classical tubular induction coilgun is essentially identical to
the simple reluctance coilgun in terms of its general construction.
The difference is that the projectile is repelled out of the coil
through the action of eddy currents induced in the projectile.
The projectile must be non-ferromagnetic (e.g. copper or aluminium)
and the starting position needs to be slightly off-centre in the
coil otherwise it wont experience a net force when the coil is
fired. The impulse experienced by the projectile depends on mutual
inductance and magnetic diffusion processes, which must be taken
into account to effect a good design. When a multistage launcher
is designed, the individual drive coils can be made short compared
to the projectile length, allowing a smoother acceleration profile.
should also be noted that the projectile need not be a solid conductor
- a projectile comprising a coil shorted on itself can also be
used and offers efficiency advantages. The projectile can also
be tubular where it rides a cavity formed by the drive coils and
an inner, rifled mandrel. The mandrel provides support to the
projectile against the radial component of the driving force,
while also imparting spin. This particular configuration has been
termed the Theta Gun.
reconnection coilgun (also known as the plate or disc launcher
) consists of two coils stationed either side of the projectile.
When a current is pulsed into the coils eddy currents form in
the projectile and the interaction of the coil current and eddy
currents propels the projectile. As the projectile leaves the
coils the flux lines from the coils 'reconnect', hence the name
[4,5]. A multi-stage cylindrical induction coilgun can also be
referred to as a reconnection coilgun.
after Elihu Thomson's jumping ring device, this is another style
of induction coilgun and it works on the same principle as the
classical induction coilgun. Again, the projectile is made from
a non-ferromagnetic material like copper or aluminium. The diagram
below shows one possible design for a single stage Thompson coilgun.
the coil is fired the ferromagnetic core becomes magnetised, and
as the magnetic flux increases in magnitude it causes a circumferential
eddy current in the ring projectile. This induced current is repelled
by the coil current and the projectile shoots off the core. The
faster the flux is increased, the greater is the induced current
and the resulting force on the projectile. For best results the
core should be constructed from either laminations, bundled rods,
or powdered material. This is necessary to minimise eddy currents
in the core and therefore permit rapid flux swings.
speaking, in order to generated a sufficiently rapid increase
in flux it is usually necessary to use potentials of several hundred
volts or more. Due to the risk of electrocution from a high voltage
capacitor these types of coilgun aren't suitable for the beginner.
Helical Coil Launcher
this type of launcher there are two coils; the drive coil (stator)
and the launch coil (armature). The armature coil is connected
electrically in series with the stator coil via brush gear that
contacts the armature's trailing arms. The projectile is orientated
such that the currents in the stator and armature travel in opposite
directions, producing a repulsive force similar to that in the
tubular induction launcher. The difference in this case is that
the duration of the repulsion isn't limited to magnetic diffusion
is possible to construct a helical coil launcher in which the
armature/projectile rides on the outside of an elongated stator.
Rails are placed either side of the projectile and brushes channel
current into the stator and projectile. The brushgear is arranged
such that there is a section of energised stator immediately behind
the armature regardless of the position of the armature on the
coilgun topology employs passive commutation of the drive coils
eliminating the need for active sensing. The armature is charged
with an initial current that persists during the launch (i.e.
the L/R time constant of the current decay is longer than the
launch time). At the start of the launch the drive coils are fired
and, as the projectile is drawn into the coils, each successive
coil has its current driven to zero. The SCR switches that commutate
current to the drive coils automatically turn off when the current
goes through zero so no 'suckback', or braking force, is produced.
J. A. Andrews and J. R. Devine, "Armature Design for Coaxial
Induction Launchers", IEEE Transactions on Magnetics, VOL.
27, NO. 1, January 1991.
T. J. Burgess, E. C. Cnare, W. L. Oberkampf, S. G. Beard, and
M. Cowan, "The Electromagnetic Theta Gun and Tubular Projectiles",
IEEE Transactions on Magnetics, VOL. MAG-18, NO. 1, January 1982.
Paul R. Berning, Charles R. Hummer, and Clinton E. Hollandsworth,
"A Coilgun-Based Plate Launch System", IEEE Transactions
on Magnetics, VOL. 35, NO. 1, January 1999
M. Cowan et al, "The Reconnection Gun", IEEE Transactions
on Magnetics, VOL. MAG 22, NO. 6, November 1986.
M. Cowan, M. M. Widner, E. C. Cnare, B. W. Duggin, R. J. Kaye,
and J. R. Freeman, "Exploratory Development of the Reconnection
Launcher 1986-1990", IEEE Transactions on Magnetics, VOL.
27, NO. 1, January 1991
Ronald J. kaye, et al, "Design and Performance of a Multi-Stage
Cylindrical Reconnection Launcher", IEEE Transactions
on Magnetics, VOL. 27, NO. 1, January 1991.
Thomas G. Engel, Dwayne Surls, and William C. Nunnally, "Prediction
and Verification of Electromagnetic Forces in Helical Coil Launchers",
IEEE Transactions on Magnetics, VOL. 39, NO. 1, January
T. G. Engel and W. C. Nunnally, "Development of a Small-Bore,
High-Efficiency, Helical Electromagnetic Launcher", 14th
IEEE International Pulsed Power Conference, June 2003, Digest
of Technical Papers, pp 1099-1102.