Section 4.4 - Orbital Assembly

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.