The Boeing KC-135 Stratotanker's primary mission is to aerial refuel compatible U.S. Air
Force, Navy and Marine Corps aircraft. It also provides refueling support to compatible allied aircraft
On 15 July 1954, Boeing's famous Model 367-80 prototype took to the air for the
first time. The four-jet, swept-wing transport provided the basis for the prolific Model 707 civil
airliner and Model 717 (C-135) tanker/transport families, and also set the design philosophy which
extended throughout Boeing's hugely successful airliner dynasty. Note: The Model 717 differs primarily
from the Model 707 by having a smaller-diameter fuselage, deletion of cabin windows, and reduced size
In September 1955, following successful trials with the "Dash Eighty" prototype
configured with a Boeing-designed flying boom under the rear fuselage, Boeing received the first order
for the KC-135A tanker. The first aircraft (#55-3118) flew on 31 August 1956. The KC-135A entered service
with the U.S. Air Force on 28 June 1957. The last tanker was delivered to the Air Force in 1965.
A total of 732 KC-135s were built in a long and efficient production run. The first
582 aircraft were built with a short tailfin, but from then on a taller fin was introduced to make the
aircraft more stable during takeoff. This feature was eventually retrofitted to all aircraft in the
Over 500 of the tankers all built by Boeing at its Seattle facilities remain
in service today. In addition, Boeing built 88 aircraft in over 30 different models for other USAF uses,
such as cargo transports, flying command posts, electronic reconnaissance and photo mapping. The last of
these special-purpose aircraft was delivered in late-1966. Responsibility for the KC-135 was transferred
to Boeing in Wichita in 1969.
During aerial refueling operations, 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 trailed behind the flying boom, is used to refuel aircraft fitted with probes. In the
early-1990s, a feasibility study was undertaken to fit the KC-135R with wing pods for the
refueling of probe-equipped aircraft from U.S. and foreign services.
The Multi-Point Refueling System Program is an effort to enhance the efficiency
and flexibility of the Air Force’s air refueling fleet. Forty-five KC-135R Stratotankers are being
outfitted to accept wingtip hose-and-drogue and air refueling pods for refueling NATO and U.S. Navy
aircraft. With the number of worldwide joint and combined military operations on the rise, the Department
of Defense directed the Air Force to outfit part of its KC-135 fleet with the capability of refueling
both probe-and-drogue and boom receptacle aircraft on the same mission. This also the capability of
refueling up to two probe-and-drogue aircraft at the same time.
While in flight, the aircraft's large flyable boom is controlled by a boom operator,
commonly called "boomer". Performed from a prone position in a fairing under the rear fuselage, there
is an excellent view of the boom and the receiver aircraft. Normally, the boomer is in radio contact with
the receiver aircraft. A small control column is provided which is linked to control surfaces on the end
of the boom. Further controls are provided for extending and retracting the boom and the operation of
director lights which provide positional information for receiver pilots. The hook-up is made by
directions given to the receiver aircraft through a system of lights located on the belly of the KC-135,
just behind the nose gear (the same system was used by the KC-97).
Internally, the KC-135 features integral wing tanks between the spars, and further
tanks in the lower lobe of the fuselage, making a total of 22. The fuel cells are made of nylon fabric
less than one-sixteenth of an inch thick a cell weighing 80 pounds will hold seven tons of fuel! The
fuel system is managed from the cockpit by the co-pilot. Fuel can be offloaded at a rate of 6,500 pounds
(2,948kg) per minute. This is enough fuel in one minute to operate an average family car for one year.
The main cargo hold provides enough room for six standard 463L-type cargo pallets,
which are loaded through a side cargo door. Depending on fuel storage configuration, the KC-135 can
carry up to 83,000 pounds (37,648kg) of cargo or 80 passengers. A crew of four is comprised of two
pilots, a navigator and a boom operator.
|Airborne Command Post
||EC-135 (Looking Glass)
||NKC-135 (Vomit Comet)
||RC-135 (Combat Sent / Rivet Joint / Cobra Ball)
||VC-137 (formerly Air Force One)
Fifty-six KC-135 tankers were converted to KC-135Qs, which feature additional navigation
and communications equipment for the support of the now-retired Lockheed SR-71 fleet. These tankers carry
high-flashpoint JP-7 fuel in addition to the standard JP-4/5 used by the tanker itself. Although the
Q-models are still capable of refueling receivers that require JP-4/5, the tanks must be purged before
carrying JP-7 (JP-4/5 will contaminate JP-7; not vise-versa). Today these tankers are often involved in
supporting F-117 Nighthawk stealth fighter operations, and are in the process of being re-engined to
Between 1975 and 1988, Boeing replaced the lower wing skins of all surviving KC-135s
to extend their useful lives to beyond the year 2020. The work involved replacing about 1,500 square feet of
aluminum on the underside of the wings, which carry most of the wing load in flight, with an improved
aluminum alloy. The original wing surface consisted of a type of aluminum more susceptible to fatigue.
Skin panels were milled, machined and contoured at Boeing. The wing then went into a rivet assembly
jig where stiffeners and skins were joined. The bottom section of the old wing was cut away and replaced
by the new sections. In addition to the skin panels, engine strut fittings were also replaced. Each
unit required a total of 564 parts, 32,200 steel fasteners and 19,500 aluminum rivets.
In 1981, work also began on re-engining a large portion of the C/KC-135A fleet with
turbofans to replace the noisy, smokey, and inefficient J57-P-59W turbojet engines. Of the two programs,
the least ambitious was that to upgrade over 160 KC-135A tankers of the Air National Guard and Air Force
Reserve (together with a small number of special mission EC/NKC/RC-135 aircraft) with refurbished TF33-PW-102
engines taken from used, commercial 707 airliners. After modification, the airplanes were redesignated
Fitment of the TF33 provides several important improvements over the original J57.
The greater thrust allows the aircraft to operate on far greater safety margins than previously possible,
and to use shorter runways. Secondly, the fan engines are more efficient, offering cost-saving and greater
fuel offloads on similar mission profiles. Also, noise and air pollution is considerably reduced, a major
factor for Guard units operating from civilian airports, and lastly the TF33s incorporate thrust-reversers for
enhanced safety during landings.
In 1980, Boeing announced a major upgrade program for the KC-135 involving the fitment
of CFM International's CFM56 (military designation F108) high bypass-ratio turbofan engine to offer far
greater fuel efficiency, noise reduction and operational flexibility than the E-model's TF33s. Also
included was the addition of a Quick Start Auxiliary Power Unit (QSAS), characterized by intake and exhaust ports on
the port side of the rear fuselage, which allows the aircraft to undertake autonomous operations from
austere locations. Under the company designation KC-135RE, the first conversion took to the air on 4
Designated KC-135R in service (the second time this designation had been applied,
having previously been used by a reconnaissance variant of the KC-135A), the first delivery took place in
July 1984. Boeing has delivered approximately 400 re-engined KC-135Rs and is under contract for about
432 re-engine kits. Each kit includes struts, nacelles, 12.2 miles of wiring, and other system
The KC-135R 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. This re-engined and enhanced
version of the venerable KC-135 tanker is now the mainstay of the U.S. Air Force's tanker fleet.
The U.S. Air Force will continue its effort to improve the KC-135 with the Multi-Point
Refueling System Program which adds wingtip, hose-and-drogue refueling pods to the tanker. The Air Force
has also acknowledged that the cockpit of the KC-135 must be modernized. It has issued a solicitation
for new Compass, Radar And Global positioning systems, known as
"Pacer CRAG", in the cockpit.
During the war in Vietnam, KC-135 Stratotankers made the air war different from all
previous aerial conflicts. Aerial refueling brought far-flung bombing targets within reach. Combat
aircraft, no longer limited by fuel supplies, were able to spend more time in target areas.
The KC-135 tanker fleet also made an invaluable contribution to the success of
Operation Desert Storm (1991) in the Persian Gulf, flying around-the-clock missions to maintain
operability of allied warplanes. The KC-135s form the backbone of the Air Force tanker fleet, meeting
the aerial refueling requirements of bomber, fighter, cargo and reconnaissance forces, as well as the
needs of the Navy, Marines, and allied nations.
Back to top