Variable Specific Impulse Magnetoplasma Rocket
The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) is an electromagnetic thruster under development for possible use in spacecraft propulsion. It uses radio waves to ionize and heat a propellant. Then a magnetic field accelerates the resulting plasma to generate thrust (plasma propulsion engine). It is one of several types of spacecraft electric propulsion systems.
The VASIMR method for heating plasma was originally developed from nuclear fusion research. It is intended to bridge the gap between high-thrust, low-specific impulse and low-thrust, high-specific impulse systems, and is capable of functioning in either mode. Former NASA astronaut Franklin Chang Díaz created the VASIMR concept and has been developing it since 1977.
Design and operation
VASIMR, sometimes referred to as the Electro-thermal Plasma Thruster or Electro-thermal Magnetoplasma Rocket, uses radio waves to ionize and heat the propellant, which is then accelerated with magnetic fields to generate thrust. This engine is electrodeless, of the same propulsion family as the electrodeless plasma thruster, the microwave arcjet, or the pulsed inductive thruster class. It can be thought of as an electrodeless version of an arcjet rocket that can reach higher propellant temperature by limiting the heat flux from the plasma to the structure. Neither type of engine uses electrodes; this eliminates the electrode erosion that shortens the life of other ion thruster designs. Since every part of a VASIMR engine is magnetically shielded and does not directly contact plasma, the durability of this engine is predicted to be greater than many other ion/plasma engines.
VASIMR has been described as a convergent-divergent nozzle for ions and electrons. The propellant (a neutral gas such as argon or xenon) is injected into a hollow cylinder surfaced with electromagnets. On entering the engine, the gas is first heated to a “cold plasma” by a helicon RF antenna (also known as a “coupler”) that bombards the gas with electromagnetic waves, stripping electrons off the propellant atoms and producing a plasma of ions and loose electrons that flow 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 generating either low-thrust, high–specific impulse exhaust or relatively high-thrust, low–specific impulse exhaust. The second phase of the engine 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 is achieved through a reduction of the magnetic field in this portion of the engine that slows the orbital motion of the plasma particles. This section further heats the plasma to greater than 1,000,000 kelvin—about 173 times the temperature of the Sun’s surface.
The path of ions and electrons through the engine approximates lines parallel to the engine walls; however, the particles actually orbit those lines while traveling linearly through the engine. The final, diverging, section of the engine contains an expanding magnetic field that drives the ions and electrons in steadily expanding spirals and ejects them from the engine, parallel and opposite to the direction of motion at velocities as great as 50,000 m/s.
Advantages and drawbacks
In contrast to the typical cyclotron resonance heating processes, VASIMR ions are immediately ejected from the magnetic nozzle before they achieve thermalized distribution. Based on novel theoretical work in 2004 by Alexey V. Arefiev and Boris N. Breizman of University of Texas at 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 hardware parts, thus eliminating electrode erosion, a major source of wear in ion engines. Compared to traditional rocket engines with very complex plumbing, high performance valves, actuators and turbopumps, VASIMR has almost no moving parts (apart from minor ones, like gas valves), maximizing long term durability.
However, new problems emerge, such as interaction with strong magnetic fields and thermal management. The relatively large power at which VASIMR operates generates substantial waste heat that needs to be channeled away without creating thermal overload and thermal stress. Powerful superconducting electromagnets, necessary to contain hot plasma, generate tesla-range magnetic fields that can cause problems with other onboard devices and produce unwanted torque by interaction with the magnetosphere. To counter this latter effect, the VF-200 consists of two 100 kW thruster units packaged with magnetic fields oriented in opposite directions, making a net zero-torque magnetic quadrupole.
Research and development
The first VASIMR experiment was conducted at Massachusetts Institute of Technology in 1983 on the magnetic mirror plasma device. Important refinements were introduced to the rocket concept in the 1990s, including the use of the "helicon" plasma source, which replaced the plasma gun originally envisioned and made the rocket completely "electrodeless"—adding to durability and long life. A new patent was granted in 2002.
In 1995, the Advanced Space Propulsion Laboratory (ASPL) was founded at NASA Lyndon B. Johnson Space Center, in the Sonny Carter Training Facility. The magnetic mirror device was brought from MIT. The first plasma experiment in Houston was conducted with a microwave plasma source. Collaboration was established with University of Houston, UT-Austin, Rice University and other academic institutions.
In 1998, the first helicon plasma experiment was performed at the ASPL. VASIMR experiment (VX) 10 in 1998 achieved a helicon RF plasma discharge as great as 10 kW, VX-25 in 2002 as great as 25 kW, and VX-50 as great as 50 kW. In March 2000, the VASIMR group was given a Rotary National Award for Space Achievement/Stellar Award. By 2005 breakthroughs were obtained at ASPL including full/efficient plasma production and acceleration of the plasma ions. VX-50 proved capable of 0.5 newtons (0.1 lbf) of thrust. Published data on VX-50, capable of 50 kW of total radio frequency power, showed ICRF (second stage) efficiency to be 59% calculated by 90% NA coupling efficiency × 65% NB ion speed boosting efficiency.
Ad Astra Rocket Company (AARC) was incorporated on January 14, 2005. On June 23, 2005, Ad Astra and NASA signed the first Space Act Agreement to privatize VASIMR Technology. On July 8, 2005, Díaz retired from NASA after 25 years. Ad Astra’s Board of Directors was formed and Díaz became chairman and CEO on July 15, 2005. In July 2006, AARC opened its Costa Rica subsidiary in Liberia on the campus of Earth University. In December 2006, AARC-Costa Rica performed its first plasma experiment on the VX-CR device, using 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 tripled the prior record of the VX-50.
Model VX-100 was expected to have NB ion speed boosting efficiency of 80%. Instead, efficiency losses emerged from the conversion of DC electric current to radio frequency power and the energy consumption of the auxiliary equipment for the superconducting magnet. By comparison, 2009 state-of-the-art, proven ion engine designs such as NASA's High Power Electric Propulsion (HiPEP) operated at 80% total thruster/PPU energy efficiency.
200 kW engine
On October 24, 2008 the company announced that the plasma generation component 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, reached 98% efficiency. The helicon discharge used 30 kW of radio waves to turn argon gas into plasma. The remaining 170 kW of power was allocated for acceleration of plasma in the second part of the engine, via ion cyclotron resonance heating.
Based on data from 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 5,000 s using low cost argon propellant. One of the remaining untested issues was potential vs actual thrust—whether the hot plasma actually detached from the rocket. Another issue was waste heat management. About 60% of input energy became useful kinetic energy. Much of the remaining 40% is secondary ionizations from plasma crossing magnetic field lines and exhaust divergence. A significant portion of that 40% was waste heat (see energy conversion efficiency). Managing and rejecting that waste heat is critical.
Between April and September 2009, tests were performed on the VX-200 prototype with integrated 2-tesla superconducting magnets. They expanded the power range of the VASIMR to its operational capability of 200 kW.
Results presented in January 2011 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% was achieved, yielding overall system efficiency (DC electricity to thruster power) of 60% (since the DC to RF power conversion efficiency exceeds 95%) with argon propellant.
The 200 kW VX-200 had executed more than 10,000 engine firings by 2013, while demonstrating greater than 70% thruster efficiency—relative to RF power input—with argon propellant at full power.
The VF-200 flight-rated thruster consists of two 100 kW VASIMR units with opposite magnetic dipoles so that no net torque is applied to the space station when the thruster magnets are working. The VF-200-1 is the first flight unit and was slated to 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 engine . In December 8, 2008, NASA and AARC entered into a Space Act Agreement that could lead to conducting a space flight test of the engine on the ISS.
From 2008 Ad Astra was working on placing and testing a flight version of the VASIMR thruster for the International Space Station (ISS). The first related agreement with NASA was signed on December 8, 2008; . a formal preliminary design review took plaace on 26 June 2013.
In March 2, 2011, Ad Astra and NASA Johnson Space Center signed a Support Agreement to collaborate on research, analysis and development on space-based cryogenic magnet operations and electric propulsion systems currently under development by Ad Astra. As of February 2011, NASA had assigned 100 people 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.
However, in 2015 NASA ended plans for flying the VF-200 to the ISS. A NASA spokesperson stated that the ISS "was not an ideal demonstration platform for the desired performance level of the engines". Ad Astra stated that tests of a VASIMR thruster on the ISS would remain an option after a future in-space demonstration. Work with NASA continued in 2015 under NASA's NextSTEP program with planning for a 100-hour vacuum chamber test of the VX-200SSTM thruster.
Since the available power from the ISS is less than 200 kW, the ISS VASIMR would have included a trickle-charged battery system, allowing for 15-minute pulses of thrust. Testing of the engine on the ISS would have been 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 might 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.
In March 2015, Ad Astra announced the award of a $10 million award from NASA to advance the technology readiness of the next version of the VASIMR engine, the VX-200SS (SS stands for steady state) to meet the needs of deep space missions.
In August 2016, Ad Astra announced completion of the milestones for the first year of its 3-year contract with NASA. This will allow first high-power plasma firings of the engines, with a stated goal to reach 100 hr and 100 kW by mid-2018. In August 2017, the company reported completing its Year 2 milestones for the VASIMR electric plasma rocket engine. NASA gave approval for Ad Astra to proceed with Year 3 after reviewing completion of a 10-hour cumulative test of the 200SS™ rocket at 100 kW.
VASIMR is not suitable to launch payloads from the Earth's surface because it has a low thrust-to-weight ratio and requires an ambient vacuum. Instead, the engine would function as an upper stage for cargo, reducing fuel requirements for in-space transport. The engine is anticipated 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, and 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). In 2010 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.
Space tug/orbital transfer vehicle
The most important near-term application of VASIMR-powered spacecraft is cargo transport. Studies have shown that, despite longer transit times, VASIMR-powered spacecraft will be much more efficient than traditional integrated chemical rockets when 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 envisions 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 expended. A comparable OTV would employ 5 VF-200 engines powered by a 1 MW solar array. To do the same job, a VASIMR-powered OTV would need to expend only about 8 metric tons of argon propellant. The total mass of such an electric OTV would be in the range of 49 t (outbound & return fuel: 9 t, hardware: 6 t, cargo 34 t).
OTV transit times can be reduced by carrying lighter loads and/or expending more argon propellant with VASIMR throttled up to higher thrust at less efficient (lower Isp) operating conditions. 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 specifications' 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.
Mars in 39 days
In order to conduct a manned trip to "Mars in just 39 days", the VASIMR would require an electrical power level available only by nuclear propulsion (specifically the nuclear electric type) by way of nuclear power in space. This kind of nuclear fission reactor might use a traditional Rankine/Brayton/Stirling conversion engine such as that used by the SAFE-400 reactor (Brayton cycle) or the DUFF Kilopower reactor (Stirling cycle) to convert heat to electricity. However, the vehicle might be better served with non-moving parts and non-steam based power conversion using a thermocell technology of the thermoelectric (including graphene-based thermal power conversion), pyroelectric, thermophotovoltaic, thermionic magnetohydrodynamic type. Thermoelectric materials are also option for converting heat energy (being both black-body radiation and the kinetic thermal vibration of molecules and other particles) to electric current energy (electrons flowing through a circuit). Avoiding the need for "football-field sized radiators" (Zubrin quote) for a "200,000 kilowatt (200 megawatt) reactor with a power-to-mass density of 1,000 watts per kilogram" (Díaz quote) this reactor would require efficient waste heat capturing technology. For comparison, a Seawolf-class nuclear-powered fast attack submarine uses a 34 megawatt reactor, and the Gerald R. Ford-class aircraft carrier uses a 300 megawatt A1B reactor.
Mars manned mission advocate Robert Zubrin has called VASIMR a hoax, claiming that it is less efficient than other electric thrusters that are now operational. He also believes that electric propulsion is not necessary to get to Mars; therefore, budgets should not be assigned to develop it. His second critique concentrates on the lack of a suitable power source. Ad Astra responded in a press release:
In the near term, using solar-electric power at levels of 100 kW to 1 MW, VASIMR propulsion could transfer heavy payloads to Mars using only one to four first-generation thrusters in relatively simple engine architectures.[...] It is abundantly clear that the nuclear reactor technology required for such missions [fast manned Mars transport] is not available today and major advances in reactor design and power conversion are needed.
As a response to VASIMR being labeled as a hoax by Zubrin, Ad Astra added a section to their FAQ:
It [the hoax claim] was made by an individual who never visited the MIT or NASA facilities where the research originated or the Ad Astra Rocket Company laboratories where the development continues and, despite an open invitation, has never bothered to see any of the prototypes being fired in the vacuum chamber and reviewed the copious amounts of calibrated and validated data available. It is unclear whether this person has read or understood the numerous peer-reviewed and published articles regarding this work.
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The VX-200 will provide the critical data set to build the VF-200-1, the first flight unit, to be tested in space aboard the International Space Station (ISS). The electrical energy will come from ISS at low power level, be stored in batteries and used to fire the engine at 200 kW.
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About 100 NASA people are now working with AAR on the project. AAR is negotiating with NASA for a launcher and the leading contender currently is Orbital Science's Taurus II. The VASIMR system will provide re-boost for the station plus it can also offer access to its 50 kWh (180 MJ) batteries when not in operation. The thruster can fire for up to 15 minutes at 200 kW. The lab prototype has exceeded thruster output by a factor of two over the requirements set for the ISS version.
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