Once a deceleration of 0.05 g was detected, the hard work of entry began. The spacecraft was in the tenuous but thickening atmosphere and there was no longer any need to hold it in a particular attitude because its inherent aerodynamic stability was dominant. Therefore P64’s first task was to discontinue attitude-hold and begin to ensure that any unwanted motions in the pitch and yaw axes were damped out. The three displays on the DSKY showed their roll angle as commanded by the guidance computer, their velocity and the g-forccs associated with their deceleration. It was no coincidence that these values were duplicated on the EMS; both systems operated independently, and if one failed, the other could be used to complete the re-entry.
The primary task for P64 wns to slow the spacecraft below’ orbital velocity, about 7.8 kilometres per second, thereby ensuring that it could not return to space on a long and lethal orbit of Earth. Within P64’s regime, the deceleration loads quickly built up to a peak above six g while the program repeatedly tested their flight path, evaluating whether a safe re-entry trajectory had been achieved.
By this time, a substantial shock wave had developed just ahead of the heatshield as the CM rammed into the tenuous gases of the upper atmosphere, instantaneously subjecting them to extreme compression and heating them to temperatures similar to the visible surface of the Sun, ionising them and surrounding the spacecraft in a sheath of plasma that effectively blocked radio communication. Eor about three minutes of the initial re-entry, this ‘blackout’ meant that mission control had no visibility into the craft’s systems and could do nothing except wait. idle. The crew’, meanwhile, unable to communicate, concentrated on monitoring their flight path, although on Apollo 14. CMP Stu Roosa reported that he could hear his Capcom "just fine”.
When Apollo 13 had to abort and return to Earth early, an unintended shallowing of their flight path caused blackout to last much longer than expected, in the process raising the tension of an already dramatic situation with the prospect that the craft had burned up. This radio-opaque plasma sheath is an inherent problem for all reentering spacecraft but the Space Shuttle managed to circumvent it by establishing communications through the rear of the sheath via a geostationary communications satellite.
‘What is that?” said Anders on his first and only space re-entry. "Airglow?”
”That’s right, you’ve never seen the airglow,” said Lovell. Both he and Borman had re-entered on Gemini 7 and. like a couple of old-timers, thought they knew’ what was coming. ”Take a look at it.”
"You can’t get your pin without seeing the airglow7.” kidded Borman, referring to the gold astronaut pin that Anders would receive after Apollo 8 landed.
The view out of the CM window during Apollo 15’s re-entry, including the jettison of
the forward heatshield and the deployment of the parachutes. (NASA)
"That’s right,” joked Lovell.
Anders laughed. "I see it, I see it. Let’s see, is this where I’m supposed to ask how many g’s, Lovell?”
"That’s right,” answered the experienced spaceman, "you ask how many g’s.”
There were no rookies on Apollo 10, but that did not inhibit their surprise at the spectacle of being inside a re-entry from lunar distances where much more kinetic energy was expressed.
"Here comes the glow, John,” said Tom Stafford as they approached the 0.05-g event.
“Here it comes, babe.’- said Eugene Cernan.
“Shit, you’d belter believe it," said Stafford. "Okay. Stand by for rc-ent…” He interrupted himself. "Oh. look at that.”
"Look at that.” repeated Cernan. "God damn. God damn.”
"Just looks like daytime.” said Stafford, who then counted the g-forccs building up. "Point two, point three, point four. We’re trimming in good.”
"Here comes some g’s. babe,” said Young.
"Oh, you’d better believe here comes some g’s,” said Stafford, "Here comes the water, too. Just sit back anyway." Water that had condensed around the cold apex of the cabin began dripping over them.
"Okay, there’s one g,” said Young. But the sudden onset of a deceleration equivalent to Earth’s gravity seemed worse than it was to a crew that had been weightless for over a week.
"Shit!” cried Stafford. "You got to be kidding. Jose.”
"It seems like about 10 [g],” estimated Cernan.
On board Apollo 8. even the ’veterans’ Borman and Lovell were brought up short by a sight once described as like being inside a fluorescent tube. "God damn, this is going to be a real ride; hang on.” called Borman as the light outside and the g-forces built up. "Eve never seen it this bright before."
"Quite a ride, huh?’’ said Anders.
"Damndest thing I ever saw.” agreed Borman. "Gemini was never like that, was it, Jim?”
"No, it was a little faster than this one,” said Lovell, referring to the length of time they were staying in the high-speed region of flight.
"I assure you I’ve never seen anything like it,” said rookie Anders. "Cabin temperature’s holding real good. Up one degree.’’
After the flight, Borman was upbeat about the experience, "’flic ionisation on these high-speed entries is fantastic. The whole spacecraft was lit up in an eerie iridescent light very similar to what you’d see in a science fiction movie. I remember looking over at Jim and Bill once, and they were sheathed in a white glow. It was really fantastic.”
But this was no sci-fi movie. This was the real thing. The Apollo 11 crew were more descriptive of the sights that accompanied the onset of re-entry. "Along about 0.05 g, we started to get all these colours past the windows.” said Collins at their post-flight debrief. "Around the edge of the plasma sheath, there are all varieties of colours – lavenders, lightish bluish greens, little touches of violet, and great variations mostly of blues and greens. The central core has variations on an orange-yellow theme. It’s sort of a combination of all the colours of the rainbow really. The central part looks like you would imagine a burning material might look. Orangish. yellowish, whitish, and then completely surrounded by almost a rainbow of colours.”
"I thought there was a surprisingly small amount of material coming off.” added Aldrin.
"That’s right; there didn’t seem to be any chunks as there were on Gemini,’’ said Collins.
“There was a small number of sparks going by." added Aldrin. “You could definitely see the flow pattern. Looking out the side window, you could get a very good indication of the angle of attack by the direction of motion of the particles. That didn’t seem to change very much. When a thruster would fire, you could pick it up immediately, because it deflected the ion stream behind you.”
Charlie Duke was surprised at how effective the thrusters were when he re-entered on Apollo 16. “When it decided to roll, boy, it just took off. You could see the horizon through the ionisation sheath, both out window five and the rendezvous window four."
He then spoke about detached Mylar strips he saw out his window. Considering the punishing temperatures being experienced on the opposite side of the spacecraft, it was remarkable that this plastic film could survive. “There was Mylar on window five that was flapping back and forth across the window that was there at touchdown. It had come up right at CM SM sep. I had seen that strip fly by. When we started getting the g’s it flopped up over the window, sort of stayed there and wiggled the whole time, which amazed me."
“Here’s tw o g’s." said Stafford. The deceleration was ramping up for CM Charlie Brown.
“Okay, baby; you keep flying it." Young urged the computer. “Three g’s."
“There’s one minute gone," said Stafford.
“Four g’s," said Young.
“Fiveg’s." they announced together.
On the g meter, they watched the deceleration peak at 6.2 g’s.
“Hang on. It’s getting better." said Cernan.
“It’s going dowm," said Stafford. “We’re starting to roll."
“Go, machine,’’ said Young. “It’s rolling good.’’
“Come on, baby; fly," urged Cernan.
“It’s good. It know s just what it’s doing." said Young as P64 manoeuvred to force the spacecraft towards Earth.
“It’s rolled lift vector down," observed Stafford.
“Go on. Keep that lift vector down," said Young. A spacecraft that was going down was one that would not fly back out into space.
“Ooh, only three g s." said Stafford. The pulse of g-force that went beyond six had been brutal.
“Oh, man." said Cernan. "That first [g-peak] was a bitch."
As high-performance jet pilots, astronauts had learned howr to breathe under such crushing conditions, tightening their chest muscles and taking short grunting breaths.
After the peak g-load had subsided. P64 maintained a four-g deceleration until it had determined that their velocity had dropped below the speed required for orbital flight. Then there was no way it could exit the atmosphere and enter orbit. It was going to land somewhere. When this condition was met, there were two possibilities for the rest of the re-entry.