Blog 58: The US Air Force’s Backbone for 60 Years: the Stratotanker
Four U.S. Air Force KC-135 Stratotanker aircraft, assigned to the 185th Air Refueling Wing of the Iowa Air National Guard, taxi prior to take off from the Sioux Gateway Airport in Sioux City, Iowa, on Sept. 3rd of 2009
Welcome back to Brooke In The Air! This week we’re exploring the backbone of the US Air Force and the US’ global dominance. It’s not a new weapon, a flashy fighter, a new tank, or brilliant bomber. No, without this, none of the above could function. This week we’re examining the Boeing KC-135 Stratotanker of the United States Air Force.
OVERVIEW
An American-origin military aerial refueling tanker aircraft that was developed from the Boeing 367-80 prototype, simultaneously produced alongside the Boeing 707 airliner, the KC-135 has a narrower fuselage and is shorter than the 707. Boeing gave the aircraft the internal designation of Model 717. The KC-135 was the United States Air Force's first jet-powered refueling tanker and replaced the KC-97 Stratofreighter. The KC-135 was initially tasked with refueling strategic bombers, but it was used extensively in the Vietnam War and later conflicts such as Operation Desert Storm to extend the range and endurance of US tactical fighters and bombers.
USAF KC-10 Extender refueling allied fighters over Afghanistan
INTRODUCTION
The KC-135 first entered service with the United States Air Force - USAF, in 1957; it is one of nine military fixed-wing aircraft with over 60 years of continuous service with its original operator. The KC-135 is supplemented by the larger McDonnell Douglas KC-10 Extender. Studies have concluded that many of the aircraft could be flown until 2030, although maintenance costs have greatly increased. The KC-135 is to be partially replaced by the Boeing KC-46 Pegasus.
ORIGIN
Starting in 1950, the Air Force operated the world's first production aerial tanker, the Boeing KC-97 Stratofreighter, a gasoline fueled piston-engined Boeing Stratocruiser (USAF military designation C-97 Stratofreighter) with a Boeing-developed “flying boom" and extra kerosene (used as jet fuel) tanks feeding the boom. The Stratocruiser airliner itself was developed from the B-29 bomber after World War II. In the KC-97, the mixed gasoline/kerosene fuel system was clearly not desirable and it was obvious that a jet-powered tanker aircraft would be the next development, having a single type of fuel for both its own engines and for passing to receiver aircraft. The 370 kph (230 mph) cruise speed of the slower, piston-engined KC-97 was also a serious issue, as using it as an aerial tanker forced the newer jet-powered military aircraft to slow down to mate with the tanker's boom.
Like its sibling, the commercial Boeing 707 jet airliner, the KC-135 was derived from the Boeing 367-80 jet transport "proof of concept" demonstrator, which was commonly called the "Dash-80". The KC-135 is similar in appearance to the 707, but has a narrower fuselage and is shorter than the 707. The KC-135 predates the 707, and is structurally quite different from the civilian airliner. Boeing gave the future KC-135 tanker the initial designation Model 717.
Boeing model 367-80
In 1954 USAF's Strategic Air Command (SAC) held a competition for a jet-powered aerial refueling tanker. Lockheed's tanker version of the proposed Lockheed L-193 airliner with rear fuselage-mounted engines was declared the winner in 1955. Since Boeing's proposal was already flying, the KC-135 could be delivered two years earlier and Air Force Secretary Harold Talbott ordered 250 KC-135 tankers until Lockheed's design could be manufactured. In the end, orders for the Lockheed tanker were dropped rather than supporting two tanker designs. Lockheed never produced its jet airliner, while Boeing would eventually dominate the market with a family of airliners based on the 707.
In 1954, the Air Force placed an initial order for 29 KC-135As, the first of an eventual 820 of all variants of the basic C-135 family. The first aircraft flew in August 1956 and the initial production Stratotanker was delivered to Castle Air Force Base, California, in June 1957. The last KC-135 was delivered to the Air Force in 1965 as Vietnam loomed.
DEVELOPMENT CONCEPT
Developed in the early 1950s, the basic airframe is characterized by 35-degree aft swept wings and tail, four underwing-mounted engine pods, a horizontal stabilizer mounted on the fuselage near the bottom of the vertical stabilizer with positive dihedral on the two horizontal planes and a hi-frequency radio antenna which protrudes forward from the top of the vertical fin or stabilizer. These basic features make it strongly resemble the commercial Boeing 707 and 720 aircraft, although it is a different aircraft.
Reconnaissance and command post variants of the aircraft, including the RC-135 Rivet Joint and EC-135 Looking Glass aircraft were operated by SAC from 1963 through 1992, when they were reassigned to the Air Combat Command (ACC). The USAF EC-135 Looking Glass was subsequently replaced in its role by the U.S. Navy E-6 Mercury aircraft, a new build airframe based on the Boeing 707-320B.
Part of a KC-135 Refueling Boom during maintenance
All KC-135s were originally equipped with Pratt & Whitney J57-P-59W turbojet engines, which produced 10,000 lbf (44 kN) of thrust dry, and approximately 13,000 lbf (58 kN) of thrust wet. Wet thrust is achieved through the use of water injection on takeoff, as opposed to "wet thrust" when used to describe an afterburning engine. 670 US gallons (2,500 L) of water are injected into the engines over the course of three minutes. The water is injected into the inlet and the diffuser case in front of the combustion case. The water cools the air in the engine to increase its density; it also reduces the turbine gas temperature, which is a primary limitation on many jet engines. This allows the use of more fuel for proper combustion and creates more thrust for short periods of time, similar in concept to "War Emergency Power" in a piston-engined aircraft.
“WAR EMERGENCY POWER”
Essentially, war emergency power (WEP) is a throttle setting that was first present on some American World War II military aircraft engines. For use in emergency situations, it produced more than 100% of the engine's normal rated power for a limited amount of time, often about five minutes. Maximum normal power would be limited by a mechanical stop, for instance a wire across the throttle lever slot. A more forceful push would break the wire, allowing extra power.
In the 1980s, the first modification program retrofitted 157 Air Force Reserve (AFRES) and Air National Guard (ANG) tankers with the Pratt & Whitney TF33-PW-102 turbofan engines from 707 airliners retired in the late 1970s and early 1980s. The modified tanker, designated the KC-135E, was 14% more fuel-efficient than the KC-135A and could offload 20% more fuel on long-duration flights. Only the KC-135E aircraft were equipped with thrust reversers for aborted takeoffs and shorter landing roll-outs. The KC-135E fleet has since either been retrofitted as the R-model configuration or placed into long-term storage ("XJ"), as Congress has prevented the Air Force from formally retiring them. The final KC-135E, tail number 56-3630, was delivered by the 101st Air Refueling Wing to the 309th Aerospace Maintenance and Regeneration Group (AMARG) at Davis–Monthan Air Force Base in September 2009.
KC-135 Refueling pod mounted in the fuselage.
The second modification program retrofitted 500 aircraft with new CFM International CFM56 (military designation: F108) high-bypass turbofan engines produced by General Electric and Safran. The CFM56 engine produces approximately 22,500 lbf (100 kN) of thrust, nearly a 100% increase compared to the original J57 engine. The modified tanker, designated KC-135R (modified KC-135A or E) or KC-135T (modified KC-135Q), can offload up to 50% more fuel (on a long-duration sortie), is 25% more fuel-efficient, and costs 25% less to operate than with the previous engines. It is also significantly quieter than the KC-135A, with noise levels at takeoff reduced from 126 to 99 decibels. This 27 dB noise reduction results in a sound pressure level of about 5% of the original level. The KC-135R's operational range is 60% greater than the KC-135E for comparable fuel offloads, providing a wider range of basing options.
UPGRADED CONSIDERATIONS
Upgrading the remaining KC-135Es into KC-135Rs is no longer in consideration; this would have cost approximately $3 billion USD, $24 million USD per aircraft.
In order to expand the KC-135's capabilities and improve its reliability, the aircraft has undergone a number of avionics upgrades. Among these was the Pacer-CRAG program (compass, radar and GPS) which ran from 1999 to 2002 and modified all the aircraft in the inventory to eliminate the Navigator position from the flight crew. The fuel management system was also replaced. The program development was done by Rockwell Collins in Iowa and installation was performed by BAE Systems at the Mojave Airport in California. Block 40.6 allows the KC-135 to comply with global air-traffic management. The latest block upgrade to the KC-135, the Block 45 program, is online with the first 45 upgraded aircraft delivered by January 2017. Block 45 adds a new glass cockpit digital display, radio altimeter, digital autopilot, digital flight director and computer updates. The original, no longer procurable, analog instruments, including all engine gauges, were replaced. Rockwell Collins again supplied the major avionic modules, with modification done at Tinker AFB.
The KC-135Q variant was modified to carry JP-7 fuel necessary for the Lockheed SR-71 Blackbird by separating the JP-7 from the KC-135's own fuel supply (the body tanks carrying JP-7, and the wing tanks carrying JP-4 or JP-8). The tanker also had special fuel systems for moving the different fuels between different tanks. When the KC-135Q model received the CFM56 engines, it was redesignated the KC-135T model, which was capable of separating the main body tanks from the wing tanks where the KC-135 draws its engine fuel. The only external difference between a KC-135R and a KC-135T is the presence of a clear window on the underside of the empennage of the KC-135T where a remote controlled searchlight is mounted. It also has two ground refueling ports, located in each rear wheel well, so ground crews can fuel both the body tanks and wing tanks separately.
KC-135R of the 912 Air Refueling Squadron belonging to the 452nd Air Mobility Wing refueling an F-35A during a Red Flag exercise.
Eight KC-135R aircraft are receiver-capable tankers, commonly referred to as KC-135R(RT). All eight aircraft were with the 22d Air Refueling Wing at McConnell AFB, Kansas, in 1994. They are primarily used for force extension and Special Operations missions, and are crewed by highly qualified receiver capable crews. If not used for the receiver mission, these aircraft can be flown just like any other KC-135R.
The KC-135R has four turbofan engines, mounted under 35-degree swept wings, which power it to takeoffs at gross weights up to 322,500 pounds (146,300 kg). Nearly all internal fuel can be pumped through the tanker's flying boom, the KC-135's primary fuel transfer method. A boom operator stationed in the rear of the aircraft controls the boom while lying prone, viewing through a window at the bottom of the tail. Both the flying boom and operator's station are similar to those of the previous KC-97. A special shuttlecock-shaped drogue (basket), attached to and trailing behind the flying boom, may be used to refuel aircraft fitted with probes, like most US Navy and Marine Corps aircraft. This apparatus is significantly more unforgiving of pilot error in the receiving aircraft than conventional trailing hose arrangements; an aircraft so fitted is also incapable of refueling by the normal flying boom method until the attachment is removed. A cargo deck above the refueling system can hold a mixed load of passengers and cargo. Depending on fuel storage configuration, the KC-135 can carry up to 83,000 pounds (38,000 kg) of cargo.
The Multi-point Refueling Systems (MPRS) modification adds refueling pods to the KC-135's wings. The pods allow refueling of U.S. Navy, U.S. Marine Corps and most NATO tactical jet aircraft while keeping the tail-mounted refueling boom. The pods themselves are Flight Refueling Limited MK.32B model pods, and refuel via the probe and drogue method common to Navy/Marine Corps tactical jets, rather than the primary "flying boom" method used by Air Force fixed-wing aircraft. This allows the tanker to refuel two receivers at the same time, which increases throughput compared to the boom drogue adapter.
A number of KC-135A and KC-135B aircraft have been modified to EC-135, RC-135 and OC-135 configurations for use in several different roles (although these could also be considered variants of the C-135 Stratolifter family).
OPERATIONAL DOCTRINE & CONCEPT
The KC-135 was initially purchased to support bombers of the Strategic Air Command, but by the late 1960s, in the Southeast Asia theater, the KC-135 Stratotanker's ability as a force multiplier came to the fore. Midair refueling of F-105 and F-4 fighter-bombers as well as B-52 bombers brought far-flung bombing targets within reach, and allowed fighter missions to spend hours at the front, rather than a few minutes, which was usual due to their limited fuel reserves and high fuel consumption. KC-135 crews refueled both Air Force and Navy / Marine Corps aircraft; though they would have to change to probe and drogue adapters depending upon the mission, the Navy and Marine Corps not having fitted their aircraft with flying boom receptacles since the USAF boom system was impractical for aircraft carrier operations. Crews also helped to bring in damaged aircraft which could sometimes fly while being fed by fuel to a landing site or to ditch over the water (specifically those with punctured fuel tanks). KC-135s continued their tactical support role in later conflicts such as Operation Desert Storm and current aerial strategy.
SAC (Strategic Air Command) had the KC-135 Stratotanker in service with Regular Air Force SAC units from 1957 through 1992 and with SAC-gained ANG (Air National Guard) and AFRES (Reserve) units from 1975 through 1992. Following a major USAF reorganization that resulted in the inactivation of SAC in 1992, most KC-135s were reassigned to the newly created Air Mobility Command - AMC.
KC-135R Digital Glass Cockpit
While AMC gained the preponderance of the aerial refueling mission, a small number of KC-135s were also assigned directly to United States Air Forces in Europe (USAFE), Pacific Air Forces (PACAF) and the Air Education and Training Command (AETC). In order to spread the planes around. All AFRC KC-135s and most of the ANG KC-135 fleet became operationally-gained by AMC, while Alaska Air National Guard and Hawaii Air National Guard KC-135s became operationally-gained by PACAF.
AMC manages a total of 396 Stratotankers, of which the AFRC (Reserve)and ANG (Air National Guard) fly 243 in support of AMC's mission as of May 2018. The KC-135 is one of a few military aircraft types with over 50 years of continuous service with its original operator as of 2009.
RESEARCH USAGE
Besides its primary role as an inflight aircraft refueler, the KC-135, designated NKC-135 for this research purpose, has assisted in several research projects at the NASA Armstrong Flight Research Center at Edwards Air Force Base, California. One such project occurred between 1979 and 1980 when special wingtip "winglets", developed by Richard Whitcomb of the Langley Research Center, were tested at Armstrong, using an NKC-135A tanker loaned to NASA by the Air Force. Winglets are small, nearly vertical fins installed on an aircraft's wing tips. The results of the research showed that drag was reduced and range could be increased by as much as 7 percent at cruise speeds. Winglets are now being incorporated into most new commercial and military transport/passenger jets, as well as business aviation jets.
REPLACING THE STRATOTANKER?
Modified KC-135E on loan to NASA for Research from the Armstrong Flight Research Center, Edwards Air Force Base, California
Between 1993 and 2003, the amount of KC-135 depot maintenance work doubled, and the overhaul cost per aircraft tripled. In 1996, it cost $8,400 per flight hour for the KC-135, and in 2002 this had grown to $11,000 USD. The Air Force's 15-year estimates project further significant cost growth through fiscal year 2017. KC-135 fleet operations and support costs were estimated to grow from about $2.2 billion USD in fiscal year 2003 to $5.1 billion (2003 dollars) in fiscal year 2017, an increase of over 130 percent, which represented an annual operating cost growth rate of about 6.2 percent.
The Air Force projected that E and R models have lifetime flying hour limits of 36,000 and 39,000 hours, respectively. According to the Air Force, only a few KC-135s would reach these limits by 2040, when some aircraft would be about 80 years old. A later 2005 Air Force study estimated that KC-135Es upgraded to the R standard could remain in use until 2030.
In 2006, the KC-135E fleet was flying an annual average of 350 hours per aircraft and the KC-135R fleet was flying an annual average of 710 hours per aircraft. The KC-135 fleet is currently flying double its planned yearly flying hour program to meet airborne refueling requirements, and has resulted in higher than forecast usage and sustainment costs. As of March 2009, the Air Force indicated that KC-135s would require additional skin replacement to allow their continued use beyond 2018.
Finally, the USAF decided to replace the entire KC-135 fleet. However, the fleet is large and will need to be replaced gradually. Initially the first batch of replacement planes was to be an air tanker version of the Boeing 767, leased from Boeing. In 2003, this was changed to contract where the Air Force would purchase 80 KC-767 aircraft and lease 20 more. In December of 2003, the Pentagon froze the contract and in January 2006, the KC-767 contract was canceled.
THE PLOT THICKENS
This move followed public revelations of corruption in how the contract was awarded, as well as controversy regarding the original leasing rather than outright purchase agreement. The then-Secretary of Defense Donald Rumsfeld stated that that move would in no way impair the Air Force's ability to deliver the mission of the KC-767, which would be accomplished by implementing continuing upgrades to the KC-135 and KC-10 Extender fleet.
In January 2007, the U.S. Air Force launched the KC-X program with a request for proposal (RFP). KC-X was the first phase of three acquisition programs meant to replace the KC-135 fleet. On 29 February 2008, the US Defense Department announced that it had selected the EADS/Northrop Grumman "KC-30" (to be designated the KC-45A) over the Boeing KC-767. Boeing protested the award on 11 March 2008, citing irregularities in the competition and bid evaluation. On 18 June 2008, the US Government Accountability Office sustained Boeing's protest of the selection of the Northrop Grumman/EADS's tanker.
Yet because of this competition-conflict, in February 2010, the US Air Force restarted the KC-X competition with the release of a revised request for proposal (RFP). After evaluating bids, the USAF selected Boeing's 767-based tanker design, with the military designation KC-46, as a replacement in February 2011. The first KC-46A Pegasus was delivered to the U.S. Air Force on the 10th of January in 2019.
NOTABLE VARIANTS
Known KC-135 Variants
Many variants exist or have existed throughout the plane’s 60 year history.
KC-135A
Original production version powered by four Pratt & Whitney J57 engines, with 732 built. Given the Boeing model numbers 717-100A, 717-146 and 717-148.
Boeing -USAF & NASA NKC-135A
NKC-135A-Airborne Laser Platform
The Boeing NC-135 and NKC-135 are special versions of the Boeing C-135 Stratolifter and Boeing KC-135 Stratotanker modified to operate on several different programs.
KC-135B
Airborne command post version equipped with turbofan engines, 17 built. Provided with in-flight refueling capability and redesignated EC-135C
KC-135D
All four RC-135As (Pacer Swan) were modified to partial KC-135A configuration in 1979. The four aircraft were given a unique designation KC-135D as they differed from the KC-135A in that they were built with a flight engineer's position on the flight deck. The flight engineer's position was removed when the aircraft were modified to KC-135 standards but they retained their electrically powered wing flap secondary (emergency) drive mechanism and second air conditioning pack which had been used to cool the RC-135As on-board photo-mapping systems. Later re-engined with Pratt & Whitney TF33 engines and a cockpit update to KC-135E standards in 1990 and were retired to the 309th AMARG at Davis-Monthan AFB, AZ in 2007.
KC-135E
Air National Guard and Air Force Reserve KC-135As re-engined with Pratt & Whitney TF33-PW-102 engines from retired 707 airliners (161 modified). All E model aircraft were retired to the 309th AMARG at Davis-Monthan AFB by September 2009 and replaced with R models.
NKC-135E
Test-configured KC-135E. 55-3132 NKC-135E "Big Crow I" & 63-8050 NKC-135B "Big Crow II" used as airborne targets for the Boeing YAL-1 Airborne Laser carrier.
KC-135Q
KC-135As modified to carry JP-7 fuel necessary for the SR-71 Blackbird, 56 modified.
KC-135R (1960s)
4 JC/KC-135As converted to Rivet Stand (Later Rivet Quick) configuration for reconnaissance and evaluation of above ground nuclear test. Serial Numbers (aircraft) 58-0126 replaced 59-1465 after it crashed in 1967). These aircraft were powered by Pratt & Whitney J57 engines and were based at Offutt AFB, Nebraska.
KC-135R
KC-135As and some KC-135Es re-engined with CFM56 engines, more than 417 converted. Modern version.
KC-135R(RT)
Receiver-capable KC-135R Stratotanker; eight modified with either a Boeing or LTV receiver system and a secure voice SATCOM radio. Three of the aircraft were converted to tankers from RC-135Ds, from which they retained their added equipment.
KC-135T refueling a C-131 Starlifter at night
KC-135T
KC-135Q re-engined with CFM56 engines, 54 modified.
C-135F
A new-built variant for France as dual-role tanker/cargo and troop carrier aircraft. 12 were built for the French Air Force with the addition of a drogue adapter on the refueling boom.
C-135FR
11 surviving C-135Fs upgraded with CFM International F108 turbofans between 1985 and 1988. Later modified with MPRS wing pods.
EC-135Y
An airborne command post modified in 1984 to support CINCCENT. Aircraft 55-3125 was the only EC-135Y. Unlike its sister EC-135N, it was a true tanker that could also receive in-flight refueling. Pratt & Whitney TF33-PW-102. Retired to 309th AMARG at Davis-Monthan AFB, AZ.
CINCCENT - Acronym for US Central Command.
