The new dawn of electric rockets, Edgar Choueiri. Electric jet engine (EPE) Electrostatic rocket engine

Lonely in the darkness of space, probe Dawn(Dawn) NASA rushes beyond the orbit of Mars towards the asteroid belt. He must collect new information about the initial stages of the formation of the Solar system: explore the asteroids Vesta and Ceres, which are the largest remnants of embryonic planets, as a result of the collision and interaction of which with each other about 4,5-4,7 billions of years ago today's planets were formed.
However, this flight is notable not only for its purpose. Dawn, launched in October 2007, is equipped with a plasma engine capable of making long-distance flight a reality. Today there are several types of such engines. The thrust in them is created through ionization and acceleration of charged particles by an electric field, and not by burning liquid or solid chemical fuel, as in conventional ones.
The creators of the Dawn probe from NASA's Jet Propulsion Laboratory chose a plasma engine because it would require ten times less working fluid than a chemical fuel engine to reach the asteroid belt. A traditional rocket engine would have allowed the Dawn probe to reach either Vesta or Ceres, but not both.
Electric rocket engines are quickly gaining popularity. Recent space probe flight Deep Space 1 NASA's approach to the comet was made possible through the use of electric propulsion. Plasma engines also provided the thrust required to attempt to land the Japanese probe. Hayabusa to an asteroid and for spacecraft flight SMART-1 European Space Agency to the Moon. In light of the demonstrated benefits, developers in the United States, Europe and Japan are choosing these engines for future missions to explore the solar system and search for Earth-like planets beyond it when planning long-distance flights. Plasma engines will also make it possible to turn the vacuum of space into a laboratory for fundamental physical research.

The era of long flights is approaching

The possibility of using electricity to create engines for spacecraft was considered back in the first decade of the 20th century. In the mid-1950s. Ernst Stuhlinger, member of Wernher von Braun's legendary German rocket team that led the US space program. moved from theory to practice. A few years later, engineers at NASA's Glenn Research Center (then called Lewis Research Center) created the first functional plasma engine. In 1964, such an engine, which was used to correct the orbit before entering the dense layers of the atmosphere, was equipped with a device that made a suborbital flight as part of the Space Electric Rocket Test program.
The concept of plasma electric propulsion engines was independently developed in the USSR. Since the mid-1970s. Soviet engineers used such engines to ensure orientation and stabilize the geostationary orbit of telecommunications satellites, since they consume a small amount of working substance.

Rocket Realities

The advantages of plasma engines are especially impressive compared to the disadvantages of conventional rocket engines. When people imagine a spaceship rushing through the black void towards a distant planet, a long plume of flame from the engine nozzle appears before their mind's eye. In reality, everything looks completely different: almost all the fuel is consumed in the first minutes of the flight, so then the ship moves towards its goal by inertia. Chemical fuel rocket engines lift spacecraft from the surface of the Earth and allow trajectory adjustments during flight. But they are unsuitable for deep space exploration, since they require such a large amount of fuel that it is not possible to lift it from Earth into orbit in a practical and economically acceptable way.
In long flights, in order to achieve high speed and accuracy of reaching a given trajectory without additional fuel costs, the probes had to deviate from their path in the direction of planets or their satellites, capable of accelerating in the desired direction due to gravitational forces (gravitational slingshot effect, or maneuver with using gravitational forces). This circuitous route limits launch capabilities to fairly short time windows to ensure precise passage of the celestial body that is supposed to act as a gravitational accelerator.
To conduct long-term research, the spacecraft must be able to adjust its trajectory, enter orbit around the object, and thereby ensure the conditions for completing the assigned task. If the maneuver fails, the time available for observations will be very short. Thus, NASA's New Horizons space probe launched in 2006, approaching Pluto nine years later, will be able to observe it in a very short period of time, not exceeding one Earth day.

Rocket equation of motion

Why hasn't there been a way to send enough fuel into space yet? What is preventing this problem from being solved?
Let's try to figure it out. To explain, we use the basic equation of rocket motion - the Tsiolkovsky formula, which experts use when calculating the mass of fuel required for a given task. It was developed in 1903 by the Russian scientist K.E. Tsiolkovsky, one of the fathers of rocketry and astronautics.

CHEMICAL AND ELECTRIC ROCKETS Chemical and electrical propulsion systems are suitable for different types of applications. Chemical ones (on the left) quickly create high thrust and therefore allow you to quickly accelerate to high speeds, but consume very large amounts of fuel. These characteristics are suitable for short distance flights. Electric rocket engines (right), in which the working fluid (fuel) is plasma, i.e. ionized gas, develop much less thrust, but consume incomparably less fuel, which allows them to operate much longer. And in the space environment, in the absence of resistance to movement, a small force acting for a long time allows one to achieve the same and even higher speeds. These characteristics make plasma rockets suitable for long-distance flights to multiple destinations

In fact, this formula mathematically describes the intuitively realized fact that the higher the rate of exhaustion of combustion products from a rocket, the less fuel is needed to carry out a given maneuver. Imagine a baseball pitcher (rocket engine) standing with a basket of balls (fuel) on a skateboard (spacecraft). The higher the speed at which he throws the balls back (the rate of combustion gases), the faster the skateboard will roll after he throws the last ball, or, equivalently, the fewer balls (fuel) he will need to increase the speed of the skateboard by a given amount. Scientists denote this increment in speed with the symbol dV (read delta-ve).
More specifically: the formula relates the mass of fuel required by a rocket to perform a specific mission in deep space with two key quantities: the rate of combustion products flowing out of the rocket nozzle and the value dV achievable by burning a given amount of fuel. Meaning dV corresponds to the energy that the spacecraft must expend to change its inertial motion and perform the required maneuver. For a given rocket technology (providing a given exhaust velocity), the equation of rocket motion allows us to calculate the mass of fuel required to achieve the required value dV , i.e. to perform the required maneuver. Thus. dV can be thought of as the "cost" of the task, since the cost of getting fuel onto the flight path usually accounts for the bulk of the cost of completing the entire task.
In conventional rockets using chemical fuel, the rate of exhaustion of combustion products is low ( 3-4 km/s). This circumstance alone casts doubt on the advisability of their use for long-distance flights. In addition, the form of the equation of motion of the rocket shows that with increasing dV the share of fuel in the initial mass of the spacecraft (“fuel mass fraction”) grows exponentially. Consequently, in an apparatus for long-distance flights requiring great importance dV , fuel will account for almost the entire starting mass.
Let's look at a few examples. In the case of a flight to Mars from low Earth orbit, the required value dV is about 4,5 km/s From the equation of rocket motion it follows that the mass fraction of fuel required to carry out such an interplanetary flight is greater than 2/3 . For flights to more distant regions of the solar system, such as the outer planets, it is required dV from 35 before 70 km/s The share of fuel in a conventional rocket will have to be allocated 99,98 % starting mass. In this case, there will be no space left for equipment or other payload. As the destinations of spacecraft become increasingly distant regions of the solar system, chemical fuel engines will become increasingly futile. Perhaps engineers will find a way to significantly increase the flow rate of combustion products. But this is a very difficult task. A very high combustion temperature will be required, which is limited both by the amount of energy released by the chemical reaction and by the heat resistance of the rocket engine wall material.

Plasma solution

Plasma engines allow much higher exhaust velocities. Thrust is created by accelerating plasma - partially or fully ionized gas - to speeds significantly exceeding the limit for conventional gas-dynamic engines. Plasma is created by imparting energy to a gas, such as by irradiating it with a laser, micro- or radio-frequency waves, or using strong electric fields. The excess energy strips electrons from atoms or molecules, which as a result acquire a positive charge, and the detached electrons are able to move freely in the gas, making the ionized gas a much better conductor of current than metallic copper. Since plasma contains charged particles whose movement is largely determined by electric and magnetic fields, exposure to electric or electromagnetic fields can accelerate its components and eject them as a working substance to create thrust. The required fields can be created using electrodes and magnets, using external antennas or wire coils, or by passing current through the plasma.
The energy to create and accelerate the plasma is usually obtained from solar panels. But for spacecraft heading beyond the orbit of Mars, nuclear energy sources will be required, because As you move away from the Sun, the intensity of the solar energy flux decreases. Today, robotic space probes use thermoelectric devices heated by energy from the decay of radioactive isotopes, but longer missions will require nuclear or even fusion reactors. They will be turned on only after the spacecraft is launched into a stable orbit, located at a safe distance from the Earth; before operation begins, nuclear fuel must be maintained in an inert state.
Three types of electric rocket engines have been developed to the level of practical application. The most widely used is the ion engine, which was equipped with the Down probe.

Ion engine

The idea of ​​ion propulsion, one of the most successful concepts in electrical propulsion, was proposed a century ago by American rocketry pioneer Robert H. Goddard, while still a graduate student at Worcester Polytechnic Institute. Ion engines make it possible to obtain exhaust velocities from 20 before 50 km/s (box on next page).
In the most common embodiment, such a motor receives energy from panels of solar cells with a barrier layer. It is a short cylinder, slightly larger than a bucket, installed at the rear of the spacecraft. From the “fuel” tank, xenon gas is supplied to it, which enters the ionization chamber, where the electromagnetic field removes electrons from the xenon atoms, creating plasma. Its positive ions are drawn out and accelerated to very high speeds by the electric field between two mesh electrodes. Each positive ion in the plasma experiences a strong attraction to the negative electrode located at the rear of the engine and is therefore accelerated in the rearward direction.
The outflow of positive ions creates a negative charge on the spacecraft, which, as it accumulates, will attract the emitted ions back to the spacecraft, reducing the thrust to zero. To prevent this, an external electron source (negative electrode or electron gun) is used to introduce electrons into the stream of outgoing ions. This ensures neutralization of the outflowing flow, leaving the spacecraft electrically neutral.

Today, commercial spacecraft (mainly communications satellites in geostationary orbits) are equipped with dozens of ion thrusters, which are used to correct their position in orbit and orientation.
The world's first spacecraft, which used an electric thrust-generating system to overcome Earth's gravity when launching from near-Earth orbit, was at the end of the 20th century. probe Deep Space 1 To fly through the dusty tail of Comet Borrelli, it needed to increase its speed by 4,3 km/s, for which less was spent 74 kg of xenon (about the same mass as a full beer barrel). This is the largest speed increase to date achieved by any spacecraft using thrust rather than a gravitational slingshot. Dawn should soon exceed the record by about 10 km/s Engineers at the Jet Propulsion Laboratory recently demonstrated ion engines that can operate continuously for more than three years.

THE BEGINNING OF THE ERA OF ELECTRIC ROCKET ENGINES

1903 g.: K.E. Tsiolkovsky derived the equation of rocket motion, widely used to calculate fuel consumption in space flights. In 1911, he proposed that an electric field could accelerate charged particles to create jet thrust
1906 g.: Robert Goddard considered the use of electrostatic acceleration of charged particles to create jet propulsion. In 1917, he created and patented an engine - the predecessor of modern ion engines
1954 g.: Ernst Stuhlinger showed how to optimize the characteristics of an ion engine
1962 g.: Published the first description of a Hall thruster - a more powerful type of plasma thruster - created based on the work of Soviet, European and American researchers
1962 g.: Adriano Ducati discovered the principle of operation of the magnetoplasma-modynamic (MPD) engine - the most powerful type of plasma engines
1964 city: Spacecraft SERT 1 NASA conducted the first successful test of an ion engine in space
1972 g.: The Soviet satellite "Meteor" made the first space flight using a Hall engine
1999 city: Space probe Deep Space 1 NASA's Inactive Thrust Laboratory demonstrated the first successful use of an ion engine as the main propulsion system to overcome Earth's gravity when launching from Earth orbit.

The characteristics of electric rocket engines are determined not only by the speed of the outflow of charged particles, but also by the thrust density - the value of the thrust force per unit area of ​​the hole through which these particles flow. The capabilities of ion and similar electrostatic thrusters are limited by space charge, which places a very low limit on the achievable thrust density. The fact is that as positive ions pass through the electrostatic grids of the engine, a positive charge inevitably accumulates between them, which reduces the strength of the electric field accelerating the ions.
Because of this, the thrust of the probe engine Deep Space 1 is equivalent to about the weight of a sheet of paper, which is a far cry from the thrust of engines in science fiction films. To accelerate a car using this force from zero to 100 km/h (in the absence of resistance to movement: a car standing on the ground, such a force will not even move from its place - approx. lane) would have taken more than two days. In the vacuum of space, which offers no resistance, even a very small force can impart high speed to the apparatus if it acts long enough.

Hall engine

A variant of the plasma thruster, called a Hall thruster (box on page 39), is free from the limitations imposed by space charge and is therefore capable of accelerating a spacecraft to high speeds faster than a comparably sized ion thruster (due to its higher thrust density). In the West, this technology gained recognition in the early 1990s, three decades later than the start of development in the former USSR.
The principle of operation of the engine is based on the use of a fundamental effect discovered in 1879 by Edwin H. Hall, who was then a graduate student at Johns Hopkins University. Hall showed that in a conductor in which mutually perpendicular electric and magnetic fields are created, an electric current (called Hall current) arises in a direction perpendicular to both of these fields.
In a Hall thruster, plasma is created by an electrical discharge between an inner positive electrode (anode) and an outer negative electrode (cathode). The discharge removes electrons from neutral gas atoms in the gap between the electrodes. The resulting plasma is accelerated towards the outlet of the cylindrical engine by the Lorentz force, which arises as a result of the interaction of the applied radial magnetic field with the electric current (in this case, the Hall current), which flows in the azimuthal direction, i.e. around the central electrode. Hall current is created by the movement of electrons in electric and magnetic fields. Depending on the available power, outflow velocities can range from 10 before 50 km/s
This type of plasma thruster is free from the limitations of space charge because it accelerates the entire plasma (both positive ions and negative electrons). Therefore, the achievable thrust density and, consequently, its strength (and therefore the potentially achievable value dV ) are many times higher than those of an ion engine of the same size. More than 200 Hall thrusters are already operating on satellites in low-Earth orbits. And it was precisely this engine that was used by the European Space Agency to economically accelerate the spacecraft. SMART 1 while flying to the Moon.

The dimensions of Hall thrusters are quite small, and engineers are trying to create such devices so that they can be supplied with the higher powers required to obtain high exhaust velocities and thrust values.
Scientists at Princeton University's Plasma Physics Laboratory have achieved some success by installing sectioned electrodes on the walls of the Hall thruster, which generate an electric field in such a way as to focus the plasma into a narrow output beam. The design reduces the useless off-axis component of thrust and allows for increased engine life due to the fact that the plasma beam does not come into contact with the walls of the engine. German engineers achieved approximately the same results by using magnetic fields of a special configuration. And researchers at Stanford University have shown that coating engine walls with durable polycrystalline diamond significantly improves their resistance to erosion by plasma. All these improvements made Hall thrusters suitable for long-distance space flights.

Next generation engine

One way to further increase thrust density is to increase the total amount of plasma accelerated in the engine. But as the plasma density in the Hall thruster increases, the frequency of collisions of electrons with atoms and ions increases, which
prevents electrons from carrying the Hall current necessary for acceleration. The use of denser plasma is made possible by a magnetoplasmodynamic (MPD) engine, in which, instead of the Hall current, a current is used that is directed mainly along the electric field (inset on the left) and is much less susceptible to destruction due to collisions with atoms.
In general terms, an MTD motor consists of a central cathode located inside a larger cylindrical anode. Gas (usually lithium vapor) is fed into the annular gap between the cathode and anode, where it is ionized by an electric current flowing radially from the cathode to the anode. The current creates an azimuthal magnetic field (surrounding the central cathode), and the interaction of the field and current generates the Lorentz force, which creates thrust.
The MTD engine, the size of a regular bucket, is capable of processing about a megawatt of power from a solar or nuclear source and allows for exhaust velocities from 15 to 60 km/s. Truly, small and brave.

Another advantage of the MTD engine is the possibility of throttling: the exhaust speed and thrust in it can be adjusted by changing the current strength or the flow rate of the working substance. This makes it possible to change the engine thrust and exhaust speed in relation to the need to optimize the flight path. Intensive research into processes that deteriorate the characteristics of MTD engines and affect their service life, in particular plasma erosion, plasma instabilities and power losses in it, has made it possible to create new engines with high performance. They use lithium or barium vapor as working substances. The atoms of these metals are easily ionized, which reduces internal energy losses in the plasma and makes it possible to maintain a lower cathode temperature. The use of liquid metals as working substances and the unusual design of the cathode with channels that change the nature of the interaction of electric current with its surface helped to significantly reduce cathode erosion and create more reliable MTD motors.
A team of scientists from academia and NASA recently completed development of a new "lithium" MTD engine called a2. potentially capable of delivering a nuclear-powered spacecraft carrying a large payload and people to the Moon and Mars, as well as providing flights of automatic space stations to the outer planets of the Solar System.

Turtle wins

Ion, Hall and magnetoplasmodynamic are three types of plasma engines that have already found practical application. Over the past decades, researchers have proposed many promising options. Motors operating in pulsed and continuous mode are being developed. In some, plasma is created using an electrical discharge between electrodes, in others - inductively using a coil or antenna. The mechanisms of plasma acceleration also differ: using the Lorentz force, by introducing plasma into magnetically created current layers, or using a traveling electromagnetic wave. One type even involves ejecting plasma through invisible “rocket nozzles” created using magnetic fields.
In all cases, plasma rocket engines accelerate more slowly than normal ones. Nevertheless, thanks to the paradox “the slower the faster,” they make it possible to achieve distant goals in a shorter period of time, since they ultimately accelerate the spacecraft to a speed much higher than chemical fuel engines with the same mass of fuel. This allows you to avoid wasting time on deviations towards bodies that provide the gravitational slingshot effect. Just as in the famous story of the slow-moving tortoise that eventually outruns the hare, in the “marathon” flights that will become more common in the coming era of deep space exploration, the tortoise will win.

Today, the most advanced plasma engines are capable of providing dV before 100 km/s This is quite enough to fly to the outer planets in a reasonable time. One of the most impressive projects in the field of deep space exploration involves the delivery to Earth of soil samples from Titan, Saturn's largest moon, which, according to scientists, has an atmosphere very similar to the one that enveloped the Earth billions of years ago.
A sample from Titan's surface will provide scientists with a rare opportunity to search for signs of chemical precursors to life. Chemical fuel rocket engines make such an expedition impossible. Using gravity slingshots would increase flight time by more than three years. And a probe with a “small but remote” plasma engine will be able to make such a journey much faster.

Translation: I.E. Satsevich

ADDITIONAL LITERATURE

Benefits of Nuclear Electric Propulsion for Outer Planet Exploration. G. Woodcock et al. American Institute of Aeronautics and Astronautics, 2002.

Electric Propulsion. Robert G. Jahn and Edgar Y. Choueiri in Encyclopedia of Physical Science and Technology. Third edition. Academic Press, 2002.

A Critical History of Electrical Propulsion: The First 50 Years (1906-1956). Edgar Y. Choueiri in Journal of Propulsion and Power, Vol. 20, No. 2, pages 193-203; 2004.