Section 4.5: Phase 4A - Low Orbit Development (page 2)

Assembly Platforms

Given a conventional method to get cargo to orbit, and possibly a Hypervelocity Launcher (previous section), the next step is an Assembly Station to collect cargo from the high gee launcher, and more sensitive cargo and humans from conventional rockets, and build larger systems there for further space projects. Assembly lowers the required size of individual launches, and thus the up-front development cost.

History

Prior manned space stations, especially the current International Space Station, have used orbital assembly as an engineering method. Particular features for assembly include, first of all, designing the parts to be assembled. Mechanical docking devices include guides to align the parts, latches and powered bolts to fasten them firmly, and electrical and other connectors which automatically join when the parts are brought together. The heavy duty physical tasks of assembly are carried out by a rail-mounted robotic manipulator arm, normally controlled by an on-board operator. Lighter duty tasks are done by humans in pressurized suits. The experience from that project is a good starting point, but it has been about 20 years since the Space Station design was set. Computers and communications have advanced a great deal since then. Additionally, design for continuous growth requires a different design philosophy.

Design Approach

The approach in this example uses smaller modular components than in the past. In the early construction stages these are assembled with remote controlled/automated robotic arms into larger units. Once sufficient facilities are in place, human crew can be added. The robotic work uses experience from the Advanced Manufacturing step. The Assembly Station starts very basic, and gradually extends it's capability by adding more modular parts, and later by manufacturing items locally, rather than just assembling delivered cargo. An important part of the design is to use standardized modular components. That way new parts can be added in any arrangement and they will still fit, and new designs are not needed for each new job. Additionally, use of standard parts makes it easier to stock spares.

Parts like truss elements, which are naturally strong, can be packed for gun launch, then assembled into a complete truss. Pressurized modules with rigid walls would not fit into a gun-launch cargo, but inflatable modules might. Alternately conical or dome sections can be nested and then assembled into complete modules. If a vacuum welder or laminated tape winder were available, module assembly from smaller pieces would be possible. So each component needs to be looked at to find the best method of delivering it, and it will likely end up a mix of launch methods.

The output of the Assembly Station would be commercial items like spacecraft or sections of spacecraft, and also internal production that would extend the range of later steps. For example, the Assembly Station could assemble a mining tug from parts, which then goes to collect materials from a Near Earth Asteroid. The Station could "reproduce" itself in a sense, by splitting off or assembling a subset which can then go off and be the seed for construction in a new location. Initial growth is by simply adding more modules of a given size. Later growth can be by using larger launch systems from Earth when the economics justify it, or by producing larger components for later "generations" of construction.

Component Types

The following is a list of parts for start up of an Assembly Station. It will require a lot more detailed analysis and design to reach a final list, but this will illustrate the types of modules that would go into such a design. First launch may be by another launch system, to enable a complete functioning system to get delivered as a unit. Later launches can be smaller elements as additions delivered part by part.

Thuster Unit

One of the first items to deliver to orbit will be a small chemical propulsion unit. It will include tanks, fuel and small thrusters, and a way to dock firmly to other structures. The docking port may be as simple as a magnet to attract another payload, and then some bolt or clamp to secure it. The propulsion unit does all the moving around to line up with the payload. Docking other payloads will automatically connect power and data lines. For a first launch, it may be feasible to launch an electronics unit and a partly fueled thruster unit as a single cargo. Otherwise a larger capacity launch system is used.

Fuel Tanks

The growing assembly station will use fuel to meet each cargo as it reaches orbit, and also to make up for drag losses from the thin atmosphere that exists at any low orbit altitude. Therefore it will need periodic fueling.

Electronics Unit

This will contain some smaller solar arrays for power, some computer systems, batteries, one or more cameras and GPS units for navigation, and radio or laser communications.

Robot Arms

The next couple of items would be robot arms to give the propulsion unit the ability to do more complex tasks controlled from the ground. Items like robotic arms would be subject to a design trade-off. They would have to be made very rugged for gun delivery, versus a lighter weight version launched by conventional rockets. Arms could be made as segments with one or two joints, which are connected in series to make more flexible units, and have replaceable tool/manipulator ends for different tasks. The arms are designed as double-ended, so that either end can attach to a base or tool, and have a split joint to go from one shaft to two or more "fingers" or "arms".

Robot Attachments

These are tools that attach to the arms, and a rail car unit to move the arm from place to place.

Structural Base

This is a set of truss elements that can be assembled into larger arbitrary structures to which other parts of the growing assembly station will be attached. One approach is a ball and stick truss, with hubs at the intersections that have fittings at 90 and 45 degree angles. These are connected with struts of standard lengths to form the framework. The base truss might have a spacing of 1 meter, with adapters to scale up or down to other grid sizes as needed. Filler plates would span a truss bay to add rigidity or provide container spaces or additional mounting locations. The plates can be either perforated or solid as needed.

The basic structural system includes rails for moving robot arms and other items from place to place. The rails would extend a short distance from the hubs, with smooth joints to allow continuous motion. Either curved or pivoting sections would enable changing the plane of motion.

Utility Grid

The concept here is to have a redundant and modular utility system with different services (power, data, fuel lines) added as needed. One approach is to use a truss column as the utility carrier, and install support brackets to hold the various lines, with insulation or meteoroid blankets on the sides. That allows for easy access for additions or repair.

Power Units

There is another trade-off to do here for photovoltaic arrays, which are not suited to gun launch, but are lightweight, versus something like a Brayton generator, which in theory can be rugged. For low orbit, the power units would need some sort of storage, i.e. batteries, since sunlight is only available 60% of the time. To start with, simply attaching PV arrays to your structural base will provide a power supply.

Electric Thruster Unit

For more extended missions that require more fuel, Ion or Plasma thrusters are added, which are more efficient than chemical thrusters.

Enclosed Modules

Some equipment, and humans, benefit from not being in vacuum. Other tasks benefit from temperature control, or keeping debris contained. For those sorts of requirements an enclosed module is needed. For early use, an inflatable module may be suitable. Finished modules are not suited to gun launch, but a fiber-reinforced aluminum tape could be launched as a spool, then formed around a mandrel to create larger shapes. Concentrated sunlight and pressure rollers can braze/solidify layers of tape until sufficient thickness is built up. That way the small cargo volume of the gun projectiles could be used to fabricate larger items. Once sufficient habitable volume and supplies are in place, then humans can start to work on the Assembly Station, but the initial construction will all be done via remote control.

Electric Propulsion

Electric propulsion typically has about ten times the fuel efficiency of chemical rockets. Thus they turn an exponential fuel requirement (fuel to push more fuel) into a nearly linear one for most Solar System missions. The timing of this step would be in parallel or soon after Orbital Assembly is started.

Near Term Electric Thruster Types

There are several kinds of electric thrusters that are good candidates for near term use. The selection here is based on state of development and usefulness:

49 Electrostatic Ion - This knocks electrons off of gas atoms making them charged, which is called "ionized". Once charged, they can be accelerated by metal screens with a large voltage difference. Ion thrusters are used on some communications satellites, and the Dawn spacecraft currently exploring the asteroids Vesta and Ceres.

51 Microwave Heated Plasma - This type uses microwave frequency heaters to heat the fuel. This is the same principle as a microwave oven, but much more intense. Above a certain temperature the heated atoms in the fuel will knock electrons off each other, turning it into a mixture of ions and electrons, which is called a Plasma. The plasma is contained and directed by magnetic fields. You need to do that because plasma is so hot it will melt anything it contacts, or cool itself down too much. In fact on Earth plasma is used as an efficient way to cut through metal. A version of this thruster is currently being developed on the ground, and will soon fly for testing on the Space Station. It's full name is Variable Specific Impulse Magnetoplasma Rocket, which is mercifully abbreviated to VASIMR. As a category they are called plasma thrusters.

72 Ionospheric Current - This operates like a motor, running a current in a wire in a magnetic field. The return path for the current is the ionosphere. This method is limited to places with suitable magnetic field and ionosphere density, but low Earth orbit fortunately is such a location. The attraction is it does not require direct fuel use, only a little leaked plasma to make electrical contact with the ionosphere. The equivalent exhaust velocity as if it were a fuel-using engine is 250 km/s. Since low orbit is the first place we want to use, developing this type of thruster is a high priority. Note it is not as fully developed as the other types.

Electric vs Chemical Thrusters

All rockets work by tossing mass in one direction, and by Newton’s Law (for every action, there is an equal and opposite reaction), the rest of the rocket gets pushed in the other direction. The faster you toss the mass, the more push (momentum) you get out of it. Conventional rockets burn fuel in a chamber then let it expand out a supersonic nozzle to get it going as fast as possible. The shape of the nozzle is governed by the physics of expanding gases, which is why they all look more or less the same. How fast it can get is limited by how hot the gas is and it's molecular weight. The best combination used today is burning Hydrogen and Oxygen in a ratio of 1:6 by weight. This produces mostly steam with a bit of Hydrogen left over to lower the average molecular weight. How fast the gas is going is technically called exhaust velocity, and is limited to about 4.5 km/s for this fuel type.

Electric thrusters are not limited by the energy produced by burning the fuel. They feed energy to the fuel from an external source, thus can get much higher exhaust velocity. This gives you more push from given amount of fuel. Since you have a finite amount of fuel to use this is more efficient in direct proportion to the increase in exhaust velocity. By analogy to automobiles, you are getting better "gas mileage".

The extremely high fuel efficiency is the key to why this type of thruster is important. If you are doing a lot of moving about in space the fuel savings outweigh (literally) the mass and cost of the power supply by a large margin. Conventional rockets only need a fairly lightweight fuel tank, but burn a lot more fuel. One drawback to electric thrusters when transporting humans is their relatively low thrust. This makes the trip times longer. There are various ways to work around that drawback. For example, passage through the Earth's radiation belts slowly would be unacceptable radiation exposure. So you can transport your main vehicle by electric thruster, taking weeks, and then deliver a crew in a small capsule taking hours once the main vehicle is outside the radiation belts.

Comparisons Between Types

All electric thruster types need an external power supply since the fuel is not self-heating as in chemical engines. The most common power supply used in space are photovoltaic panels. Those can get unwieldy at power levels of hundreds of kW or more, and their power output per area drops as the inverse square of distance from the Sun. So for some past and future missions a nuclear power source is preferred. Smaller size nuclear generators are based on isotope decay, and larger ones are full nuclear reactors. Any type of nuclear device brings both technical and political complications.

Electric thrusters cannot be used directly for launch or landing on large objects, because their thrust-to-mass ratio is significantly less than the local gravity acceleration. They can be used indirectly via space cable/elevator type systems. Chemical engines can reach vehicle thrust-to-mass ratios well above Earth gravity, which is one reason they have been the primary way to launch things to date.

Both Ion and Microwave Plasma thrusters have exhaust velocities in the range of 20 to 50 km/s, so are 4 to 10 times more fuel efficient than conventional rockets. Like electric devices on Earth, they are rated by how much power they use. The Dawn spacecraft has a 10 kW set of solar panels, and the VASIMR thruster in development is rated at 200 kW. Generally ion thrusters will maintain efficiency at lower power levels than plasma type thrusters because ion flow does not have to be restrained by containment fields, while plasma requires a field to keep it separated from the solid hardware. At small sizes the plasma volume vs total engine volume becomes small and efficiency drops.

For efficiency reasons, ion thrusters prefer high atomic weight fuels. The energy to ionize an atom is roughly constant across the Periodic Table, but does not contribute to thrust in this engine type. Thus using high weight fuels lowers the portion of total power used for ionization relative to acceleration. Typically Xenon is used as a fuel. Plasma thrusters can use most fuel types since their goal is to make the plasma extremely hot, on the order of a million degrees. By tuning the microwave generators, most atoms and molecules will absorb the energy. A key advantage of this is fuels like Oxygen or water are common in asteroids, so electric thrusters can be refueled locally, rather than having to bring all the fuel from Earth.

Electric Propulsion Applications

The following early missions can be performed starting with relatively small thrust levels, and working up to more ambitious missions.

Atmosphere Mining

This mission involves collecting air from the edge of the Earth's atmosphere for fuel and breathing. We start with a 50 kW solar array and a VASIMR type thruster which can generate 2 Newtons thrust at 40% efficiency and 20 km/s exhaust velocity. The solar arrays are assumed to use modern multi-layer cells with 30% efficiency, and have a power to mass ratio of 100 W/kg. The array will thus mass 500 kg, and we assume operates 30% of the time by intermittent use. The electric thruster can then produce an average thrust of 0.6 Newtons. At 200 km altitude, each square meter of collector generates 0.0129 Newtons of drag, so the total collector allowed area is 46 square meters to match the average thrust. This will collect 0.08 g/s, and the thruster consumes 0.03 g/s, leaving a net of 0.05 g/s. This amounts to 4.32 kg/day, or 3.15 times the solar array mass per year.

Later expansion would take the same thruster module to 200 kw power level and 5.7 N thrust at 50 km/s exhaust velocity and 60% operating time. The operating time is limited by the 40% of the orbit in the Earth's shadow. Thus average thrust is 3.42 N, and collection rate is 0.456 g/s. The thruster uses 0.114 g/s, leaving a net of 0.342 g/s. This is 29.5 kg/day or 10,785 kg/year, or 5.4 times the array mass per year. With a 15 year service life for the arrays, they can supply 75 times their mass in total. An electrodynamic thruster to make up for drag might improve on this even further.

For human transport, where speed is important going through the radiation belt, the collected air can be separated for Oxygen, and mixed with added Hydrogen from Earth in a chemical thruster. Alternately a lower exhaust velocity, higher thrust electric thruster could be used, sacrificing fuel efficiency for fast transit. There are several plasma and arc jet thrusters that could do that job.

Orbital Cleanup and Maintenance

Earth orbit has accumulated debris from spacecraft explosions and collisions, and there are a number of non-functional satellites which only need a single part repaired or new fuel to function again. This mission involves using a range of electric thruster vehicle sizes to collect the debris, repair or refuel satellites on location, or bring them to the orbital platform for maintenance. Debris mass ranges down to centimeter or less in size, so it would be inefficient to send a large vehicle to collect it. Alternately, satellites can range up to several tons in mass. Therefore we select electric vehicle sizes to match the size of what is being collected or moved. For debris collection, several pieces in similar orbits can be collected in one trip to minimize fuel use and mission time. The fuel for these cleanup missions comes from the atmosphere mining. Depending on what the target objects are, we perform one or more of the following tasks:

Collect orbital debris and either deliver it to a low enough orbit that it will decay and burn up quickly, or feed the debris into a processing unit to extract useful materials from.
Return non-working satellite hardware to the orbital platform to be salvaged for working parts.
Repair non-working satellites at the orbital platform with salvaged or new parts.
Repair, refuel, or attach a new propulsion unit to existing satellite at their current location.
Transport new cargo to higher orbits.

The tasks above are approximately in order of size and difficulty. Before salvaging used satellites, you would need to get permission from their original owners. The legal regime for broken debris pieces is unclear. If they are considered a menace to navigation, they might be removed without permission, or the original owners charged for cleanup.

Moved Text to be Merged

[MOVE to Phase 4B High Orbit] Further expansion of production may leads to power satellites, which beam energy to Earth for 24-hour power. If this can be done economically, it would likely be the largest export market to Earth. Solar-thermal with storage works in sunny climates on Earth, but many people don't live in such climates. Solar flux in space is 10 times higher than low sun climates on the ground. Energy delivered from orbit may prove cheaper overall, despite the extra cost of building in space. This is especially true if most of the materials for the satellites and their production equipment can be sourced from space, and the production is highly automated.