Everything about Kc-135 Stratotanker totally explained
The
Boeing KC-135 Stratotanker is a
United States aerial refueling tanker
aircraft. It has been in service with the
United States Air Force since 1957.
Development
The KC-135 is derived from the original
Boeing jet transport "proof of concept" demonstrator, the
Boeing 367-80 (commonly called the "Dash-80"). As such, it has a narrower
fuselage and is shorter than the
Boeing 707 jetliner. Boeing gave the tanker the designation of Model 717.
Developed in the late 1950s, the basic airframe is characterized by 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's actually a different aircraft.
The
Strategic Air Command had the KC-135 Stratotankers in service from 1957 through 1991 and with the
Air National Guard and the
Air Force Reserve from 1975 through 1991, when most were re-assigned to the newly-created
Air Mobility Command (AMC). As AMC gained the preponderance of the aerial refueling mission, a small number of KC-135s were also assigned to directly
United States Air Forces in Europe (USAFE) and
Pacific Air Forces (PACAF).
Reconnaissance and command post variants, including
EC-135 Looking Glass, Post Attack Command & Control Systems were operated by SAC from 1963 through 1991, when they were re-assigned to the
Air Combat Command.
Boeing's 367-80 was the basic design for the commercial
Boeing 707 passenger aircraft as well as the
KC-135A Stratotanker. In 1954 the
USAF's
Strategic Air Command ordered the first 29 of its future fleet of 732. 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.
Four
turbojet engines, mounted under 35-degree swept wings, power the KC-135 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 special shuttlecock-shaped drogue, attached to and trailing behind the flying boom, may be used to refuel aircraft fitted with probes. A
boom operator stationed in the rear of the aircraft controls the boom while lying prone. 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 (37,600 kg) of cargo.
In
Southeast Asia, KC-135 Stratotankers ability as a force multiplier truly came to the fore. Midair refueling brought far-flung bombing targets within reach, and allowed fighter missions the spend hours at the front, rather than just a few minutes, due to their limited fuel reserves.
Air Mobility Command (AMC) manages more than 546 total aircraft inventory Stratotankers, of which the
Air Force Reserve and
Air National Guard fly 292 in support of AMC's mission.
Design
General upgrades
The KC-135Q was the variant modified to carry the
JP-7 fuel necessary for the
SR-71 Blackbird, segregating the JP-7 from the KC-135's own fuel supply (the body tanks carrying JP-7, and the wing tanks carrying JP-4). When the KC-135Q model received the CFM-56 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.
The KC-135R/T was a receiver-capable tanker, used for pilot training and operational refueling missions. Currently all KC-135R/T models (Receiver capable) are with the 22nd Air Refueling Wing at McConnell AFB, KS. Most of these are 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.
In order to expand the KC-135's capabilities and improve its reliability, the aircraft has undergone a number of upgrades. Among these was the Pacer-CRAG program (CRAG=
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 modifications were performed by
BAE Systems at the
Mojave Airport in
California. The latest block upgrade to the KC-135 is Block 40.3 which allows the KC-135 to comply with
CNS/ATM.
Re-engining
All KC-135s were originally equipped with Pratt & Whitney J-57-P-59W turbojet engines which produced 10,000 lbs of thrust dry,and approximately 13,000 pounds of thrust wet. Wet thrust was achieved through the use of water injection on takeoff. 670 gallons of water were injected into the engines over the course of two and a half minutes. This water allowed a second set of fuel injectors to activate without melting the turbine buckets. The water turned to steam, was ejected out the rear of the engine, and increased thrust. The water and the extra fuel increased the mass through the engine, increasing thrust. The engine ran a little hotter, increasing thrust. Also noise. The A-model on takeoff was a loud jet. In the 1980s the first modification program re-engined 157 Air Force Reserve (AFRES) and Air National Guard (ANG) tankers with the
Pratt & Whitney TF-33-PW-102 engines from 707 airliners retired by United and American Airlines. The re-engined tanker, designated the KC-135E, (now retired) was 14% more fuel efficient than the KC-135A and could offload 20% more fuel on long duration flights. (The difference is that the A-model weighed only 104,000 lbs empty, while the E-model weighed 115,000 lbs empty. But the maximum takeoff weight wasn't increased for the E-model. Therefore, the A-model could takeoff with 200,000lbs of fuel, while the E-model could only takeoff with 190,000lbs of fuel.) Only the KC-135E aircraft were equipped with thrust-reversers for takeoff aborts and shorter landing roll-outs.
The second modification program re-engined 500 with new CFM International CFM56 (CF-100 for the military designation) engines produced by General Electric and Snecma. The re-engined tanker, designated either the KC-135R or KC-135T, can offload up to 50% more fuel (on a long duration sortie), is 25% more fuel efficient, costs 25% less to operate and is 96% quieter than the KC-135A.
The KC-135R's operational range is 60% greater than the KC-135E for comparable fuel offloads, providing a wider range of basing options.
No longer in consideration, upgrading the remaining KC-135E into KC-135R would have cost about three billion dollars, about 24 million dollars per aircraft..
Multi-Point Refueling System program
This program adds refueling pods to the KC-135's wings. The pods allow refueling of U.S. Navy, U.S. Marine Corps and NATO tactical jet aircraft while keeping the tail-mounted refueling boom.
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Research and development
Besides its primary role as an inflight aircraft refueler, the KC-135, designated NKC-135, has assisted in several research projects at the
NASA Dryden 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 Dryden, 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 jets, as well as business aviation jets.
NASA also has operated several KC-135 aircraft (without the tanker equipment installed) as their infamous
Vomit Comet zero-gravity simulator aircraft. The longest-serving (1973 to 1995) version was KC-135A 59-1481, named
Weightless Wonder IV registered N930NA.
Electronic warfare
A number of KC-135A and KC-135B aircraft have been modified to
EC-135 configuration for use in several different roles.
Operational history
Replacing the KC-135
As of 2006, the KC-135E fleet is flying an annual average of 350 hours per aircraft and the KC-135R fleet is 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 forecasted usage and sustainment costs.
The Air Force projects 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 before 2040, but at that time some of the aircraft would be about 80 years old. The Air Force estimates that their current fleet of KC-135s have between 12,000 to 14,000 flying hours on them-only 33 percent of the lifetime flying hour limit. Nevertheless these aircraft are over 40 years old and maintenance costs are increasing, with airframe corrosion being the worst problem.
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. The Air Force’s 15-year cost estimates project further significant growth through fiscal year 2017. For example, operations and support costs for the KC-135 fleet are estimated to grow from about $2.2 billion in fiscal year 2003 to $5.1 billion (2003 dollars) in fiscal year 2017, an increase of $2.9 billion, or over 130 percent, which represents an annual growth rate of about 6.2 percent.
The USAF has decided to replace the KC-135 fleet. However, since there are over 500 KC-135 these planes will be replaced gradually, with the first batch of about 100 aircraft to be replaced in the current buy.
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 2003, the Pentagon froze the contract and in January 2006, the
KC-767 contract was canceled. This followed public revelations of corruption in how the contract was awarded, as well as controversy regarding the original leasing rather than outright purchase agreement. This was also designed to be a cost-cutting measure and is part of a larger reorganization and redefinition of the
Air Force's mission that includes the retirement of the
E-4B fleet, the cancellation of the
Boeing 767-based
E-10 MC2A program, as well as the elimination of all but 58
B-52 Stratofortresses. Former Secretary of Defense Rumsfeld stated that this move will in no way impair the Air Force's ability to deliver the mission of the KC-767, which will be accomplished by continuing upgrades to the KC-135 and
KC-10 Extender fleet.
In January 2007, the U.S. Air Force formally launched the
KC-X program with a
request for proposal (RFP). KC-X is first phase of three acquisition programs 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 for the first stage of the program.
Operators
