During the second test flight of Saturn Five last April several of these tubes, which carry fuel to the starters for the second and third stage engines, fractured so that these engines would not start. There had been no comparable failures during tests on the ground and it took some brilliant engineering detective work to track down the trouble.
It emerged that the tubes had been broken by vibration during the flight, a contingency which the ground tests proved to have rather freakishly excluded. During these tests, before the air was pumped out of the test chamber (to simulate the vacuum of space) some of the moisture it contained froze on to the tubes. The amount of ice was minute, but it had been enough to affect the way the tubes responded to vibration and thus to prevent them from breaking.
An equally tiny defect this Christmas could mean total disaster.
Of course, spaceflight is risky. In the words of General Samuel C. Phillips, Director of the Apollo programme, “You won’t be able for a number of years to climb aboard a large rocket without having some risk involved.” But in several vital respects the coming flight will involve hazards of an entirely new order, totally different in scale from all previous manned flight.
The first hazard is the astronauts’ absolute dependence – without any possibility of support or rescue – on the spacecraft engine which they will fire as they approach the Moon to put themselves into orbit around it, and again, 10 orbits later, to put themselves on course back to Earth.
Another entirely novel danger is the time it will take to get the crew back to Earth in an emergency. Orbiting Earth, astronauts are an hour or two from safety, orbiting the Moon, they are between two and three days away.
A small defect in the cabin oxygen supply, or trouble with the temperature control which would have led to no more than an early landing from all previous flights could be fatal.
Finally, re-entry into the Earth’s atmosphere has to be achieved at a speed half as fast again as on return from Earth orbit. Apollo Eight will have to hit a corridor about 30 miles deep at 25,000 mph if it is to land safely. If it comes in too steeply it will burn up; if it comes in not steeply enough it will bounce out of the atmosphere; and since the crew will by then have jettisoned all their oxygen reserves they will die long before they can recover.
Hair-raising as this maneuver sounds, NASA officials do not consider it particularly dangerous. The long journey back from the Moon will leave ample time for computing the spacecraft’s course and making any necessary adjustments. Nevertheless the margin of error is effectively zero.
Are all these wholly new risks justified at this stage? According to General Phillips the coming flight is a “normal progression” from the previous Apollo flight. But the fact that an entirely different mission – testing the lunar landing module in Earth soil – was originally scheduled, together with the recent Russian success with unmanned flights round the Moon, makes it look very like a desperate attempt to prevent the Russians chalking up yet another first.
In fact, the rearrangement of the programme makes a great deal of sense. The lunar module that was to have been tested is not ready and to have waited for it would have set everything back by several months. The risks of a round-the-Moon flight had to be accepted at some stage and another Earth orbital flight would have made very little difference to them.
There has never been any question of flight testing the Apollo spacecraft and Saturn Five in the comprehensive way a civil airliner is tested. That would involve probably hundreds of firings which, at hundreds of millions of dollars a time, is out of the question. Instead, NASA is relying on designs with big margins of safety, very thorough ground testing, and missions planned so that they can be terminated prematurely if necessary.
The vital spacecraft engine for example, has only three parts which are not in duplicate; the fuel injector, the combustion chamber and the nozzle. If on part fails there is another to take over. Some very important valves are even quadrupled. The craft has also been put through one of the most elaborate test programmes ever devised by man. This included firing 157 explosive charges inside the combustion chamber while the engine was operating to see whether the combustion process could be unstable.
The mission itself is planned with a series of “commit points” where there is time to decide whether or not to go on with the next stage of the flights. But once it is committed to a lunar orbit, the risks really begin to escalate.
However careful the planning, thorough the testing and ingenious the design, mishaps like the broken tubes in the Saturn Five are bound to happen. In Saturn Fine there are more than 5½ million parts. Even if each one is so reliable that it fails only once in a million flights there are so many of them that several failures are certain every time the rocket is used.
There are some accidents which no amount of testing or planning can avoid and the more complicated the equipment the more likely they are. The odds on something going wrong can be called, and most certainly have been called in Houston. My own guess and it is no more than a guess is that there is one chance in three of four that the mission will not go according to plan, and perhaps one in 10 that the astronauts will not get back to Earth alive.