Beamed Propulsion and Cubesats

john hare

Watching Jonathons’ video presentation on micro depots for very small satellites even before servicing large satellites or manned vehicles, and reading a few articles on electric propulsion triggered a thought, or possibly just a memory of something suggested elsewhere. What if the micro depot had a laser or microwave mounted to not only fuel the vehicles but also to leverage the propellant capabilities.

An orbital laser for boosting just refueled cubesats would not have to be all that large to have an effect all out of proportion to its’ size. Say the kilogram cubesat just bought a half kilogram of hydrogen plus the beamed powers services sold separately for a prospecting mission to an NEO that required 5 km/sec of V to reach.  The laser would need to add enough heat to get an Isp of about 1,200 out of the hydrogen aboard the cubesat. The exhaust would still be cooler than that of any normal chemical rocket engine. The orbit of the depot would allow it to follow the cubesat being boosted around half an orbit or more. A 2,000 second burn (lase?) would only require an average of a quarter gee of acceleration. Average thrust is in the 300-400 gram range. That puts the power beam requirements at well under a megawatt depending on assumptions.

By getting high Isp performance on board the cubesat with basically a tank and nozzle instead of the higher complexity and expense of an electric propulsion system with on board power, it should be possible to get the costs really low. A small depot/beam facility could send thousands of explorers and prospectors a year way out there for relatively small change in the cubesat propulsion costs. The high Isp also considerably reduces the costs of shipping propellant to orbit compared to chemical engines.

 

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11 Responses to Beamed Propulsion and Cubesats

  1. DougSpace says:

    Would the laser have to track the cubes at while it was accelerating? How far would the cubes at be from the laser when it started accelerating?

  2. john hare says:

    Tracking would be critical to success of any beamed propulsion. Though now that you bring it up, it does seem that a vehicle could ride a (relatively) fixed beam to save a little complexity at the depot end.

    The cubes would likely start at just a few meters from the depot/laser unless there is a good reason not to. To me the challenge is on the other end when the beam rider is thousands of kilometers from the depot/laser.

  3. Andrew S. Mooney says:

    To: Mr. John Hare – I am reading a book “Destination Mars” by Martin J.L. Turner, that includes a good piece of information in respect to this scheme:

    “We can calculate the thrust of a Solar-Electric vehicle with a 1 megawatt power source and an exhaust velocity of 10km/sec. Rearranging the equation:
    F=2P/Ve
    the thrust is just under 200N. It may be noticed that as the exhaust velocity increases, the thrust for a given power decreases.” – He suggests trying to find higher values for Ve. Similar to your own project.

    But the problem with a laser equivalent would seem to be the following: A 1MW array of solar panels in LEO would need to roughly be about 3000 square metres in area, 55 metres on a side, assuming 2.94 m2/KW of sunlight at earth, and would weigh about 24 tonnes. So add on the laser and it is slightly more than that.

    As he states: “This is not impossible, but is certainly not current technology.”

    55 metres on a side is like the roof of a mid sized industrial building or a car park, and so it’s a pretty big array for this task on it’s own: You’d need something else up there that uses the array most of the time, to amortise the cost of launching it, and occasionally blag some power from it for each cube sat that is to be launched.

    Turner’s design is for a solar electric vehicle assuming an ion drive or electric thruster of some other sort. A megawatt of energy of any kind focused onto an object that at best is one Kilogram in mass is a lot of energy for that kilogram to process into heat. I think that the Isp value is a bit ambitious for heating hydrogen gas. It’s more than many of the old NERVA designs that were limited by the requirement to keep the system’s reactor core solid.

    I don’t think that it’ll work I’m afraid. But maybe this guy could help with building one:
    http://www.telegraph.co.uk/news/worldnews/asia/southkorea/9428047/South-Korean-artist-set-to-launch-homemade-satellite.html#

    It strikes me that although he can build that thing for $300, it is getting up into space that is the cost issue here.

  4. john hare says:

    Solar electric is going to be a heavy system, which is why I suggest not using it for inexpensive missions that require good performance. For the concept I suggested, Well under a hundred kw should be sufficient. A megawatt per kilogram of payload is the rule of thumb for laser launching from the Earth surface. I suspect your information on mass per kilowatt in orbit may be a bit outdated, though your point on requiring other markets for the array is well taken.

    I think Leik Myrabo or Jordan Kare might argue with you about possible Isp, feasibility, and thrust density on the vehicles. With the laser focused up the tailpipe of the rocket, much of the laser energy is absorbed by the hydrogen before it hits the physical vehicle.

    This idea is not my project, rather it is a suggestion that could be implemented by any of several qualified companies. I agree that satellites are expensive due to launch costs far more than actual construction costs.

  5. DougSpace says:

    > To me the challenge is on the other end when the beam rider is thousands of kilometers from the depot/laser.

    At those distances, one approach might be to build up a charge and then us that power to intermittently accelerate the propellant. So initially, there would be continuous propulsion but then further on our it would become pulsed.

    Yet another approach would be to have two beams in different locations. For example, one beam could be in the ISS and a second beam could be orbiting the Moon or Mars. This could double the acceleration time.

    Perhaps one could also beam the sat towards Venus, gain a slingshot which would cause it to pass by the Earth again so that the beamed power could transmit energy both coming and going.

  6. john hare says:

    I think your third option is the most feasible. An initial boost to a resonant elliptical orbit. Perigee on the ellipse collects a second beam acceleration to a lunar free return trajectory. At second perigee from lunar distance the third beam acceleration puts the probe into a fairly high energy escape that would have enough velocity to reach anything this side of Saturn.

  7. Chris (Robotbeat) says:

    This is a little over-doing it. You already have a beamed power source: the Sun. You’d get the power from it the same way you’d get it from a laser: via photovoltaics.

    Cubesats’ small size mean they can collect disproportionate amounts of sunlight. Use it. Much easier than a ginormous antenna or the large and expensive optics needed for beamed power.

  8. Chris (Robotbeat) says:

    And using hydrogen for a cubesat propulsion would just absolutely suck. Because of their enormous surface-area-to-mass ratio, it’d all boil off very quickly. Not only that, but you couldn’t fit half a kilogram in a cube sat! A cubesat unit has a volume of just one liter (practically, it’s much less than that… because that volume has to include some mounting hardware and the tank, etc)… In one liter, you could fit only 68 GRAMS of liquid hydrogen (before it very quickly boiled off), so you’re off by an order of magnitude on how much liquid hydrogen you could conceivably carry in a cubesat. And if you’re talking about pressurized hydrogen gas (because of the boiloff issue), your mass ratio will be horrendous… and then you’d be off by about two orders of magnitude on how much hydrogen you could fit in a cubesat (even a 3-u one).

    Density of propellant is just as important, perhaps more so, for cubesats. Another important factor is safety of storage… if you have a corrosive or dangerous oxidizer (basically ANY oxidizer) or a gas under pressure, there are handling constraints. One of the best benefits of some of the cubesat electric propellants is that they are solid and basically inert, so they don’t have handling/integration costs (sometimes you can’t even get a ride if you don’t have carefree handling). That they have a higher Isp is practically an afterthought. Using hydrogen for a cubesat is a nonstarter. Using beamed propulsion doubly so.

    Your best option for cubesat propulsion is usually to use the densest propellant you can find, not the lightest!

  9. Chris (Robotbeat) says:

    Here’s an example of electric propulsion for a cubesat:
    http://www.clyde-space.com/cubesat_shop/propulsion/303_cubesat-pulse-plasma-thruster

    It’s actually pretty cheap at only 10000 British pounds! I’ll have to keep this in mind when I’m writing grant proposals.

  10. Peterh says:

    The point of laser beamed propulsion is that the photovoltaic array can be used to launch many payloads, and doesn’t have to be propelled itself with the payload.

    One beamed light arrangement I’ve looked at is 2 mirrors. The first focuses sunlight at a point, and the second focuses light from there on a target. Very efficient and continuous output, but intensity and collimation limited by the initial light source. A non-imaging concentrator at the intermediate point might improve concentration some.

  11. Paul says:

    Chris,
    Minor point. I believe John intends the laser to heat/super-heat the propellant directly, not be converted to electricity by PVs on the CubeSat.

    Peterh,
    You aren’t going to get better than 1/2 degree spread from the Sun, near Earth, regardless of the mirror arrangement you use. At 1km from the mirror, the minimum theoretical focal width is over 8.5 metres (30ft). (85m at 10km, 850m at 100km.) The size of the mirror/array only affects the overall brightness, not the focus width. There’s no way of cheating this absolute.

    (Okay, meta-material mirrors may get “better than theoretical”, but real world meta-materials tend to be highly absorbing.)

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