By Al Globus, National Space Society Board of Directors
Space solar power (SSP) usually involves gathering energy in space, converting it into a microwave beam, and directing that beam at Earth to be converted into electricity. The National Space Society has promoted SSP for decades. A year or so ago the Alliance for Space Development made SSP one of its four legislative objectives, and in the last month the Beyond Earth Institute and the Progressive Policy Institute have made major moves promoting SSP, including a draft Presidential Policy Directive by Beyond Earth. Why this much interest now?
Because for many moons studies have found SSP to be technically sound, but too expensive to take off. That may be changing. In particular, two of the largest costs, launch and manufacturing, are dropping like a stone.
In 2011 the space shuttle flew its last flight in the midst of a long era of $20,000 per kg launch costs on mostly expendable boosters. Today (2021), the cheapest launch vehicle is the partially reusable SpaceX Falcon Heavy with an advertised launch price of around $1,400 per kg, a reduction of well over an order of magnitude. But there’s more.
SpaceX has been developing the fully reusable launch vehicle, Starship, with an estimated launch cost of somewhere around a few hundred dollars per kg, another order of magnitude reduction in cost if all goes well. While SpaceX frequently misses deadlines they have consistently achieved their performance goals. They build and operate the most successful launch vehicle of our day, the partially reusable Falcon 9.
This suggests it is reasonable to expect a reduction in launch cost of around two orders of magnitude when compared to the shuttle era. But there is more.
While launch costs are a major fraction of the economic problem faced by SSP, they are not the largest. Satellites are generally significantly more expensive than launch. Typical cost of a payload can range from a few thousand dollars per kg to 200 thousand per kg or more. Such high prices reflect that space payloads traditionally are handcrafted one of a kind systems. To significantly reduce costs requires manufacturing large numbers of identical components to amortize automation and achieve other manufacturing economies of scale. The problem for SSP, a new source of power, is to get economies of scale when building the first operational powersat.
The communication satellite constellations are leading the way to the first step. They consist of thousands of identical, or nearly identical, spacecraft that conform to certain communication protocols. These spacecraft can be, and are, mass produced. For even a single constellation the number of satellites is large enough that one can get economies of scale. For the SpaceX’s Starlink constellation this difference in cost is about 100 to one. Again, two orders of magnitude cost reduction. As you may have guessed, there is more.
The SPS-ALPHA (Solar Power Satellite by means of Arbitrarily Large Phased Array) design takes the next step, generating economies of scale in the manufacture of a single, albeit extremely large (km scale), satellite. SPS-ALPHA consists of more than a million modules of only 16 types, ~60,000 modules per type. The list of types changes as the design matures, but for illustrative purposes, a few of these types might be:
- An adjustable mirror module type that reflects sunlight onto an array of beam generation modules that take sunlight from one side, and emit microwaves via phased array from the other
- A beam generator module type that converts sunlight to microwaves
- A simple robot arm type that can work in groups to construct and repair the powersat by adding and removing modules
- A module type with electric propulsion for station keeping and transferring the finished powersat from a construction site in Low Earth Orbit to operations in Geosynchronous.
In addition, there are a number of trusses. Using the trusses for structure, much like bones in our bodies, modules self-assemble in space in an environment designed for robotic construction and repair. The closest analogy is a robotic warehouse where robotic forklifts move pallets around, a much easier task than navigating on the surface of Mars, which is not designed for robotic exploration!
Mass production of modules combined with the cost reduction for launch that Starship promises may at long last close the SSP business case with a two orders of magnitude cost reduction in the most expensive parts of a spacecraft: launch and manufacture. This is why there is so much interest. Even if the business case does not completely close it will certainly take a giant step in the right direction and develop valuable technology in the process.
Special thanks to John Mankins for most of the ideas and data in this piece.