INTRODUCTION
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 object’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.
CLASSIFICATION OF
ELECTRIC ROCKETS
Electro thermal rocket engines
Resist jets
Arc
jets
Electrostatic Rocket Engines
Ion engines
Colloid Thruster
Electromagnetic Rocket Engines
Magnetoplasmadynamic (MPD) Thrusters
Pulsed Plasma Thrusters
Field Emission Electric Propulsion
(FEEP) Thrusters
WORKING OF DIFFERENT TYPES OF ELECTRIC ROCKET ENGINES
Electro thermal Rocket Engines
This class of electric rocket engine works
by heating a propellant.
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.
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.
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.
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 hounds’ 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.
Electrostatic Rocket Engines
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.
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.
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.
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
Pulsed
Plasma Thrusters
These small electric thrusters have been
around for decades and have flown on many space missions performing station
keeping functions.
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.
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.
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.
CONCLUSION
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.
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