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3S E RI E S

COD W M 0 . I A l l l l l V d-

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AN EDUCATIONAL PUBLICATION OF TH ENATIONAL AERONAUTICS AND SPACE ADMINISTRATION

I s-41 13-67

J--The Countdown9~c&-

Fi-ecise checks ar e niaiiitairied w r i ai i piiases of iiie couriidowri by experts seaiea at teievision controi paneis such as this athe Mission Control Center. The countdown can be halted at any moment for as long as might be required to make a fix. If thfix i s to require a longer time than is permissible in the flight schedule for that day the mission is cancelled. After the fix is madthe mission is rescheduled and a new countdown begun:The countdown from 10 t o 0 immediately before abig space vehicle is launched has become a famil iarand very excit ing occurrence, thanks to televisionand radio. But this count measures only the lastten seconds before a launch; there is a pre-coun tstart ing as early as three days before launch, amid-c ount start ing about a day ahead and thena terminal count for the last few hours.Preparations for launching a giant Atlas, Gemini(Titan II), or Saturn launch vehicle are very exact-ing and take a great deal of time . Every system ,component and part i n the vehicle must be checkedthoroughly and declared ready so that proper func-t ion in space can be anticipated. I f even just onetiny bolt isnt in place just as it should be, then

the countdown is stopped un t i l it is in place. Consequently, it is easy to see that the last few dayand hours on the launch pad include a busy, constant schedule of f inal checks. This entire perioof t ime is cal led the countdown.A l l t imes before launch are called minus t ime,and those after are denoted as plus t ime. ThusT minus 2 minutes means that two minutes rema in before launch; T plus 9 seconds means thalaunch occurred 9 seconds earl ier. The minucountdown ends with T-t ime or zero t ime, wherocket engine igni t ion and l i f t -of f occur . Af ter Tt ime, t he countdown cont inues into the p lus counKeeping t rack of th is v i ta l span of t im e is by nmeans l imited to operations at the launch pad

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NASA TRACKING FACILITIES

This is a map of the entire tracking and communications network NASA uses to track sounding rockets, satellites, mannedspace flights and unmanned deep space missions. These stations are all located on land sites. I n addition, for projects suchas the Apollo lunar landi ng mission, numerous aircraft and ships will add air and sea area communicat ionsand tracking stations.These additional stations give NASA experts direct li ne-of-sightcommunications capabilities at all times.

0 MA NN ED SPACE FLIGHT NETWORK 4 SATELLITE T UC KI NG AND DATA ACQUISITION NETWORK A DEEP SPACE IN STlU MEN TATi ON FACILITIES 0 OPTICAL TUC Ki NG YETWORK

Stat ions on the t rack ing network around the worldare alerted well in advance and are kept not i f iedof the exact count at al l t imes. This is very import-ant because the t rack ing s tat ions must be tuned inand ready to report the pos i t ion and communicatewith the space vehicle as it passes on each revo-lut ion.To assure mission success, every func t ionin g partof the spacecraft and i ts launch vehicle must besynchronized-timed perfec t ly to operate in har-mon y with al l other p arts. There is a well planned,minu te-by-minu te wri t ten schedule of s tart ingevents for p repara t ion of th e rocket launching. Dur-ing this per iod the many t iny motors and otherdevices are checked and swi tched on in turn . There-fore, the countdown is actual ly a checkoff l ist

used so that launch operat ions may be performedwith accuracy. Everything to be done i s wri t tendown opposi te a set t ime for that act ion.A t any t ime dur in g the countdown, it is possiblet o i n troduc e a delay or hold. I f a repair , adjust -ment , or replacement is needed in the spacecraftor the launch vehic le, or i f there is a problem some-where on the wo rld-wide t rack ing network, the coun tis s topped for whatever t ime is needed for correc-t ive act ion. For example, at T minus 2 hours, 6minutes, an adjustment to the engine plumbing m aybe needed. If it is a major adjustment indicat ingan hour or more for repair , the countdown wi l l bestopped at that m inute. Later, when repairs arecomplete, the countdown cont inues or resumes,beginning at T minus 2 hours, 6 minutes. In case

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the delay causes a necessary reche cking of otheroperat ions al ready performed, the countdown maybe recyc led, or turned back to al low more testt im e before launch. For example, the countdownmay be reset to say, T minus 2 hours, 36 minutes.During the last minute the countdown is read of fin seconds. This last minute inc ludes a large num -ber of s ta rt ing funct ions no t only in the rocket butalso in the ground equipment , in the immediatelaunch area, and throughout the world-wide bandof t ra ck in g s tat ions. Thus, at the mome nt of launch,there is assurance that every thing has been checkedand is in go condi t ion.Af ter l i f tof f , another equal ly important wr i t tenschedule for al l events tak ing place dur ing f l ightcomes into use. Exact t im in g is also extremely im -portant af ter the rocket is launched. Whi le therocket is on the launch pad, a delay may be easi lycal led to correct a m al funct ion or to resynchronizesystems, b ut after it is in fl ight proper t ime funct ionmust be automat ic.

Countdown procedures di ffer for var ious k inds ofspace vehicles. A ma nned vehicle usu ally is readytor f inal checkouts about 48 hours before launch;the checkout i n th is case involves more compl icatedfunct ions, and far more varied parts. When thereis a countd own for f l ight to the moon-manned orunmanned-it may extend into days before launch .For unmanned research and development spacevehicles, countdowns may last as long as 8 t o 10hours.The service structure includes a series of plat-forms mounted one atop another on a movablest ructure. These plat forms prov ide work ing levelsso that ground crewmen may have access to a l lparts of the vehic le. The men work ing f rom theplat form are known as the launch operat ionscrew, because the ir operat ions on the vehicle du r-ing this cr i t ical t ime great ly determine ei ther thenecess ity for holds or the go-ahead s ignal . Probably the most cr i t ical , and the most exc i t -ing, moments of the ent i re countdown procedure

are those of the last s ixty seconds before launch.This cr i t ical last minute is announced al l over thelaunch area. Loudspeakers and intercom s blarethe magic sound T minus 60 seconds! It ispassed on to outposts a l l over the wo rld-trackingstat ions, communicat ions networks, telemetry postsof al l k inds. And of course it is announced onmi l l ions of radios and telev is ion sets throughoutthe w orld.This second-by-second procedure inc ludes manyexact ing processes. Al l moto red or electrical de-vices necessary to the funct ioning of the ent irevehicle system m ust be started. These ignit ion smust be accompl ished in proper sequence andt im ing before launch. Cont inuous checks of fueland oxidants are needed for quanti ty, temperature,and stabi l i ty.Often, especial ly during a manned f l ight , the lastmome nts before T-t ime are controlled by an auto-matic sequencer. This is a ful ly autom atic systemwhich monitors in spl i t seconds the step-by-stepprogression of the f inal countdown events. It workson the principle of perfect sequence of act iv i t ies;it wii i stop the iaunch operat ions i f an event occursout of sequence. Autom atic sequencers great ly addto the safety of launching, because they h elp avoidhuman errors and overs ights dur ing the last sec-onds of the countdown, when the swi tching se-quence is rapid.Af ter the last 60 seconds have elapsed and therocket i s safely on i ts way, th e countdo wn of plust im e begins. Engine igni tions and cutof fs are t ime dto th e second in order tha t the correct veloc ity andfl ight path may be maintaine d. Each step of theent i re f l ight i s scheduled according to the f l ightplans. So, the plus t ime countdown cont inuesthroughout the f l ight , and ends only when the miss ion is completed. This plus t ime is cal led groundelapsed t ime by space f l ight control lers.QUESTIONS:1. When does the countdown begin and end?

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2. Why is it important for various stations al l overthe wor ld to be a ler ted to the countdown proce-dures?3. What is th e checkoff l ist?4. What i s meant by hold and recycle?5. Explain the automatic sequencer, and the rea-sons for i ts use.ACTIVITY:Make u p a l ist of actions, each taking only a secondor two, such as closing a book, moving a penci lf rom one p lace t o another, sett ing an object in acertain place. First, t im e three short operationsby a stopwatch t o ca lcu late the min imu m t ime foreach. Then ma ke up a t imetable inc luding abouta ha lf dozen of these activi t ies, beginning each a ta particu lar second. Set the stopwatch for, say, 20seconds; set each dut y beside a certa in second, andcount down from 2 0 by the stopwatch. I f two par-t icipate, have one count down, and have the otherperfo rm the actions as they are l isted. This activi tyi l lustrates the precision with which the various ac-tions in a countdown for launching a space vehiclemust be performed.REFERENCES:

Chapter I.First Edit ion, NASA SP-7.Grayson Merri l l , pages 162-3.

F i rs t Men to the Moon, by Wernher von Braun,Dictionary of Technical Terms for Aerospace Use,Principles of Guided Missi le Design, edited byDictionary of Guided Missi les and Space Fl ight.

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