Aeroplane September 2017

(Brent) #1

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 system 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

78 http://www.aeroplanemonthly.com AEROPLANE SEPTEMBER 2017

BRIEFING FILE


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


T


he desire to achieve
effective fi repower in
aerial combat has always
been challenged by
the requirement of delivering
the critical weight of fi re onto
the target in fl eeting, three-
dimensional, dynamic combat.
By the end of World War Two it
was clear that a new solution was
required for ranging still higher
volumes of fi re in ever briefer ‘on-
target’ opportunities.
Fitting groups of heavy guns to
aircraft had fallen out of favour,
and the ability to speed up the
rate of fi re of a single weapon was
found to be limited by both the
fundamental mechanical time
strictures of chambering, fi ring
and extracting a round, as well as
the heating and wear of the barrel
that was exacerbated by fast — if
brief — bursts of fi re.
By the war’s end, many
innovative attempts to increase
the weight of fi re had been tried,

gun from a museum and coupled
it to an electric motor drive,
briefl y achieving a remarkable
5,000-round per minute (rpm)
rate of fi re before, unsurprisingly,
breaking down.
Building on this, the Gatling
rotary cannon concept was re-
engineered from scratch. The
fi rst trial batches of guns were
delivered in 1950, initially in
0.60in, 20mm and 27mm calibres.
The calibre settled on was 20mm,
and the Vulcan gun was adopted
by the US Air Force in 1956.
The gun is normally used at a
selected rate of 4,000 or 6,000rpm
though the design is able to fi re
at rates of up to 7,200rpm. This
obviously takes place in very short
bursts, usually less than a second,
both because the objective is
the maximum weight of fi re in
brief periods, and since carrying
ammunition for extended fi ring
would be weight-prohibitive as
well as wearing out the barrels.

VULCAN ROTAR Y CANNON


mostly running to dead ends.
But one remarkable project was
to revolutionise aircraft gunnery.
In June 1946, the Armament
Division of the General Electric
(GE) company was awarded a US
military defence contract which
GE entitled Project Vulcan.

Unusually, it looked to historic
technology for the answer,
revisiting the long-obsolete
original Gatling gun, which had
proven highly reliable. That had
been externally powered, usually
by a hand crank. The project
team borrowed a 1903 Gatling

An M61A1 Vulcan cannon as fi tted to current USAF fi ghters. USAF

78-79_AM_BRIEFING_Sept17_cc C.indd 78 31/07/2017 15:33

Free download pdf