Variable Specific Impulse Magnetoplasma Rocket
The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) is an electro-magnetic thruster for spacecraft propulsion. It uses radio waves to ionize and heat a propellant, and magnetic fields to accelerate the resulting plasma to generate thrust. It is one of several types of spacecraft electric propulsion systems.
The method of heating plasma used in VASIMR was originally developed as a result of research into nuclear fusion. VASIMR is intended to bridge the gap between high-thrust, low-specific impulse propulsion systems and low-thrust, high–specific impulse systems. VASIMR is capable of functioning in either mode. Costa Rican born American scientist and former NASA astronaut Franklin Chang Díaz created the VASIMR concept and has been working on its development since 1977.
- 1 Design and operation
- 2 Research and development
- 3 Potential future applications
- 4 Criticisms of VASIMR by Robert Zubrin
- 5 See also
- 6 References
- 7 Further reading
- 8 External links
Design and operation
The Variable Specific Impulse Magnetoplasma Rocket, sometimes referred to as the Electro-thermal Plasma Thruster or Electro-thermal Magnetoplasma Rocket, uses radio waves to ionize and heat propellant, which generates plasma that is accelerated using magnetic fields to generate thrust. This type of engine is electrodeless and as such belongs to the same electric propulsion family (while differing in the method of plasma acceleration) as the electrodeless plasma thruster, the microwave arcjet, or the pulsed inductive thruster class. It can also be seen as an electrodeless version of an arcjet, able to reach higher propellant temperature by limiting the heat flux from the plasma to the structure. Neither type of engine has any electrodes. This is advantageous in that it eliminates problems with electrode erosion that cause rival designs of ion thrusters to have relatively shorter life expectancy. Furthermore, since every part of a VASIMR engine is magnetically shielded and does not come into direct contact with plasma, the potential durability of this engine design is greater than other ion/plasma engine designs.
VASIMR can be most basically thought of as a convergent-divergent nozzle for ions and electrons. The propellant (a neutral gas such as argon or xenon) is first injected into a hollow cylinder surfaced with electromagnets. Upon entry into the engine, the gas is first heated to a “cold plasma” by a helicon RF antenna (also known as a “coupler”) which bombards the gas with electromagnetic waves, stripping electrons off the argon or xenon atoms and leaving plasma consisting of ions and loose electrons to continue down the engine compartment. By varying the amount of energy dedicated to RF heating and the amount of propellant delivered for plasma generation VASIMR is capable of either generating low-thrust, high–specific impulse exhaust or relatively high-thrust, low–specific impulse exhaust. The second phase is a strong electromagnet positioned to compress the ionized plasma in a similar fashion to a convergent-divergent nozzle that compresses gas in traditional rocket engines.
A second coupler, known as the Ion Cyclotron Heating (ICH) section, emits electromagnetic waves in resonance with the orbits of ions and electrons as they travel through the engine. Resonance of the waves and plasma is achieved through a reduction of the magnetic field in this portion of the engine which slows down the orbital motion of the plasma particles. This section further heats the plasma to temperatures upwards of 1,000,000 kelvin — about 173 times the temperature of the Sun’s surface.
Motion of ions and electrons through the engine can be approximated by lines parallel to the engine walls; however, the particles actually orbit those lines at the same time that they are traveling linearly through the engine. The final, diverging section of the engine, contains a steadily expanding magnetic field which forces the ions and electrons into steadily lengthening spiral orbits in order to eject from the engine parallel and opposite to the direction of motion at speeds of up to 50,000 m/s, propelling the rocket forward through space.
Benefits and drawbacks of design
In contrast with usual cyclotron resonance heating processes, in VASIMR ions are immediately ejected through the magnetic nozzle, before they have time to achieve thermalized distribution. Based on novel theoretical work in 2004 by Arefiev and Breizman of UT-Austin, virtually all of the energy in the ion cyclotron wave is uniformly transferred to ionized plasma in a single-pass cyclotron absorption process. This allows for ions to leave the magnetic nozzle with a very narrow energy distribution, and for significantly simplified and compact magnet arrangement in the engine.
VASIMR does not use electrodes; instead it magnetically shields plasma from most of the hardware parts, thus eliminating electrode erosion—a major source of wear and tear in ion engines. Compared to traditional rocket engines with very complex plumbing, high performance valves, actuators and turbopumps, VASIMR eliminates practically all moving parts from its design (apart from minor ones, like gas valves), maximizing its long term durability.
However, some new problems emerge, like interaction with strong magnetic fields and thermal management. The relatively large power at which VASIMR operates generates a lot of waste heat, which needs to be channeled away without creating thermal overload and undue thermal stress on materials used. Powerful superconducting electromagnets, employed to contain hot plasma, generate tesla-range magnetic fields. They can present problems with other on board devices and also can produce unwanted torque by interacting with the magnetosphere. To counter this latter effect, the VF-200 will consist of two 100 kW thruster units packaged together, with the magnetic field of each thruster oriented in opposite directions in order to make a zero-torque magnetic quadrupole.
Research and development
The first VASIMR experiment was conducted at MIT starting in 1983 on the magnetic mirror plasma device. Important refinements were introduced to the rocket concept in 1990s, including the use of the "helicon" plasma source, which replaced the initial plasma gun originally envisioned and made the rocket completely "electrodeless"—an extremely desirable feature to assure reliability and long life. A new patent was granted in 2002.
In 1995, the Advanced Space Propulsion Laboratory (ASPL) was founded at NASA Johnson Space Center, Houston in the building of Sonny Carter Training Facility. The magnetic mirror device was brought from MIT. The first plasma experiment in Houston was conducted using a microwave plasma source. The collaboration with University of Houston, University of Texas at Austin, Rice University and other academic institutions was established.
In 1998, the first helicon plasma experiment was performed at the ASPL. The decision was made regarding the official name of VASIMR and VASIMR experiment (VX). VX-10 in 1998 ran up to 10 kW helicon discharge, VX-25 in 2002 ran up to 25 kW and VX-50—up to 50 kW of RF plasma discharge. In March, 2000, the VASIMR group was given a Rotary National Award for Space Achievement / Stellar Award. By 2005 major breakthroughs were obtained at the ASPL including full and efficient plasma production, and acceleration of the plasma ions in the second stage of the rocket. The VASIMR engine model VX-50 proved to be capable of 0.5 newtons (0.1 lbf) of thrust. Published data on the VX-50 engine, capable of processing 50 kW of total radio frequency power, showed ICRF (second stage) efficiency to be 59% calculated as: 90% NA coupling efficiency × 65% NB ion speed boosting efficiency. It was hoped that the overall efficiency of the engine could be increased by scaling up power levels.
Ad Astra Rocket Company (AARC) was incorporated in Delaware on January 14, 2005. On June 23, 2005, Ad Astra and NASA signed the first Space Act Agreement to privatize the VASIMR Technology. On July 8, 2005, Franklin Chang Díaz retired from NASA after 25 years of service. Ad Astra’s Board of Directors was formed and Dr. Díaz took the helm as chairman and CEO on July 15, 2005. In July, 2006, AARC opened the Costa Rica subsidiary in the city of Liberia at the campus of Earth University. In December 2006, AARC-Costa Rica performed first plasma experiment on the VX-CR device utilizing helicon ionization of argon.
The 100 kilowatt VASIMR experiment was successfully running by 2007 and demonstrated efficient plasma production with an ionization cost below 100 eV. VX-100 plasma output was tripled over the prior record of the VX-50. In the same year, the AARC moved out from the NASA facility to its own building in Webster, TX.
Model VX-100 was expected to have the NB ion speed boosting efficiency of 80%. There were, however, additional (smaller) efficiency losses related to the conversion of DC electric current to radio frequency power and also to the superconducting magnets' auxiliary equipment energy consumption. By comparison, 2009 state-of-the-art, proven ion engine designs such as NASA's HiPEP operated at 80% total thruster/PPU energy efficiency.
Development of the 200 kW engine
On October 24, 2008 the company announced that the plasma generation aspect of the VX-200 engine—helicon first stage or solid-state high frequency power transmitter—had reached operational status. The key enabling technology, solid-state DC-RF power-processing, has become very efficient reaching up to 98% efficiency. The helicon discharge uses 30 kWe of radio waves to turn argon gas into plasma. The remaining 170 kWe of power is allocated for passing energy to, and acceleration of, plasma in the second part of the engine via ion cyclotron resonance heating.
Based on data released from previous VX-100 testing, it was expected that the VX-200 engine would have a system efficiency of 60–65% and thrust level of 5 N. Optimal specific impulse appeared to be around 5,000s using low cost argon propellant. One of the remaining untested issues was potential vs actual thrust; that is, whether the hot plasma actually detached from the rocket. Another issue is waste heat management. About 60% of input energy ends up as useful kinetic energy. A large portion of the remaining 40% will be secondary ionizations cost from plasma crossing magnetic field lines and exhaust divergence. A significant portion of that 40% would end up as waste heat (see energy conversion efficiency). Managing and rejecting that waste heat is critical to allowing for continuous operation of the VASIMR engine.
Between April and September 2009, tests were performed on the VX-200 prototype with fully integrated 2-tesla superconducting magnets. They successfully expanded the power range of the VASIMR up to its full operational capability of 200 kW.
During November 2010, long duration, full power firing tests were performed with the VX-200 engine reaching the steady state operation for 25 seconds thus validating basic design characteristics.
Results presented to NASA and academia in January 2011 have confirmed that the design point for optimal efficiency on the VX-200 is 50 km/s exhaust velocity, or an Isp of 5000 s. Based on these data, thruster efficiency of 72% has been achieved by Ad Astra, yielding an overall system efficiency (DC electricity to thruster power) of 60% (since the DC to RF power conversion efficiency exceeds 95%) with argon as the propellant.
Testing on the space station
On December 8, 2008, Ad Astra signed an agreement with NASA to arrange the placement and testing of a flight version of the VASIMR, the VF-200, on the International Space Station (ISS). As of June 2012[update], its launch is anticipated to be in 2015; the Antares rocket has been reported as the "top contender" for the launch vehicle. Since the available power from the ISS is less than 200 kW, the ISS VASIMR will include a trickle-charged battery system allowing for 15 min pulses of thrust.
Testing of the engine on ISS is valuable because it orbits at a relatively low altitude and experiences fairly high levels of atmospheric drag, making periodic boosts of altitude necessary. Currently, altitude reboosting by chemical rockets fulfills this requirement. The VASIMR test on the ISS may lead to a capability of maintaining the ISS or a similar space station in a stable orbit at 1/20th of the approximately $210 million/year present estimated cost.
The VF-200 flight-rated thruster consists of two 100 kW VASIMR units with opposite magnetic dipoles so that no net rotational torque is applied to the space station when the thruster magnets are working. The VF-200-1 is the first flight unit and will be tested in space attached to the ISS.
In June 2005, Ad Astra signed its first Space Act Agreement with NASA which led to the development of the VASIMR engine. In December 10, 2007, AARC and NASA signed an Umbrella Space Act Agreement relating to the space agency's potential interest in the VASIMR, providing a framework for collaboration between the parties, setting out the general conditions governing aspects of their ongoing relationship. In December 8, 2008, NASA and AARC entered into a Space Act Agreement that could lead to conducting a space flight test of the VASIMR on the ISS. In March 2, 2011, Ad Astra and NASA Johnson Space Center have signed a Support Agreement to collaborate on research, analysis and development tasks on space-based cryogenic magnet operations and electric propulsion systems currently under development by Ad Astra. As of February 2011[update], NASA had 100 people assigned to the project to work with Ad Astra to integrate the VF-200 onto the space station. On December 16, 2013, AARC and NASA signed another five-year Umbrella Space Act Agreement.
Potential future applications
VASIMR is not suitable to launch payloads from the surface of the Earth due to its low thrust-to-weight ratio and its need of a vacuum to operate. Instead, it would function as an upper stage for cargo, reducing the fuel requirements for in-space transportation. The engine is expected to perform the following functions at a fraction of the cost of chemical technologies:
- drag compensation for space stations
- lunar cargo delivery
- satellite repositioning
- satellite refueling, maintenance and repair
- in space resource recovery
- ultra fast deep space robotic missions
Other applications for VASIMR such as the rapid transportation of people to Mars would require a very high power, low mass energy source, such as a nuclear reactor (see nuclear electric rocket). NASA Administrator Charles Bolden said that VASIMR technology could be the breakthrough technology that would reduce the travel time on a Mars mission from 2.5 years to 5 months.
In August 2008, Tim Glover, Ad Astra director of development, publicly stated that the first expected application of VASIMR engine is "hauling things [non-human cargo] from low-Earth orbit to low-lunar orbit" supporting NASA's return to Moon efforts.
Use as a space tug or orbital transfer vehicle
The most important near-future application of VASIMR-powered spacecraft is transportation of cargo. Numerous studies have shown that, despite longer transit times, VASIMR-powered spacecraft will be much more efficient than traditional integrated chemical rockets at moving goods through space. An orbital transfer vehicle (OTV)—essentially a "space tug"—powered by a single VF-200 engine would be capable of transporting about 7 metric tons of cargo from low Earth orbit (LEO) to low Lunar orbit (LLO) with about a six month transit time. NASA envisages delivering about 34 metric tons of useful cargo to LLO in a single flight with a chemically propelled vehicle. To make that trip, about 60 metric tons of LOX-LH2 propellant would be burned. A comparable OTV would need to employ 5 VF-200 engines powered by a 1 MW solar array. To do the same job, such OTV would need to expend only about 8 metric tons of argon propellant. Total mass of such electric OTV would be in the range of 49 t (outbound & return fuel: 9 t, hardware: 6 t, cargo 34 t). The OTV transit times can be reduced by carrying lighter loads and/or expending more argon propellant with VASIMR throttled up to higher thrust at a less efficient (lower Isp) operating condition. For instance, an empty OTV on the return trip to Earth covers the distance in about 23 days at optimal specific impulse of 5,000 s (50 kN·s/kg) or in about 14 days at Isp of 3,000 s (30 kN·s/kg). The total mass of the NASA specs' OTV (including structure, solar array, fuel tank, avionics, propellant and cargo) was assumed to be 100 metric tons (98.4 long tons; 110 short tons) allowing almost double the cargo capacity compared to chemically propelled vehicles but requiring even bigger solar arrays (or other source of power) capable of providing 2 MW.
As of October 2010[update], Ad Astra Rocket Company is working toward utilizing VASIMR technology for space tug missions to help "clean up the ever-growing problem of space trash." They hope to have a first-generation commercial offering by 2014.
Criticisms of VASIMR by Robert Zubrin
Robert Zubrin is critical of the VASIMR, claiming that it is less efficient than other electric thrusters which are now operational. Zubrin also believes that electric propulsion is not necessary to get to Mars; therefore, budgets should not be assigned to develop it. His second point of criticism concentrates on the lack of a suitable power source. Ad Astra subsequently responded to the criticism in a press release:"It is abundantly clear that the nuclear reactor technology required for such missions is not available today and major advances in reactor design and power conversion are needed". 
- Comparison of orbital rocket engines
- Electrodeless plasma thruster
- Helicon Double Layer Thruster
- Magnetoplasmadynamic thruster
- Ion thruster
- Pulsed plasma thruster
- Solar panels on spacecraft
- Spacecraft propulsion
- List of plasma (physics) articles
- Nano-particle field extraction thruster
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|Wikimedia Commons has media related to VASIMR.|
- Ad Astra Rocket Company
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- "Plasma Rocket" (Video). Brink. Science. December 18, 2008.
- NASA documents
- Technical Paper: Rapid Mars Transits with Exhaust-Modulated Plasma Propulsion (PDF)
- Variable-Specific-Impulse Magnetoplasma Rocket (Tech Brief)
- Advanced Space Propulsion Laboratory: VASIMR
- Propulsion Systems of the Future