Gas stations in the sky: 90 years of aerial refueling (pictures)

There's only so much fuel you can put into a given airplane at any given time, and by extension, only so far that airplane can fly once it takes off. Unless, that is, a rendezvous can be arranged with a flying gas station. Nowadays, that's become par for the course for the US Air Force and other military branches, a skill that's regularly trained and a key facet of both wartime and peacetime operations. Consider, for instance, the B-52s that flew from Louisiana to Iraq and back in Operation Desert Storm in 1991, or the B-2s that traveled from Missouri to South Korea (and back) earlier this year -- an astonishing round trip of 13,000 miles -- for a little geopolitical muscle-flexing.

Here we see a US Navy F/A-18 Hornet getting itself into position behind a KC-10 Extender aerial tanker for a fill-up in 2010 while thousands of feet in the air over Afghanistan. But the story of aerial refueling stretches back a full 90 years. Let's travel back in time....

This is what aerial refueling looked like in the summer of 1923. In June of that year, the two de Havilland DH-4B biplanes you see here took to the skies over San Diego's Rockwell Field to make history, with the first midair transfer of gasoline from one plane to another. A hose was dangled below and behind the lead plane to be grabbed by one of the two Army Air Service officers in the open, rear cockpit of the receiving plane.

Two months later, in late August 1923, the two men who'd been in the receiving plane -- Lowell Smith and John Richter -- set an endurance record with a flight that lasted 37 hours, 15 minutes, with help from 16 midair refueling maneuvers, according to the National Museum of the US Air Force. In October of that year, the duo flew from the Canadian border to the Mexican border, with three aerial refuelings. The novelty was quickly becoming well-established reality.

Military aircraft of yesteryear are better known for having their fuselages adorned with pinup girls than punctuation, but not for one particular mission that began on January 1, 1929. It was an era of endurance record-setting, and the Army Air Corps wanted to show just how long it could keep a plane in the air. (It wasn't just to demonstrate aerial refueling skills, but also the reliability of its aircraft engines and the hardiness of its air crews.)

So it was that an Atlantic-Fokker C-2A airplane was dubbed the Question Mark, in an exercise that was part research project, part publicity stunt. The question was this: just how long could this plane stay in the air before it finally had to come back down to Earth?

Here we see one of the Question Mark's crew members, Lt. Elwood "Pete" Quesada, making an adjustment to a gas line. With refuelings from a pair of the Douglas C-1 transport planes, the Question Mark aircraft stayed aloft until January 7, flying nonstop over Van Nuys, Calif., for a total of 150 hours, 40 minutes. During those six and a half days, the C-1 tankers made more than 40 refuelings, about a quarter of them at night. While the Douglas C-1 lowered a hose in order to dispense the fuel, it used a rope to pass the Question Mark crew oil, water, and food. Oh, and none of the aircraft involved was equipped with a radio, so the crews communicated with each other via hand and flashlight signals and messages written on blackboards.

A year later, according to the National Museum of the US Air Force, a civilian plane carrying two men remained in the air for 647 hours, 28 minutes -- that is, 27 days.

Experiments in aerial refueling continued through the 1930s, but other technological advances took some of the urgency out of finding ways to gas up while flying. For instance, metal-bodied monoplane designs -- more aerodynamic, and with greater cargo and fuel capacity -- were beginning to take over from the wood-and-fabric biplanes that predominated in aviation's early days. In the 1940s, military leaders fighting long-range campaigns in World War II experimented with steps such as converting B-24 Liberator bombers (like the one above) into tankers, but production and crew-training capacities were already seriously overtaxed. In addition, newer aircraft coming into service like the B-17 Superfortress had greater range and capacity than earlier bombers, and in the Pacific, US forces were gradually capturing islands from the Japanese, meaning airfields could be established closer to their targets. Aerial refueling was not used operationally in World War II.

After World War II ended, the US turned its geopolitical attentions to an assertive Soviet Union, underscored by fears of communist encroachment in nations hither and yon. Long-range bombers would soon enough prove crucial to the posture of a US military capable of tackling threats, nuclear or otherwise, around the globe. Those long-range bombers would need to be refueled in flight.

Against that backdrop, in 1949 a US Air Force B-50A bomber known as the Lucky Lady II (seen here) carried out the first-ever nonstop round-the-world flight. The circumnavigation took 94 hours, 1 minute -- 2 hours less than four full days -- from February 26 to March 2, and required the assistance of four pairs of KB-29M tankers, which carried out an unspecified number of refuelings.

The Lucky Lady II flew 23,452 miles in its global circuit. Here's how the National Museum of the US Air Force describes the aerial refueling maneuver:

"The fuel delivery aircraft (KB-29M) would fly above and forward of the receiver aircraft (B-50A) and unreel a long refueling hose. The crew of the B-50A would extend an apparatus from the rear of the aircraft designed to snag the refueling hose trailing behind the KB-50M. Once the fuel hose was captured, it was reeled into the B-50A where the crew connected it to the refueling manifold. Once the fuel transfer was complete, the hose was released and the KB-29M reeled in back."

While the Air Force worked the kinks out of aerial refueling, it tried out some other methods for extending the range of fighter aircraft. One of them, in an experiment known as Operation Tip-Tow, involved a larger plane towing smaller planes. Here, two F-84D fighters are clamped onto the wingtips of a modified B-29 bomber. The process was not an easy one, and Tip-Tow came to an end not long after a fatal crash involving a B-29 and an F-84 in April 1953.

One of the first big postwar successes for aerial refueling came in July 1952 during the Korean War. In what was called Operation Fox Peter One, three squadrons of F-84G fighters made the long trip west from Turner Air Force Base in Georgia to the Korean theater of operations, getting refueled in midair during the initial legs of the journey. The tankers were KB-29P aircraft, two of which are pictured here.

Two F-84G fighters approach a KB-29P over Texas during Operation Fox Peter One. The three squadrons of the 31st Fighter Escort Wing got aerial fill-ups en route from Georgia to Travis AFB in California, and then again on the next leg -- across 1,860 nautical miles of open ocean -- from California to Hickam AFB in Hawaii. For the rest of the journey across the Pacific, the fighters island-hopped to Japan via Midway Island, Wake Island, Eniwetok, Guam, and Iwo Jima.

The KB-29P tanker aircraft, which went into service in September 1950, were B-29 bombers outfitted with the flying boom system. The boom operator (or boomer) worked in what had been the tail turret.

The P models were an upgrade over the M models, which had been in service for a couple of years at that point. The B-29M used a loop-and-hose system, the hose of which had a 2.5-inch-diameter refueling hose that transferred fuel at a relatively modest 110 gallons per minute. That would prove inadequate for the new high-speed, high-altitude bombers coming into service, on top of being tricky to operate.

"Interestingly enough, Boeing already had a better system in mind," according to an Air Force post describing the transition to the KB-29P. "The company developed a 'flying boom,' which featured a telescoping pipe with fins at the nozzle end. The fins were termed 'ruddervators' because they functioned as both rudders and elevators. The boom operator, sitting in the B-29's converted tail turret, literally flew the boom into a receptacle on the upper fuselage of the receiver aircraft. This design allowed more positive control of the air-to-air refueling operation and, with the boom's four-inch diameter, it offered much faster fuel transfer."

The KB-29Ms were later converted to a more modern probe and drogue system.

As the Cold War became an enduring fact of life in the early 1950s, the US needed to show that its military might could reach deep into farflung lands whenever the need might arise, putting the Soviet Union, its chief antagonist, on notice. Aerial refueling of bombers helped drive that message home. Here, KB-29P tanker refuels an RB-45C Tornado of the 91st Strategic Reconnaissance Wing.

The real workhorse tanker for the Strategic Air Command in the 1950s was the Boeing KC-97 Stratotanker; more than 800 were built. But the propeller-driven KC-97 couldn't adequately match the speed or altitude of the bombers it was refueling, often requiring a dangerous diving maneuver called "tobogganing," and two or more of the tankers were needed to refuel the powerful B-52 bombers that came into service in the middle of the decade. Here, a B-47A bomber gasses up at a KC-97.

If you were the pilot of a receiver aircraft getting fuel from a KC-97, this would have been your view -- the underside of the tanker's fuselage, the boomer pod, and right up close, the flying boom.

The modern era of aerial refueling began in the mid- to late 1950s, when a new Stratotanker went into service -- the jet-powered KC-135, which paired up much more effectively with the B-52. In fact, through the 1960s and 1970s, the Strategic Air Command inventory showed a nearly a one-to-one ratio of KC-135s to B-52s. And like the enduring B-52, the KC-135 continues its mission for the Air Force after more than 50 years, though the tanker's days are now numbered.

There are about 400 KC-135s in the Air Force inventory today, including those in active duty, in the Air Force Reserve, and in the Air National Guard. The Air Force says it last took delivery of a KC-135 in 1965, but the venerable aircraft have had their share of modifications and updates over the years in areas ranging from the engines to navigation and surveillance systems. In this photo from 2011, a pilot runs through preflight checks in KC-135 based in Southwest Asia.

A KC-135 Stratotanker can carry about 200,000 gallons of fuel for transfer, plus 37 passengers and 83,000 pounds of cargo. This one is assigned to the 121st Air Refueling Wing in Columbus, Ohio.

The KC-135 can transfer 5,000 pounds of fuel per minute, under the watchful eye of the boom operator.

With some air-to-air refueling help from the KC-135 just ahead, in the skies over Newfoundland, the pilot of a NATO E-3 Sentry aircraft on August 1 of this year was able to fly from Geilenkirchen, Germany, to Seattle, on the far side of the US, a 13.5-hour journey.

Here's the view from the other angle, as a KC-135 boom operator sizes up the approach of an F-35A Lightning II in May of this year.

An F-15E Strike Eagle eases up to a KC-135 during a training mission over the Atlantic Ocean in September 2012.

The other mainstay of aerial refueling today is the KC-10 Extender -- this one's flying a mission over Southwest Asia a few years back -- which can carry a total of more than 356,000 pounds of fuel in six tanks, almost twice as much as the KC-135, along with up to 75 people and nearly 170,000 pounds of cargo. A modification of the commercial DC-10, the KC-10 has been in service since 1981, and there are about 60 of them in the Air Force inventory.

The boom operator of a KC-10 Extender opens up his sighting door in preparation to refuel a C-17 Globemaster III in May 2012.

Aerial refueling doesn't always require a jet-driven KC-135 or KC-10. Sometimes the right plane for the mission is an HC-130J Combat King II, seen here gassing up a pair of HH-60G Pave Hawk helicopters near Davis-Monthan Air Force Base, Ariz.

And for that matter, a thirsty flying machine doesn't necessarily need a flying gas station to get a fill-up. For some US Navy helicopters, such as this MH-60R Sea Hawk, you just need to sidle up to a handy guided-missile destroyer (this is the USS Roosevelt, aka DDG 80) with a long hose.

Before too much longer, unmanned aircraft will be doing air-to-air refueling, too. That will be an especially useful skill for drones expected to stay on station in prolonged feats of persistent surveillance and reconnaissance at high altitudes. In its KQ-X project that wrapped up in September 2012, DARPA investigated the potential for autonomous midair refueling with a pair of RQ-4 Global Hawk UAVs. No actual refueling took place, but the two aircraft did operate safely in close proximity and provided Pentagon researchers with stacks of performance data.

"During its final test flight," DARPA said, "two modified Global Hawk aircraft flew in close formation, 100 feet or less between refueling probe and receiver drogue, for the majority of a 2.5-hour engagement at 44,800 feet. This demonstrated for the first time that High Altitude Long Endurance (HALE) class aircraft can safely and autonomously operate under in-flight refueling conditions."

The Navy, meanwhile, had hoped to carry out aerial refueling tests with its X-47B drones in 2014, but it looks now like that will not come to pass.

The future of aerial refueling for the Pentagon belongs to the KC-46A. In February 2011, after a decade-long competition, Boeing beat out rival EADS to win a contract that could eventually be worth $30 billion to build a fleet of the next-generation tankers that will replace both the KC-135 and the KC-10. For starters, Boeing is getting $3.5 billion to build the first four KC-46A aircraft, based on its 767 design. On Friday, Boeing said that it had begun assembly of the second KC-46A.

This artist's rendering by Boeing shows what the aerospace giant had been calling, before the contract was awarded, the NewGen Tanker, refueling a B-2 Spirit stealth bomber.

Boeing on Friday laid out this timetable for the KC-46A: "The first test aircraft is expected to roll out of the factory in January 2014, while the second is scheduled to leave the factory in March. Boeing plans to fly the fully provisioned tanker for the first time in early 2015 and make the first delivery in 2016. The company expects to build and deliver the first 18 KC-46As by 2017 and a total of 179 by 2027 if all options under the contract are exercised."