The Full Wiki

F-100 Super Sabre: Map


Wikipedia article:

Map showing all locations mentioned on Wikipedia article:

The North American F-100 Super Sabre was a supersonic jet fighter aircraft that served with the United States Air Force (USAF) from 1954 to 1971 and with the Air National Guard (ANG) until 1979. As the first of the Century Series collection of USAF jet fighters, it was capable of supersonic speed in level flight. The F-100 was originally designed as a higher performance follow-on to the F-86 Sabre air superiority fighter.

Adapted as a fighter bomber, the F-100 would be supplanted by the Mach 2 class F-105 Thunderchief for strike missions over North Vietnam. The F-100 flew extensively over South Vietnam as the Air Force's primary close air support jet until replaced by the more efficient subsonic A-7 Corsair II. The F-100 also served in several NATOmarker air forces and with other US allies. In its later life, it was often referred to as "the Hun," a shortened version of "one hundred."

Design and development

The underside of a YF-100 (s/n 52-5754)
The cockpit of an F-100D

In January 1951, North American Aviation delivered an unsolicited proposal for a supersonic day fighter to the United States Air Force. Named Sabre 45 because of its 45° wing sweep, it represented an evolution of the F-86 Sabre. The mockup was inspected 7 July 1951 and after over a hundred modifications, the new aircraft was accepted as the F-100 on 30 November 1951. Extensive use of titanium throughout the aircraft was notable. On 3 January 1952, the USAF ordered two prototypes followed by 23 F-100As in February and an additional 250 F-100As in August.

The YF-100A first flew on 25 May 1953, seven months ahead of schedule. It reached Mach 1.05 in spite of being fitted with a de-rated XJ57-P-7 engine. The second prototype flew on 14 October 1953, followed by the first production F-100A on 9 October 1953. The USAF operational evaluation from November 1953 to December 1955 found the new fighter to have superior performance but declared it not ready for widescale deployment due to various deficiencies in the design. These findings were subsequently confirmed during Project Hot Rod operational suitability tests. Particularly troubling was the yaw instability in certain regimes of flight which produced inertia coupling. The aircraft could develop a sudden yaw and roll which would happen too fast for the pilot to correct and would quickly overstress the aircraft structure to disintegration. It was under these conditions that North American's chief test pilot, George Welch, was killed while dive testing an early-production F-100A on 12 October 1954. Another control problem stemmed from handling characteristics of the swept wing at high angles of attack. As the aircraft approached stall speeds, loss of lift on the tips of the wings caused a violent pitch-up. This particular phenomenon (which could easily be fatal at low altitude where there was insufficient time to recover) became known as the "Sabre dance".

Nevertheless, delays in the F-84F Thunderstreak program pushed the Tactical Air Command to order the raw F-100A into service. TAC also requested that future F-100s should be fighter-bombers, with the capability of delivering nuclear bombs.

The North American F-107 was a follow-on Mach 2 development of the F-100 with the air intake moved above and behind the cockpit. It was not developed in favor of the F-105 Thunderchief.

Operational history

An F-100D showing its oval air intake
F-100D of the 50th TFW (Wing Commander's aircraft), at Toul Air Base, France in 1958

The F-100A officially entered USAF service on 27 September 1954 with 479th Fighter Wing at George AFBmarker, CA. By 10 November 1954, the F-100As suffered six major accidents due to flight instability, structural failures, and hydraulic system failures, prompting the Air Force to ground the entire fleet until February 1955. The 479th finally became operational in September 1955. Due to ongoing problems, the Air Force began phasing out the F-100A in 1958, with the last aircraft leaving active duty in 1961. By that time, 47 aircraft were lost in major accidents. Escalating tension due to construction of the Berlin Wallmarker in August 1961 forced the USAF to recall the F-100As into active service in early 1962. The aircraft was finally retired in 1970.

The TAC request for a fighter-bomber was addressed with the F-100C which flew in March 1954 and entered service on 14 July 1955 with the 450th Fighter Wing, Foster AFBmarker, TX. Operational testing in 1955 revealed that the F-100C was at best an interim solution, sharing all the vices of the F-100A. The uprated J57-P-21 engine boosted performance but continued to suffer from compressor stalls. On a positive note, the F-100C was considered an excellent platform for nuclear toss bombing because of its high top speed. The inertia coupling problem was more or less addressed with installation of a yaw damper in the 146th F-100C, later retrofitted to earlier aircraft. A pitch damper was added starting with the 301st F-100C, at a cost of US$10,000 per aircraft.

The addition of "wet" hardpoints meant the F-100C could carry a pair of 275 US gal (1,040 l) and a pair of 200 US gal (770 l) drop tanks. However, the combination caused loss of directional stability at high speeds and the four tanks were soon replaced by a pair of 450 US gal (1,730 l) drop tanks. The 450s proved scarce and expensive and were often replaced by smaller 335 US gal (1,290 l) tanks. Most troubling to TAC was the fact, that, as of 1965, only 125 F-100Cs were capable of utilizing all non-nuclear weapons in the Air Force inventory, particularly cluster bombs and AIM-9 Sidewinder air-to-air missiles. By the time the F-100C was phased out in June 1970, 85 had been lost in major accidents.

The definitive F-100D aimed to address the offensive shortcomings of the F-100C by being primarily a ground attack aircraft with secondary fighter capability. To this effect, the aircraft was fitted with autopilot, upgraded avionics, and, starting with the 184th production aircraft, the Sidewinder capability. In 1959, 65 aircraft were modified to also fire the AGM-12 Bullpup air-to-ground missile. To further address the dangerous flight characteristics, the wing span was extended by 26 in (66 cm) and the vertical tail area was increased by 27%.

The first F-100D (54-2121) flew on 24 January 1956, piloted by Daniel Darnell. It entered service on 29 September 1956 with 405th Fighter Wing at Langley AFBmarker. The aircraft suffered from reliability problems with the constant speed drive which provides constant-frequency current to electrical systems. In fact, the drive was so unreliable that USAF required it to have its own oil system to minimize damage in case of failure. Landing gear and brake parachute malfunctions claimed a number of aircraft, and the refueling probes had a tendency to break away during high speed maneuvers. Numerous post-production fixes created such a diversity of capabilities between individual aircraft that by 1965 around 700 F-100Ds underwent High Wire modifications to standardize the weapon systems. High Wire modifications took 60 days per aircraft at a total cost of US$150 million. In 1966, Combat Skyspot program fitted some F-100Ds with an X band radar transmitter to allow for ground-directed bombing in inclement weather or at night.

In 1961, at England AFB, LA, (401st Tactical Wing), there were four fighter/bomber squadrons. These were the 412th, 413th, 414th and the 415th (Fighting Tigers). The 415th aircraft were modified to be the only self starting fighters in the world. This was accomplished with the addition of a large canister to the underside of the aircraft. This canister contained a black powder compound. The canister was ignited (electro-mechanical) and drove the jet engine to minimal ignition point. During the Berlin Crisis (approximately 09/61) the 615th was deployed to Rammstein AB, Germany to support the Germans. At the initial briefing, the 415th personnel were informed that due to the close proximity of the USSR, if an ICBM were to be launched, they would only have thirty minutes to launch the 415th aircraft and retire to the nearest German bunker.

In 1967, the USAF began a structural reinforcement program to extend the aircraft's service life from the designed 3,000 flying hours to 7,000. Over 500 F-100Ds were lost, predominantly in accidents. After one aircraft suffered wing failure, particular attention was paid to lining the wings with external bracing strips. During the Vietnam War, combat losses constituted as many as 50 aircraft per year. On 7 June 1957, an F-100D fitted with an Astrodyne booster rocket making 150,000 lbf (667.2 kN) of thrust successfully performed a zero length launch. The capability was incorporated into late-production aircraft. After a major accident, the USAF Thunderbirds reverted from F-105 Thunderchief to the F-100D which they operated from 1964 until it was replaced by the F-4 Phantom II in 1968.

F-100D in trial of zero-length-launch system.
Pilot is Maj R.
(USAF photo)
A USAF KB-50D of the 622d Air Refueling Squadron carrying out the first triple-point refuelling operation with three F-100Cs in 1956

The F-100 was the subject of many modification programs over the course of its service. Many of these were improvements to electronics, structural strengthening, and projects to improve ease of maintenance. One of the more interesting of these was the replacement of the original afterburner of the J-57 engine with the more advanced afterburners from retired Convair F-102 Delta Dagger interceptors. This modification changed the appearance of the aft end of the F-100, doing away with the original "petal-style" exhaust. The afterburner modification started in the 1970s and solved maintenance problems with the old type as well as operational problems, including compressor stall problems.

The F-100F two-seat trainer entered service in 1958. It received many of the same weapons and airframe upgrades as the F-100D, including the new afterburners. By 1970, 74 F-100Fs were lost in major accidents.

By 1972, the F-100 was mostly phased out of USAF active service and turned over to tactical fighter groups and squadrons in the Air National Guard. In Air National Guard units, the F-100 was eventually replaced by the F-4 Phantom II, A-7 Corsair II, and A-10 Thunderbolt II, with the last F-100 retiring in 1979. In foreign service, Royal Danish Air Force and Turkish Air Force F-100s soldiered on until 1982.

Over the lifetime of its USAF service, a total of 889 F-100 aircraft were destroyed in accidents, involving the deaths of 324 pilots. The deadliest year for F-100 accidents was 1958, with 116 aircraft destroyed, and 47 pilots killed.

After Super Sabres were withdrawn from service, a large number were converted into remote-controlled drones (QF-100) under the USAF Full Scale Aerial Target (FSAT) program for use as targets for various anti-aircraft weapons, including missile-carrying fighters and fighter-interceptors, with FSAT operations being conducted primarily at Tyndall AFBmarker, FL. A few F-100s also found their way into civilian hands, primarily with defense contractors supporting USAF and NASA flight test activities at Edwards AFBmarker, CA.

Project High Wire

High Wire was a modernization program for selected F-100Cs, Ds and Fs. It consisted of two modifications:
  1. Electrical rewiring upgrade
  2. Heavy maintenance and IRAN upgrade.
Rewiring upgrade operation consisted of replacing old wiring and harnesses with improved maintainable designs. Heavy maintenance and IRAN (inspect and repair as necessary) included new kits, modifications, standardized configurations, repairs, replacements and complete refurbishment.

This project required all new manuals (TOs) and incremented (i.e. -85 to -86) block numbers. All later production models, especially the F models included earlier High Wire mods. New manuals included colored illustrations. All manuals will have the Roman numeral (I) added after the aircraft number (i.e. T.O. 1F-100D(I)-1S-120, 12 January 1970).

Vietnam War

F-100Ds of the 416th Tactical Fighter Squadron at Da Nang Air Base, South Vietnam, in 1965
Two USAF F-100Ds over South Vietnam in 1967
A USAF F-100F of the 416th TFS at Phu Cat Air Base, South Vietnam

On 16 April 1961 six Super Sabres were deployed from Clark Air Force Basemarker in the Philippines to Don Muang Airfield in Thailand for air defense purposes; the first F-100s to enter combat in Southeast Asia. From that date until their redeployment in 1971, the F-100s would be the longest serving US jet fighter bomber to fight in the Vietnam War. Serving as MIGCAP escorts for F-105 Thunderchiefs, MISTY FACs, and Wild Weasels over North Vietnam, and then relegated to close air support and ground attacks within South Vietnam.

On 18 August 1964, the first F-100D to be shot down by ground fire was piloted by 1st Lt Colin A. Clark, of the 428th TFS; Clark ejected and survived. On 4 April 1965 as escorts protecting F-105s attacking the Thanh Hoa Bridgemarker, F-100 Super Sabres fought the USAF's first air-to-air jet combat duel in the Vietnam War, in which an F-100 piloted by Capt Donald W. Kilgus shot down a North Vietnamese Air Force MiG-17, using cannon fire, while another fired Sidewinder missiles. The surviving North Vietnamese pilot reported 3 of his planes were shot down. Although recorded by the US as one probable kill, this represented the first aerial victory by US forces in Vietnam. However, the small force of 4 MiG-17s had evaded the F-100s to claim two F-105s. The F-100 was soon replaced by the F-4C for Mig CAP which pilots noted suffered for lacking built-in guns for dogfights.

The Vietnam War was not known for utilizing activated National Guard or other US Reserve units; but rather, had a reputation for conscription (military draft) during the course of the war. During a confirmation hearing before Congress in 1973, USAF General George S. Brown, who had commanded the 7th Air Force (7 AF) during the war, stated that five of the best Super Sabre squadrons in Vietnam were from the Air National Guard. This included the 120th Tactical Fighter Squadron (120 TFS) of the Colorado Air National Guard, the 136 TFS of the New York Air National Guard TFS, the 174 TFS of the Iowa Air National Guard and the 188 TFS of the New Mexico Air National Guard. The fifth unit was a regular AF squadron manned by mostly Air Guardsmen.

The Air National Guard F-100 Squadrons increased the regular USAF by nearly 100 Super Sabres in theater, averaging, for the Colorado ANG F-100s, 24 missions a day, delivering ordnance and munitions with a 99.5% reliability rate. From May 1968 to April 1969, the ANG Super Sabres flew more than 38,000 combat hours and more than 24,000 sorties. Between them, at the cost of seven F-100 Guard pilots killed (plus one staff officer) and the loss of 14 Super Sabres to enemy action, the squadrons expended over four million rounds of 20mm cannon, 30 million pounds of bombs and over 10 million pounds of napalm against the enemy.

The Hun was also deployed as a two-seat F-100F model which saw service as a "Fast-FAC" or "Misty-Fac" (forward air controller) in North Vietnam, spotting targets for other fighter-bomber aircraft and conducting SAR (Search and Rescue) missions as part of the top-secret Commando Sabre or "Misty" Operation based out of Phu Cat Airbase. It was also the first Wild Weasel SEAD (air defense suppression) aircraft whose specially-trained crews were tasked with locating and destroying enemy air defenses. Four F-100F Wild Weasel Is were fitted with an APR-25 vector radar homing and warning (RHAW) receivers, IR-133 panoramic receivers with greater detection range, and KA-60 panoramic cameras. The APR-25 could detect early-warning radars and, more importantly, emissions from SA-2 Guideline tracking and guidance systems. These aircraft deployed to Korat Royal Thai Air Force Base, Thailandmarker in November 1965, and began flying combat missions with the 388th Tactical Fighter Wing in December. They were joined by three more aircraft in February 1966. All Wild Weasel F-100Fs were eventually modified to fire the AGM-45 Shrike anti-radiation missile.

By war's end, 242 F-100 Super Sabres had been lost in Vietnam, as the F-100 was progressively replaced by the F-4 Phantom II and the F-105 Thunderchief. The Hun had logged 360,283 combat sorties during the war and its wartime operations came to end on 31 July 1971.

Algerian war

French Air Force Super Sabres flew combat missions, with strikes flown from bases within France against targets in Algeria.

Cyprus crisis

Turkish Air Force F-100 units were used during the invasion to Cyprus in 1974. Together with F-104G Starfighters, they provided close air support to Turkish ground troops and bombed targets around Nicosia.

Notable achievements

Source: Knaack
  • The first operational aircraft in United States Air Force inventory capable of exceeding the speed of sound in level flight.
  • On 29 October 1953, the first YF-100A prototype set a world speed record of 755.149 mph (656.207 kn, 1,215.295 km/h) at low altitude.
  • On 20 August 1955, an F-100C set the first supersonic world speed record of 822.135 mph (714.416 kn, 1,323.098 km/h).
  • On 4 September 1955, an F-100C won the Bendix Trophy, covering 2,235 mi (2,020 nmi, 3,745 km) at an average speed of 610.726 mph (530.706 kn, 982.868 km/h).
  • On 26 December 1956, two F-100Ds became the first-ever aircraft to successfully perform buddy refueling.
  • On 13 May 1957, three F-100Cs set a new world distance record for single-engine aircraft by covering the 6,710 mi (5,835 nmi, 10,805 km) distance from Londonmarker to Los Angelesmarker in 14 hours and 4 minutes. The flight was accomplished using inflight refueling.
  • On 7 August 1959, two F-100Fs became the first-ever jet fighters to fly over the North Polemarker.
  • On 16 April 1961, the first USAF combat jets to enter the Vietnam War.
  • On 4 April 1965, the first USAF aircraft to engage in aerial jet combat during the Vietnam War, while escorting F-105 Thunderchiefs to target.
  • The U.S. Air Force Thunderbirds operated the F-100C from 1956 until 1964. After briefly converting to the F-105 Thunderchief, the team flew F-100Ds from July 1964 until November 1968, before converting to the F-4E Phantom II.


The costs are in contemporary United States dollars and have not been adjusted for inflation.

!F-100A F-100C F-100D F-100F
R&D 23.2 million for the program or 10,134 prorated per aircraft
Airframe 748,259 439,323 448,216 577,023
Engine 217,390 178,554 162,995 143,527
Electronics 8,549 12,050 10,904 13,667
Armament 19,905 21,125 66,230 66,332
Ordnance 20,807 12,125 8,684 3,885
Flyaway cost 1,014,910 663,181 697,029 804,444
Additional modification costs 224,048 110,559 105,604
Cost per flying hour 583 583
Maintenance cost per flying hour 215 249 249 249


Prototype YF-100A (Serial number: 52-5754)
F-100A with the original short tail fin
A QF-100D over Tyndall Air Force Base, Florida (USA), in 1981

Prototype, Model NA-180 two built, s/n 52-5754 and 5755.
9 test unmanned drone version: 2 D-models, 1 YQF-100F F-model,see DF-100F, and six other test versions.
Single-seat day fighter; 203 built, Model NA-192.
RF-100A (Slick Chick)
F-100A modified for photo reconnaissance, six modified in 1954. Unarmed, with camera installations in lower fuselage bay. Retired from USAF service in 1958. Four transferred to Republic of China Air Force, retired in 1960.
See North American YF-107
Proposed interceptor version of F-100B, did not advance beyond mock-up.
Seventy Model NA-214 and 381 Model NA-217. Additional fuel tanks in the wings, fighter-bomber capability, probe-and-drogue refueling capability, uprated J57-P-21 engine on late production aircraft. First flight: March 1954; 476 built.
One F-100C converted into a two-seat training aircraft.
Single-seat fighter-bomber, more advanced avionics, larger wing and tail fin, landing flaps. First flight: 24 January 1956; 1,274 built.
Two-seat training version, armament decreased from four to two cannon. First flight: 7 March 1957; 339 built.
This designation was given to one F-100F that was used as drone director.
Three F-100Fs used for test purposes, the prefix N indicates that modifications prevented return to regular operational service.
Specific Danish designation given to 14 F-100Fs exported to Denmarkmarker in 1974, in order to distinguish these from the 10 F-100Fs delivered 1959-1961.
Another 209 D and F models were ordered and converted to unmanned radio-controled FSAT (Full Scale Aerial Target) drone and drone directors for testing and destruction by modern air-to-air missiles used by current Air Force fighter jets.
Unbuilt all-weather export version for Japan.
Unbuilt variant with a J57-P-55 engine.
Unbuilt version with simplified avionics.
Proposed French-built F-100F with Rolls-Royce Spey turbofan engine.


It was the only allied air force to operate the F-100A model. The first F-100 was delivered in October 1958. It was followed by 15 F-100As in 1959, and by 65 more F-100As in 1960. In 1961, four unarmed RF-100As were delivered. Additionally, 38 ex-ANG F-100As were delivered later, to bring total strength to 118 F-100As and four RF-100As. F-100As were retrofitted with the F-100D vertical tail with its AN/APS-54 tail-warning radar and equipped to launch Sidewinder air-to-air missiles. Several were lost in intelligence missions over the People's Republic of China.

Retired Danish F-100F Super Sabre
It operated total 72 aircraft. 48 F-100Ds and 24 Fs were delivered to Denmark from 1959 to 1974. The last Danish F-100s were retired from service in 1982. The F-100s were replaced by Saab F-35 Draken and Lockheed Martin F-16s. Some Danish F-100s were transferred to Turkey (21 F-100Ds and two F-100Fs).

The Armee de l'Air was the first Allied air force to receive the F-100 Super Sabre. The first aircraft arrived in France on 1 May 1958. A total of 100 aircraft (85 F-100Ds and 15 F-100Fs) were supplied to France, and assigned to the NATO 4th Allied Tactical Air Force. They were stationed at German French bases. French F-100s were used on combat missions flying from bases in France against targets in Algeria. In 1967 France left NATO, and German-based F-100s were transferred to France, using bases recently vacated by the USAF.

The Turk Hava Kuvvetleri received about 206 F-100C, D and F Super Sabres. Most came from US stocks, and 21 F-100Ds and two F-100Fs were supplied by Denmark. Turkish F-100s saw extensive action during the 1974 invasion of Cyprus.

List of F-100 Units of the United States Air Force



On display
  • French Air Force F-100D 42185 is displayed at the Schwäbisches Bauern Technical Museum, Eschachmarker-Seifertshofen, Germanymarker
  • French Air Force F-100D 42136 is displayed at the Schwäbisches Bauern Technical Museum, Eschach-Seifertshofen, Germany
  • F-100F 56-3944 of the USAF is on display at the The Virtual Museum, Flugausstellung Leo Junior, Hermeskeil, Germany


On display


On display


On display


On display

United Kingdom

On display

United States

A CH-54 an F-100A on its last flight at Hill Air Force Base, Utah (USA), prior to being placed on static display, in 1979.
On display

Specifications (F-100D)

An F-100D of the 308th TFS, being loaded with Mk 117 750 lb bombs at Tuy Hoa, South Vietnam, in early 1966.

See also



  1. F-100: "Designed originally to destroy enemy aircraft in aerial combat"
  2. Global Security A-7: "The aging low-payload F-100 was the Air Force's primary air-to-ground CAS airplane at the time."
  3. Boeing Co. F-100 History
  4. Video
  5. Martin Caidin's book Thunderbirds was written while the team flew F-100s. He was the only journalist to ever fly with them.
  6. Official USAF F-100 accident rate table (PDF) at
  7. USAF F-100 Super Sabre - Flight Manual - Technical Order: 1F-100D(I)-1S-120; 12 January 1970)
  8. Anderton 1987, p. 57.
  9. Anderton 1987, p. 71.
  10. Anderton 1987, p. 136.
  11. Anderton 1987, p. 144.
  12. Anderton 1987, pp. 136, 145.
  13. Hobson 2002
  14. Thompson 2008, pp. 73–74.
  15. Thompson 1999, p. 64.
  16. Baugher's: QF-100 Drone
  17. HaseGray: FSAT
  18. Baugher's: RF-100As in ROC-TW
  19. "F-100F on Display." Das Virtuelle Luftfahrtmuseum. Retrieved: 4 September 2009.
  20. Wilberg, Thomas. "North American F 100 Super Sabre." The Aircraft Museum. Retrieved: 4 September 2009.
  21. Newark Air Museum: Aircraft list
  22. " North American F100F Super Sabre 563938 11-MU." Lashendene Air Warfre Museum. Retrieved: 4 September 2009.
  23. "North American F-100C Super Sabre." Pima Air & Space Museum. Retrieved: 4 September 2009.
  24. Carolinas Aviation Museum: F-100D in Restorations.
  25. [1]
  26. [2]


  • Anderton, David A. North American F-100 Super Sabre. London: Osprey Publishing Limited, 1987. ISBN 0-85045-622-2.
  • Davies, Peter E. North American F-100 Super Sabre. Ramsbury, Wiltshire, UK: Crowood Press, 2003. ISBN 1-861265-778.
  • Drendel, Lou. Century Series in Color (Fighting Colors). Carrollton, Texas: Squadron/Signal Publications, 1980. ISBN 0-89747-097-4.
  • Green, William. The World's Fighting Planes. London: Macdonald, 1964.
  • Gunston, Bill. Fighters of the Fifties. Osceola, Wisconsin: Specialty Press Publishers & Wholesalers, Inc., 1981. ISBN 0-933424-32-9.
  • Hobson, Chris. Vietnam Air Losses: United States Air Force, Navy and Marine Corps Fixed-Wing Aircraft Losses in Southeast Asia, 1961-1973. North Branch, Minnesota: Specialty Press, 2002. ISBN 1-85780-1156.
  • Pace, Steve. X-Fighters: USAF Experimental and Prototype Fighters, XP-59 to YF-23. Oscela, Wisconsin: Motorbooks International, 1991. ISBN 0-87938-540-5.
  • Thompson, Kevin F. "North American NA-180>NA-262 YF-100A/F-100A/C/D/F Super Sabre." North American: Aircraft 1934-1999 - Volume 2. Santa Ana, CA: Johnathan Thompson, Greens, Inc., 1999. ISBN 0-913322-06-7.
  • Thompson, Warren E. "Centuries Series: F-100 Super Sabre." Combat Aircraft, Volume 9, Issue 3, June–July 2008, London: Ian Allan Publishing.

External links

Embed code:

Got something to say? Make a comment.
Your name
Your email address