Air Force Tasks Rhea Space Activity to Build Rapid-Response Lunar Communications Spacecraft

July 19, 2021

 Rhea Space Activity (RSA)  has been selected by the United States Air Force (USAF) for a Phase I, Small Business Innovation Research (SBIR) 2021 Space Force Pitch Day award to investigate a bi-modal, solar-thermal propulsion system that would provide rapid repositioning capabilities for a future United State Space Force (USSF) deep space communications spacecraft.

The proposed craft, dubbed SCORPIUS, will use an origami-inspired, unfolding solar reflector to heat a tungsten block that will vaporize propellant to generate its main propulsion. This multi-role solar reflector will also act as a large-area communications antenna that can also redirect solar light to generate power for all of the spacecraft's subsystems. This architecture will allow for the USSF to rapidly reposition SCORPIUS in deep space to conduct offensive and defensive communications operations.

SCORPIUS is intended to provide a radical, cost-effective solution to a range of problems now facing USAF planners as they consider the challenges of deep-space travel. As U.S. and international spacecraft operations gradually extend past traditional geosynchronous orbits, spacecraft will require increased propulsion capabilities.

At present, to reach destinations beyond geosynchronous orbit, chemical propulsion is only capable of delivering small amounts of 'payload' over a short distance in a short period of time, while electric propulsion is capable of delivering significantly more payload, but much more slowly, taking months or even 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), aims to develop a nuclear-thermal propulsion system, which would, in theory, deliver a high-thrust, high-efficiency spacecraft able to move large amounts of payload quickly. DRACO, however, is hampered by the safety and policy challenges of working with nuclear reactors. (The term "cislunar" refers to the vast area of space between the Earth and the Moon).

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

Thus, SCORPIUS is intended to free up significant mass for larger spacecraft payloads, allowing the USSF to move assets through cislunar space in a much more responsive timeframe. SCORPIUS could potentially enable missions such as 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 now under development 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. 

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.

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