Military outposts need a lot of power, but they’re frequently located in places without easy access to electricity. The U.S. Army can’t exactly build a field of solar arrays every time it sets up shop in remote locales, and fossil fuels can be expensive and difficult to transport out to the field.
What the Army needs is an energy source it can tap into from anywhere on Earth with limited infrastructure, and the Air Force Research Laboratory believes it might just have the solution: A constellation of satellite-mounted solar arrays that can collect energy on orbit and beam it down to Earth. Terrestrial service members would be able to plug into that energy source from anywhere on the planet with mobile equipment, enabling them to power an entire forward operating base or simply charge a radio.
It sounds like wild science fiction, but AFRL engineers say it’s possible, and they’re working toward a technology demonstration as early as 2024.
The initiative is called the Space Solar Power Incremental Demonstrations and Research, or SSPIDR.
“SSPIDR is sort of a portfolio of tech development. So ultimately, our goal is solar to RF [radio frequency] power beaming — taking power from the sun that we collect in space, converting it to RF on orbit, and beaming that to the ground where it would then be converted back to power by a rectifying antenna, or rectenna,” SSPIDR chief engineer Mandy Self told C4ISRNET.
AFRL believes that’s achievable, but getting there will be a journey. SSPIDR takes an incremental approach, maturing several technology elements of the notional system and leading to three demonstrations. The flagship effort is Arachne — named after a woman who was turned into the first spider, according to Greek mythology — which will consist of an on-orbit demonstration in 2025.
The primary contractor for SSPIDR is Northrop Grumman. The company received a $100 million award focused on the solar-to-RF conversion. AFRL is also working with other companies on high-efficiency solar cells, deployable space structures, and more, said Self.
Some technologies involved are already mature, said Self, but they need to be miniaturized. In fact, miniaturization is one of the primary challenges with making SSPIDR a reality. The final size of the system in space is still a conversation, but it depends largely on how well the lab can miniaturize the various components and how much it can condense the total structure into a launch payload. Currently, some of the available technologies are simply too heavy to use for a space-based mission, so a big part of AFRL’s effort is reworking those solutions to reduce mass and size.
Which leads to the most important aspect of any effort to collect energy from the sun: The solar panels. While the technology is always improving, there’s generally a correlation between the surface area of the solar panel and how much power it provides. To power a small traffic signal, perhaps a 1 square foot panel will suffice. SSPIDR will need significantly more power and paneling than that.
“When it comes to solar-RF conversion, bigger is always better, so the more we can put in there, the better,” said Self.
SSPIDR communications officer Rachel Delaney said AFRL’s goal is to be able to generate 1,000kW of power — enough to run a forward operating base according to a study conducted by the Naval Research Laboratory. For context, a GPS III satellite boasts four satellite arrays spread over its 48 foot wingspan that together generate about 4,500 watts of power. SSPIDR will need to scale up the efficiency of the solar paneling as well as the surface area of its arrays to generate the amount of power engineers are looking for.
To make that possible, AFRL wants to use foldable solar panels that can be condensed into a relatively small payload for launch, and then unfolded once in place on orbit. That’s not an unusual approach for satellites, but the scale of SSPIDR’s solar arrays could be novel.
AFRL recently opened a new Deployable Structures Lab at Kirtland Air Force Base, New Mexico, which will help engineers develop solar arrays that are not only large enough to generate 1,000kW, but can also be effectively retracted for launch. The new facility has enough space for the team to deploy a full solar array while offsetting Earth’s gravity, helping them simulate the zero-gravity space environment the satellite will operate in. That will also enable AFRL to build the entire satellite with lighter, weaker materials that may not hold up under Earth’s gravity but will operate sufficiently on orbit.
An important space-saving innovation is a new “sandwich panel” designed by AFRL that both collects solar energy and converts it to RF.
“One side’s the solar, one side’s the RF, and the magic happens in the middle,” explained Self. “And we’re really trying to make that as thin as possible and light.”
On the terrestrial side of things is the rectenna, which will take the RF signal being beamed from space and convert it back into usable power. That technology is incredibly flexible, said Self. Larger rectennas can be installed on every building of a forward operating base, supplying a continual source of energy from space. But it can also be used on a much smaller scale. Perhaps, explained Self, a smaller rectenna could be built into soldiers’ tents, allowing them to charge their radios and other electronic devices while out in the field. AFRL has even discussed a simple umbrella-like design which can be carried into the field — simply unfold it and there’s power.
Anyone in the path of the beam can access that power if they have the right equipment — no cords necessary. And the beam is large. The final size will depend on the end product AFRL is able to develop, but right now engineers expect it to provide a coverage blanket scaling tens of kilometers. And that’s with just one satellite. AFRL envisions an entire constellation of 12 satellites called Scorpion in medium Earth orbit beaming power down to any location on the planet.
AFRL has also partnered with the Naval Research Laboratory, which was working on a similar project. The Navy Research Laboratory sent its own demonstration project — the Photovoltaic Radio-frequency Antenna Module, or PRAM — into space aboard the secretive X-37B space plane in May 2020. PRAM is testing a solar-to-RF conversion panel developed by the Navy.
AFRL’s own demonstration project, Arachne, will take that farther by actually beaming the RF to another location. The experiment will test out the first Northrop Grumman sandwich panel in space. The first panel is expected to be delivered in fiscal 2024, with a launch later that year or in early 2025.