Category How Apollo Flew to the Moon

The measurement of radial velocity relied on the Doppler effect, whereby the movement of a transmitter altered the received pitch or frequency of whatever wave was being transmitted. This is familiar to most people as the change in pitch of a constant note, perhaps from an engine or a horn, as it passes the listener. We hear its pitch drop as it passes by and departs. The same effect applies to radio transmissions where the frequency of the received signal changes slightly according to whether the transmitter is approaching or receding from the receiver. This frequency can be accurately measured and, by knowing the precise frequency with which it was transmitted, the velocity of the transmitter can be calculated. The system is similar in some respects to radar speed traps used to catch speeding car drivers.

To use the Doppler effect well, the frequency of the transmitted signal from the spacecraft had to be known with great precision. Onboard equipment to generate a signal of sufficient accuracy – one whose frequency was precise and stable despite the thermal extremes of space, w’ould have been excessively heavy and power-hungry. However, an elegant solution existed that kept the heavy equipment on the ground, yet could yield a measurement that was inherently more accurate.

Given that a powerful, aeeurate radio signal would in any case be sent to the spacecraft to carry voice and data from mission control, engineers simply arranged that it be modified on board in a known way. and retransmitted back to the ground, this lime carrying voice and data from the spacecraft. If the frequency of the signal from Earth was precisely known, then so was that from the spacecraft if it were not moving.

For Apollo, the ground station transmitted data and voice signals from mission control to the spacecraft on a carrier signal called the uplink. This carrier was synthesised from a very accurate frequency standard installed at the station. Ground stations supporting the Apollo programme had some of the most accurate frequency standards available at the Lime. For the CSM, the carrier had a frequency of 2.106.4 MIIz while that for the LM was 2,101.8 MHz. On reception by the spacecraft antenna, an onboard transponder Look this signal, multiplied its frequency by the ratio of 240/221 (about 1.086) and sent it back to Earth, using this new signal as the carrier for the downlink.

When received by the ground station, the precise frequency of the downlink was measured and compared to the uplink. If the precise 240/221 relationship was maintained, the spacecraft was neither approaching nor moving away from the ground station, such as when moving across the face of the Moon perpendicular to the line of sight to the ground station. A higher received frequency meant that the spacecraft was approaching; a lower frequency indicated receding motion. This was a very powerful system because it measured Doppler shift over both the up and down legs of the signal’s journey, doubling the sensitivity of the system to the point where it could even detect the velocity change caused by the minuscule thrust that was generated when the crew’ dumped their urine overboard.

Buried within the pie-shaped struc­ture of the service module were two (later three) tanks each of oxygen and hydrogen. Although these two sub­stances are excellent propellants for rocket engines, in this case propulsion was not their purpose.

A common notion is that space­craft usually derive their electrical power from the Sun via large arrays of photovoltaic cells. While this is

generally true for automatic spacecraft in the inner solar system and for the International Space Station, the high power demands of a typical Apollo flight would have required such large panels as to make them cumbersome. This size would not have been a problem during a coasting flight, but when the spacecraft’s large engine was fired, the mechanical stress from the aeeeleration would have required the panels to be folded away at the very time that their power was most needed. Л second alternative is to use storage batteries to bring eleetrical power from Harth. Although they could have supported a short flight, as they did for the early manned Gemini flights, they could not support the Apollo spacecraft for two weeks without being prohibitively heavy.

It was up to the Gemini programme to prove the concept of a third alternative, the fuel cell, as a source of clcctrieiiy for long-duration flights, f irst developed before the Second World War in Britain, the operation of the alkali fuel cell is remarkably simple. It acts like a battery by using the chemical reaction of two substances, in this case oxygen and hydrogen, and it makes the energy of the reaction available in the form of prodigious quantities of electricity. However, unlike a conventional battery, the reactants can be replenished constantly. As long as fresh reactants are fed past the electrodes the fuel cell does not run down. Even more remarkable is the fact that the waste product of this reaction is water that is sufficiently pure to drink.

The adoption of the fuel cell in Apollo therefore killed two design quarries with one stone. Not only did it produce lashings of electricity (a single fuel cell could generate well over one kilowatt of electricity at peak demand), the water it produced became a sort of lifeblood of the spacecraft. It quenched the thirst of the crew and rchydraicd their food in metered amounts through a pistol-style ‘squirt gun’ on the end of a hose. It also supplemented the cooling of the spacecraft’s electronic equipment by being evaporated into space, taking heat with it. Any excess was periodically discarded through an orifice in the spacecraft’s hull.

The high-energy reaction that occurs when hydrogen and oxygen are burned in the combustion chamber of a rocket makes it greatly favoured by rocket engineers. In the Apollo fuel cell, most of this energy was expressed as electricity, but although it could reach efficiencies of 70 per cent, the reaction still yielded significant amounts of heat. Some of this was used to warm the extremely cold reactants before they entered the cell; the rest was rejected through eight radiator panels around the upper circumference of the service module. An early version of the fuel cell flew on seven of the Gemini flights which gave engineers a chance to iron out the teething troubles with this promising technology. By the time the Apollo programme finished Apollo 13’s oxygen tank explosion notwithstanding – no Apollo flight suffered from a failure of their fuel cells. It was one of the many technologies and techniques for which the Apollo programme depended on Gemini to pioneer.

Among the limitations of the Apollo fuel cell was that it was very sensitive to the presence of impurities in the reactants. Even with hydrogen and oxygen of the highest purity that NASA could procure, the build-up of contaminants required that the cells be purged from time to time to avoid the resultant loss of electrical power. Oxygen purges were carried out daily, while hydrogen purges happened every second day. Three switches on the LMP’s side of the main display console allowed the gases to he routed to any of the three cells for this function, hlcclric heaters were included to ensure that the purging gas was warm enough to avoid it freezing the water in the cells.

Apollo’s implementation of black-and-white television w as, by far, the simpler of the tw;o systems, and the less greedy of radio bandwidth. The camera, as used on Apollos 8, 9 and 11 had just one imaging tube and operated at scan rates that would normally be called slow-scan television. The frame rate used was only 10 frames per second with 320 lines per frame. There was no interlace. The bandwidth of the signal (which dictated how well the image handled fine detail) was restricted to a very low value of 0.4 MHz (as compared to about 5 MHz for contemporary broadcast TV). The camera used a vidicon type of imaging tube that was notorious at the time for its excessive image lag, and this caused a ghostly smear to trail behind the moving image.

When the pictures reached Earth at this non-standard frame rate, they were electronically incompatible with just about every TV system on the planet. Converters were installed at chosen ground stations to generate standard US television signals from the lunar TV. The converter worked in two stages. The first simply consisted of another vidicon TV camera aimed at a small television screen. The screen displayed the images from the Moon at 10 frames per second while the camera, which ran at 60 fields per second, was allowed to capture the screen only when a full image had been completed, which was every tenth of a second. In other words, only one field in six from the camera contained a picture.

The second stage was to recreate the missing five fields. The single good frame from the TV camera was recorded onto a magnetic disk which then replayed it five times to reconstruct the full 60-fields-per-second TV signal, ready for distribution to Houston. The repetition of the fields and additional lag from the second camera added to the ghostly impression left by Apollo ll’s moonwalk coverage.

A question that is often asked is, if Neil Armstrong was the first man on the Moon, who operated the TV camera? ft is a spurious question because it assumes that all cameras must have a cameraman behind them. In fact, Eagle’s camera was mounted inside a fold-down panel next to the ladder. At the top of the ladder, Armstrong pulled a lanyard to open the panel and thereby reveal the camera. The most ergonomic and lightweight way to mount the camera on this panel was upside – down, so on Earth, the conversion equipment had a switch which the operator flicked to right the upside-down picture. Once both crewmen were on the surface, Armstrong lifted the camera from its mount and placed it on a stand from where the TV audience could watch proceedings. The operator threw his switch back to restore the image’s orientation.