George H. Diller                               October 19, 1992
          Kennedy Space Center
          407/867-2468


          KSC Release No. 145-92


          COUNTDOWN FOR STS-52 LAUNCH OF COLUMBIA BEGINS TODAY


               The countdown clock for  the  STS-52  flight  of  the  Space
          Shuttle  Columbia is scheduled to begin at 4 p.m.  (EDT) today at
          the T-43 hour mark.  The  countdown  includes  24  hours  and  16
          minutes  of  built-in  hold time leading up to the opening of the
          launch window at 11:16 a.m.  (EDT) on Thursday, October 22.   The
          launch window extends until 2:21 p.m. that day.

               At  the  start  of the countdown,  the launch team in firing
          room 3 of the Launch Control Center verifies the Shuttle  vehicle
          is  powered  up  and  that  the data processing system and backup
          flight control system are operating.  Flight software  stored  in
          the  orbiter's twin memory banks will be reviewed,  computer con-
          trolled display systems will be activated,  and the backup flight
          system general purpose computer will be loaded.

               Preparations  also  start  at  the  liquid oxygen and liquid
          hydrogen storage farms for loading the external tank  with  super
          cold,  or cryogenic,  propellants. The main propulsion system and
          shuttle main engines are prepared for cryogenic loading,  orbiter
          navigation  aids  are  turned  on  and  tested,  and the inertial
          measurement units are activated.

               In parallel with these activities,  the launch team prepares
          for  loading Columbia's onboard fuel cell storage tanks with liq-
          uid oxygen and liquid hydrogen reactants.  Also performed at  the
          start  of the countdown is the final stowage,  microbial sampling
          and water level adjustment of the crew waste management system.

               At T-27 hours,  the countdown will enter its first  built-in
          hold.  This is a 4-hour hold which extends from 8 a.m. to 12 noon
          on Tuesday,  October 20.  When the countdown resumes,  the launch
          pad  will  be cleared of all personnel in preparation for loading
          the fuel cell storage tanks.

               Servicing of the liquid oxygen and liquid hydrogen fuel cell
          supply tanks is scheduled to start at the T-25 hour mark. Servic-
          ing activities take approximately five hours.

               When servicing of the fuel cell tanks  has  been  completed,
          the pad will be reopened for normal work,  and the countdown will
          enter the second planned hold.  During this four-hour hold at the
          T-19 hour mark,  the orbiter mid-body umbilical unit used to load
          the super cold fuel cell  reactants  into  the  orbiter  will  be
          demated.    This  hold will extend from 8 p.m.  to 12 midnight on
          Tuesday.

               When the countdown resumes,  technicians will complete final
          vehicle and facility closeouts and begin activating the orbiter's
          communications  systems  and  configuring  Columbia's cockpit for
          flight.

               The orbiter's flight control system navigation aids and com-
          munications systems will be activated,  and switches in the cock-
          pit  will  be  configured  for loading of the external tank.  The
          stowable mission specialist seats will be installed in the flight
          and mid-decks.

               The countdown will enter the  next  built-in  hold  at  T-11
          hours  at 8 a.m.  on Wednesday.   During the hold,  time critical
          equipment will be installed in the orbiter's cockpit and the  in-
          ertial  measurement  units  will  be activated and warmed up.  At
          about 11 a.m.,  the rotating service structure will be moved away
          from  the vehicle to the launch position.  This hold is scheduled
          to last 12 hours, 56 minutes, or until 8:56 p.m. on Wednesday.

               At T-9 hours, the onboard fuel cells will be activated,  and
          the launch team will begin evacuating the blast danger area at T-
          8 hours, or about 11:56 p.m. Wednesday. At T-7 hours, 45 minutes,
          conditioned air that is flowing through the orbiter's payload bay
          and  other areas will be switched to gaseous nitrogen in prepara-
          tion for loading the external tank with super-cold liquid propel-
          lants.  The inertial measurement units will transition  from  the
          warm  up  stage to the operate/attitude determination mode at T-6
          hours and 45 minutes.

               The countdown will enter another planned  built-in  hold  at
          the  T-6  hour mark at approximately 1:56 a.m.  Thursday.  During
          this one-hour hold,  final preparations for loading the  external
          tank will be completed and a pre-tanking weather briefing will be
          conducted.

               Chilldown  of the lines that carry the cryogenic propellants
          to the external tank will begin at  the  T-6  hour  and  counting
          mark,  at 2:56 a.m. Thursday. Filling and topping of the external
          tank should be complete at the beginning of the planned  hold  at
          T-3 hours which will start at 5:56 a.m. Thursday.

               This  hold is two hours in duration.  During this time,  the
          ice inspection team will perform a survey of the tank's outer in-
          sulation,  and the closeout crew will begin configuring the  crew
          module  and  white  room  for the flight crew's arrival.   Liquid
          oxygen and liquid hydrogen will be in  a  stable  replenish  mode
          during this time to replace the propellant that "boils" off.

               The  STS-52  flight crew is scheduled to be awakened at 6:21
          a.m. Thursday and have breakfast. Their breakfast is scheduled at
          6:51 a.m.

               After eating,  the crew will receive a briefing  on  weather
          conditions  around  the world via satellite from Mission Control-
          Houston.

               The flight crew will  suit  up  in  their  partial  pressure
          suits,  then  leave the Operations and Checkout Building at about
          8:01 a.m.,  at T-3 hours.  They will arrive at the pad white room
          at about 8:31 a.m. where they will be assisted by white room per-
          sonnel in getting into the crew cabin.

               Just  prior  to  the  T-1  hour mark,  the test team and the
          flight crew will get another weather update,  including  observa-
          tions  from an astronaut flying in a Shuttle Training Aircraft in
          the KSC area.

               The last two built-in holds will be 10 minutes in duration
          and will occur at the T-20 minute mark or at 10:36  a.m.  and  at
          the T-9 minute mark or at 10:57 a.m.  During the final hold,  the
          flight crew and ground team receive the  NASA  Launch  Director's
          and the Mission Management Team's final "go" for launch.

               Milestones  after  the  T-9 minute mark include start of the
          ground launch sequencer;  retraction of the orbiter access arm at
          T-7 minutes,  30 seconds;  start of the orbiter's auxiliary power
          units at T-5 minutes; pressurization of the liquid oxygen tank at
          T-2 minutes,  55 seconds;  pressurization of the liquid  hydrogen
          tank  at  T-1  minute,  57  seconds;  and  the electronic "go" to
          Endeavour's  onboard  computers  to  start  their  own   terminal
          countdown sequence at T-31 seconds.  The orbiter's three main en-
          gines will start at T-6.6 seconds.


                                COUNTDOWN MILESTONES

                        Launch - 3 Days (Monday, October 19)

               Perform the call-to-stations at T-43 hours.  Begin check out
          of  the backup flight system and review flight software stored in
          mass memory units and display systems.  Astronauts arrive.

               Load backup flight system  software  into  Columbia's  fifth
          general purpose computer.

               Begin   stowage  of  flight  crew  equipment.   Inspect  the
          orbiter's mid and flight decks and remove crew module  platforms.
          Start  external  tank  loading  preparations.   Close payload bay
          doors.


               Launch - 2 Days (Tuesday, October 20)

               Enter first planned built-in hold at T-27 hours for a  dura-
          tion of 4 hours.

               Resume countdown. Start preparations for servicing fuel cell
          storage  tanks and begin final vehicle and facility closeouts for
          launch.

               Clear launch pad of all personnel and load liquid oxygen and
          liquid hydrogen  reactants  into  Columbia's  fuel  cell  storage
          tanks.

               Enter  second  planned  built-in hold at T-19 hours for four
          hours.

                 After the loading operation, the pad will be reopened for
          normal work and orbiter and ground  support  equipment  closeouts
          will resume.

                Demate  orbiter  mid-body  umbilical  unit used during fuel
          cell loading. Resume countdown.   Retract rotating service struc-
          ture.



               Launch - 1 Day (Wednesday, October 21)

               Activate orbiter communications systems,  flight control and
          navigation systems.  Install mission and payload specialist seats
          in  crew  cabin.  The  tail  service masts on the mobile launcher
          platform will be closed out for launch.   Pepare Shuttle main en-
          gines for propellant tanking and flight.

               Enter  planned hold at T-11 hours (8 a.m.) for a duration of
          12 hours and 56 minutes.

               Perform orbiter ascent switch list  in  crew  cabin.  During
          this  hold,  the orbiter's inertial measurement units will be ac-
          tivated and kept in the "warm up" mode and film will be installed
          in the numerous cameras on the launch pad.  In  addition,  safety
          personnel  will  conduct a debris walkdown and the pad sound sup-
          pression system water tank will be filled.

               The rotating service structure will be  moved  to  the  park
          position during this hold at 11 a.m.

                Resume countdown at 8:56 p.m. Install time critical flight
          crew  equipment  and  perform the pre-ingress switch list.  Start
          fuel cell flow-through purge.

               Activate orbiter fuel  cells.  Configure  communications  at
          Mission  Control Houston for launch.  Clear the blast danger area
          of all non-essential personnel and switch Columbia's purge air to
          gaseous nitrogen.


               Launch Day (Thursday, October 22)


               Enter one-hour planned built-in  hold  at  T-6  hours  (1:56
          a.m.).

               Resume countdown.   Launch team verifies there are no viola-
          tions of launch commit criteria prior to cryogenic loading of the
          external tank.  Start loading the external  tank  with  cryogenic
          propellants.

               Complete  filling  the external tank with its flight load of
          liquid hydrogen and liquid oxygen propellants.  Perform open loop
          test with Range Safety and conduct gimbal profile checks of orbi-
          tal maneuvering system engines.

               Perform  inertial measurement unit preflight calibration and
          align Merritt Island Launch Area (MILA) tracking antennas.

               Enter 2 hour hold at T-3 hours. Closeout crew  and  ice  in-
          spection team proceeds to launch pad 39-B.

               Resume  countdown at T-3 hours (7:56 a.m.) Complete closeout
          preparations in the white room and cockpit switch configurations.
          Crew departs astronaut quarters at Operations and Checkout Build-
          ing for the pad.

               Flight crew enters orbiter. Astronauts perform air-to-ground
          voice checks with Mission Control Houston.  Close Columbia's crew
          hatch.  Begin Range Safety final network open loop command check,
          perform hatch seal and cabin  leak  checks,  begin  the  inertial
          measurement unit preflight alignment and range safety closed loop
          test. The white room is closed out and the closeout crew moves to
          fallback area. Primary ascent guidance data is transferred to the
          backup flight system.

               Enter planned 10-minute hold at T-20 minutes.

               Resume  countdown.  Transition  orbiter onboard computers to
          launch configuration and start fuel  cell  thermal  conditioning.
          Close orbiter cabin vent valves. Backup flight system transitions
          to launch configuration.


               Enter planned 10 minute hold at T-9 minutes.


               Resume countdown.



          Start automatic ground launch sequencer (T-9 minutes).
          Retract orbiter crew access arm (T-7:30).
          Start mission recorders (T-5:30).
          Start APU's. Arm SRB and ET range safety safe and arm devices (T-
          5).
          Start liquid oxygen drainback (T-4:55).
          Start orbiter aerosurface profile test (T-3:55).
          Orbiter transfers to internal power (T-3:30).
          Start MPS gimbal profile test (T-3:30).
          Pressurize  liquid  oxygen (LO2) tank/retract gaseous oxygen vent
          arm (T-2:55).
          Fuel cells to internal reactants (T-2:35).
          Pressurize liquid hydrogen (LH2) tank (T-1:57).
          Deactivate SRB joint heaters (T-1:00).
          LPS go for start of orbiter automatic sequence (T-0:31 seconds).
          Start SRB gimbal profile test (T-0:6.6).
          SRB ignition and liftoff (T-0).