# Space Transport and Engineering Methods/Energy
## Energy in General

All of civilization requires energy to function, including about 8 MJ/day (~2000 Calories) of food energy per person. Space systems are no exception, They require energy both for propulsion and for other systems like life support, computers, and materials processing. The general field of energy is vast, involving many fields of engineering. Energy for space projects involves an equally wide range. So we can only provide an introduction, and supply references for further study. In this section we survey the range of possible energy sources for all types of space systems. Since the book is oriented to future space projects, we list a number of sources that are not developed yet, but are possible according to known physics. These sources can then be used for the propulsion methods tabulated in Part 2, and for other engineering purposes described in Part 3 and later. By making a two dimensional table of energy sources vs propulsive forces, we can categorize all possible propulsion methods, and we do this at the start of Part 2 of the book. We have not yet developed a similar table to neatly categorize the other systems in a space project.
Proximate vs Ultimate Energy
The principle of Conservation of Energy states that energy can neither be created nor destroyed, merely transformed from one kind to another. So all energy for a project must come from a preexisting source. For a given project you can distinguish a proximate energy source, which is in the form consumed by the project, and an ultimate energy source, which traces the proximate source back through previous transformations to it's original form. Ultimately, all energy traces back to the creation of the Universe, but for engineering purposes we rarely go back that far, and are generally concerned with proximate sources.
For current space systems the energy is typically stored internally as chemical energy in the case of launch vehicles, or uses sunlight in the case of satellites. In the future, energy needs are likely to change, and the sources of energy will also change. Permanent locations, such as a large orbiting habitat or surface base, generally need continuous sources of energy to operate. Devices like batteries become unwieldy at that scale to supply power for the night part of Earth orbit or the two week Lunar night. Future projects may also need much greater power levels for tasks like processing of local materials. So the following headings attempt to include all potential energy sources, including many not yet used, but which may become useful in the future space projects. We list them all so that designers know the full range of possibilities, from which they can then select viable options for a given task. We exclude sources such as human and animal power from consideration here, both due to their low power levels, and because living things are not subject to the same kind of engineering design as we apply to non-living system. We also exclude sources like wind and geothermal, which mostly apply to Earth. Last, we includes some energy storage methods, which are not strictly sources. Energy storage, however, is often a necessary and important part of system design.
Energy References
A starting point for understanding energy sources in general, not just as they apply to space projects, is a National Academies book America's Energy Future, 2009. About 150 other books are available for free download in the Energy and Conservation topic from the same site. Encyclopedic references on the topic of energy, include the Encyclopedia of Energy, Encyclopedia of Energy Engineering and Technology, and the Macmillan Encyclopedia of Energy. These are often expensive reference books, so library or other sources are recommended to get access. There are numerous engineering books on more specific aspects of energy systems. Wikipedia also has an Outline of Energy article, with many links. Some of the concepts listed below are currently theoretical, so they are not well covered in reference books about current energy use or engineering. Information about them will mostly be found in research reports and scientific/technical papers.

## Mechanical Sources

Mechanical Energy includes energy stored by virtue of previous work, as in compressed gases, and that which exists by virtue of position (potential) and motion (kinetic). Objects in orbital motion have a combination of potential and kinetic energy.

#### A. Compressed Gas

Although Pressure vessels are strictly an energy storage method, for space missions the tanks are normally pre-filled. So they operate as a proximate energy source in flight. The available energy, W, stored in a pressurized tank, can be found from
    $W=p_{B}v_{B}\ln {\frac {p_{A}}{p_{B}}}$
where B represents the high pressure and A represents the low pressure, and p and v are pressure and volume respectively. So a 1 cubic meter tank with a high pressure of 20 MPa and a low pressure of 10 Mpa would provide 13.8 MJ of available energy. Compressed gas is a low density energy storage method. It is often used in space vehicles for tasks like cold gas thrusters and pressurizing liquid fuel tanks. Its chief advantages are simplicity, requiring just a storage tank and a valve, and rapid release of the stored energy. When larger total amounts of energy are needed, a higher density but more complex system is often preferred.

#### B. Potential Energy

Potential Energy is the ability of a system to do work by virtue of it's position or configuration. In space projects this is usually position relative to the gravity well of a massive object such as a planet. A simple hypothetical example is a stationary space elevator cable. While raising a cargo, electricity is converted to potential energy of height. When lowering a cargo, the potential energy can be extracted back to electricity. The formula for potential energy U was given in Section 1.1 – Physics as
    $U=-G{\frac {m_{1}M_{2}}{r}}$
The difference in energy at two radii gives the amount of potential energy stored or released over the distance. For small changes in radius (height) relative to the distance r, the potential difference can be approximated by the average gravitational force (weight) times height. On planetary surfaces, large amounts of available mass can be used to store potential energy. On Earth this is done with dams for hydroelectric power. On other bodies, a mountain and a pile of rocks can serve the same purpose. Transporting the rocks up and down the mountain can serve to store or release energy.

#### C. Kinetic Energy

Kinetic Energy is that which an object possesses by virtue of it's motion. Its formula was given in Section 1.1 – Physics as
    $KE={\tfrac {1}{2}}mv^{2}=Fd$
where KE is the kinetic energy, m is the object's mass, and v is the velocity. It is also equal to an accelerating force F times the distance d it is applied over. An object in orbit has both kinetic energy in it's orbital velocity and potential energy in it's altitude. In an elliptical orbit, it continuously exchanges altitude for velocity. So it also exchanges potential and kinetic energy, but the combined total stays the same.
Rotating objects such as a space station or reaction flywheel have a form of kinetic energy in it's motion around an axis. Rotational energy is E(k) calculated by
    $E_{k}={\frac {1}{2}}I\omega ^{2}$
Where $\omega$ is the Angular Velocity in units of radians/second, and $I$ is the Moment of Inertia of the mass about the center of rotation. The moment of inertia is the measure of resistance to Torque, or rotational force, applied on a spinning object. The higher the moment of inertia, the slower it will spin when a given force is applied. Moment of inertia depends on the distribution of mass in the rotating object. The farther out a given portion of the mass is, the larger the contribution. Formulas for many shapes are found in the List of Moments of Inertia. For complex shapes, the total moment can be found by dividing it into simpler parts and summing the individual moments.
Some examples of moment of inertia formulas are:
    $I={\frac {1}{2}}mr^{2}$ for a solid cylinder,
    $I=mr^{2}$ for a thin-walled empty cylinder, and
    $I={\frac {1}{2}}m({r_{external}}^{2}+{r_{internal}}^{2})$ for a thick-walled empty cylinder
Kinetic energy can be exchanged for other forms of energy by gravitational forces, as in gravity assist maneuvers; or electromagnetic forces, as in many electromechanical devices. Potential energy can also be exchanged for kinetic energy via the Oberth effect by expending propellant deep in a gravity well.

## Chemical Sources

Chemical sources are an arrangement of atoms in a higher energy state which are converted to a lower energy state by a Chemical Reaction, releasing the difference. Combustion is the most common way to release this energy, accounting for over 80% of total human energy use. Batteries are characterized by a reversible reaction, so that the same device can store and release energy multiple times.

#### D. Fuel-Atmosphere Combustion

The Earth's atmosphere, neglecting the variable amounts of water vapor, contains 20.95% Oxygen (O$_2$) molecules, which react with many other compounds to release energy. This Oxygen is the byproduct of Photosynthesis in living things. In the case of aircraft, a hydrocarbon fuel such as Kerosene is reacted with the atmospheric Oxygen in an engine. Since only the fuel is carried internally to the vehicle, the energy released, about 43 MJ/kg, is about three times as much as when both ingredients are carried internally, such as in a typical rocket. Large amounts of Oxygen in an atmosphere is unstable, because it is so chemically reactive. It only exists on Earth in this form because plants constantly produce it. So this energy source is not available on other bodies. The reverse option is available on a body such as Titan, which has a hydrocarbon atmosphere. In that case, Oxygen can be the carried ingredient, and burned with the surrounding atmosphere. For atmospheres which are mostly CO$_2$ (Venus and Mars), which is an end product of combustion, or bodies with no atmosphere at all, this energy source is not available.

#### E. Fuel-Oxidizer Combustion

The energy source in conventional rockets is Combustion, where both the fuel and oxidizer are supplied from internal sources. The ingredients with the highest reaction energy, Hydrogen and Oxygen, provide 15 MJ/kg of propellant. Although lower in Specific Energy than D. Fuel-Atmosphere Combustion, it is not restricted to operating in the Earth's atmosphere. Liquid rocket engines also have extraordinary power-to-mass ratios. This enables launch trajectories from large bodies like planets. Combustion can also be used as a secondary power source in Auxiliary Power Units. Because the rate of energy release is very high, combustion is useful when high power levels are needed. The efficiency of combustion engines is typically 1/3 to 2/3, so other options may be preferred when that is an important factor.

#### F. Chemical Battery

An Electric Battery is a device which converts stored chemical energy to electricity, and in a storage battery also reverses the reaction. Common examples of chemical batteries include the Lead-Acid type used in automobiles, and the Lithium-Ion type used in many portable devices. Depending on battery type, they generally store less than 1 MJ/kg, considerably less than combustion. The ability to cycle energy in and out multiple times can outweigh the lower energy density. An example is the International Space Station, where large batteries supply power in the shadowed part of its orbit.
A Fuel Cell is a type of battery where the reactants are stored in external tanks, rather than in a sealed battery case. It can have high specific energy because the tanks are lightweight compared to electrolyte solutions. Fuel cells have therefore been used in space projects, such as the Space Shuttle Orbiter. A Hydrogen-Oxygen fuel cell combined with an Electrolysis unit to convert the resulting water back to Hydrogen and Oxygen can supply energy storage with a specific in the range of 3-10 MJ/kg. Sealed batteries are simple and reliable, and can be made in very small sizes. Fuel cells have higher specific energies, but are more complicated devices, since they need valves and a way to pump and store the various chemicals.

## Thermal Sources

Thermal Energy is the internal energy of a system due to its temperature. It comes from the kinetic and vibration energies, and the attractive potentials of the molecules or other particles making up the system. Thermal energy can be stored for later use, or added from an outside energy source for immediate use. We sense high temperatures as heat, and even higher temperatures as visible light. Energy naturally flows from higher to lower temperature areas by conduction, radiation, and convection. When a temperature difference exists, some of the thermal energy can be converted to other forms and used. An example is a steam turbine that generates electricity from the difference between hot steam and the cooled outlet.

#### G. Thermal Storage Bed

For locations like the Lunar surface, which has a long night, solar power is not effective half the time. So storing heat in a Thermal Energy Storage system may be a viable option. Heat is put into rocks during the daytime and extracted from them at night to run a generator. The rock bed is enclosed in a container, and gas transfers heat to a turbine for generation, and from a solar collector for storage. Since the rock can be obtained locally, the energy stored per mass of installed equipment is fairly high. Environment temperatures during the Lunar night are quite low, and this can be enhanced by thermal shields between a radiator and the ground and daytime Sun. So the temperature difference between the storage and rejection temperature, and therefore efficiency, can be fairly high.
Some bodies, like Earth and Jupiter's moon Io, have relatively high interior temperatures. They serve as a natural thermal storage bed by the low thermal conductivity of surface rock. The source of heat can be radioactive decay or tides. This energy can be put to use by drilling down to high temperature regions, and exploiting the difference between those and surface temperatures.

#### H. Concentrated Light

A number of industrial tasks require heating, which is easily done in space by concentrating sunlight. Examples are heating of raw materials to extract volatiles, or maintaining temperature and growing ability in a Mars greenhouse. The concentration ratio determines the maximum Black Body temperature that can be reached, up to the temperature of the light source. In the case of the Sun, the upper limit is the Sun's surface temperature, 5,775 K, less reflection losses and radiation losses from the object you are heating. Since Tantalum-Hafnium-Carbide, the highest melting point substance known, melts at 4200 K, concentrated sunlight should be sufficient for most industrial processes.
For space transport, a reaction mass can also be heated by concentrated solar or artificial light. Lighter molecules can be used than the exhaust products of chemical reactions, so higher performance can be reached. Lack of powerful enough lasers limits their use for propulsion at present, but sunlight is widely available in space.

## Electrical Sources

Electricity is the set of phenomena associated with the presence and flow of electric charge. Common examples are Electric Current, in the form of electrons moving in a conducting wire, and Lightning, a powerful electrostatic discharge through a plasma channel in a storm. Electricity is a very versatile energy source because it can be converted to other forms efficiently, controlled in both tiny and large amounts, and moved about from place to place relatively easily. There are a number of ways to produce and distribute electrical energy.

#### I. Power Line

Most space projects use electrical energy in some form. The parts of a project on Earth are often by far the largest. These include factories to produce the vehicles and spacecraft, launch sites, and control centers. Typically they get their electricity from a network of Electric Power Transmission and Distribution lines. This is distributed to the point of use by local Electrical Wiring. What distinguishes these three is the scale of power, P, in Joules/second, or Watts, and the voltage and distance the power is moved. Most wires have electrical Resistance, R, which is a measure of the difficulty in carrying a current, I, using a voltage V. They are related by the formulas
    $V=IR\qquad and\qquad P=VI$
The resistance causes some of the energy to be converted to heat. The amount of power converted, P, is found by
    $P=I^{2}R$
When efficient delivery of energy, and not heating, is the intended purpose, you want to minimize the resistance heating. Since resistance is a material property, by this formula you want to use a low current, I. By the previous formula, the useful power P = VI, so a low current implies a high voltage. Therefore long distance lines are operated at higher voltages, and voltage changes are provided by Transformers as needed.
The same principles will apply to the space portion of a project when the distance between the point of generation and the point of use is large. Additionally, mass is usually a factor for space systems. So besides minimizing losses from resistance and transformer efficiency, you want to minimize the mass of the wires. The International Space Station is an example where generation and use are separated by an average of about 50 meters. This is because the Station is intended as a zero gravity laboratory, but the solar arrays need to rotate to follow the Sun. Since they are quite large, they are placed to the sides with rotating joints.This option mostly applies to fixed locations rather than vehicles. This is not a large enough distance to require high voltage lines for efficiency. Examples of future projects with longer power lines include mining water ice in shadowed craters at the Lunar poles. Solar arrays at the crater rim may have continuous sunlight while the mining area has none. So a transmission line can bridge the gap. Another example is a nuclear power source for nighttime power for a Mars Base. In that case the source is separated from the rest of the base for safety. Finally, large orbital habitats and industrial plants may use centralized generators that are lighter and more efficient, and need long power lines due to the size of the facility.
All types of wires need isolation from other system elements and each other, to prevent power leakage, shorts, arcing, and for safety. In a vacuum or non-conducting atmosphere, which the Earth's mostly is, isolation can be provided by mechanical gaps and spacing of wires. When the wires must be spaced close together, an Electrical Insulator can provide isolation, and combinations of spacing and insulation can also be used.

#### J. Electric Generator

An Electric Generator converts mechanical energy to electric energy. The two general types are a Dynamo which produces direct current, where electrons flow in one direction; and an Alternator which produces alternating current, where electrons flow in both directions in an alternating cycle. Most of the Earth's electric power is produced by large alternators. The mechanical energy enters the device via a rotating shaft, and an arrangement of magnetic fields induces a current in coils of wire. The mechanical energy can come from any of a number of sources. On Earth it is usually from high pressure steam or falling water acting on a turbine whose shaft is connected to the generator. In the case of steam, it is created by burning fossil fuels or a nuclear reactor, and more recently, from concentrated sunlight. A growing number of Wind Turbines are being used to produce electricity. The wind rotates aerodynamic blades mounted on a central shaft, and the shaft is connected to a generator.

#### K. Magnetic Storage

Some space projects, such as electromagnetic launch from Earth, require very high electric power levels for short periods of time. These power levels can exceed what is available from the power grid. Magnetic storage accumulates energy over a longer period of time, then releases it quickly when needed. It uses a 'Superconducting or high inductance/low resistance coil to store energy in a magnetic field. Superconductors eliminate resistance heating losses, but require cryogenic refrigeration to maintain the superconducting state. A large coil, cooled to lower resistance but not cryogenic, may be sufficient for some purposes. The energy, E (in Joules) stored in a magnetic field can be found from
    $E={\frac {1}{2}}LI^{2}$
L is inductance in Henries, and I is current in Amperes. Storing energy in this way causes structural loads from the field back to the coil, so the total storage amount is limited by the strength of the structure. Some uses in space, like pulsed plasma propulsion systems, can benefit from smaller magnetic storage units to produce high power pulses from a lower power steady source.

#### L. Semiconductors

Photovoltaic cells convert light, usually from the Sun, into direct current electricity using semiconductor materials. This technology is rapidly developing and has multiple materials and techniques (Figure 1.4-1). Conversion efficiency from sunlight of the best research cells, using multiple layers to capture different wavelengths, has reached 46.0% as of 2015. Production panels for use in space, made of multiple cells each, are near 30% efficiency, and the more common but less expensive single layer panels on Earth are typically 20% or less. Note that the efficiency on Earth vs in space are based on different solar intensity and spectra, because the Earth's atmosphere absorbs some wavelengths.
Thermophotovoltaic devices convert infrared and visible light from any hot object into electricity. They use similar semiconductors as photovoltaic cells, but optimized for the lower temperature source. A Thermoelectric Generator uses semiconductors to convert a temperature difference into electricity. The most common use in space is generating power from radioisotope decay, in locations where solar panels are cumbersome, is not available all the time, or is too dim, such as beyond Jupiter. An isotope like Plutonium-238 produces decay heat on a steady basis, which thermoelectric cells convert to electricity. For space applications, pure efficiency is not the only significant measure. Variation with temperature, radiation exposure, and the specific power (W/kg) are also important. Given the trend of past improvements, it is expected semiconductor devices will continue to improve, at least in the short term. The latest data should be checked for current performance.
(The following are old references, and should be updated)
* Anonymous "Conference Record of the Nineteenth IEEE Photovoltaic Specialists Conference- 1987", New Orleans, Louisiana, 4-8 May 1987.
* Anonymous "NASA Conference Publication 2475: Space Photovoltaic Research and Technology 1986: High Efficiency, Space Environment, and Array Technology", Cleveland, Ohio, 7-9 October 1986.
* Chubb, Donald L. "Combination Solar Photovoltaic Heat Engine Energy Converter", Journal of Propulsion and Power, v 3 no 4 pp 365-74, July-August 1987.

#### M. Solar-Driven Turbine/Generator

In space, most electrical power so far comes from photovoltaics, since solar panels are lightweight and simple for small to medium amounts of power. For large scale power, Brayton Cycle turbines have been proposed, because of their potential high efficiency and low mass. The turbine shaft then drives an electric generator. The high and low temperatures for the cycle would be produced by solar concentrators and radiator panels. Stirling type engines have also been proposed for space use. Sunlight is abundant in space, and lightweight reflectors to concentrate it and feed a heat engine may be lower mass than photovoltaics.
(The following is an old reference, and should be updated)
* Spielberg, J. I. "A Solar Powered Outer Space Helium Heat Engine", Appl. Phys. Commun. vol 4 no 4 pp 279-84, 1984–1985.

#### N. Rectenna Array

A rectifying antenna, or Rectenna is an antenna that is used to convert electromagnetic energy into direct current electricity. A single antenna element can be a Dipole, with a Diode connected across the dipole arms. The incoming electromagnetic waves induce alternating currents in the dipole. Since diodes conduct in only one direction, a direct current is passed on. Many antenna elements are combined into an array to capture the whole of the incoming energy. Rectennas have been proposed as the receiving element of a long distance microwave power transmission system, such as from Earth orbit to the ground. Much more solar energy is available in space, which results in more net energy delivered on the ground, despite conversion losses. The beam can also travel from the ground to orbit to deliver power to a satellite.
The length of a dipole antenna scales with the Wavelength of the incoming energy. In principle, microscopic antenna arrays can be made by the same methods used for integrated circuits. This would allow for the direct conversion of infrared or visible light. Small scale antennas are in an early stage of research. Their advantage for long-range transmission is in a smaller transmitter for a given distance. Microwave technology, by comparison, is well developed, and high efficiency rectenna conversion has already been demonstrated.

## Beam Sources

Entropy is a measure of disorder in a system. A directional beam of energy has low entropy because the waves are highly ordered (parallel). Useful work can be extracted from a low entropy system. This results in increased entropy (disorder), typically as random motion of atoms in thermal equilibrium and random thermal emission. Beams can be natural or artificial, and consist of electromagnetic waves or particles. Energy beams can be used for various kinds of Beamed power propulsion, or for powering more stationary activities.

#### O. Sunlight

At increasing distances from the Sun, around 14 million km or more, sunlight becomes highly directional. The source, which is 1.392 million km in diameter, then fills a small angular part of the sky. At the Earth's distance it appears 0.5 degrees in width. The small angle allows directional reflection as a controlled propulsion method. It also allows for concentration by lenses or mirrors, to generate high temperatures for industrial or propulsion purposes. This source includes direct use of sunlight, while the items under electrical sources are for sunlight converted to electricity.
The center of the Sun is at about 15.7 million degrees K, and has a core density of about 160,000 kg/m$^3$. Under these conditions Hydrogen undergoes Nuclear Fusion to Helium, releasing 3.846 x 10$^{26}$ Watts of energy. This energy works its way from the core to the surface, where the temperature has fallen to 5,780K. At this point the intensity is 63.1 MW/m$^2$. At greater distances the same energy flow is spread over larger spherical surfaces, reaching 1362 W/m^2 at the Earth.
At a distance of more than 550 AU, the Sun acts as a Gravitational Lens, bringing the light of other stars to a focus. Light which passes farther from the Sun's edge is bent less, and comes to a focus farther away. This creates a radial focal line of concentrated light from every other star or light source in the sky. The same process happens around every other star that has visible neighboring stars. These Star Lines may be intense enough to be useful, since they concentrate light from the whole circumference of a star to a point.

#### P. Laser

A Laser emits light through a process of optical amplification by stimulated emission. The output of a laser is coherent and collimated, allowing it to be tightly focused, or travel long distances without spreading out. The output can also be in a very narrow range of wavelengths. Because of the narrow wavelength, it can be coupled efficiently to an absorber, or a high reflectivity reflector for that specific wavelength. It can also be coupled to a photovoltaic device with high efficiency. As an energy source for propulsion it can supply higher intensity light than natural sources like stars. High power lasers have been proposed for launch from Earth, but sufficiently high power ones to make that use practical do not yet exist. Lower power lasers can augment natural sunlight falling on spacecraft solar arrays. Very high power lasers focused by the Sun's gravity have been suggested to power interstellar vehicles, but that use would far in the future.

#### Q. Microwave

This energy source involves direct use of the microwave beam, while item N. Rectenna Array converts it to electricity. A microwave beam can be absorbed and converted directly to heat, or it can be used to create photon pressure. Any suitable wavelength can be used to create a directional energy beam. However some wavelengths are absorbed by the Earth's or other atmospheres. Shorter wavelengths can be focused more easily, since that depends on the ratio of antenna size to wavelength. The efficiency of producing shorter wavelengths is typically lower, and generators with high enough power to be useful may not be available. Microwave band equipment is developed enough to not suffer from these limitations.

#### R. Neutral Particles

A Particle Beam is a collimated stream of high energy particles to deliver energy from one place to another. The concept was originally developed as a weapon, but less lethal amounts of energy can be used as a power source. Charged particles, such as protons in a Particle Accelerator repel each other, so a beam would spread out once it leaves the confinement of the accelerator. To prevent this, the charged particles are allowed to combine with electrons to form neutral atoms, or neutral particles like Neutrons are used. Particle beams are in an early state of development, and mostly for military use, rather than energy delivery.

## Nuclear Sources

Nuclear energy sources involve a change in one or more types of Atomic Nucleii, with the release of net energy. The protons and neutrons in a nucleus are bound together by the strong nuclear force. A change in their arrangement involves typically a million times as much energy than rearranging electrons, which is what chemical reactions do. So nuclear energy sources are potentially very powerful. Although the Sun operates by nuclear fusion, we consider it a source of light energy. The fusion happens in the Sun's core, where it is not accessible, and what reaches us is blackbody radiation from the surface.

#### S. Radioactive decay

Radioactive Decay is the spontaneous change of unstable atomic nucleii by the emission of particles or electromagnetic energy. Unstable natural elements were created before the formation of the Solar System, most likely in supernova explosions. The less unstable ones, such as Uranium and Thorium, still survive after billions of years, and continue to decay at a steady rate. Artificial radioactive materials, such as Plutonium-238, are created in nuclear reactors or particle accelerators. They are more unstable, and thus decay faster (an 88 year half-life in the case of Pu-238). This element produces 500 Watts/kg of heat, when fresh, through radioactive decay, making it a useful energy source. It has been used for this purpose on a number of planetary exploration missions. Other elements with very long decay times in their natural state are too weak to use as energy sources.
[This is an old reference and should be updated]
* Lockwood, A.; Ewell, R.; Wood, C. "Advanced High Temperature Thermo-electrics for Space Power", Proceedings of the 16th Intersociety Energy Conversion Engineering Conference, v 2 pp 1985–1990, 1981.

#### T. Nuclear Fission

The low natural decay rate of some elements can be increased by artificial means. A Nuclear Reactor is a device for doing this in a controlled way for the production of energy. The two main ways to do this are Nuclear Fission, the splitting of heavy nucleii into smaller parts, and Nuclear Fusion, the merger of lighter nucleii to form a heavier one. The reason for the two types can be found in the Binding Energy per nuclear particle in a nucleus (Figure 1.4-2). A higher binding energy means the particles are more strongly held together and more stable, so energy can be released in forming that nucleus. The binding energy has a peak at Iron-56, so reactions from either the light end (fusion) or heavy end (fission) towards the middle both produce energy. Fission reactors are a significant source of electrical power on Earth. In space, a few small-scale reactors have been used, and work is in progress on developing larger scale versions with higher power demands.
[These are old references and should be updated]
* El Genk, M.S.; Hoover, M. D. "Space Nuclear Power Systems 1986: Proceedings of the Third Symposium", 1987.
* Sovie, Ronald J. "SP-100 Advanced Technology Program", NASA Technical Memorandum 89888, 1987.
* Bloomfield, Harvey S. "Small Space Reactor Power Systems for Unmanned Solar System Exploration Missions", NASA Technical Memorandum 100228, December 1987.
* Buden, D.; Trapp, T. J. "Space Nuclear Power Plant Technology Development Philosophy for a Ground Engineering Phase", Proceedings of the 20th Intersociety Energy Conversion Engineering Conference vol 1 pp 358-66, 1985.

#### U. Artificial Nuclear Fusion

Natural fusion occurs in stars, and the resulting light output has been addressed above under beamed power sources. This item is for artificial energy sources. Fusion has been achieved momentarily in nuclear bombs, but steady state operation has proved difficult. The most researched approach uses Tokamaks, which are doughnut shaped magnetic fields which contain a hot plasma. This approach has not yet produced a working device, although various research machines have been built or are under construction. A Tokamak type power reactor would be too massive for a reasonable propulsion system. For stationary projects it would be as reasonable on another planet as on Earth. A number of alternate intermittent and steady state fusion devices are under varying levels of research, but all at much lower funding than the work invested in the Tokamak type devices. Some of those might yield a lightweight enough device for propulsion.
All fusion reactions combine light atomic nuclei into heavier ones. As shown in Figure 1.4-2, the largest energy release is in the first few elements, from Hydrogen to Boron. What is required to achieve fusion is to bring the positively charged nuclei close enough together against their electric repulsion for the nuclear forces to take over. This requires the equivalent of millions of degrees K, or particle kinetic energy in the tens of kilo electron Volts (keV).
[These are old references and should be updated]
* Miley, G. H. et al "Advanced Fusion Power: A preliminary Assessment, final report 1986-1987". National Academy of Sciences report #AD-A185903, 1987.
* Eklund, P. M. "Quark-Catalyzed Fusion-Heated Rockets", AIAA paper number 82-1218 presented at AIAA/SAE/ASME 18th Joint Propulsion Conference, Cleveland, Ohio, 21-23 June 1982.

#### V. Muon-Catalyzed Fusion

Muon-Catalyzed Fusion is a method of catalyzing fusion reactions at temperatures far below the millions of degrees K otherwise required. A beam of muons is directed at a deuterium/tritium mixture, where they cause multiple fusion reactions. This heats the gas, which can then drive a generator for electricity. Although this method and more complicated systems based on it are sound from a physics standpoint, a practical system from an engineering point of view has not been developed yet. It must be considered a possible future energy source at this time.

#### W. Nuclear Explosions

Unfortunately, explosive Nuclear Weapons are all too well developed. Various concepts have been proposed to use their high energy output for space projects. These include a nuclear-powered launch device, where the explosive heats gas in an underground chamber. This then propels a projectile up a barrel. Another idea is to detonate small nuclear explosions behind a space vehicle, directly pushing it with the blast wave. These concepts are speculative at present, because there is no way to safely test them on Earth, and treaties prohibit nuclear weapons in space.

## Matter Conversion Sources

In physics, Mass-Energy Equivalence is the idea that mass is related to energy by the formula E = mc$^2$. Since the speed of light, c, is a large number – 299.8 million m/s, the square is very large: 8.9875 x 10$^{16}$ Joules/kg. This is equal to the output of a nuclear power plant for 2.85 years for each kg of mass converted to energy. In theory, total matter conversion provides the highest amount of energy per unit mass. In practice, however, this is not so easy.

#### X. Antimatter

Antimatter is composed of antiparticles, which have the same mass as normal particles, but opposite charge and other properties. When a particle and anti-particle meet, they destroy each other and are converted to other particles or photons, releasing large amounts of energy in the process. Our universe is almost entirely made of matter. So use of this material as an energy source requires making it artificially. This requires at least as much energy as is later released by the annihilation. Antimatter is therefore an extreme type of energy storage. Antimatter is made in small amounts today in particle accelerators, and used for physics research. We do not have practical ways to make and store it in large enough amounts for space projects. Conceptually, a space vehicle would store some amount of antimatter, then use it to produce energy for propulsion. If the storage system is light enough, the energy per mass would then be higher than nuclear fusion or other methods.
[These are old references and should be updated]
* Hora, H.; Loeb, H. W. "Efficient Production of Antihydrogen by Laser for Space Propulsion", Z. Flugwiss. Weltraumforsch., v. 10 no. 6 pp 393-400, November-December 1986.
* Forward, R. L., ed. "Mirror Matter Newsletter", self published, all volumes, contains extensive bibliography.

#### Y. Black Hole

A Black Hole is a region of Spacetime with such strong gravity that nothing can travel from inside to outside it. Two forms of energy extraction are possible for black holes. The first is infall energy, generated as material in an accretion disk around the black hole heats by friction and emits energy. It is essentially converting potential energy into heat. Since the gravitational potential of a black hole is extreme, this can release a lot of energy. The second is Hawking Radiation by quantum tunneling from hypothetical quantum black holes. Black holes can form by the collapse of a large star at the end of it's life, or a sufficiently dense and massive region at the center of a galaxy. Quantum black holes are smaller, and hypothesized to have formed during the creation of the Universe. The nearest known stellar-mass black hole is 2800 light years from Earth, and quantum black holes have not been discovered, nor is there a known way to make them. So use of black holes for space projects is theoretical at present.