X-37B now on the 6th mission of the space plane program.
Credit: Boeing

That classified mission of the X-37B robotic space plane operated by the United States Space Force has winged past 250 days in Earth orbit. Launched on May 17, 2020, this X-37B flight is also known as Orbital Test Vehicle-6 (OTV-6).

One known experiment that the space plane carries is called the Photovoltaic Radio-frequency Antenna Module Flight Experiment (PRAM-FX) – a Naval Research Laboratory (NRL) investigation into transforming solar power into radio frequency microwave energy.

Naval Research Laboratory (NRL) has pioneered “sandwich” modules that are far more efficient for space solar power.
Credit: NRL/Jamie Hartman

PRAM-FX is a 12-inch square tile that collects solar energy and converts it to RF microwave power.

NRL’s Paul Jaffe, the Innovation Power Beaming and Space Solar Portfolio Lead, told Inside Outer Space last year that the experiment is not beaming microwave energy anywhere.

“The focus of the experiment on X-37B is not establishing an actual power-beaming link,” Jaffe said. “It is more on the performance of the sunlight to microwave conversion.”

PRAM, while it does generate RF energy, that energy does not go to an antenna due to a potential for interference with other OTV-6 payloads. To be measured is how the PRAM is performing from an efficiency standpoint and also a thermal performance standpoint, Jaffe said.

NRL’s Paul Jaffe holds a module designed for space solar power in front of a customized vacuum chamber used to test the device.
Credit: NRL/Jamie Hartman

Preliminary results

PRAM-FX is the first test in orbit of an element for sandwich module space solar architectures.

The first preliminary results from PRAM-FX aboard OTV-6 have been published as part of a review paper co-authored by Jaffe in the Institute of Electrical and Electronics Engineers (IEEE) Journal of Microwaves.

While a very small sample of data has been received at the time of the paper’s writing, “preliminary indications compare quite favorably to pre-flight testing performed at NRL’s Washington D.C. location.”

The experiment is operating in a low Earth orbit that is altered throughout the course of the X-37B’s mission. “The experiment on occasion uses heaters to simulate the thermal environment in a geosynchronous orbit (GEO), where the sun would be nearly continuously shining on the solar array and thermally heating the sandwich module,” the paper notes.

Efficiency rating

The preliminary data received at this point is solely from the GEO thermal simulation data set. The maximum RF power achieved to date is 8.4 W, at an angle of 32 degrees from zenith. This corresponds, the paper explains, to a total module efficiency of approximately 8 percent.

“Though these results are preliminary, they compare favorably with the performance documented in ground testing, which also demonstrated 8% total module efficiency. As the experiment proceeds, a full picture of the module’s performance under different illumination and temperature conditions in the space environment will be uncovered,” the IEEE paper points out.

Credit: Microwave and Millimeter Wave Power Beaming, Rodenbeck, et al.

Future in-orbit demonstrations

The IEEE review paper also underscores the construction of multiple U.S. in-orbit demonstrations — planned for 2023 launch — that will demonstrate key technologies for space-based solar power.

The Air Force Research Laboratory (AFRL) is executing a major demonstration project with the goal of beaming power collected in space to expeditionary forces on Earth, the paper points out. The AFRL project is labeled as Space Solar Power Incremental Demonstrations and Research (SSPIDR).

Rectenna demonstration targeting high efficiency at low incident power densities for early SSPIDR demonstrations of space-to-earth power beaming.
Credit: Microwave and Millimeter Wave Power Beaming, Rodenbeck, et al.

As outlined in the IEEE paper, these demonstrations include: (1) Arachne, (2) SPINDLE, and (3) SPIRRAL.

Arachne will be the world’s first space-to-ground power beaming demonstration of a solar-to-RF modular panel with on-the-spot surface-shape measurement to optimize beam formation. The solar-to-RF panel technology is designed to scale to very large apertures and to support high volume, low-cost manufacturing. Arachne is planned to fly in 2023.

SPINDLE will test on-orbit structural deployment of a sub-scale version of the operational system. SPINDLE is designed to test deployment kinematics and deployed structural dynamics.

SPIRRAL will test thermal management approaches to ensure a long-lasting, high-performance system. The SPIRRAL experiment is planned to launch in 2023 via the Materials International Space Station Experiment (MISS-E) Flight Facility. MISS-E is an in-orbit platform from Alpha Space Test and Research Alliance deployed externally onboard the ISS.


To review the review paper – “Microwave and Millimeter Wave Power Beaming” — in IEEE Journal of Microwaves, January 2021, go to:


In a related development in the field of wireless power transmission, the Advanced Research Projects Agency-Energy (ARPA-E) program is looking at a potential funding opportunity this year on wireless power transmission technologies.

For more information on the Advanced Research Projects Agency-Energy (ARPA-E) program, go to:


Also go to this informative video on OPEN 2021: ARPA-E’s Marina Sofos Discusses Wireless Power Transmission at:


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