Development of argon plasma rocket engine continues


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Capitalism in space: The argon-fueled plasma rocket engine being developed by the Ad Astra Rocket Company has advanced its test engine firings from 30 seconds long to five minutes long.

Their $9 million contract with NASA calls for a 100-hour-long engine firing by 2018. At the moment the company says they are on schedule to meet that goal.

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24 comments

  • Edward

    This is an exciting development. A Variable Specific Impulse Magnetoplasma Rocket (VASIMR) thruster can provide more thrust, although at lower efficiency, when it is deeper in a gravity well, such as in low Earth orbit. The same thruster can provide more efficiency, although lower thrust, when it is farther out of the gravity well. This allows a spacecraft to (relatively) quickly escape a planet then efficiently travel between planets.

    The efficiency is in terms of propellant mass needed, not necessarily the electrical energy used.

    Another possible advantage is that this type of thruster is expected to have a longer lifespan than ion thrusters.

    https://en.wikipedia.org/wiki/Variable_Specific_Impulse_Magnetoplasma_Rocket
    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.

  • Cotour

    Gravity, good old gravity, good old consistent gravity.

    I wonder if the human race will ever discover a way to neutralize it in some fashion?

  • Steve

    One thing to remember is that faster travel through the solar system means you also have to consume fuel to slow down if you’re planning on entering orbit or landing.

  • wayne

    Clarify please…
    Is this argon-ion or argon-plasma? or is that the same thing?

    -This is an extremely interesting, brief background, on ion-engine physic’s. Lots of factoids on Nobel gases & propulsion.
    Why Do Ion Thusters Use Xenon?
    KSP Doesn’t Teach…..
    Scott Manley 2017
    https://youtu.be/Cb_U_CbQ5sc
    (8:56)

  • Wayne: Others will know better than I, but from what I understand the argon is the fuel which is ionized to produce the plasma that is ejected to provide the thrust.

  • Dick Eagleson

    I’ve long hoped Ad Astra would pull its VASIMR rabbit out of the hat eventually. It’s been a long, bumpy road, but it would seem Big Things are now in near-term prospect.

    One of the reasons I like VASIMR so much is that, unlike most ion thruster designs, it was designed to run on Argon from the get-go. Xenon is very rare and quite expensive. Argon, on the other hand, is trivially producible in industrial quantity as a by-product of air liquefaction given that it accounts for roughly a half-percent of Earth’s atmosphere. Argon is also used in industrial quantity as a shielding gas for welding so one can already buy it by the tanker-load all over the world. With large, cheap methalox launchers about to debut (New Glenn, SpaceX BFR/BFS), Argon is going to be the high-efficiency, low-cost propellant of choice when embarking on voyages beyond cis-lunar space.

    Even better, Argon accounts for about one percent of the Martian atmosphere. So, in addition to methane, LH2 and LOX propellant, future Martians can also make a self-sustaining business supplying Argon reaction mass to the rest of the Solar System. Mars may never have that million-person metropolis Elon wants to build there, but it will likely be one of the principal propellant sources for the rest of extraterrestrial humanity. Mars may neverhave its own New York, but it seems likely to have several Galvestons.

  • wayne

    Mr. Z., Thank you, yes, you are correct. (I can’t believe I asked that question!) Ionized gas is Plasma by definition.

    Dick-
    Good stuff.
    (My son-in-law interned at Liquid Air Products in the early ’00’s.)

    tangentially– The Air Liquide Corp people have pure Argon available, in quantities “up to 15 metric tons per month.”
    https://industry.airliquide.us/argon

    Referencing the Scott Manley video-clip I referenced above;
    I’m confused as to the relative performance of either Argon or Xenon, the Mass, and specific-impulse achieved of both. But that’s just me, I need to re-watch it & jot down some notes.

  • wayne

    Way out of my bailiwick!

    But, this is a fascinating Topic, and I almost…. have my head wrapped around it. (almost):
    [I’m having Periodic Table flashbacks, from HS chemistry!!]

    Argon ionizes at 15.75 eV, Xenon at 12.13eV
    Argon- atomic mass = 39.95, Xenon- atomic mass = 131.29.

    With a 2,500 volt potential difference;
    Argon exhaust velocity about 100km/sec
    Xenon exhaust velocity about 55km/sec
    (Concurrently, I get completely lost when the Momentum of Xe/At are introduced into the equation.)
    The price differential between the gases, is intriguing as well.

    Q: What is the preferred electricity source for ion-engines?
    (I would think an RTG would be optimal?)

  • Joe

    Gas temperatures are crazy high from these ion plasma engines, this is a materials problem, what keeps this thing from melting down?

  • wayne

    Joe-
    (I’m just a rank amateur.)

    This might help–
    http://www.adastrarocket.com/aarc/VASIMR

    Looks like, the magnetic-field insulates the internal parts by confining the plasma. (??) And there are no electrodes in physical contact with the plasma.

  • Joe

    Thanks Wayne, so much to learn!

  • wayne

    Joe-
    Thanks. I know I’m (way) outa my league on this specific stuff, but I’m trying! (and I almost think I understand it.)(highly enjoyed taking apart burnt out vacuum-tubes as a kid! How to they work?!ha.)

    referencing this Scott Manley dude, I reference; he’s an actual Physicist by education, software developer for his day-job, and Kerbal Space program wizard on the interweb.
    Gameplay & Fun aside, he has a lot of straight educational shorts that are very good, challenging but clearly presented. Mainly rocket related but not exclusively.

  • wodun

    Wayne, you might be interested in this. Dr James Woodward Not VASIMR but…

    Dr. James Woodward is an adjunct professor of physics professor and history professor at Cal St. Fullerton and is widely published on the Mach-Effect. Dr. Woodward wants to create technology that will allow mankind to cheaply travel to the stars. Dr. Woodward prefers to give credit of these propulsion devices to Mach. Therefore Dr. Woodward does not refer to the theory as the Woodward Effect (as do others) and instead uses the phrase Mach Effect.

  • Tom Bilings

    wayne asked: “Why Do Ion Thusters Use Xenon?”

    Just as temperature of the trust chamber inner surface is a basic limitation on chemical rocket engines, ion rocket engines have their own basic limitation, called “Space Charge”.

    https://en.wikipedia.org/wiki/Space_charge

    The result of this is that only so many ions of the same charge can exist in a given volume at one time. Once that limit is hit inside the acceleration volume, things start to go badly wrong. That, in turn, means that only so many ions can be in the acceleration volume of an ion engine. Once that limit is hit, either you have to expend 4 times the energy to double the thrust by doubling the velocity (Kinetic energy= mass*V^2), or you have to have more mass in each ion particle. The deeper you are in any gravity well, the greater your gravity losses are from the gradual accelerations of of ion engines. So, you want higher thrust, without having a massive power plant to produce more electricity, then Xenon is one way to do that.

    The reason is that Xenon is a noble gas, whose stable isotope has an atomic mass of 136, much higher than most propellants. Since it is a noble gas, you can recollect it and reuse it when you are developing the engine on the ground, without having to clean the vacuum chamber of a poisonous substance after each run before you can work on the engine. Mercury was proposed before, but its toxicity made it horrid to work with in ground test equipment. Iodine, which vaporizes at 485º is now being used in development and is substantially less toxic than Mercury, but more than Xenon.

    Xenon has the one problem of being very expensive, but as long as the majority of it is expended in ground tests for development, as has been the case so far, it can be recycled and the high costs do not happen until you actually fly in Space. IMHO, that operational cost, as more operational ion engines are in use, will eventually make propellants like Iodine more attractive. However, since Congress began structuring the aerospace industry 70 years ago so that its major profit center is development, not operations, we have not seen this have any effect just yet.

    The Argon in the VASIMR engines has the nice advantage of being a noble gas available on Mars as well as Earth at cheap rates. Also, since it’s used as a multi-charge plasma, rather than as a same-charge ion flow, its atomic mass of 40 is little disadvantage in comparison to Xenon.

  • wayne

    wodun-
    thank you.

    Tom– great stuff.
    Highly informative facts.

  • Edward

    wayne,
    You wrote: “Ionized gas is Plasma by definition.

    I would phrase it the other way around. A plasma is a very hot gas, so hot that it becomes ionized. You can have ions without a plasma, but a plasma requires that the gas has become ionized. When you think plasma, also think hot. The solar wind is essentially a plasma.

    As you noticed, argon can be accelerated to almost twice the speed as xenon, which means that the mass efficiency of argon is almost twice as much, because mass efficiency (specific impulse, or Isp) is directly related to the velocity of the propellant exhaust. However, as Tom Billings noted, the energy required to accelerate the propellant is related to the square of the velocity, so argon takes almost four times as much energy to get that higher Isp. Mass and energy requirements are a trade-off that must be considered during mission planning. Solar arrays may be a superior energy source in the inner solar system (e.g. out to Mars, although Curiosity uses a nuclear power source at Mars), and a nuclear power source may be superior farther out (e.g. Jupiter and farther, although Juno uses solar arrays at Jupiter).

    Steve is correct. The propellant needed to enter orbit or land at the destination must be considered. The faster the journey, the faster the approach, and the more propellant needed to slow down during arrival. It is yet another aspect of mission planning trade-offs.

  • wayne

    Edward-
    gotcha. thank you.

  • Big gap between 5 minutes and 100 hours. Fingers crossed.

    What I’m dying to know, what sort of velocity can we expect from 100 hours at a power level of 100 kilowatts??????

  • Edward

    H. Hunter asked: “What I’m dying to know, what sort of velocity can we expect from 100 hours at a power level of 100 kilowatts?

    https://en.wikipedia.org/wiki/Variable_Specific_Impulse_Magnetoplasma_Rocket#200_kW_engine
    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 5000s. 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.

    With an exhaust velocity of 50 km/s, there should be some rather nice acceleration of the spacecraft for the (argon) propellant expended. Of course, the spacecraft’s velocity change after 100 hours depends upon the mass of the spacecraft, but the Wikipedia article suggests that the 200 kilowatt engine should produce 5 Newtons of thrust: “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.” I like that it is 60% efficient in the use of electricity.

    Specific impulse (Isp) is measured in seconds, and the easiest way to understand it is that a 5 Newton (N) thruster with an Isp of 5000 seconds would take 5000 seconds to expend 5000 Newtons of (argon) propellant. Another example is the Saturn V first stage F1 engines, all five of which generated, at sea level, 7.9 million pounds of thrust at an Isp of 263s for 165 seconds, taking the rocket to a speed of about 1.5 miles per second. They burned almost 5 million pounds of fuel and oxidizer during the first stage of each launch.

    An Isp of 5000 seconds means that the VX-200 thruster would take 83 minutes and 20 seconds to expend 1/2 kg of propellant while producing 5 Newtons of force. Over the course of 100 hours, the thruster would expend 36 kg of argon.

    (Comparing this to the 3 thousand tonne Saturn V first stage, about 1/19th as much propellant, about 263,000 pounds, would have been needed — but of course, the VASIMR thruster couldn’t have gotten the Saturn off the ground.)

    Let’s assume that there is one thruster for every 5 tonnes of spacecraft, then the acceleration at 5N thrust would be 0.001 meters/sec. Over the course of 100 hours (360,000 seconds), that would be 360 meters per second (if I have the decimal places right), or about 0.2 miles per second.

    Assuming that the 100 kw thruster produces 1/2 that thrust, then 0.1 miles per second would be the expected velocity change.

  • Edward: You’re the math guy. Can you do the math to give us an idea how quickly such an engine could get a spacecraft to Mars?

  • wayne

    Edward-
    thanks for expending the effort! highly interesting.

    Q: (just thinking out loud)
    -say we run it for 200 hours, do we get a linear increase in velocity, or what?

  • Edward

    Robert,
    It is much easier to calculate elliptical (non-powered) orbits, and I am way out of practice, having taken one class on the subject two decades ago. Calculating a trip that is powered the whole way would be quite hard to do.

    There is the acceleration away from the first planet, the trip where the sun’s gravity is dominant, and the deceleration into orbit at the destination planet. These calculations are relatively easy (emphasis on “relatively”) for instantaneous rocket burns and elliptical (or hyperbolic) orbits, but a powered trip constantly changes the trajectory.

    Leaving Earth (a great title for a book, by the way http://behindtheblack.com/books/leaving-earth/ ), you need a rocket burn, preferably at perigee, that puts you in a hyperbolic orbit faster than escape velocity and with enough extra velocity to get you to Mars. Once the Sun’s gravity becomes dominant, you calculate an elliptical orbit (such as a Hohmann Transfer Orbit) that gets you to Mars. As you approach Mars and its gravity becomes dominant, you calculate the hyperbolic approach orbit and the rocket burn that you need to puts you into an elliptical or circular orbit around the planet.

    That’s the easy calculation, and it is much more fun to remember the hours spent to calculate it for my Orbital Mechanics class than it was to actually make the calculations.

    Of course, these days there are easier computer programs for this type of mission planning (although I did not see anything about determining orbits for ion-propulsion spacecraft; maybe that will be a future feature).
    http://www.agi.com/orbit-determination-tool-kit

    However, the following link is an article that suggests that with a nuclear power supply, VASIMR engines theoretically could make the trip to Mars in 39 days.
    https://www.newscientist.com/article/dn17476-ion-engine-could-one-day-power-39-day-trips-to-mars/

    I have usually heard figures closer to three months or so, but those were for a more generic ion thruster. I’m not sure what it would take for a manned spacecraft, which would necessarily be heavy and hard to push with wimpy ion thrusters. The author of the book “The Martian” did some research on that application of ion thrusters. His hypothetical spacecraft, Hermes, took 124 days to arrive at Mars, accelerating at 2mm/sec.
    https://www.insidescience.org/news/inside-spaceflight-martian

    Since orbital mechanics is so much fun (unless you are actually doing the calculations), here is another option that someone calls “ballistic capture transfer, or a weak stability boundary (WSB) transfer” (no math in this article to tangle us up).
    https://www.space.com/30749-the-martian-faster-way-to-mars.html

    wayne,
    The acceleration would be mostly constant, although as the propellant is expended and the spacecraft becomes lighter, the acceleration would increase over time proportional to the craft’s original mass relative to the lighter mass (without the expended propellant mass).

    Last year, I read a science fiction novel called “Saturn Run” in which a VASIMR engine was used. The authors seemed to do quite a bit of research, and they noted several technical problems to overcome, such as the need to radiate away a lot of excess heat from their nuclear reactor. They (hypothetically) got all the way to Saturn in about a year. The farther you go, the more time you have to accelerate, so you get to higher speeds and you save more time over the conventional unpowered orbital trajectories.

  • Edward, thank you for taking the time to go through all that for us. I almost understand it. VELOCITY, not acceleration, would be .1 miles/second?

  • wayne

    Edward-
    Good stuff! (highly appreciate your efforts)
    I almost understand it as well.

    Purely for fun….
    One Giant Leap:
    “If History Had Gone Differently: Mars in 1981.”
    Kerbal Space Program
    https://youtu.be/lnId8E–ZN4
    (25:02)

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