oliver@vf.jsc.nasa.gov (04/27/91)
This post is intended to provide a little background into why space shuttle launches are processed the way they are. I'm going to try to not get too detailed so forgive me if I leave out your favorite launch activity. LAUNCH WINDOWS You may have noticed that the launch window is different for every flight. There are many concerns which must be taken into account when determining the launch window for a flight. One of the primary considerations is the lighting and thermal conditions for the payloads. The Shuttle often caries multiple paylaods and the requirements of each must be considered. For many payloads the angle of the sun (the lighting condition for photography) plays an important role. The sun angle, and whether you are in daylight or darkness plays a role in the temperature of the payload. For deployed payloads the position of the shuttle in space is a major consideration. There also are major Orbiter lighting considerations. The flight designers have to worry not only the lighting at KSC but also at the trans Atlantic abort sites and Edwards AFB. Launch abort planning can impact when a launch window opens and how long that window stays open. For a planatary mission, like Galileo, the launch window is only a few minutes long. For STS-39, the launch window remains open for 3 1/2 hours. THE COUNTDOWN In recent posts questions have been raised as to why the countdown clock is stopped and why does NASA have built in holds. The answer is most events in the countdown are sequenced to happen at certain times in relation to other events. However, due to unforseen circumstances, the execution of a procedure may not be completed on time. Also, in a vehicle as complex as the Shuttle and its ground support equipment, it is not uncommon for something to break. The built in holds allow for small items to be repaired and for procedures which are running behind to complete. If the countdown were running off of a clock which didn't stop, the timing for the sequenced events would have to be redone each time there was a delay. The built in holds are inserted into the countdown at places where there are no sequenced events waiting to occur. What good are built in holds? When launching any rocket one must always be wary of the weather. There are weather constraints for loading propellent onto the vehicle as well as weather constraints for launching. For the Shuttle, propellent loading begins shortly after the count is resumed from the T-6 hour hold. If the weather is bad, i.e. lightning in the area, the hold can be extended without affecting the timing for the sequencing of events for the last six hours of the count. The T-9 minute hold provides a good waiting point for weather violations to the launch. Remember in order to launch the Shuttle you not only have to have good weather at the launch pad at the time of launch, but also at the Shuttle Landing Facility some thirty minutes later to support a return to launch site abort. Then there is the weather at the trans-Atlantic abort sites about 20 minutes after launch that also figures into the weather forcast. Other considerations which must be taken into account is the length of the crew day and when the crew wakes up. There are many activities that the crew must perform on any mission on the first flight day, and in general the maximum crew day is 16 hours. And that 16 hour clock starts ticking when the crew wakes up. Also, the crew is not allowed to spend more than 2 1/2 hours on their backs before launch, so when the crew is inserted into the Orbiter plays a port as to how long the hold times can be extended. I hope all this clears up more questions than it creates. And for those that do not know what goes on during a Space Shuttle countdown I am including a typical launch countdown. My thanks to the folks at Rockwell International who produced this countdown (I think it came out of an old press kit) for it does an excellent job of explaning Orbiter systems and what is actually going on during the count and the holds. This sample countdown picks up at the T-6 hour point which for the case of STS-39 is about L-08:20. Depending on the mission, the placement and duration of the planned holds may change. PRELAUNCH COUNTDOWN T - (MINUS) HR:MIN:SEC TERMINAL COUNTDOWN EVENT 06:00:00 Verification of the launch commit criteria is complete at this time. The liquid oxygen and liquid hydrogen systems chill-down commences in order to condition the ground line and valves as well as the external tank (ET) for cryo loading. Orbiter fuel cell power plant activation is performed. 05:50:00 The space shuttle main engine (SSME) liquid hydrogen chill-down sequence is initiated by the launch processing system (LPS). The liquid hydrogen recirculation valves are opened and start the liquid hydrogen recirculation pumps. As part of the chill- down sequence, the liquid hydrogen prevalves are closed and remain closed until T minus 9.5 seconds. 05:30:00 liquid oxygen chill-down is complete. The liquid oxygen loading begins. The liquid oxygen loading starts with a "slow fill" in order to acclimate the ET. Slow fill continues until the tank is 2-percent full. 05:15:00 The liquid oxygen and liquid hydrogen slow fill is complete and the fast fill begins. The liquid oxygen and liquid hydrogen fast fill will continue unti that tank is 98-percent full. 05:00:00 The calibration of the inertial measurement units (IMUs) starts. The three IMUs are used by the orbiter navigation systems to determine the position of the orbiter in flight. 04:30:00 The orbiter fuel cell power plant activation is complete. 04:00:00 The Merritt Island (MILA) antenna, which transmits and receives communications, telemetry and ranging information, alignment verification begins. 03:45:00 The liquid hydrogen fast fill to 98 percent is complete, and a slow topping-off process is begun and stabilized to 100 percent. 03:30:00 The liquid oxygen fast fill is complete to 98 percent. 03:20:00 The main propulsion system (MPS) helium tanks begin filling from 2,000 psi to their full pressure of 4,500 psi. 03:15:00 Liquid hydrogen stable replenishment begins and continues until just minutes prior to T-0. 03:10:00 Liquid oxygen stable replenishment begins and continues until just minutes prior to T-0. 03:00:00 The MILA antenna alignment is completed. 03:00:00 The orbiter closeout crew goes to the launch pad and prepares the orbiter crew compartment for flight crew ingress. 03:00:00 Begin 2-hour planned hold. An inspection team examines the ET Holding for ice or frost formation on the launch pad during this hold. 03:00:00 Two-hour planned hold ends. Counting 02:30:00 Flight crew departs Operations and Checkout (O&C) Building for launch pad. 02:00:00 Checking of the launch commit criteria starts at this time. 02:00:00 The ground launch sequencer (GLS) software is initialized. 01:50:00 Flight crew orbiter and seat ingress occurs. 01:50:00 The solid rocket boosters (SRBs) hydraulic pumping units gas generator heaters are turned on and the SRBs aft skirt gaseous nitrogen purge starts. 01:50:00 The SRB rate gyro assemblies (RGAs) are turned on. The RGAs are used by the orbiter's navigation system to determine rates of motion of the SRBs during first-stage flight. 01:35:00 The orbiter accelerometer assemblies (AAs) are powered up. 01:35:00 The orbiter reaction control system (RCS) control drivers are powered up. 01:35:00 Orbiter crew compartment cabin closeout is completed. 01:30:00 The flight crew starts the communications checks. 01:25:00 The SRB RGA torque test begins. 01:20:00 Orbiter side hatch is closed. 01:10:00 Orbiter side hatch seal and cabin leak checks are performed. 01:10:00 IMU preflight align begins. 01:00:00 The orbiter RGAs and AAs are tested. 00:50:00 The flight crew starts the orbiter hydraulic auxiliary power units'(APUs') H20 (water) boilers preactivation. 00:45:00 Cabin vent redundancy check is performed. 00:45:00 The GLS mainline activation is performed. 00:40:00 The eastern test range (ETR) shuttle range safety system (SRSS) terminal count closed-loop test is accomplished. 00:40:00 Cabin leak check is completed. 00:32:00 The backup flight control system (BFS) computer is configured. 00:30:00 The gaseous nitrogen system for the orbital maneuvering system (OMS) engines is pressurized for launch. Crew compartment vent valves are opened. 00:26:00 The ground pyro initiator controllers (PICs) are powered up. They are used to fire the SRB hold-down posts, liquid oxygen and liquid hydrogen tail service mast (TSM), and ET vent arm system pyros at lift-off and the SSME hydrogen gas burn system prior to SSME ignition. 00:25:00 Simultaneous air-to-ground voice communications are checked. Weather aircraft are launched. 00:22:00 The primary avionics software system (PASS) is transferred to the BFS computer in order for both systems to have the same data. In case of a PASS computer system failure, the BFS computer will take over control of the shuttle vehicle during flight. 00:21:00 The crew compartment cabin vent valves are closed. 00:20:00 A 10-minute planned hold starts. Hold 10 All computer programs in the firing room are verified to Minutes ensure that the proper programs are available for the final countdown. The test team is briefed on the recycle options in case of an unplanned hold. The landing convoy status is again verified and the landing sites are verified ready for launch. The chase planes are manned. The IMU preflight alignment is verified complete. Preparations are made to transition the orbiter onboard computers to Major Mode (MM)-101 upon coming out of the hold. This configures the computer memory to a terminal countdown configuration. 00:20:00 The 10-minute hold ends. Counting Transition to MM-101. The PASS onboard computers are dumped and compared to verify the proper onboard computer configuration for launch. 00:19:00 The flight crew configures the backup computer to MM-101 and the test team verifies the BFS computer is tracking the PASS computer systems. The flight crew members configure their instruments for launch. 00:18:00 The Mission Control Center-Houston (MCC-H) now loads the onboard computers with the proper guidance parameters based on the prestated lift-off time. 00:16:00 The MPS helium system is reconfigured by the flight crew for launch. 00:15:00 The OMS/RCS crossfeed valves are configured for launch. The chase aircraft engines are started. All test support team members verify they are "go for launch." 00:12:00 Emergency aircraft and personnel are verified on station. 00:10:00 All orbiter aerosurfaces and actuators are verified to be in the proper configuration for hydraulic pressure application. The NASA test director gets a "go for launch" verification from the launch team. 00:09:00 A planned 10-minute hold starts. Hold 10 Minutes NASA and contractor project managers will be formally polled by the deputy director of NASA, National Space Transportation System (NSTS) Operations, on the Space Shuttle Program Office communications loop during the T minus 9-minute hold. A positive "go for launch" statement will be required from each NASA and contractor project element prior to resuming the launch countdown. The loop will be recorded and maintained in the launch decision records. All test support team members verify that they are "go for launch." Final GLS configuration is complete. 00:09:00 The GLS auto sequence starts and the terminal countdown begins Counting The chase aircraft are launched. From this point the GLSs in the integration and backup consoles are the primary control until T-0 in conjunction with the onboard orbiter PASS redundant-set computers. 00:09:00 Operations recorders are on. MCC-H, Johnson Space Center, sends a command to turn these recorders on. They record shuttle system performance during ascent and are dumped to the ground once orbit is achieved. 00:08:00 Payload and stored prelaunch commands proceed. 00:07:30 The orbiter access arm (OAA) connecting the access tower and the orbiter side hatch is retracted. If an emergency arises requiring flight crew activation, the arm can be extended either manually or by GLS computer control in approximately 30 seconds or less. 00:05:00 Orbiter APUs start. The orbiter APUs provide pressure to the three orbiter hydraulic systems. These systems are used to move the SSME engine nozzles and aerosurfaces. 00:05:00 ET/SRB range safety system (RSS) is armed. At this point, the firing circuit for SRB ignition and destruct devices is mechanically enabled by a motor-driven switch called a safe and arm device (S&A). 00:04:30 As a preparation for engine start, the SSME main fuel valve heaters are turned off. 00:04:00 The final helium purge sequence, purge sequence 4, on the SSMEs is started in preparation for engine start. 00:03:55 At this point, all of the elevons, body flap, speed brake and rudder are moved through a preprogrammed pattern. This is to ensure that they will be ready for use in flight. 00:03:30 Transfer to internal power is done. Up to this point, power to the space vehicle has been shared between ground power supplies and the onboard fuel cells. The ground power is disconnected and the vehicle goes on internal power at this time. It will remain on internal power through the rest of the mission. 00:03:30 The SSMEs nozzles are moved (gimbaled) through a preprogrammed pattern to ensure that they will be ready for ascent flight control. At completion of the gimbal profile, the SSMEs nozzles are in the start position. 00:02:55 ET liquid oxygen prepressurization is started. At this point, the liquid oxygen tank vent valve is closed and the ET liquid oxygen tank is pressurized to its flight pressure of 21 psi. 00:02:50 The gaseous oxygen arm is retracted. The cap that fits over the ET nose cone to prevent ice buildup on the oxygen vents is raised off the nose cone and retracted. 00:02:35 Up unti this time, the fuel cell oxygen and hydrogen supplies have been adding to the onboard tanks so that a full load at lift-off is assured. This filling operation is terminated at this time. 00:01:57 Since the ET liquid hydrogen tank was filled, some of the liquid hydrogen has turned into gas. In order to keep pressure in the ET liquid hydrogen tank low, this gas was vented off and piped out to a flare stack and burned. In order to maintain flight level, liquid hydrogen was continuously added to the tank to replace the vented hydrogen. This operation terminates the liquid hydrogen tank vent valve is closed, and the tank is brought up to a flight pressure of 44 psia at this time. 00:01:15 The sound suppression system will dump water onto the mobile launcher platform (MLP) at ignition in order to dampen vibration and noise in the space shuttle. The firing system for this dump, the sound suppression water power bus, is armed at this time. 00:00:38 The onboard computers position the orbiter vent doors to allow payload bay venting upon lift-off and ascent in the payload bay at SSME ignition. 00:00:37 The gaseous oxygen ET arm retract is confirmed. 00:00:31 The GLS sends go for redundant set launch sequence start. At this point, the four PASS computers take over main control of the terminal count. Only one further command is needed from the ground, go for main engine start, at approximately T minus 9.7 seconds. The GLS in the integration console in the launch control center still continues to monitor several hundred launch commit criteria and can issue a cutoff if a discrepancy is observed. The GLS also sequences ground equipment and sends selected vehicle commands in the last 31 seconds. 00:00:28 Two hydraulic power units in each SRB are started by the GLS. These provide hydraulic power for SRB nozzle gimbaling for ascent first-stage flight control. 00:00:21 The SRB gimbal profile is complete. As soon as SRB hydraulic power is applied, the SRB engine nozzles are commanded through a preprogrammed pattern to assure that they will be ready for ascent flight control during first stage. 00:00:21 The liquid hydrogen high-point bleed valve is closed. 00:00:18 The onboard computers arm the explosive devices, the pyrotechnic initiator controllers, that will separate the T-0 umbilicals, the SRB hold-down posts, and SRB igniton, which is the final electrical connection between the ground and the shuttle vehicle. 00:00:16 The aft SRB multiplexer/demultiplexer (MDM) units are locked out. This is to protect against electrical interference during flight. The electronic lock requires an unlock command before it will accept any other command. The MPS helium fill is terminated. The MPS helium system flows to the pneumatic control system at each SSME inlet to control various essential functions. The GLS opens the prelift-off valves for the sound suppression water system in order to start water flow to the launch pad. 00:00:15 If the SRB pyro initiator controller (PIC) voltage in the redundant-set launch sequencer (RSLS) is not within limits in 3 seconds, SSME start commands are not issued and the onboard computers proceed to a countdown hold. 00:00:10 SRB SRSS inhibits are removed. The SRB destruct system is now live. LPS issues a 'go' for SSME start. This is the last required ground command. The ground computers inform the orbiter onboard computers that they have a "go" for SSME start. The GLS retains hold capability until just prior to SRB ignition. 00:00:09.7 Liquid hydrogen recirculation pumps are turned off. The recirculation pumps provide for flow of fuel through the SSMEs during the terminal count. These are supplied by ground power and are powered in preparation for SSME start. In preparation for SSME ignition, flares are ignited under the SSMEs. This burns away any free gaseous hydrogen that may have collected under the SSMEs during prestart operations. The orbiter goes on internal cooling at this time; the ground coolant units remain powered on until lift-off as a contingency or an aborted launch. The orbiter will redistribute heat within the orbiter until approximately 125 seconds after lift-off , when the orbiter flash evaporators will be turned on. 00:00:09.5 The SSME engine chill-down sequence is complete and the onboard computers command the three MPS liquid hydrogen prevalves to open. (The MPS's three liquid oxygen prevalves were opened during ET tank loading to permit engine chill-down.) These valves allow liquid hydrogen and oxygen flow to the SSME turbopumps. Command decoders are powered off. The command decoders are units that allow ground control of some onboard components. These units are not needed during flight. 00:00:06.6 The main fuel and oxidizer valves in each engine are commanded open by the onboard computers, permitting fuel and oxidizer flow into each SSME for SSME start. All three SSMEs are started at 120-millisecond intervals (SSME 3, 2, then 1) and throttle up to 100 percent thrust levels in 3 seconds under control of the SSME controller on each SSME. 00:00:04.6 All three SSMEs are verified to be at 100 percent thrust and the SSMEs are gimbaled to the lift-off position. If one or more of the three SSMEs do not reach 100-percent thrust at this time, all SSMEs are shut down, the SRBs are not ignited, and an RSLS pad abort occurs. The GLS RSLS will perform shuttle and ground systems safing. Vehicle bending loads caused by SSME thrust buildup are allowed to stabilize before SRB ignition. The vehicle moves towards ET and the entire stack is displaced approximately 25.5 inches. 00:00:00 The two SRBs are ignited under command of the four onboard PASS computers, the four hold-down explosive bolts on each SRB are initiated (each bolt is 28 inches long and 3.5 inches in diameter), and the two T-0 umbilicals on each side of the spacecraft are retracted. The onboard timers are started and the ground launch sequence is terminated. All three SSMEs are at 104-percent thrust. Boost guidance in attitude hold. 00:00 Lift-off. -- Pat Oliver - Lockheed Engineering and Sciences Company at NASA JSC 2400 NASA Rd One, Houston, TX 77058 (713) 483-3323 OLIVER@vf.jsc.nasa.gov
stanfiel@testeng1.misemi (Chris Stanfield) (04/30/91)
In article <1991Apr26.154621.1@vf.jsc.nasa.gov> oliver@vf.jsc.nasa.gov writes: >This post is intended to provide a little background into why space shuttle >launches are processed the way they are. I'm going to try to not get too >detailed so forgive me if I leave out your favorite launch activity. stuff deleted > Also, the crew is not allowed to spend more than 2 1/2 hours on >their backs before launch, so when the crew is inserted into the Orbiter plays >a port as to how long the hold times can be extended. I am interested in this 2.5 hour time limit for the crew. Has this always existed for manned flights? I thought that some of the early manned flights had the crew in the capsule (long time ago!) for longer than 2.5 hours prior to lift-off. Also, this limit must have an effect on the actual launch window. No matter how long the launch window is, if the crew are in the orbiter, then the launch window ends .5 hours after the scheduled lift-off (the crew enters 2 hours before lift-off, right?). So, am I right about the above? Any of it? Facts are welcome, flames are pointless. Chris Stanfield, Mitel Corporation: E-mail to:- uunet!mitel!testeng1!stanfiel (613) 592 2122 Ext.4960 We do not inherit the world from our parents - we borrow it from our children.
oliver@vf.jsc.nasa.gov (05/02/91)
> In article <1991Apr26.154621.1@vf.jsc.nasa.gov> oliver@vf.jsc.nasa.gov writes: >> Also, the crew is not allowed to spend more than 2 1/2 hours on >>their backs before launch, so when the crew is inserted into the Orbiter plays >>a port as to how long the hold times can be extended. > > I am interested in this 2.5 hour time limit for the crew. Has this > always existed for manned flights? I thought that some of the early > manned flights had the crew in the capsule (long time ago!) for > longer than 2.5 hours prior to lift-off. Also, this limit must have an > effect on the actual launch window. No matter how long the launch > window is, if the crew are in the orbiter, then the launch window ends > .5 hours after the scheduled lift-off (the crew enters 2 hours before > lift-off, right?). So, am I right about the above? Any of it? Facts > are welcome, flames are pointless. I must apologize for the misleading way the first statement was written. The crew is allowed to remain on their backs for 2.5 hours after the nominal liftoff time. The actual time that the crew is strapped in could be 4.5 hours or so depending on when they entered the spacecraft. -- Pat Oliver - Lockheed Engineering and Sciences Company at NASA JSC 2400 NASA Rd One, Houston, TX 77058 (713) 483-3323 OLIVER@vf.jsc.nasa.gov