Section 4.12 - Interplanetary Transfer
With a network of systems developed around Earth and the Moon, the next step is to extend the network out to Mars and the Main Asteroid Belt. A similar process of setting up seed factories and adding facilities is followed as before.
The system concept has the following main parts:
- Use free flying electric thruster powered vehicles to reach new locations that have useful resources.
- Set up seed factories in each location to build up industrial capacity, including more ships and seed factories for the next location.
- Produce fuel, life support supplies, and habitats at each location so they can be occupied permanently.
- Build more Skyhooks to provide fast velocity changes, but get the benefit of electric propulsion efficiency.
High Earth Orbit Skyhook
In a previous step we defined a Low Earth Orbit (LEO) Skyhook with a tip velocity of 2400 m/s. Since its orbit velocity is 7474 m/s, at the top of its rotation it could release a cargo at a total of 9,874 m/s. At an altitude of 1,226 km, or a radius of 7,604 km from the Earth's center, with a Standard gravitational parameter of 398.6 x 10^12, we get an escape velocity of 10,239 m/s. Thus our LEO Skyhook is only 365 m/s short of reaching escape velocity. The LEO Skyhook can thus deliver cargo to an elliptical transfer orbit with a semi-major axis of the orbit of 54,278 km, and thus a high point of 100,952 km.
A circular orbit at that altitude has a velocity of 1,987 m/s, and the transfer orbit arrives at 743 m/s. The difference is 1,244 m/s. A tip velocity of 1,500 m/s or more would allow injection to Mars and Main Asteroid Belt transfer orbits, and any closer transfer orbits to the Moon and low inclination Near Earth Asteroids. A High Earth Orbit (HEO) Skyhook can therefore serve as a launch platform to any desired inner Solar System destination orbit by selecting the radius, and thus velocity, and the time, which gives the direction, of release. This location is outside of the Earth's radiation belts, but it is also unprotected by the Earth's magnetosphere from solar and cosmic radiation. So human habitats will need radiation shielding. Such a high orbit is relatively easy to reach from the Moon or NEO's, so bulk matter for shielding will likely be brought from one of those sources.
The construction sequence would start with fetching Near Earth Orbit asteroid material and placing it in High orbit, and delivering equipment from Earth. Once a processing plant, factory, and habitat are set up, carbon from carbonaceous asteroids is used to make carbon fiber for the Skyhook. Initial velocity capacity would not be as high, and so more vehicle propulsion would be needed, but as the Skyhook grows, it can reach a wider range of orbits. Momentum changes are not free, so a substantial power supply and thruster set will be needed at the Skyhook. But since those do not have to be carried along with the vehicles who are getting their orbit changed, the propulsion can be as large and heavy as needed.
Near Earth Objects have a limited range of Solar orbit velocities in the ecliptic plane by definition. They also have a range of orbit inclinations, which results in a velocity component when crossing the ecliptic. The Inclination Station is a second Skyhook located in the vicinity of Earth, such as at one of the Lunar Lagrange points, and oriented perpendicular to the ecliptic plane. Therefore it can deliver cargo to and from inclined orbits to reach NEOs with less fuel and mission time, while the first HEO Skyhook operates in the ecliptic plane to reach Mars and the main Asteroid Belt. Additionally, flybys of planets can be used to further change orbit inclinations and reach other groups of asteroids. The Station itself will react to the average of cargo orbit changes. It will therefore need some propulsion to maintain position, but that will use less fuel than each vehicle doing it's own orbit maneuvers separately.
Given sufficient traffic, it may make sense to have other Inclination Stations set up at different tilts to generate different combinations of ecliptic plane and inclination velocity changes. The velocity difference between the various high orbit Skyhooks will be very low, and release from less than their full radius will be sufficient to get between them. Since trip times between them will be short, the first one can have the bulk of the habitat and production facilities. The later ones can serve mostly as transit hubs, and not be as built up.
Transfer orbits to NEOs or Mars will require trip times measured in months. For human passengers there is the risk of exposure to radiation, and also a need for food and life support. If you expect to make multiple trips, it makes sense to have habitats permanently in transfer orbits to those destinations. Then the mass of the shielding and greenhouses does not matter that much, as they are not moving once set up. The passengers would use a small vehicle between the HEO Skyhook and the Transfer Habitat when it passes near Earth, then ride in the habitat until it is near the destination, and then again use a small vehicle for arrival. Since only the passengers and cargo need to change velocity the total mass transferred per mission is greatly reduced.
All objects in the Solar System are in motion relative to each other, and transfer orbits only line up properly when, for example, the Earth and Mars are in the right relative positions. Thus a network of multiple Transfer Habitats in different orbits will be needed to deliver passengers and cargo to the right destinations at the right time. The source materials to build the Habitats would come mostly from whichever asteroids are already in the closest orbits. Depending on size, the NEO would be mined for materials, or if small, moved entire to the desired transfer orbit.
Optionally a transfer habitat would have a Skyhook attached to it to enable added delta-V for arriving or departing vehicles. This also provides an artificial gravity environment for the habitat. If that is not provided, then part of the habitat would be rotating to create artificial gravity. Whether to use a Skyhook or not will take more detailed analysis of the complete transportation network.
When the habitats are not ferrying passengers to Mars, they are exploiting Near Habitat Asteroids. Just like there is a set of asteroids whose orbits are "Near Earth", and so easy to reach for mining purposes, there will be a different set which are close to any given Transfer Habitat orbit. So you can busy yourself producing fuel, setting up manufacturing, and eventually have a space city there, just one that happens to get close to Mars periodically. When it does, you drop off humans and accumulated hardware at Phobos, where you are also building up facilities, and thence onward to Mars itself.
Since we are starting from Earth, we want the Habitat to pass by Earth on a regular basis. If we set the orbit period to be 1.50 years, it will do so every second orbit. The orbit will be an ellipse, and the long axis will be 2.62 AU. If the near point of the orbit is at Earth (1.00 AU), then the far point will be 1.62 AU, which is slightly past the average distance of Mars (1.52 AU). A velocity change of 3,340 m/s beyond Earth orbit is required to reach this orbit. This comes from a combination of the HEO Skyhook, Lunar gravity assist, propulsion on the transfer vehicle carrying crew and cargo between points, and possibly a Skyhook at the Transfer Habitat. The Habitat will align with Mars once every 7.5 years, and a velocity change of 4,440 m/s is needed at that end to match orbit. Again this would use a combination of propulsion, including Mars gravity assist. On early trips the transfer vehicle would need to do more work, but later a Mars orbit Skyhook can take up more of the velocity changes.
Since once per 7.5 years is not very often, you can place multiple habitats in a given orbit track, and use multiple orbit tracks spaced equally around the Earth's orbit at their near points. This will give more frequent opportunities to go back and forth from Earth to Mars. For 80% of their orbit cycle the habitats would would be doing mining and construction around the habitat, accessing asteroids in nearby orbits. The other 20% of the time they would also be carrying passengers and cargo for the trips to and from Mars, when they happen to line up. There may be a better arrangement of orbits and habitats to increase the fraction of time they can be used as a ferry service.