The zero-length launch system or zero-length take-off system (ZLL, ZLTO, ZEL, ZELL)
It was a system whereby jet fighters and attack aircraft were intended to be placed on short-burn duration, often solid-fuel, “drop-away” rocket booster units, deployed with mobile launch platforms.
The U.S. Air Force, the Luftwaffe, and the Soviets’ VVS all conducted ZELL experiments. The first manned aircraft to be ZELL-launched was an F-84G in 1955. German Air Force conducted ZELL experiments on F-104 Starfighter aircraft. The Soviets’ conducted ZELL experiment on MiG-19s
In the above video, you can see a German Air Force (Luftwaffe) F-104 Starfighter aircraft being tested as part of Short Air Field for Tactical Support (SATS) and zero-length launch system or zero-length take-off system programs.
ZELL was a system whereby jet fighters and attack aircraft were intended to be placed on large jet-assisted takeoff (JATO) rockets attached to mobile launch platforms.
Furthermore, Most zero-length launch experiments took place in the 1950s, during the Cold War.
The SATS was a test program for take-off and landing on short runways and featured a runway with an aluminum surface of interlocking lightweight metal alloy planking, a catapult and a carrier deck-type arresting gear.
In addition, the tests were carried out at the Naval Air Test Facility in Lakehurst
U.S. where a few hundred take-offs and landings were successfully completed.
In the above video, you can see F 84 Thunderjet zero-launch variant
Advantage of ZELL
The primary advantage of ZELL was the elimination of the need for a vulnerable airfield for takeoffs.
Furthermore In the event of a sudden attack, air forces could field effective air defenses and launch airstrikes even with their own airbases destroyed.
Although launching aircraft using rocket boosters proved to be relatively trouble-free
if aircraft were required to land at the same base, a runway was still required.
Bulky mobile launching platforms also proved to be expensive to operate and difficult to transport. Security would also have been an issue with mobile launchers, especially if equipped with nuclear-armed strike fighters.
The American tests with the F-84s started with using the Martin MGM-1 Matador
Solid-fuel boost motor of some 240 kilonewtons thrust output which burned out seconds after ignition and dropped away from the manned fighter a second or two later
The larger F-100 Super Sabre and MiG-19/SM-30 “Farmer” tests (with the SM-30 using the Soviet-design PRD-22R booster unit)
used similar short-burn solid fueled boost motors, of much more powerful 600 kN thrust-class output levels.
Furthermore, All works upon ZELL aircraft were abandoned due to logistical concerns and the increasing efficiency of guided missiles.
The desire to field combat aircraft without depending on vulnerable landing strips also motivated the development of aircraft capable of vertical (VTOL) or short (STOL) takeoffs and landings.
In addition, Examples of these include British Hawker Siddeley Harrier, Soviet Yakovlev Yak-38.