Conference 7.286::space

724.0. "Shuttle Countdown Sequence" by 58453::SKLEIN (Nulli Secundus) Sun Apr 28 1991 08:33

I read this article on usenet and thought that fellow noters would be 
interesting in reading the countdown sequence for a typical space shuttle
launch. The article is very long and detailed, but certainly opened my
eyes to what happens during the countdown.

Susan

--
From: [email protected]
Subject: Launching a Space Shuttle
Date: 26 Apr 91 21:46:21 GMT
Organization: NASA Johnson Space Flight Center 
 
Pat Oliver -  	Lockheed Engineering and Sciences Company at NASA JSC
		2400 NASA Rd One, Houston, TX 77058 (713) 483-3323
		[email protected]
--
 
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.

Thanks...this is a nice listing...I'm glad you posted it.

Just a few points/questions:

1.  The author never answered the "real" question about built in holds.  He
answered the question for holds in general.  Of course you stop the clock when
things are behind schedule or whatever.  The question is, why should you PLAN
to stop the clock at T-9 even if things are just fine.  Why not count straight
down EXCEPT when you have a problem?

2.  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. 

I didn't know it was burned.  Does anyone know where the flare stack is located?



3.  "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."

I don't think "stabilize" is the right word.  During the very first Readiness
Firing before STS-1, they timed the "bending loads", i.e. the "twang" of the
shuttle bending forward and then rebounding.  They then adjusted the engine
start time in the countdown so that the shuttle would be pointing straight
up again at T-0.  In other words, it has not stabilized.  They just launch
after exactly 1/2 oscillation cycle.

B

RE: .1

That's the first thing I noticed the first (and every) time I see the IMAX
film "The Dream Is Alive".  That side view of the shuttle launch is awesome.
You can just see the whole structure bend in the direction of the ET at main
engine start (T-6), then just as it "rocks" back to the vertical, *BLAM*!  SRB
ignition and the whole thing just jumps off the pad.  I get chills just thinking
about it (and teary-eyed each time I see it).  Unbelievable power!

...Roger...

  I believe that the built in holds are to allow extra time to solve problems
that may come up. If the problems do come up, they can launch on time by just
having a shorter hold. In the unlikely event that no problem comes up, they
hold for the maximum planned time and still hit the window.

  Because of oribtal mechanics, NASA can't leave early to get somewhere on time
so this is the next best thing. 

  George

    I don't have any insider knowledge, but I think having "built-in holds"
    instead of a longer countdown is psychological.  If someone isn't sure
    that a problem he notices is important, he may be reluctant to speak up
    and stop the count, since if he's wrong he looks bad.  By contrast, a
    problem can be discussed during a hold to see if permission should be
    given to exit from the hold, and permission can be withheld as long as
    there is any doubt.
    
    The difference isn't real, of course, only a change in the human
    factors.  However, human factors are very important when you've got
    hundreds (?) of people working closely together to launch a manned
    rocket safely.
        John Sauter

Hmmm...  I always thought the holds were a way of accomodating the humans
in the loop (or reality, if you will).

A countdown sequence is more than a clock ticking towards 0 - each tick of
the clock triggers the launch processing system to perform a number of checks
on the launch vehicle, communications systems, etc.   The numbers all have to
fall into place according to the plan (e.g., pressures have to ramp up or
down, temperatures have to rise/fall/stay within limits).  [Remember, there
are times when the clock can stop and there are times when it would be a real
good idea for it not to stop -- likewise, you can't always stop the clock and 
pick it up with the same time).

There is a big book with exactly what happens at which clock tick - and the
machinery and humans have to follow this.

When things go wrong (weather, subsystems, people) you have to deal with
this without rewriting the book each launch (or launch attempt).   I've felt
that the holds are where the new or unfinished business gets taken care of,
retaining the standard sequence for each launch attempt.   To me it makes
sense - if there are points in the countdown that can be stretched for
a variety of reasons - stop the clock.

- dave

    Part of the launch strategy is to plan for the unplanned.  This is
    particularly the case with the diverse payloads that are carried by
    the shuttle.  Prelaunch trouble-shooting and servicing of these payloads
    are a distinct possibility.  The holds allow for this contingency
    without disrupting orbiter processing.  Another major factor is that
    the holds mark "points of no return" in the countdown.  In other words,
    once the sequence has passed a certain hold (and other points in the
    countdown), turnaround times for launches cancelled at that point
    change due to what systems were running and need reservicing.  The
    turnaround times can range from 1 to 6 days depending how far along
    they were.
    
    FYI, the H2 burnoff stacks are between 39A+39B.

RE:      <<< Note 724.1 by DECWIN::FISHER "Pursuing an untamed ornothoid" >>>

>3.  "Vehicle bending loads caused by SSME thrust buildup are allowed...

>In other words, it has not stabilized.  They just launch
>after exactly 1/2 oscillation cycle.

This means that at release, the shuttle stack has a non-zero pitch rate. In
other words, the stack will continue to pitch somewhat past vertical.

Does anyone know if this is actively cancelled (e.g., by elevon deflection), or
naturally damped out (by the general lift and drag forces on the orbiter wing),
or cancelled by the off-center thrust from the SSMEs?
						-- Tom

My guess would be that it's all part and parcel of the normal guidance.  The
interial guidance senses a pitch rate different that what it is supposed to be
so it does whatever it normally does.  In this case, I think the major factors
are the gimballs on the SRBs, although I'm sure they also play with the SSME
gimbals and the aerosurfaces as well.  (I seem to remember the aerosurfaces
are preset to compensate for the off-center thrust of the SRBs)

Burns

Re .0

>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.

Dare I ask but what would happen if one or more of the four hold-down explosive
bolts didn't fire?

    Re:.9
    
    	The answer is simple:
    
    CRACK!!! EAEEEEEEEEEEEEEERRRRRRR!!!! CRASH! BOOOOOOOOOOOOM!!!!!!!
    
    				Drew
    

  Worse yet, what would happen if one of the SRB's didn't light.

  I guess the significant point is that lighting the solid fuel and explosive
bolts has been very reliable. The probability of loss of spacecraft and crew do
to other factors is much larger. 

  George

If one of the bolts doesn't fire - some damage to the pad and/or SRB skirt,
but it'll get off the ground just fine.   This has happened once, so there
isn't a lot of speculation about this.  The probability that several would
fail is "very high indeed".

I'm not certain how many would have to fail before the assertion of .10
would come true.

An SRB not lighting is a catastrophic failure (crew, vehicle, and probably
the launch platform).

- dave

RE:              <<< Note 724.12 by PRAGMA::GRIFFIN "Dave Griffin" >>>

>If one of the bolts doesn't fire... This has happened once

Can you think of a reference where I could read about this? Sounds like it
would be very interesting.
						-- Tom

re .12

>If one of the bolts doesn't fire - some damage to the pad and/or SRB skirt,
>but it'll get off the ground just fine.   This has happened once, so there
>isn't a lot of speculation about this.
    
    Which mission was that?
    
>An SRB not lighting is a catastrophic failure (crew, vehicle, and probably
>the launch platform).
    
    Can you explain why? Would it depend on whether one or both SRBs failed to
    ignite? I imagined that if both failed then they could just cut the main
    engines and abort the launch.
    
    Do we have a posting somewhere in this conference describing all known
    catastrophic failure situations? I thought I'ld ask these questions
    between missions so as not to give anyone nightmares. :-)

    Are there any failsafe mechanisms to ensure that either both SRBs
    ignite, or that neither do? It seems to me that it would have to be an
    all-or-nothing situation.
    
    Are the bolts really strong enough to prevent the Shuttle from
    separating from the pad? One of my lecturers in Space Physics at
    Leicester University (this was back in 1982) reckoned that the total
    thrust from the shuttle main engines and SRBs would be sufficient to
    drag the launch pad with it, if the bolts failed. I assumed that this
    was a slight exaggeration, but I wonder how much of one?
    
    Dave Hazel

    
    If one SRB ignited but not the other... Bad news ... BUT
    
    I would think that the graples should be strong enought to keep it place 
    until burn out...   I mean it isn't very difficult to this. They even do
    it during some of the SRB tests.
    
    Gil 

Re: .13

I'll try to dig up my reference on this.  I can't recall it right now.  It
might have been sci.space.shuttle chatter now that I think about it.  [I'll
temporarily retract my comment as hearsay.]


Re: .12

I explicitly said if one doesn't light there is a problem.


Re: others

I believe the discussion about the hold-down posts holding down a lit stack
is academic.   From what I know about the launch sequence, the commands to
ignite the SRBs and the commands to the detonators on the frangible nuts
which keep the hold-down posts attached to the SRB's are given either in
rapid succession or they are part of the same command.  No monitoring of
the SRBs is done to see if the ignition sequence worked before letting
the bolts go.   So basically, if they both don't light, the stack falls over,
goes boom.

It's sort of odd that people concentrate on it not lighting, while most (well,
a lot) of the engineering effort is put into not lighting them!
There are numerous safety features from keeping the SRBs from lighting EARLY.
Three electrical signals are required for the arm and fire devices to work,
plus a manual pin must be pulled prior to launch, and there are mechanical
arming systems in there as well which are activated at T-5 minutes.
Note that all 3 SSMEs must report ready to the sequencer before the ignition
signals are sent to the SRB stacks.

The ignitors use two redundant NSDs (NASA Standard Detonators) to ignite a
"pyro booster charge" - which then shoots a flame down the *entire* length
of the SRB.   Two redundant NSDs are on each hold-down post.  Bolt tension,
gravity, and the gas pressure from the detonators shoots the bolt into a
catch area on the Mobile Launch Platform.

The arming voltages for all the detonators is constantly monitored by the
Launch Processing System - if any of them goes too low, a hold is immediately
generated.

- dave

Re: .16

Most SRB tests are horizontal on test stands.

Also, test stands are designed to not let things go -- the mobile launch
platform is not a test stand.   I don't see how you can make the assumption
that they can hold it down (I don't really know one way or the other), but
I'm moderately sure that they don't have enough water to keep the MLP
from melting for the entire burn time (or, more likely, the acoustic
pressure would shake the whole thing to pieces).

As to whether or not the shuttle would take off with the MLP attached...
the entire platform (fueled shuttle and all) weighs in at 12,700,000 pounds
(that's 5,760,682 kg.)...  The reader is welcome to solve the rest.


- dave

    Re .18 (Dave)
    
    I would be supprised if NASA didn't take the additional and relativly
    inexpensive measures to insure that the pad can hold and survive a full
    burn in place of any or all of the booster elements.
    
    I mean they already have the graples and the cooling water system, all
    they have to do is use them a little longer then normal.
    
    If I remember correctly the take off sequence goes something like this:
    
    1-  Fire up the SSMEs, and make sure they are working ok. 2,3 secs.
    
    2-  Then fire the SRBs, hold everything for 1,2 secs to insure that the 
        SRBs are burning uniformly, etc.
    
    3-  Only then, release the shuttle stack for take off.....
    
    Gil

RE:          <<< Note 724.19 by MAYDAY::ANDRADE "The sentinel (.)(.)" >>>
    
>    2-  Then fire the SRBs, hold everything for 1,2 secs to insure that the 
>        SRBs are burning uniformly, etc.
>    3-  Only then, release the shuttle stack for take off.....

Nope. The stack is released the moment the SRBs are lit. There's no point in
waiting to see if they're burning uniformly because there's nothing that can be
done if they're burning incorrectly.
						-- Tom

    Furthermore, the timing is based on the way the stack 'twangs' after
    main engine start. The stack pitches forward and the solids light as
    the stack passes through vertical on the rebound. You may recall that
    some of the early launches that did not do this 'walked' across the
    pad, causing a lot of extra damage to the pad.
    
    The water, by the way, is called the sound suppression system and that
    is its main function - protecting the vehicle from the reflected sound
    of SRB ignition.
    
    gary

    Re. .17:
    
    Thanks for the detailed information.
    
    With all the effort put into ensuring that the SRBs only ignite when
    they are supposed to, I imagine that the chance of one of them failing
    to ignite when told to do so is pretty minimal. It looks to me as
    though the system is as near failsafe as they can make it.
    
    
    Dave Hazel

        The difference between the SRB thrust and the SSME thrust is on
        the order of 10:1. Thus there is no problem holding down the
        shuttle while the SSMEs build up thrust and are checked out.

I read this on USENET and thought it appropriate to add it here, it is just
a continuation of the launch. Maybe, one of you space gurus can boil it down 
to layman terms. The author seems to be someone in the know at NASA.

Susan

From: [email protected] (Kenneth C. Jenks [GM2] 483-4368)
Subject: Roll Program
Date: 23 Aug 91 16:59:23 GMT
Organization: NASA/JSC/GM2, Space Shuttle Program Office
 
I received some e-mail asking for an "authoritative" explanation for
the roll maneuver during ascent.  I dug around in my flight controller
training manuals, and I found the "Ascent Guidance and Flight Control
Training Manual" ASC G&C 2102, dated August 1989, which states:
 
    "The first-stage trajectory is divided into two phases.
      * Vertical rise phase
      * Tilt phase
     This trajectory is shown in figure 2-2 [which shows a shuttle
     launching, rolling, tilting, up to SRB sep].  During the vertical
     rise phase, the launch pad attitude is commanded until an I-loaded
     V(subscript rel) [relative velocity] sufficient to assure launch
     tower clearance is achieved.  Then, the tilt maneuver (roll
     program) orients the vehicle to a heads down attitude required to
     generate a negative q-alpha, which in turn alleviates structural
     loading.  Other advantages with this attitude are performance
     gain, decreased abort maneuver complexity, improved S-band look
     angles, and crew view of the horizon.  The tilt maneuver is also
     required to start gaining downrange velocity to achieve main
     engine cuttoff (MECO) target in second stage."
 
If you need a copy, send a Freedom of Information Request to NASA/JSC,
Public Affairs Officer, Mail Code AP, Houston, TX 77058, and ask for
the document I cited above.
 
 
-- Ken Jenks, NASA/JSC/GM2, Space Shuttle Program Office
      [email protected]  (713) 483-4368
 
     "...back to the moon, back to the future,
      and, this time, back to stay." -- George Bush