Category How Apollo Flew to the Moon

On 3 September 2002, astronomer Bill Yeung discovered a faint, 16th magnitude object that was orbiting Earth.1 Initial excitement about this apparent asteroid, designated J002E3, centred on the remote possibility that it might, one day. impact Earth. As more data on its orbit was gathered, analysis showed that it could not have been in Earth’s vicinity for long and had probably been in a heliocentric orbit before being captured by Earth. Additionally, spectroscopic studies revealed that its surface colour was consistent with titanium oxide, the pigment in white paint. It was no asteroid.

Projecting the orbit ОҐ. Г002ЕЗ around the Sun backwards in time showed that it had previously been in the Earth-Moon vicinity in 1971, around the time of the Apollo 14 mission. However, since all of the components of that mission had been accounted for. it could not have come from Alan Shepard’s flight. Suspicion shifted to the Apollo 12 S-IVB.

After Richard Gordon had completed his TD&E exercise, the two Apollo 12 spacecraft. Intrepid and Yankee Clipper, continued on their path to the Moon in November 1969. NASA intended that Apollo 12’s S-IVB should go the same way as the previous Moon-bound third stages by having its residual propulsion slowr it dowm sufficiently to pass the Moon’s trailing limb and be slung into heliocentric orbit. Unfortunately, a guidance error by mission control resulted in a burn that lasted too long and the stage was slowed more than intended. It therefore passed too far from the Moon to achieve a proper slingshot and instead entered a large Earth orbit from which, owing to a later encounter with the Moon, it subsequently escaped. As far as anyone can tell, it was this S-IVB that had returned for Bill Yeung to catch in his telescope that September night.

As the design of the G&N system was being finalised in the early 1960s, a eonllict arose between those who designed the equipment and those who would fly with it. The designers at the Instrumentation Laboratory at MIT led the field of applying mathematics to the problems of guidance, whether in a submarine, an aircraft, a nuclear-tipped missile or the exploration of space. They saw the problem from a wide perspective in which all guidance could be reduced to equations that modelled the solar system as it sat surrounded by the stars. Their fundamental point of view’ was an inertial one. which was expressed during Apollo’s gestation by an intention that the spacecraft’s attitude should be displayed as a set of numbers with respect to inertial space. The crews, on the other hand, w’ere pilots, and pilots see flight largely in terms of movement with respect to the horizon of wTiatever planet (usually Earth) they are flying over. Their point of view’ dealt with a local frame of reference that stayed aligned with the ground beneath their spacecraft, even as they flew around a curved planet.

These two viewpoints on spacecraft control influenced the Apollo guidance and navigation system as it evolved at a tremendous pace during its development; the inertial point of view’ dictating its fundamental structure, but w’ith the astronauts’ preferences heavily influencing the final mode of operation because they had experience on their side. They had cut their teeth on the Gemini flights of 1965 and 1966 during which NASA learned how to fly in space. They pointed out that most manoeuvres needed to be carried out with respect to the ground below, especially the all-important rendezvous manoeuvres on which the Moon-bound flights relied. As the design of the Apollo G&N system had been largely settled by the Lime this operational experience w-as gained, both hardware and software modifications had to be made to meet the expectations of the crews. These included the ‘8-ball‘ display and the ORDEAL add-on to turn it into an artificial horizon that the pilots preferred.

Space is a strange place for those of us who are used to the warmth of Earth. Here on our planet, the air. the oceans and the land absorb the heat from the Sun and give it up at night, thereby moderating temperatures. We know instinctively the importance of air in the transportation of heat, whether it is between the sea and land, within the rooms of our houses or inside the equipment we possess. In space, things are very different.

Imagine placing an object in cislunar space, not too near Earth, sitting motionless. The side facing the Sun will become warm. How much depends on its characteristics but as it gradually warms, it also radiates heat. The warmer it gets, the more heat it radiates until it reaches a point where it radiates as much heat as it receives. At this point, it is at thermal equilibrium and its surface temperature, probably quite high, is constant. The side of the object opposite the Sun will also radiate whatever heat it had. but this will not be replenished. The surface temperature will gradually fall until the minimal sources of heat available to it become comparable to the heat it is losing. Given time, and assuming that little heat leaks through the object from the sunward side, this area will become extremely cold. These extremes of temperature easily coexist in an environment where there is no air to transport heat.

In the Apollo spacecraft, there were various reasons why it was undesirable to allow these temperature extremes to exist for long. For example, tests had shown that the heaishicld material around the command module would crack and flake if it were allowed to become too cold, while the tanks for the RCS thrusters had to be kept at moderate temperatures at all times to prevent freezing or overpressurisation. The simple solution was to rotate the spacecraft gently around its long axis, side-on to the Sun. This technique was formally known as passive thermal control (PTC) but for many commentators, a far more descriptive term was the ‘barbecue’ mode.

Apollo 8 was the first to try to set up a PTC roll. Mission control gave Frank Borman an initial altitude that would place the spacecraft side-on to the Sun whilst avoiding gimbal lock and maintaining good communications. Once aligned, he began a constant, slow roll about the spacecraft’s longitudinal axis of only 0.1 degree per second which Look an hour to make a full rotation. However, physics abhors such a rotation, at least in the long term, and especially when large quantities of fluid are involved. With Lime, the rotation axis itself began to rotate so that the long axis sw’ept out a cone with an ever-increasing angle – a motion appropriately known as coning.

It was soon found that this simple method of initiating and maintaining PTC would not be appropriate for later missions. The addition of a lunar module would lengthen the stack further and make the simple roll manoeuvre even more difficult to maintain. Instead, use was made of the tracking programs in the command module’s computer to carefully control the overall attitude as the rotation progressed. Another change for later missions was to generate a reference orientation for the platform, a REFSMMAT, which was particularly suited to the manoeuvre.

The final three Moon-bound Apollo missions. Apollos 15 to 17, had one special task to perform prior to arrival in lunar orbit. Sector 1 of their service modules contained a scientific instrument module, or SIM bay for short. It housed a variety of cameras and instruments to investigate the Moon and its environment, and would be operated by the CMP during his lonely vigil while his crew-mates explored the lunar surface.

Hidden as it was behind the external skin of the service module, the SIM bay had to be exposed to space by removing one of the spacecraft’s panels. Rather than implementing door-like mechanisms with latches and hinges, engineers decided that a more reliable solution was to jettison the bolt-on panel by blowing it clear with pyrotechnic charges. This occurred before the spacecraft entered lunar orbit in order

that the jettisoned door would not enter lunar orbit and become a possible collision hazard. Explosive cord had been laid within a groove around the door’s edge. This was detonated to cleanly cut the aluminium skin, while further charges were set off to push the severed door clear. While the spacecraft then eased itself into lunar orbit, the door coasted around the Moon to emerge on a trajectory which would return it to the vicinity of Earth and. in all likelihood, eventually enable it to slip into an independent solar orbit.

Apollo 15 provided the first occasion for the detonation of these fireworks and Capcorn Joe Allen made light of the situation: “By the way. is that the manoeuvre where the SIM bay door jettisons the spacecraft?’’

In the Newtonian environment of space, it was as valid to say that the door was jettisoning the spacecraft as the other way around. A1 Worden agreed: "It has been variously known as that kind of a manoeuvre, yes.”

In fact, just as the spacecraft had pushed the door away, the door also pushed the spacecraft away, and engineers on Earth could detect this tiny trajectory change in their tracking. “15, just out of interest, we saw a good healthy jolt in our Doppler data down here during jcLL time," informed Allen.

“Gee, that’s very interesting,” replied David Scott, “because I would say that the jolt in here was very minor.”

As a precaution, the crew of Apollo 15 put their suits on in ease the shock of the explosives caused a breach in the cabin for some reason. This reflected wariness by programme managers following a Soviet space tragedy. A month earlier. Soyuz. 11 had departed the first Salyut space station. The shock of the pyrotechnic charges that jettisoned the orbital and service modules of the spacecraft immediately following the de-orbit manoeuvre had inadvertently opened a ventilation valve intended for use in the atmosphere, enabling the air to escape and quickly asphyxiating the crew’. Although the automatic systems brought the capsule to a pinpoint landing on Earth, the ground personnel found that the crew were dead when the hatch was opened. It was decided that in future Soyuz crews would be provided with pressure suits to be worn for launch and entry.

Even as Kennedy announced that the Moon would be the destination for America s aerospace community, managers still had to decide how to make the trip. At first, two competing methods, or modes, were investigated, both of which had powerful advocates and detractors. A third plan struggled for attention and was often mocked.

The first plan was known as direct ascent. Although it appeared at first glance to be the simplest solution, it was the most audacious of all. It entailed the development of a truly monumental booster that could hurl a large spacecraft directly to the Moon without pausing in Earth orbit. This Apollo ship would carry everything needed to complete the mission and get back home; landing gear, supplies for the trip and for the lunar surface, and engines to lower the entire vehicle to the Moon and then lift off again for the journey home. The proponents of this brute-force method said it was the simplest and easiest proposal to realise within the time allowed, avoiding complexity where possible. But on the minus side, it would have required the development of the Nova, a rocket of simply stupendous proportions to execute – one that would have dwarfed even the mighty Saturn V that was eventually built. For a time, the direct mode was championed by Robert Gilruth, leader of the Space Task Group, which was a small organisation within NASA that included Faget and Maynard and which would go on to form the core of the Manned Spacecraft Center, now known as the Johnson Space Center, in Houston.

The charismatic German rocket engineer Wernher von Braun – the director of the Marshall Space Flight Center in Alabama – had different ideas. He and his team had been brought to the United States after the war and had helped the US Army to develop its first useful rockets. They then formed part of an effort to create a family of large launch vehicles collectively known as Saturn. While some at Marshall welcomed direct ascent and the Nova, von Braun preferred Earth orbit rendez­vous (EOR), believing it to be more readily attainable. This called for a sequence of the smaller Saturn vehi­cles to place the components of the ship into Earth orbit where they would be assembled for the flight to the Moon. When the launch facilities Wernher von Braun beside an early Saturn at Merritt Island near Cape Canaveral launch vehicle. (NASA) were being laid out by von Braun’s

compatriot Kurt Debus, EOR appeared to be the best way to achieve the lunar goal. The perceived need for launches to occur in quick succession, and the associated processing of the launch vehicles, defined the layout of the new Moon port. In the event, these capabilities would barely be brought to full use.

As engineers and designers studied the options, huge problems became evident in both of these mission modes, and these shortcomings threatened to slip the success of the project past the deadline set by President Kennedy. A major headache was the sheer size of the Nova rocket. Building, transporting, fuelling and finally launching such a gargantuan rocket was becoming difficult to comprehend. One engineer put it in plainer terms: "It would have damn near sunk Merritt Island.” Contractors had to make a start on building the launch facilities, and the type of launch vehicle would be crucial to the layout. One of the larger Saturn derivatives on the drawing board, the C-5, itself around 36 storeys tall, seemed to be a much more sensible solution. This vehicle was later renamed Saturn V, pronounced as ‘Saturn Five’.

Both direct ascent and EOR envisaged sending a single large Apollo spacecraft to the Moon, and its shape and layout were proving to be an equal headache. Seated in a heavy conical Apollo command module mounted at the top of a huge rocket – powered landing stage, the crew would find that all their windows looked towards the sky when, like all pilots, they would rather look down at their approaching landing site. It slowly dawned on the potential astronauts that the CM’s shape could hardly have been less suited to a lunar landing.

At this time, Gilruth’s Space Task Group was based at NASA’s aeronautical centre in Langley, Virginia. Another research group at Langley, who were studying possible trajectories to the Moon, had pointed out the huge weight savings that could be made by using a lunar parking orbit within the mission. In parallel with

Lunar orbit rendezvous 11

engineers at Vought Astronautics, they devised a daring but highly efficient means of travelling to the Moon using only a single Saturn C-5 vehicle. It was this third mode that eventually won the day and became America’s path to a new world.

Launched on 22 January 1968, Apollo 5 is the flight that history treats almost as a footnote. It was neither manned nor did it have the remarkable Saturn V as its launch vehicle. It used the AS-204 launch vehicle that had been intended to lift Apollo 1, but it is important to the story because, as a В-mission, it tested the first Apollo lunar module, LM-1. The test allowed engineers to verify the lunar module’s structure and its response to the launch environment, and it gave them their first

opportunity to test the spacecraft’s two engines in the space environment.

In the case of the ascent engine, it was NASA’s first opportunity to try out a fire-in-the-hole burn when they ignited the ascent engine just as the descent stage was being jettisoned. In their effort to give crews the best possible chance of escape from any reasonable failure of equipment, the LM’s designers planned that if the descent engine should fail while a crew were descending to the Moon, the ascent engine should fire and lift the crew back to the safety of an orbit. For this to happen, its engine would have to ignite while the descent stage was still in place. Despite some problems, the legless module successfully demon­strated everything that was asked of it,
and a second В-mission was cancelled. The second test lander. LM-2, is now on display at the National Air and Space Museum in Washington DC. 1’he next spacecraft to fly, LM-3, would be entrusted with the lives of two men.

Three weeks prior to launch brought rehearsals for launch day. The most important of these was the Countdown Demonstration Test. Simply put, the Countdown Demonstration Test was a complete stab at preparations for the launch of the space vehicle up to, but not including, the ignition of the F-l engines of the first stage. The spacecraft was fully powered, fuelled with its highly toxic propellants, and occupied by the prime crew. All the Saturn’s propellants were loaded according to plan, including the cryogenic hydrogen and oxygen, and all the tanks were pressurised. Everyone in the nearby launch control room and at the Mission Control Center in Houston were at their consoles, each ensuring that their system was operating within limits. This attention to detail and procedure paid off by the excellent record the launch team would attain throughout the Apollo/Saturn period.

Countdown to launch

The numbers that express the scale of the Saturn V are often quoted: 110 metres tall, 10 metres wide, weighing about 3,000 tonnes at launch. But there was something about it which surpassed quantitative expression. The Saturn V was a sleek, white, slender ship that rose to the heavens like no other machine before or since. It was not only
functional; it was beautiful and seemed to be perfectly styled for the task of taking mortals to heavenly realms.

Moreover, its beauty was set against the ugliness of the steel towers that nursed it to the point of its departure.

Chock full of extreme technologies, this ship hid many ways to kill or injure the men who would ride it. Yet it, and the smaller Saturns that served with it – all swords turned to ploughshares; peaceful ships derived from military technology – had an excellent record of success, in some cases flying on in the face of failure and danger to accomplish their peaceful goals.

The launch of a Saturn V was orchestrated around the familiar countdown, a timeline leading up to the moment of launch and beyond, during which everyone and everything associated with getting the rocket off the ground coordinated their tasks.

German film maker Fritz Lang is usually credited with introducing the concept of the countdown as a device to raise suspense in his 1929 film Frau im Mond (The Girl in the Moon). It was adopted by the rocket pioneers in the German rockery club, the VfR, who maintained its use after their move to the United States.

The countdown was not continuous as it progressed towards the launch. At preplanned points it was deliberately paused to allow engineers to catch up with tasks and resynchronise their preparations. In many cases, these holds allowed small technical gremlins to be analysed and rectified. If a problem seemed to require a longer time to correct, a hold could be extended, but only up to the point where the delay would push the time of launch beyond acceptable limits.

Although the countdown has been retained in the American rocket industry, its precise implementation can vary. In the case of the Saturn V, the descending count eventually led to the point where the vehicle left the pad. With other rockets, such as the Titan II that lifted the Gemini spacecraft to orbit, the zero point was when the engines were ignited.

The job of the S-IC and its five F-l engines was to lift the stack to an altitude of 70 kilometres and accelerate it to a speed of about 8,500 kilometres per hour. As it did so, the acceleration felt by the crew gradually increased. In common parlance, acceleration is stated in terms of g-forces, because the force we feel on Earth due to gravity is directly comparable to the force imparted by the acceleration of a vehicle. Therefore, it is useful to relate acceleration forces to something of which everyone has a lifetime’s experience. In this manner, when the Saturn was sitting on the pad, the crew felt an acceleration of 1 g, due entirely to Earth’s gravity.

Because the Saturn V weighed down on the launch pad with almost as much force as the engines were pushing up, the stack initially rose quite gently with g-forces barely above 1. But the consumption of over 13 tonnes of propellant per second lightened the vehicle considerably as it flew. This decreased the mass the engines had to push against and, since they did not throttle, increased the acceleration forces imposed upon the crew.

A small additional source of rising acceleration came from an improvement in the efficiency of the F-l engines as they rose through the atmosphere. A rocket engine works by burning propellants in a combustion chamber. The heat of combustion causes the gases to expand very rapidly and this exerts massive pressure on the walls of the chamber. If one of those walls is missing (because someone has placed a nozzle there), the pressure within the chamber becomes unbalanced, resulting in a force.

Graph of g-forces during first stage flight. (Redrawn from NASA source.)

However, at sea-level, the pressure of Earth’s atmosphere has the effect of slightly capping the open end of the nozzle, somewhat inhibiting the high-speed flow of exhaust gases and reducing the thrust that the engine can generate. By the time the virtually empty S-fC gave its final push, the atmosphere had become essentially a vacuum, which reduced the back-pressure against which the exhaust gases had to contend as they left the nozzle and this had the effect of improving the thrust by almost 20 per cent. Each engine, which had started out with a thrust equivalent to 690 tonnes, was pushing with 815 tonnes force just prior to the exhaustion of the first stage.

In response to these two effects – an increasingly light S-fC and five increasingly efficient engines – the acceleration continued to ramp up ever faster until about 2 ‘A minutes into the flight when, having reached nearly 4 g, it was reduced by the shut­down of the S-fC’s centre engine to a little over 3 g. Under the power of the remaining engines, the g-force resumed its rise towards 4 g as the vehicle lightened further. The early shut-down of one engine not only curtailed the rising acceleration, it also lessened the jolt felt by the stack and the crew when the remaining engines cut out. Additionally, it flagged the Saturn’s computer to think about starting Timebase 2, which eventually began once a specified speed had been gained. The computer then sensed propellant levels in the nearly empty tanks and prepared to shut down the outboard engines. By the time the stage was expected to shut down, about 25 seconds after the centre engine cut-off, the acceleration would again be approaching

4 g.

When Apollo was blazing its pioneer­ing trail to the Moon, the nascent space industry had yet to set up a comprehensive, worldwide communi­cations network using geostationary satellites and ground stations. It would take the efforts of another generation to arrange an infrastruc­ture that would allow crews to at least talk to mission control at any point in their orbit. Apollo crewmen could talk to mission control only for intervals of up to seven minutes at each ground station as they passed over a scattering of them along their orbital path. As with many aspects of Apollo, the exact configuration of these stations changed from mission to mission as operational experience was gained and priorities changed.

Early missions supplemented their coverage with extra ground sites. A scattering of specially equipped ships filled the gaps between the main sites.

Stations were sited on islands or on board ships strung across the Atlantic Ocean leading from Cape Canaveral to provide coverage for the ascent to orbit. A station on one of the Canary Islands off the coast of Africa permitted communications on the opposite side of the Atlantic, and another on Madagascar was used during the early missions for coverage heading out over the Indian Ocean. An outpost near Canberra in eastern Australia gave coverage on the opposite side of the world. An important station was set up on Hawaii, in the middle of the Pacific Ocean, which covered at least part of the spacecraft’s departure for the Moon. This was supplemented with ships and Apollo range instrumentation aircraft (ARIA) which filled in the gaps before a siring of stations across the continental United States gave constant coverage to the Atlantic. The ARIA were EC-135 jets – similar in structure to the Boeing 707 jetliner that were specifically equipped to support Apollo communications by relaying voice and recording telemetry.

During each short period of communication, data about the state of the crew and spacecraft were exchanged with updates from mission control. Another vital job for some of the ground stations at this time was to use large radar antennae to track the speed and position of the spacecraft as accurately as possible by reflection off its skin. This refined mission control’s knowledge of iis trajectory; information that was necessary to ensure an accurate burn towards the Moon. In particular, the station on the Canaries could provide an initial orbital determination and Carnarvon in Australia refined the determination antipodal to insertion.

When Neil Armstrong and Buzz Aldrin went outside the lunar module Eagle for their historic moonwalk, one of their tasks was to place a seismometer on the surface that would study moonquakes after they departed. However, the project to produce this instrument was conceived in a hurry. Its power came from two small panels of

The apparent brightness of astronomical objects is slated in magnitudes. A bright star is about magnitude 0. one at the limit of human eyesight is magnitude 6 while the faintest star visible with an Harth-based telescope is about magnitude 25

solar cells and, unfortunately, although it had small radioisotopic heaters, it was seriously damaged by the chill of its first lunar night. It was turned off during the next lunar day.

It fell to the next crew, from Apollo 12, to place on the Moon the first full science station, known as ALSEP, which included a seismometer that drew its power from a self-contained power unit. Subsequently, all missions that reached the Moon’s surface, with the exception of Apollo 17, emplaced seismometers to create a network of stations spread across the near side. From Apollo 13 onwards, all S-IVB stages were steered onto trajectories that led to a violent end, each forming a new crater on the Moon’s surface.

Flight controllers had two major sources of propulsion with which to control the trajectory of the spent S-IVB. The two APS modules had some leftover propellant, and there was still a small quantity of LOX that could be jettisoned through the J-2 engine nozzle under pressure from whatever heat was leaking into its tank. Minor additional thrust could be achieved by dumping the remaining hydrogen from the fuel tank and the helium gas from the pressurising system through two propulsive vents.

Control of the nearly dead stage was seldom very accurate and controllers never brought their rocket stage down on the Moon closer than 150 kilometres from the planned target. Nevertheless, they were able to track them accurately to their end and the impacts pro­vided lunar geologists with seismic events of known energies occurring more or less in known locations.

With each successful S-IVB impact sending lunar shockwaves to in­creasing numbers of seismometers, the quality of information that could be derived from the travel time of the sound waves improved.

The final network could provide triangulated readings from any im­pact, natural as well as those due to the S-IVBs and the discarded ascent stages of the lunar modules, yield­ing detailed information about the The lunar crater formed by the impact of Apollo Moon’s interior. 13’s S-IVB north of Mare Cognitum. (NASA)