The zero-length launch system or zero-length take-off system (ZLL, ZLTO, ZEL, ZELL) was a method whereby jet fighters and attack aircraft could be near-vertically launched using rocket motors to rapidly gain speed and altitude. Such rocket boosters were limited to a short-burn duration, being typically solid-fuel and suitable for only a single use, being intended to drop away once expended.
The majority of ZELL experiments, which including the conversion of several front-line combat aircraft for trialling the system, occurred during the 1950s amid the formative years of the Cold War. As envisioned, the operational use of ZELL would have employed mobile launch platform to disperse and hide aircraft, reducing their vulnerability in comparison to being centralised around established airbases with well-known locations. While flight testing had proved such systems to be feasible for combat aircraft, no ZELL-configured aircraft were ever used operationally. The emergence of ever-capable missiles had greatly reduced the strategic necessity of aircraft for the nuclear strike mission, while questions over practicality had also played a role.
During the second world war, Germany experimented with the Bachem Ba 349, but due to them losing the war development was cut short.
According to aviation author Tony Moore, the concept of the zero-length launch system became popular amongst military planners and strategists during the early years of what is now known the Cold War. Conventional aircraft, reliant on large and well-established airbases, were thought to be too easily knocked out in the opening hours of a major conflict between the superpowers, thus the ability to remove this dependence upon lengthy runways and airbases was highly attractive. During the 1950s, various powers began experimenting with a diverse range of methods to launch armed fighter jets, typically using some arrangement of rocket motors. In some concepts, such a fighter could be launched from a trailer from virtually any location, including those that could be camouflaged or otherwise concealed up until the moment of launch.
The primary advantage of a zero-length launch system is the elimination of the historic dependence on vulnerable airfields to facilitate air operations. In the event of a sudden attack, air forces equipped with such systems could field effective air defenses and launch their own airstrikes even with their own airbases having been destroyed by an early nuclear attack. Although launching aircraft using rocket boosters proved to be relatively trouble-free, a runway was still required for these aircraft to be able to land or else be forced to crash. The mobile launching platforms also proved to be expensive to operate and somewhat bulky, typically making them difficult to transport. The security of the mobile launchers themselves would have been a major responsibility in and of itself, especially in the case of such launchers being equipped with nuclear-armed strike fighters.
The United States Air Force, the Bundeswehr's Luftwaffe, and the Soviets' VVS all conducted experiments in zero-length launching. The first manned aircraft to be ZELL-launched was an F-84G in 1955. The Soviets' main interest in ZELL was for point defense-format protection of airfields and critical targets using MiG-19s. The American tests with the F-84s started with using the Martin MGM-1 Matador solid-fuel boost motor of some 240 kilonewton (52,000 lbf) thrust output, which burned out seconds after ignition and dropped away from the manned fighter a second or two later. Tests of the larger F-100 Super Sabre and SM-30 (MiG-19) (with the SM-30 using the Soviet-design PRD-22R booster unit) used similar short-burn solid fueled boost motors, albeit of a much more powerful 600 kN (135,000 lbf) thrust-class output levels.
Testing proved that the F-100 was capable of a ZELL launch even while carrying both an external fuel tank and a single nuclear weapon mounted on its hard points. The conceived mission profile would have been for the pilot to have launched a retaliatory nuclear strike against the attacker before attempting to return to any available friendly airbase, or having to eject from the aircraft if a safe landing site could not be reached. Despite the extremely high thrust generated by the rocket motor, the F-100 reportedly subjected its pilot to a maximum of 4g of acceleration forces during the takeoff phase of flight, reaching a speed of roughly 300 mph prior to the rocket motor's depletion. Once all fuel had been exhausted, the rocket motor was intended to slip backwards from its attachment points and drop away from the aircraft. However, testing revealed that this would sometimes fail to detach or cause minor damage to the aircraft's underside when doing so. Despite such difficulties being encountered, the F-100's ZELL system was considered to be feasible, but the idea of its deployment had become less attractive as time went on.
Eventually, all projects involving ZELL aircraft were abandoned, largely due to logistical concerns, as well as the increasing efficiency of guided missiles having rendered the adoption of such aircraft to be less critical in the eyes of strategic planners. Furthermore, the desire to field combat aircraft that lacked any dependence upon relatively vulnerable landing strips had motivated the development of several aircraft capable of either vertical takeoff/landing (VTOL) or short takeoff/landing (STOL) flight profiles; such fighters included production aircraft such as British Hawker Siddeley Harrier and the Soviet Yak-38, as well as experimental prototypes such as the American McDonnell Douglas F-15 STOL/MTD.
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