AEROPLANE SEPTEMBER 2017 http://www.aeroplanemonthly.com 79
The GAU-4/A’s gas drive mechanism
The SUU-23/A pod
Had no internal gun
but regularly carried
a cannon in a
centreline pod
GAU-4/A
20mm cannon
Ammunition drum
Capacity: 1,200 rounds
Drive Connects gun rotation to conveyor
system. Also, links starter unit to gun
Conveyor system Moves
rounds from drum to gun
Feed
chute
Return
chute
Brake Prevents gun
overspeeding
Starter unit Rotates gun until
gas drive sytem takes over
RAF McDonnell Douglas Phantom FGR2
Battle of
the Ruhr
How the mechanism works
1
2
3
4
5
5
7
8
6
An electrical contact is made through the firing
pin to a round and it is fired
Exhaust gas is ported from barrel to gas drive
cylinder, pushing the camshaft piston forward
The cam motion causes a cam follower
attached to the barrels, breech bolts and rotor
to rotate the whole assembly (note that camshaft
piston, shaft and cam do not rotate)
Other barrel ports at the opposite end of the
cylinder ensure the camshaft reciprocates back
and forth, maintaining rotation
Meanwhile, cam followers on the breech
bolts follow a fixed cam path on the inside of
the gun housing, pulling them backwards as
they rotate
As the breech bolts near the bottom of their
rotation the empty cases are ejected
At the same time, on the other side of the rotor,
new rounds are guided into the breech bolts
via the conveyor system
As they rotate, the breech bolts are cammed
forward until the rounds are chambered ready
for firing
GAU-4/A
cannon
SUU-23/A pod
1
2
3
4
7
8
6
Round Breech
Barrel Exhaust gas bolt
Camshaft piston Camshaft Cam
Cam follower
Cam
follower
Fixed Rotor
cam path
Gas drive
cylinder
NEW FEATURE
Our new bi-monthly series, examining in depth an aspect of aviation technology or tactics. This time our subject is the
Vulcan rotary aircraft cannon
Modern aircraft versions are
mostly externally powered by
electric power rather than by
the conventional gas or recoil
operation of most other fi rearms.
Thus the gas-operated version
illustrated here is conventional in
fi rearms terms, but unusual for an
aircraft rotary cannon. This was
chosen to ensure that the entire
unit was self-contained in the pod.
The RAF’s McDonnell Douglas
Phantom was, like the original
F-4 design, not equipped with
an internal gun (though other
Phantoms were). The RAF
obtained the SUU-23/A pod
which had the GE GAU-4 Vulcan
gun as a self-contained unit. This
could be fi tted to the FGR2,
F-4J(UK) and the ex-Royal Navy
FG1 versions, though originally
the Navy’s FG1s were not so
equipped. Unlike other fi xed
mountings of the Vulcan family
on aircraft, it was designed not
to use any of the aircraft’s power
to rotate the barrel unit, but
was gas-operated with a pod-
mounted internal electric inertia
starter to start the fi ring run. This
accelerated the gun to a fi ring rate
of 5,400rpm before automatically
disengaging; the gun’s gas
operation runs up to 6,000rpm.
The ammunition drum at the
rear of the pod contains 1,200
rounds of electrically primed
20mm ammunition of ball,
armour-piercing or incendiary
type. The six barrels are rigidly
clamped together to achieve
a minimum dispersal pattern
though other clamp confi gurations
can be used to provide a greater
spread. The pod is hung from two
suspension units with the electric
power, ammunition state and
fi ring circuits run through them
VULCAN ROTAR Y CANNON
WORDS: JAMES KIGHTLY
ARTWORK: IAN BOTT
from the aircraft. The attachment
is the weakest aspect of the unit,
early examples jolting off target
due to the recoil.
Like many ideas, once successful
the rotary gun concept spawned
several spin-offs, ranging from the
famous Volkswagen Beetle-size
30mm cannon as fi tted to the
Fairchild A-10, and a range of
original Soviet designs, to small
‘mini-gun’ machine gun versions
used from the Vietnam War
onwards. Currently a US company
offers a pod-mounted mini-gun
rotary cannon on the open market,
which can be fi tted to your own
aircraft, or even your car.
The podded Vulcan cannon as used on an RAF Phantom FGR2.
DENIS J. C ALVERT
Correction: In note 6 of the
Oboe Briefi ng File feature (July
2017), the Morse codes for ‘B’
and ‘C’ were accidentally
reversed. ‘B’ should be _... and
‘C’ _._. when correct. Thanks to
former radio offi cer David
Fletcher, Lt Col (retired).
78-79_AM_BRIEFING_Sept17_cc C.indd 79 31/07/2017 11:10