By Paul Rincon

Science reporter, BBC News

Please turn on JavaScript. Media requires JavaScript to play. Advertisement Sitting in a broad armchair in the front room of his home near Folkestone, UK, Reg Turnill recalls the moment he realised Apollo 13 was in trouble. The former BBC aerospace correspondent had been covering the flight from the Johnson Space Center in Houston - home to Nasa's mission control. On the evening of 13 April 1970, he had been at dinner with his wife Margaret, but later returned to JSC to "check all was well" and record a one-minute report for the BBC's morning bulletins. That visit would lead to one of the biggest scoops of his long and distinguished career. "I was almost out of the door when I heard 'Houston we've had a problem'," he tells me. "What I did was go back to my desk, a few yards away, and there I stayed for the next three days." Apollo 13's commander Jim Lovell, along with his colleagues Fred Haise and Jack Swigert, launched on what was to have been Nasa's third mission to the lunar surface. Lovell and Haise were to become the fifth and sixth people to walk on the Moon, hiking from their landing site to explore the rugged Fra Mauro hills. Mission control instructed the crew how to deal with the rising CO2 levels

None of this happened, of course, because, nearly 56 hours into the mission, an explosion aboard the spacecraft plunged the crew into a fight for their survival. During lift-off, a potentially dangerous problem caused one of the Saturn V rocket's second stage engines to shut down two minutes earlier than expected. This forced other engines to burn longer in order to get the ship to orbit. Nearly two days into the flight, things were going so smoothly that Joe Kerwin, on duty at mission control, told the crew: "The spacecraft is in real good shape as far as we are concerned. We're bored to tears down here". That sentiment would not last long. We had this cascade of systems failures throughout the spacecraft It was all at one time - a monstrous failure

Sy Liebergot

Apollo EECOM Several hours later, the crew was instructed to carry out a routine stir of the spacecraft's cryogenic tanks. They heard a sharp bang and a shudder. The exchange that followed between the Apollo 13 crew and Charles Duke in Houston contained five words which would enter our lexicon. Swigert: "Okay, Houston, we've had a problem here." Duke: "This is Houston. Say again please." Lovell: "Houston, we've had a problem. We've had a main B bus undervolt." Duke: "Roger. Main B undervolt." Reg Turnill explains: "There was an edge to those words, as spoken by Jim Lovell, and I knew at once, this was the big one." "An hour after it happened, I broke the story to the world on BBC World Service at about 5am London time."

On board, warning lights flashed, informing the crew that two out of three fuel cells had been lost. When Jim Lovell checked the oxygen pressure and quantity gauges, he discovered one tank had an "empty" reading, while the other was plunging inexorably towards zero. Unsure whether these were just instrument failures, the crew and the engineers battled to comprehend what had happened. Thirteen minutes into the crisis, Jim Lovell looked out of the hatch and saw gas venting into space. One of two oxygen tanks housed in the centre of the service module had exploded, damaging the plumbing to the other tank. The blast crippled the craft's main power supply and left oxygen and water reserves critically low. Please turn on JavaScript. Media requires JavaScript to play. "Within less than a minute we had this cascade of systems failures throughout the spacecraft It was all at one time - a monstrous failure," says Sy Liebergot, Nasa's lead "EECOM" flight controller for the Apollo missions, which meant that he looked after the life support systems in the spacecraft's command and service modules. Mr Liebergot was an integral member of the "tiger team" that worked around the clock at mission control to come up with the solutions required to bring the astronauts back alive. "We normally liked to start with the problem and work backwards. But, as one of my backroom guys said despairingly: 'I don't know where to start'. My feelings exactly," he says. Reg Turnill says: "Nasa was very quick to react. Engineers and astronauts - some of the most famous ones such as Neil Armstrong and Buzz Aldrin - all flooded in and got in their simulators to start working out how on Earth you could get these astronauts home." Liebergot instructed the crew to switch off the power to as many systems as they could in order to reduce the electrical load on the remaining fuel cell. This in turn would reduce its consumption of oxygen and hydrogen. The teams at mission control in Houston were elated

A plan was agreed to use the lunar lander as a lifeboat: "It was a team effort to figure out how to get [the astronauts] back with the tools we had left. Fortunately we had the lunar module with its full set of consumables: battery power, water, oxygen. So that was a matter of determining how long we could stretch those," says Mr Liebergot. In an interview with the BBC, Jim Lovell said the astronauts never talked about the possibility that they would not get home: "We tried to figure out what happened. If I had just waited for some miracle, I'd still be up there." The spacecraft looped around the Moon, using its gravity to return to Earth. This allowed crew members to take amazing pictures of the far side, but, of course, there was to be no landing. Even as the crew prepared to make the fiery re-entry into Earth's atmosphere, there were some last minute fears the explosion might have damaged the protective heat shield on the command capsule, potentially causing the spacecraft to burn up. Fears about the heatshield proved to be unfounded "There was this period when we didn't know whether the astronauts would live or die as they re-entered," says Reg Turnill, "That was one of the tensest moments." Millions of people followed the drama on television, hoping for the best. But the wait would have been most agonising for the astronauts' families - who remained extraordinarily dignified under an intense media spotlight. Slung under parachutes, the cone-shaped command module appeared through the clouds, and exhausted workers at mission control were finally able to breathe their sighs of relief, raise their hands and cheer. The capsule splashed down in the Pacific Ocean near Tonga, where the USS Iwo Jima was waiting to recover the three crew members. After the crisis was over, it became clear the accident could have been much worse. Post mission analysis showed that when oxygen tank 2 exploded, it generated a pressure of around 60,000 psia (pounds per square inch absolute). Sy Liebergot says this was roughly equivalent to detonating 7lbs of TNT and enough to level a 3,000 sq ft (279 sq m) house. Home at last: the astronauts were brought aboard the USS Iwo Jima

"If that panel had not blown off when it did, there is speculation that the pressure could have built up between the heat shield of the command module and the top of the service module," Mr Liebergot told me. This, he explains, could have blown apart the three tension ties that held the service module to the command module. It would have left the two vehicles connected only by the umbilical which supplied electrical power and oxygen to the command module. "Now you would have the command module flopping, hinged by the big umbilical between the two vehicles. Now you're dead," he says. "You're just hanging there and you can't orient the spacecraft you are unable to use the lunar module." In addition, Sy explains: "The explosion probably happened at the best time it could. If it had happened a lot earlier, before the crew had docked with the lunar module, that would have been the end. And if it had been after the crew had used the lunar module supplies, that would have been the end." Although the mission was not a success from a conventional perspective, it was a triumph of ingenuity and determination by the engineers and astronauts. Jim Lovell explains: "I think one of the things that showed the people of the world was that even if there is a great catastrophe, good leadership and teamwork, initiative and perseverance - these things make for getting an almost certain catastrophe into a successful recovery." Paul.Rincon-INTERNET@bbc.co.uk



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