In large measure, of course, the success of the Apollo landing missions was due to lessons learned and experiences gained during the unmanned and manned missions that preceded them. Robot spacecraft like the Rangers and Lunar Orbiters provided close-up, map-quality images of the lunar surface; and, then, beginning with Russia's Luna 9 in 1966 and continuing through the Surveyor series (31 Mb PDF) of that same year, soft-landers determined something of the chemical, mechanical, and bearing properties of the surface layers and, as well, provided ground level views of crater-and-rock-strewn lunar terrains. And, during this same period, Mercury and Gemini flights were used to develop some of the basic operational knowledge needed for the manned lunar flights.
In the very early years of the Space Age, cautious planners had demanded unmanned demonstration flights to prove that humans could survive, at least, the rigors of spaceflight; but, after a few launches of canine and primate stand-ins, it wasn't long before the Mercury and Gemini astronauts demonstrated that they could control and maneuver a spacecraft and, also, could perform the critical maneuvers necessary to rendezvous and dock with a passive target.
As mentioned previously, the tight deadline that Kennedy had established for accomplishing the first landing dictated the design choice of having an astronaut stay in lunar orbit in the Command-and-Service Module while a two-man landing crew flew the LM down to the surface. This choice minimized the amount of mass that had to be launched from Earth and, indeed, for almost any other mission mode, NASA would have been faced with the choice of either designing a bigger booster than the Saturn V or making multiple Saturn V launches to accomplish a single mission. Neither alternative was really acceptable and ultimately, NASA realized that only with the design choice of a CSM/LM combination could they meet Kennedy's challenge. However, the choice also meant that, when a LM crew lifted off from the Moon, they would have to rendezvous and dock with the CSM and, while theoretical studies of orbital rendezvous were reasonably well developed, it took the ten-mission Gemini program to prove that it could be done. Of the ten Gemini crews, six crews had rendezvous targets available on orbit and all six conducted successful rendezvous. The Gemini 11 crew - Pete Conrad and Dick Gordon - were even able to rendezvous and dock with their target during their first orbit of Earth.
The Gemini program provided opportunities to prepare for Apollo in other ways as well. The crews of Gemini 5 and 7 spent eight and fourteen days in space, respectively and, although they had far less elbow room than the Apollo crews, they proved beyond any doubt that there were no physiological or operational barriers to the conduct of a ten-day lunar mission. Five of the Gemini astronauts ventured outside their spacecraft and, when they tried to do strenuous work, discovered that the air-cooling system used in the Gemini suits wasn't going to be adequate for Apollo. An astronaut sitting inside a cramped capsule simply couldn't work hard enough to generate much body heat and, consequently, could be kept cool by oxygen flowing through the suit. But when he got outside and flexed his arms and legs against the internal pressure of the suit, the extra heat load quickly overwhelmed the airflow cooling system. What was needed was a system which could carry away the excess heat; and, as a result of the Gemini experience, NASA and its contractors developed a system for circulating cooled water through tubes woven into an undergarment worn next to the astronaut's skin.
The first manned flight test was delayed for twenty months because of the tragic Apollo 1 launch-pad fire that took the lives of Gus Grissom, Roger Chaffee, and Ed White in January 1967. By October 1968, however, the CSM design had been modified to eliminate the fire hazard - and to incorporate a number of essential upgrades - and the crew of Apollo 7 (Wally Schirra, Donn Eisele, and Walter Cunningham) put the CSM through its paces in low orbit around the Earth. Two months later, the crew of Apollo 8 (Frank Borman, Bill Anders, and Jim Lovell) made an epochal Christmas voyage to lunar orbit. In part, they flew this early voyage to the Moon - leap frogging CSM/LM tests in low and high Earth orbits - for the simple reason that development of the LM was lagging behind the rest of the program by five months and there wasn't a spacecraft ready to fly. And, more importantly, the Russians had sent unmanned Zond spacecraft around the Moon - although not into lunar orbit - in September and November and then had brought them back to Earth. The Zond missions were generally seen as precursors to a manned orbital mission and, in large measure, Apollo 8 was flown to prevent a Russian first which would have greatly diminished the psychological impact of the first landing. Global politics aside, however, Apollo 8 also provided a fortunate opportunity to prove out flight control procedures at lunar distances. It gave NASA vital experience in tracking a spacecraft orbiting the Moon and also provided an opportunity to expand the photographic coverage of potential landing sites. It was a great confidence builder for the entire Apollo team; and it was shared with an international audience of hundreds of millions of people. Few who witnessed it will ever forget the crew's Christmas Eve reading from Genesis that accompanied the TV pictures of the lunar surface passing below. And, as well, there were the still photos the crew brought home, the stunning pictures of the Big Blue Marble, our home planet Earth, rising above the limb of the Moon. It was a taste of other stunning moments and pictures yet to come.
The LM was like no other vehicle that had ever been flown. It was a new sort of beast; and it is, perhaps, not surprising that it was the last major piece of Apollo hardware to be ready for flight test. It was a two-stage vehicle, one stage for the descent from lunar orbit to the landing, and the other for the return from the surface. Because the LM would never have to fly on its own through Earth's atmosphere, it didn't need the structural strength or aerodynamic shape of the CSM. What the LM had to be was lightweight and reliable and, although NASA successfully tested the propulsion and guidance systems during an unmanned, earth-orbital flight in January 1968, it was excess weight and other design problems that delayed the first manned test. There was no room in the weight budget for much sophistication and, indeed, every kilo that could be shaved from the spacecraft gave the crew another tenth of a second or so to find a safe landing spot, another half hour to spend on the Moon, or the opportunity to bring a few more priceless samples back to Earth. The search for weight savings and improved engine performance continued virtually until Apollo ended and the payoff was not only a flawless series of LM missions - nine of them in all - but also a dramatic increase in productivity with each succeeding flight.
Because of the tight deadline that Kennedy had set, the final rehearsals for the lunar landing mission came in rapid-fire order. As hardware became available for flight test, it was immediately pressed into service. During Mercury, NASA had conducted a flight on the average of once every four months and then, during Gemini, had increased the rate to one every other month. But, while each of the crews built on the successes of their predecessors, at no time during Mercury or Gemini was so much new equipment or so many new procedures brought on line so rapidly as during Apollo. At three month intervals, Apollo crews made great strides along the road to the lunar surface and it was a combination of determination, plentiful resources for design and testing, and also a good measure of luck that made it all possible.
The Apollo team did its best to make sure that equipment and procedures would work the first time they were tried in flight; and the successes of the unmanned test series and of Apollo 7 and Apollo 8 continued unabated into the final year of the decade. For the first time, on Apollo 9, a crew performed the delicate maneuver of separating the Command Module from the Saturn V, then moving away and turning around to dock with the stowed LM and pull it free of the Saturn. On the second day of the flight, they fired the big Service Module engine, much as later crews would do to enter lunar orbit, and tested the bond between the two spacecraft. On the third day, in what would later prove to have been a rehearsal for the Apollo 13 rescue, they test fired the LM's Descent Engine for six minutes while still docked with the Command Module. On the fourth day, Schweickart spent nearly an hour outside on the LM's front porch, testing the Portable Life-Support System or PLSS - the backpack that the lunar surface crews would wear. And then, finally, on the fifth day of the mission, McDivitt and Schweickart pulled away from the CSM and fired the Descent Engine to boost themselves to higher orbit around Earth. This was about as close a simulation of the lunar descent that could be flown in Earth orbit; and, once the maneuver was complete, McDivitt fired the Descent Engine for a third and final time to position the LM for a simulated ascent back to lunar orbit. Then, all that remained was to jettison the Descent Stage and fire up the Ascent Engine for a rendezvous with Scott. It all went flawlessly.
A lunar landing was a complex undertaking and, although the procedures had been reviewed and practiced countless times in simulations, there was nothing quite like going to lunar orbit and then flying almost down to the surface to make sure that everything - the propulsion systems, the guidance, radar, and communications systems - was ready for the actual landing. Twenty-plus years after the fact, Jack Schmitt says that he has no doubt that a final dress rehearsal would have been flown, no matter what it might have meant in terms of meeting the Kennedy deadline. However, in his excellent book, Carrying the Fire, Michael Collins says that, had it been his decision, he would have delayed Apollo 10 until a landable LM was ready for Stafford and Cernan to fly. Certainly, there was a good deal of talk and debate before a final decision was made to fly one last rehearsal.
According to Gene Cernan, "When I got assigned to Apollo 10, I remember Stafford telling me, 'You know, it depends on how the hardware flows, but we may be the first guys to land on the Moon.' And we went into that flight not knowing whether we would or would not. Apollo 10 was not designed to be a dress rehearsal until LM-5 began to have problems. LM-5 was the lighter weight LM that would have flown on Apollo 10 had Apollo 8 not flown without one and which eventually flew on Apollo 11. It was at least two months behind and it was at that time that there were discussion of 'Do we wait for the LM and take Apollo 10 all the way to the surface of the Moon? After all, if we're going to go through the risk of launch and TLI, and go a quarter of a million miles away, and go down to some distance above the surface, why not take the rest of the risk and land. Let's wait for the LM that's going to be able to do that.' And the other theory was 'Well, let's take it step by step. Let's put it through all those risks, but let's just not take "the big risk" and put them down on the surface.' And we, the crew, were in discussions - Stafford in particular - as to what direction this should head. And I think Tom tended, once LM-5 was slipping and the opportunity to go on Apollo 10 came, to think that that was the thing to do. Tom was not so adamant about being first on the Moon. He never looked at it that way. He wanted to do what was the best thing to do to have a coordinated, planned program. And when he got involved in those discussions, I think he tended to agree with the people that said 'Let's go to the Moon, but not land.'"
Stafford, Young, and Cernan were launched from Earth on May 18, 1969; and, after spending a couple of hours in Earth orbit checking out all the spacecraft systems, they fired the Saturn V's third stage engine to send them toward the Moon. As with Apollo 8, they flew what is known a "free return" trajectory, one that would take them around the backside of the Moon and then, if the Service Module engine failed to fire, directly back to Earth. However, as it did on all of the Apollo missions, the Service Module engine worked perfectly. As the crew swung around the back of the Moon, flying only 60 nautical miles (110 kilometers) above the center of the lunar Farside, they fired the engine for about four minutes and entered lunar orbit. Twenty minutes later they came around the eastern limb of the Moon and regained radio contact with Earth. The timing of their re-appearance was all that was needed to confirm that they were in the planned orbit.
For the next several hours, Stafford and Cernan checked out the LM. Then, after a night's rest, they separated from Young and, over the Farside, fired the Descent Engine for thirty seconds and put themselves in an orbit which would take them down to 47,000 feet over a point 300 miles east of the planned Apollo 11 landing site. As they approached the low point of their orbit, they got close enough to the ground to check out the LM radar and to make a visual examination of the area in case it was needed as a backup site for the first landing. Then, rather than fire the Descent Engine against their forward motion for a landing, they fired it in the opposite direction and boosted themselves into a new orbit which would take them out to 190 nautical miles above the Farside and then back down for a final pass, this time over the planned Apollo 11 site.
Two hours later, at the end of another lunar orbit, Stafford and Cernan were ready to rehearse the ascent and rendezvous. Once again passing low over the southwestern corner of the Sea of Tranquility, they jettisoned the Descent Stage and fired the Ascent Engine for about 15 seconds and, soon thereafter, made a perfect rendezvous with Young. Almost everything had gone perfectly. There had been one anxious moment when, at the moment of staging, the Ascent Stage began to spin and pitch. As it turned out, a control switch for the small steering rockets had been improperly set and it took Stafford eight long seconds to regain control of the spacecraft and then fire the Ascent Engine.
"I saw the lunar horizon go by about seven or eight times in ten seconds," Cernan says. "That's a hair-raising experience. That's when I said 'Son-of-a-bitch, what the hell happened?'
"Now, this story applies to some things we're going to talk about later, so it's worth going into it in some depth. Although I was the Lunar Module Pilot on 10, I prided myself that I could fly the Command Module. Not as good as John Young; because I didn't understand all the systems in the depth that he did. But I had trained in the Command Module simulators, especially for the rendezvous, and I certainly understood how to operate it. It was my nature to do that. I wanted to know what John was doing so that, if we had a problem, I'd understand what his problem was and I'd know what our problem was. And in the Lunar Module simulators, sometimes I would fly on the left seat (that is, stand on the left side of the sumulator - with someone else on the right side - and do the Commander's tasks), just so I would know what Tom was doing. I'd flown with Tom before, so I knew him pretty well. I know how he acts and reacts pretty well. And I felt I knew the Lunar Module on both sides, very well. Including handling the PGNS (Primary Guidance and Navigation System, pronounced "pings"), as well as anybody. I also had the responsibility of operating the AGS (Abort Guidance System, pronounced "ags"). And a lot of times on Apollo 10, I would operate the PGNS as well as the AGS.
And this is what happened on Apollo 10, to the best of my recollection. When we staged (separated the Ascent Stage from the Descent Stage), there were a number of things we had to do, including changing programs in the PGNS and changing switch positions. At this late stage, twenty five years after it happened, neither Tom or I can be sure but, when it came time to stage, some program change blipped up on the PGNS (display) and I acknowledged it from my right-hand seat - or there was some switch (actually, the AGS Mode Control Switch) that had to be changed, and I changed it. And I'd be willing to bet that, if we could recreate that moment in history - and I think Tom would say the same thing - I put the switch in the new position and Tom went ahead and moved it back to the old one. His action was to move the switch. I had already done it for him. But he didn't know that and, when he moved the switch, he just moved it back where it was. And, in effect, we created the problem.
"When the spacecraft began to gyrate, neither one of us realized what the other had done. And so we looked at failed software, we looked at stuck thrusters, we looked at all kinds of things. It happened right at the moment of staging and, my God, it was not a passive moment, it was an active moment. And we did do some wild gyrations (maximum rates of motion were 19 degrees per second in pitch and greater than 25 degrees per second in yaw and roll). I think Tom probably damped it out in less than twenty seconds (actually, within eight seconds of staging). Pretty quick. He handled it extremely well. But maybe this is one area we didn't train enough on. And I was going through the procedures and I moved the switch. And I don't remember whether I should have or not. And that's what happened.
"Now, that problem could not and would not have happened on the lunar surface because, had we been ready to stage on the lunar surface, we would have looked at all the switches. We were very meticulous. On Apollo 17, we were ready 30 minutes to an hour ahead of time. We went over the checklist more than once. We would have verified all the switches. So that problem was not relevant to staging on the surface. But, when we came to Apollo 17, there was an invisible line at the right-hand edge of the PGNS and I said to Jack, 'Don't ever - don't ever, ever, ever, ever, ever, ever - touch anything to the left of that line, unless I know about it.' And the reason I said that is because I was on his side once and I touched a switch. Tom and I operated a little bit more interdependently, maybe, than Jack and I did, and I didn't want Jack to get ahead of me, throw a switch and the same thing happen to us that happened to Tom and me. And that was my unwritten rule. And that had nothing to do with Jack's abilities, because Jack knew the computer, he knew the systems. He was an outstanding Lunar Module Pilot. It was just that, in times of dynamic action and reaction and people trying to help each other, they can over help each other. Like on Apollo 10. And so the rule was, 'Jack, don't touch anything left of the right-hand edge of the PGNS.' And it worked out perfectly."
Once Stafford and Cernan regained control of the LM, they fired the Ascent Engine and, two orbits later, rendezvoused and docked with Young in the Command Module. Another orbit later, after Stafford and Cernan were safely back in the Command Module, they jettisoned the LM which Houston then sent into a solar orbit with a final firing of its engine.
After a good night's sleep and a day in orbit to do landmark tracking, the crew of Apollo 10 headed back to Earth. Although the crew of Apollo 10 encountered problems during the mission, they were the sort of problems that inevitably plague test flights. The important point is that there had been no showstoppers, and the net result of the Apollo 10 rehearsal was that the stage was now set for a try at a landing on Apollo 11.
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