Republished by F-111.net with the express permission of Carlo Kopp. More articles here.
Expanding the Tanker Fleet
Part 1 Problem Issues in Fleet Expansion
Text and Diagrams © 2001 Carlo Kopp, Aersopace Publications, Pty Ltd.
Should Australia aim to address the stated capability goals in the new White Paper, the RAAF will face four major capability growth challenges over the coming decade. These will be the integration of AEW&C, selection of a Hornet replacement, deployment of a strategic reconnaisance capability and importantly, significant expansion of aerial refuelling capabilities.
Aerial refuelling has been, in a sense, a Cinderella in the public defence debate. Fighters evoke intense emotion in public debate, AWACS impresses all and sundry with its high technology sophistication, while satellite and UAV reconnaissance systems carry the mystique of secrecy. Tankers offer none of these exciting attributes, yet they are the backbone of any modern air force, and perhaps the decisive element in every aerial confrontation since Vietnam.
In this feature we will take a broader look at the problems the RAAF will face in this
area, and illustrate that in its own way, aerial refuelling capability is no less
challenging than other components of modern air power.
Fighters and Aerial Refuelling
Aerial refuelling is used to support fighters in a number of ways. Long range deployment will see fighters cross intercontinental distances with tanker support. Defensive Counter-Air (DCA) will see fighters and supporting AWACS/AEW&C orbit for many hours beyond their design endurance on internal fuel, refuelled by tankers. With tankers, Offensive Counter-Air (OCA) and Land/Maritime Strike operations will see fighters reaching out to engage air and surface targets at distances well beyond their design combat radius. Range and endurance unlimited? Not quite.
The basic constraint to all aerial refuelling of fighters is that whenever a fighter lines up on a tanker to take on fuel, it should have enough fuel remaining in its tanks to make it back to a friendly runway safely. If this is not the case, failure of the refuelling equipment, be it probe/drogue or receptacle/boom, will see the fighter lost due to fuel exhaustion. While such failures are infrequent, they do happen. The RAAF on two occasions had to jettison hoses with damaged drogues, and the USAF snaps a tanker boom every once in a while. As infrequent as such events are, they are a fact of life. The cost of fighters and high risks to aircrew mean that prudent operational practice is to budget with this possibility.
In this sense, aerial refuelling is not a substitute for large fighter operating radius. Consider the simple scenario of a fighter Combat Air Patrol (CAP) orbiting some distance over the ocean, to stop a potential cruise missile attack. This is a `real world' scenario given developing regional capabilities.
The scenario can be approached in two ways. The first is that a tanker is orbiting in the CAP station area, CAPs fly out to station, top up their fuel to gain time on station, and then depart once their planned mission duration is up or they have expended their missile load in combat. In this model, the tanker only performs the task of providing endurance on station, while the distance to the CAP station is limited wholly by the fighter's internal and external fuel capacity.
An alternative approach to this model is to sortie a tanker with the CAP, and have the fighters continually top up until they reach their CAP station, upon which they continue top up to maintain enough fuel to safely recover. This approach allows the CAP to orbit at greater distances, but also forces the need for more frequent aerial refuelling to remain on station. At all times the fighters must carry enough fuel to safely recover, plus their combat gas reserve for an engagement, and fuel to burn on orbit until the next refuelling.
Smaller fighters fall down very quickly in this model, since they will be going into an engagement at weights much heavier than their optimal or design combat weight, thus losing most of their agility. Indeed, all fighters have structural G limitations at higher weights and many may be unable to manoeuvre aggressively if loaded up to 75% or more of total internal and external fuel capacity.
Larger fighters, with `big bore' engines, can accommodate this regime of operations much better. Designed for larger combat radii, they will be operating at weights much closer to their design or optimum combat weight. The structural redesign of the F-15E for 9G, against the 7.33 G rated F-15C, was no accident.
Large fighters do provide for some economies in aerial refuelling, but this is not for the simplistic reason that they can operate without refuelling when small fighters cannot. The reality is that the cruise fuel burn of a F/A-18 sized fighter does not differ dramatically from that of an F-15 sized fighter. Cruise burn is dominated by transonic drag, and whatever total weight advantage the small fighter has is offset by the higher drag resulting from more external gas tanks.
A large fighter will therefore need to take on a similar amount of gas over the sortie, to what the small fighter does, to achieve similar endurance on station.
The difference results from the reality, that a CAP of small fighters must fly out, remain on station, and return `towed by a tanker' since they need to continually top up to maintain a safe margin of fuel to get home with. A large fighter can operate at similar CAP radii without being closely tied to the tanker. Indeed, the tanker may be sortied independently of the large fighters, which only take on fuel once on station, to extend endurance.
Therefore, large fighters allow the economy of smaller numbers of tankers, and larger tankers which produce economies in aircrew, total support and fuel burn per tonne of fuel offloaded. Small fighters push the numbers of tankers up, and push their size down, since the fuel needs to be moved around in smaller chunks, and more tankers need to be in more places at once.
Another issue is the potency of a fighter. Consider a fighter fleet comprising 80 F-15 fighters - if they are replaced with 40 F-22s this yields a greater total fleet combat capability, yet the demand for tanker fleet size is close to halved. Large numbers of low capability fighters will always impose bigger demands upon tanker fleet size than smaller numbers of high capability fighters, to achieve the same total effect. If tanker fleet size is factored in, then `small cheap fighters' really amount to `big expensive fleet' fighters. Finding enough pilots to fly them and the supporting tankers is yet another issue.
These issues underscore the complexity of tanker fleet sizing analysis, and the implicit inter-relationships between fighter size, capability and tanker size.
If strike radius and CAP station radius matter, then the most general conclusion which
can be drawn is that `bigger is better', both for fighters and tankers, and economies
accrue to those players who exploit this cleverly.
Refuelling Other Types
Maritime patrol work involves long endurance sorties, and the RAF has frequently used aerial refuelling to stretch the on station endurance of its Nimrod aircraft. Whether the AP-3C replacement is another P-3 derivative, or a 737-700 IGW derivative, the latter in the running for the USN's P-3 replacement, a 1,500 NMI radius with 3-4 hrs on station can be significantly stretched by the use of tankers. This can be especially valuable when defending sea lanes, or prosecuting a submarine once contact has been made.
Airlift operations can also benefit from aerial refuelling. While the ADF is unlikely to be challenged with the global deployment needs of the USAF, some peacekeeping commitments will require global range. Of more concern is that many runways in the nearer region are of woeful quality and length, and this will often severely limit airlifter takeoff and landing weights. Having the opportunity to gas up after takeoff from such a runway removes this problem as a critical constraint to operations.
Whether tanking is used to extend AEW&C, maritime or airlift resources, the common factor in all is that large volumes of fuel may need to be offloaded from the tanker. The receiver aircraft will carry 20-30 tonnes of internal fuel, and heavier airlifters even more. Therefore a top up by a tanker might require anything from 10 to 20 tonnes of fuel, many times the offload required per fighter.
The central conclusion is that a robust tanker fleet can produce large gains, no matter
which way it is used.
Crewing a Tanker Fleet
During the latter part of 1999, the author produced a 140 page study on strategic tanking, published in March 2000 by the RAAF Aerospace Centre. The four part tanker series in AA drew largely on this study. Extensive computer simulations were performed to gather hard numbers for this task.
The most interesting conclusion of this study was the required number and size of aerial refuelling tankers to address the RAAF's current shortfall in capability. Three separate models were used to produce a fleet size estimate. One was based wholly upon defensive Combat Air Patrols in the Pilbara, another was based upon regional long range power projection, and the third was based upon scaling the ratio of fighters to tankers, used by the US Air Force and Royal Air Force, against the number of F/A-18s and F-111s in the RAAF force structure. All three estimation models yielded almost identical results.
These results indicated that should the RAAF aim to properly address this capability, it would need to field either 12-16 heavy tankers (in the class of the Boeing 747, Boeing DC-10 or Airbus A340), or 25-30 medium tankers (in the class of the Boeing KC-135R, Boeing 707-320, Boeing 767 or Airbus A330), or some High-Low mix of heavy and medium tankers. A heavy tanker can offload roughly twice the fuel which a medium tanker can offload.
While we can quibble about specific assumptions made in the sizing analysis, the order of magnitude will not change significantly. The USAF experience since the 1950s is the bottom line - for every 100 fighters, around 25 medium tankers or 12.5 heavy tankers are needed to provide robust aerial refuelling support. The commitment to 5 tankers in the White Paper is a best a `half-measure' and this will need to be addressed with proper funding over the next decade.
The big issue the RAAF will face with aerial refuelling is that of crewing a fleet. Difficulties will arise especially in training qualified tanker commanders. On the other hand, the capital costs of 12-25 tankers are comparable to those of 12-25 new fighters, which are significantly lower than upcoming programs such as AIR 6000.
The US Air Force Doctrine Document 2-6.2, `Air Refuelling', provides an excellent indication of needs in tanker crewing, empirically learned over five decades.
For `intertheater' aerial refuelling, comparable to long range land and maritime strike aerial refuelling support in the ADF context, an aircrew to aircraft ratio of 1.65:1 allows full utilisation of the available aircraft. This assumes the aircraft each fly 9.9 sorties per week, each of 12 hours duration, under which conditions the aircraft spend 70% of the time airborne.
For `intratheater' aerial refuelling, comparable to defensive combat air patrol aerial refuelling support in the ADF context, an aircrew to aircraft ratio of 1.62:1 allows full utilisation of the available aircraft. This assumes the aircraft each fly 19.4 sorties per week, each of 4 hours duration, under which conditions the aircraft spend 46% of the time airborne. Extending the average duration of the sortie will provide better utilisation of the aircraft, although this is unlikely to strongly impact the ratio of aircrew to aircraft.
Should allowances be made for spare aircrew, instructor pilots and pilots redeployed in staff positions, an aircrew to aircraft ratio much closer to 2:1 would result. In the simplest of terms, for every tanker aircraft close to two aircraft commanders and two copilots would be required.
If the fleet is to be sustainable, that is have the capacity to perform high intensity operations for several consecutive months, as may occur in a crisis situation, then even a 2:1 crewing ratio may not be adequate. This is because crews can only put in a finite number of flight hours per week and month, not to exceed safe fatigue levels.
The US Air Force makes substantial use of the Air Force Reserve (AFRes) and Air National Guard (ANG) to crew their fleet of around 600 tankers. Roughly 50% of the tanker fleet is crewed by AFRes and ANG personnel, many of whom have `weekday' jobs as commercial pilots.
Should the RAAF aim for a tanker fleet with credible aggregate size, depending on the mix of medium and heavy tankers used, a total of 24 to 48 command qualified pilots, and 24 to 48 first officer qualified pilots would be required. Should one half of these be drawn from active reservists flying for the airlines, the RAAF would still need to maintain 12 to 24 full time pilots in either category. If a minimal ratio of aircrew to aircraft is maintained, such as 1.25:1 as the US Air Force do at this time, the requirement drops to about 15 to 30 pilots in total, in either category.
These order of magnitude numbers cannot be substantially altered, at best the burden can be shifted to a higher proportion of active reserve aircrew in the employ of the commercial airlines. Withdrawing, at short notice, such a number of reservists from the airlines would however impact operations for the airlines, especially if they are also subjected to ADF demands for second echelon civil airlift. Reliance upon the US Air Force to provide adequate aerial refuelling assets is yet another instance of shifting the burden elsewhere, in this instance upon the US taxpayer. Given that the US Air Force tanker fleet is already undercrewed, such a strategy incurs the risk of the US not being able to provide the resource in a crisis situation. Needless to say, leasing the tankers changes the equation very little, if the aircraft are to be flown in a combat situation.
Of all the obstacles the RAAF will face in a future expansion of aerial refuelling
capabilities, the crewing problem will by far be the most difficult one to resolve.
Given judicious choices in aircraft types and powerplants, most of the support burden can be shifted to existing airline operators, who already maintain substantial fleets of similar or identical types. Indeed, sharing facilities such as flight simulators further improves upon this.
A much bigger issue for the RAAF is the provision of adequate aviation kerosene supplies to the more remote bases in the north of Australia. Strategically, the four most important bases are RAAF Learmonth, RAAF Curtin, RAAF Tindal and Darwin airport. Only Learmonth and Darwin at this time have runways which are rated for the use of heavy tankers at high gross takeoff weights.
Sustained high intensity operations by a substantial proportion of the RAAF fast jet force, fully supported by proper numbers of tankers, would consume many hundreds of tonnes, or more, of aviation kerosene per 24 hour cycle. While underground storage tanks with 10,000 - 20,000 tonnes capacity, installed at Darwin, Learmonth and possibly Curtin, would buffer the demand during short notice or surge operations, proper replenishment would be required to sustain operations at this tempo. This is unlikely to be feasible using tanker trucks on northern highways.
Fortuitously, the two most relevant runways, Darwin and Learmonth, are situated sufficiently close to the coast to allow for a shipping pipeline to be provided to a loading jetty. In this manner a tanker ship can be used to replenish on site storage at a sustainable rate.
Another important factor to consider is the recently initiated Syntroleum Sweetwater project for a Gas-To-Liquids (GTL) synthetic crude oil production plant, being built close to Karratha in the Pilbara (http://www.syntroleum.com). Such plants use a derivative of the synthetic fuel production technology used by Germany in WW2, replacing the coal feedstock with natural gas. This plant and technology are very attractive for two reasons. The first is its proximity to Learmonth and Curtin, which cuts fuel transportation costs and shipping delays. The second reason is that a synthetic JP-8 / Jet A type fuel could be made with virtually zero sulphur and heavy metal content, and precisely controlled aromatic content, thereby reducing jet engine corrosion and `coking' in combustors and burners, respectively.
The current thrust in the GTL industry is to produce `environmentally friendly' synthetic gasoline, diesel and high purity hydrocarbon feedstocks. Aviation kerosene as a smaller market has yet to attract the industry's focus, but given the high cost of engine overhauls and progressively declining quality of natural crude oil stocks, the long term drivers would favour such a fuel.
A good case can be made for Australia to be self sufficient in the production of aviation kerosene, which is vital for the ADF and the commercial airlines. The 10,000 BBL/day total capacity Sweetwater plant, even if configured for aviation kerosene production, would not have the capacity to meet the needs of high intensity RAAF operations in the north-west. However, should plant capacity be increased 5-10 fold in coming years, then it would be able to meet such needs.
It is worth noting that the Allied Force air campaign against Serbia relied critically upon the NATO network of fuel pipelines, which were used to replenish NATO air bases in Italy and Germany. The availability of substantial fuel supplies to bases in Saudi Arabia was pivotal to the sustainability of the Desert Storm campaign in 1991.
What the cost of proper fuel replenishment might be remains to be exactly determined.
Underground concrete fuel tanks and plumbing are not particularly expensive. The political
complexities of securing domestic self sufficiency or secure wartime kerosene supplies may
prove to be the bigger issue.
Choice of Aircraft for Tanker Conversions
The principal risk faced in tanker buys is that of injudiciously choosing an aircraft type which disappears from the commercial fleets well before the air force owned tanker variant runs out of airframe fatigue life. If the disparity is too great, the long term cost of supporting the tanker variant can become prohibitive.
This is because military tankers accrue flight hours, and thus airframe fatigue, much more slowly than their airline operated siblings. The US Air Force are now facing this very problem with their fleet of KC-10 Extender tankers, based on the commercial DC-10. The commercial DC-10 fleet will run out of life around 2010-2015, leaving the USAF with the burden of supporting an orphaned aircraft with one half or less of its fatigue life expended. The USAF KC-135 fleet is cited at 14,500 hrs airframe time, on average, yielding a fatigue life until 2040.
A useful side effect of the different rate at which fatigue is accrued in tankers is that 7-12 year old used airliners are a much better buy than new build aircraft, and at this age most used airliners cost a small fraction of the cost of a new aircraft.
Minimising the cost of a tanker fleet over its life thus drives a buyer in the direction of those airliner types which are most widely used and most likely to thus remain supportable cheaply over the longer term. Further cost reductions will also accrue from the opportunity to share training facilities and exploit reservists flying commercial models of the same aircraft.
In this context, heavy tankers are a better buy than medium tankers, since typically half as many are required to deliver the same load of fuel, thus reducing support costs and aircrew numbers. Medium tankers look better only in the training role, and low intensity operations, where their lower fuel burn reduces operating costs.
The principal cost driver in a tanker conversion is the Non Recurring Expenditure (NRE) incurred in the design, prototyping and flight testing of a conversion. For a small number of tankers, this overhead can rival the cost of the fleet. Therefore, the only economically viable choices for the ADF are established tanker conversions, where the manufacturer or another air force has absorbed the overhead.
At this time there are few such conversions in the market. Other than refurbished US Air Force KC-135Rs, the only tested and flown choices are the Boeing 707, Boeing DC-10 and Boeing 747. While the Boeing 767 and Airbus A310 have been vigorously marketed, to date no conversions have been performed as the manufacturers seek customers which are prepared to absorb the NRE of a conversion design, prototyping and test.
The only medium tanker which incurs zero NRE is the KC-135R, rebuilt from mothballed KC-135A tankers held by the US Air Force in their desert boneyard. While the Boeing 767 and Airbus 330 would be viable alternatives with ample numbers appearing in the used aircraft market in coming years, until another party pays for the NRE of the tanker conversion they would be quite expensive. The KC-135R is an excellent choice in terms of performance, but the basic airframes are very old and thus problems similar to those seen in the RAAF 707-338C may need to be addressed in the medium to long term.
In terms of heavy tankers, only the Boeing 747 and DC-10/MD-11 incur negligible NRE. The latter is not flown in Australia and is thus not viable, by default. The 747 is abundant and cheap in the used aircraft market, with -200 and -300 series models similar or lower in cost to the much smaller 767 series. Boeing recently delivered the first 747-300SF freighter rebuild. With the impending production of the Longer Range 747-400 (AKA 747-400X), 747X and stretched 747X, extended range 777 and Airbus A3XX series, we can expect to see the 747-400 soon appear in the used market at affordable unit prices as it is displaced from its most competitive niche. The 747-400 provides much better performance than the -200 and -300 models. The Airbus A340 falls well below the 747 in performance, but provides substantial commonality with an A330 tanker. Like the A330, it incurs full NRE.
Cost and support factors aside, another important issue is performance. Jet airliner/tanker airframes can be broadly divided into two categories, by wing aerodynamic design.
These are airframes with `fast' wings, with quarter chord sweep angles between 35 and 37.5 degrees, optimised for fast cruise at Mach 0.84-0.86, and airframes with `slow' wings, with quarter chord sweep angles between 27 and 31 degrees, optimised for slow cruise at Mach 0.79-0.82. The 747, DC-10/KC-10, 707 and KC-135 fall into the fast category, the 767 and Airbus types the slow category.
A tanker with a slow wing does best when loitering in an refuelling orbit, as fuel burn is minimised, but is slower in transit to station and cannot keep up with fighters on long range deployment or strike sorties. A tanker with a fast wing can match the cruise climb profiles of fighters in transit and long range sorties, but burns slightly more fuel when parked in an orbit. So the tactical and operational advantages largely go to tankers with fast wing designs, accepting that in loiter intensive situations the aircraft is slightly less efficient than a slow winged tanker.
What the optimal choices in this game?
1.Tankers with fast wing designs are operationally superior, with some penalty to be paid in fuel burn.
2.New build airframes may not pay for themselves within a supportable aircraft life, given the slow rate of fatigue accrual.
3.A High/Low mix of medium and heavy tankers falls in between the operational economic advantages of a heavy tanker fleet and flexibility and training cost advantages of a medium tanker fleet.
4.A High/Low mix of medium and heavy tankers with a bias to heavy tankers is cheaper in offload per total dollar than a mix biased to medium tankers.
5.Established tanker conversions are much cheaper to acquire and support than new design conversions.
Where does this leave the RAAF? If the aim is to robustly address the White Paper capability goals, then some High/Low mix of aircraft using established conversions and fast wing airframes yields the optimum package, with the exact mix ratio yet to be determined.
Of course, if we place other criteria above these, then other answers might fit. However, such a strategy either increases total package cost or compromises operational capability. As always, there are no free lunches.
Part 2 will explore the various aircraft types available for fulfilling the role of medium tankers.
Pic.1 RAAF 707-338C in grey (not enclosed)
The RAAF has four Boeing 707-338C tankers, to provide a training and limited operational capability. In numbers these aircraft are inadequate to meet future needs, despite their excellent airframe design for the role, and their age is beginning to show.
Pic.2 RAAF F/A-18 (not enclosed)
The F/A-18A is not well suited for long range combat or long endurance combat air patrols with CAP stations hundreds of nautical miles from a runway. Appropriate aerial refuelling capacity can however alleviate these problems and give the aircraft some credibility during the remaining decade or more of its operational life.
Pic.3 RAAF F-111 (not enclosed)
An important advance in the recent White Paper is a firm commitment to provide boom equipped tankers to support the F-111 fleet in the long range strike and sea control roles, vital to the regional `denial strategy' which underpins the document. With tanker support, USAF F-111F and EF-111A aircraft flown in the El Dorado Canyon strike against Libya remained airborne for almost 14 hours, this amounting to an effective combat radius of about 3,000 nautical miles.
Pic.4 RAAF 707 cockpit shot with crew (not enclosed)
The most difficult problem the RAAF will face in future tanker fleet expansion will the the recruiting, training and retention of adequate numbers of pilots, especially qualified tanker commanders. Full utilisation of a tanker fleet requires a 1.6:1 or greater ratio of crews to aircraft, with each crew comprising a commander, copilot, most likely also a flight engineer and refuelling equipment operators.
Reliability theorists use the `bathtub curve' to illustrate the failure rate behaviour of equipment. The `infant mortality' period is associated with failures due to manufacturing defects, the `active life' period covers time when the equipment can be economically used, while the `wearout' period is when fatigue and corrosion introduce an increasing frequency of failures. For tanker conversions, the optimum is to select airframes which are likely to remain in production for as long as possible, yet are mature enough for affordable 7-12 year old used aircraft to be bought (Author).
An alternative to the 747 series for the heavy tanker role is a KMD-11CF/F,
converted from a used commercial MD-11 trijet. In terms of fast cruise and fuel offload
performance, it is very close to the KC-10A and 747 tankers, depending on the
configuration of lower deck fuel cells. Because boom conversions for the KDC-10 and pod
conversions for the KC-10A can be utilised, the overheads of tanker conversion are
minimised. Its disadvantages are the lack of Australian MD-11 operators and much lower
main deck capacity for bulky airlift, which may be an issue should airlift capacity become
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