I largely agree the logistics are probably unrealistic, but this isn’t necessarily true, is it?
Envision a scenario where Plane 1 takes off without a full tank of fuel so as to maximize cargo capacity. Plane 2 can top off Plane 1 midair and potentially still produce an overall savings in fuel usage partly by reducing the number of trips required to haul X amount of weight. Add the possibility that Plane 2 can go on to top off Planes 3, 4, 5, etc, and the savings can pile up. Plus the fees for the extra cargo you can now carry help offset the cost as well.
Of course, this adds a considerable logistical headache and probably cost to the proceedings, but potential overall fuel savings are real.
ETA: I also see there is a new attempt at this concept over in europe right now. Who knows if it’s actually economical, but there’s some people willing to give it a go.
Basically the paper is an engineering study, not a business case study: the paper hypothesizes that if a passenger aircraft were optimized for more cargo, less fuel, and it was refuelled at the optimal time, there could be 30-40% savings. There’s lots of graphs relating to how weight, altitude and other factors can be optimized, but the fact is that the article is predicated on refuelers being at the right place at the right time every time to make sure that the short-range aircraft can be refueled in an optimal way.
The general scenario is that a civil aircraft takes off and flies for approximately 3,000 miles, at which location there is an airbase where an Air to Air Refueler flies “a minimum outbound leg to its first rendezvous,” after which the AAR refuels two more aircraft in an optimal manner, and the AAR does not return to its original base. Now, the odds of having airbases synched up so that an eastbound flight from San Francisco can get refueled after 3,000 miles, but also that an eastbound flight from Atlanta can get refueled after 3,000 miles, just doesn’t seem practical… not to mention that the efficiency of the AARs is enhanced by the expectation that they don’t have to return to base. They would seemingly just keep going until they could land at the next airbase in that direction.
To say it another way, the engineering of fuel efficiency, weight and other factors in that article seem plausible. The logistics and the business case of that engineering study seem totally unrealistic.
I suspect the 35% to 40% fuel savings stated is for the flight of the aircraft being refueled and does not take into account the fuel required to run the tanker. Of course, yes, a tanker might be more efficient if it can refuel several aircraft with one take-off. that would require some good timing, or if the one refueling is delayed, the next flight would have to circle to stay in the area of the tanker (which kind of wastes the advantage in time and fuel).
I don’t see the issue - you take off from London to Sydney, if by the time you are halfway the fuel is running low and the tanker is not around, you land in Delhi or Bangkok like normal flights. Even NYC to Perth, you land in Tahiti or New Caledonia if there is a problem (or Africa or Sri Lanka the other way). Aircraft always will (must) have enough fuel for their alternate landing. It’s not like they refuel in mid-ocean when fuel is nearly gone. If they can make 2/3 of the semi-circle now, they only need to top up somewhere after 1/3 and before 2/3 of the flight, preferably while they still have safety options.
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Potential fuel savings in the range of 35-40% have been estimated for long haul flights (including the fuel used during the tanker missions).
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One situation where I could see it coming in handy is if anyone ever gives the SST concept another go. The big limiting factor with supersonic aircraft is having to lift the large amounts of fuel up to cruising altitude with airframes and engines that aren’t very efficient in lower altitude subsonic flight. If they could take off with just enough fuel to reach a tanker at 30 or 40 thousand feet, that could massively increase their payload and range. It’d just be a matter of if the refueling can be done quickly enough to stay competitive versus subsonic flights, but for long hauls it should still be a reasonable proposition.
That’s not logical. The perfect refueling would be from an airplane launched from airport in line with the flight.w That’s the minimum fuel burn. That means a very large airplane has to take off and climb to an acceptable altitude and then return to the airport. This is the reverse of a plane landing for refueling. What has changed is the fuel and expense of an extra airplane and crew.
It’s the same fuel burn to transition an airport PLUS the expense of another plane. That can be a shared expense if you have multiple airplanes that need fuel within the flight window of the tanker. But it’s a considerable expense.
It would make more sense to send up a combi plane with passengers and extra fuel instead of freight. The tanker plane makes a short-haul run with people and transloads fuel to the long-haul plane en-route.
Let me make it clear that for various logistical and other reasons outlined in various posts in this thread I don’t think inflight refuelling is going to happen. However…
I think you are missing a very big point. Sure, hub and spoke is a great general solution, but the fact is that if there is a sufficient market for a direct flight, airlines will schedule a direct flight because if they don’t their competitors will, because customers very (very) much like direct flights. Your final sentence just quoted I would not believe without hard data. The kangaroo route (Sydney-London) is huge. If direct flights on that route were possible, I think it would save several hours and I think all else being equal customers would flock to it. You would fill enough planeloads to make the route viable to schedule at least some direct flights per week is my guess.
IME if you take a flight to Singapore out of Brisbane, about maybe a quarter of the plane will get off that flight and onto a flight to London, and there are many, many Brisbane- Singapore flights a day. If there was a direct flight, then those quarter plane loads of people going Brisbane-Singapore-London would consolidate onto the Brisbane-London flight, and fill it.
The same is true but moreso for Sydney-London.
Sure. As long as it had plenty of fuel to get to an intermediate airport if the tanker didn’t show up.
Would you not also need enough fuel to get to an alternate if the refuel plane did not get up & the weather was below minimums when you got to the ‘oops, the tanker did not get here airport’. )
Depends on how far the alternate airport from your refueling point. You’d be surprised where you can find good places to put a plane down in an emergency, especially after they built all those airfields in WWII to get planes across the oceans.
Option c is frequently used, but the plane has spend 5,000 miles carrying extra fuel. You can have the tanker fly from a low cost airport, avoid the delays of setting down momentarily at a high cost airport, and avoid the cost of carrying fuel for 5,000 miles.
The logistics and numbers may not work out in reality, but it’s not inherently ridiculous to consider.
From what I have read, the most efficient usage of tanker aircraft is refuelling shortly after takeoff. As, the greatest amount of fuel taken is to get to cursing altitude. If you top up from there, the airline had much greater range.
A lot of the reason air-to-air refueling is practiced by the military is because the fuel needs of military aircraft are far less predictable than those of civilian airliners. Airliners are going to a fixed destination and usually repeat the same flight paths many times, allowing precise data to be gathered. Military aircraft often are searching for a target of some sort so don’t know exactly where they are going when they take off, have to keep a reserve for combat maneuvering (which burns lots), and rarely fly any given flightpath more than a handful of times. Having a tanker available has saved the ass of countless military pilots who thought they had enough fuel for their mission when they took off but burned more fuel than planned.
Robin Olds (triple-ace pilot who flew in WWII and Vietnam) nearly ended up running out of fuel over Vietnam that way. Got caught up in a big dogfight, didn’t realize how much fuel he was burning until it was way past the time they should have broken off and left.
Ended up calling for a tanker flying over Thailand that flew into North Vietnamese airspace to meet them (rather more dangerous than the unarmed tankers usually went in for). When he made visual contact with the tanker, the pilot was putting his KC-135 (more or less a militarized Boeing 707) almost on its wingtip to bring it around to line up with the approaching fighters. They tanked up, and were saved the indignity of running out of fuel and bailing over enemy territory. As it happens, the tanker used up so much of its fuel getting to them that they ended up having to divert to the fighters’ base instead of its home base.
So the story goes, Robin Olds offered to fill the entire tanker up with bourbon in gratitude.
In addition to the bit a few posts above about the plane having to carry the weight of the fuel (more fuel is more weight, more weight means the engines have to work harder to drag everything through the air, and that burns more fuel). Hence, airliners tend to fly with as little fuel as they can (allowing for a reserve in case they need to divert)
Also, most aircraft are capable of flying while carrying far more weight than they can actually take off or land with, due to limitations with the landing gear and runway lengths (heavier planes need a longer takeoff or landing run because they need to move faster). So that’s another reason that a plane would hit the tanker immediately after takeoff. Launch light, then max out the fuel load for their mission.
I just watched a short newsreel of inflight refueling from the 1940’s. It was a for a London to Bermuda flight. The process was unbelievably complex. First, a hauling line was lowered from the tanker and then the the passenger ship had to fire a line that captured the hauling line. The passenger ship then had to maneuver around the line so that the line was trailing behind. Now, the tanker hauled in the line and connected the line from the passenger plane to the fuel hose. The hose was then hauled to the passenger plane and connected. The fuel then transferred via gravity. I can’t imagine this getting done in bad weather or at night.
