Most modern combat aircraft have fixed, forward firing armament. While this arrangement is good for air to air combat it is less suited to attacking ground targets. To attack a target on the ground requires the aircraft to dive in the direction of the ground, something that in most other situations it is normally prudent to avoid. Flying straight at your intended target also makes it easier for the intended target to shoot back. As veteran pilot Walter Bjourneby comments:-
......As for ground targets for a gun - the lack of functional armor on a fighter makes even massed AK47 fire dangerous. A gun is indeed nice to have but inside a modern FEBA - getting that low is troublesome to me. Ok fighting Taliban, but against a modern foe? Ouch. The A10s learned not to do that in Iraq. They sidelined that big 30mm and used Mavericks. And they had real armor, too.
On his webpageCarlton Meyer poses the question Why don't modern fighters have turrets?. The short answer appears to be that modern designers feel that a turret imposes too great a weight and space penalty on a modern high performance jet. For slower aircraft such as those intended for ground attack turrets may not prove to be such a hinderance. Aerodynamic drag is proportional to the square of velocity, which means that an aircraft designed to move at half the speed of another will only be subject to a quarter of the drag.
The prototype of the Russian Su-25 Frogfoot Attack aircraft had a 23mm internal gun that could be depressed. The production version of this aircraft has a fixed 30mm cannon but can still use SPPU-22 pods with 23mm guns that can be depressed to engage ground targets.
The Su-25 was also sometimes fitted with cannon pods containing a twin-barrelled NR-23 23 millimeter cannon, with 260 rounds per pod and the ability to pivot the cannon 30 degrees down from the aircraft centerline. The pod could be mounted backwards to provide covering fire as the Su-25 left the target area, discouraging the Mujahedin habit of popping up out of hiding and firing on the aircraft from the rear. Reports indicate that the cannon pods were often mounted with one firing forward and another firing backward.
During the Vietnam war the US made use of fixed wing gunships based on transport aircraft such as the C-47 Dakota, C-119 Flying Boxcar and C-130 Hercules. All of these mounted armament that fired from hatches and doors on their port side so used a technique known as Orbiting fire. To attack a linear target such as a road the aircraft simply flew parallel to it. To attack a point target the aircraft simply flew a broad circle with the target at its centre. One of the benefits of using orbiting fire was that the aircraft could fire at a target for an prolonged time while never having to move any closer to the target. This helped keep the aircraft out of range of small arms fire and reduced the effectiveness of other types of weapons.
While the aircraft that used orbiting fire in Vietnam were mainly large transports side-mounted weapons have also been used on smaller aircraft such as the Cessna U-27 Caravan.
When I was working for Cessna on the U-27A project, I had an engineer from General Dynamics ask me what kind of sight we needed for the GAU-19, 50 Cal three barrel gun that we had test fired from the airplane. I told him none. He looked stunned and wondered how we were going to aim the gun. I took him through the drill.
Air Force pilots are no longer trained on how to execute the standard commercial manuever called the pylon turn. Since they are not familiar with the concept of pivitol altitude and sight picture, they have to have a digital ballistic computer with drift and ground speed variable to aim their guns. I can give a pilot thirty minutes of ground instruction and fifteen minutes of practice and turn him into a gunship pilot.
We found that the GAU-19 slowed down to 1000 shots per minute had 400 lbs of thrust. From the cargo door of the Caravan I, we aimed the gun five degrees forward of the lateral axis and the gun hit the target when the pilot hit the thumb switch on the stick. The sight consisted of a greasepencil dot on the side window. We could take off with six hours of fuel and 4000 rounds of ammo. The electronic geeks went nuts.
Simple is good. Simple is effective. Simple does not win contracts because it is not sexy.
In the events depicted in the book Blackhawk Down the movements of the Rangers were watched by the video cameras of a P3 Orion 4,000 ft above. If this aircraft had mounted just a couple of mini-guns in the fashion of the AC-47 Spooky it could have provided instant fire support for the ground troops against the threats that it witnessed.
Is it possible to add an orbiting fire capability to other aircraft? The simplest way to do this would be to fit side-firing guns but many attack aircraft lack the interior space that most transport aircraft have.
The second option that suggests itself is for the aircraft to mount a ventral turret. Although combat aircraft with turrets are a rarity since the Second World War they are not unknown. The OV-10 Night Observation Gunship (NOGS) was a USMC OV-10A modified to include a turreted FLIR sensor and turreted M-197 20-mm gun slaved to the FLIR aimpoint. Successful in combat in Vietnam, NOGS evolved into the NOS OV-10D, which included a laser designator.
The turret systems used on modern helicopter gunships give us some idea of what the turret of a modern attack plane might be like. The chin turret of the AH-64 Apache is designed to collapse should the aircraft have to crash land. A turret that can do the same or be jettisoned would be a good feature for a fixed wing aircraft. Even more useful would be a turret mount that can either fold back or retract when not needed, decreasing drag. How a turret is used will depend on the aircraft's design. For a two-man aircraft the second crew man can operate the turret, freeing the pilot to fly the aircraft. For a one-man aircraft turrets will probably be designed to slew to preset positions for firing. The turret would then be fired in the same manner as the side-mounted gun described for the U-27 but with the added advantage that it can be set to fire to either side of the aircraft. If an aircraft flying at 2,000 ft sets its turret to a traverse of 45° and a depression of 30° it can engage ground targets a mile away (1,600m) and never have to pass any closer than 1,100m.
The most practical way to create a Gunhawk would be to build a gun mount that can be fitted on to any UH-60 and fires from the side doors. Many Insurgents in South America have learnt to fire on helicopters on their right side since door machine guns are always mounted on the left. The Gunhawk kit should be capable of being fitted on either side of the aircraft or a kit mounted on both the port and starboard of the aircraft. Alternately the kit could be mounted so that it can be traversed to fire from either door.
The most important element of the Gunhawk mounting is that it is stabilized to deliver accurate fire at long ranges. To assist in this role it is also provided with a laser rangefinder, thermal imager and high magnification optics.
Likely weapons for the Gunhawk mount are the .50 Gecal, 30mm ASP cannon or 40mm AGL. The Gecal is effective against personnel and thin-skinned vehicles and has the advantage that a large number of rounds can be carried. The ASP uses bulkier rounds but will be more effective against hard targets. The Grenades of the AGL have a long flight time and curved trajectory but in certain applications the AGL has advantages over both the MG and cannon. The lower velocity means that rounds are less likely to bury themselves in the ground when fired down from a high altitude. A 40x53mm version of the 40x46mm M397 bounding airburst round might be useful for the Gunhawk. On impact a small charge in the nose ejects the main explosive section of the grenade from the grenades base to a height of 1½ -2m where it explodes. This was of variable effectiveness in the soft ground of Vietnam but may be more effective in other applications and with a more modern design. Similar grenades such as the Russian VOG-25P Frog have a small kicker charge in the nose which throws the whole grenade up into the air. There is also the 40x53mm M684 Airburst round with an electronic proximity fuse. Use of programmable airburst ammunition may be possible with both the ASP and AGL.