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Credible Sport: The Super-STOL Hercules

Iran’s 1979 Islamic revolution should never have caught the U.S. by surprise. But because Iran had been a key client state, the American government turned a blind eye to not only the Shah’s brutalities, but also to the growing popular discontent against him. Then, suddenly, the revolution happened, the Shah fled, and not long after that, fifty two American diplomats were taken prisoner by Iranian militants.

On April 24, 1980, a rescue of the hostages was attempted. The operation, codenamed “Eagle Claw,” called for infiltrating Delta Force teams into Tehran to assault the U.S. embassy and the foreign ministry compounds where the Americans were being held. Once freed, they would be taken across the street to Amjadien soccer stadium, where they would be airlifted out by Navy RH-53D Sea Stallion helicopters. It was a highly complicated plan that involved C-130s loaded with commandos, equipment and fuel meeting up with the eight Sea Stallions at “Desert One,” a remote staging area in the Iranian desert, before refueling and being ferried by helo to their pre-assault “hides.”

Credible Sport Test Flight

YMC-130H Credible Sport 1, firing its take-off rockets in a test of its STOL capabilities at Wagner Field, Fla. DoD photo

But things started going wrong soon after the operation got under way. Two helicopters dropped out en route due to mechanical problems. The remaining six then flew into a dust storm, causing their engines to overheat. By the time they reached Desert One, only five were still mission-capable, one less than the absolute minimum needed to carry out the mission.

The decision was made to abort. Then, as the helicopters prepared to depart, one collided with a C-130 loaded with fuel bladders. There was a huge fireball, destroying both aircraft and killing eight crewmen, whose bodies could not be recovered. Without encountering the enemy, the daring rescue attempt had turned into a humiliating debacle.

Carter immediately ordered another rescue attempt. This time, the Pentagon decided they’d use a “super-STOL” version of the C-130, able to land inside a stadium and then take off, nearly straight up, fully loaded with hostages, rescuers and crew. The aircraft would then need to land on an aircraft carrier so that casualties could receive immediate medical treatment.

The problem was, of course, that a “super-STOL” C-130 didn’t yet exist. But C-130s had been making near-vertical takeoffs for years using JATO (Jet Assisted Take Off – actually rockets) and were renowned for short runway landings. As for landing on carriers, that had been successfully done two decades earlier. Of course a soccer stadium with 30-foot-high walls is not the same thing as a carrier deck, but at least, conceptually, there was something to work with.

The secret program was given the name  “Credible Sport” and headquartered at Eglin Air Force Base’s Auxiliary Field #1. It would be run by the Air Force and Lockheed Georgia. Four C-130s were delivered. Two would be modified, the third aircraft would act as a technology demonstrator, and the fourth used as an interior mock-up for simulation training.

Quickly they realized that the JATO idea wouldn’t work. The fifty-eight JATO bottles needed for both takeoffs and landings was too much additional weight. They decided to use rocket motors from missiles instead.

Operation Credible Sport

The prototype YMC-130H Credible Sport 1, firing its upper forward-facing retrorockets in a test of its STOL capabilities at Wagner Field, Fla. in 1980. DoD photo

The first task was reinforcing the airframe’s structure to withstand the intense stresses that the firing rockets would produce. After that they began mounting the rocket motors to different points on the fuselage. Eight forward-pointing Navy anti-submarine rocket (ASROC) motors were mounted on the forward fuselage for decelerating the aircraft. Eight downward-pointing Shrike (anti-radiation missile) motors were mounted underneath to brake descent. Eight rear-mounted MK 56 motors from RIM-66 Standard Missiles were mounted at the rear of the aircraft for takeoffs. Four ASROC motors were mounted on the wing pylons to control yaw, and two more ASROC motors were mounted on the underside of the tail to prevent it from striking the ground during takeoff.

To improve the aircraft’s handling at low speed, a larger dorsal fin and two ventral fins were added to the rear fuselage. The flaps and ailerons were also increased in size to aid low-speed handling. An arresting hook was installed to enable the aircraft to land on a carrier. A new radome was added in the nose as well as the same avionics package used aboard Combat Talon aircraft, along with a chin-mounted FLIR and special terrain following radar.

Three weeks after the program’s start, the test-bed aircraft made its first flight. A month later, the first fully modified aircraft was delivered for testing. Numerous flights were made, and each time the aircraft performed perfectly. The double-slotted flaps allowed the aircraft to make its landing approach at 85 knots and with a steep glide slope descent.

By October 29, everything was ready to go with the rescue. But first the team needed to do full-up tests of the rockets. The takeoffs went very well, but as they prepared to start the landing phase, they decided that the computers running the firing sequence were not properly calibrated, so they would perform it manually the first time. At twenty feet from the ground, they would fire the forward-facing rockets on top of the aircraft, followed by the forward-facing rockets located lower on the fuselage once the aircraft had actually touched down. But the flight engineer in charge was blinded by the rockets firing topside and misjudged the timing for the lower rockets, firing them off too early, while the aircraft was still airborne. No longer aerodynamic, the aircraft crashed to the ground, breaking off the starboard wing while the rockets were all still firing. The aircraft burst into flames, but luckily no one was hurt.

A few days later the presidential election was held, and Ronald Reagan won by a landslide. Immediately the Iranians signaled that they would be freeing the hostages. Credible Sport was cancelled and promptly forgotten.

By

Brendan McNally is a journalist and writer specializing in defense and aerospace. Brendan began his career...