Aerial-refueling aircraft in the Indian context: a capability review
At the time of this writing, the A-330 MRTT is about to be chosen to provide the Indian Air Force (IAF) with a fleet of modern aerial-refueling tankers. There have been substantial time delays to this program on the basis of cost. The A-330 MRTT is not cheap. But when it joins the IAF, it will initially supplant, and then later replace, the existing fleet of six IL-78MKI aircraft currently in use. The purpose of this analysis is to determine what the A-330 MRTT brings to the table compared with its other contemporaries, especially the IL-78MKI. The analysis will compare general flight performance and refueling capabilities of the aircraft. Further, the analysis will discuss what alternatives to the A-330 MRTT exist (such as the A-310 MRTT) as well as debunk some myths about its performance relative to the powerful KC-10 extenders of the U.S. Air Force (USAF).
Ground rules
Comparing fixed wing aircraft is an inherently complex process. As such, this effort will focus on two key features of the aircraft involved: time-on-station (TOS) / time-of-flight (TOF) and Transfer Fuel Load (TFL). The general flight performance of the aircraft involved is obtained from the manufacturer’s literature or the end-user literature (where applicable). The analysis is simplified to allow the casual reader to grasp the essentials of the performance metrics. The modeling of the performance of each of the aircraft considered is based on known metrics for the propulsion, structural weights and aerodynamics of the baseline vehicles upon which the tanker variants are based. Note, however, that simplifications made to the models for ease of interpretation mean that the numbers will have some error built into them. However, such error is fixed for all the aircraft compared so the relative trends of the aircraft will be similar. Validation data is provided where available. Correction to publicly available data is also provided, where such errors have been found.
The Multi-Role Tanker Transport
As the name implies, the designation applies to aircraft that have the capability to behave as a transport as well as an in-flight refueling tanker, albeit under different conditions. For example, the IL-78MKI in Indian service is a modification of the IL-78MK, which is a tanker aircraft when the fuselage is equipped with additional fuel tanks instead of cargo. When the fuselage is filled with cargo instead of the fuel tanks, the aircraft behaves primarily as a long-range transport with short-range tanker capabilities. Other aircraft, such as the A-330 and the KC-10, have cargo capacities that are independent of the fuel carriage space, but because of the overall Maximum Take-Off Weight (MTOW) limitation, allow the aircraft to either carry some cargo plus large fuel capacity or vice versa, depending on the requirements. If the cargo is bulky but light (such as fighter aircraft or helicopters) then these large aircraft can accomplish both roles simultaneously. Smaller, more ad-hoc conversions, such as the A-310 MRTT, are merely medium transport aircrafts with a light tanker capability.
Note: since almost all contenders for the tanker program have some genesis also as transports or airliners, for the sake of readability, the following article will refer to each aircraft by its simpler alpha-numeric designation without the MRTT attached at the end.
In the Indian context, the global tanker market can be simplified down to the following options: the A-330 and the IL-78MKI. The argument for the A-330 is based on performance, availability and political factors. The Russian IL-78MKI is already in service within the IAF. The KC-10 is not available and the KC-135 is outdated and being replaced. The A-310 is included in this analysis as an example of what can be possible if available short/medium-haul airliner aircraft in India are converted to tankers at cheaper costs. The KC-767 is not included here because it is a runner-up to the A-330, which outclasses it.
The reluctant tanker
When the IAF began operating the IL-78MKI in 2003, the service had little choice in the matter. Options were few and capital for costly purchases even less so. However, a tanker was needed, and a tanker based on an airframe that was already operated by the service in large numbers meant that the Russian IL-78 entered Indian service. A new Squadron was created and the IAF was in the in-flight refueling business.
The IL-78MKI has been in Indian service since 2003. And as such, the IAF is intimately familiar with its capabilities. The IL-78MKI is a modified IL-78MK with Israeli-made aerial refueling pods. The MK version of the aircraft in itself is a MRTT derivative of the M version (which was a dedicated tanker aircraft that could not be converted back to a transport). The IL-78 is modified in its internal design to enhance the transferable fuel load of the baseline IL-76 design on which it is based. And the result is a three-point probe and drogue tanker aircraft. The IL-78M has a wing fuel-tank capacity of 57,720 kg. Earlier versions of the IL-78 had the ability to carry two fuselage hold tanks, but the IL-78MK has enhancements to allow the installation of three removable fuel tanks of 18,230 L capacity each. This increases the total offload fuel capacity of the IL-78MK to 105,720 kg when all three fuselage tanks are installed. The external look and feel of the IL-78 is very similar to that of the IL-76, which the IAF is also intimately aware of, having operated the purely transport version of the aircraft since the 1980s.
The baseline IL-78 with two fuselage fuel tanks and a three-point refueling system (left); One of the internal fuselage fuel tanks (center); The remaining cargo space behind the internal fuel tanks on an IAF IL-78MKI (right; note the port-off-center probe-and-drogue unit)
The issue with the IL-78MKI is its overall dismal performance relative to its size. The IL-76 has, until recently, been plagued by the inefficient D-30KP engines. The aerodynamics of the aircraft are not meant for high-endurance and efficiency, designed as they were, for ruggedness and fast transport of military cargo. The much more modern and higher-performance PS-90 engines may improve general performance of the aircraft, but the IL-78MKI has been a poor candidate for a high-performance tanker aircraft from the very beginning. And such a lineage as that shared by the IL-76 will not do it any good. It should be mentioned, for the record, that the IL-76MD aircraft has performed sterling service for the IAF as a strategic transport aircraft, ferrying troops and cargo to the most difficult of places, but that in itself does not yield into making the aircraft a good tanker. In fact, it works against it in many ways.
The layout and size of the IL-78 also poses problems. Because the fuselage fuel tanks are loaded into the cargo hold, the cargo-carrying capacity of the aircraft is dramatically reduced (see above images). In order to carry cargo instead of fuel, the fuselage tanks have to be detached and offloaded before the cargo can be loaded in. This takes time to do and the turnaround between the tanker and transport versions of the same aircraft are then limited by available ground-support equipment. On the other hand, on aircraft conceptually designed to be long-range, long-endurance troop/cargo transports, such problems are not encountered. The fuselage cargo and fuel spaces are separate and the option to transfer the aircraft from a tanker to a transport is as simple as offloading fuel from the tanks. This requires minimum ground-support equipment and increases turn-around time.
On the plus side, the military transport lineage of the IL-78 holds itself in good stead when used as a transport. The cargo bay is large and accessible via a rear cargo bay and high tail section. The high-wing design allows lower ground clearance and therefore ease of access. And its cavernous interiors are very useful for bulky military cargo. All of these features are not available on converted airliner designs such as the A-330, A-310 and the KC-10. These latter aircraft serve primarily as troop transports and for carriage of cargo pellets similar to that done by civilian airliners. Outsized military cargo remains the domain of the IL-76/78 type of aircraft.
The question then looms: is the IL-78 a tanker first or a transport? If it is the latter, is that how things should be?
The replacement
The IL-78MKI will need a replacement. And the future of the tanker in the Indian context apparently belongs to the A-330 MRTT. Supporters of the A-330 MRTT program have long sought to compare it with the true heavyweight of the tanker class of aircraft: the KC-10 Extender. Unfortunately, this comparison is quite flawed in that the A-330 MRTT is not a replacement for the KC-10 in a one-for-one basis. When loaded fully for the tanker role, the KC-10 has greater offload fuel capacity than any other tanker aircraft except the KC-747 concept. The latter is not in use except in a limited capacity by the Iranians. The KC-10 can offload a maximum of 160,000 kg of fuel when loaded in that configuration. The A-330, on the other hand, is limited to about 110,000 kg of fuel. But the A-330 has the same endurance as the KC-10 when both aircraft have access only to their onboard fuel. Additionally, the A-330 is a much more modernized design and is more efficient, whereas the KC-10 approaches the end of its life in the years to come. Further, with A-330s operating out of Indian airports, the ground support infrastructure can be easily expanded to the military arena as well.
The modification of the A-330 into an MRTT has led to other Airbus airliners also being suitably modified. The A-310 MRTT is another example. It is possible, in theory, to modify some of the older airlines in the short/medium-range class to an MRTT design for cheaper prices than the long-range A-330 MRTT. The technical feasibility has been demonstrated by the A-310 MRTT project. However, just how good a tanker can be made out of such older designs, remains to be seen. The A-310 MRTT is added here for analysis to demonstrate the viability of such options from a performance standpoint.
Performance comparisons
The performance of the IL-78MKI, the A-330 MRTT, the KC-10 and the A-310 MRTT are presented below. The results are summarized in the form of flight-time/Fuel plots. Because the vertical axis of the plots is TOS and the horizontal axis is the required fuel, the more vertical the curve for a particular aircraft gets, the more time on station it has, but lesser is the fuel available for transfer. The flatter the curve, the lesser time on station it has for a given fuel amount. All flight-time data is extracted at normal cruise speeds for the aircraft involved.
A-330 versus Midas
First, let’s compare the IL-78MKI versus the A-330. The IL-78MKI has a maximum endurance (when using all internal wing fuel + fuselage fuel; theoretically) of about 10.5 hours at its normal cruise speed. But comparison, the A-330 runs out of fuel after flying for about 15 hours at its normal cruise speed. In both cases, the overall transferable fuel amount is roughly similar, but because the A-330 has a lower fuel consumption rate, it can transfer a lot more fuel. For example, at 10.5 hours, when the IL-78MKI is out of fuel, the A-330 can still transfer 40,000 kg of fuel to the fighters it is supporting. At lower endurance hours, the difference between the two aircraft lowers, and the IL-78MKI starts catching up with the A-330 in terms of transferable fuel. At 5 hours of flight (including outbound and inbound flight time from the airbase to the station), the IL-78MKI can transfer off about 60,000 kg of fuel and the A-330 can transfer 80,000 kg of fuel. At 1 hour into the flight, both the IL-78MKI and A-330 can transfer about ~90,000 kg of fuel if required and then will have to return to refuel from the ground.
Consider a practical scenario where a flight of Su-30MKIs need to be refueled. A typical Su-30MKI would need about 10,000 kg of overall fuel in its tanks, but refueling with the tanker means that there is at least some fuel already existing in its tanks. If we assume that the Flanker is down to its last 10% fuel, meaning about 1,000 kg, it would need 9,000 kg of fuel to top up. If four such aircraft are to be refueled in the flight, the corresponding IL-78MKI and A-330 would have different loiter times associated with them. For the IL-78MKI, the offloading 36,000 kg of fuel to the four fighters would mean that its overall loiter time (assuming its hovering within 30 minutes of flight time from its home base or about 300 km radius around its launch point), would be 6 hours. For the A-330 it would be 10 hours or more. This is a substantial improvement in performance for the Indian tanker fleet.
A-330 versus Extender
Now, let’s compare the A-330 with the KC-10. This is particularly important given how these two aircraft are offered as the heavy-hitters of the tanker class of aircraft. Notice how the KC-10 has a very high offload capacity well in excess of the A-330. At 8 hours of flight (including outbound and inbound flight time from the airbase to the station), the KC-10 can offload 90,000 kg of fuel and the A-330 can offload about 60,000 kg of fuel. In other words, at 8 hours, the KC-10 can offload about 50% greater fuel quantity that the A-330. But because the KC-10 is guzzling fuel at a higher rate than the A-330, both aircraft run out of fuel at around the same endurance of about 15 hours. This result is testament to the highly efficient engines of the A-330 which allows it to do more with less. However, for a given fuel offload capacity, the KC-10 can stay on station longer.
Midas versus A-310
The viability of short/medium-range airliners to serve in the dedicated tanker role is highlighted by the performance of the A-310 MRTT versus the other contenders. The A-310 is able to match either the fuel capacity or loiter time only under fractional conditions. For example, at 1 hour flight time, the A-310 MRTT will offload about 35,000 kg of fuel whereas the A-330 will offload about 100,000 kg of fuel and the IL-78MKI will offload about 90,000 kg of fuel. It will therefore take three A-310 MRTTs to cover what one IL-78MKI or A-330 can cover in terms of tanker support to front-line fighters.
Validation
There is no such thing as enough validation for a simulation model. When this analysis was being compiled, the author took the following chart provided by the Airbus team for the A-330 MRTT program. Essentially, the document aimed to highlight the performance of the A-330 MRTT under realistic NATO conditions. This chart is reproduced below.
The reader should take note of how the performance metrics are provided for a typical mission. This includes a mission radius away from a launch airbase and loiter time at that holding point. The remaining fuel is available for transfer. We have two data points in the chart below to consider. The first one assumes a radius (outbound and inbound) of 1,852 km. We assume this to be done at the typical cruising speed for the A-330, which is about 871 km/hr. So the time take to reach and return from the holding point plus loiter time for the case provided comes to about 9-10 hours. Looking up in the simulation plot above, this corresponds to a transfer maximum of about ~50,000 kg. This is in line with the Airbus data. Similarly, consider the second data point. Here we have a radius of 926 km and a station time of 5 hours. This corresponds to about 8 hours total flight time. From the plot above, that comes to about 60,000 kg of transfer fuel. Again, we are in line with the Airbus data. These two points should help provide the reader with some sense of how realistic the simulations are for this analysis.
In the same Airbus document, the following quotation is provided:
The reference to the KC-10 is clearly out of place here. With respect to the Air Commander, either he meant the KC-135 (for which the statement about double offload fuel capacity would be accurate), or he meant the KC-10 when it’s not configured to maximum potential as a tanker. It is very well true that the A-330 has higher loiter efficiency than the KC-10, but twice the TOS and offload is absolutely incorrect.
Conclusions
The A-330 MRTT clearly provides superior performance for the existing fleet of Indian Air Force tankers. The improvement in performance is more the result of increased efficiency of the A-330 airframe and propulsion systems than of overall fuel load capacity. However, the differences between the two aircraft become more visible at very long endurance and ranges. For short-range and short-endurance refueling operations, the two aircraft perform almost identically. When operating in the tanker role, the A-330 MRTT does not sacrifice its cargo carriage space, although the amount of cargo is limited as a result of MTOW similar to the IL-78MKI currently operated by the Indian Air Force. As a result, the turnaround time between the tanker and transport variants of the A-330 MRTT is very short and requires the minimal of ground-support equipment. The fuel stowage can be varied depending on whether cargo or fuel is required for the next flight. This allows the Airbus aircraft to permit long-range expeditionary operations for the Indian Air Force wherein carriage of troops, cargo and fuel can be easily interchanged. It should be noted, however, that long-range strategic transports will still be required to carry out-sized military cargo which is not possible for the A-330 series aircraft.
Dr. Vivek Ahuja
source The Beta Coefficient: Aerial-refueling aircraft in the Indian context: a capability review
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