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CHEMICAL ROCKET ENGINES
Chemical rocket engines, like those on the space shuttle, work by burning two gases to create heat, which causes the gases to expand and exit the engine through a nozzle. In so doing they create the thrust that lifts the shuttle into orbit. Smaller chemical engines are used to change orbits or to keep satellites in a particular orbit. For getting to very distant parts of the solar system chemical engines have the drawback in that it takes an enormous amount of fuel to deliver the payload. Consider the Saturn V rocket that put men on the moon: 5,000,000 pounds of its total take off weight of 6,000,000 pounds was fuel. The problem is that all the energy for chemical engines comes from the energy stored in the propellants.
Electric rocket engines use batteries, solar power, or some other energy source to accelerate and expel charged particles. These rocket engines have extremely high specific impulses, so they are very efficient, but they produce low thrusts. The thrusts that they produce are sufficient only to accelerate small objects, changing the objectâ„¢s speed by a small amount in the vacuum of space. However, given enough time, these low thrusts can gradually accelerate objects to high speeds. This makes electric propulsion suitable only for travel in space. Because electric rockets are so efficient and produce small thrusts, however, they use very little fuel. Some electric rockets can provide thrust for years, making them ideal for deep-space missions. Satellites or other spacecraft that use electric rockets for propulsion must be first boosted into space by more powerful chemical rockets or launched from a spacecraft.
2. CLASSIFICATION OF ELECTRIC ROCKETS
2.1. Electro thermal rocket engines
2.1.1. Resist jets
2.1.2. Arc jets
2.2. Electrostatic Rocket Engines
2.2.1. Ion engines
2.2.2. Colloid Thruster
2.3. Electromagnetic Rocket Engines
2.3.1. Magnetoplasmadynamic (MPD) Thrusters
2.3.2. Hall Thrusters
2.3.3. Pulsed Inductive Thrusters
2.3.4. Pulsed Plasma Thrusters
2.3.5. Field Emission Electric Propulsion (FEEP) Thrusters
3. WORKING OF DIFFERENT TYPES OF ELECTRIC ROCKET ENGINES
3.1. Electro thermal Rocket Engines
This class of electric rocket engine works by heating a propellant.
3.1.1. Resist jets
A resist jet simply uses electricity passing through a resistive conductor, to heat a gas as it passes over the conductor. As the conductor heats up the gas is heated, expands, exits through a nozzle and creates thrust.
FIGURE:3.1. RESIST JET
In real resist jets the conductor is a coiled tube through which the propellant flows. This is done to get maximum heat transfer from the conductor to the propellant. Almost any gas and even some liquids can be used as fuel, the most common being hydrazine (N2H4). Hydrogen, nitrogen, ammonia and many other fuels have also been used. For a given engine and power level, the lighter the propellant the higher the specific impulse and the lower the thrust. Hydrogen produces very high specific impulses (as high as 400 sec.) This may not sound very high but resist jets are designed for small-thrust missions like orbital station keeping and the best chemical engines in this range only have specific impulses of 200 sec or less. They use so little power, 350 watts or less and then only intermittently, that they can operate using residual electrical power already available on the satellite. Because of their increased specific impulse they need hundreds of pounds of fuel less than the next best chemical engine. That's weight that can be used for more propellant, so the satellite can remain on orbit years longer, or for extra payload. Resist jets produce thrusts on the order of small fractions of a pound.
As with any electrical device, resist jets are not perfectly efficient. Typically they convert 50 percent of the electric energy passed through them into thrust energy.
The problem with resist jets is that the physical limitations of the conductor means that the maximum temperature they can achieve is 1800 degrees C. run them hotter than this and they start to melt. Fortunately, there's a solution: the arc jet.
3.1.2. Arc jets
An arc jet is simply a resist jet where instead of passing the gas through a heating coil it's passed through an electric arc.
FIGURE: 3.3. ARC JET
Because arcs can achieve temperatures of 15,00 degrees C. this means the propellant gets heated to much higher temperatures (typically 3,000 degrees C.) than in resist jets and in so doing achieve higher specific impulses, anywhere from 800 sec for ammonia to 2,000 seconds for hydrogen. Arc jets tend to be higher power devices, typically 1 to 2 kilowatts, and used for higher thrust applications, like station keeping of large satellites. Several are currently in orbit.
The largest arc jet used in space was a 26 kilowatt engine operating on ammonia with a specific impulse of 800 sec. It was part of the USAF's ESEX space experiment program. Arc jets can run at up to 35 percent efficiency.
Two problems houndsâ„¢ arc jets: the electrodes run glowing hot causing erosion and this heat can get conducted to the spacecraft heating it to unacceptable levels. For station keeping missions they aren't on long enough for the heating to be a serious problem. But it could be for large engines designed to operate for long periods of time. Arc jets don't scale down as easily as resist jets.
3.2. Electrostatic Rocket Engines
3.2.1. Ion engines
Rub a balloon against your hair or shirt and then hold it near your arm, the hairs on your arm will feel tingly and be attracted to the balloon. Bring the balloon near the carpet and bits of lint will be pulled to it. What's happening is that electrons have been deposited onto or removed from the balloon depending on what it was rubbed against, giving it an electrostatic charge, which creates an electrostatic field. A similar field can be used to produce thrust in a rocket engine called an ion thruster.
As propellant enters the ionization chamber (the small ns on the left), electrons (small -s in the middle) emitted from the central hot cathode and attracted to the outer anode collide with them knocking an electron off and causing the atoms of the propellant to become ionized (+s on the right). This means that they have an electric field around them like the balloon. As these ions drift between two screens at the right hand side of the ionization chamber, the strong electric field of the "+" side repels them and the "-" side attracts them, accelerating them to very high velocities. The ions leave the engine and since the engine pushes on them to accelerate them, they in turn push back against the engine creating thrust. Ion thrusters typically use Xenon (A very heavy, inert gas) for propellant; have specific impulses in the 3,000 to 6,000 range and efficiencies up to 60 percent. An average thruster is one to two feet in diameter, produces thrust on the order of small fractions of a pound and weighs some tens of pounds.
Downstream of the exhaust is a hot cathode emitter that injects electrons into the exhaust stream. Without this, the exiting ions would slowly cause a charge to build up in the spacecraft that could interfere with its operation and create a pull on the ions that would reduce the thrust.
Ion thrusters are well developed and have been used on a few space missions, such as a comet encounter. With their high specific impulses they are well suited to deep space types of missions.
3.2.2. Colloid Thruster
A colloid is a micro droplet like inkjet printers use to spray their ink on paper. Given an electrical charge, these micro droplets, or colloids, can be accelerated in a thruster similar to an ion thruster. The advantage of a colloid thruster is that because the individual particles being accelerated are so much larger and heavier than the atoms in a regular ion engine, the specific impulse can be lowered and thrust increased to make a better fit for a particular mission. Also, the variety of propellants that can be used is much greater. Although colloid thrusters have been around almost as long as ion engines they have not been developed to flight status. In the laboratory they typically have specific impulses around 1,000 sec.
FIGURE 3.7. SINGLE COLLOID THRUSTER EMITTER
Many of these would be tied together to create a single large thruster.
3.3. Electromagnetic Rocket Engines
This is by far the largest group of electric thrusters with many different techniques used to create thrust. As widely divergent as these thrusters may seem they all use the same principle: the Lorenz force.
If you have an electric current flowing perpendicular to a magnetic field, the magnetic field will push against the current. If the current is flowing through a solid conductor or even a gas the gas will be pushed out as well. This is the Lorenz force.
3.3.1. Magnetoplasmadynamic (MPD) Thrusters
MPD thrusters are unique among the electric rocket engine fraternity because they are capable of producing thrusts as high as 50 pounds in an engine small enough to fix in a large shoe box. The problem with them is the electrodes wear out from handling all the current and they eat up enormous amounts of power: on the order of megawatts. There is currently no space power system that comes even close to this level. Typical performances numbers are 30 percent efficiency at 2,500 sec specific impulse. In laboratories they usually run on argon, but anything that can be pumped into them can be used. (When I was working in the lab I always wanted to run one of vaporized sodium metal.) Using hydrogen would push the specific impulse into the 15,000 sec or higher range. Because of their compact size and potential for high thrust MPD thrusters are one of the few viable options for primary propulsion on high-mass, deep space mission
3.3.2. Hall Thrusters
These engines are popular with the Russians and over 100 have been used on their space missions.
FIGURE 3.9. DIAGRAM OF A HALL THRUSTER
Electromagnets around the outside cylinder and inside core create a magnetic field pointing radically inward. The interplay of this magnetic field and the electric field between the anode propellant injectors and the electron cloud created outside of the thruster causes a current (called the Hall current) to be induced to flow azimuthally around the open annulus in the thruster. The magnetic field pushes on the current and accelerates it, and the gas it's traveling through, out of the thruster to create thrust.
Typical thrusters in US laboratories are a foot or so across, use 1 to 5 kilowatts of power, operate at 2,200 sec specific impulse, produce less than one pound of thrust, and are 50 to 60 percent efficient. They are noted for their durability.
New designs using vaporized bismuth can have efficiencies as high as 70 percent making them the efficiency rulers in the electric propulsion world. Hall thrusters come in two main variants: the Stationary Plasma Thruster (SPT) and the Thruster with Anode Layer (TAL). The SPT has insulating walls on the acceleration chamber and is longer; the TAL has conducting material lining the walls and is shorter.
3.3.3. Pulsed Inductive Thrusters
Imagine an electromagnet sitting on its end on a table. Now place a metal ring on top of it. Pulse current through the coil and a second current will be induced to flow through the ring. This induced current will flow around the ring in the direction opposite to the coil. We now have another case of a current (flowing in the ring) moving perpendicular to a magnetic field (created by the coil and directed along the coil's axis) so the Lorenz law tells us there will be a force on the ring wanting to push it away. If enough current is forced through the coil the ring will shoot straight up into the air. If the current in the coil is high enough and increases fast enough, the ring will be vaporized and ionized. Even in the gaseous state it'll still conduct the current and be accelerated away from the coil. That's what a pulsed inductive thruster is .
FIGURE 3.10. PULSED INDUCTIVE THRUSTER
The only difference between actual PIT thrusters and the coil analogy is that a special valve and nozzle unit mounted in the center of the coil directs a short pulse of gas down to cover the face of the coil. The current pulse through the coil is synchronized with the gas pulse so that the gas is ionized and accelerated away, creating thrust, before it can dissipate into the vacuum of space.
Pulsed inductive thrusters are big, beautiful, sexy looking thrusters up to a meter in diameter. They operate in a pulsed mode at up to 1,000 pulses per second with specific impulses between 2,000 and 5,000 sec and thrusts of fractions of a pound to tens of pounds. Efficiencies can be as high as 50 percent.
Although the inductive coupling between the engine and plasma should imply that there is no erosion as there is in the MPD thruster, at high pulse rates the large surface area of the engine will be exposed to the thermal loading of having a virtually constant plasma mere centimeters from it. The coils could heat to the point where increased electrical resistance could cause problems or even melting. These engines are also power gluttons, eating up tens of kilowatts to megawatts of electricity.
3.3.4. Pulsed Plasma Thrusters
These small electric thrusters have been around for decades and have flown on many space missions performing station keeping functions.
FIGURE 3.11. PULSED PLASMA THRUSTER DIAGRAM
In the pulsed plasma thruster, a bar of solid propellant (could be anything but Teflon is the usual fuel of choice) is spring loaded against two stops near the exit of the thruster. When it's desired to fire it, an energy storage unit discharges an arc across the face of the propellant, ablating a small amount of the Teflon bar. Just like the rail gun and Magnetoplasmadynamic thruster, the current flowing through the vaporized propellant ionizes it and reacts with the magnetic field created by the current to accelerate the propellant out of the engine, creating thrust. As the propellant bar is eroded, the spring pushes it forward for the next pulse.
These engines are extremely simple, reliable, and robust. They have to be operated in the pulsed mode but can be pulsed rapidly to provide almost continuous thrust. They typically use 30 watts or less power, have efficiencies around 30 percent, specific impulses of 1,000 seconds, and thrust levels measured in micro pounds to millponds.
3.3.5. Field Emission Electric Propulsion (FEEP) Thrusters
These are extremely small thrusters that operate somewhat like a colloid thruster in that they have sharp propellant emitters. The difference is that in the FEEP the emitter is so small that individual ions are pulled from the emitter instead of droplets. Also, ionization occurs as a side effect of the emission process so an ionization chamber isn't required.
FIGURE 3.12. FIELD EMISSION ELECTRIC PROPULSION THRUSTER
Because the emitter hole or slit is so small, only 0.001 millimeters across, capillary action both draws the liquid propellant into it and prevents it from exhausting into space; therefore a valve is not required.
FEEPs typically have specific impulses from 6,000 to 12,000 seconds and use melted indium as a propellant.
4. PROBLEMS WITH USING ELECTRIC PROPULSION FOR PRIMARY PROPULSION MISSIONS
First, multi-megawatt space power systems continue to be too heavy to make EP for primary propulsion attractive. Second, even with a multi-megawatt power supply the thrust provided by the highest thrust engines is only measured in tens of pounds. While such low thrusts can achieve enormous velocities if operated for years, such slow delivery speeds are unattractive for manned missions and politically difficult to sell. Only on very long missions to the outer planets do long thrusting electric propulsion systems show a time advantage over chemical systems. Third, thermal rejection systems for the engine and power conditioning unit (needed to convert the power from the power system into a flavor that the thruster prefers) can drive the weight of such a system beyond the practical limit.
Hardly a year goes by in the electric propulsion world without someone thinking up a new concept. These are always variations of the thrusters outlined in this page and attempt to get around one problem or another through an innovative geometry, ionization scheme, or other concept. It would be impossible to chronicle all of them but I hope the thrusters that have been represented on this page provide a basic understanding of the world of electric propulsion.
The explanations of the thrusters on this page are oversimplifications of what are in fact extremely complex devices. It takes a PhD and many years of working with these devices to understand them at the current state of the art.
Electric propulsion research is extremely expensive. While many of the thrusters can be manufactured for a few thousands of dollars, the enormous vacuum chamber required to test one of them can easily top $1,000,000 to build and hundreds of thousands of dollars a year to operate.
4. Yahya S.M., Fundamentals of Compressible Flow with Aircraft and Rocket
Propulsion Third Edition