Variable Specific Impulse Magnetoplasma Rocket

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VASIMR test bed

The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) is an electro-magnetic thruster for spacecraft propulsion. It uses radio waves to ionize a propellant and magnetic fields to accelerate the resulting plasma to generate thrust.

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-American scientist and former astronaut Franklin Chang-Diaz created the VASIMR concept and has been working on its development since 1979.

Basic design

The Variable Specific Impulse Magnetoplasma Rocket, sometimes referred to as Electro-thermal Plasma Thruster, uses radio waves to ionize and to heat propellant and magnetic fields to accelerate the resulting plasma to generate thrust. This type of engine can be viewed as a variation (differing in the method of plasma acceleration) of the electrodeless plasma thruster. Both types of engine do not have any electrodes. The main advantage of such designs is elimination of problems with electrode erosion. Furthermore, since every part of a VASIMR engine is magnetically shielded and does not come into direct contact with ionized plasma, the potential durability of this engine design is greater than other ion engine designs.

The engine design encompasses three parts: 1) turning gas into plasma via helicon RF antennas; 2) energizing plasma via further RF heating in an ICRF booster; 3) using electromagnets to create a magnetic nozzle to convert the plasma's built-up thermal energy into kinetic force. 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.

VASIMR diagram

It is worth mentioning that, 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 & compact magnet arrangement in the engine.

Performance

Current VASIMR designs should be capable of producing specific impulses ranging from 3,000 to 30,000 seconds (jet velocities 30 to 300 km/s). The low end of this range is comparable to some existing ion thruster designs. By adjusting the manner of plasma production and plasma heating, a VASIMR can control the specific impulse and thrust. VASIMR is also capable of processing much higher power levels (Megawatts) than existing ion thruster electric propulsion designs. Therefore, it can provide orders of magnitude higher thrust, provided a suitable power source.

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 transport.
in-space refueling.
in space resource recovery.
ultra high speed transportation for deep space missions.

Other applications for VASIMR such as rapid transportation of people to Mars requires a very high power, low mass energy source, such as nuclear.

In August 2008, Tim Glover, Ad Astra director of development, has 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.

Current development

The principal developer of the VASIMR has been the Ad Astra Rocket Company. Currently, efforts have been focused on improving the overall efficiency of the engine by scaling up power levels. According to company's data, current VASIMR efficiency is at 67%. Published data on the VX50 engine, capable of processing 50kW of total radio frequency power, shows efficiency to be 59% calculated as: 90% N_A ion generation efficiency × 65% N_B ion speed boosting efficiency. Model VX100 is expected to have an overall efficiency of 72% by improving the N_B ion speed boosting efficiency to 80%. There are, however, additional (smaller) inefficiency losses related to the conversion of DC electric current to radio frequency power and also to the superconducting magnets' energy consumption. By comparison, current state-of-the-art, proven ion engine designs like NASA's HiPEP operate at 80% total thruster/PPU energy efficiency. Published test data on VASIMR engine model VX50 show it to be capable of 0.5 N thrust. The Ad Astra Rocket Company plans to ground test a prototype rocket in early 2008; the VX-200 rated at 200 kW total radio frequency power, to demonstrate the required efficiency, thrust and specific impulse.

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, has 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, we can expect that the VF-200 engine (to be installed on ISS) will have a system efficiency of 60-65% and thrust level of 5N. Optimal specific impulse appears to be around 5000s using low cost argon propellant. The specific power is estimated at 1.5 kg/kW meaning that this version of the VASIMR engine will weigh only about 300 kg. One of the remaining untested issues is: potential vs actual thrust. That is, whether or not the hot plasma actually gets detached from the rocket. This will be confirmed in 2009 when a VX-200 engine will be installed and tested in a large enough vacuum chamber. Another issue is waste heat management (60% efficiency means about 80kW of unnecessary heat) critical to allowing for continuous operation of VASIMR engine.

On December 10, 2008 Ad Astra Company 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). Its launch is expected to be in 2011-2012.

The ISS VASIMR engine will operate in burst mode. Since ISS's power generation is not great enough, the system will include a trickle-charged battery system allowing for 10 min pulses of thrust. This however, is expected to be sufficient to maintain ISS altitude, eliminating the need for costly, periodic chemical rocket reboosting operations.

As of 31st May 2009, tests have begun on the VX-200 prototype with fully integrated superconducting magnets. They intend to expand the power range of the VASIMR up to its full operational capability. If these tests prove successful, Ad Astra will develop a flight capable VASIMR to be installed on the ISS.

Space tug : 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 OTV (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 long 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 tonnes 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 tonnes of argon propellant. The OTV transit times can be reduced by flighting with lighter loads and/or expanding more argon propellant with VASIMR throttled down to lower Isp. For instance, an empty OTV on the return trip to Earth covers the distance in about 23 days at optimal specific impulse of 5000s or in about 14 days at Isp of 3000s.

External links

NASA documents

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Published in July 2009.

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