It’s a special week this week – fifty years since the beginning of the Apollo 11 mission that took Neil Armstrong, Buzz Aldrin and Michael Collins to the Moon. Although dates around the world differ because of the magic of the international date line and time zones, by NASA’s timing they launched on 16 July, entered lunar orbit on 19 July, landed on the 20th, left lunar orbit on 21 July, and landed safely in the Pacific on the 24th.
The mission was hugely risky. In his evocative autobiography Carrying the Fire (1976), Collins figured he would likely be OK, staying in orbit as Command Module Pilot. But he gave the whole mission only a 50/50 chance because his colleagues Neil Armstrong and Buzz Aldrin had to come back from the lunar surface.
They were trying something never done before, in the most complex machines built to that time by humanity. And despite every effort to think through all possibilities, despite endless hours of training, nobody knew how things would pan out. NASA had, indeed, decided to make Apollo 11 the landing mission only after the success of Apollo 10’s ‘dress rehearsal’. It didn’t have to be; they had enough time in the schedule to run another practice mission if anything major had gone wrong with Apollo 10, without missing the end-year deadline.
The fact remained, of course, that irrespective of the preparation, whichever mission went for first landing was always going to be poking into the unknown.
In the event Apollo 11’s landing met a raft of issues, any one of which could have aborted the mission or killed the astronauts. All were overcome through skill and forethought. During the powered descent phase to the lunar surface, the autopilot overloaded first with a ‘1202’ and then a ‘1201’ warning, codes neither astronaut knew: there were tense moments while someone in Mission Control scrabbled through the manual.
Then, as they made final approach, Armstrong realised they had overshot the target and were going to land in a boulder field. He took manual control and flew the LM out of danger. That, too, was fraught; the LM had little reserve fuel. Duty CAPCOM, Charlie Duke – back in Mission Control – called the ‘sixty second’ warning.
To ‘stage’ the lunar module and fire the ascent motor took between two and four seconds. A 1966 NASA study indicated that in a low-altitude abort there was a high risk of the ascent module being damaged by gas pressure or debris if the descent stage exploded on impact.
Armstrong saved the day: with ice cool skill he flew over the hazard zone, looked for a place – actually said: ‘looks like a good area here’. Then – with lunar dust being kicked up by the engine (visible in the 16mm film taken from the LM window) and as the fuel dropped to the twenty-second warning – he lowered the module to the surface.
Post-mission analysis put the autopilot overshoot down to the fact that the LM was 5 km (3 miles) out of place when powered descent was initiated, due to (as a post-flight report put it), ‘uncoupled RCS attitude maneuvers and cooling system venting not accounted for by the propagation of the predicted navigates state at PDI‘. Um… OK. I know what that’s meant to mean.
As the astronauts shut down the descent stage and prepared for the moon-walk, Mission Control noticed an alarming pressure-rise in the descent engine fuel system, which operated by helium gas pressure. The remaining helium was meant to vent after landing, but it turned out that the fuel lines had frozen and the question was whether a ‘burst disk’ – designed to rupture if the tanks over-pressured – would do so.
That was resolved, but then there was trouble opening the LM cabin hatch, likely due to residual pressure in the cabin after venting. The LM was lightly built: if they wrenched the hatch open, they might damage the delicate structure and prevent the cabin being re-pressurised. Aldrin finally, carefully, managed to open it.
They still weren’t out of the woods. After the moon-walk, the astronauts discovered one of them had bumped a circuit-breaker push-switch with his back-pack and snapped it – something easily done in the cramped cabin. This push-switch was needed to liven the ascent engine ignition circuit. It was a safety precaution, meaning the ignition circuit couldn’t be accidentally triggered prematurely. Now it couldn’t be triggered at all.
NASA had prepared for the possibility of the astronauts being stuck on the Moon. The mission plan included shutting down contact to give them privacy. A speech had been prepared for President Richard Nixon, just in case.
Fortunately, the broken arm/disarm switch wasn’t a killer. The unit was a thin mechanical rod designed to be pushed down and hold the circuit-breaker closed, inside the control panel. Any similar-shaped object would do the trick, and Aldrin had a ballpoint pen. So they left the lunar surface with the pen jammed into the place where the broken circuit breaker had been.
In point of fact, the possibility of being stuck on the Moon was something both NASA and the LM’s builders, Grumman, put a good deal of thought into. There wasn’t weight allocation for a backup ascent engine, but the Bell/Rocketdyne unit chosen was a development of the reliable Bell 8247 engine, with a solid track record in the Agena rocket stage. For lunar work, every major component except the combustion chamber was doubled. The propellants – hydrazine and di-nitrogen textroxide – burned on contact. The only major risk was that power might not be available to trigger the LM’s staging system and, simultaneously, get the motor started.
NASA had various ways of getting that circuit livened, including a last-ditch option involving an unscheduled EVA without back-pack and only a 30-minute emergency oxygen supply, to take a pair of jumper leads from a descent stage battery back up to terminals inside the cabin. These went straight to the staging/ignition circuit. As soon as the clips hit the terminals – wham, the lunar module would fire its staging pyros and the engine would ignite, sending the astronauts skywards, still in vacuum and with the loose end of the jumper leads – still clipped at the other end to the descent stage – whipping out through the open door. Woah! Luckily this never had to be tried.
The breaker switch wasn’t the last critical thing that went wrong on the Apollo 11 mission either. On 24 July, as the trio were re-entering Earth’s atmosphere, Collins looked out the window and saw their own Service Module, jettisoned a few minutes earlier, breaking up and burning rather too close to their Command Module. It wasn’t supposed to be there. The same thing, it turned out, had happened to Apollos 8 and 10, the missions that had returned from the Moon during the run-up, and nobody noticed. Afterwards, NASA changed the procedure to make sure the discarded module kept its distance.
I’ve got a couple more posts lined up this week to remember Apollo 11 – watch this space!
Copyright © Matthew Wright 2019