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Space Engineering

Impulse in space

Researchers at the UnB construct a new version of a thruster rocket for satellites

UNBElectron emitter: Correcting the orbit with high efficiencyUNB

When they arrive in space, satellites need small mechanisms called thruster rocket that initially position them and from time to time correct their orbits. The fuel used in these cases is made of a combination of solid or liquid chemical substances that, when suffering a process of thermal expansion, send jets of gas out of the satellite in order to position it with greater precision. The problem is that the action of these fuels uses up a vast amount of electrical energy, shortening the satellite’s useful life. To find creative and viable solutions to this question is the task of various research groups throughout the planet, who are searching for alternatives that will make the satellite’s thrusters last longer. One of these groups is located at the Plasma Laboratory of the Physics Department of the University of Brasília (UnB), which has developed a new thruster, which uses plasma as a fuel. Known as the fourth state of matter, plasma is a type of ionized gas because it loses electrons from part of its atoms and assumes distinctive properties of the solid, liquid and gaseous states. “Stars, lightning and auroras find themselves in a state of plasma”, exemplifies Professor José Leonardo Ferreira, the team’s coordinator.

Electricity with magnets
The prototype of the plasma rocket thruster, dubbed Phall-01, and developed at the UnB, has some 20 centimetres in diameter and 10 centimetres in height. “It will be even smaller, as our laboratory developments advance”, Professor Ferreira adds. The Brazilian thruster has already shown itself efficient when compared to other similar pieces of equipment built in other countries “It reduces by up to 50% the electrical power needed for steering the satellite, besides doubling the useful lifespan of space vehicles in orbit, which currently ranges from two and four years.” A performance that is directly linked to the innovation proposed by the UnB researchers: an arrangement of magnets, with permanent magnetic fields, planar (like the magnets on a home refrigerator) to one of the sides of the thruster that dispenses with the use of batteries as the source of electricity.

The electrical energy of a satellite, according to Professor Ferreira, is very small. An device of medium size (500 kilos), for example, must not spend more than 1.5 kilowatts (kW) (the equivalent of 25 household bulbs of 60 watts each) in the propulsion system, because it does not generate much more this power. The small availability of energy has been of constant concern for researchers at the laboratories and space research centers since the  60’s.

“Propulsion by plasma in satellites is brought about by thrusters that speed up accelerate ions (atoms that have lost electrons) of xenon gas to huge velocities. The mechanism consists of an ionization chamber into which the xenon is injected”, as explained by Ivan Soares Ferreira, a physicist who is studying for his Ph.D. at the National Institute for Space Research (Inpe) and worked on the development of Phall-01. In all of the versions of plasma propulsion, the electrons are emitted by an electron emitter of high efficiency and they collide with the xenon gas creating positive ions, pushing the spacecraft in the opposite direction. The adjustment to the orbit is made through the actions of different thrusters that exist on the spacecraft. The ions are accelerated and ejected out of the ship at more than 10,000 meters per second (m/s). “In conventional thrusters, based on chemical reactions, the acceleration is only 700 meters per second (m/s)”, Professor Leonardo compares. The tests with the plasma thruster at the UnB are carried out in vacuum chambers that simulate the low pressure of the residual atmosphere of the earth, found in space, of up to one million times less than that compared to the pressure at sea level.

Just a short time ago only the Russians had made use of plasma propulsion of the Hall type in a version using electro-magnets. The North American Space Agency (NASA), after the disappointments of various tests on satellites and space exploration vehicles, managed to get their plasma thruster working in space in 1998 with the launch of the probe Deep Space 1 (DP1), which flew close to the Borrelly Comet and provided pictures and scientific data never before obtained concerning this type of asteroid. The Europeans have been increasing  their investments in the space research and have been successful on various missions, including that of the Small Missions for Advanced Research in Technology (Smart), satellite launched on the 25th of September of 2003 from the French Guiana as the first mission of the European Space Agency (ESA) on a visit to the Moon.

The economic viability and the efficiency of the Phall model places this technology as an important alternative for the Brazilian Space Program, because Brazil needs to make use of various satellites as a consequence of the growing demand of the industries of telecommunications, meteorology, information technology and the environment. The problem for an advance in the research lies in the lack of funding for this project. Since 1999, the researchers have only been supported through the Physics Department of UnB and by resources obtained from the foundations associated with the university. “We receive annual assistance from the university to the tune of R$ 2 million for all the research activities”, Professor José Leonardo wraps up.