Variable Specific Impulse Magnetoplasma Rocket


Variable Specific Impulse Magnetoplasma Rocket
Artist's impression of several VASIMR engines propelling a craft through space

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 scientist and former astronaut Franklin Chang-Diaz created the VASIMR concept and has been working on its development since 1977.[1]

Contents

Design and operation

Vasimr schematic

The Variable Specific Impulse Magnetoplasma Rocket, sometimes referred to as the Electro-thermal Plasma Thruster or Electro-thermal Magnetoplasma Rocket, uses radio waves[2] to ionize and heat propellant, which generates plasma that is accelerated using magnetic fields to generates 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. The main advantage of such designs is elimination of problems with electrode erosion that cause rival designs of ion thrusters which use electrodes to have a short 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.[1]

The engine design encompasses three parts: turning gas into plasma via helicon RF antennas; energizing plasma via further RF heating in an ion cyclotron resonance frequency (ICRF) booster; and 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.[3]

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.[3]

VASIMR does not use electrodes and 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.[citation needed]

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.[4] 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 quadrapole.[5]

Criticisms of VASMIR

Robert Zubrin is critical of the VASMIR design listing several serious problems and points out that it is less efficient than other electric based thrusters which are now operational. Zubrin believes that electric propulsion is not best way to get to Mars and therefore budgets should not be assigned to develop it. His second point of criticism concentrates on lack of suitable power source.[6][7]

At the Mars Society 2011 Dallas Convention, Zubrin invited the VASIMIR inventor, Dr. Franklin Chang Diaz, to debate the merits of his thruster. Dr. Diaz did not attend to defend his ideas. [8]

Research and development

The testing vacuum chamber, containing the 50kW VASIMR

After many years researching the concept with NASA, Franklin Chang-Diaz set up the Ad Astra Rocket Company in January 2005 to begin development of the VASIMR engine. Later that year, the company signed a Space Act Agreement with NASA, and were granted control of the Advanced Space Propulsion Laboratory.[9] In this lab, a 50 kW prototype was constructed, and underwent testing in a vacuum chamber. Later, a 100 kW version was developed, and this was followed by a 200 kW prototype. After a long period of rigorous testing in a 150 m3 vacuum chamber, the latest configuration was deemed space-worthy, and it was announced that the company had entered into an agreement to test the engine on the International Space Station, in or before 2013.

The first VASIMR engine model VX-50 proved to be capable of 0.5 newtons (0.1 lbf) thrust.[citation needed] Published data on the VX-50 engine, capable of processing 50 kW of total radio frequency power, showed thruster 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.[citation needed]

Model VX-100 was expected to have a thruster efficiency of 72% by improving the NB ion speed boosting efficiency to 80%.[10][11] 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.[12] Ongoing improvement to the the engine design concentrate on increasing power level which should lead to higher efficiency level. In September 2011 the Ad Astra Company has published the results of first full scale tests of VX-200 engine. They confirm theoretical perditions measuring thruster efficiency at 72% [+/-9%] at 200kW power level with specific impulse of 4900s with corresponds to trust level of 6N. This represents much higher thrust and power level than any other currently existing prototype of electric propulsion system.

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.[13]

Based on data released from previous VX-100 testing,[4] 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 5000s using low cost argon propellant. The specific power estimated at 1.5 kg/kW meant that this version of the VASIMR engine would weigh only about 300 kg. One of the remaining untested issues was potential vs actual thrust; that is, whether the hot plasma actually detached from the rocket. Another issue was waste heat management (60 % efficiency means about (100 %-60 %)/100 %*200 kW = 80 kW of unnecessary heat) critical to allowing for continuous operation of 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.[14]

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.[15] 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 70 % has been deemed by Ad Astra to be achievable, yielding an overall system efficiency (DC electricity to thruster power) of 60 % (since the DC to RF power conversion efficiency exceeds 95 %).[16]

Testing on the space station

On December 8, 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).[17] As of February 2011, its launch is anticipated to be in 2014,[18][19] though it may be later.[9] The Taurus II has been reported as the "top contender" for the launch vehicle.[18] 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. If the tests of VASIMR reboosting of the ISS goes according to plan, the increase in specific impulse could mean that the cost of fuel for altitude reboosting will be one-twentieth of the current $210 million annual cost.[9] Hydrogen generated by the ISS as a by-product is currently vented into space but will be redirected to the VASIMR to act as the fuel in place of the current Argon.

VF-200

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 thrusters are firing. The VF-200-1 is the first flight unit and will be tested in space attached to the ISS.[5]

NASA partnership

As of February 2011, NASA has 100 people assigned to the project to work with Ad Astra to integrate the VF-200 onto the space station.[18]

Potential future applications

VASIMR magnetic field

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 months to days.[20]

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.[21]

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 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 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)[22] allowing almost double the cargo capacity compared to chemically propelled vehicle but requiring even bigger solar arrays (or other source of power) capable of providing 2 MW.

As of October 2010, 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.[23]

See also

References

  1. ^ a b Billings, Lee (September 29, 2009). "A Rocket for the 21st Century". Seed. http://seedmagazine.com/content/article/a_rocket_for_the_21st_century/. Retrieved September 30, 2009. 
  2. ^ Noelle Stapinsky (May 13, 2010), Nautel’s space oddity (RF generator for Ad Astra), Canadian Manufacturing, http://www.canadianmanufacturing.com/plant/features/industry/article.jsp?content=20100513_153743_3324 
  3. ^ a b Tim W. Glover, et al. (February 13–17, 2005). "Principal VASIMR Results and Present Objectives". Space Technology and Applications International Forum. http://www.adastrarocket.com/TimSTAIF2005.pdf. Retrieved 2010-02-27. 
  4. ^ a b Jared P. Squire, et al. (September 5–6, 2008). "VASIMR Performance Measurements at Powers Exceeding 50 kW and Lunar Robotic Mission Applications". International Interdisciplinary Symposium on Gaseous and Liquid Plasmas. http://www.adastrarocket.com/ISGLP_JPSquire2008.pdf. Retrieved 2010-02-27. 
  5. ^ a b "International Space Station Mission". Ad Astra Rocket Company. 2011. http://www.adastrarocket.com/aarc/VASIMRISS. Retrieved 2011-02-08. "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). It will consist of two 100 kW units with opposite magnetic dipoles, resulting in a zero-torque magnetic system. The electrical energy will come from ISS at low power level, be stored in batteries and used to fire the engine at 200 kW." 
  6. ^ http://www.spacenews.com/commentaries/110711-vasimr-hoax.html
  7. ^ http://www.marssociety.org/home/press/tms-in-the-news/thevasimrhoax
  8. ^ http://www.marssociety.org/home/join_us/convention/2011-convention-schedule
  9. ^ a b c "Executive summary". Ad Astra Rocket Company. 24 January 2010. http://www.adastrarocket.com/EXECUTIVE%20SUMMARY240110.pdf. Retrieved 2010-02-27. 
  10. ^ Edgar A. Bering, et al. (9–12 January 2006). "Recent Improvements In Ionization Costs And Ion Cyclotron Heating Efficiency In The VASIMR Engine". AIAA Aerospace Sciences Meeting and Exhibit. http://www.adastrarocket.com/AIAA2006.pdf. Retrieved 2010-02-27. 
  11. ^ Jared P. Squire, et al. (September 17–20, 2007). "High Power VASIMR Experiments using Deuterium, Neon and Argon". International Electric Propulsion Conference. http://www.adastrarocket.com/Jared_IEPC07.pdf. Retrieved 2010-02-27. 
  12. ^ Frederick W. Elliott, et al. (11–14 July 2004). "An Overview of the High Power Electric Propulsion (HiPEP) Project". AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit. ftp://ftp.grc.nasa.gov/users/ep/ion/publications/2004/onsite/aiaa-2004-3453.pdf. Retrieved 2010-02-27. 
  13. ^ Press release (October 24, 2008). "VASIMR VX-200 first stage achieves full power rating.". Ad Astra Rocket Company. http://www.adastrarocket.com/Release241008.pdf. Retrieved 2010-02-27. 
  14. ^ Press release (September 30, 2009). "VASIMR VX-200 reaches 200 kW power milestone.". Ad Astra Rocket Company. http://www.adastrarocket.com/Release_200kW_01Oct2009Final.pdf. Retrieved 2010-02-27. 
  15. ^ Benwl (December 15, 2010). "Video of VASIMR VX-200 firing for 25 seconds at full power rating.". Ad Astra Rocket Company. http://www.youtube.com/watch?v=BRS26DcQhq0. Retrieved 2011-01-04. 
  16. ^ VASIMR VX-200 Performance and Near-term SEP Capability for Unmanned Mars Flight, Tim Glover, Future in Space Operations (FISO) Colloquium, 2011-01-19, accessed 2011-01-31.
  17. ^ Nasa.gov - NASA Administrator Hails Agreement with Ad Astra (Dec. 17, 2008)
  18. ^ a b c Lindsay, Clark (2011-02-07). "RLV and Space Transport News". http://www.hobbyspace.com/nucleus/index.php?itemid=27038. Retrieved 2011-02-08. "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 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." 
  19. ^ Aviation Week - Vasimr Prototype Makes New Strides (Jun 16, 2010)
  20. ^ Morring, Frank (2010). "Commercial Route". Aviation Week & Space Technology (McGraw Hill) 172 (6): 20–23. 
  21. ^ Irene Klotz (7 August 2008). "Plasma Rocket May Be Tested at Space Station". Discovery News. http://dsc.discovery.com/news/2008/08/07/plasma-rocket.html. Retrieved 2010-02-27. 
  22. ^ Tim W. Glover, et al. (September 17–20, 2007). "Projected Lunar Cargo Capabilities of High-Power VASIMR Propulsion". International Electric Propulsion Conference. http://www.adastrarocket.com/Tim_IEPC07.pdf. Retrieved 2010-02-27. 
  23. ^ Rocket Company Launches Stock Offering, TicoTimes (San Jose, Costa Rica), 2010-10-01, accessed 2010-10-02.
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