back to article Japan successfully propels steam-powered spacecraft

Japan's space agency has successfully used water to propel a spacecraft and claimed it represents "the world's first successful orbit control beyond low-Earth orbit using a water propellant propulsion system." The craft in question is EQUULEUS, the 6U CubeSat that rode along on NASA's Orion mission. After flying past the Moon …

  1. Sorry that handle is already taken. Silver badge


    Technically, the water isn't the fuel, it's the working fluid and propellant. Not all rockets use a fuel but when they do, it's the energy source and its reaction products are the propellant.

    The demonstration of water-propelled engines strikes me as one of those "obvious in hindsight" things that could turn out to be very useful for very cheap smallsats, for which even the use of noble gases is (apparently) not viable.

    1. Lars Silver badge

      Re: EQUULEUS

      Yes, like claiming a Jet ski uses water as fuel.

    2. DJO Silver badge

      Re: EQUULEUS

      Came for the same pedant pendant although to be technically correct (the best kind of correct) the water is "reaction mass".

      Water is cheap but low mass which is why Xenon is often used, not so cheap but easy to handle and a lot heavier (nearly 6 times heavier than water) so you can get a lot more thrust from the same volume of reaction mass. If when designing your craft if volume is more important than mass then Xenon is the way to go, if mass is more important than volume then water is good.

      Also with water you have to stop it freezing which adds complexity over gaseous reaction mass.

      1. ArrZarr Silver badge

        Re: EQUULEUS

        Isn't radiating excess heat the problem in space? Could dump waste heat into the water tanks and use water vapor as the propellant. Same mass but free energy.

        Excuse me while I start my designs for a device powered by a literal steam engine in space.

        1. DJO Silver badge

          Re: EQUULEUS

          Yes but "excess heat" is only an issue when doing things that generate heat, most of the time a small craft will be coasting with instruments on standby and then the problem is keeping everything warm enough to come out of standby.

          1. bombastic bob Silver badge

            Re: EQUULEUS

            some kind of carefully designed "thermos bottle" might retain heat well enough (on small amounts of propellant) to make this practical, so that you collect heat whenever it is practical to do so, then carefully retain it inside the specially designed tank containing a limited amount of propellant, enough to maneuver anyway. Remaining energy could also be accumulated from solar panels when batteries are fully charged (i.e. heat it electrically with spare trons).

            in this case you need no vacuum container (space vacuum works) but you'd minimize the thermal conduction area and insulate as much as possible to avoid radiative cooling.

            1. DJO Silver badge

              Re: EQUULEUS

              Insulation can and is added but you will still need active heating unless your payload is constantly powered up. Insulation adds weight and volume both of which are at a premium.

              Every space probe is different and has different requirements and restrictions, the balance between active heating and passive insulation will be optimised for each use case.

      2. John Brown (no body) Silver badge

        Re: EQUULEUS

        "Also with water you have to stop it freezing which adds complexity over gaseous reaction mass."

        On the other hand, there's likely a lot of frozen water already out there in space that eventually may be able to be collected in enough quantity to use as further reaction mass so long as you have enough fuel to do the convert to steam bit, eg solar panels or nuclear.

        1. Michael Wojcik Silver badge

          Re: EQUULEUS

          This is one of the plot points of Stevenson's Seveneves. I don't want to post a bunch of spoilers just in case (though the novel's been out for a while), but some of the characters find themselves in need of reaction mass, and fortunately some is available in the form of a not-too-distant comet.

          (This part of the book contains one of its best lines: "Physics is physics, but some physics is nuclear physics".)

      3. M.V. Lipvig Silver badge

        Re: EQUULEUS

        The freezing is what I'm wondering about. Using it as a reaction mass means spraying it out the opposite side they want the craft to move, but at those temps the water would freeze instantly, possibly freezing inside the nozzle before it actually leaves the spacecraft. Can't wait to see the real-world results on this test.

    3. bombastic bob Silver badge

      Re: EQUULEUS

      might get more efficiency if you can put something in the water that retains more heat, weighs more than pure water, does not gunk things up,l and does not significantly increase the boiling point. That way you get more impulse for the same wattage, and fuel tanks can be smaller.

      an alcohol of some kind might do the trick, maybe propanol/isopropyl or heavier...?

      1. DJO Silver badge

        Re: EQUULEUS

        You are not going to get more thrust for the same power, it does not (and can not) work like that.

        The issue is thrust per volume unit of reaction mass. Water is OK for short lived probes or ones with plenty of space, Xenon at nearly 6 times the density is excellent if space is at a premium and being inert is not too tricky to contain, Iodine at nearly 5 times the density of water is an interesting reaction mass, it's common and easily vaporised and being stored in solid form makes containment really simple and with no liquid sloshing around guidance is simplified.

      2. John Brown (no body) Silver badge

        Re: EQUULEUS

        Beer powered spaceship? That gassy "Bud Lite" from the US should work well and at least it would be put to doing something useful for once :-)

        As an aside, I "won" a so-called "basket bonus" after shopping at the local supermarket. The prize? a 4-pack of Bud Lite "for watching the World cup". I didn't even bother claiming my "prize" :-)

        1. Michael Wojcik Silver badge

          Re: EQUULEUS

          Wait, are you saying Bud Lite contains beer?

  2. Filippo Silver badge

    Dang. I was hoping for a massive bronze-and-wrought-iron steampunk contraption, slowly rising from the ground on the power of mighty boilers.

    1. Anonymous Coward
      Anonymous Coward

      That'll be the steam-propelled K-class submarine (

      "they gained notoriety and the nickname of "Kalamity class" for being involved in many accidents. Of the 18 built, none were lost through enemy action, but six sank, with significant loss of life, in accidents"

    2. jmch Silver badge
      Thumb Up

      This absolutely needs to happen!! where's the kickstarter page?

    3. Arthur the cat Silver badge

      I was hoping for a massive bronze-and-wrought-iron steampunk contraption, slowly rising from the ground on the power of mighty boilers.

      Ditto. Or at least launched from the top of a nuclear powered airship.

      1. seven of five

        The dark side of the moon. And call it "Gotterdammerung".

        Heil Vogler...

  3. Anonymous Coward
    Anonymous Coward

    "Right, that's the design, construction plan and finance finalised... now what about the acronym?"

    1. Sceptic Tank Silver badge

      "Right, that's the acronym finalised... now what about the design, construction plan and finance ?"

  4. Andy The Hat Silver badge

    "... the city-state's white collar crime investigation agency – the Commercial Affairs Department"

    Brain automatically did a random search/replace and I had an image of Liquidator Brunt of the Ferengi Commerce Authority introducing himself ... :-)

  5. Ian Johnston Silver badge

    Mad Mike Hughes is boiling in his grave ...

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