v1.0.4 / 01 oct 04 / greg goebel / public domain
* Many strange and interesting ideas have been pursued through the history of aircraft development. None were much more strange than the "zero-length launch (ZEL)" concept, in which a jet fighter was blasted off the back of a truck with a solid-fuel rocket. This document describes the origins, development, and ultimate fate of the ZEL concept.
* During the Cold War, the vulnerability of airfields to attack was obvious to military planners, and a number of schemes were dreamt up to develop ways to fly aircraft without using airfields. Early experiments with vertical take-off and landing (VTOL) aircraft, the "flying pogos", focused on aircraft that could take off straight up and land on their tails. Landing was a frightening procedure and the concept did not inspire enthusiasm.
Another approach was considered: put a rocket on a conventional aircraft and simply blast it into the sky, without need for a runway. "Jet-assisted take-off (JATO)" rockets had been used to help get heavily-loaded aircraft off the runway since the end of World War II, and developing a bigger rocket to do the whole job didn't seem like too big a stretch. Besides, by 1953, contemporary cruise missiles were being launched with precisely such a technology. The idea materialized in the form of a program named "Zero Length Launch / Mat Landing (ZELMAL)". Not only would a Republic F-84G Thunderjet fighter be blasted into the sky by a big booster rocket, the fighter would also land, gear up, on a huge inflatable mat, 25 x 245 x 1 meters (80 x 800 x 3 feet) in size, and snag an arresting cable to stop.
Tests were begun in December 1953 at Edwards Air Force Base in California. The launches were performed off a trailer normally used to launch a Matador cruise missile, and it is plausible that the Matador RATO booster was used as well. Take-offs were surprisingly easy, but the mat landings were another matter. The inflatable mat, which was transported on a couple of trailer trucks, leaked the first time it was set up, and portions had to be sent back to Goodyear, the manufacturer, for repairs.
The first mat landing didn't take place until 2 June 1954. It was a fiasco. The F-84G's arresting hook tore up the mat, and the aircraft was so badly damaged that it had to be written off. The test pilot, Robert Turner, was laid up with back injuries for months. Two more mat landings were performed, and though the results were nowhere near as bad as the first landing, the idea was still clearly unworkable.
George Rodney, one of the test pilots, described the mat landings: "We tied ourselves into the seat real well, so we wouldn't pitch forward into the control column and the instrument panel, but unfortunately your head, it goes into a big arc and comes down on your chest." Rodney suffered a neck injury that bothers him to this day. ZELMAL was terminated after 28 launches. Rodney commented: "It needed some work, that was for damn sure." The Air Force abandoned the concept for a time.
* Although the mat landings were a bad idea, the rocket take-offs had actually worked pretty well, and in 1957 the Air Force decided to revive that part of the concept. The idea was to launch a nuclear-armed strike aircraft from a truck trailer, bomb a target, then have the pilot bail out over friendly territory. The acronym was shortened to "ZEL".
The F-100 Super Sabre was selected for tests, which posed some problems. The F-100 weighed over twice as much as the F-84, and required a really big booster rocket to get it into the air. The booster was built by Rocketdyne and generated almost 578 kN (59,000 kgp / 130,000 lbf) of thrust for four seconds, providing a maximum acceleration of 4 gees. The aircraft would be instantly airborne and would be flying at an altitude of 120 meters (400 feet) and almost 450 KPH (275 MPH) at rocket burnout.
Engineers conducted preliminary launch tests with an "iron bird", a structure of steel and concrete that simulated an F-100. The iron bird performed some really impressive maneuvers, such as backward somersaults, that demonstrated the importance of precisely aligning the booster.
The first actual shot with an F-100 went fine. The test pilot, a remarkably laid-back guy named Al Blackburn, found it "exhilirating" and "better than any ride you can find at Disneyland." The second launch didn't go so well. The booster didn't separate, and nothing Blackburn could do would shake it off. He had to eject and let the aircraft crash into the desert. A post-mortem showed the booster had hung up on the attachment bolts, which were supposed to shear off, but didn't. The attachment scheme was modified with explosive charges that could blow the bolts off on command.
Fourteen more flights were performed between March and October 1958. All these shots went well, and became perfectly routine. One pilot performed a ZEL launch for a public demonstration and did a slow roll immediately after booster separation. There was no doubt of the technical feasibility of ZEL, but by this time there were doubts that sending a nuclear-armed fighter around the countryside on a truck was such a good idea. In consequence, a few launches were performed from inside a hardened shelter at Holloman AFB in New Mexico, with the very last launch on 26 August 1959. There were no problems.
148 F-100s were modified for ZEL. In 1963, the German Luftwaffe sponsored similar launch experiments with a single F-104G strike fighter, with Lockheed test pilot Ed Brown performing the flights. Brown was impressed with the scheme, saying: "All I did was push the rocket booster button and sit back. The plane was on its own for the first few seconds and then I took over. I was surprised at the smoothness, even smoother than a steam catapult launch from an aircraft carrier."
Nothing came of any of it. Nobody wanted to field ZEL. The idea was sexy, it worked well, but it was expensive, and had nasty logistical and security concerns associated with it. In the end, the capability offered by the system was provided by battlefield missiles and the Harrier VTOL "jump-jet" that didn't need a booster rocket. Nobody but test pilots ever blasted a jet fighter into the sky on a rocket booster. It wasn't a practical idea, but it had certainly been a ride that Disney could never have built in a thousand years.
* As was typical of the Cold War, what the Americans would do, the Soviets would often do in response, and the reverse. It is therefore not surprising that the Soviets performed ZEL experiments of their own, using their counterpart to the American F-100, the MiG-19.
The Soviet motivation was different, however. Instead of dispersing nuclear-armed strike aircraft to protect them from a first strike, the Soviets wanted to launch air-defense interceptors from remote locations and forward battle areas. The concept was suggested by the Mikoyan-Gurevich (MiG) design bureau, and development of a trailer-launched MiG-19 system was approved in 1955.
The result was a specially-built version of the MiG-19, designated the "SM-30". The SM-30 was reinforced to allow it to tolerate the stresses of rocket-boosted takeoff, incorporated a special headrest to protect the pilot from whiplash, and replaced the single ventral fin of the MiG-19 by a pair of fins straddling a PRD-22 solid-fuel booster. Like the F-100's booster rocket, the PRD-22 generated about 578 kN (59,000 kgp / 130,000 lbf) of thrust.
The SM-30 was transported to a remote site on a large trailer, and then placed on a separate trailer for launch. A trench had to be dug behind the trailer to contain the exhaust, which would otherwise kick up a cloud of debris that would be visible from far away. The aircraft was secured to the launch rail by bolts that would, in principle, shear when afterburner was engaged.
The first test launch, using a remote-controlled SM-30, was conducted in the fall of 1956. The launch went smoothly, but the launch trailer was wrecked, and so a blast shield was devised. The first piloted launch was in April 1957, with test pilot Georgi M. Shiyanov at the controls. Shiyanov had trained for the launch on a special launch simulator catapult. He had endured 18 gees on one test when the technician arming the catapult made an error. Shiyanov suffered no real harm from the incident; sources make no mention of any consequential injury to the technician.
As with the American ZEL efforts, launches proved surprisingly easy. Landings were a bit trickier. The standard drag chute for the MiG-19 wasn't adequate for the rough forward-based landing strips envisioned for the ZEL fighter, and so an arresting-cable scheme was implemented and partially tested.
The tests were terminated when the Soviet government lost interest in the concept, for reasons somewhat similar to those that killed the American ZEL project. The requirement for a landing strip reduced the attractiveness of ZEL, while attempting to transport such a big and bulky system across the countryside was difficult, particularly when confronted by obstacles such as tunnels. The development of effective battlefield surface-to-air missiles proved to be a much more workable solution to the problem of forward air defense, and the Soviet ZEL went nowhere.
* Sources for this document include:
ZEL was one of those nifty ideas in circulation when I was a kid, in the early 1960s, and reading the SMITHSONIAN article brought back memories that inspired me to write this document. Model kits were available not only for the F-100 ZEL concept, but for the F-104 implementation as well. I even recall an episode of the 1960s MAN FROM UNCLE TV spy series in which a gang of terrorists attempted to seize a nuclear-armed ZEL fighter hidden in an underground silo. Interestingly, I recollect the head terrorist was Slim Pickens, immortalized for his role as Major "King" Kong in Stanley Kubrick's movie DR. STRANGELOVE, riding an H-Bomb to glory in one of cinema's most famous images.
Sometimes it's interesting find out what really happened to those crazy ideas dreamed up decades ago that had their moment of fame, and then vanished into the haze.
* Revision history:
v1.0 / 13 oct 96 / gvg
v1.1 / 07 jun 98 / gvg / Added comments on Soviet ZEL.
v1.2 / 30 oct 99 / gvg / Minor cosmetic update.
v1.0.3 / 01 oct 02 / gvg / Minor cosmetic update.
v1.0.4 / 01 oct 04 / gvg / Minor cosmetic update.