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Surely, mid-air refueling is a cool thing, but is it worth it?

It's obvious that the military, for one, has some requirements to keep aircraft airborne so that they can keep patrolling, for example.

However, that is due to the need to keep the aircraft airborne and is not necessarily because the method is cost-effective. Flying another aircraft carrying lots of fuel just doesn't seem very money/environment-friendly – you waste more fuel needed to power the fuel-carrying aircraft in addition to the fuel you would already need.

  • Is mid-air refueling cost-effective under any circumstances? Or is it just an emergency practice?
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We're looking for long answers that provide some explanation and context. Don't just give a one-line answer; explain why your answer is right, ideally with citations. Answers that don't include explanations may be removed.

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    $\begingroup$ Eco-friendly? Surely not. An important point is that it adds range though and would allow specialized aircraft to reach targets it otherwise wouldn't have. It's a lot more a question of logistics than of money. $\endgroup$ – falstro Feb 14 '15 at 18:25
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    $\begingroup$ There is at least one airplane that can only takeoff and do half its initial climb before it needs more fuel. Without midair refueling that plane would be a paperweight. $\endgroup$ – casey Feb 14 '15 at 23:46
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    $\begingroup$ @reirab I believe it is. At the very least, that's true of the SR-71 -- that's partly because it leaks fuel at low speeds, though (the fuel tank was designed to be leaky at low speeds and temperatures, because at Mach 3 it would expand and this way there was room for it to do that), and partly because it took off with less than full fuel to reduce weight at takeoff. $\endgroup$ – cpast Feb 15 '15 at 9:18
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    $\begingroup$ @casey I prefer my paperweights to not drip fuel on my papers. $\endgroup$ – David Richerby Feb 15 '15 at 9:57
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    $\begingroup$ Why would it be inefficient? I guess a large transport aircraft has much better fuel efficiency per wight than a fighter aircraft. I don't have any numbers, but as an educated guess, I'm fairly certain that a fighter has to burn much more fuel than a large transport aircraft to transport (or even take off with) x kg of extra fuel. $\endgroup$ – vsz Feb 15 '15 at 11:55

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In some contexts, the economics depends on how special the aircraft you're refueling is.

Consider something like an AWACS mission. You have this very complex specialized aircraft, with a large, highly trained, crew in addition to the flight crew. If you didn't refuel that during the mission, it might need to spend a lot of its active hours being unproductive en route to and from the war. You would need more of them (and more crews) to reach any given level of in-theater coverage.

On the other hand with aerial refueling you need some tankers scuttling back and forth with fuel for your sentries. But they are cheaper and simpler to operate, cheaper and simpler to crew, and so forth. So even though you may use slightly more aircraft hours in total, the total costs can be lower.

This doesn't hold to just the same extent for fighters and bombers, because they have to return to base from time to time to stock up on munitions anyway. There are still "readyness" types of missions it would be relevant for. The price tag for an F-35 seems to be in about the same ballpark as for a KC-767, but a KC-767 can service many F-35s in a single mission, so having the tankers do the scuttling-back-and-forth can still be an economic win.

Also, it's a good thing for a fighter that gets into combat not to be lugging around enough fuel to get all the way home by itself.

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    $\begingroup$ One more good point that would make this excellent answer a complete one; very often, bases for fighters cannot always be located within range of the operating theater, due to political or practical concerns. In the first Desert Storm, the USAF operated primarily from Riyadh Air Base in Saudi Arabia and a couple carriers in the Gulf, with a few FABs for A-10s and helicopters further north. The F-15, the U.S's primary deep strike weapon for that campaign, could not get from Riyadh to Baghdad and back without IAR if the mission called for 2000-lb GBU-10s to be delivered to the target. $\endgroup$ – KeithS Apr 27 '15 at 23:48
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First to the issue of cost-effectiveness: If fighter or strike aircraft would need to take more fuel with them, they would have to be larger and more fuel-hungry themselves. This is hard to quantify, but with air refuelling you can cap the size at a much smaller value and still be sure that all future range and endurance requirements can be met.

Next, modern air campaigns are extremely complex. It takes months of planning for a single day of multi-national operations, and aircraft from several countries have to be staged at some distance to then fly their sorties with utmost precision. Air refueling makes it much simpler to do this planning, because it allows to build in waiting times or to use scarce ressources multiple times during a day. Take electronic countermeasures aircraft: Instead of flying back to a remote home base, the aircraft can refuel at an airborne tanker and be back to support the next wave of attacking aircraft.

And whoever said that concerns about their environmental impact have ever influenced the choice of means of the military?

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    $\begingroup$ Environmental impact isn't the major consideration but, sure, it's an influence. The British Royal Navy has scrapped its single-hulled tanker ships, for example. And military budgets are under pressure in most places, so burning less fuel is certainly something that's taken into account. $\endgroup$ – David Richerby Feb 15 '15 at 10:53
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    $\begingroup$ Kind of a quasi-Tsiolkovsky situation; Space rockets have to lift their own fuel, and they don't have the option of in-air refueling, so for each additional pound of payload, you need the additional fuel to get that pound into orbit, plus all the fuel needed to get the additional fuel at least partway. The same ultimately applies to an aircraft, albeit with lower speeds; to increase your range without midair refueling, you need the fuel to go the extra X miles, plus the extra fuel to get back those extra X miles, plus the fuel needed to get and keep that extra fuel in the air. $\endgroup$ – KeithS Apr 27 '15 at 23:30
  • $\begingroup$ Do you think the weight of cargo comes into play as well? I'm just considering, why should a fighter or strike aircraft overload itself with fuel? They can just carry more ammo and little fuel (hence, the need to refuel without returning to base). $\endgroup$ – IROEGBU Jun 30 '15 at 11:48
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    $\begingroup$ @iroegbu: Fighter aircraft are space limited. The internal volume is kept to a minimum to improve supersonic performance and keep structural mass low. Everything else is hung on external stores. So you're right in a way, but it is the number of hard points that sets the limit, and not the weight. $\endgroup$ – Peter Kämpf Jun 30 '15 at 15:59
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It's not a question of saving money, or emergencies. The reason militaries use mid-air refueling is so they can reach anywhere in the world. Aircraft have a limited range, without mid-air refueling they would have to have enough fuel to get to and from their area of operations back to a base. This would limit where military aircraft (or espionage aircraft) could reach, even with bases all over the world.

With mid-air refueling aircraft can fly great distances from their bases over any territory, complete their mission, and get back. It greatly increases a military air force's capability.

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    $\begingroup$ There's one other important reason why military aircraft use airborn refueling. Naval aviation is limited by maximum takeoff weights that are much lower than similar ground based aircraft with access to longer runways can achieve. As a result, they often take off with fuel tanks only partially filled and then immediately tank afterwards. At the extreme, for bombing missions with heavy ordinance loads the on board fuel tanks may be almost empty with the majority of fuel being provided after takeoff. $\endgroup$ – Dan Neely Feb 14 '15 at 21:56
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    $\begingroup$ Another way to put it would be to say that if a journey would require refueling, it will be necessary to have someplace along the way to refuel. Establishing such a place on open water, or on another country's territory, is often difficult or impossible compared with establishing such a place in the air. Additionally, ground facilities are much more subject to attack by enemy ground forces than would be a mid-air "fueling depot". $\endgroup$ – supercat Feb 14 '15 at 22:08
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    $\begingroup$ @DanNeely, re naval: But how does the tanker get to be there in the first place? It can't have launched from the carrier itself, under the same restrictions? If you own an airbase close enough to the war for your tanker aircraft to be able to get there, what do you need a carrier on the scene for? $\endgroup$ – Henning Makholm Feb 14 '15 at 22:33
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    $\begingroup$ @HenningMakholm Dedicated tankers also tend to have much greater range than strike aircraft (a side effect of being big slow airliner derivatives, instead of things meant to stand up in combat), and can be shared across friendly air forces. You could well have a tanker based further from the scene than your carrier, and still have it work out better than not using a tanker. $\endgroup$ – cpast Feb 15 '15 at 9:15
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    $\begingroup$ Some countries have political issues with allowing other countries armed aircraft to land and refuel. An unarmed tanker may be a piece of military hardware, but no arms makes for an easy political situation. $\endgroup$ – CGCampbell Feb 15 '15 at 13:53
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Cost effective?

Look up the Black Bucks raids, then work out how much that refuelling cost.... Then compare that to the cost to the UK of losing the Falklands Islands.

Those raids were absolutely not possible without midair refuelling. Similarly Cold War patrols and reconnaissance missions. It's not about whether it's cost effective, it's about whether the mission can be undertaken without refuelling.

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    $\begingroup$ +1: The military is not a for-profit enterprise. Their job is to provide capabilities that their government needs; money is just a means to that end, not an end in itself. $\endgroup$ – cpast Feb 15 '15 at 9:11
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    $\begingroup$ The British treasury would be better off without the Falklands. All they provide is pride and prickly relations with Argentina, for the cost of £300,000 per family. $\endgroup$ – Peter Kämpf Apr 26 '15 at 20:42
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    $\begingroup$ Firstly I'd like to see a citation for that £300k/year claim: but the oil around the island is potentially far more lucrative. More importantly though, you're right: Pride. Name me another country which would meekly hand over territory and those citizens resident there? I'd expect my government to protect my home, cost is irrelevant in that case. You don't ignore your people just because they aren't profitable. $\endgroup$ – Jon Story Apr 26 '15 at 21:05
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In addition to other answers, tankers can provide specialization for their task -- i.e. circling for a long time where no one is shooting at you. Many military aircraft have to massively compromise fuel efficiency and capacity to do their job. Dedicated tankers tend to be variants of airliners; they have lots of carrying capacity for fuel and can be designed with fuel efficiency as a concern. The other aircraft can then optimize for things besides fuel capacity, and don't need to worry so much about being extremely heavy and (for fighters) having unaerodynamic drop tanks.

To use the SR-71 as an example, it couldn't carry much fuel at all and had horrendously inefficient engines (it was a plane that routinely cruised on afterburner) in order to do its job of flying faster and higher than anything else in the sky that wasn't a rocket. You could give it a high fuel capacity, but since it leaked fuel at lower speeds (the fuel tanks only sealed under thermal expansion) there wouldn't be much point, and you'd have to deal with a really heavy plane not optimized for low-speed flight during takeoff. However, the KC-135Q that refueled it did not have to fly at Mach 3, so it could be designed to be able to take off with plenty of fuel. You could not build a plane to do what the SR-71 was supposed to do and be able to operate without tanker support; the designs for "able to take off with all the fuel you'll need" and "able to do your job" conflict, and the best way to resolve it is to split them into separate aircraft. So in that case, it would be far, far more expensive to build something to approximate the same capabilities without a tanker.

(The main reason, though, is what Jon Story said: Midair refueling is used when you have a job to do, not when you're trying to make money through your aviation operations. If you're trying to make money by flying things around, you wouldn't be doing the missions that require mid-air refueling. You only use it when your guiding principle is "here's your job, here's a budget, go and do it," not "go and make money the best way you can.")

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    $\begingroup$ The SR-71 wasn't that inefficient, was it? Usually afterburners are inefficient because they're designed for emergency use, not sustained use. But on planes like the SR-71, you can optimize for afterburner use. Besides, " inefficient" is relative. What is efficient at Mach 3.2? $\endgroup$ – MSalters Feb 16 '15 at 16:00
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    $\begingroup$ @MSalters Exactly. There isn't really a way to make an efficient plane to fly at Mach 3.2. So you can fly more fuel on a plane that is efficient (KC-135Q), and then not need to push even more fuel through the sky at Mach 3.2. $\endgroup$ – cpast Feb 16 '15 at 18:35
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Technically, the purpose of inflight refuelling is to act as a force multiplier.

Say I have a requirement for a combat air patrol (two aircraft on station all the time) at a given location 250 miles away. The aircraft available have a range of 1000 miles and the ground turn-around takes the same time as flying 250 miles.

I need two in the air going there, two on station, two coming back and two on the ground - minimum of 8 of which 6 are not patrolling. Now put up a tanker and even with only one refuel per patrol I can now keep between 2 and 4 aircraft on patrol at all times with only 4 aircraft.

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Thinking of it from logical standpoint, we can define the exact set of circumstances in which it would be cost effective to do this.

1) The plane to be refueled is incapable of accomplishing its mission by going from any valid (=reachable given time and distance constraints) launch spot to any valid landing spot;
AND
2) The plane is capable of accomplishing its mission by going from any valid launch spot to a valid landing spot, if one or more refueling tankers can be got to some point along the route;
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3) The tanker(s) CAN get to that point along the route;
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4) The value of the mission is greater than the costs, even with the refuelling.

There are also cases where these rules don't necessarily have to hold. For example, if a plane can get to the target and back on its own power, but would have to fly a more dangerous route, the reduced risk of using a tanker may be worthwhile. At $2.4Bn per B2 bomber, a mere 0.04% reduction in mission risk is worth over a million dollars in hardware alone: not counting costs from failing the mission, payload value, potential lost-pilot-rescue costs, new-pilot-training costs, political costs, and opportunity costs for being one plane down.

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I'll build on @HenningMakholm's excellent answer. Mid-air refueling brings lots of cost-benefits beyond the obvious "make planes go farther". First, tankers can provide operational flexibility and efficiency compared to having to set up closer airbases.

  • You don't have to set up a closer airbase.
    • You don't have to build it.
    • You don't have to conquer and secure that space (very costly).
  • Your airbases can be farther away from the front lines.
    • Immune from artillery and ground attack.
    • More layers of air defenses.
    • More warning of air attack.
  • You don't have to keep moving and rebuilding your airbases as the front line moves.
    • And all their fuel, ordinance, personnel, hangars, buildings, bunkers, defenses...
    • You can respond faster to changes on the ground providing better support.
  • You can strike much, much deeper into enemy territory.
  • You can attack from unexpected directions.
    • Put a tanker where you have no airbases, attack from that direction.

But does it save fuel?

Central point number one: combat aircraft are inefficient, tankers are efficient and somebody has to drag that fuel out there. Might as well be a tanker.

While keeping a tanker in the air might seem like a waste of fuel, and it does have to burn fuel while in the air, it likely saves a lot of fuel because tankers are so efficient and combat aircraft are so inefficient. A tanker is typically based on a civilian airliner. Once it's at altitude and in position, it can run racetrack laps at its most efficient speed and altitude (many combat aircraft have trouble going slow enough for the tanker) sipping fuel compared to the ordinance laden (ie. lots of drag), high performance aircraft it's fueling.

If that very efficient tanker didn't drag that fuel out there, those same laden, inefficient combat aircraft would have to and this leads to what is known as The Tyranny Of The Rocket Equation which says to carry more fuel you need even more fuel. If you have a lot of inefficient aircraft, it can save fuel to have them take off with half empty tanks and top up at an efficient tanker on the way. The Rocket Equation is so tyrannical you can save fuel even in civilian flights, here's a paper on the subject (sorry about the paywall).

Central point number two: make as few combat trips as possible to destroy a target. A plane can only carry so much, and more fuel means less weapons. Less weapons means more trips. More trips means more fuel. More importantly it means more times the pilot and airframe are exposed to danger (and if you need to take two trips you've probably lost the element of surprise), and more aircraft needed to attack the same number of targets.

Take off is another consideration. Taking off and climbing to altitude is one of the most fuel hungry parts of a flight. Combat and long haul aircraft typically take off near maximum weight requiring full engine power and a long climb. Taking off with less fuel means you burn less fuel getting to altitude.

Heavier aircraft need a longer runway to take off from. That runway length may not be available at your forward base, or it may be damaged. Less fuel means heavier weapon loads with shorter runways which means more flexibility.

Combat aircraft can only carry so much fuel internally before they need drop tanks. Drop tanks add drag making the aircraft less fuel efficient and generally maneuver like a pig. Drop tanks also take up precious hard point space for weapons which means more round trips which means... you get the idea.

You can take this idea too far. The biggest problem with the F/A-18 Hornet was its measly fuel load. The long ranges typically required at sea meant it practically required a tanker to operate effectively. Carriers can't carry big tankers, so they had to resort to inefficient buddy packs mounted on smaller aircraft. This problem was resolved with the F/A-18 Super Hornet which is practically a different aircraft.

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Lets say you're on a carrier and you want to bomb a target 200 miles inland. What do you do?

Carriers normally stay a few hundred miles out to sea for safety--if they got too close they would be vulnerable to attack by Harpoonskis. Oops, you don't have the fuel to reach your target with a load of bombs under combat conditions. If you want to hit that target you're going to have to launch with tankers and you very well might need to meet tankers on the way back also.

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It can be a regular non-emergency military practice.

One major early example was US airstrikes on the Hanoi area during the Vietnam war, which were launched from far to the south, and would refuel just before crossing into North Vietnamese airspace, so that the planes would have as much fuel available as possible during the mission.

During air combat, particularly for aircraft who have missions that aren't just to go to one place and return immediately (e.g. to engage flying enemy aircraft), the amount of fuel carried (above what's needed to get there and back) determines the amount of time that aircraft can do tactically-useful things. Being able to top off closer to the target maximizes that time. For aircraft with afterburners, this also contributes to the amount of time they can use those, as using afterburners consumes fuel very quickly.

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You either fly the airborne plane from it's current location to land/refuel and back to the previous current location, or make the refuelling plane do it.

Either way, the same journey is being made.

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Fuel is cheap.

Some more valuable resources that may be rendered useless or vulnerable during refueling include combat aircraft (which cost orders of magnitude more than refueling planes), nuclear missiles, training combat pilots or presidents.

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Even if your fighter has to go half-way home to meet a tanker in the middle to refuel, that's half the journey it would otherwise have had to make. That's a huge saving in time and if you want your plane to be close to where it needs to be then you don't really have much choice. You can't have it spending half its mission going back and forth between home bases.

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A very common practice in the military is to take off with almost zero fuel and to refuel right after the initial climb to altitude, when operating fast movers from forward operating bases.

The practice occurs for fighter jets because their wings are designed for high speed, thus would require a very long runway to be able to carry their full weapons compliment into the air with their fuel. By sending up the fuel on cargo style planes, they can overall reduce the runway length required to fly fighters with a full combat load out (as the cargo planes are designed to lift things).

Once in the air the fighters are moving much faster, so they can carry the extra weight easily. The extra weight in the combat load out was decided to be fuel simply because it is easier to refuel in mid air than it is to rearm in midair (missiles cannot be easily piped).

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protected by voretaq7 Feb 17 '15 at 6:31

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