Category How Apollo Flew to the Moon

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)

The designers of the Apollo spacecraft were always careful to build redundancy into their systems to ensure that a single point failure could not put the crew in jeopardy a philosophy that extended to the guidance and navigation system. The designers were very aware that any of its exotic components could fail at any point in the mission. To this end. the command module had a second control system which, although it shared many components with the G&N system, could operate entirely independently. This stabilisation and control system (SCS) could maintain attitude and allow’ the crewr to make accurate manoeuvres and, if necessary, even manually control the SPS engine. Like the G&N system, it used gyroscopes, but these w:ere arranged in a different way to the gyroscopieally stabilised IMIJ.

The gyroscopes for the SCS were not attached to a stabilised platform. Instead, they were fixed to the spacecraft structure and therefore had to move with it. Being mounted in this way prompted the name body-mounted attitude gyros (BMAGs). Like all gyros, they had a tendency to want to remain in one attitude, and when the spacecraft rotated, they exerted a force on their mountings. As this force was a measure of the rate of rotation, the BMAGs were very suitable for measuring how fast the spacecraft was rotating rather than yielding absolute attitude. However, by processing the rate information within electronic boxes it was possible to derive the absolute attitude. The resultant values for attitude were highly prone to drift, much more so than those from the IMIJ, so it was important to regularly realign them to match. Prior to intensive use of the SCS. the crew would press a button to update the BMAGs’ electronics with the spacecraft’s attitude from the IMlJ’s platform. If the IMU ever became unusable, the crew had an emergency procedure whereby the attitude information from the BMAGs could be aligned by sighting on the stars.

One of the regular tasks for the CMP was the perfectly routine stirring of the service module’s tanks that contained the cryogenic oxygen and hydrogen reactants for the fuel cells. Each tank was essentially an efficient vacuum flask whose contents were best described as being a very dense fog rather than a liquid. As the gas was drawn off for the fuel cells or for the cabin air, the pressure in the tanks reduced slightly. If gas pressure falls but the volume stays the same, then according to the gas law that shows how pressure, volume and temperature are related, the temperature will also fall. Therefore, electrical heaters, w’hich could be switched on automatically or manually as required, were installed to help to maintain the tanks at their operating pressure.

Two long devices ran through the middle of each tank. One was a set of heating elements wrapped around a supporting tube. Two fans w’ere mounted, one at either end of the tube, to stir the tank’s contents. The other was a probe that determined the quantity of gas remaining in the tank. It consisted of a tube within a Lube and it measured the electrical characteristics across the gap between – a quantity known as capacitance. The capacitance of the probe depended on the density of the gas between the tubes, and this could be calibrated to infer how’ much gas was present in the tank. However, in the weightless environment of space, the gas tended to gather in layers of differing densities against the probe, which skewed the readings. This was where the fans came in. At regular intervals, they were switched on to stir the contents of the tanks in order to homogenise its density and allow’ an accurate reading. When EECOM Sy Liebergot asked Capcom Jack Lousma to ask CMP Jack Swigert on Apollo 13 to stir the tanks in Odyssey s service module almost 56 hours into the mission, the result became part of popular culture.

Over the final few hours before they entered lunar orbit, the Apollo crews worked through an exhaustive series of checks and adjustments, interrogating the spacecraft’s systems about their ability to sustain life while in the Moon’s clutches, and on the engine’s readiness to do its job properly.

Another important task in the build-up to LOI was to change the spacecraft’s knowledge of which way was ‘up’. During the five or six minutes of the burn, the crew’ would want to avoid any appreciable errors in the direction of the engine’s thrust. Additionally, they needed to ensure that the guidance system could measure the effect of the burn on their velocity. As was usual before a burn, the CMP performed a P52 to check the alignment of the guidance platform, but this time special procedures were applied. Up to this point, the platform had been aligned with an orientation that suited the coast to the Moon and made the barbecue rotation easier to set up and maintain. Now the platform would be realigned to match a new

REFSMMAT[3] that suited the LOI burn, and so obviously it was known as the LOI RHFSMM AT.

First, the CMP carried out a realignment to refine the platform’s orientation in terms of the REFSMMAT they had been using. This yielded a measure of its inherent drift, a parameter that was always carefully monitored and no opportunity was missed to gain another data point. Once the amount of drift had been measured, the platform’s orientation was torqued around align with the new RFFSMMAT. This one had been chosen to match the attitude in which the spacecraft would make the upcoming burn. By lining up the coordinate systems of the platform and the spacecraft, the crew’s job of monitoring attitude during the burn became a lot easier. Their 8-ball attitude indicators would now read zero on all three axes w’hich made them much simpler to interpret, к is wise to be certain that your ship is pointing in the correct direction when you make major engine burns near planets (especially ones without atmospheres) as a mistake can lead to a crash.

Next, they put the entry monitor system (FMS) through a test to demonstrate that it could still accurately measure the change in speed brought about by the burn. This feature of the FMS, its ‘Dclla-v‘ display, w-as one of the redundant methods by which the engine could be commanded to shut down once it had achieved its task.

The spacecraft’s cooling circuits w’ere next to be checked. At first glance this may appear to be one of the less exotic systems, but if any flaw were to be found in cither the main or the backup circuit – especially if any leaks had formed in the radiator pipes due to micromcLcoroid damage – the crew would return directly to Earth.

More checks followed w’hich covered the caution and warning system, the tanks and valves associated w’ith the manoeuvring thrusters on both the service module and the command module, and the spacecraft’s supplies of oxygen, water and power. Once these essential tasks had been completed, the crew could begin to implement the burn itself.

Any journey in space is heavily influenced by the propellant available to achieve it. At the same time, the amount of propellant required is largely determined by the mass of the object that is to make the journey and how quickly the journey has to be undertaken. In simple terms, mass is everything. The alternative scheme, known as lunar orbit rendezvous (LOR) sought to limit the amount of mass that had to be propelled at each key point in the journey. A reduction in the quantity of propellant required for the Apollo spacecraft would also minimise the initial mass that would begin the journey, and thus bring the entire mission within the capability of a single Saturn C-5.

The advantages are best understood by working backwards through a mission. The only part of the spacecraft that could return to Earth was the heatshield – protected command module. To propel it out of lunar orbit required the propulsion capability of the service module and their combined mass defined the amount of propellant required for the task. Next, instead of taking a lot of redundant mass down to the Moon’s surface just to bring it up again, a dedicated lander would be designed specifically for the task, leaving the Apollo mothership, the CSM. in lunar orbit with the consumables and propellant to get home. This lander would only Lake two of the crew down to the surface, leaving the third to take care of the CSM. Moreover, there was no need for the engine, landing gear and the empty tanks that had enabled them to land on the surface, to come back up to lunar orbit. The crew with its gathered lunar treasures could return to the mothership in only the Lop part of the lander using a smaller engine and the propellant required for the task. As there would be no need to bring this remaining part of the lander back to Earth, it, too, could be discarded at the Moon. Therefore, the final propellant load for the CSM was made up by the fraction required to get the entire assemblage into lunar orbit, plus the fraction required to get itself to Earth. At each key point in the journey, the engines would work against only the mass that was absolutely necessary, and everything else would be discarded when its function had been fulfilled.

The cumulative weight savings made the LOR scheme highly attractive in engineering and cost terms, but it caused NASA to face certain operational realities which, in the early days of space flight, seemed daunting. As with EOR. having separate spacecraft meant learning how to rendezvous in orbit when both were travelling at what wrere then perceived to be incredible speeds. The ships would have to join together, or dock, to allow crewmen and cargo to transfer from one craft to the other. Neither of these techniques had yet been demonstrated in Earth orbit, but the LOR concept was calling for them to occur nearly half a million kilometres away in the lonely vicinity of the Moon. A failure of the rendezvous would doom the occupants of

the lander to certain death in lunar orbit, while a failure of the docking would require crewmen to don spaccsuits and move from one craft to another by going outside. At a time when no one knew what challenges the weightless environment would present to a crewman in a bulky pressure suit, this seemed to be a very risky thing to do.

Many in the burgeoning space community were aghast at the audacity of LOR. It seemed foolhardy and dangerous. However, convinced of the benefits, and with an almost religious /.cal, its leading advocate, John Houbolt. drove through layers of NASA bureaucracy and the entrenched positions of its various centres, in an effort to convince the organisation that there was little chance of getting to the Moon within the decade unless LOR was adopted.

NASA debated the mode issue for more than a year after Kennedy had laid dow n his challenge, during which Lime, direct ascent and its incredible Nova launch vehicle was largely discarded, leaving EOR, championed by von Braun, and LOR, which, because it included a specialied lander, had become Gilruth’s preferred option, as the competing schemes. As work on the spacecraft could not begin in earnest until the matter was settled, Joseph Shea from NASA headquarters asked each side to report on the other’s scheme – a management strategy that enabled von Braun to recognise the benefits of LOR. In June 1962, at a large meeting at Marshall, NASA acceded to Iloubolf s campaigning and chose LOR as the means by which they would get to the Moon.

With the mission mode settled, the definition, design and construction of the spacecraft could begin. The command and service modules would be built by North American Aviation. These craft were already well into their initial development, but their role could now’ be precisely defined; there being no need for a landing stage on the SM, for example. Major components for the SM had already been designed. It was decided to leave the thrust of its propulsion system at its original design value and Lake this capability into account in mission planning. Two versions of the CSM were to be built. The Block I spacecraft would be incapable of supporting a mission to the Moon, but w’ould allow experience to be gained in Earth orbit until the Block II became operational. The Block II would be the Moonship proper. Complete with fuel cells for power, hardware for docking, deep-space communications and a fully capable guidance and navigation system, the Block II CSM would be the linchpin in the Apollo story, ferrying a spidery landing craft to another w’orld. In a sense, the CSM was a mini-planet, providing everything three men w’ould need for Lw’o w’eeks in space during which they would undertake a journey that had been a dream of humans over the ages. In the event, the design of the Block II w’ould be forged in the lessons learned from the fatal flaw’s that would prevent the Block I from flying a manned mission.

By the spring of 1968, with two flights completed, the Apollo programme seemed to be hitting its stride. It had demonstrated all three stages of the Saturn V worked, the command module had survived its high-speed re-entry, and an early version of the lunar module had performed satisfactorily. Before the Saturn V could be declared fit to carry astronauts, a second А-mission was required. This flight was named Apollo 6 and. once again, events unfolded that threatened to stop the programme in its tracks.

After a successful lift-off on 4 April 1968. the first problem appeared towards the end of the SIC’s flight. Rockets have always been prone to vibrations along their length, but for about ten seconds immediately before the first stage was to shut down, the longitudinal shaking of the entire vehicle (known as pogo) became alarming. Meanwhile, at the front end of the rocket, a conical aerodynamic shroud that would normally protect the lunar module (not carried on this flight) was losing chunks of its outer surface. Since this section had to support the mass of the CSM multiplied by the g-forces of acceleration, its structural integrity was of some concern.

Halfway through the flight of the S-II stage, one of its five J-2 engines began to falter, prompting the instrument unit to shut it down. As it did so. another engine that had been showing no distress also shut down, causing the thrust from the other three to be applied asymmetrically. Considering that the Saturn’s control system had been programmed only to deal with a single-engine failure, it did a remarkably good job of compensating for the off-axis thrust and burned the remaining engines for longer on the residual propellant. The first burn of the S-IVB third stage successfully pul the vehicle into orbit, but a subsequent command to restart the engine failed. Some of the flight’s objectives were met, but if the problems could not be fixed, NASA would not dare to put men on top of the next Saturn V. as was being considered instead of a third Л-mission test.

In the event, engineers managed to find solutions for all these problems. The first stage vibrations were suppressed by the addition of helium gas to cavities in the LOX feed lines, which damped out pressure oscillations. Elaborate tests on the J-2 engine discovered a design fault in a liquid hydrogen fuel line that had not only caused one of the engines on the S-II to shut down but also prevented the S-IVB from restarting. Compounding the S-II problem, a wiring error had sent the shutdown command from the Saturn’s instrument unit to the wrong engine, shutting it down unnecessarily. The aerodynamic shroud had failed because frictional atmospheric heating as the rocket went supersonic caused trapped moisture and air within its aluminium honeycomb sandwich skin to expand, in turn causing the skin to peel off in sheets. This problem was remedied by making small ventilation holes in the shroud’s skin and adding cork insulation.

The launch vehicle issues apart, the CSM-020 spacecraft successfully performed a number of remote-controlled manoeuvres and was recovered from the Pacific Ocean. Preparations for Apollo 7 continued because it would use a Saturn IB launch vehicle. It was decided that if this mission went well, the third Saturn V would indeed carry a crew.