The fact that human beings reached the Moon and walked on its pristine surface just 8 years after President John F. Kennedy committed America to that goal is nothing short of amazing. The first manned spacecraft to reach the Moon was Apollo 8, at Christmastime 1968. The crew was Frank Borman, James Lovell, and William Anders. As they orbited the Moon that Christmas Eve, they each read passages from the first chapter of Genesis, a fitting gesture as they flew in heavenly proximity to the handiwork of creation. I remember that flight, going outside to see if my 7-year-old eyes could spot the spacecraft near the Moon. I was sure I saw it - a tiny speck, an insignificant dot very close to our good satellite. Such is the expectant vision of youth (Fig. 10.3)!
This first flight to the Moon was a bold move, sending three brave men hurtling away from Earth like never before. The Apollo 8 spacecraft flew without a Lunar Module, because the subcontractor, Grumman, was still perfecting the final details. This meant that if anything went wrong with the basic spacecraft - the combined Command and Service Modules - the crew could well be stranded in Lunar orbit, or worse yet, in Solar orbit. But everything went as planned, and the crew returned safely.
For Apollo, missiles and modules were the key to success. This, together with a brand new technique called Lunar orbit rendezvous, enabled the first manned Moon shots before the year 1970. For Lunar landing missions, there were eight separate rocketships that helped man reach the Moon. These were the three main launch stages, the launch escape tower, the Moon ship, and the Command ship. The last two were made of two pieces each. By leaving the descent stage of the Lunar Module on the Moon, and rendezvousing with the Command ship in the ascent stage, a minimum of weight would have to be rocketed off the surface of the Moon. By minimizing the various landing or liftoff weights, the total amount of required propellant could be kept down as well, making the job that much easier for the Saturn V launch vehicle. The Saturn V was the largest rocket built to date. It stood 363-ft tall, weighed 6/ million pounds at liftoff, and was able to generate 7/ million pounds of thrust. It rose slowly, majestically off the launchpad, but afterward accelerated rapidly to boost its payload into orbit.
Why did the missiles and modules concept work so well in getting Apollo to the Moon? The entire thrust behind this idea was to launch each mission as a self-contained unit and to minimize propellant usage during the flight. There was never a plan to perform in-space refueling. The Saturn V lifted off with everything the mission would need, every drop of propellant, every piece of equipment that it would take to get two astronauts onto and off of the Moon while a third waited in Lunar orbit. The plan worked admirably, but there was a price to pay. Not one piece of the Apollo/Saturn space vehicle was reusable. Every piece was thrown away after it had done its job. So, although the astronauts always got to fly brand new equipment, the costs were tremendous.
It took three enormous rocket stages to get the Saturn V stack off the ground. The first stage used five huge F-1 engines burning kerosene and LOX and delivering 1/ million pounds of thrust each. The dense fuel kerosene was used here because it could deliver greater thrust to boost the vehicle off the pad, although the propellant consumption in terms of mass flow rate was higher, and the specific impulse was lower than with liquid hydrogen. Still, at this stage of the flight raw thrust was more important than specific impulse, because the launch vehicle was still moving relatively slowly. When the first stage burned out and dropped off, the five J-2 engines of the second stage took over, burning LH2 and LOX, now that the speeds had increased to the point that specific impulse became more important than thrust alone. Remember that specific impulse is simply a rocket's thrust divided by its propellant weight flow rate. Liquid hydrogen, with its higher specific energy content and lower density compared to kerosene, delivered a higher specific impulse. So this was the fuel of choice for both the second and third stages. Once the second stage had expended all of its propellants, the lone J-2 engine of the
Saturn's third stage finished the job and delivered the remaining stack into LEO. This engine could be shut down and restarted, a feature that allowed the third stage to boost the Apollo spacecraft complex to the Moon.
Once the third stage, the S-IVB, had done its job and the spacecraft was established on its high-speed coast toward the Moon, the Command ship disengaged, turned around, and docked nose-to-nose with the Lunar Module. The Command ship then pulled the LM out of its docking adapter - the shroud that had protected it during the atmospheric ascent - and together they continued their uphill ride. The S-IVB was now cast aside, since it could serve no more useful purpose. See Chap. 4, Figs. 4.7 and 4.9.
At this point, the original eight-piece stack was down to four. The launch escape tower - intended to pull the crew capsule away from the stack in case the rocket blew up - had been jettisoned during the boost to orbit (Fig. 4.8). The remaining modules consisted of the Command Module, the Service Module, and the Lunar Module descent and ascent stages. The service propulsion system engine, with its large area-ratio nozzle mounted at the aft end of the SM, would do the job of getting the spacecraft into and out of Lunar orbit. If this engine failed, especially if it failed after the Lunar Module's propellants had been expended, the crew would be marooned in space. Once in Lunar orbit, two of the astronauts transferred from the Command ship into the Lunar lander, undocked, and flew down to the surface. Using the descent stage's engine only, they navigated to a preselected touchdown zone. Coming out of orbit, the LM used its descent engine initially to reduce orbital, or forward, speed (Fig. 10.4). This allowed the Moon's gravity to pull the craft toward the surface. By carefully adjusting the attitude of the lander, the descent engine could be used to reduce vertical velocity as well as horizontal, so that just before touchdown, the LM was hovering with little or no forward speed, over the landing zone (Fig. 10.5).
When Lunar explorations were complete, the crew climbed back into the ascent stage, used the descent stage as a launchpad, and blasted back into orbit. If this engine failed for any reason, the crew would be stranded on the Moon. The two Moon explorers now met up with the lone astronaut in the Command ship (even though the commander of the mission got to walk on the Moon), transferred rocks and samples, and jettisoned the LM ascent stage.
Now the spacecraft, with all three astronauts back together, was down to two modules. The Service Module's high area-ratio engine (tailored for efficiency in vacuum conditions) pushed the spacecraft out of Lunar orbit to begin the long downhill coast back to Earth. By the time the spaceship reached the atmosphere, it had accelerated to around 25,000 mph, or 7 miles per second. The cylindrical Service Module was discarded just before reentry, with the conical Command Module entering the atmosphere alone, protected by an ablative heat shield and a detached bow shock wave. The astronauts endured 6.8 G's during this phase of the mission, until drogue and main parachutes were deployed and the Apollo crew capsule splashed down at sea.
Unlike the pilots of the Space Shuttle, or any aircraft for that matter, the crew now relied completely on rescue-recovery from the US Navy (Fig. 10.6). Frogmen
were dispatched from helicopters, flotation collars were attached to the spacecraft, and life rafts were inflated for the astronauts. The frogmen helped the crew out of their spacecraft, into the life raft, and finally onto the deck of an aircraft carrier.
The missiles and modules of Apollo won the space race to the Moon. The system worked beautifully, and Apollo accomplished its objectives. For its time, the modular concept was the correct choice. But what of the future?
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