Advanced Technology in Propulsion - VASIMR MODE

VASIMR MODE

               The variable- specific-impulse magneto plasma rocket (VASIMR) is high powered, electro thermal plasma

rocket, capable of modulating the exhaust at constant power. An electrode less design enables the rocket to operate at power densities much greater than those of more conventional magneto plasma or ion engines. VASIMR is intended to bridge the gap between high-thrust, low-specific impulse propulsion systems and low-thrust, high-specific impulse systems. Therefore it is capable of functioning in either mode placing the VASIMR far ahead of anything available today. This rocket utilizes hydrogen as its propellant which can be operated at relatively low cost.

            The VASIMR is expected to be commercially useful for boosting communication satellites and other Earth-orbiting spacecraft to higher orbits, retrieving and servicing spacecraft in high orbits around the Earth, and boosting high-payload robotic spacecraft on very fast missions to other planets. The greatest potential of the VASIMR is expected to lie in its ability to significantly reduce the trip times for human missions to Mars and beyond. This reduction in times is expected to enable long-term exploration of outer space by humans — something that conventional rocket designs now preclude.

            The VASIMR contains three major magnetic cells — the forward, central, and after cells. A plasma is injected into these cells, then heated, then expanded in a magnetic nozzle.  During operation of the VASIMR, a neutral gas (typically, hydrogen) is injected into the forward cell, where it is ionized. The resulting plasma is then heated further in the central cell which serves as an amplifier, to the desired temperature and density, by use of radio-frequency excitation and ion cyclotron resonance. Once heated, the plasma is magnetically and gas-dynamically exhausted by the aft cell to provide modulated thrust.

The VASIMR offers numerous advantages:

• Its unique electrode less design provides not only high thrust at maximum power but also highly efficient ion-cyclotron-resonance heating, and high efficiency of the VASIMR
• The residual magnetic field of the engine and the hydrogen propellant will be effective as a shield against radiation.
• The variability of thrust and Isp at constant power will afford a wide range of capabilities to abort.
• Because hydrogen is the most abundant element in the universe, the supply of hydrogen could likely be regenerated.
• The VASIMR is flexible and adaptable to both fast transfers of humans and slower high-payload robotic missions; hence, there would be no need to develop separate propulsion systems for missions of each type, and costs would be held down accordingly.

       

 Because the VASIMR is a high-Isp rocket, the VASIMR concept can be expected to lead to lower initial mass in low Earth orbit, relative to nuclear, thermal, and/or chemical rockets.

          The VASIMR engine could also even help protect astronauts from the dangerous effects of radiation during their trip. In the less-distant future, VASIMR could even help keep the International Space Station (ISS) in orbit without requiring extra fuel to be brought up from Earth 

          It even features an "afterburner" mode that sacrifices fuel efficiency for additional speed. Possible fuels for the VASIMR engine could include hydrogen, helium, and deuterium

TETHER BASED PROPULSION

          

         Tether propulsion systems are proposals to use long, very strong cables (known as tethers) to change the velocity of spacecraft and payloads. The tethers may be used to initiate launch, complete launch, or alter the orbit of a spacecraft.

           Spaceflight using this form of spacecraft propulsion may be significantly less expensive than spaceflight using rocket engines.

          Tethers are kept straight by either rotating end for end, with very high tips speeds (several km/s), or by the difference in the strength of gravity over their length (tidal stabilization). Tethers require strong, light materials. Some current tether designs use crystalline plastics such as ultra high molecular weight polyethylene, aramid, diamond or carbon fiber. A possible future material would be carbon nanotubes, which have an estimated tensile strength between 140 and 177 GPa 

           Such a "space tether" principle can be used for a surprising range of applications, such as payload transfer, power generation, and orbital propulsion. To achieve maximum performance and low cost, tethers need to be made of materials with the combination of high tensile strength and low density.