Aerial Refuelling - Why (not)?

My previous post highlighted the reality that direct flights with modern aircraft generally consume less, at least no more fuel than making an en-route refuelling stop - https://www.rawaviationconsulting.com/single-post/direct-vs-1-stop-long-haul-flights-theory-vs-reality

Some researchers propose aerial refuelling of civil transport aircraft to reduce mission fuel usage. It achieves some of the same benefits of a refuelling stop at an airport while largely eliminating its ~2-3 hour time penalty, thus maintaining current scheduling patterns.

The military often uses aerial refuelling of transport aircraft to extend their payload/range. However, I assume the prime benefit to be reduced deployment flight time and higher payloads traded for fuel in the initial TOW. The lack of available, secure landing sites en route with suitable support equipment may also be a factor.

I am familiar with aerial refuelling of transport class aircraft. Many years ago, I wrote a report on the various refuelling systems. It included spending time on an RAF Boeing E-3D Sentry’s flight deck observing various pilots practise boom refuelling. I took the photo when the receiver aircraft was correctly positioned.

THEORY

Consider a modern transport aircraft flying 6000nm and consuming 70t of fuel. Making a refuelling stop at 3,000nm yields about a 5% theoretical fuel burn advantage (3.5t saving) compared to a direct flight. However, my previous post described how operational practicalities essentially eliminate these benefits.

For simplicity, let’s assume no benefit for making a refuelling stop.

Aerial refuelling should boost the theoretical benefit relative to stopping by about 2-3% at 6,000nm, about 1.7 tonnes. It eliminates the fuel required to land, taxi and take-off that does not contribute to moving the aircraft towards its ultimate destination.

Suppose the receiver aircraft flies directly over the tanker airport, a modern tanker will consume about 6 tonnes of fuel to start, taxi take-off and climb 100-150nm to the receiver aircraft, cruise for 20-30 minutes to rendezvous and transfer the 35-40t fuel, before descending 100-150nm back to base and taxiing in.

Consuming 6t of tanker fuel for a 1.7t receiver fuel saving is clearly nonsensical.

If the tanker carries 120t of fuel to resupply 3 receiving aircraft, its fuel consumption will be closer to 7t due to the extra tanker take-off weight.

7t tanker fuel > 3 x 1.7t receiver fuel-saving still makes no environmental sense.

REALITY BITES

Routing: If the optimal direct routing is 1000nm from the fixed tanker airport due to great circle distance and wind effects, either the tanker or receiver fuel must increase its flight time to make a rendezvous, burning more fuel. The overall penalty should be less than the receiver landing at the airport, but it will not change the overall outcome of higher fuel for aerial refuelling.

North Pacific routings are often 1000nm south of Anchorage, with no closer viable tanker base option available. Flying this distance also reduces the fuel available for offload, perhaps permitting only 1 or 2 refuellings, further penalising aerial refuelling.

If the refuelling point is not at the mid-point, the erosion of fuel burn benefits seen for stopping also occurs.

Given the theoretical benefit does not exist, the routing inefficiencies make it even worse.

Refuelling Equipment Failures: Imagine 2-3 aircraft flying over the Pacific Ocean some 500-1000 away from the tanker base aiming for a tanker rendezvous when the refuelling equipment fails on either aircraft! Is there an airport close enough to divert to and refuel? Does the receiver need to carry extra fuel for such an event, thus reducing the point of refuelling?

If the tanker fails, is a spare tanker acting as a flying reserve, or do other tankers operating at the same time must carry additional fuel to cover this eventuality. Both scenarios result in significant fuel burn penalties. Possibly a rolling overcapacity reduces this, if the traffic is dense enough.

Tanker Fleet Sizing: The tanker fleet needs to be about 50% of the maximum traffic flow (50 tankers for 100 receivers), assuming each tanker services an average 2 receivers per flight per flight.

Long haul flights tend to operate like sea tides. High traffic flows occur in one direction in a relatively short period before the same traffic flow returns later. The Earth’s rotation imposes airport curfews at different times, and airline network planning needs to connect with feeder traffic.

Some of the busiest long haul routes see peak traffic flows in both directions close to the flights’ mid-point at about the same time, e.g. Europe-Asia and across the North Pacific Ocean. Upper atmosphere winds might drive the two flows 1,000nm apart, requiring two tanker fleets.

The peaky traffic flows size the tanker fleet accordingly as a reasonable average 4 hr cycle time (including turnround) usually limits a single cycle per peak flow. If the peak flows are separate, 2 cycles per day should be possible. If they coincide, just one cycle will be possible.

The economics of this are terrible

Aircraft Design for Reduced Range

Many single-stop and aerial refuelling studies also include substantial benefits from redesigning long-haul aircraft for reduced design range. It further reduces the mission fuel consumption.

However, the benefits are not great enough for modern aircraft – a maximum of 3-5% and this will not make an environmental or economic case for AAR – a future post will address reduced design range specifically.

Conclusion I believe aerial refuelling to be unviable for modern transport aircraft.

Using older retired transport aircraft as tankers for economic reasons just increases tanker fuel consumption, making

it even less attractive environmentally.

Aerial refuelling includes many other minor disadvantages such as safety and the logistics of supporting a large tanker fleet in an often remote location. However, given the lack of compelling fuel burn benefits, these effects are not quantified.