Ion thruster

Ion thrusters require power sources, either nuclear or solar.

Electrothermal Propulsion Systems

Arcjets

An arcjet heat a propellant using an electric arc rather than a chemical reaction. It is therefore a thermal engine. The ISP from an arcjet can be higher than for a chemical rocket, but remains at around 500s, one order of magnitude less than what is required for the Enterprise.

http://www.nasa.gov/offices/oce/llis/0736.html

Electromagnetic Propulsion Systems

Magnetoplasmadynamic (MPD) Thrusters

A gas is ionised, turned into a plasma and fed into a acceleration chamber, where the interaction between an electrical current in the plasma and the magnetic field produced by electromagnets pushes the plasma up to high speeds. Vasimir is an application of this principle.

This is one of the best candidates for the Enterprise propulsion. However, since it is more efficient at larger sizes, the lack of a suitable power source to test the principle in space has hampered the development of this technology. Thrusters with powers up to 500 N are possible, reducing the number of thrusters down to a few hundred units for the Enterprise.

http://en.wikipedia.org/wiki/Magnetoplasmadynamic_thruster

VASIMR

The Vasimr (Variable Specific Impulse Magnetoplasma rocket) engine, as per 2011, has an optimum specific impulse of 5000. Required power is 200 kW, with an efficiency of 60%, for a thrust of 6 N. 14 000 engines would be required for the Enterprise. The fuel is argon but other gases can be used. The concept should be scalable up to 500N per unit, bringing the number of engines down to 168. (Sizing information required)

Pulsed Plasma Thrusters

A material is transformed into a burst of plasma by a short lived electric arc (think of a spark plug), and the plasma is accelerated by the electric field between an anode and a cathode. This is a simple but inefficient type of thruster that, at 10% efficiency, is not suitable for Mars transportation.

http://en.wikipedia.org/wiki/Pulsed_plasma_thruster

However, in the following paper : http://alfven.princeton.edu/papers/tem_jpc2002.pdf a proposal is made for a much more powerful version, that, although still only 50% efficient, might be further upgraded to provide the required thrust. The proposed fuel would be lithium. The design is very simple, and might be very light.

Electrostatic Propulsion Systems

Hall effect

The Hall effect thruster is a (mostly) Russian technology. Over 200 units have been flown. Engine performances are comparable to ion grid. Hall effect thrusters are physically smaller than ion grid thrusters. This is a distinct advantage for the Enterprise configuration. Wikipedia cites efficiencies up to 75%

Ion grid

The ion grid thruster is a mature technology that has performances very close to the BTE mission requirements.

The current NASA model is the NEXT thruster. . The engine thrust is very small, at 0,2 N per motor, with 6,9 kW and 70% efficiency. To supply the 84 000 N required by the Enterprise 400 000 thrusters would be required. The fuel is Xenon gas. With a size of about 600mm wide per unit, about 80 000 m2 are required to mount the thrusters, a surface of 280m x 280m.

The Hipep Ion engine has an efficiency of 80% and similar characteristics than the NEXT. The HIPEP is rectanglar and can be assembled in tight grids. The model tested was 600mm x 1200mm (approx, to be confirmed). 124 000 HIPEP thrusters would be required, for an area of about 90 000 m2.

NanoFET

This is a technology in the very early stages of developement. The fuel is composed of tiny droplets of semi conductors, encased in a shell of protein. The propulsion method uses electric fields to accelerate the particles. Efficiencies may be very high. Tho envisioned market is micro satellites, but it might be possible to 'print out' large boards of these micro thrusters using micropressor production technologies and eventually reach the required thrust (with millions of thrusters).

Propellants

Hydrogen

Hydrogen is an effective propellant, very appropriate for high ISP thrusters.

Xenon

Xenon is an inert gas that is relatively easy to ionise and denser that other inert gases. It is the best choice for the fuel of most types of ion engines and may be used as well as an ionizing agent in the coolant system, to provide the required plasma for the MHD generator. Xenon is quite expensive, at about 20$ per liter (6g). At current prices, if Xenon was the sole fuel of the Enterprise, the 25 000 tons of Xenon required would cost over 80 billion dollars. A possible source of Xenon would be the oxygen plants required for the fuel of the rockets used to carry up the materials for the Enterprise into space. There are about 45 billions tons of Xenon in Earth's atmosphere, allowing for about 2 millions trips on the scale of the Enterprise trip to Mars. Due to cost and availability concerns, argon may be a better choice that Xenon.

Argon

Argon is a inert gas that composes almost 1% of Earths atmosphere. It can be used instead of Xenon as propellant for the Enterprise. It should also be possible to use it as the ionising agent in the cooling system for the MHD generator operation. The Vasimir engine uses argon as propellant.

Water

Water

Helium

Helium is

Liquid metals

Sodium, Lithium, lead, lead bismuth, mercury,

Liquid salts

Nanofluids

Nanoparticles in suspension in a carrier fluid can have interesting propulsive properties.