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A Laser-Powered Ion Engine for Deep Space

When we kick around ideas for deep space propulsion, we have to keep in mind that the best solutions may involve hybrid technologies, leveraging the best of several methods. JAXA (Japan Aerospace Exploration Agency) demonstrates this with its ambitious plan to take a sail like IKAROS to Jupiter, for a study of the Jupiter trojan asteroids. Like the original, the upgraded IKAROS will use liquid crystal reflectivity control devices as a means of attitude control.

But operating at the limits of solar sail functionality, the new JAXA sail will also carry a high specific impulse ion engine to facilitate its maneuvers among the trojans. Here we have a mission that couldn’t be flown with just a sail, because the numerous trajectory changes required at destination demand a reliable thruster, one that in this case will be fed by some 30,000 solar panels in the form of thin film solar cells attached to the sail membrane.

What of missions into still deeper space? We’d like to return with robust technologies to the outer planets, not to mention the need to follow up the Voyagers and New Horizons with craft specifically designed to study the interstellar medium beyond the heliosphere.

For that matter, we’re seeing increased interest in exploring the Sun’s gravity lens, whose effects can be examined beginning at 550 AU. That last is a hot and controversial topic, as the spirited debate between Slava Turyshev (JPL) and Geoff Landis (NASA GRC) showed at the recent TVIW conference in Huntsville (keep an eye on the TVIW 2017 video page, where these discussions will soon be available). To resolve the matter, we need to actually go there.

JPL’s John Brophy has been exploring another form of hybrid technology to make such missions possible — I talked about this one when it surfaced last April (see NIAC 2017: Interstellar Implications). Now working under a Phase 1 grant from the NASA Innovative Advanced Concepts (NIAC) program, Brophy proposes using lasers to provide the power source for a spacecraft’s ion engines, beaming to solar panels on the craft. The idea here is to take advantage of a power source separate from the spacecraft in return for major gains in weight and efficiency.

Brophy’s ion engine infrastructure depends upon a 10-kilometer 100 MW laser array capable of beaming power across the Solar System. The beam would be captured by a 70% efficient photovoltaic array tuned to the laser frequency and producing power at 12 kV, according to this precis prepared for the NIAC program. As Brophy has noted, the array output here far surpasses our best solar arrays today, found on the International Space Station, which produce 160 volts. With an areal density of 200 grams per square meter, the 10-kilometer array would be heavier than today’s solar sails but substantially lighter than existing solar arrays.

The laser array in question has its roots in high-power laser concepts of the kind Philip Lubin has advocated for Breakthrough Starshot, only Brophy’s array is in space, avoiding the numerous issues raised by Starshot’s ground-based installation (and creating construction issues of its own). What you achieve with this kind of configuration is the ability to deploy powerful ion engines in the outer Solar System, as Brophy is quick to note in his NIAC precis:

Our innovation is the recognition that such an array increases the power density of photons available to a spacecraft illuminated by the laser beam by two orders of magnitude relative to solar insolation at all the solar system distances beyond 5 AU, and that this enormous power can then be used to great effect by driving a highly-advanced ion propulsion system.

Image: The laser-powered ion thruster concept as developed by John Brophy and colleagues.

The thruster being powered by the laser beam is a lithium-fueled gridded ion propulsion system that does away with what would otherwise be heavy power processing hardware and associated thermal radiators. The 58,000 second specific impulse — compare this to Dawn’s 3,000 seconds — goes well beyond current state of the art in spacecraft systems — about 20 times — and takes advantage of the fact that lithium is both easily ionized and easily stored. The result:

This allows the thruster to be operated with nearly 100% ionization of the propellant which effectively eliminates neutral gas leakage from the thruster and the production of charge-exchange ions that are responsible for thruster erosion and current collection on the photovoltaic arrays. This key benefit enables very long thruster life and facilitates the development of the 12-kV photovoltaic array.

Brophy believes such a system could achieve velocities of 260 kilometers per second, which would make missions to Jupiter feasible within one year of flight time, while reaching the gravity focus would be a matter of 10 to 12 years. If that isn’t tantalizing enough, he also talks about a Pluto orbiter mission with a travel time in the area of 4 years. Thus the hybrid concept ramps up the performance of ion engines in places far enough from the Sun to pose serious power issues, and also taps into a laser infrastructure that could one day drive missions system-wide.

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{ 25 comments… add one }
  • ljk November 7, 2017, 14:54

    Quoting from the main article:

    “Brophy believes such a system could achieve velocities of 260 kilometers per second, which would make missions to Jupiter feasible within one year of flight time, while reaching the gravity focus would be a matter of 10 to 12 years. If that isn’t tantalizing enough, he also talks about a Pluto orbiter mission with a travel time in the area of 4 years.”

    The Orion nuclear pulse vessel could go to Pluto and back with a manned crew in just one year. :^)

    • Alex Tolley November 7, 2017, 19:27

      It remains to be seen how a km sized laser array in space compares in cost to an Orion ship. While I think the cost is likely to be rather high, the ion ship is robotic and therefore not just lighter, but desn’t need to support a crew and return them home safely after a reasonable mission time.

      For trips to the outer solar system, the Kuiper belt and the 500 AU for a FOCAL mision, an unmanned vessel is the way to go. Exploding nukes behind an Orion is not going to be tolerated by our space treaty signatories. What could possibly go wrong?

      • ljk November 8, 2017, 10:00

        What could go wrong with a giant powerful laser in space in the wrong hands? What could go wrong with just about any means of accessing space being turned into a weapon? Of course the irony is that most of the history of the rocket has been as a weapon.

        If you want to colonize the Sol system in bulk, Orion would be the way to go. And there is one nation which has the capability to build such a vessel and did not sign any treaties. And they very much want to dominate space.

        • Harry R Ray November 9, 2017, 10:16

          If you want to find out the answer to your question, watch “Geostorm” when it comes out.

  • Ron S. November 7, 2017, 15:33

    How are those high voltages achieved? That precis doesn’t say, or suggest new cell technology. The basic physics and chemistry of a photovoltaic cell produces only several volts. If many cells are chained in series to deliver high voltage there are many single points of failure. That is, one cell malfunction can disrupt a large segment of the array, especially since they are by necessity highly exposed. I would think that parallel cells followed by DC to DC conversion better allow for detection and isolation of failed cells that would otherwise disrupt much of the array. What am I missing?

    • Alex Tolley November 7, 2017, 19:22

      As mention4ed by Brophy, the configuration of the ISS solar arrays allows a 160V output. The arrays on the ion drive are “disposable”, i.e. lightweight and not built to last, so presumably failure is not considered a major issue.

    • Brett Bellmore November 8, 2017, 7:20

      The obvious way to do it is parallel/series: You have a whole bunch of individual cells in parallel, and the bunches are in series, with everything wired so single point failures don’t obstruct the current, and fused so that local shorts get isolated. Not really any different from high voltage battery packs in electric cars.

      You do tend to lose efficiency where not all the cells are performing identically, though. Quality control would have to be good.

      A big advantage of running at such high voltage is that it reduces the weight of the wiring, because it’s current that causes Ohmic losses, and you need less current to carry the same power at high voltages.

      • Ron S. November 8, 2017, 12:45

        I understand all your points. This is routine design. That’s even how the 160 VDC is reached. I don’t even see how this can be presented as innovative in the proposal. So I think my question stands. Many more points of failure are created. One cell failure affects a greater fraction of the total array. Will lower reliability not materially impact the mission objectives? How many failures and of what type can be tolerated? Hopefully the full proposal quantifies this.

  • Bert Molloy November 7, 2017, 20:42

    I always find it confusing and disheartening when such exciting projects which have a fairly extended projected timeline, (~15 to 35 years), fail to make any mention of a Space Elevator as a possible method of accessing NEO at a fraction of the current AND projected costs of ALL existing and planned access-to-space methods !
    Space Elevators, if possible- and that is looking quite promising, will lead to an immediate reduction in costs of 90% or more!
    Further, by allowing on-orbit construction of nuclear-powered spaceships, the cost of access to ‘space’ could be reduced by perhaps 99% or more !! (Can you say 1.5 million times the available energy at a mass penalty of ‘only’ 100 or so !)

    Bert Molloy – Member of International Space Elevator Consortium

    • Mauldred November 8, 2017, 8:04

      The main problem I see for a space elevator (appart from the fact that the required material does not exist yet) is that the cable is exposed to the weather (potentially equatorial typhoons/cyclones/hurricanes) in its lower part, to space debris and micrometeorites in its upper part, besides, it requires a simultaneous international cooperation and effort to build it, maintain it and protect it. One cable fails and huge damage is to be expected on the ground below, besides anything on the falling cable (cabins and people inside) will be destroyed unless one cabin is attached to several cables in a fail-safe manner, but the risk of losing more than one or all cables at once is also very high (a collision with a used rocket stage in low orbit can easily damage the redundant cables if they are close enough from each other, which is necessarily the case if one cabin has to be attached to all of them) and it will result in a material, political and economical disaster of enormous proportion. I don’t think anyone would want to take such a risk. Compared to that, the risk of a rocket launch failure is largely acceptable.

    • ljk November 8, 2017, 10:43

      Bert, a Space Elevator certainly seems like an elegant and much more civilized way of placing payloads into space than those barbaric and messy exploding bombs called rockets.

      However, I must ask the following, if for no other reason than to help figure out a way to get around or remove the obstacles that have kept SEs from becoming a reality so far:

      Who would pay for this?

      Who would maintain it?

      How long would it take for the SE to pay for itself? Or start making a profit?

      If we want to put an SE all the way to geosynchronous orbit, what materials could keep it in place out to 22,300 miles? Do they exist yet?

      How fast could the elevators move to get payloads into space?

      How much is payload weight a consideration, at least at the base of the SE?

      What would it take to keep the SE safe from terrorists? If you read about the SE in the Red Mars SF trilogy, it was knocked down by terrorists and the result was devastation around the entire equator of Mars. Yes, this was fiction, but still something to consider.

      Would an SE work better on less massive worlds?

      Thank you.

    • Alex Tolley November 8, 2017, 11:14

      Space elevators are OT for this thread, but certainly not OT for Centauri-Dreams. You can look at past posts on elevators uusing the search function.

      If Brad Edwards’ book “The Space Elevator” (2002) was the start of the modern rekindled interest in the subject partly because of the discovery of CNT, what real progress has been made since? AFAIK we don’t see industry creating kilometer-long spools of CNT threads to be woven into tape, or elevator cars able to climb more than a short distance from the ground at very modest speeds. I fear that your 15-35 year development cycle will prove as optimistic as commercial fusion power forecasts. Elevators for the Moon and possibly Mars seem more likely than for Earth at this point.

  • Dmitry Novoseltsev November 8, 2017, 0:20

    Perhaps it will be the same, only on a smaller scale: https://iz.ru/663104/dmitrii-strugovetc-anastasiia-sinitckaia/roskosmos-sozdaet-orbitalnuiu-aes.
    For ultralight probes power may be sufficient.
    In any case, will be able to perfect the technology.

  • matterbeam November 8, 2017, 9:03

    Hello!
    Could I just clarify some of the figures cited here?
    A 100MW beam is generated by an array at a rate of 1.27W/m^2. The array masses 200 grams per m^2, so its power density is 6.37W/m^2. It would also mass a total of 15700 tons.

    The photovoltaics receiving this laser handle 100MW with 175m diameter. That’s 4157W/m^2 before 30% is lost to inefficiency. It is stated that the power density achieved is 0.25kg/kW, so each square meter masses 1.039kg or 1.484kg.

    My questions are:
    Why is the laser so weak? Could they not have used a simpler laser generator + focusing array arrangement, with the focusing array being a thin reflector?
    How is 70% efficiency possible? What about the Shockley-Queisser limit?
    Why do the panels weigh so much? Is the lithium propellant included in the panel mass?

    • Brett Bellmore November 8, 2017, 13:16

      Shockley-Queisser is usually calculated for sunlight. In a system where the cells are driven by a monochromatic laser, you can match the energy of the photons to the bandgap, and achieve much higher efficiencies, because you don’t have the spectrum losses to deal with.

    • Andrei November 9, 2017, 10:57

      Your numbers must be very off, a square meter of a weight 1.039 kg = One metric ton. 1.484 kg = almost 1.5 metric ton!

  • Harry R Ray November 8, 2017, 10:29

    Rendezvous with Oumuamua? Another fascinating mission using this technology would be sending a probe to station keep(orbit not possible due to low mass)and possibly land on the first discovered extrasolar asteroid!

  • David H November 8, 2017, 10:57

    One reason why Starshot wanted their propulsion laser to be on Earth is that it couldn’t be used as a weapon. In the age of assassin drones, a 100MW orbital laser with perfect aim would be a very tempting weapon indeed.

    • Brett Bellmore November 8, 2017, 13:19

      The back side of the Moon might be feasible, too, with substantial advantages for the optics, because you don’t have all that air to deal with.

    • Randy Chung November 8, 2017, 13:52

      The power beamer satellite can be designed so that the solar cells are on the “front” side, and the lasers on the back side. The lasers won’t get power when pointed toward the sun. By putting the satellite in an orbit outward from the Earth, or by putting the satellite in a sun synchronous dawn-dusk Earth orbit, the laser wouldn’t get power when pointed at the Earth.

  • Jim Early November 8, 2017, 11:03

    This makes an interesting intermediate goal compared to the Starshot design with its 100 GW laser. The description seems to confuse the size of the laser array and the laser launch optics. Laser beam optics of 5 m would easily handle a beam at 500 W/cm2 so the scale lengths in the laser system are meters not kilometers. The beam would then be expanded to 10 km in the launch optics which would have to be a transmissive Fresnel lens.
    This same launch optics could provide 600m resolution of Pluto which makes orbiter missions of lessor value. Perhaps the real value within the solar system is the ability to deliver heavy payloads for landers.

    The real propulsion cash cow beyond LEO is delivering payloads to GEO. For laser beam powered ion propulsion the power levels drop another three orders of magnitude to 100 kw (which would match the Hall thruster of your previous article). The optics also drop into the 10 meter range. The down side of this mission is the very slow passage through the Van Allen belts which may be acceptable. This may be an achievable next step.

  • Michael November 8, 2017, 15:53

    We could use a large thin parabolic reflector to focus the laser light onto a smaller solar panel. Also we could still build the laser on Earth perhaps as a pre starshot design and use a sterrable balloon reflector high in the atmosphere to allow for a longer period of illumination. I would not be surprised if the ground based laser could be used to launch orbiting reflectors due to the power of it.

  • ljk November 13, 2017, 12:05

    As usual there are going to be problems with humanity’s latest starship plan, in this case Breakthrough Starshot, and the technical issues are the least of them:

    https://www.bostonglobe.com/ideas/2017/11/11/here-why-plans-for-death-laser-space-have-been-put-hold/SwlSwr6G8VkYo9SIOR60mO/story.html

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