Tankers tor the RAAF?

These tests proved that air refuelling contacts could be made safely and consistently by day and night, and that pilot fatigue would not restrict absolute aircraft endurance. However, a practical application for the concept did not arise until the late 1930s, when several countries began a major effort to provide a high-speed mail service across the North Atlantic. Britain had two possible schemes to link Southampton and New York: the Short Mayo composite aircraft in which a large flying boat would act as a launching platform for a heavily-laden Mercury floatplane, and a flight-refuelled flying boat. Just one month before the outbreak of war in 1939, Imperial Airways made eight double crossings of the Atlantic, using two Short C-Class flying boats which were refuelled on 15 occasions by four Handley Page Harrow bombers on the leg between Newfoundland and Shannon (Eire).

Following the end of hostilities in 1945, Britain again turned to the development of flight refuelling for commercial services. In 1947 the Ministry of Civil Aviation ran a series of tests with Lancasters to prove the feasibility of a nonstop service between London and Bermuda, these trials included 11 weekly return flights. In the following year BOAC made 15 return flights between London and Montreal, using a Lancaster-refuelled Liberator.

Flight refuelling at that stage relied on a looped-hose system, in which the receiver aircraft trailed a weighted line with a grapnel at the end. The tanker fired a line across this, to engage the grapnel and allow the receiver to haul in the refuelling hose. In 1948 some systems produced by Flight Refuelling Ltd in the UK were purchased by the US and employed early in 1949 for the first nonstop flight around the world. In this a Boeing B-50 named “Lucky Lady II” took off from Fort Worth, Texas, and landed back 94 hours later, having covered 36,973 km. Refuelling was provided by pairs of KB-29M tankers stationed at the Azores, Dhahran, the Philippines, and Hawaii.

However, Boeing was already working on a more advanced concept – the flying boom. In this a telescopic tube universally mounted on the underside of the tanker was to be “flown” by an operator controlling sharply dihedralled wings termed “rendezvous”. The boom was approximately 7. 6 m long retracted and up to 13. 7 m when extended. A convergent receptacle in the upper surface of the receiver aircraft would guide the boom nozzle into the fuel coupling. The Boeing concept made possible a much higher rate of fuel flow and eliminated the need for precise positioning of the receiver aircraft. It became the standard fit on the USAF’s first production tanker, the piston-engined Boeing KC-97, and its replacement, the jet-engined KC-135. Perhaps the most dramatic demonstration of its use took place in 1957, when three Boeing B-52s, refuelled by KC-135s, flew around the world, covering 38,920 km in 45 hours 19 minutes.

The British probe and drogue system came on the scene in 1949, a Meteor receiver staying aloft for 12 hours refuelled by a Lancaster tanker. In September 1950 two probe-equipped F-84 Thunderjets flew from Manston (Kent) to Maine, USA, using one Lancaster, a Lincoln, and a B-29 as tankers. Although limited to slower flow rates, this system had the advantage of being easily fitted to any aircraft, and it allowed three aircraft to be refuelled simultaneously. On balance, it is probably better suited to refuelling tactical fighters, whereas the boom is well suited to bombers. The probe and drogue also made possible “buddy” refuelling for naval aircraft. Since many US Navy aircraft and some older USAF aircraft use refuelling probes, USAF KC-135s can accept on the end of the boom a 2. 75 m hose with a drogue.

Flight refuelling thus became a military reality in the early 1950s, gaining significance firstly in extending the range and endurance of SAC bombers and later playing a crucial role in the Vietnam War, making possible fighter strikes against North Vietnam from distant airfields in the South and from Thailand, and bomber missions from as far afield as Guam. At least one battle-damaged F-105 was towed back to base by a KC-135. Flight refuelling was also used by the Israelis in the 1967 war, to make possible long, low-level sorties outflanking Egyptian defences.

Today the US is still the leading exponent of the art, with SAC operating most of the KC-135s. On the US Navy side, in addition to buddy refuelling, each major carrier has four Grumman KA-6D Intruders (replacing the Douglas KA-3D Skywarrior). The US Marine Corps uses the Lockheed KC-130F Hercules. In the Soviet forces flight refuelling is employed primarily for long-range maritime reconnaissance, the tankers a mix of Tupolev TU-16s, Mya-4s, and (more recently) Ilyushin ll-76s.

Britain uses flight refuelling for its V-bombers, interceptors, and tactical fighters, the current emphasis being on sustaining F-4 patrols over the Iceland-Faroes gap, although tankers are also used routinely to ferry Harriers nonstop to Belize in Central America. Flight-refuelled land-based fighters were also supposed to provide cover for Royal Navy units after the cancellation of CVA-01, but this has not proved satisfactory, hence the development of the Invincible Class carriers and Sea Harrier. The RAF currently has 23 Victor K2 tankers, which are to be joined by five VC10 K2s (ex-Gulf Air) and four VC10 K3s (ex-East African Airways). All 32 aircraft will have three-point hose and drogue units, but (unlike the KC-135) none of them can be. used in the transport role, the VC10 carrying its extra fuel in tanks attached to the cabin floor.

The French Air Force has 11 C-135 Avions cisterns, fitted with hose and drogue at the end of the boom. Purchased specifically for the Mirage IVs of the force de frappe, they are also used to deploy Mirage F1s and Jaguars to Africa. The tanker version of the Boeing 707 is also used by Canada (with two Beech hose and drogue units, to ferry F-5s to Norway in wartime), by Iran (six converted 707-320Cs, with centre-line boom and two Beech drogues, to refuel F-4s and F-5s), and Israel (five ex-TWA 707s). Something of a mystery surrounds Iran’s use of tankers, in that three ex-TWA 747s were on order for this role, and trials reportedly took place, but Boeing now denies the existence of a tanker 747. Following limited use of the KC-130 Hercules as a tanker by both the USAF (refuelling the HH-3 rescue helicopter) and UMSC, the type has been exported in small numbers. Argentina has two KC-130Hs, as has Brazil (to refuel its F-5s), Indonesia reportedly had two C-130Bs converted in 1967, Israel has two KC-130Hs, Saudi Arabia has four KC-130Hs operating with its F-5s, and Spain has three KC-130Hs to support its F-4s. There are thus at least 12 countries operating tankers, aside from those with buddy refuelling equipment for aircraft such as A-4s.

Tankers On Offer

The McDonnell Douglas KC-10A Extender, which first flew on July 12, represents the top end of the market. It was developed primarily to solve the problem of flying aircraft directly from Continental US (CONUS) bases to the Middle East. For example, the recent deployment of 12 F-15s from the US to Saudi Arabia via the Azores with 115 tons of equipment and 209 support personnel took 16 KC-135s, three C-141s, and two C-5s, whereas it could have been accomplished with the support of only four KC-1Os. Given the planned force of 26 KC-10s (of which only six are yet contracted for). the USAF will be able to deploy four 15-aircraft fighter squadrons with 800 support personnel and 450 tons of equipment over 12,000 km in one day.

The KC-10A is derived from the DC-10-30CF with which it retains 88 per cent commonality. The principal changes are the addition of a refuelling boom with digital FBW system, and operator station, seven extra fuel cells forming two tanks in the cargo hold, and an AAA receptacle in the upper front fuselage, so that the KC-10A can itself be refuelled in flight. Most of the cabin windows are eliminated. The aircraft differs from the KC-135 in that conversion to hose and drogue can take place in flight, and that the ARO (aerial refuelling operator) sits, rather than lying prone. The KC-10A boom is also longer, 11 m stowed and 17. 8 m extended, and can transfer fuel at 5,677 litres/min, compared to 3400 for the KC-135. The Bonanza-type tail of the earlier boom is replaced by a U-shaped assembly. Boom refuelling is to be tested with the C-5, F-15, F-4, and A-10, and hose and drogue with the Navy’s S-3, F-14, and A-4. The KC-1QA can offload 110,000 kg of fuel at up to 2000 km radius, decreasing linearly to zero at approximately 9000 km.

The latest Lockheed Hercules tanker variant is the KC-130H, known to the Marines as the KC-130R, and used to refuel aircraft such as the F-4, A-4, and OV-1. It has an additional 13,635-litre fuel tank fastened to the floor of the hold, two probe and drogue refuelling pods 27. 5 m apart under the wings, and enlarged observer windows in the aft personnel doors. It can quickly be converted back to the transport role, carrying 18. 5 tonnes of cargo, or 92 armed troops, or 64 paratroops and their equipment. The hoses are 26 m long, and each pod can transfer fuel at 1,135 litres/min from the fuselage tank or 568 litres/min from the wing tanks. The KC-130H can transfer 23,800 kg of fuel at 1000 km radius, or 17,235 kg at 2000 km. The typical refuelling speed is 225 knots IAS at 25,000 ft, increasing to 245 knots at 20,000 ft.

Despite the development of the KC-10 and steady sales of the KC-130, the Boeing KC-135 Stratotanker will dominate the Western tanker scene until the end of the century. This aircraft has been one of the great aviation success stories of the postwar era: a basically sound design, manufactured in large numbers with few changes, and with a series of improvements now planned to multiply its life by a factor of four.

The KC-135 story might be said to have begun with the first flight of the Boeing Model 367-80, the forerunner of both the tanker and the 707 transport. Within a month the USAF had announced its intention to order a tanker derivative to replace the piston-engined KC-97, the last of 888 of these aircraft rolling out only one month before the KC-135A made its first flight on August 31, 1956. The first of the 732 KC-135As produced entered service with SAC at mid-1957.

In a recent count-up, there were 459 KC-135As still flying with SAC, 98 with the Air National Guard, 30 with the Air Force Reserve, and one with TAC, making a total of 588. In addition, there are 55 KC-135Qs which take only the JP-7 fuel burned in SAC’s SR-71 reconnaissance aircraft, and the French Air Force has 11 C-135Fs. Rate of utilisation for the KC-135 is approximately one-fifth that of its 707 stablemates, and ages are in a similar ratio. Although a handful of KC-135s have as many as 30,000 hours “on the clock”, the majority are in the 5000-10,000 hour bracket, with an average around 9000 hours, increasing at only 360 hr/yr. For comparison, the 707s are currently spread between 40,000 and 65,000 hours, with an average of around 45,000 hours.

The current KC-135A has a gross weight of 135,000 kg, and is powered by four J57 turbojets of 6,235 kg thrust. It can transfer a maximum of 54,420 kg of fuel at up to 2000 km radius, this offload falling linearly to zero at approximately 6500 km. Refuelling normally takes place in the 25,000-30,000 ft bracket, at an IAS of 250-300 knots. Since the additional fuel is carried in tanks below the cabin floor, the aircraft is readily switched to the transport role, taking 25 tonnes of cargo or 80 armed troops.

The KC-135 had one serious limitation from the outset, in the form of the material used for the wing lower skin. The 7178-T6 alloy used to save weight was adequate for the original 10,000-hour design life, but provided no scope for life extension, due to its low durability and damage tolerance, and poor crack resistance. The first major improvement for the KC-135 has therefore been to replace this lower skin, a program begun in 1975. This adds 25,000-30,000 hours to its safe life.

The second limitation is in the use of the old J57 turbojet, which limits take-off weight, is uneconomical by modern standards, and poses major environmental problems with its noise and smoke. In January a contract was therefore awarded to Boeing to retrofit a KC-135 with four GE1SNECMA CMF56-181 turbofans of 10,885 kg thrust, in the same way that one 707 is already fitted as a trials installation aircraft (first flight November 27, 1979).

Other changes envisaged for the so-called “KC-135RE” (re-engined) include a larger horizontal tail, undercarriage modifications to suit higher weights, more powerful fuel pumps, a self-contained starting system to improve reaction time, and improvements to the boom control system and aircraft flying controls. It is claimed that the KC-135RE will save up to 25 per cent of cruise fuel, corresponding to over 500 million litres per year at current utilisation rates. It will also eliminate smoke, and reduce noise to commercial standards (FAR Pt 36). The predicted 90 EPNdB footprint will be reduced in area by 96 per cent! In broad terms, it is reckoned that one KC-135RE will do the work of two KC-135As in long-range deployments, eg in taking four F-4Es from CONUS to Europe. Over shorter distances, the improvement is less pronounced. For example, in supporting Phantoms in ground attack missions, refuelling both pre-strike and post-strike, one KC-135A would support six F-4Es and one KC-135RE would support nine. In terms of transferable fuel, a KC-135RE will be able to offload over 65,000 kg at a 1500 km radius.

Preliminary estimates quote $US 7m as the cost of converting each KC-135A to -135RE standard. Boeing is pushing for 100 aircraft to be converted by mid-1985 (the first production aircraft would be delivered at end-1983), but present indications are that the USAF will opt for a slower program, and will not re-engine all the KC-135As now available.

A third major improvement for the KC-135 may come with the addition of winglets. These devices are estimated to save 7 per cent-8 per cent of fuel used, which for the SAC fleet is worth approximately 140 million litres per year. One KC-135 is already testing such winglets under a NASA research program.

Operational Considerations

Due to the high unit cost of the KC-10, it currently seems that purchases will be limited to a handful for SAC, primarily to support emergency deployments from the US to the Middle East. At the opposite end of the spectrum, the limited performance of the KC-130 and its restriction to hose and drogue refuelling appear likely to limit its acceptance. However, the very low hours on most KC-135s, and their potential for life extension to around 40,000 hours, mean that this aircraft will form the hard core of the SAC tanker fleet for the foreseeable future. If the USAF is willing to release half a dozen of these aircraft to the RAAF for underwing reskinning, CFM56 re-engining, and the installation of winglets, Australia could end up with some very useful aircraft for both the tanker and transport roles.

One of the problems in assessing the value of flight refuelling is that, given a tanker such as the KC-135, the duration of the fighter is limited by the fatigue of its pilot, rather than fuel supply. If, for example, the aim is to establish a combat air patrol (CAP) at 1600 km from the base (which is around the realistic maximum), then two hours on station may correspond to a flight time approaching six hours, which would be preceded by lengthy briefings. Fatigue on CAP would be a function of cockpit discomfort and piloting tasks. If the man is flying with his head down in the radar scope, his effectiveness will clearly go downhill faster than if he can sit back and listen to the music, relying on Jindalee to detect the target and a Grumman E-2C to specify its precise position, height, heading, speed, etc. However, even in this case, a six-hour flight is probably a reasonable limit for a fighter pilot.

On the basis of back-of-envelope calculations, a fighter on station for two hours at 1600 km radius might take on something in the region of 5500 kg of fuel. To have two aircraft on station throughout a 24-hour period would require 24 sorties, demanding perhaps 36 fighters on the ground, and four KC-135RE sorties, which might be possible with only two aircraft. These rough calculations suggest that five or six tankers represents a sensible ratio for a force of 75 fighters, with some margin to supplement the two existing 707s in the transport role.

What does a batch of tankers add up to for Australia? Firstly, both the F-111s and the NTF could be ferried to SE Asia quickly and without depending on permission to land to refuel or to overfly territory. Tankers would extend the strike radius of the F-111 to cover virtually any foreseeable target. The NTF aircraft could maintain a forward CAP over convoys or naval units while in coastal waters, or against stand-off attacks on Australian cities. The CAP could certainly be provided at a 1000 km radius, and possibly as far out as 2000 km. To put this in perspective, the Torres Strait is roughly 1250 km from Darwin and Townsville, and a 2000 km radius from these bases takes in the whole of PNG.

A tanker squadron would thus make Australia’s ability to support SE Asia from home bases more credible, and it would considerably enhance the deterrent value of the F-111. Flight refuelling would enable land-based fighters to augment the overwater defences provided by fixed-wing naval aircraft, and to protect forward Army units and the civilian population. A few years ago stand-off attack could be prevented by intercepting at around 500 km, but the advent of accurately-guided cruise missiles makes it highly desirable to have a first line of defence at much greater radius. The tanker is thus a force-multiplier in the sense of increasing the fighter’s time on station, and it is a means by which a comparatively inexpensive fighter can achieve the same objective as a far more expensive one, by intercepting the launch aircraft rather than a number of small missiles.

Tankers would thus play a vitally important role in Australia’s defence, and the KC-135RE appears to be the best choice, taking into account the low cost of the basic airframe and the operational flexibility provided by the number that could be bought.