We rely on satellites every day for navigation, communication, weather data, TV, radio and internet. In the last few years, there has been an exponential growth in the number of satellites launched from earth for exploration, commercial and defense purposes. Typical satellites can be the size and weight of a large automobile (Fig. 1). They are launched to space on rockets and placed in orbit anywhere from a few hundred to many thousands of miles above Earth. Some large satellites also travel to other planets, asteroids and even beyond our solar system. Access to space can cost tens or hundreds of thousands of dollars per pound.
Small satellites are changing the nature of spacecraft and space-based science. The recent miniaturization of electronics and sensors means small satellites can now be developed at a lower cost and on a more rapid development cycle than traditional satellites. Nanosatellites (1–10 kg) in particular have caught the attention of scientific and commercial entities for a wide range of tasks from imagery to weather to ship tracking.
Since their introduction a decade ago, “CubeSats” have come to be the dominant nanosatellite design. A single CubeSat unit (U) is a cube with 10 cm sides (Fig. 2). Most CubeSats consist of Us (1U, 3U, 6U, etc.) Because of the adoption of this standard, construction costs have been reduced, standardized hardware can be manufactured, and flight heritage can be established across the many programs that use CubeSats.
As a result, where traditional satellites are prohibitively expensive to build and launch, CubeSat programs support emerging industries, universities, countries without large space programs, as well agencies such as NASA and The European Space Agency.
THE NEED FOR PRIMARY PROPULSION
While 100% of large satellites use some form of propulsion, current smallsat propulsion is less advanced because of the difficulties in miniaturizing conventional thruster designs. Chemical propulsion requires far more propellant mass than can be stored in such a small craft while conventional electric propulsion devices such as Hall thrusters and ion engines lose effectiveness when scaled down. This technological gap requires a new sort of electric propulsion device designed to be small, low mass, and low power.
Phase Four’s RF Thruster (RFT) is designed specifically to fulfill these requirements.
WHAT IS PLASMA?
THE FOURTH PHASE OF MATTER
We interact with three states of matter every day: solids, liquids, and gases. But there is a fourth state of matter–plasma–that is created by ionizing gas molecules and forming a mixture of ions and electrons. Plasma has all of the properties of a gas (expanding to fill its container, compressing under a force) but can also be manipulated with electric and magnetic fields. Many plasma applications use electric and magnetic fields to move the ions and electrons around to do something useful like creating light in a plasma TV, etching silicon for a computer chip, or in our case, creating a plasma jet for satellite propulsion.
Plasma propulsion has been used on satellites for over 50 years. All chemical rockets operate on the same principles and therefore look very similar; plasma propulsion devices have a tremendous variety of designs. Today’s most commonly used plasma devices on new satellites are ion engines and Hall thrusters. Both produce a stream of ions to generate thrust but ion engines do this with a pair of closely spaced, electrically biased grids while Hall thrusters rely on trapping electrons in an annular channel. The use of plasma propulsion for conventional satellites has exploded in recent years, most notably with the 2015 launch of the first entirely plasma propelled satellites, a testament to the industry’s faith in this technology.
MOVING FORWARD: BEYOND HALL THRUSTERS AND ION ENGINES
The P4 RFT is an entirely different sort of plasma propulsion device. It uses magnetic fields to shape and direct the plasma, through a compact and scalable design. Crucially, no metal parts are exposed to the plasma, eliminating components which are Hall thrusters’ and ion engines’ main failure points. A lack of electrodes also allows for a wide range of propellants to be used with the P4 RFT, especially next generation propellants such as iodine and water.