White Paper is centred upon the use of air power to control the
maritime and air approaches to Australia, and deliver long range
strikes against hostile assets and bases which might be capable of
threatening Australia. Both of these central tenets of our new defence
strategy rely upon the RAAF having the reach and endurance to do the
job, which over the longer term will demand a substantial fleet of
The optimal strategy for the RAAF is to aim for some High-Low mix of
medium and heavy tankers, as this provides an appropriate balance
between the flexibility, training and low intensity operational use
economies of a medium tanker, against the crewing and high intensity
operational use economies of a heavy tanker. Fiscal realities will
however most likely force the RAAF in the direction of a single type
tanker fleet, which makes careful choices all the more important. In a
single type fleet, crewing requirements for a full fleet strength
strongly favour heavy tankers. The RAAF was at the time of writing
uncommitted to a specific tanker fleet model.
In terms of candidate airframes for the heavy tanker role, the short
and medium term optimum is a 747 derivative, with an MD-11 derivative
in the required offload class but presenting some longer term support
issues. Used 747s are cheap and plentiful, indeed quite a few Qantas
-200s, -300s and SPs may become soon available. As a strategic tanker
the smaller A340 will always be disadvantaged by its wing design, even
if it becomes economically viable and supportable in Australia. The
A340 is out of its league against the 747 as an airlift asset.
Interested readers are invited to visit
http://www.defence.gov.au/aerospacecentre/publish/paper82.htm for a
more detailed analysis of these issues.
If the RAAF opts for a High-Low mix strategy, or decides that it does
not wish to ever economically expand its tanker fleet to full strength,
opting for a long term strategy of limiting its growth potential to a
half measure fleet, then medium sized tankers would need to be
acquired. Unlike the situation in heavy tankers, where both 747 and
MD-11 derivatives are well known quantities, in medium tankers the
situation is more complex.
While the new White Paper commits the RAAF to new technology tankers,
it is worth examining the 707-338C to place this decision into its
proper context. The RAAF's existing fleet of four 707-338C tankers was
intended to provide a training and limited operational capability.
These aircraft were converted to tankers during the early nineties, by
IAI/HdH in Melbourne.
The AAR hardware installation is based upon a design produced by the
Bedek Division of Israel Aircraft Industries for the Israeli Defence
Force, which has several tanker aircraft in service. The system design
had some detail changes to meet the RAAF's engineering requirements.
Most of the hardware was sourced in the US and UK, with remaining
components supplied as upgrade kits by IAI. The Mk.32B refuelling pods
were manufactured and supplied by Flight Refuelling Ltd in the UK.
Internal modifications to the 707-338C systems were necessary. The AAR
system uses hydraulically powered fuel pumps to drive fuel to the pods,
which in turn feed the fuel via hose to the receiver aircraft. Four
submerged J.C.Carter fuel pumps are situated in the centresection fuel
tank and these feed fuel into 3" pipes via a crossfeed valve
arrangement which allows either pod to be fed by any pump. This
functional redundancy was adopted to minimise the likelihood of fuel
pump failure interrupting an AAR hookup. Under operational conditions
each pod will be supplied by a selected pump. The 3" pipes are
installed through the wing main spar box using attachments designed to
decouple mechanical loads from the wing structure. The pipes then
attach to mounting flanges within the wingtip pylons, the pylons are
structurally attached to the forward and aft main spars.
The fuel management strategy used during AAR operations differs from
that adopted for regular 707-338C operation, as fuel from the inboard
and outboard wing tanks is pumped into the centresection tank from
where it is offloaded to receiver aircraft.
Hydraulic fluid for the pumps is supplied via 1.25" pipes and hoses
from two redundant utility hydraulic systems, designated UT1 and UT2.
UT1 is the basic 707-338C hydraulic system which is powered by two Abex
engine accessory drive hydraulic pumps fitted to inboard engines #2 and
#3. Typically one fuel pump will be driven by UT1, together with
remaining aircraft systems such as flight controls. UT2 is a new
installation carried out as part of the upgrade and involves, other
than the necessary plumbing, the installation of another two Abex
hydraulic pumps on outboard engines #1 and #4. Again, under operational
conditions, UT2 will in turn supply the second pod. This highly
redundant strategy is designed to allow system operation with full or
partial capability in the event of hydraulic or fuel pump failures. The
system is designed to accommodate a boom or a third hose/drogue system.
The upgrade also incorporated dual redundant Litton LN-92 ring laser
gyro equipment, dual redundant Collins SIT-421 IFF transponders, and a
forward facing Hazeltine AN/APX-76B(V) IFF interrogator with dipoles
mounted on the existing weather radar antenna. A Collins 150 Tacan
system was installed, consisting of AN/ARN-118 and APN-139 subsystems
which allow receiver aircraft to locate and rendezvous with the tanker.
Additional communications equipment included a Magnavox AN/ARC-164 UHF
transceiver and a Collins DF-301E/F UHF DF set. The refuelling operator
was provided with a steerable camera in a lower fuselage turret.
The aircraft do not have lower deck auxiliary fuel cells, since at
158,000 lb of internal fuel, the sixties technology JT3D-3B engines
would be unable to get the aircraft airborne from most runways with the
additional weight of fuel.
In terms of basic airframe aerodynamic performance, the 707-338C is
without doubt the best medium tanker in existence. Indeed, the never
implemented KC-135H and KC-135X upgrades would have seen the 707-320B
wing retrofitted, with either TF-33 or CFM56 engines, to KC-135A
airframes. The 707-320B is the basis of the E-3 AWACS, E-6A TACAMO, E-8
JSTARS and KE-3A tanker.
The RAAF's 707-338Cs have little remaining fatigue life, with serious
fatigue problems in key structural components of the wings and
centresection. Moreover, other problems have arisen. The JT3D-3B
engines, which are too noisy for most civilian airfields and
underpowered, are now being obsoleted by the manufacturer. Old age is
also taking its toll, with corrosion in some places and the electrical
wiring approaching the end of its safe life. It is likely that the
steamgauge cockpit instrumentation will be become harder to support
over time, as the USAF puts Pacer CRAG glass cockpits into its 707 and
KC-135 derivative fleet.
Replacing the engines is not a difficult task, the candidates being
either the CFM56 common to the 737 and KC-135R, or the JT8D-219 common
to the MD-80 series. Both engines deliver similar 21,000 lb class
takeoff thrust, with the CFM56 offering slightly better SFC due to its
much higher bypass ratio, while using a bulkier and less convenient
nacelle size. While the CFM56 is the better performer, a retrofit is
costlier due to the need for structural work to fit a very different
pylon design. An engine replacement would come to around USD 30M per
aircraft. Replacing the electrical wiring is also straightforward, but
potentially costing several million per aircraft, as the wiring is
embedded in the structure in many places and difficult to access.
The biggest long term issue for the 707 will be corrosion. The USAF
greatly regretted its decision to accept a political directive to use
refurbished 707-320B airframes for the E-8 JSTARS, instead of new build
707-320/E-3/E-6 airframes. Repair of all airframe corrosion pushed the
price of the E-8 refurbishment close to that of new build
The JSTARS program yielded a large database of corrosion statistics on
the 707 airframe. The most severe corrosion was generally found in four
hot spots, the worst by far being around the nosewheel well and forward
fuselage. Other problem areas were in the lower fuselage, at three
points aligned with the leading edge of the wing, and ahead and behind
the main undercarriage wells. Lesser but still significant corrosion
was found in the wings, especially around the engines, and upper wing
roots. These corrosion hot spots differ from those seen in the RAAF
fleet, which has experienced most of its corrosion in the upper wing
skins, fuselage roof and tail surfaces.
Herein lies the crux of the issue, in that the reskinning, structural
repairs and rewiring required to give the 707s another 20 or more years
of airframe life could prove to be as expensive as USD 65M per
aircraft, pushing the price close to that of a very good used 747, 767
or Airbus. Add in engines, a boom, avionics updates and the cost
becomes very close to a 747, 767 or Airbus conversion.
In a sense this is unfortunate, since the 707 is aerodynamically a
superb fast tanker, especially if fitted with new engines, a boom and
lower deck fuel cells, indeed it would be much like the proposed
Boeing built 820 KC-135A and derivative special purpose airframes
between 1957 and 1965. No less than 732 were KC-135As. At this time the
USAF has 609 KC-135s of various models in service with USAF, AFRes and
ANG squadrons, and around 60 airframes of various subtypes remain at
the AMARC boneyard. It remains the most numerous large aircraft in the
The KC-135 is the forerunner to the 720/707 series, using a single lobe
fuselage which is several inches narrower and less tall than the 707,
and wing very similar to the now extinct 720/707-100 series. Up to
31,200 USG of fuel is carried in 12 wing tanks and nine fuselage tanks,
only one of which is above the floor.
The aircraft have been through extensive upgrades over their service
life. Known structural fatigue problems were addressed by a number of
modifications. The WS360 fix alleviated a problem in a wing splice
plate, ECP 405 replaced 7178-T6 alloy lower skins between the engines
with stronger 2024-T351 alloy skins, and replaced around 62% of wing
structure, including many spars, rib cords and stiffeners. These
modifications add 26,000 hrs to the structural fatigue life of the
An ongoing upgrade has seen the replacement of the immersed fuel pumps
with a safe dry running type. A number of aircraft were retrofitted
with yaw dampers cannibalised from retired 707s.
Two major upgrade programs are active at this time. The most
significant of these are the KC-135R and KC-135T programs, which
incorporate the replacement of the J57 turbojets with CFM56-2B1
(F108-CF-100) fans, the installation of a Flight Control Augmentation
System (FCAS) incorporating a yaw damper and improved pitch trim
control, the addition of a pair of T-62T-40 auxiliary power units (APU)
in the rear fuselage for unassisted ground starting, improved brakes
and electrical power generation, strengthened undercarriage, aft
fuselage blister windows, a refuelling receptacle above and behind the
cockpit, and the new technology improved refuelling boom using an
extruded tube structure.
The original CFM56-2B1 had the thrust reversers removed, and the
retrofit requires structural strengthening of the front wing spar, new
pylons and unique nacelles. The engine has a static rating of
4,970/22,000 lbf and cruise SFC of 0.662 lb/lb/h (cf JT8D-219 at
The other important upgrade is Pacer CRAG, which fits a modern
technology glass cockpit, flight management system and GPS, intended to
remove the need for the navigator. In practice, high workload sorties
may still require a third flight crew member.
An optional upgrade applied to a small number of USAF KC-135Rs, is the
Boeing Multi-Point Refuelling system, essentially an wingtip
installation of Mk.32B pods similar to that in the RAAF 707s.
The ANG KC-135E upgrade involved the retrofit of TF-33-PW-102/JT8D-3
fans, in part cannibalised from retired 707s. Many ANG units are now
The KC-135R is the best medium tanker available in the market at this
time, in terms of capabilities and performance, and the nearest
equivalent would be a 707-320 series with a similar package of upgrades
applied to it.
The cost of a KC-135R is nominally around USD 53M or slightly more with
the Mk.32B pods and Pacer CRAG fitted, while the KC-135E is nominally
worth USD 30.6M, and the KC-135A nominally USD 26.1M. Therefore the
cost of raw boneyard KC-135A is similar to that of a pre-loved 747-200
The big issue for the USAF is affordably stretching the life of the
fleet to its expected fatigue life expiry in 2040. As the airframes are
of similar age to the RAAF 707-338C and USAF 707-320B JSTARS, the USAF
has major concerns about corrosion and other deleterious effects of old
age on the airframe and systems. While the USAF has plans for a new
technology KC-X tanker to be fielded after 2013, the cost of replacing
around 600 KC-135s with even a lesser number of new KC-X tankers makes
any life extension effort on the KC-135 highly profitable. Even should
an affordable long term fix to the corrosion problem be found, long
term support will be hampered by poor availability of other system and
airframe components. We may yet see the KC-X program initiated earlier
than previously planned, and many reports from the US suggest the
program may be accelerated.
In considering the KC-135R as a medium tanker alternative for the RAAF,
the principal issue will not be performance or capabilities both of
which are excellent, but rather long term support costs, the same
problem faced with the 707-338C. There would be a genuine risk that a
KC-135R buy would soak up USD 50M per airframe of corrosion repair and
refurbishing costs per unit, at some time during the next 2 decades.
Current statements from the RAAF and DMO would indicate that the KC-135
will thus not be considered.
Boeing 767 and Airbus
Both Boeing and Airbus have actively marketed tanker/transport
derivatives of their widebody twins, as the KC-767 and MRTT
respectively. Both types offer similar or better offload performance
than the KC-135R (a KC-767-300ER cca 15% better), and are also much
better in the secondary airlift role due to larger internal volume and
greater floor strength. In terms of simple metrics such as payload
range, long range variants of both aircraft make for excellent medium
tanker, at the higher end of the performance scale. With a large
support base worldwide in commercial use and large numbers of used
airframes, albeit a little expensive at this time, both would be
relatively easy to acquire and support.
The snag with both the KC-767 and an Airbus MRTT-310/330 derivative is
that nobody has yet paid for the design, prototyping and flight test of
the conversion, which is likely to run into a considerable sum. Flight
test against large numbers of inventory military types to be refuelled
can be particularly resource hungry. Until this overhead is paid for
either by the manufacturers or another air force, this cost overhead
would make both types an expensive proposition for an RAAF fleet. A
used 767-300ER costs between USD 50-90M per unit, pushing the cost with
an refuelling conversion close to the USD 70M-110M mark, without the
overhead of conversion design, prototyping and flight test.
An A330-200/300 derivative would offer slightly better offload
performance than an equivalent 767-200/300, and pod installation on the
wings is simplified by structural commonality with the A340. However,
the smaller operating base will push the unit cost up. As recently
announced, Qantas intend to operate seven A330-200 and six A330-300.
The key issue for the A330 will be the cost of used airframes, even the
oldest of which can fetch around USD 90M per unit.
At the time of writing, Italy was yet to announce whether it had chosen
the KC-767 or an Airbus MRTT-A330 derivative for its tanker
requirement. Japan is recently reported to have authorised funding for
tankers, the KC-767 being the most likely prospect given Japan's use of
the E-767 AWACS, but as yet no announcements have been made on specific
choices for the JASDF. Boeing have been very actively marketing the
KC-767 as a direct KC-X replacement for the USAF KC-135R.
The limitation of both the KC-767 and MRTT-310/330 is the Mach
0.78-0.82 optimised wing, which is a byproduct of the original domestic
medium haul design optimisation of both types. This is especially an
issue for the Airbus designs, and makes both families of aircraft less
than ideal for military use where transit and dash speed is an issue,
such as supporting reactive long range CAPs and maritime or long range
strike sorties, or emergency refuelling.
Another key issue for both of these twin engine airframes is mission
reliability on long duration or long range over-water sorties. While
the loss of an engine does not mean the the loss of the tanker, which
can straggle home on one engine, it would most likely result in a
mission abort since the aircraft could not be expected to continue
its refuelling mission on one engine alone. The practical consequence
of this is that more airborne spare tankers will be required to ensure
that a tanker abort does not result in a complete strike package or CAP
mission abort. With the White Paper capping the fleet to 5 tankers,
this would significantly complicate what the RAAF could do with a
medium tanker based fleet.
Therefore, should the RAAF opt for either family of aircraft to replace
the 707-338C as the standard medium tanker, the aircraft will be
relatively expensive in offload per dollar due to commercial demand for
used airframes, and tactics will need to be adapted to accommodate the
dash speed performance limitations of these types, and the limitations
of two engines over water.
Airlifters as Tankers
An alternative which is frequently raised in the public debate on
aerial refuelling is the use of airlifters such as the C-130 Hercules,
C-17 or A400M as tankers, by equipping these with refuelling equipment.
Airlifters are a poor choice for aerial refuelling, since they cruise
at much lower speeds than fighters, and cannot compete against
airliners in fuel offload performance, the principal measure of a
tanker's worth. Moreover, if committed to refuelling they are
unavailable for airlift, and vice versa.
To place this in perspective, a C-130 Hercules airlifter equipped as a
tanker delivers about 1/3 the offload performance of a medium tanker
like a 707 or KC-135. Other than niche roles such as refuelling
helicopters or close air-support fighters, the C-130 is not very useful
as a tanker.
It follows that the RAAF would have to commit its whole airlift fleet
to aerial refuelling operations to achieve the effect of a small number
of genuine tankers, imposing thus much greater demands in aircrew and
airframe time per tonne of fuel offloaded.
The other side of this argument is that a robust fleet of RAAF
tanker/transports can address much of the strategic airlift needs, thus
freeing up the C-130 fleet almost wholly for tactical Army support work.
Electronic Warfare Self
At this time USAF tankers do not carry either Radar Warning Receivers
(RWR) or Defensive Electronic CounterMeasures (DECM)/Expendables.
Therefore these aircraft are wholly dependent upon defence by fighters
and supporting AWACS.
This remains a very contentious issue in the US. In every air war since
the 1960s, tankers have had to perform emergency refuelling
penetrations of contested airspace to rescue fighters with empty tanks,
most recently during Allied Force. Without RWR/DECM, these aircraft are
sitting ducks for mobile SAMs or fighters which may penetrate the
defensive CAPs. Some years ago the author discussed this issue with a
USAF ANG tanker captain who flew in Desert Storm. Their operational
practice was to carry extra crew to maintain a lookout using binoculars!
In this day and age of 80 NMI range ramjet BVR missiles such as the
R-77M RVV-PD (ramjet AA-12 Adder), the safety margins for forward
operating tankers have been significantly eroded. US operational
experience clearly shows that fuel management by fighter pilots in the
heat of combat will always be an problem issue, so tankers will always
be confronted with the need to either skirt or penetrate into dangerous
The inevitable conclusion is that a prudent operator will install some
defensive EW capability.
What represents the best EW package is an excellent question. An RWR is
a must, preferably one with decent detection range performance against
a 10-20 kW class air intercept radar. A towed decoy package would be an
excellent defensive measure against long range radar guided AAMs and
SAMs. Whether expendables are justified is open to debate, insofar as a
fighter getting close enough to use a heatseeking AAM might just as
well use his gun. At the speeds, altitudes and ranges tankers will be
operating at, the primary threats will remain long range SAMs like the
SA-10/12/20 and BVR missile shots by fighters.
If the tanker has a secondary role as an airlifter, then shoulder
launched SAMs do become an issue and a flare dispenser or infrared
jammer is justified.
Equipping a tanker with a robust defensive package is a large cost
overhead, which could fall into the USD 5-20M cost range, depending
upon how elaborate the package needs to be. Should the RAAF consider
equipping its future tanker fleet with a defensive package, there would
be much merit in using as much common hardware as is possible against
the F-111 and F/A-18. Should the domestic ALR-2000 series be used, this
would at least introduce some economies of scale into the equation.
Given the airframe size of a tanker, antenna and towed decoy placement
is not an issue.
Funding Tanker Fleet Expansion
As always, funding expansion of an existing capability will result
inevitably in arguments over money. The basic cost of a robustly sized
tanker fleet would vary significantly with the mix of aircraft chosen
and level of capability per aircraft. For instance in heavy tankers,
provision of full freight capability can add USD 12M-20M per aircraft.
While plumbing and wiring for wing pods costs around USD 2.5M-3M per
aircraft, an all up tanking package including a boom, manifold and pump
system, auxiliary tanks, AAR receptacle/probe and single point ground
refuelling falls into the USD 30M-35M range. Additional avionics such
as JTIDS/MIDS terminals, secure military comms, military GPS and a
minimal RWR would run into millions per aircraft. Depending on the type
and age of the airframe chosen, between USD 30M and 90M could be spent.
Therefore a medium or heavy tanker could cost between USD 75M and 145M,
medium tankers not necessarily being cheaper to buy than heavies. With
fleet numbers between 12 and 28 the total package cost could vary
between USD 0.7B and 4B, depending on choices made. Therefore doing a
proper tanker fleet expansion would be a large project, albeit much
smaller than AIR 6000 and at most similar to AIR 5077 in costs.
There are numerous ways in which this could be implemented. The
classical model would be to order the aircraft to be delivered over a
3-5 year period, and expend around USD 0.5B annually over that period.
Alternately, the buy could be spread over a 10 year period, halving the
annual cost to about USD 250M and also providing plenty of time to
build up aircrew and training systems.
Another strategy is private financing (PFI), where the Commonwealth
would contract a supplier such as an airline to provide the aircraft
and crews on demand, either for the whole tanker fleet or a portion of
it. Numerous alternatives exist in PFI schemes, ranging from a
dedicated RAAF use only tanker fleet, to dual use aircraft which are
swapped between military tanking and commercial transport work.
In a dual use PFI arrangement, the aircraft would be owned by the
contractor, and flown as commercial freighters or airliners, with pods,
military comms and EW equipment removed, but retaining the tanker
plumbing, wiring, pylons and boom. The performance hit resulting from
the extra weight/drag would be offset in the cost of the contract.
Regular training, large exercises and crisis situations would see some
or all of the dual use PFI aircraft withdrawn from commercial use and
reconfigured for RAAF operations.
The PFI model is attractive to our political leadership since it
spreads the cost of the fleet over time, and does not incur the large
budgetary hit of a single large purchase. Whether it would be cheaper
overall than doing it the classical way remains to be determined.
Indeed, some very complex contractual arrangements may be required to
keep both parties happy, especially with a dual use PFI model. The
risk is that poor choices either by contractor or Commonwealth could
land either with a large long term contract which doesn't work for them
in the intended manner, resulting in litigation and profit destroying
Crewing a dual use PFI fleet raises other issues. While commercial
pilots may be viable for training operations, crisis or wartime use
would demand reservists.
Other complexities may also arise with dual use PFI schemes, such as
aircraft in commercial use overseas being impounded by allies of an
opponent should a dispute arise. The withdrawal of the aircraft from
commercial service for mobilisation would be a dead giveaway of
military intent, and the time and cost overhead of reconfiguring them
for military use would make any fleet mobilisation an expensive
Most of these complexities vanish with a dedicated military PFI fleet,
which in effect becomes a form of a wet lease arrangement. Such
arrangements are potentially much simpler, but do not offer the
political attractions of commercial work to offset costs.
While a PFI fleet may have the potential to ease or remove the funding
crunch of an upfront buy, it does introduce numerous complexities which
need to carefully considered. As always, there are no free lunches.
Tankers are the backbone of a modern air force, and by this measure the
RAAF is at this time in a very weak position. More tankers are
essential to put genuine credibility into the RAAF's force structure,
and meet the stated capability goals in the new White Paper.
Without a robustly sized tanker fleet, the RAAF would be unable to
perform medium and high intensity air defence operations in the cruise
missile launch belt north-east of the Pilbara and Timor Sea, and would
be hard pressed to effectively escort the F-111 to the outer bounds of
its combat radius. Indeed, genuine independent air operations over the
air sea gap are contingent upon having a proper number of tankers. The
order of magnitude in tanker numbers is at least 12.5 heavy tankers, 25
medium tankers, or some High/Low mix of either.
In terms of medium tanker choices, cheap options do not exist in the
foreseeable future. Many longer term alternatives are in some respects
inferior performers to established tankers such as the KC-135R and
707-338C, both of which would need over the longer term very expensive
rejuvenation. In heavy tankers, the 747 remains the most practical and
cheapest choice, with the KMD-11 a viable alternative. Capital
acquisition unit costs to a government or a private contractor would be
of the order of USD 2B for a 50/50 medium/heavy mix and comprehensive
The biggest issue for the RAAF in tanker fleet expansion will be
crewing, even with the most aircrew efficient choice of heavy tankers.
Another issue will be the provision of adequate fuel replenishment to
northern bases. A 747 rated runway close to Karratha and its planned
synthetic fuel plant could prove to be a very useful asset in this
None of these problems are insurmountable, nor unreasonably expensive
should the government make judicious long term choices. Nevertheless,
they will present challenges within a Canberra political culture which
has a very poor literacy level in these issues and is very nervous
about short term expenditure.
The RAAF's Boeing
tankers have much remaining performance and capability potential which
could be exploited by fitting CFM56 or JT8D-219 turbofans, lower deck
fuel cells, an APU package, a refuelling boom and a glass cockpit,
utilising hardware common to the KC-135R series. The decisive issue
which will lead to their retirement is dealing with corrosion,
structural fatigue, electrical wiring deterioration and aging of other
aircraft components, the repair costs alone being similar to the market
value of a used 767 or 747 aircraft (© 1989 -
2010 Carlo Kopp).