Richard Lawrence - The Mammoth Book of Space Exploration and Disasters

Scott, meanwhile, was busy aviating and navigating. The California Capcom, Al Shepard, took over voice communications for the retrofire sequence, one minute away:

“Seven, this is Cap Com. Are you in retroattitude?”

Carpenter replied: “Yes. I don’t have agreement with ASCS in the window, Al. I think I’m going to have to go fly-by-wire and use the window and the [peri]scope. ASCS is bad. I’m on fly-by-wire and manual.”

Capcom responded: “Roger. We concur.”

But in going to fly-by-wire, Scott forgot to shut off the manual system that he’d activated during the pre-retro checklist over Hawaii as backup for the automatic system. So his efforts to control attitude during retrofire were accomplished on both fly-by-wire and manual control modes, spewing out fuel from both tanks. Halfway through his fifteen-minute flight the year before, Al himself had committed the identical error. Retrosequence was coming up.

Capcom radioed: “About ten seconds on my mark… 6, 5, 4, 3, 2, 1.”

Carpenter reported: “Retrosequence is green.”

Green is good. Green means that everything is set right for automatic retrofire. But not in this case, because the automatic system was locked out and the gyros were caged. Scott would have to fire the retros manually, throwing switches upon Al’s count, coming up fast. Suddenly a critical intervention from Al. If Scott’s gyros were caged, Al reported, he would have to “use attitude bypass.” His gyros were off, Scott answered, and Al repeated his remark:

“But you’ll have to use attitude bypass and manual override.”

Carpenter reported: “Roger.”

Then two seconds. Al counted down, “4, 3, 2, 1, 0.”

Before, during, and after the retrofire firing, Al offered Scott two crucial observations. Because of the instrument failures, the cockpit was in a configuration never before envisioned, and Al perceived the effect it would have on the required cockpit procedure. His contribution to Scott’s safe reentry was a resounding endorsement of the decision made a few years before to place astronauts in the communication loop, as knowledgeable buffers between the ground-control people and the man doing the flying. Al’s insight at a crucial moment probably kept Scott’s landing from being even farther off target than it was.

Carpenter continued:

The last thirty minutes of my flight, in retrospect, were a dicey time. At the time I didn’t see it that way. First, I was trained to avoid any active intellectual comprehension of disaster – dwelling on a potential danger, or imagining what might happen. I was also too busy with the tasks at hand. Men and women who enter high-risk professions are trained to suppress, or set aside, their emotions while carrying out their duties. After the job, and after the danger has passed, is the time for emotions.

Without the ability to detach oneself from the peril in a situation, one has no chance of surviving it. What perils did I face? They were the same perils faced by Al, Gus, and John – and later by Wally and Gordo. The retros might not fire. They might explode or not burn properly. The heat shield might not work. The drogue or the main chute might not deploy or reef properly. Thinking about all the things that could go wrong, no one would ever climb into a spacecraft. A pilot counts on all those things going right, not because he needs to believe in a fairy tale, but because he has confidence in the hardware, in the systems, in the men and women on the ground. In himself.

Still, I do remember being surprised by the power of my detachment. It felt as though I were watching myself, with fascination and curiosity, to see how my great adventure might turn out. The rockets were supposed to fire automatically. I watched the second-hand pass the mark, and when they didn’t, punched the retrobutton myself a second later. An agonizing three seconds passed until the reassuring sound and vibration of firing retrorockets filled the cabin. I was prepared for a big boot, which never came. Deceleration was just a very gentle nudge, not at all the terrific push back toward Hawaii that John had reported feeling from his own retrofire.

Al was still in voice range and we continued to transmit information about retrosequence. I noticed smoke in the cabin and the smell of metal. Two fuses had overheated. I was worried about the delayed firing of the retrorockets. At that speed, a lapse of three seconds would make me at least fifteen miles long in the recovery area. Al asked me if my attitudes held, and I said, “I think they are good,” but I wasn’t sure, adding that “the gyros are not quite right.”

My visual reference was divided between the periscope, the window, and the attitude indicators. My views out the window and the periscope helped me attain the desired pitch of 34 degrees, nose down, although the attitude indicator read minus-ten. I tried to hold it there, at minus-ten degrees, a false but at least steady reading, throughout retrofire, continually cross-checking with the window and the scope. The long hours on the Mercury Procedures Trainer were paying off.

I had commented, many times, that on the MPT you cannot divide your attention between one attitude reference system and another and still do a good job in retrofire, although it appears I pretty much nailed the pitch attitude. But the nose of Aurora 7, while pitched close to the desirable negative 34 degrees, was canted about 25 degrees off to the right, in yaw, at the moment of retrofire. By the end of the retrofire event I had essentially corrected the error in yaw, which limited the overshoot. But the damage was already done.

In the end I was 250 miles long. The yaw misalignment alone caused the spacecraft to overshoot the planned impact point by about 175 miles. The three-second delay in firing the retrorockets added another fifteen miles to the error in trajectory. Under-thrusting retrorockets piled an additional sixty miles onto the overshoot.

“But retrojettison,” I said to Al, changing the subject. It was a question. “Roger,” he told me, “ten seconds until retrojettison.” Right on time. I heard barely audible ignition sounds from the retrojettison rocket. I reported on my fuel supplies, now at 20 percent for manual control, 5 for automatic. My next job was to damp out any oscillations the spacecraft might develop during the next stage of reentry. But in a little more than a minute, the manual control system would be out of fuel and thus worthless. I still had the fly-by-wire system: “I am out of manual fuel, Al.”

As always, Al was cool. I had merely reported the depletion of manual fuel.

As it happens, Max Faget had foreseen the reentry dangers posed by an aerodynamically unstable craft. He prepared for this theoretical possibility of attitude-control failure by designing a near-perfect reentry body. The vehicle was designed, Max explains, “so that in the event the attitude control system failed, we would still make reentry.”

Fuel starvation had rendered my attitude control system ineffective. Still, during the final stages of reentry, I remember thinking the capsule’s oscillations were being damped, or suppressed, without any control inputs I was attempting. I remember thinking Faget’s design was working: the Mercury capsule had positive aerodynamic stability – as advertised back in 1959, when we were invited to volunteer for this grand adventure. My safe reentry was virtually guaranteed.

Aerodynamic stability is a good thing, but there were still things I could do to improve my situation. At 04 35 13.5, the capsule was oscillating between plus or minus 30 degrees in pitch and yaw, but I was able to use my fly-by-wire controls to further damp the oscillations. A postflight report noted that “by manually controlling the spacecraft during retrofire,” I had “demonstrated an ability to orient the vehicle so as to effect a successful reentry, thereby providing evidence” that human beings can “serve as a backup to malfunctioning automatic systems of the spacecraft.”