Near Term Laser Space Propulsion to Move Twenty Times Faster in the Solar System

This is an update of the system under development by John Brophy and his team and it will use a kilometer-scale, multi-hundred-megawatt phased-array laser to beam power to a vehicle that converts it to electrical power for a multi-megawatt electric propulsion system that produces a specific impulse of 58,000 s. This will enable velocities of 100 to 200 km/s, and would enable a mission to the solar gravity lens location of 550 AU in less than 15 years. This will be nine to eighteen times faster than the Dawn Mission (11.5 km/s).

Brophy’s talk starts 25 minutes into the video.

John discusses using 3-micron thick thin-film solar cells. They could be used to generate power at below one kilogram per kilowatt. They could have a mass of 100 to 200 grams per square meter. The more power that be generated for less weight the faster you can travel.

Even Thinner Solar Cells Have Been in MIT Lab Since 2016

There has been 1.3 micron thick solar cells with 3.6 grams per square meter mass. The thinner solar cells have not had production scaled up and have not been deployed to space.

Flexible plastic films have been studied as alternatives to glass substrates for solar cells for years. Examples include variants of polyethylene terephthalate (PET)—used in soda bottles and plastic wrap—and polyethylene naphthalate (PEN)—used in beer bottles and sailcloth. But these plastics are typically flattened mechanically (“top-down”) into sheets and are thus difficult to make extremely thin (~1 micron thick) and uniform; they also require extra cleaning steps before solar cells can be fabricated on them.

MIT has been using ultrathin films of a transparent polymer known as parylene as alternative substrates for lightweight, flexible solar cells. Parylene is a commercially available plastic coating used widely to protect implanted biomedical devices and printed circuit boards from environmental damage. Transparent, clean (contaminant-free) parylene films can be formed by “bottom-up” chemical vapor deposition (CVD) on nearly any solid surface with precise thickness control. No extra cleaning steps are required.

They deposit a thin (~1 micron) layer of parylene on glass, fabricate thin-film solar cells on the parylene, then peel the entire structure off the glass—parylene and all. The resulting devices are the thinnest complete solar cells demonstrated—less than 0.05% of the thickness of equivalent devices on glass substrates—yet they convert sunlight into electricity as efficiently as their rigid counterparts, with a specific power (power-to-weight ratio) of 6 W/g or higher—among the highest achieved with any PV technology. With further development, parylene and other flexible polymer substrates could open the door to new form factors and new applications for solar photovoltaics.

If this technology could be used it would enable improvements to Brophy’s design.

Brophy’s Space Propulsion

They need to be able to build structures in the 2-4 kilometer size range which would be 20 to 40 times larger than the international space station. This can be very achievable with the fully reusable SpaceX Super Heavy Starship that should enable 10,000 times more material to be taken to space in a year by 2025. This would be 5 million tons instead of 500 tons per year. The lightweight materials and space manufacturing capabilities are all doable.

Building even larger and having more power would help.

The system would have accelerations of 5-8 mm per second squared out to about Saturn. The system could send a 50-ton manned mission out to Jupiter in 2.8 years and Saturn in 4 years. The system could send unmanned missions to Uranus in 2 years, Neptune in 3 years or Pluto in 4 years.

The Phase I study investigated all of the key assumption made in the original proposal including: the feasibility of developing photovoltaic arrays with an areal density of 200 g/m^2; the feasibility of developing a highpower electric propulsion system with a specific power of less than 0.3 kg/kW; the feasibility of developing photovoltaic cells tuned to the frequency of the laser with efficiencies of greater than 50%; and the feasibility of being able to point the laser array with the required accuracy and stability necessary to perform the reference mission to the solar gravity lens location. The Phase I work identified plausible approaches for achieving each of these technology goals. The original proposal postulated the existence of a phased-array laser with a 10-km diameter aperture, a 100-MW output power at a laser frequency of 1064 nm. This laser was assumed to power a 70-MW electric propulsion vehicle with a 175-m diameter photovoltaic array directly coupled to lithium-fueled ion thrusters operating at a specific impulse of 58,000 s.

The Phase I scaling work indicated that a better approach would be a laser with a 2-km diameter aperture with an output power of 400 MW at a laser frequency of 300 nm driving a vehicle with a 110-m diameter photovoltaic array powering a 10 MW electric propulsion system at a specific impulse of 40,000 s.

In Phase II they want to develop a system specifically for a solar gravity lens mission.

They will address the remaining technical feasibility issues including:
(1) Demonstrating that photovoltaic (PV) coupons can be operated at more than 6 kV in the plasma environment created by the lithium-ion propulsion system.
(2) Demonstrating PV cell efficiency of 50% or greater for monochromatic inputs.
(3) Modeling the characteristic of the lithium plasma plume created by the ion propulsion system.
(4) Demonstrating operation of a small aperture (0.3 m to 1 m dia.), low power (a few hundred watts) phased array with long a coherence length and beacon feedback that is scalable to large apertures.
(5) Investigation of beacon phase locking for long round-trip light time delays.
(6) Investigating laser location impacts on cross-track thrust. They are developing a technology roadmap including technology demonstration missions recommended as stepping stones to get to the final system architecture.

SOURCES- NASA, NIAC, John Brophy
Written By Brian Wang, Nextbigfuture.com

19 thoughts on “Near Term Laser Space Propulsion to Move Twenty Times Faster in the Solar System”

  1. The answers are probably given in the slides that Brian has screenshotted, but I can’t read the low resolution writing.
    And I’m not watching a 70 minute video, not even on 2x speed.

    But we do have this quote in the text:

    The Phase I scaling work indicated that a better approach would be a laser with a 2-km diameter aperture with an output power of 400 MW at a laser frequency of 300 nm driving a vehicle with a 110-m diameter photovoltaic array powering a 10 MW electric propulsion system at a specific impulse of 40,000 s.

    So, a 110m diameter PV array (9500 sq. metres) giving 10 MW when hit with optimized laser light (just over 1 kW per sq.m. So maybe 5 times the output of a standard solar cell under direct Earth Surface level sunlight.)

    They don’t mention the thrust that I can see, but 10 MW, giving 40 000 s ISP, at 97% efficiency.
    So that’s 9.7 MW of energy in the ion beam, at an exhaust velocity of 40 000s x 9.8 = 392 000 m/s
    Energy = 9.7 MW = 1/2 mass flow x velocity^2
    9.7 MW = 1/2Q x 153.7 billion
    Q = 0.126 gram/sec

    Impulse = Q x v = 49 N of thrust.

  2. Thanks for clarifying the objective as increase travel speeds vs maintaining a sustainable energy source. The follow up question for the above described propulsion system and energy source is how many megawatts of power are required for each kilogram of thrust and how many square kilometers of ultra thin film PV arrays would need to be assembled in space (?) to keep it functional.

  3. Light pressure gives just under 6.7 millinewtons per MW, so it is quite small compared to the thrust of the electric propulsion.

  4. Correction to my previous post: the electric propulsion is 1.76 N/MW. I divided by 9.8 instead of multiplying. So their system would only manage a few tens of newtons or less.

  5. Why not just a mirror to focus sunlight on rocket nozzle. With hydrogen you could get ISP up to 5000. You would need a large capital project like a multi-miles diameter laser.

  6. They speculate they can get the PV panels to generate more than 1kW/kg, maybe up to 6 kW/kg.

    I don’t know any space faring reactor that comes close to those numbers.

    eg. the https://en.wikipedia.org/wiki/Kilopower

    Design study reactor plans to get 10 kW for 1500 kg

    That’s 1/150 times as good as the starting point for the laser PV.

  7. Relying purely on the momentum transfer from the photons gives the highest possible ISP, and could at relativistic speeds be the most efficient method. But at the very low (relative to C) speeds envisaged here and now, it’s much better to throw away most of the ISP “efficiency” to get more thrust at low speeds, which you get via using that energy to run a more normal ion drive or similar.

  8. Given the power of their laser, I’m not so sure. But you may be right.

    On the other hand, electric propulsion doesn’t produce all that much thrust either. At 58000 s Isp, it would be 170 N/MW. So a “multi-megawatt electric propulsion system” would manage a few kN at best.

  9. It *might* turn out that with the laser-photovoltaic system the part of the power supply that is on the spacecraft would be less massive than a nuclear power supply for the same amount of power. If not then I don’t see any advantage.

  10. Could a dish shaped mirror reflecting solar energy away from the rear of a vessel also serve as a solar sail? Front facing gallium arsenide PV cells could feed the laser described herein, but it would seem that in the vacuum of space where there is no forward resistance other than cosmic radiation, the tail end repulsive force that can be gleaned from rear facing mirror would be difficult to improve upon considering the reflective efficiencies mirror.

  11. The laser is powering an electric rocket of some sort rather than pushing the spacecraft. So as long as the laser can hit the PV panels at destination distance, slowing down isn’t a problem.

  12. John discusses using 3-micron thick thin-film solar cells. They could be used to generate power at below one kilogram per kilowatt. They could have a mass of 100 to 200 grams per square meter.

    A 3-micron thick film has a total volume over 1 square metre of 3 millionths of a cubic metre. If that weighs 100-200 grams then the density of the material is
    100 g/0.000003 = 33 333 kg/cubic metre at the light end, to 66 666 kg/m^3 at the high end.

    For comparison, the densest material known is Osmium at 22 000 kg/m^3.

    They’ve got their numbers wrong.

  13. We already have power plants on earth that exceed this power generation. Consequently the solar collector and the laser would logically be in space.

  14. Not really a “fusion” story, but interesting power beaming. I think the laser is on Earth, by the way, in this plan.

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