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.

Written By Brian Wang,