Section 1.11 - Future Projects< Space Transport and Engineering Methods
Previous sections have described general engineering process and organization. This section lists the categories of space activities and mission objectives that future projects and programs may want to incorporate. A given project, location, or mission may include more than one of these purposes. Later sections will then describe the component pieces and processes to design, build, and operate systems that can implement these goals. These activities are not independent. Some of them can link to others, such as building new exploration or mining vehicles from previously accumulated materials. A complex program has multiple purposes or interlinked elements, and that is addressed as the last major section of the book. The order listed below not rigidly defined. It is somewhat in terms of idealized complexity, but an actual program that implemented multiple goals would likely choose a different order that makes sense from the design standpoint.
Bulk mass is matter which is undifferentiated into manufactured components, either in the raw unprocessed state, or processed into a refined product. The quantity of bulk matter is a main variable, as all the bulk material of a given type is equivalent.
Propellants or fuels have several general applications: space transport, ballistic transport, aerodynamic transport, surface transport, and portable power. Due to the wide range of applications and environments, there are a wide range of potential propellants and fuels. While originally all such materials have been supplied from Earth, using local resources will often prove less expensive, if for no other reason than reduced overhead for delivery. Specific propellants for space transport have been discussed in Part 2 of this book. The other applications are listed below.
Of particular interest for advanced uses are nuclear fuels such as Uranium or Thorium, and if fusion power is developed, light elements for those reactions. The use of fuel depots at intervals along a trip has general utility for any space mission. This is because if all the fuel is carried from the start, more of it has to undergo a velocity change, and thus the overhead of using fuel to move fuel. By refueling at intervals, the on-board inventory is less, and thus the overhead is less.
On an undeveloped or heavily cratered terrain, attempting to drive across the surface will be difficult, and for long distances will be slow. Ballistic transport uses similar methods as space transport, but instead of going to and from orbit, or between orbits, is used in sub-orbital trajectories to go from one point to another. The lower velocities potentially allow for simpler propulsion or higher payloads. This method is especially useful on smaller bodies where the velocity requirements are lower.
On bodies with a sufficiently dense atmosphere, you can potentially use aerodynamic lift or buoyancy and a combustion engine for transport, similar to how we use aircraft on Earth.
Solar power, while long-lived, does not have a very high power to area ratio, and so is not well suited for powering faster surface vehicles. A fuel cell or combustion engine can provide more power in a compact device and be more suitable for such vehicles. Stationary fuel stations can refuel a vehicle withing an operating range.
Solar power is suitable for small portable devices which do not need high power levels. Higher power levels for short periods of time may be satisfied by propellant or fuel sources.
- Ramohalli, K.; Ash, R.; Dowler, W.; French, J. "Some Aspects of Space Propulsion with Extraterrestrial Resources", Journal of Spacecraft and Rockets v 24 no 3 pp 236-44, 1987.
Natural space radiation comes from 4 sources: cosmic rays, solar wind and plasma events, trapped particles in magnetic fields, and concentrations of radioactive elements. Human-caused radiation comes from nuclear and other high energy devices. Humans, materials, and electronics are sensitive to radiation exposure. A thickness of roughly 1 meter of loosely packed unprocessed material can reduce exposure to reasonable levels for humans. Shielding varies by composition, with light elements protecting better for some types of radiation, and heavy elements for others.
The surface or subsurface of large bodies has sufficient local material to provide radiation shielding. Habitats with thick walls or soil may provide sufficient shielding without adding extra shielding. For vehicles or lightweight habitats additional shielding can be added from propellant, water, and food supplies, unprocessed raw material, stockpiles of processed materials, and wastes after processing (slag).
An Ore is any natural material containing enough of a desired product to be economic to extract. On Earth, crude oil, iron ore, and crushed stone are all ores used to make other products, and by mass are the largest volume transported. In space, bulk ores are also likely to be a major transport item by mass. Depending on economics and technology, bulk ore can be transported in it's raw state, concentrated in the desired components, called Beneficiation, and then transport the concentrate, or processed in place to a final material.
Ore Delivery to EarthEdit
If you want to import large amounts of Iron to the Earth, the simplest method is to aim pieces from metallic asteroids at a selected spot, and just collect the bits that make it to the ground. This requires no processing. You can find examples of surviving pieces in museum asteroid collections. No processing, extraction, etc. Non-metal slag and volatiles will tend to burn off during re-entry. Choose a size, likely around 10-50 tons, for the pieces so that re-entry drag will slow them down, and you don't get a big crater. There are plenty of places on Earth with few people and decent access for shipping. The market for steel is enormous. The trick is delivering it to Earth for around $1/kg or less. You likely will need to redirect and chop up a megaton (60 meter) metallic asteroid, but the yield is worth $1 billion ( 1 million tons steel @ $1000/ton), which in theory would be enough to cover operating cost.
Ore Delivery to Space LocationsEdit
Because of the current high cost of launch to orbit, material in a desired space location is worth much more than most materials on Earth. What is required is to move it from where it naturally occurs to where it is needed. The Earth's gravity well requires a lot of energy to climb out of, and to date launch systems use inefficient rockets to do it with. Transporting materials which are already nearby and don't require traversing a steep gravity well, thus can use efficient electric thrusters, can be a much less expensive solution.
Oort/Rogue Object DeliveryEdit
Interstellar missions require a lot of propellant due to the high velocity required. In this concept, several comets or unbound rogue objects, or parts thereof, are intercepted by a propulsion unit that comes from the main vehicle, or is sent ahead from the launch point. The propulsion unit consumes part of the mass to bring the rest of the mass up to the speed of the main vehicle. The delivered mass is used to further accelerate the main vehicle and resupply other materials. This allows somewhat better velocities than starting with all the fuel at the start of the mission, since the main vehicle has less mass to accelerate. For this to work, you need to know where the objects are ahead of time, or trust that their density is sufficient to find them along your path as needed.
This category is using material and energy resources to build equipment and facilities to do processing and manufacturing in space. A simple example is making pressure vessels from metallic asteroid material, which are then used in chemical processing of ores.
Bootstrapping Seed FactoriesEdit
A Seed Factory uses a small initial set of equipment (a seed) to build a larger and more complex set, and eventually end products, from space materials. The growth can come from making copies of it's own equipment, making larger versions of the equipment, or making new equipment of different types to expand the range of production. Some amount of added items from outside besides the starter set is likely required, though in theory this should reduce over time. Because space locations are not uniform in energy or material resources, the seed equipment and what it makes will likely be distributed in a trade network where components are optimized for location. This is the smallest feedback loop among space projects, since the seed factory uses its output to expand itself.
The concept of self-expansion using local energy and materials applies everywhere, on Earth as well as in space. It is an important idea, and this book is about space systems, therefore a separate book is being written about it
- Freitas and Gilbreath, eds. Advanced Automation for Space Missions, NASA Conference Publication 2255, 1982.
Once you have set up an industrial capacity, you then want it to make useful products. There are as many possible products as there are on Earth. Which ones make sense to make at a space location depends on mass and complexity of production.
A variety of structural materials can be made from local materials in space, thus reducing the amount of material that has to be brought from Earth. Examples include Iron-Nickel shapes made from metallic type asteroids or Lunar regolith, cast or sintered rock, made using solar or microwave heating to melt, and high strength Basalt fibers extruded from native Lunar surface Basalts.
Solar Sails from Metallic AsteroidsEdit
To recover large amounts of material from the asteroids, Iron-Nickel alloy found in metallic types can be rolled into foil, and then used to make solar sails. If what you want to extract is steel, then it sails itself back to where you want it. If you want some other material, you can make larger amounts of sail area and use it as a cargo tug. To make the sails, you need the functions of a rolling mill - a way to heat the material and a way to force it between two rollers to make thin sheets. A drawback is steel sails are not as light as aluminum-magnesium alloy, and it is not as good a reflector in it's natural state. Solar sails are also somewhat limited in the directions they can apply thrust.
As an advantage it is readily available in large quantities from the asteroids themselves, and does not need a lot of processing to make into a useable form. An Aluminum or Aluminum-Silicon alloy coating can be added to increase reflectivity if desired.
Besides the obvious use as windows for greenhouses, glass can be used for fiber optic cable, and for inert reaction vessels, including those which concentrated sunlight is sent through.
Brick and ConcreteEdit
Brick is made by heating a mixture of sand and clay until the particles partly melt and bond together, a process called Sintering. Building elements can be made the same way in space, provided a sufficient source of heat and right type of ingredients can be found. Its chief advantage is simplicity.
Concrete is a class of artificial stone made from varying size crushed stone, called Aggregates, and a binder material to hold them together. On Earth the most common binder is Portland Cement, a mixture of shale and limestone heated to high temperature and then ground to a fine powder. The usefulness of concrete is based on it's relatively low cost, and the ability to be cast in a variety of shapes which then harden. For space projects alternate binders may be found that will work, as Portland Cement requires particular minerals and water.
This includes plastics, chemical reagents, lubricants, and other items made by chemical processing.
This of course includes food, but also non-food items like wood, and chemical outputs of micro-organisms. The oldest example of the latter is alcohol from yeast, but modern biotechnology can produce a wide variety of items.
There are several needs for energy in space. First is to operate transport and manufacturing systems. Next is to operate habitats and other end items. Last is to export energy back to Earth or other locations.
Photovoltaic or solar thermal power systems
Orbital Solar Power StationsEdit
This concept is to supply power to Earth from collecting stations in orbit. Sunlight in space is not affected by night, clouds, or atmospheric absorption. Thus a solar collector will generate on average 7 times as much power in space as on Earth. A large solar power plant in orbit can send the power to the ground using an efficient microwave beam. Alternate uses would be to beam power to a Lunar surface base from orbit to supplement nighttime power. Advantages of orbital solar power are nearly 100% operating time, and lack of Carbon emissions or nuclear risks. A disadvantage is the size of the collector on the ground is governed by the transmission wavelength and distance of the orbital station, so there is a minimum size for it to function efficiently. This can be counteracted to some extent by using shorter wavelengths or lower orbits. To be feasible for Earth, the total system (orbital and ground collector) needs to be less than 7 times as expensive as solar panels on Earth, otherwise using terrestrial panels would be less expensive.
At current launch costs, it makes economic sense to beam power *up* to space by swapping the transmitter and collector locations, as power in orbit is worth more than power on the ground. In the form of visible light or microwaves this would supplement on-board power obtained from sunlight. For orbital tugs at low altitude, the supplement is especially useful as the Earth's shadow covers 40% of typical low orbits.
Given a suitable atmosphere, for example Carbon Dioxide rich ones like Venus and Mars, the atmosphere might be used as a lasing medium to generate powerful beams. This concept has not been explored in detail as far as is known.
Engineered environments use structures and equipment to provide a suitable combination of pressure, temperature, gravity level, and other parameters. Even on Earth, those capable of reading this book spend most of their time in buildings or automobiles which moderate the environment to a comfortable condition. In some circumstances the environment is set up for plants (high CO2 ratio), or machines, rather than humans.
Humans evolved on Earth, so it is the only place we know of with the correct environmental parameters for us to survive unaided. Even parts of the Earth, such as much of Antarctica, are inhospitable without an engineered environment. Environments specifically set up to house living things are called habitats, and those that house humans in more or less comfortable conditions are called human habitats. The smallest habitats shade into clothing. Examples are diving and space suits. The largest may be described as artificial planets. In between they have names like pressurized modules, surface base, or space city. Those names are more descriptive of the size and population of a habitat. Since the environment parameters that humans prefer is fairly narrow, habitats will have similar functions regardless of size.
Engineered environments are much less mass intensive than natural ones. For example, the Space Station uses roughly 100 tons to support each person, while the Earth uses about 5 trillion tons. Thus habitats constructed in the Solar System can either support vastly larger numbers of people, allow vastly larger living space and energy use to current population, or only use a small fraction of available resources. The following items describe the range of internal parameters needed for comfortable human habitats:
- Radiation Level:
- Food Supply:
- Water Supply:
- Waste Disposal:
Closed Life SupportEdit
By recycling part or all of the materials used to sustain life, the amount of stored supplies or newly delivered supplies can be reduced. If coupled with local extraction of life support supplies, this can reduce the amount of extraction required. Water, air, and food are the principal items that can be recycled. Closed life support does not have to be directly coupled to human habitats. A greenhouse might be optimized for plant growth conditions, including high CO2 levels, and deliver oxygen and food products via sealed containers.
For launch from a planet it may be useful to collapse a structure into a small package. Once on location it is inflated or assembled to form the finished object.
Recycled Vehicle TanksEdit
A conventional rocket takes the final stage, along with the payload, into orbit. By re-fueling the stage, or by converting the stage tanks and structures to another use (such as an occupied pressurized module), some payload weight and volume is saved. The Skylab space station was made from a converted Saturn V 3rd stage. A number of studies have been done on re-using Space Shuttle external tanks for other uses such as pressurized living space. Re-fueling of upper stages has been studied, but not tested.
Transport is needed for delivery of new equipment and facilities, cargo, and raw materials. Transport systems are described in Part 2 of this book. The category here is transport systems built from space resources, rather than launched from Earth.
Human Hazards - Orbital debris from failed satellites which collide with other things in orbit. Nuclear waste and biological samples which are too hazardous to store on Earth.
Asteroid Hazards - Diverting asteroids and comets which are on dangerous paths. Extensive consideration has been given to Earth-approaching objects. Lunar impacts are often neglected, but more mass can be tossed off the Moon, because it's smaller, to end up sucked into the giant gravity well nearby called Earth. You get just as dead being hit by a 1 ton Lunar fragment as by a megaton asteroid, but the deaths are more distributed in time and space.
Comet Hazards - Long period comets are undetectable with current technology until they get within about 10 AU of the Sun. If one was headed towards Earth, there is not time to arrange a shift in it's orbit, so the only reasonable way to deal with it is to use an interceptor with one or more large nuclear bombs to fragment or destroy it. Comet trajectories are hard to predict because they have natural rocket thrusters in the form of gas jets.
The energy to transmit the description of an object to another star, even at an atom by atom level, is about a million times less than the energy to physically move the object from one star to another. Thus, after the first probe sets up a receiving/replication station at the other star, other objects are more efficiently scanned, transmitted, and reconstructed at the receiving end. Using atomic scale technology (such as scanning tunneling microscopes) it may be possible to eventually scan and send people this way. The subjective time to travel at the speed of light is zero, although the actual transit time is still governed by the speed of light.
Science and ExplorationEdit
Exploration is performed for scientific purposes, and to find additional resources.
Use of space for communications relay is well known. If costs were lower, it would be used more often.
Gravity Lens Relay - Massive objects like stars bend light via gravity. If you travel a sufficient distance from the star, 550 AU for the Sun, that light reaches a focus. You can then use the star as a giant lens, to focus communications from one star to the other. For optimal relays, you would use such gravity lenses at both stars.
This category would include tourism, zero-g sports, and other activities purely for entertainment.