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PLASMA ROCKETS


Magnetoplasmadynamic (MPD) Thruster
Tech level: 11
Variable Specific Impulse Magnetoplasma Rocket
Tech Level: 12
Electron Beam Plasma Propulsion
Tech Level: 14

Plasma rockets are currently being researched by NASA, specifically at Johnson Space Center’s Advanced Propulsion Laboratory. Ion drives are a specialized type of plasma rocket; see that section for further details.

THE BASICS

The hotter the exhaust of a rocket, the more thrust it will generate per unit of fuel. Plasma rockets heat their fuel to a plasma state, a state of matter where atoms are stripped of their electrons. Plasma exists at temperatures many thousands, even millions, of degrees beyond our everyday experience. Plasma usually occurs in environments of high pressures and temperatures, such as the Sun.

Since most standard materials cannot withstand such incredible temperatures, one of the primary features of a plasma rocket is containing the ignition reaction and directing the exhaust by means of electrostatic or electromagnetic fields. Since plasma is easily electrically charged, it can be readily shaped and directed by fields of sufficient strength.

The key phrase above, of course, is "sufficient" strength. Containing and channeling the kind of material fueling the stars is no easy task, and is probably the single greatest technological hurdle to making high-temperature plasma rockets a reality.

Plasma, Fusion, and Fission Rockets all produce high-temperature plasma exhaust. Plasma Rockets do have one distinct advantage over the other two, at least as far as manned missions are concerned: Plasma rocket reactions do not produce high radiation requiring heavy shielding, nor is their exhaust radioactive.

Plasma rockets are currently considered as a form of deep space propulsion and will probably first be used in that capacity, but they could also be used for boosting payloads into orbit once the technology matures. Plasma rockets are thought to be able to eventually produce specific impulses in excess of 100,000 seconds.


MAGNETOPLASMADYNAMIC (MPD) THRUSTER
Tech Level: 11

MPD Thrusters are also known as Lorentz Force Accelerators and were proposed as far back as 1964. It consists of a central columnar cathode surrounded by an annular anode. The magnetic field and current creates a Lorentz body force on the plasma particles, accelerating them along the thruster nozzle. The anode can be supplemented by an external magnetic field, greatly enhancing the plasma acceleration.

A cutaway diagram of an MPD thruster

MPD Thrusters can be operated in either a steady state or pulsed mode. The pulsed mode takes advantage of high-current capacitors discharging every few hundred microseconds. This involves a more complicated power conditioning system design than the steady-state mode, but allows for higher instantaneous discharge powers for a given steady-state power level. MPD Thrusters operating up to 30 megawatts and providing up to 200 Newtons of force per pulse are currenty being developed at NASA’s Glenn Research and Technology Center.

One of the major limitations of MPD Thrusters is that the end of the cathode is subjected to extreme temperatures, often in excess of 2500 degrees Celsius, causing rapid wear and tear. The main propellants for MPD Thrusters are typically Argon, Lithium, and Xenon.


VARIABLE SPECIFIC IMPULSE MAGNETOPLASMA ROCKET (VASIMR)
Tech Level: 12

The VASIMR being researched at the Advanced Propulsion Laboratory uses powerful microwaves to heat its hydrogen fuel into a plasma state, and manipulates it using three powerful superconducting magnets, which further ionize and heat the plasma as it flows through. A full diagram is provided below.

The operating specifics are recounted here, taken directly from NASA’s VASIMR webpage (see "Related Information" section below for link.) Exact author is unknown.

"The rocket system consists of three major magnetic cells, denoted as "forward," "central" and "aft." This particular configuration of electromagnets is called an asymmetric mirror. The forward end-cell involves the main injection of gas to be turned into plasma and the ionization subsystem; the central-cell acts as an amplifier and serves to further heat the plasma up to 50,000 degrees Celsius. The aft end-cell ensures that the plasma will efficiently detach from the magnetic field. Without the aft end-cell, the plasma would tend to follow the magnetic field and provide only a small amount of thrust. With this configuration, the plasma can be guided and controlled over a wide range of plasma temperatures and densities.

"To operate the rocket, neutral gas, typically hydrogen, is injected at the forward end-cell and ionized. Then it is heated to the desired temperature and density in the central-cell, by the action of electromagnetic waves, similar to what happens in microwave ovens. After heating, the plasma enters a two-stage hybrid nozzle at the aft end-cell where it is exhausted to provide modulated thrust."

Since the fuel used is kept at cryogenic temperatures, it can be used to not only help cool the engine as a whole but also to keep the superconducting materials within the magnets at proper operating temperature, as well.

Designers also forsee the powerful magnetic fields of the engine acting as a supplementary radiation shield during interplanetary flight. Scaled-down versions of the VASIMR design may eventually be used for satellite station-keeping.


ELECTRON BEAM PLASMA PROPULSION
Tech Level: 15

This engine injects a relativistic (sped up to near-light speed by a linear particle accelerator) electron beam into a compressed reactive fluid, most typically hydrogen, superheating it to a plasma state. Powerful magnetic fields hold both the ignition reaction and directs the plasma out of the vehicle for thrust. The extreme high energy of the beam heats the hydrogen explosively fast compared to the VASIMR model, requiring very powerful magnetic fields to contain and direct it. Like MPD thrusters, Electron Beam Plasma Rockets would give their best performance in pulsed mode, taking advantage of high-current capacitors to achieve best possible thrust energy, firing every few milliseconds.

This scheme is often mentioned as a Fusion Rocket (q.v.) option, but can be scaled down to "mere" plasma-rocket levels to avoid the complications of radiation shielding and radioactive exhaust that a fusion process would engender.

 


RELATED INFORMATION

Two articles on the VASIMR scheme:

http://spaceflight.nasa.gov/mars/technology/propulsion/aspl/vasimr.html

>http://www.nasatech.com/Briefs/Sep01/MSC23041.html

A lecture on both ion drives and plasma thrusters:

http://rigel.neep.wisc.edu/~jfs/neep602.lecture30.plasmaProp.96/neep602.lecture30.plasmaProp.96.html

Further details on MPD Thrusters:

http://www.islandone.org/APC/Electric/15.html

A brief article on the Electron Beam concept:

http://www1.msfc.nasa.gov/STD/propulsion/research/fusion/fusion_ebeam.html




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