Robotics

Using robots will avoid most of the problems associated with life support systems, all though some protection against cold, acceleration and vacuum will still be necessary.

Spaceship Propulsion

The major challenge presented by space travel is propulsion, both in deep space and on effecting planetary takeoff and landing. A number of propulsion methods has been proposed: some of these are currently in use, some are under development and others are still (on Earth) only at the theoretical stage. All are likely to appear in the game, although some may require considerable research and resources before they are usable, meaning that a group of players may choose to co-operate to launch a vessel.

Rocket Power

Rockets provide thrust by throwing out mass in the form of high-pressure gas. This produces a reaction force on the rocket, pushing it in the opposite direction. The mass comes from the mass of the fuel that the rocket engine burns. The burning process heats the fuel atoms so that they exit the rocket nozzle at high speed.

The fuel can be of several types, but always requires the mixture of a fuel and an oxidizer so that the fuel can be ignited without requiring an external air supply.

Solid Fuel Rockets

A solid block of fuel is ignited: it burns very rapidly, propelling the rocket forward. This has the advantages of simplicity and low cost. It is relatively safe under power since the fuel in theory will not explode, but it can become unstable so that a sudden impulse will ignite it, making it unsuitable for very high acceleration applications. The other disadvantages are that the thrust cannot be controlled, and the engine cannot be stopped or restarted once ignited. Solid fuel rockets are therefore useful for short lifetime tasks, or booster systems which are separated from the main spacecraft as soon as they have launched it from the ground.

Liquid Fuel Rockets

A fuel and an oxidizer, for example liquid hydrogen and liquid oxygen, are pumped into a combustion chamber where they burn to create a stream of hot exhaust gases which can be channeled through a nozzle to propel the spacecraft.

Multi-Stage Rockets

The biggest problem with rockets is that they have to carry their reaction mass with them. So the rocket must carry enough fuel to propel the fuel itself, which may weigh 20 times more than the payload. Hence it is common to stack several rockets, the first of which is jettisoned once its fuel has been consumed, leaving the smaller secondary stage craft to continue with a reduced mass burden.

Multi-use rockets

Ideally, instead of abandoning the booster rocket stage once it has served its purpose of lifting the payload up from the ground, the rocket would be refuelled and reused.

Ion drive

Ion thrusters work on the same basic principle as rockets, pushing themselves forward by throwing mass out the back. The reaction mass is usually a heavy, stable substance such as xenon. Atoms are charged by an electric arc which removes an electron. These ions are then accelerated by passing them through highly-charged grids. The molecules are expelled from ion engines at much greater speed than from chemical engines, so that far less reaction mass is needed. However, because the electrical power requirements are high, the amount of thrust that can be generated is low. Hence ion drives are suitable for interplanetary missions, but not for operating within an atmosphere, or for lifting a payload into orbit.

Solar sails

A solar sail is a very large mirror that reflects sunlight. The photon of sunlight strike the sail and bounce off, transferring momentum to the sail and so pushing the sail along. The force is slight but continuous, so that the spacecraft accelerates gradually, potentially to a higher velocity than is usually achieved using rockets. The biggest advantage of solar sails are that neither fuel nor reaction mass needs to be carried, and the power to trim the sails can be solar generated. However, they cannot operate within an atmosphere or lift a payload into orbit. Changes of direction will be slow.

Laser power

Ground-based lasers are fired at the craft, which uses mirrors to receive and focus the incoming beam. The beam heats air at the back of the craft to so high a temperature that it converts to a plasma state and then explodes, propelling the vessel forwards.

The craft needs to spin rapidly in order to stabilize it during flight, which means that the vessel must be small and must tolerate the rapid spinning. The vessel would also only work within an atmosphere, making it potentially a good way to lift a small, robust payload into orbit but not suitable for interplanetary flight.

Microwave power

A microwave-powered craft relies on power beamed down from orbiting, solar power stations. Millions of tiny antennae covering the top of the craft would convert the microwaves into electricity. This electricity can be used to ionize air, which can then be manipulated electrically and used to propel the ship in a similar way to ion thrusters. This method of propulsion can only operate within an atmosphere, although it would be possible to receive beamed microwave power out in space and use it to power a different propulsion method.

Balloons

An atmosphere could potentially be used as a lift method in itself, rather than being a hindrance to achieving orbit. Giant helium balloons could be used to raise a spacecraft to the edge of space. Here it would deploy ion thrusters or another conventional propulsion method to attain orbital speed, without having to expend energy pushing through the lower atmosphere.

It would be possible to maintain a permanent upper-atmosphere base supported by balloons, which would act as a way station for craft leaving and re-entering the atmosphere.

Tether

Long, strong strings can be used to change the orbits of spacecraft. There are a number of uses to which tethers may be put:

• Tidal Stabilization: the tether is attached to a satellite at one end, and a small mass at the other. Two types of tidal forces then act on the system. Firstly, the mass and the satellite are forced by the tether to move at the same speed, even though the satellite, which is further out, would move at a different orbital speed if not connected to the weight. The resulting centripetal forces pull the satellite upwards and the mass downwards. Secondly, the gravitational acceleration experienced by the satellite is less than that on the weight, because it is further from the planet. As a result, the satellite is kept in a stable orientation, although damping is required to prevent vibration.
• Electrodynamic Tethers: if the tether is made of a conducting material, then it can be used either to accelerate or brake an orbiting spacecraft. When the tether cuts the planet’s magnetic field, it generates a current and experiences a force, thereby slowing the spacecraft. When direct current is passed through the tether, it will exert a force against the magnetic field and accelerate the spacecraft. The more rapidly the planet revolves, the more effective the tether will be, either for generating electricity or accelerating the spacecraft.
• Rotovators: a rotating tether could be used to transfer momentum to spacecraft, and the tether itself would then be spun back up to speed by running a current through the wire, or by slowing an incoming cargo.
• Skyhook: a satellite in geosynchronous orbit could let one tether down towards the planetary surface, while sending another tether of equal length out into space. The lower end of the tether could be anchored on the planet’s surface, so that the cable become a space elevator. Orbiting satellites could be released from the parent satellite, and vessels could be launched into space by continuing along the tether outwards, gaining relative momentum as they approached the end and ultimately flying off into deep space.

Plasma Rockets

A precursor to fusion rockets, plasma rockets work by ionizing a gas such as hydrogen, heating it with radio waves until it forms a plasma, and then expelling it at high speed to provide thrust.

Fusion/Fission

Fusion engines work on the same principle as rockets, pushing themselves forward by throwing matter out the back. The energy is provided by nuclear fusion or fission of suitable materials, heating them to the point where they can propel a spacecraft.

Fusion is more difficult to achieve than fission, requiring enormous pressure and temperature which need to be contained within powerful magnetic fields. Its major advantage is that the fuel is hydrogen, which is readily available from the atmosphere of many planets, whereas fission requires heavy elements. Fusion additionally produces less radiation and the fuel has a greater energy density.

Matter-Antimatter

The annihilation of a few grammes of antimatter would release enough pure energy (in the form of photons) to propel a spacecraft over interplanetary distances. The antimatter would need to be stored in magnetic rings to keep it from contacting normal matter until required, and the antimatter would have to be manufactured since it does not appear to exist in our universe in appreciable quantities.