ZERO-LENGTH LAUNCH

The use of Rocket Assisted Take-Off (RATO) boosters became very common shortly after the Second World War because the early jet engines delivered relatively low thrust when the aircraft was moving slowly. Heavy jet aircraft in particular needed a bit of help to get going. But soon the take-off thrust of jet engines had increased to the point that RATO was only required for heavy cargo aircraft using short runways or airfields in hot places and at high elevations (they are still in use today, but only in very limited circumstances).

A different idea for mixing rocket and jet propulsion that was investigated was to use a powerful rocket booster to shoot an airplane straight into the sky, dispensing with the need for a runway. With this ultimate stretch of the RATO concept, fighter aircraft could be launched anywhere and anytime, even from a truck trailer!

For Cold War military planners the vulnerability of airfields and their concentration of aircraft was always a major issue, especially for the early, underpowered jets that could not take off from rough fields and required especially long runways. This concern led to the need for Vertical Take-Off and Landing (VTOL) aircraft: jet fighters that (as with helicopters) would not need much more than a clearing in a forest to operate from. Most of the VTOL fighter concepts proved to be impractical but the developments ultimately led to aircraft like the swivel-nozzle Harrier, which can take off and land virtually anywhere whilst operating as a conventional jet fighter when in the air.

During the 1950s, blasting jet fighters into the air using rockets was a simpler way of liberating aircraft from their dependence on runways for take-off (although the returning aircraft would normally still need a prepared airfield for landing). The idea had already been pioneered by the British ship-launched, rocket-catapulted Hurricat fighter of the Second World War, but the new jet fighters were much heavier than the old Hurricane propeller fighter and required much larger boosters to get airborne: so large, in fact, that they were impossible to fit into the airframe and (like RATO units) had to be attached externally and jettisoned immediately after use. This had several benefits though. One was that the heavy rocket equipment did not need to be taken with the aircraft during its entire mission. Another was that not much modification to existing aircraft would be needed to accommodate the external rocket boosters. Also of benefit was that because the rockets would be jettisoned soon after take-off, they would not need to be especially light and efficient: relatively simple solid propellant boosters similar to those used in surface-to-air missiles would suffice. This concept became known as Zero-Length Launch (ZEL).

In the early 1950s the US Air Force began a program called ‘Zero Length Launch, Mat Landing’ (ZELMAL) in which a Republic F-84G Thunderjet fighter was to be shot into the air using a large solid propellant rocket booster. (The F-84 was selected because it was sufficiently light that it could be launched by already available rocket boosters.) To solve the problem of the need for a landing strip upon return, the idea was for the fighter to be equipped with a hook to snag an arresting cable suspended close to the ground in order to come to a quick stop, rather like on an aircraft carrier, except that instead of rolling to a halt the aircraft (without lowering its undercarriage) would smack down onto an inflatable mattress measuring 25 x 245 meters (80 x 800 feet) and 1 meter (3 feet) thick. An additional perceived benefit of this technique was that ZELMAL aircraft would not need an undercarriage and so would be lighter than comparable conventional fighter aircraft.

The Glenn L. Martin Company was selected to manage the development of the system, with the Goodyear Tire & Rubber Company making the air-filled mat. Tests started at Edwards Air Force Base, California, on 15 December 1953 with a pilotless F-84G being launched from a trailer normally used to fire Matador cruise missiles (it seems that the same type of rocket booster was employed). As planned, the aircraft was lobbed into the air and then crashed onto the hard desert floor. The next test less than a month later, on 5 January 1954, was equally promising. It was manned by test pilot Robert Turner and the G-levels that he experienced during the launch were no worse than during a conventional catapult launch from an aircraft carrier although he accidentally jerked one hand and throttled the engine back, almost stalling it. Turner made another flight on 28 January. Both flights went surprisingly well, and surviving movie footage (which is on the Internet) shows a very smooth operation with a fluent acceleration and a clean separation of the booster. This indicated that rocket-boosted take-offs were a feasible operational military possibility. In both tests Turner landed conventionally. Landing on the mat would prove much more problematic. The first time the rubber mat was inflated after being transported on a couple of trailer trucks, it was found to leak so badly that parts of it had to be sent back to the manufacturer. The first mat landing on 2 June 1954 became a fiasco when the aircraft’s arresting hook tore the mat wide open and caused a very hard landing. The plane was damaged beyond repair and Turner was rendered inactive for months due to back injuries. Two more mat landings were conducted but the sudden impact on the mat remained too hazardous. Test pilot George Rodney suffered a neck injury from his mat landing: “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.” After 28 rocket launches ZELMAL was terminated.

The sudden lift-off of a ZEL launch must have been rather strange for the pilots: they were sitting in the cockpit of a familiar aircraft but rather than seeing a runway in front of them they were looking up into the sky at a steep angle. And instead of the reassuring, slowly growing push of the jet engine there was a sudden explosion of power hurtling them into the air. It is a bit like sitting in your own car but with an additional dragster racing car’s engine in the boot.

Despite the termination of ZELMAL the idea of launching a fighter with a rocket booster was still believed valid. The Air Force initiated a new program in 1957 that dispensed with the mat landing and so was simply named ZEL. It was to involve the launch of a nuclear-armed strike aircraft from a truck trailer which, since it could be hidden anywhere, would be hard for the enemy to destroy during a first strike. After dropping his bomb, the pilot of the undercarriage-less aircraft would simply bail out over friendly territory. To be able to carry a heavy atomic bomb into Soviet territory the aircraft would have to be much larger than the F-84. The selected F-100 Super Sabre was about twice as heavy as the F-84, so a new rocket booster was developed by Rocketdyne. This solid propellant rocket could deliver a thrust of almost 578,000 Newton for 4 seconds and accelerate the F-100 at about four G. It was affixed under the aircraft’s rear fuselage, at a slight angle so that its thrust was aimed through the center of gravity and would thus not cause any rotation. At burnout, the plane would be flying at 450 km per hour (280 miles per hour) at an altitude of 120 meters (400 feet). Preliminary tests were started with a so-called ‘iron bird’, a structure of steel and concrete that simulated the weight and the mass distribution of the F-100. These tests showed that if the booster were not precisely aligned with respect to the center of gravity of the entire contraption it could perform some very impressive backward summersaults. But this was soon fixed, and the earlier problem of the pilots’ hand on the throttle moving backwards due to the acceleration was remedied by introducing a fold-out handle which the pilot could slip his hand into.

The first manned launch of an F-100 at Edwards Air Force Base on 26 March 1958 went perfectly. Test pilot A1 Blackburn said he found the flight “better than any ride you can find at Disneyland”. On his second launch, however, the rocket did not separate, even when he tried to shake it free using wild maneuvers. He had to use his ejection seat and let the plane crash in the desert because it was impossible to land it with the big booster attached. The investigation showed the attachment bolts had not sheared off as they were intended to. Thereafter explosive charges were provided that could blow the boosters off on command. Another 14 successful flights were made by October. A sign on the trailer claimed it to be the ‘World’s Shortest Runway’. The tests were not kept secret: footage of one of these launches was used in the Steve Canyon television series. There was even a public demonstration of the system, with the pilot showing off by performing a slow roll immediately after booster separation. The technical feasibility of the ZEL concept was proven, but its operational role was not so clear.

First of all there were the practical as well as safety and security issues relating to driving a fighter with a nuclear weapon on a truck through dense forests, over narrow roads and through tunnels. Critics said it would be better to launch ZEL aircraft from fixed, protected positions. To test this idea, a few launches were performed out of a hardened shelter at Holloman Air Force Base in New Mexico, the last of which took place on 26 August 1959. But launching from fixed positions denied the flexibility and elusiveness of the mobile system. A more serious threat to the project was that the idea of sending nuclear-armed fighter aircraft into the Soviet

An F-100 ZEL with its impressive rocket booster [US Air Force].

Union was rapidly being made obsolete by the increasing reliability and accuracy of unmanned ballistic missiles. Although 148 F-lOOs were modified to enable ZEL launches, the program went nowhere.

The concept of a ZEL nuclear-armed strike fighter was picked up once more, this time by the German Luftwaffe in 1963. Working with Lockheed, they organized rocket-launch experiments using an F-104G Starfighter at Edwards Air Force Base. The Rocketdyne booster could push the Starfighter to a speed of 500 km per hour (310 miles per hour) in just 8 seconds. Lockheed test pilot Ed Brown, who performed the flights, was very impressed: “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.” The successful experiments were followed up by further tests at the German Air Force base at Lechfield. But this project was also canceled for the same reasons as the F-100 ZEL and because the tests using the expendable rocket boosters were quite expensive. In addition, the much more practical YTOL Hawker Siddeley GR. 1 Harrier was by then under development. This made its first flight in December 1967. It could not only take off vertically without rocket booster assistance, it could also land vertically. A German F-104G equipped with a rocket booster and a dummy nuclear missile is on display at the Luftwaffe museum in Berlin-Gatow in Germany.

In France there was a proposal for a ZEL version of the mixed-power Durandal in which the aircraft would be launched from a mobile trailer using a cluster of sohd propellant rockets attached at an angle on the tail, but this was not developed into a real system.

Soon after the US started to experiment with rocket launched F-84Gs, the Soviets initiated a very similar project using their MiG 19 (which, as described earlier, was also converted into a high-altitude mixed-power interceptor around this same time). Rather than launching nuclear-armed strike aircraft, the Soviets intended their rocket-launched interceptor to play a role similar to that they had envisaged for their earher rocket propelled aircraft, namely a fast-reaction point-defense interceptor. Launching these from truck trailers with large rocket boosters would make it possible to station them at remote locations or in battle areas where there were no suitable runways. The MiG design bureau prepared a proposal for a trailer-launch system for the MiG-19. This was given the go-ahead in 1955. MiG came up with a modified version of the MiG-19S, designated the SM-30, which had a reinforced structure to handle the high rocket thrust, the ventral fin was replaced by two new fins straddling the rocket booster, and a special headrest to protect the pilot from whiplash at the onset of the sudden acceleration.

Like the US Air Force and the Luftwaffe, the Soviets used a large, jettisonable

A standard MiG 19 at the Letecke museum near Prague, Czech Republic [Michel van Pelt],

solid propellant rocket booster which was mounted on the rear fuselage and pointed slightly downwards. The PRD-22 booster had sufficient thrust to shoot the 8,000 kg (17,000 pound) plane into the sky. The SM-30 could be transported on a large trailer-truck combination but had to be placed on another type of trailer for the launch. The plane was connected to this trailer by bolts that would shear and release the aircraft upon ignition of the booster. When launched from soft surfaces, such as from within a forest, soldiers would be required to dig a trench behind the booster to prevent the powerful exhaust from creating a fountain of loose dirt that would be visible to the enemy from far away. The first test launch was performed using a remote-controlled unmanned airplane in the autumn of 1956. The launch went smoothly (just like the Americans had experienced) but the trailer was wrecked, thus proving that it needed to be fitted with a blast shield for protection. On 13 April 1957 test pilot Georgi M. Shiyanov made the first manned flight, which was a big success. He had trained with a special launch simulator catapult (even successfully enduring an excessive 18 G acceleration during one test when a technician made an error arming the catapult). Several more SM-30 launches were performed, all of which were successful (film of the launches is on the Internet). However, landing the heavy fighter on rough and small landing strips like those envisioned to be available near forward battle areas was not so easy; getting the plane to stop before it ran out of runway was very tricky using the standard MiG-19 drag parachute and brakes. An arresting-cable system (Uke on an aircraft carrier) was therefore tried out.

In the end, however, the SM-30 project was terminated because of the difficulty of driving the large, heavy aircraft across the countryside and because an airplane which does not require a runway for take-off but does require one for landing is not all that useful. Just as the F-100 and F-104G ZEL were rendered obsolete by the introduction of long-range nuclear missiles, the SM-30 turned out to be less effective for forward air defense than the new mobile battlefield surface-to-air missiles.