (30-40 percent) can be two to eight times larger than that for a pure rocket design (5-12 percent). From a design perspective, this provides a substantial apparent advantage to the airbreathing approach. It seems obvious that an airbreathing design where 30-40 kg of every 100 kg of takeoff mass can be hardware would be easier to design and build compared with a pure rocket design that only permits 5-12 kg of every 100 kg of takeoff mass to be hardware.

By subtracting the theoretical propellant fraction from 100 percent, the remaining fraction is the empty mass fraction objective—what the design engineers need to achieve. If a safe, operable, and buildable design can be defined where the predicted empty mass fraction is equal to or less than this objective, the design is said to “close.”

Challenges in closing the design of single-stage, scramjet-powered systems

Today, the American aerospace industry has the ability to design and build two-stage, fully-reusable space access systems that are believed to “close” based on the results of government and industry conceptual design studies using current technologies. Generally, these designs use a vertical takeoff, horizontal landing approach that is pure rocket powered. The original fully-reusable design for the Space Shuttle from the early 1970’s, before the design was changed to the expendable External Tank and Solid Rocket Boosters, is an example of this configuration.

While various ideas for single-stage, scramjet-powered aerospaceplanes have been proposed since the 1960’s, closed single-stage designs using current technologies and capable of aircraft-like safety and operability have not yet been achieved. Why is this? The problem is that the use of scramjets adds significant complexity to the design and verification of a single-stage aerospaceplane while at the same time adding uncertainty to the estimation of the required propellant fraction. Essentially, it makes the problem of predicting design closure and then demonstrating a successful design much harder, as discussed in the following examples.

- Added propellant tank mass

One design challenge is that scramjet airbreathing propulsion adds significant additional empty mass that is not needed for a pure-rocket system. While the addition of airbreathing propulsion may decrease the required propellant fraction, the average propellant density may also decrease. This can result in disproportionally larger total propellant tank volume and larger tank total mass.

This design dilemma results from the fact that a scramjet requires hydrogen as the fuel for flight speeds greater than about Mach 7. Hydrogen, it is hoped, will have the ability to sustain combustion in the scramjet’s supersonic combustor at these higher Mach numbers where no other fuel appears to be able to sustain combustion unaided. Liquid hydrogen, however, has the disadvantage of having a very low density—a beverage cup that holds 16 oz. of water only holds about 1 oz. of liquid hydrogen. Liquid hydrogen also is very cold with a temperature close to absolute zero. This requires added tank insulation and special fuel tank pressurization management. Increased tank volume, increased tank insulation, and special fuel tank pressurization management generally yields increased tank mass per kg of propellant carried.

- Added thermal protection subsystem mass

Closely related to the likely increase in total propellant tank mass is the additional thermal protection subsystem (TPS) mass required for hypersonic scramjet operations. All reusable space access systems require thermal protection during reentry. During ascent under rocket power, the thermal loads are comparatively moderate and no additional thermal protection is normally required. The thermal protection required during scramjet-powered ascent can, however, exceed that required for reentry. As a result, the total TPS mass will usually increase, compared with a pure-rocket design, due to the higher temperatures, higher aerothermal loads, the increased need for active cooling, and the larger total propellant tank surface area.

- Added propulsion subsystem mass

Another empty mass design issue is the increase in the mass fraction of the propulsion system to incorporate airbreathing propulsion. In oxygen-kerosene rocket engines, for example, the installed engine thrust-to-weight ratio is about 60-80 while it is about 50-60 for oxygen-hydrogen rocket engines. The installed thrust-to-weight of the best supersonic turbofans is about 5-8 and about 8-12 for the best subsonic turbofans. The complexity of the design of the scramjet, combined with the need to retain a rocket engine capability, means that the installed thrust-to-weight of the integrated airbreathing-rocket propulsion subsystem for an aerospaceplane may only be in the range of 3-5. Compared with a pure-rocket design where the engines may comprise about 2-3 percent of the takeoff mass, the integrated airbreathing-rocket engine may 10X heavier, comprising 20 percent or more of the takeoff mass, or about half of the allowable empty mass fraction.

- Uncertainty in predicting the required propellant fraction

Design challenges also arise on the propellant fraction side of the design closure ledger. To develop a thorough closed design, good confidence in the accuracy of the required propellant fraction is needed—typically, 3-4 significant figures. This then enables the design engineers to confidently calculate the empty mass fraction objective and have a good definition of the specific flight conditions that need to be addressed—dynamic pressures, aerothermal loads, etc.

Unfortunately, modeling and verifying the performance of an integrated airbreathing propulsion subsystem from Mach 0-15 remains very challenging. Limited ground and flight scramjet test data means that there remains substantial uncertainty about the actual achievable performance, especially at high Mach numbers. While further scramjet technology development and testing may resolve this issue, substantial further improvement in scramjet performance modeling is required before the conceptual design closure of scramjet-powered aerospaceplanes can be achieved.

Scramjet testing design and verification challenges

Rocket and airbreathing engines are extensively tested to verify performance, safety, and operability. New jet engine designs routinely undergo thousands of hours of ground test while 500-1000 test firings of a new rocket engine design is the norm. For an airbreathing propulsion system that includes scramjets, the wide range of airbreathing operation from Mach 0 to 15 will require more extensive testing than is typically done for jet engines.

One challenge hindering scramjet development is that ground test facilities for full-scale testing of airbreathing propulsion systems are quite limited for Mach numbers greater than about Mach 3-4. Achieving the correct combinations of airflow velocity, mass flow rate, temperature, and sustained test conditions in test sections of the needed size for large scramjets is very difficult. Because of this, more extensive use of flight testing would be the presumed alternative approach. As the recent X-43A test program has shown, scramjet flight testing is very expensive and time consuming. The seven-year X-43A program, using three small expendable test vehicles, cost approximately $230 million. Two of the three tests conducted were successful in demonstrating scramjet operations for a total of about 30 seconds of test data. Relying on flight testing to very high Mach numbers, assuming this is successful, will add significantly to the length of time and cost required to successfully develop a scramjet for an aerospaceplane.

Scramjet-influenced airframe structural design and verification challenges

A key element of a successful airframe structural design is one that can be economically built and tested. Fabrication and test considerations are as important to the airframe’s design as are the aero-thermal-inertia loads. An innovative structural design engineer may be able to conceptualize a highly efficient, low-mass airframe design, but it may not be able to be successfully built and/or tested. This is especially important for aerospaceplanes because of the significant aerothermal loads acting on the vehicle during descent and, with scramjet propulsion, during ascent.

An example of the design and verification testing challenges that will arise with scramjet-powered aerospaceplanes comes from the National Aerospace Plane (NASP/X-30) program of the late 1980’s. It focused on a scramjet-powered, horizontal takeoff and landing, single-stage system that would have a takeoff mass of about a half-million kilograms—about the same as the new Airbus A380.

New airframe designs go through a series of component and full-scale ground tests to verify structural integrity. This is done for X- and Y-aircraft’s airframe design as well as for the production aircraft’s airframe design. Such testing involves the initial static tests to establish the strength and stiffness of the structure followed by cyclic testing to establish the damage tolerance and durability of the structure. The need for scramjet-powered systems to carry liquid hydrogen, combined with the time-dependent aero-thermal loads from the use of high-Mach airbreathing propulsion during ascent, adds significant complexity to the airframe’s structural testing.

While most airframe structural testing is conducted in ambient air environments, this will probably not be possible for scramjet-powered aerospaceplanes. Because no other liquid properly simulates the thermal response and mass characteristics of the liquid hydrogen in the fuel tank, structural testing of the airframe of scramjet-powered aerospaceplanes may be expected to require testing with, perhaps, several hundred tons of liquid hydrogen onboard. Any type of structural testing involving flammable liquids poses additional safety constraints. Testing of hydrogen is, perhaps, the most difficult because of hydrogen’s flammability in air at low concentrations and hydrogen’s ability to escape through small leaks when other gases cannot—such as small cracks in the tank walls or internal piping. The risk of even small structural failures that release hydrogen requires that such testing be done in special inerted facilities, possible the size of a domed football stadium, located several kilometers away from inhabited buildings.

High Mach airbreathing propulsion also introduces the variable of time in the structural verification testing because the internal airframe/tank temperatures and internal loads are time dependent. When combined with the changing external aero-thermal-inertia loads acting on the airframe as the vehicle accelerates to the Mach 15 scramjet cutoff velocity, this presents a very complex structural test matrix consisting of, potentially, hundreds of different test points covering both ascent, reentry, and emergency flight conditions.

Using past plans for NASP/X-30 structural testing as the example, the quantity of liquid hydrogen in the tanks would be set to the amount remaining at that point in the flight profile and the internal structure would be brought to the design internal temperature conditions using, probably, internal electrical heating elements. The external thermal and mechanical loads would then be applied over hundreds of square meters of surface area. At the same time, the airframe and scramjet’s active cooling system, using gaseous hydrogen at pressures up to 5,000 psi, would be started to simulate the cooling of much of this external surface area to ensure the correct airframe internal thermal loads and structural deformation. Obviously, such structural testing involving large quantities of combustible fluids near absolute zero, radiant thermal loads developing external TPS temperatures of over 1000 F, hot hydrogen gas at high pressures and temperatures flowing through hundreds of square meters of minimum-gage, actively cooled structures, and large mechanical loads that induce significant structural deflection, will be challenging to undertake.

Further complicating the structural integrity tests is that they will not be comprised of a single test for each design point. As mentioned, the initial static strength and stiffness verification of the structural integrity of the airframe may involve hundreds of test points to release the system for flight test and support envelope expansion. Some of these test points will probably need to be retested using actual flight-measured temperature and aero loads. Also, verification of the resistance of the airframe to loss of structural integrity brought about by temperature cycling and operational usage will involve further repetitive load testing of hundreds to thousands of cycles when the time comes to verify the structural integrity of the production configuration.

These structural design and testing challenges indicate that, as with scramjet engine testing, the structural development of scramjet-powered aerospaceplanes will be much more complex than that required for the more traditional airframe design expected to be used in two-stage, rocket-powered aerospaceplanes.

Scramjet-powered aerospaceplane program length and cost

The final challenge facing the development of scramjet-powered aerospaceplanes is the length and cost of their development programs. One estimate, prepared in 2003, stated that the projected cost through 2018 for “large-scale scramjets and turbojets,” as part of the National Hypersonics Initiative, was $8-14 billion. (Turbojets would be needed to accelerate the vehicle to the Mach 4-7 speed necessary for the ramjet/scramjet to start to operate.) The additional technology development demands for the airframe, thermal protection system, propellant tanks, and other flight-critical subsystems influenced by the use of hypersonic airbreathing propulsion will add significantly to the necessary technology risk reduction investments that would also be required.

A primary consideration in laying out a projected development timeline for a scramjet-powered aerospaceplane is the recognition that much of the needed technology development and maturation will involve substantial flight testing. The reality of scramjet propulsion is that test size counts due to the influence of the boundary layer characteristics on the engine operation. Hence, small test engine sizes, such as used on the X-43A, have inherent limits on their contribution to technology maturation. Near full-scale engines will be needed to verify engine performance and operability and near full-scale flight systems will be needed to demonstrate the ability to actually achieve orbit.

It may be expected that following the completion of the preliminary round of technology development efforts—perhaps by 2020 assuming sufficient funding—a near full-scale X-aircraft using all representative flight hardware designs, covering the full airbreathing range from Mach 0 to Mach 15, would be built. This would be followed by a pre-production Y-aircraft that is near full-scale and includes the rocket propulsion system to verify the ability to achieve orbit and reenter safely. Once the Y-aircraft program is successfully completed, the production program would start. With a typical design, build, test cycle of 10 years for advanced flight systems and starting in 2020, the X-aircraft program could be completed in 2030, the Y-aircraft program in 2040, and the production system could become operational about 2050.

Conclusion

With the impact of the upcoming termination of Space Shuttle operations on American space operations, it is very apparent that the United States needs substantially improved passenger and cargo space access. It is also clear that only a fully-reusable system design will enable improved “aircraft-like” safety and operability to be achieved. The alternative to the scramjet-powered, single-stage aerospaceplane approach is the two-stage, rocket-powered, fully-reusable aerospaceplane. The technologies required to start development of this approach are sufficiently mature that production development of the first generation could start now. By 2020, perhaps as early as 2016, such a two-stage aerospaceplane system could become operational and America’s ability to access space will leap forward more than three decades earlier than pursuing the scramjet-powered, single-stage approach.

Getting passengers and cargo to space with “aircraft-like” safety and operability are the primary needs of a true spacefaring nation. Hopefully, further technology development will yield single-stage capabilities in the future. While such technology development should continue, it is important that America’s future in space not be held hostage by the imposition of the scramjet technology path to space. Achieving aircraft-like safety and operability are the only design prerequisites and these will bring the desired lower costs. Starting the development of rocket-powered, two-stage, fully-reusable aerospaceplanes is, today, America’s readily implementable option to meet its space access needs as a true spacefaring nation.

21 - Space solar power and America's energy future (Part 4)

Mike Snead — Tue, 01 Apr 2008 12:18:01 GMT

Minor typo correction made 20080401

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The United States consumes about 100 Q-BTU of energy per year while the total world consumes about 450 Q-BTU. But what is a Q-BTU or quadrillion British Thermal Units? This blog addresses this question.

British Thermal Units

British thermal units or BTU is defined as the amount of heat (meaning energy) required to be added to one pound of water to increase its temperature by one degree Fahrenheit. In the United States, it has remained as a common unit for expressing the energy release associated with fuels.

A quadrillion BTU is 1,000,000,000,000,000 BTU. This is written in scientific notation as 1 x 10^15 BTU. In this series, this is abbreviated as Q-BTU, instead of the abbreviation "quad" used by some, just to make sure that it is understood that BTU is being discussed.

For these discussions of America's energy future, the Q-BTU is the unit used by the U.S. government to report and forecast energy consumption, as shown in Part 1 on this series (SA blog 15).

Work and power

To be useful, energy must be transformed through some mechanical means into work. Work performed in a period of time is power.

A human climbing stairs works to increase his or her potential energy. The amount of work performed is equal to person's weight times the height climbed. Climbing two flights of stairs requires twice the work of climbing the first flight. The body's muscles convert sugar (the energy source) into mechanical force that lifts the weight of the human.

The amount of power generated as the stairs are being climbed is the work performed divided by the time required. If the stairs are climbed twice as fast, then twice the power has been generated even though the total work performed was the same.

The U.S. consumes about 100 Q-BTU of energy, in all forms, per year. This is an expression of energy per unit of time, which is power. The common unit of power is the watt, named after James Watt.

One watt is defined as one joule of energy per second. To convert BTU per year into watts, as is done below, requires a conversion of BTU into the metric unit of energy - the joule. 1 BTU = 1055.056 joules. Hence, 1 BTU per second = 1055.056 joules per second or 1055.056 watts.

A person climbing stairs at a normal rate is expending about 200 watts of power or 200 joules per second.

When discussing electrical power, the common units in the U.S. are:

Kilowatt (kW) = 1,000 watts

Megawatt (MW) = 1,000,000 watts or 1,000 kW

Gigawatt (GW) = 1,000,000,000 watts or 1,000 MW or 1,000,000 kW

Quadrillion BTU to Gigawatt conversions

1 Q-BTU (energy) = 1 x 10^15 BTUs x (1055.056 joules per BTU) = 1055.056 x 10^15 joules (energy)

1 Q-BTU of electricity produced per year (power) = 1055.056 x 10^15 joules per year / (365 days per year x 24 hours per day x 60 minutes per hour x 60 sec per minute) = 33.5 x 10^9 joules/sec or watts (power) = 33.5 x 10^6 kilowatts (kW) = 33.5 X 10^3 Megawatts (MW) = 33.5 Gigawatt (GW) of continuous production

Sustained electrical power generation

Electrical power generation is a sustained operation. To define the amount of power being continuously generated, the watt-hour is defined as one watt of power produced for one hour. This concept is also applied to yield kW-hour, MW-hour, and GW-hour with the kW-hour (or kW-hr) being the common unit for commercial power generation.

1 Q-BTU of electricity per year = 33.5 x 10^6 kW per year x (365 days per year x 24 hours per day) = 293.5 x 10^9 kW-hrs = 293,500 GW-hrs

Electrical power costs

Quite often comparisons of alternative forms of electrical power generation are made in terms of the cost per kW-hr.

1 Q-BTU of electricity per year = 293.5 x 10^9 kW-hr x $0.01 per kW-hr = $2.9 billion @ $0.01 per kW-hr

1 Q-BTU of electricity per year = 293.5 x 10^9 kW-hr x $0.075 per kW-hr = $22 billion @ the avg. U.S. retail electricity price of $0.075 per kW-hr

Comparisons of oil, coal, natural gas, and nuclear

Using the Q-BTU as the basis for comparison, the cost of these four energy sources can be estimated.

1 Q-BTU (oil) = 1 x 10^15 BTU / 5.8 x 10^6 BTU per barrel of oil = 172 million barrels

1 Q-BTU (oil) = 172 x 10^6 barrels x $100 per barrel (spot market price 20080220) = $17.2 billion

1 Q-BTU (coal) = 1 x 10^15 BTU / 22.4 x 10^6 BTU per short ton of coal = 44.6 million tons

1 Q-BTU (coal to produce electricity) = 44.6 million tons x $34.3 per ton (avg. 2006 price for electric utilities) / 0.35 (avg. plant generation efficiency) = $4.4 billion

1 Q-BTU (natural gas) = 1 x 10^15 BTU / 1031 BTU per cubic ft = 970 x 10^9 cubic ft of natural gas

1 Q-BTU (natural gas to produce electricity) = 970 x 10^9 cubic ft / 1000 x $7 per 10^3 cubic ft / 0.35 (avg. plant generation efficiency) = $19.4 billion

1 Q-BTU (uranium in the form U3O8 used to power nuclear reactors) = 1 x 10^15 BTU / 50 x 10^9 BTU per short ton of U3O8 = 20,000 short tons of U3O8

1 Q-BTU (U3O8 to produce electricity) = 20,000 short tons U3O8 x 2000 lb per short ton x $75 per lb of U3O8 (spot market price 20080220) / 0.35 (avg. plant conversion efficiency) = $8.6 billion

20 - What to tell the next President about realizing America's potential in space

Mike Snead — Mon, 04 Feb 2008 12:26:49 GMT

The following is an essay published in The Space Review on February 4, 2008.

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The start of the new presidential administration in 2009 will be a critical time for America's non-military space efforts. The Vision for Space Exploration (VSE) has pronounced Bush administration fingerprints and may not have the public support needed to weather the upcoming political debate on its future. More importantly, as the presidential debate shifts from foreign affairs to the domestic economy, no presidential candidate has yet seized on the potential of America's industrial future in space as a key economic and technological catalyst for reestablishing America's competitiveness and world leadership, and substantially helping America to become energy independent. American political leaders apparently remain blind to the true potential of space in the coming decades of the 21st century and do not yet publicly recognize that a new race for space industrialization is starting among the world's technological nations.

The clear importance of space to America's future is summarized in the current U.S. National Space Policy: "In this new century, those who effectively utilize space will enjoy added prosperity and security and will hold a substantial advantage over those who do not. Freedom of action in space is as important to the United States as air power and sea power. In order to increase knowledge, discovery, economic prosperity, and to enhance the national security, the United States must have robust, effective, and efficient space capabilities."

The Eisenhower Center for Space and Defense Studies, associated with the United States Air Force Academy, conducts public forums to discuss issues of importance to America's future in space. This past fall, for example, they hosted a conference on Space-Based Solar Power (SBSP) at which the results of the informal SBSP study, sponsored by the National Security Space Office, were presented.

On 7-8 February 2008, the Center will hold this year's National Space Forum, "Space Challenges Facing the New American Administration of 2009." The focus of the opening panel, including Ambassador Roger Harrison, Director of the Eisenhower Center, and General C. Robert Kehler, Commander, U.S. Air Force Space Command, will address the "key decisions that will need to be made in regard to space policy when a new American Administration comes to power in 2009." The panel members will respond to questions about military, commercial, and civil space provided by the moderator and forum participants.

As this presidential campaign unfolds, it is up to the American pro-space industrialization community to proactively identify and communicate the importance of space to the candidates. Our responsibility is to provide the next president with a substantially improved understanding of the true potential of space industrialization and what new initiatives can and should be pursued. The upcoming National Space Forum is an excellent opportunity to advance this critical discussion. Given the opportunity, I would ask the opening panel participants these three questions:

1. Has a new strategic race for space started between the world's leading nations to gain the knowledge, discovery, economic prosperity, and national security advantages that true spacefaring nations will enjoy over non-spacefaring nations?

2. What specific steps, actions, policies, and new initiatives should the U.S. government and American aerospace industry undertake to enable America to effectively utilize space?

3. What changes in American spacefaring logistics infrastructure are needed to provide the U.S. government and commercial space industry with the robust, effective, and efficient space capabilities needed to access and operate in space safely, routinely, and profitably?

Certainly, there are many other space-related issues that also need to be addressed: ITAR, VSE, extended use of the Space Shuttle, space science, COTS, etc. However, the real strategic issue on the table for the next administration is about the needed transformation of America into a true spacefaring nation. We are not yet at this point and our current civil and commercial programs are not bringing us closer to this goal where Americans, as spacefarers, will be able to safely and routinely access and operate in space. To change course to enable America to successful compete and win the new space race, the next American president will need to set new strategic goals for civil and commercial space. The American pro-space industrialization community must take a leadership position to help to define the near-term options for substantially improving America’s ability to utilize space.

Mike Snead

19 - Space solar power and America's energy future (Part 3)

Mike Snead — Sun, 23 Dec 2007 10:43:36 GMT

Part 1 is here.
Part 2 is here.

In Part 2 of this series, I identified my list of priorities for the criteria to select long-term future energy sources for America. This list is:

Energy acceptability
Energy economic opportunity
Energy assuredness and energy affordability
Energy sufficiency and energy availability

Of this list, the bottom two are technology-driven while the second is driven by business and market considerations. The top priority is driven by public perception and political agreements.

Personal biases

I preface the following remarks with three points. First, I fully support the development of renewable energy sources and the adoption of sensible energy conservation measures. I've held this view since the mid-1970's when I helped to start a local alternative energy association to promote discussion and understanding of renewable energy and energy conservation. Both are, I believe, morally correct positions following the old adage of "waste not, want not."

Second, I am not yet convinced that any change in the global average temperature is happening and, should it be happening, that anthropogenic influences are significant. There is a growing body of reported data and observed weather conditions that are inconsistent with the use of the term "global warming" to mean increasing average temperatures. Further, estimates of the influence of anthropogenic emissions on current and predictions of future global average temperatures or variations in local weather conditions, such as the drought in the Southeastern U.S., are less than fully convincing as the cause-effect relationship is unclear. I don't, however, discount the importance of gaining a better understanding global temperature changes and conditions and responsible natural and anthropogenic influences. Nor do I dismiss the need to take reasonable preventative measures, especially with regard to the formulation of future energy policies, while the fundamental scientific information is still being assessed.

Third, I believe it is important to gain a good understanding of global warming, but not be pushed to hysterical responses, as the words by some would support. The U.S. and the world need sound mid- and far-term energy policies and practices that enhance freedom, security, and an improving standard of living. Hysterical responses, particularly those driven by underlying political agendas, will not move the U.S. and the world towards these needed sound policies and practices.

Energy political balls in the air

Energy politics in the United States has been juggling three "balls" while the American political process attempts to find sufficient consensus on an energy strategy and, then, an energy policy. The first ball was been, since the end of World War II, the increasing U.S. dependence on imported petroleum. The second ball has been the high and increasing cost of petroleum driven, apparently, by the increasing world-wide demand for petroleum. This ball popped up in the 1970's. The third ball is the potential of a looming world-wide shortage of conventional petroleum should predictions that world total production of conventional petroleum has or is about to peak. Now we have a fourth energy ball being tossed into the political air. This is the "ball" of anthropogenic-driven global warming due to the increasing industrial consumption of non-renewable carbon fuels—primarily coal, conventional petroleum, and, increasingly, non-conventional petroleum (e.g., tar sands).

Recent Bali conference on global warming

This past two weeks a conference on "global warming" was held in Bali, Indonesia. Its purpose was to define and adopt a new international treaty on mitigating anthropogenic-caused global warming as a follow-up to the ill-fated Kyoto Protocol. This conference was a response to this year's release of the update of the report by the United Nation's Intergovernmental Panel on Climate Change.

The United States delegation to the conference was under strong political pressure, both internationally as well as domestically, to adopt a formal treaty binding the U.S. to rapid and significant reductions in "greenhouse gas" emissions. In response, the U.S. led a minority position that it was too early to adopt quantitative goals—especially if the panel's recommendations of cuts of 25 to 40 percent compared with 1990 emission levels, are to be mandated by 2020. Instead, the U.S. proposed and gained grudging acceptance of a two-year delay pending further discussions and negotiations.

The strength of the emotion-driven arguments at the recent Bali conference emphasize the growing public interest in collective world-wide environmental protection. This feeling stretches, in America, back to establishment of the first national parks to preserve places of unique natural beauty, such as Yellowstone, for future generations to enjoy. Most recognize and support the wisdom of such environmental protection efforts provided they are implemented in a practical manner.

With the heightened public awareness of changes in the world's environment, we see that the American public's political center-of-gravity on this issue is shifting towards "green." One result was that the U.S. Congress just passed and the president signed legislation mandating improved gas mileage for cars. We also see that the public is adopting personal energy conservation measures such as converting to compact fluorescent lights and purchasing more fuel efficient cars. These steps indicate increasing political support for positive greene change.

Bali's importance to American presidential politics

Whether intentional or not, the U.S.-led effort to delay adoption of specific quantitative reductions in "greenhouse gas" emissions will move this debate squarely into the U.S. presidential campaign. Already, eco-activists are initiating a write-in campaign for former Vice President Al Gore in the upcoming New Hampshire presidential primary to help highlight global warming as a political issue. While probably too late to impact the primaries, this issue may arise during the fall's campaign.

How should the U.S. respond to the demands of most other nations that we cut our greenhouse gas emissions by 25-40 percent within 12 years? The stage has been set for the next president to squarely address this issue and to deal with the selected implementation throughout their administration. Add this to the issues of U.S. dependency on imported petroleum, the high cost of petroleum, and the pending world-wide shortage of conventional oil, and it is possible that energy and the environment could move to center stage of the American presidential campaign. Voters may see this as a central "pocket-book" issue with greater personal importance than Iraq and illegal immigration.

Bali's impact on the potential for space solar power

What is important to advocates of space solar power is that, returning to the list of selection criteria for future energy sources, the public's feelings regarding energy acceptability is moving towards the acceptance of the need for new energy sources. Thus, the public may be open to new information on space solar power as a new and acceptable energy source. Heightened Congressional interest in how the U.S. will respond to the Bali and the likely debates of this issue during the fall will provide an important opportunity to introduce and expand on the discussions of the potential of space solar power for meeting mid- and far-term U.S. and world energy needs.

Response to reader comments on SA Blog 17

Mike Snead — Fri, 14 Dec 2007 15:57:02 GMT

A comment provided on my SA blog 17 asked several questions. This blog provides my responses.

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As the former NASA manager who ran the SSPS studies in the late 70s, a few words. SBSP will never get off the ground, to coin a phrase. Why? Here are a few reasons.

1. On the equinoxes SBSPs in GEO spend some time every day in the shadow of the Earth, no power transmission. Back up?

Response: Twice each year, for several days, satellites in GEO pass through the Earth's shadow. The shadow is about 12,800 km across and the satellites are traveling at about 3 km per sec. The transit time has a maximum of about 1.2 hours. The continental U.S. spans about 50 degrees of longitude. At GEO, the length of the 50 degree arc is about 31,000 km. At maximum, the shadow will cover about 40 percent of this arc meaning that about 60 percent of the SSP satellites will still be producing power. The transit of the shadow happens at local midnight when consumer demand for power is reduced. This combined with the ability to transmit power through terrestrial power grids over long distances will enable the power reductions to be managed. Unlike terrestrial power losses due to droughts, storms, cloud cover, strikes, etc., these SSP down times are very predictable in terms of timing, length, and number of satellites impacted.

2. Using reasonable conversion factors, to deliver 1 GW will require a 10 square km satellite. A 5 GW SBSP would require 30-50,000 tons in orbit. Assuming a 100 ton payload for your "aircraft-like" system (~X a shuttle payload) the number of launches to LEO is very large, number of launch sites needed, many if one is planning 100s of SBSPs! Frequency of launches at each site, weekly, monthly?

Response:

a. My assumption is that the SSP satellites will be divided into two payload classes where 95 percent will fit within the payload bay of the fully-reusable, two-stage aerospaceplane and 5 percent will require a Saturn-V class partially-expandable launch system. The assumption used during the study was that the satellite mass would be 20,000 tons. This value is doubled to add mass for propellants, on-orbit assembly support, payload holding structure, etc. Assume 40,000 tons for each SSP satellite needs to be transported to LEO and 20,000 tons to GEO.

b. Assume each aerospaceplane transports four 10-ton modules. Each module will fit within a 40-foot intermodal container for ease of land or sea transport from the manufacturing site to the launch site. Each 40-foot container contains about 2,500 cubic feet. The 20,000 lb module has a density of about 8 lbs per cubic foot - about the maximum for general cargo. 1,000 missions are required to transport 40,000 tons to LEO for one SSP satellite.

c. Assume each aerospaceplane orbiter (second stage) is capable of flying once a week ,with two weeks in depot each year, on average. In other words, each second stage can fly about 50 missions per year. (This will be a second generation aerospaceplane designed specifically to transport the modules.) Assume that one SSP satellite per year is needed. 1000 missions / 50 missions per year per orbiter requires 40 orbiters. (The number of needed first stage systems, which turn around faster, will be somewhat less.) Assume 25 percent extra orbiters to cover those out of service for depot maintenance or repairs. 50 orbiters are needed to support one SSP satellite per year. 40 missions per week are flown, on average, or about 6 per day.

d. Assume two LEO space assembly locations, 180 degrees out of phase with each other, located in a logistics orbit. (A logistics orbit is a circular, constant altitude orbit that, by selection of the inclination and altitude, has a repeating ground track. This is not a microgravity orbit like the ISS or Hubble.) This provides two launch windows per day from the primary launch site and one from a suitably-located secondary site. This provides for 6 missions per day to be flown. The primary site would be at KSC and the secondary site would be on the coast of Texas. (Note: the aerospaceplanes may not launch from sites on land, but may use floating platforms that move offshore to minimize sound and enhance safety. For LOX/kerosene fueled systems, they would be fueled at the launch point from sea-anchored terminals similar to those used to off-load commercial tankers. This will enable the platforms to be moved to sea at a reasonable speed and enhance safety near the shore.)

e. SSP satellite production rate scale up would require a larger aerospaceplane fleet size, increased capacity at the terrestrial launch sites, and added LEO space assembly depots. At some point, shifting to extraterrestrial resources/manufacturing will be needed.

3. Then the LEO tonnage must be boosted to GEO for assembly, not in LEO. This will require additional thousands of tonnes of propellant and boosters in LEO. Are you counting the launches needed?

Response: The additional propellants are factored into the total tonnage discussed above. The current expectation is that conventional rocket-propelled space tugs will give the assembly modules an initial Delta-V and then the modules will use electric propulsion to complete the transfer to GEO. The space tugs are fully-reusable and based at the space assembly depots. Electric propulsion may be provided by a separate tug or this may be integrated into the assembly module using the solar arrays.

4. Power transmission will require a steerable antenna ~ 1 km in diameter containing millions of elements. You can get one from Radio Shack, right?

Response: Well, certainly not from just one Radio Shack store. Seriously, my focus is on the spacefaring logistics needed to support the assembly of the SSP satellites. The design of the antenna is being looked at by the appropriate experts. I believe that a phased array approach may be the preferred configuration. Either way, the antenna will be assembled in LEO at a special space dock and then moved to GEO. This will certainly be a challenge but we have about 15 years to work out the design, construction, assembly, and transport details.

5. Then there is the small problem of receiving the transmission and integrating it into the power grid. The rectenna size, including a buffer zone, would be ~ 15 square km. Lots of empty space in US, however not near the consumers. Solution, long transmission lines = power loss plus problem of right of way for transmission lines.

Response: Recent information that I have found indicated that transmission distances of several thousand miles can be performed with acceptable loses. Also, the U.S. currently has a highly integrated electrical power transmission system that will be increasingly used to move electricity from distant sources to the market near major urban areas. Whether we use SSP satellites or not, the U.S. electrical power generation system already had hundreds of GW class generation stations integrated into common power networks. The U.S. will need to add many new generation sites. I do not see any specific problems with these being SSP terrestrial receiving arrays.

6. Environmental impact? Many SPSBs in orbit will not be accepted by the astronomy community and others. I could go on. Suggest you obtain a copy of Don Rapp's draft report to SMD "Assessment of Concepts for Utilizing Lunar Resources." In the meantime, of course, there are much cheaper alternatives that require no R&D like renewables, nuclear power, and perhaps breeder reactors. I don't place much hope on fusion power but potentially it may be much easier and cheaper than SPSB to finally make it cost effective.

Response:

a. I do not see the environmental impact issue as being significant. Modern astronomy is mostly digital which would imply to me that suitable computer controls can be used to eliminate any adverse impact from the SSP satellites. While the satellites are large, they will still occupy a very small fraction of the observable sky. The primary question is how do such considerations stack up against the nation's need for new acceptable energy supplies. Also, the same spacefaring logistics infrastructure that enables SSP satellites to be built will enable large space telescopes to be built and serviced. By the time the SSP satellites become numerous, current large terrestrial telescopes will be 3-5 decades old.

b. I agree that extraterrestrial resources and manufacturing of SSP satellites are likely to be used in the future. Building SSP satellites from primarily terrestrial resources will constitute the first era of space industrial operations. Building them from extraterrestrial resources will constitute the second era.

c. In one of the other SA blog entries discussing SSP, I address the issue of magnitude of energy needed by the U.S. While there are other technological options for new energy production, it is not clear that they are suitable for new electrical base load power generation of the magnitude needed.

- Terrestrial solar and wind have the disadvantage that they are non-continuous and would require some form of substantial storage to enable continuous power generation under varying weather conditions. They may prove to be useful, under some circumstances, for offsetting CO2 production, but they do not provide a replacement for the coal-fired powered plants which would still be needed to provide power at night and/or when the wind velocity is low. Few people live in a climate where terrestrial solar and/or wind can meet there energy needs 365/24.

- Nuclear has issues associated with the cost and availability of uranium, the disposal of nuclear waste, the location of the nuclear power plants due to geography and cooling water availability, and the production of plutonium in some fuel cycles. The extent of the acceptance of a significant growth in the number of nuclear power plants is not yet clear.

d. While there are engineering issues to be solved with SSP, the general technology readiness level is substantially higher than any form of fusion at this time. This means that the engineering development of SSP can proceed while further fundamental research into fusion is needed. There have been, however, some interesting new fusion technology concepts. This may be the ultimate energy source of the future -- how far in the future is not clear.

e. As the world economy grows, the demand for high quality energy - electricity - will increase significantly. For non-hydrocarbon energy production of base load electricity, only hydroelectric, geothermal, biomass, nuclear, and space solar power will be able to be used. SSP provides one renewable energy production system that can be readily exploitable as a turn-key process

Mike Snead