Howe Industries has submitted a design for a solar-powered steam rocket engine for propelling CubeSats to the National Science Foundation (NSF). With only two moving parts, the ThermaSat engine uses a thermal condenser to flash boil distilled water into superheated steam.
Made up of standardized cube-shaped units measuring four inches (10 cm) on a side, CubeSats have the potential to revolutionize how satellites operate, by replacing large, complicated spacecraft with constellations of small, simple ones that can be deployed quickly and inexpensively.
However, their small size also means that there is a trade off when it comes to design. Larger satellites can carry years or even decades of propellant to keep them on station or alter their orbits as required, but CubeSats don't have much room for thrusters, let alone the propellant and power systems to run them. Worse, CubeSats generally go into orbit as a ride-along with a primary rocket payload, which means that they have to avoid flammable, explosive, pressurized, or toxic thruster fuels.
This is unfortunate because most CubeSats are deployed in low-earth orbit, which means that without thrusters their orbits rapidly decay, and they often burn up in the atmosphere. This limits the usefulness of the tiny spacecraft, and even though CubeSats are usually designed to be inexpensive to build and launch, the costs still add up.
To overcome this, Howe Industries is developing the ThermaSat, which is a separate module that can be installed on one face of the CubeSat. The principle behind it is so simple that it was first demonstrated by Hero of Alexandria in the first century AD – water is heated until it turns into steam, which generates thrust.
The tricky bit is creating the steam without a boiler, and without an internal power supply or large solar reflectors to provide the heat. Instead, the ThermaSat concentrates sunlight using less surface area than reflectors and stores the heat with a rather clever thermal capacitor.
"The heart of the system is the unique thermal capacitor, made from phase-changing materials, which concentrates and stores the solar heat collected from just 20 square inches (129 sq cm) of exposed surface area," says Jack Miller, R&D engineer for the ThermaSat program. "Using a combination of photonic crystals and gold-tinted mirrors, the completely inert capacitor reaches a blistering operating temperature of 1,052 K (1,433° F, 779° C). This results in a specific energy comparable to a lithium-ion battery, but without the potential for explosion. The side-mounted solar panels are for the electromechanics, standby heating and as a power reserve for the payload.”
According to the company, the ThermaSat weighs 5.4 lb (2.4 kg) including a standard propellant load of 2.2 lb (1 kg) of distilled water, and is made up of two CubeSat units (2U) measuring 0.07 cu ft (2,000 cubic cm), though it is also available in one- or four-unit configurations. These can provide propulsion for spacecraft up to 16 CubesSat units in size. When the water turns to steam, it generates 1.02 N of thrust and a specific impulse of 203 seconds. This is enough to keep a CubeSat in low-Earth orbit for over five years or in very-low-Earth orbit for months instead of weeks.
In addition, the ThermaSat provides a CubeSat with the ability to change its orbit and do so very stealthily because water vapor is undetectable. This means that the spacecraft can fly in formation, deorbit on command, and avoid colliding with other satellites.
Howe Industries has developed the design for the NHS as part of a Phase I Small Business Innovation Research (SBIR) grant, and now hopes to build prototypes under a Phase II grant ahead of orbital test flights.
Source: Howe Industries
