Military spaceplane experiment sheds light on space solar satellites

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An experiment launched May 17 onboard the U.S. Air Force’s X-37B Orbital Test Vehicle-6 is based on over a decade of work focused on a modular space solar satellite capable of beaming energy to Earth.

Developed by the U.S. Naval Research Laboratory (NRL) in Washington, the hardware is called the Photovoltaic Radio-frequency Antenna Module, or PRAM for short.

PRAM is an outgrowth of NRL effort in developing “sandwich” modules where one side receives solar energy with a photovoltaic panel, electronics in the middle convert that direct current to a radio-frequency signals and the other side has an antenna to beam power away.

The Photovoltaic Radio-frequency Antenna Module (PRAM) is an experiment onboard the U.S. military’s X-37B spaceplane, shown here in 2017. Credit: U.S. Air Force

NRL’s Paul Jaffe, the Innovation Power Beaming and Space Solar Portfolio lead, said the PRAM aboard X-37B is not establishing an actual power-beaming link. Rather, the 30-centimeter module is dedicated to evaluating its energy-conversion ability and the thermal performance of the device in Earth orbit. While PRAM does generate RF energy, that energy does not go to an antenna due to a potential for interference with other payloads aboard X-37B, he told SpaceNews.

Data deliveries

“We’re testing a functional component that would be part of one class of solar power satellite that would ultimately send energy from space to Earth,” Jaffe, PRAM’s principal investigator, said. “We anticipate publishing something in several months once we get back some data and have a chance to analyze that information.”

There will be regular data deliveries from the vehicle that’s hosting PRAM, said Chris DePuma, NRL electronics engineer and PRAM program manager. “The advantage of their [X-37B] platform is that we don’t have to create our own communication system. They collect our data in a package for us to analyze.”

Given PRAM results, a next step would be fabricating a fully functional system on a dedicated spacecraft to test the transmission of energy back to Earth that could potentially help power remote installations like forward operating bases and disaster response areas.

Hitting a baseline

In an October report, “Opportunities and Challenges for Space Solar for Remote Installations,” an NRL study group explored the concept of providing power to military and remote installations via solar power. The study determined that there remain significant unresolved technological, economic, legal, political, operational, organizational, and schedule challenges inherent in the development of a deployable space solar capability.

However, because of the potential game-changing nature of space solar power for terrestrial applications, the study team recommended investments in several critical areas, the foremost of which was power beaming technology.

The PRAM hardware is the first orbital experiment designed to convert sunlight for microwave power transmission for solar power satellites. Credit: U.S. Naval Research Laboratory

Jaffe said there remain open questions with power beaming technology and its level of maturity, hence the X-37B experiment. The PRAM is viewed as the first orbital experiment designed to convert sunlight for microwave power transmission for solar power satellites.

“You can certainly make a case for solar power satellites in many circumstances where a laser link, not a microwave link, would be preferable,” Jaffe said. “One application is getting energy into the permanently shadowed regions of the moon,” he said, where expected water-ice there can be processed into drinkable quantities to sustain crews, as well as break that resource into components of rocket fuel.

NRL’s DePuma said the main goal of the PRAM experiment on the X-37B is hitting a baseline and proving the concept works and they are not missing any major issues.

“The reason behind the sandwich module architecture is to modularize the space solar satellite system. You could send up components a few at a time and assemble them on orbit. You build a very large structure with multiple small launches,” DePuma said. “It’s a good way to approach getting to the larger systems.

This article originally appeared in the June 15, 2020 issue of SpaceNews magazine.