![Flux switching alternator](https://upload.wikimedia.org/wikipedia/commons/0/06/Flux_switching_alternator.gif)
A flux switching alternator is a form of high-speed
alternator
An alternator is an electrical generator that converts mechanical energy to electrical energy in the form of alternating current. For reasons of cost and simplicity, most alternators use a rotating magnetic field with a stationary armature.Go ...
, an
AC electrical generator, intended for direct drive by a
turbine
A turbine ( or ) (from the Greek , ''tyrbē'', or Latin ''turbo'', meaning vortex) is a rotary mechanical device that extracts energy from a fluid flow and converts it into useful work. The work produced by a turbine can be used for generating e ...
. They are simple in design with the rotor containing no coils or magnets, making them rugged and capable of high rotation speeds. This makes them suitable for their only widespread use, in
guided missile
In military terminology, a missile is a guided airborne ranged weapon capable of self-propelled flight usually by a jet engine or rocket motor. Missiles are thus also called guided missiles or guided rockets (when a previously unguided rocket ...
s.
Guided missiles
Guided missiles require a source of electrical power during flight. This is needed to power the
guidance
Guidance may refer to:
Arts and media
* ''Guidance'' (album), by American instrumental rock band Russian Circles
* ''Guidance'' (film), a Canadian comedy film released in 2014
* ''Guidance'' (web series), a 2015–2017 American web series
* "G ...
and
fuzing systems, possibly also the high-power loads of an
active radar seeker
Active radar homing (ARH) is a missile guidance method in which a missile contains a radar transceiver (in contrast to semi-active radar homing, which uses only a passive radar, receiver) and the electronics necessary for it to find and track ...
(i.e. a transmitter) and rarely the missile's
control surfaces. Control surface
actuator
An actuator is a component of a machine that is responsible for moving and controlling a mechanism or system, for example by opening a valve. In simple terms, it is a "mover".
An actuator requires a control device (controlled by control signal) a ...
s for a high-speed missile require a high force and so these are usually powered by some non-electric means, such as tapping propellant
exhaust gas
Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline (petrol), diesel fuel, fuel oil, biodiesel blends, or coal. According to the type of engine, it is discharged into the atmosphere through an ...
from the missile's motor.
Rare exceptions where electrically powered control surfaces are used are mostly medium-range subsonic naval missiles, e.g.
Exocet
The Exocet () is a French-built anti-ship missile whose various versions can be launched from surface vessels, submarines, helicopters and fixed-wing aircraft.
Etymology
The missile's name was given by M. Guillot, then the technical director ...
,
Harpoon
A harpoon is a long spear-like instrument and tool used in fishing, whaling, seal hunting, sealing, and other marine hunting to catch and injure large fish or marine mammals such as seals and whales. It accomplishes this task by impaling the t ...
and
Martel Martel may refer to:
People
* Andre Martel (1946–2016), American politician and businessman
* Anne-Marie Martel (1644–1673), founder of what is now the Congrégation des Sœurs de l’Enfant-Jésus.
* James B. Aguayo-Martel, ophthalmologist ...
.
The total load varies for different missiles between around 100W to several kW.
The electrical supply for a missile must be reliable, particularly after long storage. Depending on the missile type, it may also be required to start delivering power almost immediately after start-up, or even before launch to allow gyroscopes to be accelerated to speed,
and to provide power for varying lengths of time.
Small
anti-tank
Anti-tank warfare originated from the need to develop technology and tactics to destroy tanks during World War I. Since the Triple Entente deployed the first tanks in 1916, the German Empire developed the first anti-tank weapons. The first deve ...
or
air-to-air missile
The newest and the oldest member of Rafael's Python family of AAM for comparisons, Python-5 (displayed lower-front) and Shafrir-1 (upper-back)
An air-to-air missile (AAM) is a missile fired from an aircraft for the purpose of destroying a ...
s may only require power for a few seconds of flight. Others, such as
tactical missiles or
ICBM
An intercontinental ballistic missile (ICBM) is a ballistic missile with a range greater than , primarily designed for nuclear weapons delivery (delivering one or more thermonuclear warheads). Conventional, chemical, and biological weapons c ...
s, may require power for several minutes. Turbojet-powered
cruise missile
A cruise missile is a guided missile used against terrestrial or naval targets that remains in the atmosphere and flies the major portion of its flight path at approximately constant speed. Cruise missiles are designed to deliver a large warhe ...
s have the longest flight times (being long-ranged, yet also slowest in flight); however, these also have engines that are capable of driving a more conventional generator.
Two technologies are used in practice to power missiles: batteries and generators. The batteries used are usually esoteric types rarely found outside missiles, such as
silver-zinc or
thermal batteries
Molten-salt batteries are a class of battery that uses molten salts as an electrolyte and offers both a high energy density and a high power density. Traditional non-rechargeable thermal batteries can be stored in their solid state at room temper ...
. The generators used are simple high-speed generators, driven directly by a turbine rotor that is powered by either the rocket motor's exhaust, or else a dedicated
gas generator
A gas generator is a device for generating gas. A gas generator may create gas by a chemical reaction or from a solid or liquid source, when storing a pressurized gas is undesirable or impractical.
The term often refers to a device that uses a ...
.
Alternator principles
The generator is required to be rugged and capable of very high speeds, as it is driven at the turbine's speed, without reduction gearing. The rotor must thus be simple in design and there can also be no sliding contacts to
slipring
A slip ring is an electromechanical device that allows the transmission of power and electrical signals from a stationary to a rotating structure. A slip ring can be used in any electromechanical system that requires rotation while transmitting p ...
s or other
brushgear.
Although the power requirement for the missile may be a largely DC supply, the AC alternator and its need for a
rectifier
A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The reverse operation (converting DC to AC) is performed by an Power ...
is still favoured for its mechanical robustness.
Unusually, both the
field coil
A field coil is an electromagnet used to generate a magnetic field in an electro-magnetic machine, typically a rotating electrical machine such as a motor or generator. It consists of a coil of wire through which a current flows.
In a rotating ...
s and the armature winding are carried on the fixed stator. The rotor is a simple toothed wheel, with no windings or electrical components.
In the simplest case, the stator has four poles and the field coils and armature windings are arranged alternately around the stator between the poles. The field magnets are arranged with their poles opposing each other, i.e. one armature is between the two North poles, one between the two South. The rotor is a simple toothed disc of magnetic, but unmagnetized, iron. As it rotates between poles, it links the flux between a single pair of opposing poles. The
magnetic circuit
A magnetic circuit is made up of one or more closed loop paths containing a magnetic flux. The flux is usually generated by permanent magnets or electromagnets and confined to the path by magnetic cores consisting of ferromagnetic materials l ...
of the stator is thus a pair of triangles, each containing a field, an armature and a shared path through the rotor. Flux passes in each circuit from one field and through one armature. As the rotor turns, the other triangular path is formed, switching the flux from one pair of field and armature to the other and also reversing the direction of the flux in the armature coil. It is this reversal of flux that produces the alternating
emf.
The rotor must bridge the path between opposing pole pieces, but must never bridge all four simultaneously. It must thus have an
even number
In mathematics, parity is the property of an integer of whether it is even or odd. An integer is even if it is a multiple of two, and odd if it is not.. For example, −4, 0, 82 are even because
\begin
-2 \cdot 2 &= -4 \\
0 \cdot 2 &= 0 \\
41 ...
of poles, but this must not be divisible by four.
Practical rotors use six poles.
As the rotation of one tooth pitch is sufficient to generate one AC cycle, the output frequency is thus the product of the rotation speed (in revs. per second) and the number of rotor teeth.
Early AC systems used the standard frequency of 400 Hz, which limited alternators to two pole rotors and a maximum rotation speed of 24,000 rpm. The use of higher frequencies, from multi-pole rotors, was already recognised as a future means to achieve greater power for the same weight. The
Seaslug missile alternator used a speed of 24,000 rpm to produce 1.5 kVA of electricity at 2,400 Hz.
The field may be supplied by either permanent magnets or by field coils. Regulation of the output voltage is achieved by controlling the current through a winding, either the field coil, or a control winding around a permanent magnet.
Alternator drive
Propulsion motor
The simplest solution taps off some hot exhaust gas from the propulsion motor and routes it instead through the generator turbine.
This gas may also be used to power the control surface actuators, as was used for Vigilant.
This is one of the simplest and lightest electrical power supplies available for a missile.
Bleeding exhaust gas from the motor increases the amount of fuel required, but this effect is trivial, around 1%. The exhaust is hot, possibly as hot as 2,400 °C, and at pressures varying from 2,600 psi at the
boost phase A ballistic missile goes through several distinct phases of flight that are common to almost all such designs. They are, in order:
* boost phase when the main boost rocket or upper stages are firing;
* post-boost phase when any last-minute changes t ...
to 465 psi during
sustain
In sound and music, an envelope describes how a sound changes over time. It may relate to elements such as amplitude (volume), frequencies (with the use of filters) or pitch. For example, a piano key, when struck and held, creates a near-immedi ...
.
A more serious drawback is the amount of sooty particulates in the exhaust, which requires a filter to keep them from the turbine.
As such filters may themselves clog, this method is best suited for short flight durations.
Gas generator
A
gas generator
A gas generator is a device for generating gas. A gas generator may create gas by a chemical reaction or from a solid or liquid source, when storing a pressurized gas is undesirable or impractical.
The term often refers to a device that uses a ...
is a chemical device that burns to provide a supply of gas under pressure. Although still hot, comparable to rocket motor exhaust, this gas can be cooler and cleaner of particulates than rocket efflux.
Both solid and liquid-fuelled gas generators may be used.
Advantages of a gas generator drive, rather than motor exhaust are:
* Cleaner, cooler exhaust, which is less likely to cause turbine problems.
* Ability to start the gas generator before launching, allowing time for
gyroscope
A gyroscope (from Ancient Greek γῦρος ''gŷros'', "round" and σκοπέω ''skopéō'', "to look") is a device used for measuring or maintaining orientation and angular velocity. It is a spinning wheel or disc in which the axis of rota ...
s to be spun up to speed, power for control surfaces etc.
* Ability to continue power generation after the motor has burned out, during a ballistic
coast phase
An intercontinental ballistic missile (ICBM) is a ballistic missile with a range greater than , primarily designed for nuclear weapons delivery (delivering one or more thermonuclear warheads). Conventional, chemical, and biological weapons c ...
.
Development history
The first alternators of this type began with the first missiles requiring considerable electric power, those using radar seekers (initially
semi-active radar homing
Semi-active radar homing (SARH) is a common type of missile guidance system, perhaps the most common type for longer-range Air-to-air missile, air-to-air and surface-to-air missile systems. The name refers to the fact that the missile itself is ...
). Development of these began in the late 1940s, with air-to-air missiles such as
Sparrow.
Sparrow was a relatively large missile with an airframe 8 inches in diameter. By the late 1950s, turbine-driven alternators were also being used in lightweight anti-tank missiles such as
Vigilant.
Vigilant has a body diameter of 4 inches, including a inch central jetpipe. The alternator and turbine were fitted into a remaining annular space of only 1 inches.
[, Cutaway drawing of Vigilant missile's main components]
Permanent magnet magnetos
An alternative high-speed generator is the permanent magnet
magneto
A magneto is an electrical generator that uses permanent magnets to produce periodic pulses of alternating current. Unlike a dynamo, a magneto does not contain a commutator to produce direct current. It is categorized as a form of alternator, ...
. Achieving the output needed depends on the use of modern
rare-earth magnet
Rare-earth magnets are strong permanent magnets made from alloys of rare-earth elements. Developed in the 1970s and 1980s, rare-earth magnets are the strongest type of permanent magnets made, producing significantly stronger magnetic fields than ...
s, such as
samarium cobalt or
neodymium
Neodymium is a chemical element with the symbol Nd and atomic number 60. It is the fourth member of the lanthanide series and is considered to be one of the rare-earth metals. It is a hard, slightly malleable, silvery metal that quickly tarnishes i ...
. The output coil is formed as a stator, with axial magnetic flux from a rotating multi-pole ring magnet.
See also
*
Alexanderson alternator
An Alexanderson alternator is a rotating machine invented by Ernst Alexanderson in 1904 for the generation of high-frequency alternating current for use as a radio transmitter. It was one of the first devices capable of generating the continuou ...
*
Variable reluctance sensor
*
Switched reluctance motor
References
{{Reflist
Alternators
Electrical generators
Missile technology