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This week, energy and astrophysics startup Rhea Space Activity (RSA) announced that it has received a Small Business Innovation Research (SBIR) award from the U.S. Air Force. The project will explore RSA’s solar-thermal propulsion system–an alternative to nuclear-fueled space propulsion–as a possible solution for rapid repositioning of U.S. Space Force (USSF) deep-space communications spacecraft, currently under development.

Rhea Space Activity’s proposed spacecraft, known as SCORPIUS, is based on an unfolding solar reflector, modeled after origami, to heat a block of tungsten that will then vaporize propellant in order to generate propulsion.

This reflector will also act as a large-area communications antenna that redirects solar light to generate power for all of the spacecraft’s subsystems. Ideally, this architecture would allow the U.S. Space Force to rapidly reposition SCORPIUS in deep space to conduct offensive and defensive communications operations.

As U.S. and international spacecraft operations gradually extend past traditional geosynchronous orbits, RSA notes, spacecraft will require increased propulsion capabilities.

Currently, in order to reach destinations beyond geosynchronous orbit, traditional chemical propulsion can only deliver small amounts of ‘payload’ over a short distance in a short period of time. Alternatively, electric propulsion can deliver more payload, but much more slowly: as in, months or years to arrive at its destination.

To solve the payload and deployment time issue, the Defense Advanced Research Projects Agency (DARPA) program known as “Demonstration Rocket for Agile Cislunar Operations” (DRACO), is working to develop a nuclear-thermal propulsion system, with sufficient thrust to move heavy payload quickly. But DRACO has major liability in the safety and policy challenges of working with nuclear reactors.

SCORPIUS addresses some of these challenges by offering capabilities similar to DRACO, but without using radioactive material to achieve high-performance propulsion.

Thus, SCORPIUS is intended to free up significant mass for larger spacecraft payloads, allowing the USSF to move assets through cislunar space (i.e., the space between Earth and Moon) in a much more responsive timeframe. Rhea Space Activity says this could enable missions like patrolling the Earth-Moon “Lagrange points” (defined as areas of open space in which objects remain stationary), ferrying satellites between low Earth orbit and the geosynchronous belt, or removing space debris from strategic Earth orbits. During the Phase I effort, RSA and its team worked with the USSF to identify missions of interest and ways to refine the SCORPIUS concept to improve propellant storability and lifetime.

The novel design of SCORPIUS hinges on origami unfolding solar concentrators, and a “ThermaSat+” solar thermal propulsion system currently being developed by Howe Industries, a SCORPIUS project partner with RSA. SCORPIUS uses the large solar concentrators to heat up the tungsten block of the ThermaSat+ system, melting encapsulated boron in the tungsten and storing significant amounts of energy in the phase change from solid to liquid. Once fully charged, the tungsten block vaporizes propellant at temperatures hot enough to melt steel and generates enough thrust to perform an impulsive burn.

“Solar thermal propulsion has been extensively studied since the 1990s but was considered to be too underpowered to be of much use at the time,” said Troy Howe, President of Howe Industries. “With SCORPIUS, we can use high-thrust maneuvers instead of the old low-thrust, continuous burn concept, and take advantage of the Earth-Moon gravitational environment. By building on well-established techniques, we can provide an innovative new way of approaching spaceflight.”

SCORPIUS will also harvest electrical energy from the solar concentrators to power an electric ion engine. This bi-modal capability allows SCORPIUS to further conserve propellant during non-urgent maneuvers, and to easily make small station-keeping maneuvers without heating up the tungsten block.

“Developing a high-performance propulsion system that can operate in high-thrust/low-impulse and low-thrust/high-impulse modes allows for a wide variety of flight envelopes in cislunar space,” RSA Aerospace Engineer Beau Rideout said in announcing the Air Force contract. “Additionally, multiple uses for the unfolding origami-type solar-reflector allows for a complete rethink of satellite comms and power systems. In many respects, this system promises to offer multiple applications and breakthroughs for satellites and spacecraft far into the future.”

Rhea Space Activity is also partnering on the SCORPIUS project with Space Micro, a San Diego-based electronics firm that will provide space communications and avionics expertise, including propulsion controller electronics, space processing and Command and Data-Handling, software defined radios, and star tracker/cameras.

As part of its pitch for the Space Force/SBIR project, RSA also plans to advance “constellation architecture” recommendations to show how its new solar-thermal-based capability will fit into current U.S. defense practices, and the broader U.S. space environment.

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