Variable Specific Impulse Magnetoplasma Rocket

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.[3]

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.[6] 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.

According to Ad Astra as of 2015, the VX-200 engine requires 200 kW electrical power to produce 5 N of thrust, or 40 kW/N.[5] In contrast, the conventional NEXT ion thruster produces 0.327 N with only 7.7 kW, or 24 kW/N.[5] Electrically speaking, NEXT is almost twice as efficient, and successfully completed a 48,000 hours (5.5 years) test in December 2009.[7][8]

New problems also emerge with VASIMR, such as interaction with strong magnetic fields and thermal management. The inefficiency with which VASIMR operates generates substantial waste heat that needs to be channeled away without creating thermal overload and thermal stress. The superconducting electromagnets necessary to contain hot plasma generate tesla-range magnetic fields[9] that can cause problems with other onboard devices and produce unwanted torque by interaction with the magnetosphere. To counter this latter effect, two thruster units can be packaged with magnetic fields oriented in opposite directions, making a net zero-torque magnetic quadrupole.[10]

The required power generation technology for fast interplanetary travel does not currently exist and is not feasible with current state-of-the-art technology.[11]

In 1998, the first helicon plasma experiment was performed at the ASPL. VASIMR experiment (VX) 10 in 1998 achieved a helicon RF plasma discharge of up to 10 kW and VX-25 in 2002 of up to 25 kW. By 2005 progress at ASPL included full and efficient plasma production and acceleration of the plasma ions with the 50 kW, 0.5 newtons (0.1 lbf) thrust VX-50.[13] Published data on the 50 kW VX-50 showed the electrical efficiency to be 59% based on a 90% coupling efficiency and a 65% ion speed boosting efficiency.[13]

The 100 kilowatt VASIMR experiment was successfully running by 2007 and demonstrated efficient plasma production with an ionization cost below 100 eV.[14] VX-100 plasma output tripled the prior record of the VX-50.[14]

The VX-100 was expected to have an ion speed boosting efficiency of 80%, but could not achieve this efficiency due to losses from the conversion of DC electric current to radio frequency power and the auxiliary equipment for the superconducting magnet.[13][15] In contrast, 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.[16]

VX-200 plasma engine at full power, employing both stages with full magnetic field

On 24 October 2008, the company announced in a press release that the helicon plasma generation component of the 200 kW VX-200 engine 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.[17]

Based on data from VX-100 testing,[9] it was expected that the VX-200 engine would have a system efficiency of 60–65% and a potential 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 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.[18]

Between April and September 2009, 200 kW tests were performed on the VX-200 prototype with integrated 2-tesla superconducting magnets.[19] During November 2010, long duration, full power firing tests were performed, reaching steady state operation for 25 seconds and validating basic design characteristics.[20]

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 I_sp of 5000 s. Based on these data, thruster efficiency of 72% was achieved,[21] yielding overall system efficiency (DC electricity to thruster power) of 60% with argon propellant.[22] VX-200 generates a thrust of around 5.4 N at 200 kW total RF power, and 3.2 N at 100 kW RF power.[21]^:5

The 200 kW VX-200 had executed more than 10,000 engine firings with argon propellant at full power by 2013, demonstrating greater than 70% thruster efficiency relative to RF power input.[23]

In March 2015, Ad Astra announced a $10 million award from NASA to advance the technology readiness of the next version of the VASIMR engine, the VX-200SS to meet the needs of deep space missions.[24] The SS in the name stands for "steady state", as a goal of the long duration test is to demonstrate continuous operation at thermal steady state.[25]

In August 2016, Ad Astra announced completion of the milestones for the first year of its 3-year contract with NASA. This allowed for first high-power plasma firings of the engines, with a stated goal to reach 100 hr and 100 kW by mid-2018.[26] 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 VX-200SS engine at 100 kW. It appears as though the planned 200 kW design is being run at 100 kW for reasons that are not mentioned in the press release.[27]

In August, 2019[28] Ad Astra announced the successful completion of tests of a new generation radio-frequency (RF) Power Processing Unit (PPU) for the VASIMR engine, built by Aethera Technologies Ltd. of Canada. Ad Astra declared a power of 120 kW and >97% electrical-to-RF power efficiency, and that, at 52 kg, the new RF PPU is about 10x lighter than the PPUs of competing electric thrusters (power-to-weight ratio: 2.31 kW/kg)

On 10 December 2007, AARC and NASA signed an Umbrella Space Act Agreement relating to the space agency's potential interest in the engine.[31] On 8 December 2008, NASA and AARC entered into a Space Act Agreement that was intended to result in a space flight test of the engine on the ISS.[32] A formal preliminary design review took place on 26 June 2013.[23]

In 2 March 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.[33] By February 2011, about 100 NASA people were working with Ad Astra to integrate the VF-200 onto the International Space Station.[34] On December 16, 2013, AARC and NASA signed another five-year Umbrella Space Act Agreement.[35]

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.[36] 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.[37]

A possible application of VASIMR-powered spacecraft is cargo transport. As with all ion engines, higher efficiency is balanced against longer transit times. An orbital transfer vehicle (OTV) powered by a single VF-200 engine could 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 envisioned 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, expending 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 I_sp) 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 I_sp 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)[38] 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.

In order to conduct a proposed crewed trip to Mars in 39 days,[39] the VASIMR would require an electrical power level projected to be developed only by nuclear propulsion in an application of nuclear power in space.[40] This kind of nuclear fission reactor might use a traditional Rankine/Brayton/Stirling engine to convert heat to electricity. Examples of such reactors include the SAFE-400 and the DUFF Kilopower.

However, any power generation technology will produce waste heat. The necessary 200 megawatt reactor "with a power-to-mass density of 1,000 watts per kilogram" (Díaz quote) would require extremely efficient radiators to avoid the need for "football-field sized radiators" (Zubrin quote).[41] For comparison, a Seawolf-class nuclear-powered fast attack submarine uses a propulsion plant with a shaft output of 34 megawatts (MW), and the Gerald R. Ford-class aircraft carrier uses two A1B reactor plants, each with a 700 MW thermal output and an approximately 385 MW useful output.[42] Naval reactors have the advantage of an essentially infinite heat sink - the ocean - which can be easily accessed, and can be constructed of heavy materials such as steel and lead.