A family of jet-propelled air refueling tankers, the KC-135 Stratotanker's are approaching the end of their service life. They are not filled with fuel, but carry both fuel and cargo. The KC-135, like other tankers, does not have a fuselage filled with fuel, which would make it too heavy to take off. It has additional fuel tanks from which it can transfer fuel to other aircraft.
Originally built from a modified Boeing 707 airframe, some models are reaching the end of their structural life. While the Air Force has some additional tankers such as the KC-10 Extender, replacing the 500-plus KC-135s has to be done over time, and has stalled over contractual controversies. Air Mobility Command manages an inventory of more than 481 Stratotankers, of which the Air Force Reserve and Air National Guard fly 294 in support of AMC's mission.
The Boeing Company's model 367-80 was the basic design for the commercial Boeing 707 passenger plane as well as the KC-135A Stratotanker. In 1954 the Air Force purchased the first 29 of its future 732-plane fleet. The first aircraft flew in August 1956 and the initial production Stratotanker was delivered to Castle Air Force Base, Calif., in June 1957. The last KC-135 was delivered to the Air Force in 1965. 157 Air Force Reserve and Air National Guard tankers were re-engined with the TF-33-PW-102 engines. The re-engined tanker, designated the KC-135E, is 14 percent more fuel efficient than the KC-135A and can offload 20 percent more fuel.
A mechanical upgrade, over a 13-year period ending in 1988, improved performance and extended service life. The chief aspect involved replacing the lower wing surfaces with an improved aluminium-alloy skin. In addition, new engine struts and a wide range of small parts were replaced.
Many of the aircraft still had 1950s-vintage electronics, which were less reliable, more costly, and more labor-intensive (for both flight crew and maintenance) personnel than commercial-off-the-shelf (COTS) electronics. The first major modification was the Pacer CRAG (Compass Radar and Global Positioning System) Program, which extends the KC-135 refueling fleet 2040, the airframe’s projected decommissioning date. The Pacer CRAG avionics upgrade to the KC-135 fleet is a commercial off-the-shelf modification program will eliminate the need for a navigator on most missions. Pacer CRAG upgrades allow the aircraft to be flown by a pilot, co-pilot and boom operator. Older KC-135s are flown by a four-member aircrew, which includes a navigator. The new design could also quickly be reconfigured for a navigator if the mission requires.
Another set of electronics enhancement, called Block 30, will be installed with Pacer CRAG. This includes enhanced ground proximity warning system, which uses aircraft position and a digital terrain database to provide look-ahead awareness to the aircrew. The Reduced Vertical Separation Minima, which equips the fleet to operate in reduced vertical separation airspace. This includes an additional digital air data computer, new digital altimeters and digital airspeed indicators. The navigation and safety modification installs a flight data recorder, cockpit voice recorder and emergency locator transmitter. Installing Pacer CRAG and Block 30 at once saves downtime, and also simplifies documentation. 
Of the original KC-135A's, more than 415 have been modified with new CFM-56 engines produced by CFM-International. The re-engined tanker, designated either the KC-135R or KC-135T, can offload 50 percent more fuel, is 25 percent more fuel efficient, costs 25 percent less to operate and is 96 percent quieter than the KC-135A. Of the series, only the KC-135R/T models can be refueled as well as refuel other planes, a valuable feature for extremely long missions. The R and T have new engines with much greater efficiency.
The U.S. Air Force, as distinct from the other U.S. military services and other nations, uses a rigid, steerable "boom", manually controlled by a boom operator, for high-volume transfer. There were legitimate reasons for doing so, especially for the long-range bombers that were the original priority for refueling. Nearly all internal fuel can be pumped through the flying boom, the KC-135's primary fuel transfer method. The boom enters a receptacle on the receiving aircrft.
Much more common than the "boom" is "probe and drogue", in which the receptacle is in a flexible drogue attached to a trailing hose. Originally, the KC-135 could only have one probe and drogue, but the Multipoint Refueling System (MPRS) puts one probe-and-drogue on each wing, anchored to an underwing probe. The probe and drogue is the only way U.S. Navy, U.S. Marine Corps, and most allied national aircraft can be refueled.
There have been variants both for specialized air-to-air refueling, as well as using the basic airframe for very different purposes. The KC-135Q, for example, was a variant for refueling the now-retired SR-71 Blackbird, which used a special fuel that had to be kept separate from the main fuel lines.
Other designs use the airframe and engines, but have no refueling capability. These include EC-135 electronic warfare and C3I aircraft, RC-135 specialized reconnaissance platforms, and various one-of-a-kind versions, as unique as a flying ice sprayer used to test the in-flight performance of other aircraft under icing conditions.
Operational procedure improvements
"Lean fueling operations" reduce the overhead of air refueling operation. A study asked if KC-135s carry more fuel than they will use or dispense, and, if so, what should be done about it? A literature review confirmed they did, and that considerable efficiencies would come by not loading with a standard quantity of fuel, but an amount precomputed for each flight.
- Primary Function: Aerial refueling and airlift
- Prime Contractor: The Boeing Company
- Power Plant: KC-135R/T, CFM International CFM-56 turbofan engines; KC-135E, Pratt and Whitney TF-33-PW-102 turbofan engines
- Thrust: KC-135R, 21,634 pounds each engine; KC-135E, 18,000 pounds each engine
- Wingspan: 130 feet, 10 inches (39.88 meters)
- Length: 136 feet, 3 inches (41.53 meters)
- Height: 41 feet, 8 inches (12.7 meters)
- Speed: 530 miles per hour at 30,000 feet (9,144 meters)
- Ceiling: 50,000 feet (15,240 meters)
- Range: 1,500 miles (2,419 kilometers) with 150,000 pounds (68,039 kilograms) of transfer fuel; ferry mission, up to 11,015 miles (17,766 kilometers)
- Maximum Takeoff Weight: 322,500 pounds (146,285 kilograms)
- Maximum Transfer Fuel Load: 200,000 pounds (90,719 kilograms)
- Maximum Cargo Capability: 83,000 pounds (37,648 kilograms), 37 passengers
- Pallet (i.e., cargo) Positions: 6
- Crew: Three: pilot, co-pilot and boom operator. Some KC-135 missions require the addition of a navigator. The Air Force has a limited number of navigator suites that can be installed for unique missions.
- Aeromedical Evacuation Crew: A basic crew of five (two flight nurses and three medical technicians) is added for aeromedical evacuation missions. Medical crew may be altered as required by the needs of patients.
- Unit Cost: $39.6 million (fiscal 98 constant dollars)
- Date Deployed: August 1956
- Inventory: Active duty, 187; Air National Guard, 217; Air Force Reserve, 77
- "U.S. Air Force Fact Sheet: KC-135 Stratotanker", Air Force Link
- Boeing, KC-135 Stratotanker: KC-135 Skin-Replacement
- "KC-135 Stratotanker: Pacer CRAG (Compass, Radar And GPS)", Globalsecurity.org
- "KC-135 Stratotanker: Block 30", Globalsecurity.org
- Heseltine, Bruce P., Jr. (Winter, 2007), "Analysis: KC-135 Lean Fueling Operations", Air Force Journal of Logistics