Your typical Space Shuttle Mission Part 3: Getting to space (AKA Countdown, Launch, Ascent and Orbit Insertion)

After reading all I wrote, now you know how a space shuttle roughly works, and all the people involved. A quick note, a shuttle crew, the astronauts, are usually a minimum of 2: The Commander (CDR on the radio) and the Pilot (PLT). If a mission requires extra hands, Mission Specialists are also included in the crew (MS1, MS2, and so forth). Up to now, the maximum number of crewmembers to ever have flown on the space shuttle is 7 (The CDR, The PLT, MS1, MS2, MS3, MS4 and MS5). The commander pilots the shuttle and speaks for the entire crew, while the pilot assists in piloting. To understand this, in normal flying, the CDR is the pilot, and the PLT is the co-pilot.

On to actual launch procedures!!!

COUNTDOWN

3 days before actual launch. Shuttle Discovery is in the pad, covered by the rotating service structure, protecting it from the weather. Countdown-wise, at T-43 hours (yet, 3 days before… remember, the holds), the countdown clock is started, and everyone is called to their stations. The Shuttle Test Team, the Launch team and the Mission Management Team start to work 24/7 to make sure the shuttle and crew are ready to launch, and that the systems and weather conditions allow for it, beggining with final checkouts to the shuttle and launchpad, and software loading into the shuttle’s computers.

The first built-in hold is at T-27 hours, and lasts 4 hours. During this hold, the pad is cleared of all non-essential personnel and the shuttle’s power reactant storage and distribution system is filled with cryogenic (read: uber cold) propellants. When the hold releases and it counts down again, the fuel cells’ storage tanks are filled with said cryogenic propellants.

The countdown holds again at T-19 hours, another 4-hour hold, unless the distribution system needs time to do an offload. They cut off the umbilical to the orbiter, clean and vacuum the crew cabin, and some other tasks.

The hold releases from T-19 hours. Now the 3 Main Engines are prepared for the fuel tanking and actual flight, the closing out of the orbiter and launchpad continues, and the Sound Suppression System tank is filled with water.

(A side-note: The SSS (sound suppress system) is composed of a 300,000 gallon water tank several meters away from the launchpad. This tank is connected via tubes and pumps to the launcher platform, which has several jets of water, some of them for quick-flooding the space below the launcher platform, and some that suppress fire from the launch as soon as the vehicle clears. The REAL purpose is flooding the compartments below the launcher platform, so the sound shockwave generated by the engines and boosters is dampened by the water, and doesn’t harm the orbiter on the rebound. The entire system, on peak maximum pumping, shoots a whooping 900,000 gallons of water per minute (meaning it’ll be over in less than half a minute, considering there’s only 300,000 gallons). This is the sudden rush of water you see flowing from below the platform at 16 seconds prior to liftoff. Watch a Youtube video of a launch to see what I mean)

The hold once again hold at T-11 hours. This is the LOOOOOOOOOONGEST hold, lasting 13 or 14 hours. This is where the rotating service structure opens up to reveal the shuttle, and sets itself on the ready launch park position, as pictured below.

Now, weather and engineering briefings occur during this hold, and the pad and surrounding area is checked for debris. The late flight crew equipment is stowed at this point, the IMUs (inertial measurement units, for the shuttle’s position) and the communication systems are activated, and the shuttle’s switches are put in the correct initial position for startup and launch.

Finally, the countdown resumes, and the orbiter’s fuel cells are finally activated. The blast area is cleared of all nonessential personnel for the duration of the count, and the orbiter purges its air to clean it.

We reach the T-6 hour hold, which lasts 2 hours, or one, if the launch was scrubbed and this is another next-day attempt. This is an important hold, where the Mission Management and Launch teams receive weather briefings. Launch team verifies that everything is ok for launch before ordering the External Tank fueling (called Tanking). Once everything is ok, the pad is cleared of all personnel, the go for tanking is given, and the fuel lines are chilled down to the temperatures of the liquid hydrogen and oxygen, and finally, tanking starts, where hydrogen and oxygen are pumped slowly, then quick. Usually, when tanking starts, the hold is released, and the countdown reaches T-3 hours, where another hold occurs.

At the T-3 hour hold, the tanking process finally reaches “Stable Replenish” mode. This is because a bit of the liquid hydrogen and oxygen will evaporate and escape the tank via exhaust lines. The fuel pumps keep pumping just a small stream of the liquids to replace whatever gets evaporated. All in all, the tanking is complete.

The IMUs are calibrated, and the Merritt Island Launch Area (MILA) tracking antennas are aligned. The final inspection team arrives at the pad and checks for possible ice damage (read: uber-cold liquid hydrogen and oxygen) and other issues. Finally, the Closeout Crew goes to the pad and prepares the orbiter for the astronauts’ entry (ingress).

The hold releases, and the astronauts depart to the launchpad, and they go up to the crew access arm and reach the “white room”, a small room at the end of the crew access arm that is put against the orbiter’s hatch. It has air conditioning for the comfort of the astronauts and closeout crew and it has a hose that blasts cool, clean air inside the orbiter. One by one, the astronauts are helped inside the orbiter, seated and harnessed into their seats. Once seated, they perform a basic voice check with both the NASA Test Director and Mission Control. All communications are in this format: the callsign of the one you’re calling, your callsign, and the situation, if any. This is an example of the commander’s voice check:

Mission Commander: “NTD, CDR, Comm check.”
NASA Test Director: “CDR, NTD, Loud and clear. Good afternoon!”
Mission Commander: “NTD, CDR, loud and clear. Glad to be here.”

After a brief pause, the commander checks communications with Mission Control, AKA “Houston”.

Mission Commander: “Houston, CDR, Comm check.”
Mission Control Center: “CDR, Houston, Loud and clear. Good day, and good luck.”
Mission Commander: “Houston, CDR, thanks!”

The Pilot, whose callsign is PLT, and the mission specialists, MS1, MS2, etc, also conduct their voicechecks. This is to ensure communications are possible to the ground, and that their mike lines are properly connected.

Once all astronauts are seated, the Closeout crew closes the orbiter hatch, latches and seals it for flight, and pressurizes the cabin, finally checking for leaks, before closing out the white room and leaving the pad. The astronauts perform several checks and configurations during this time.

Finally, we reach the second to last hold, at T-20 minutes, only lasting 10 minutes. The NTD makes a final launch team briefing during this hold, and the orbiter’s IMU alignment is complete.

The countdown timer resumes, and the crew is given the go to transition all GPCs to OPS 101, the terminal countdown phase of the OPS 1 (launch ops on the GPCs, remember?), and the cabin vent valves of the crew cabin are closed for flight.

And we finally reached the T-9 minute hold!! This hold typically lasts 45 minutes, but the time is adjusted to the launch window at this point (the precise moment when the shuttle must launch to reach a certain point in orbit by using the least amount of fuel possible, usually nowadays to catch up to the ISS). In this hold, the flight recorders (black boxes?) are activated. And the final flight readiness polls are done independently of all 3 teams. In the final 7 to 5 minutes of the hold, the official, final flight readiness poll is conducted as follows, assuming everything is ok for launch (transcript from STS-123 Endeavour launch poll, with names removed):

NTD: “Attention on the net, this is the NTD, conducting the Launch Status Check. All stations verify ready to resume count and go for launch. OTC?”
OTC: “OTC is Go.”
NTD: “TBC?”
TBC: “TBC is Go.”
NTD: “PTC?”
PTC: “PTC is Go.”
NTD: “LPS?”
LPS: “LPS Go.”
NTD: “Houston Flight?”
Houston: “Houston Flight is Go.”
NTD: “MILA?”
MILA: “MILA’s Go.”
NTD: “STM?”
STM: “STM is go.”
NTD: “Safety Console?”
Safety Console: “Safety is Go.”
NTD: “SPE?”
SPE: “SPE’s Go.”
NTD: “LRD?”
LRD: “LRD is Go.”
NTD: “SRO?”
SRO: “SRO is Go, you have a range clear to launch.”
NTD: “…and CDR?”
Mission Commander: “CDR is Go.”
(At this point, the Shuttle Test Team is Go, and will report to the Launch Director immediately confirming the CDR’s Go for launch. Launch will then poll his team.)
NTD: “Launch Director, NTD.”
Launch: “Launch Director.”
NTD: “Launch Team is ready to proceed.”
Launch: “Ok, I copy, thank you. Chief Processing Engineer, verify no constrains for launch.”
CPE: “No constrains, we’re ready.”
Launch: “Ok, thanks. KSC Safety Mission Assurance?”
KSMA: “KSC Safety Mission Assurance is ready to go.”
Launch: “Thank you. Payload Launch Manager?”
Payload: “The <insert whatever important cargo is hauled to space and the teams overseeing the operations> is ready to proceed.”
Launch: “Thank you. Range Weather?”
Weather Officer: “Weather has no constrains for launch.”
Launch: “Thank you. And OPS Manager?”
(The OPS Manager reports the readiness of the Mission Management Team)
OpsMan: “Ops Manager, Launch Director, the MMT is working no issues, you are go for launch.”
Launch: “Thank you, sir. Endeavour, Launch Director.”
Mission Commander aboard Endeavour: “Go ahead, Launch Director.”
Launch: <Insert praises to team, crew and weather, and gives the go for launch>
Mission Commander: <Thanks Launch Director, etc etc.>
Launch: “And NTD, you are clear to launch Endeavour.”
NTD: “Copy, clear to launch, thank you.”

And with this, the shuttle has been given the green light, the Go for Launch. The countdown usually resumes 2 to 4 minutes following the end of the Launch Status Check.

NTD: “Countdown will resume on my mark… 3…. 2…. 1… mark.”

T-9 minutes and counting.

This is the point where everything gets very dynamic, fast, and tense. Several things happen at the point of the hold releasing the 9-minute countdown. First of all, duh, the countdown timer starts to tick the final countdown to launch. Second, everyone is now checking their systems VERY CLOSELY for any problems. And third, and most importantly, all control of the countdown and all shuttle and pad systems have been transferred to the Ground Launch Sequencer Computer, or GLS. From T-9 minutes to T-31 seconds, the GLS can hold the countdown if requested by any launch controller or if it detects a problem. After that, only a cutoff and a launch scrub is possible, because the GLS gives control to the 5 GPCs aboard the orbiter.

At T-9 minutes, GLS Auto Sequence is initiated and it starts to check all systems, and perform several duties. The GLS console operator is monitoring and calling out anything the GLS reports to him.

T-7 minutes, 30 seconds. The tension rises.

“GLS is go for retract orbiter access arm.”

The crew access arm, complete with white room, slowly swings out of the way, leaving the astronauts truly confined to the orbiter. Within 30 seconds, the arm is out of the way.

T-6 minutes, 15 seconds. The pilot is given an instruction.

OTC: “PLT, OTC. Go for APU Pre-Start.”
PLT: “PLT, in work.” (a few seconds later after working on the corresponding switches on his panel) “APU Prestart complete.”

The countdown keeps rolling. Steadily it makes it to 5 minutes.

T-5 minutes.

GLS: “GLS is go for APU Start.”
OTC: “PLT, OTC. Start APU.”
PLT: “PLT in work.” (some seconds later) “APU Start complete.”

The Auxiliary power units are enabled. The shuttle’s aerosurfaces become active.

T-4 minutes, 55 seconds.

The GLS does two simultaneous checks. One is to check if the SRB’s ignition is armed and ready, and the other, which we hope never to use, is the SRB Range Safety System. This, in case the shuttle strays off course into a populated area while latched onto the SRBs, an army officer will have no choice but to activate the Range Safety System, which will detonate the SRBs and the External Tank, destroying everything, which of course results in the loss of the orbiter and the crew. Like they say, we’d rather lose 7 astronauts than hundreds or thousands of civilians.

The GLS also terminates the Liquid Oxygen Stable Replenish mode and shuts down the pump.

T-3 minutes, 55 seconds.

The GLS commands the shuttle to begin a series of programmed motions of its aerosurfaces to test them, and for warm hydraulic fluid to course through them.

T-3 minutes, 30 seconds.

With the aerosurface profile test done, the 3 main engines are gimballed (Moved around in place. The nozzles can be aimed if you didn’t know) in a preset motion pattern.

T-3:03. The gimbal check is complete, and the 3 engines are spread wide apart to avoid colliding with each other at ignition. The body flap under the engines is verified to be in launch position.

T-2:55. The GLS commands the External Tank to begin pressurization of the liquid oxygen. 5 seconds later, the cap on top of the ET, that removed whatever evaporated oxygen came out and drawed it out of the way, is removed, and moved out of the way of the shuttle.

T-2:35. The cryogenic reactant distribution system no longer fuels the fuel cells, and the fuel cells begin to take their reactants off their internal orbiter tanks to generate its own power.

T-2:00.

OTC: “OTC, crew. Close and Lock Visors.”

That means that the crew must close and lock their helmet visors for their suits to become pressurized, in the unlikely event of cabin depressurization.

T-1:57. Liquid hydrogen stable replenish terminated, all tank valves closed.

T-1:46. The External Tank begins Liquid Hydrogen pre-pressurization.

T-50 seconds. Ground power is cut from the shuttle, letting it use its internal power.

And this is the moment of truth. T-31 seconds.

“GLS is go for Auto-Sequence Start.”

From this point on, GLS arms the Cut-off command, the only command available from this point to T-0. It also initiates the Redundant Set Launch Sequencer, which will monitor that everything goes smoothly with the last 4 commands to be given by the shuttle’s computers, otherwise it’ll force a cut-off to the GLS. The countdown control is transferred to the shuttle’s computers as well, which continue the final countdown, the Auto-Sequence for launch.

T-28. The SRB APUs are started, which control the nozzles of the SRBs. 7 seconds later, the SRBs do a quick gimbal test.

T-16. The Sound Suppression System activates. A loud whistling is heard as 300,000 gallons of water are quickly pumped into the SRB and SSME’s trenches under the Mobile Launcher Platform, to mitigate the sound shockwave from the engines’ roar and lessen damage to the orbiter. From afar, a torrent of water is seen blasting down from under the platform, as you can appreciate in the picture.

T-10 Seconds.

Sparks begin to fly. But not yet from the engines. A system called the Free Hydrogen Burn Off System ignites, aiming hundreds of sparks at the engine nozzles, burning off any residual hydrogen that may be in its vicinity. It’s done like this so the ignition of the engines is controlled, and no errant explosions occur that may crack or break the engine nozzles. At the same time, the GLS gives its green light to the GPCs onboard the shuttle to start its engines.

“GLS is go for Main Engine Start.”

Everyone is on the edge of their seats. Will the engines ignite? The RSLS system monitors for the successful ignitions of all 3 engines. Everyone holds their breath.

T-6.6 seconds. The GPC sends Main Engine #3 the start command.
T-6.48 seconds. Main Engine #2 is ordered to start.
T-6.36 seconds. Main Engine #1 is last.

Almost instantly, a loud whoosh and rumble comes out of the launchpad as red hot flames shoot out of the 3 main engines. The instant incredible heat hits the water in the trench, which is divided into 2 sections. A massive cloud of water vapor shoots out of the front of the pad as the water from the SSS is flash-evaporated. At first, the flames look out of control. But just as quickly they settle into a normal down-blasting direction, and the flame blast is almost invisible, save for the white halo core ‘inside’ the flame blast. The engines have ignited!! The entire stack makes a motion, a small nod. If you look at the top part of the ET, you see it lurches forward a bit, then settles back up straight in time for T-0. The movement is completely imperceptible from a moderate range.

Everyone still holds their breath, though. The engines have to reach 100% power within 3 seconds, or the RSLS will cut off the launch. It has happened 5 times in NASA’s shuttle flight record. Everyone silently prays this isn’t a 6th time. 6….5….4…3…

The engines reached 100%. RSLS is standing by for the final command. …2…1…

The point of no return is here.

T-0 seconds. Booster ignition.

LAUNCH

Simultaneously, the SRBs are given the ignite command. Almost instantly, a mass of white-hot flames shoot out of each of the SRBs’ nozzle, flash-evaporating the water on their side of the trench, shooting a massive cloud to the back of the platform. And at the same time, the bolts holding down the entire shuttle stack are detonated, releasing the entire shuttle. Since the SRBs have a powerful thrust, the shuttle literally leaps up and out of the launchpad.

…and LIFTOFF!!!!

ASCENT

The countdown is no more. It is now the mission elapsed time counter. The count is now read as “x days, x hours, x minutes, x seconds into the flight”.

Now begins the ascent part of the flight. As soon as the shuttle clears the launchpad, Kennedy Space Center instantly surrenders all responsibility, communications and monitoring of the shuttle to Mission Control Center at Johnson Space Center, Houston, Texas.

All functions from this point to MECO (Main Engine Cut-off) are controlled by the shuttle’s onboard GPCs, and is fully automatic, unless there’s an emergency. Different milestones occur during ascent. Let’s assume Discovery’s the one lifting off.

NOTE: All times from this point on are estimates based on a real mission. It will vary from mission to mission.

T+0. The GPCs are transitioned into OPS102, the “First Stage Ascent” Phase of OPS1. Discovery lifts off thanks to the combined power of both SRBs and the 3 Main Engines.

T+6.5 Seconds. Discovery clears the tower around this timeframe. Instantly, Mission Control Center at Houston, Texas assumes control of the mission.

T+11 seconds. Discovery does a roll and pitch maneuver. Basically, the engine nozzles move so that the shuttle turns until the top side of Discovery is aiming at the desired flight path. Then it begins to pull its nose up, aiming the nose at the flight path, which is up and away from the pad. It seems like a backflip, because Discovery ends flying upside down. It remains like this for almost all of the ascent. Once the roll maneuver is underway, Discovery’s CDR calls it out.

Discovery: “Houston, Discovery. Roll Program.”
Houston: “Roger Roll, Discovery.” (it seriously almost sounds like rock and roll…. look up STS-119 launch on Youtube and you’ll see… er, hear what I mean, immediately after CDR Lee Archambault’s callout of the roll program.)

T+30 seconds. Discovery’s already flying at 365MPH, 1 mile in altitude, and 7 miles downrange (meaning away) from KSC.

T+45 seconds. Discovery begins to go past the speed of sound, and the main engines are commanded to drop to 60% power to reduce the stress the strong aerodynamic pressure is causing on the orbiter. If you are viewing the launch, at this point, you’ll see the indication of the shuttle breaking the sound barrier because of the vapor-shockwave-like cloud forming around it, usually when something goes past the speed of sound. Later on, the atmosphere is already thinning out, and the stresses are no more.

T+1:05. Discovery’s engines are commanded to throttle up to 104%, its maximum power.

Houston: “Discovery, Go at throttle up.”
Discovery: “Discovery copies. Go at throttle up.”

At this point, the shuttle travels at a speed that makes the atmosphere cause the highest amount of aerodynamic pressure on Discovery. This region, which is quickly passed as the atmosphere keeps thinning out, is called Max Q.

All systems are in good shape. The engines are still going, the fuel cells are healthy and the APUs are working perfectly.

T+1:16. A little over a minute, and Discovery’s already at 1800MPH, 10 miles high and almost 12 miles away from KSC.

T+2:05. The SRBs have burned all of their useful propellant. At this point, explosive bolts are blown that cut off the SRBs from the External Tank. They go up for a bit due to the speed they were going, before dropping down to the ocean, where the SRB Salvage Ships, the Liberty Star and the Freedom Star, are waiting. The GPCs onboard Discovery transition to OPS 103, “Second Stage Ascent”. Now, several abort options are called out, the one available at this point being the “Return To Launch Site” abort, or RTLS abort, if one or 2 engines fail. I’ll explain the abort options on the next post. But note that if in any moment of the ascent, until when the shuttle’s path is permanently increasing in altitude and the shuttle goes past 300,000 feet, all 3 engines fail, the only option is to ditch the orbiter into the ocean

At this point, Discovery is now flying under the power of its 3 main engines. The flight path goes up, remains level, then seems to drop, but then, it goes level, then shoots up in altitude. This is because the 3 main engines don’t have that much power to keep Discovery climbing at first, and the climb decreases until it begins to fall back to Earth. However, since it’s wasted so much propellant at that point, and it keeps wasting it, Discovery begins to get extremely lighter, until the main engines are more than enough to propel Discovery clear out of the atmosphere. It stops dropping, and again starts to climb at an ever increasing rate. Either way, it keeps developing horizontal speed, which is very important. It needs to reach over 17,000 MPH by the end of the ascent.

A few seconds later after the SRBs are released, the OMS engines (remember, the 2 small engines on either side of the back used for orbit maneuvers) fire for a few minutes, providing an assist boost to the 3 Main Engines.

T+2:15. Thanks to the assistance of the SRBs, Discovery’s at 3600MPH, 32 miles high and 43 miles away from KSC, and now accelerating quickly.

T+2 minutes, 39 seconds.

Houston: “Discovery, 2-engine Moron.”
Discovery: “2-engine Moron.”

No, Houston didn’t just insult Discovery’s engines. This is a call to Discovery indicating that they are sufficiently high and fast to make it to a Transatlantic abort landing site if just one of the engine fails. Still, if 2 engines fail, it’s an RTLS abort. If all 3 fail, I already told you. There are several transatlantic landing bases, which are checked before launch and selected due to weather. The two sites usually chosen are Zaragoza and Moron Air Force Bases in Spain. Right now, for this flight path, Moron AFB is chosen. However, all 3 engines are still doing great.

T+3:05. Discovery’s speed is 4,300MPH, 48 miles high and 83 miles away from KSC.

T+3:42. Discovery’s at 5000MPH, 56 miles high and 127 miles away from KSC.

T+3 minutes, 55 seconds.

Houston: “Discovery, Negative Return.”
Discovery: “Discovery copies, Negative Return.”

The first major milestone in the second stage ascent has been reached. Discovery is too high and too fast to be able to do an RTLS abort. Tensions rise, because in this period, 2 engine failures would be catastrophic, but Moron AFB can be reached if just one fails. However, the engines are still burning and at 104%.

T+4:18. Discovery’s at 6000MPH, 62 miles high, 177 miles downrange.

T+4:55. Discovery’s at 7000MPH, 65 miles in altitude, and 235 miles downrange.

T+5 minutes, 17 seconds.

Houston: “Discovery, Press to ATO.”
Discovery: “Discovery. Press to ATO.”

Finally, the shuttle is flying sufficiently fast and high to reach a safe lower orbit than planned if an engine fails. Then it’s just a matter of adjusting the orbit with the OMS engines and the mission goes as planned. But, all engines are still good. To date, only one mission suffered an ATO abort. Tensions rise again, because once again, a 2-engine failure would be catastrophic.

T+5 minutes, 39 seconds.

Houston: “Discovery, Single Engine OPS 3.”
Discovery: “Single Engine OPS 3.”

A welcome sound. At this point, an engine burnout would still initiate an ATO abort (which, when you come to think of it, isn’t really an abort, just a plan to reach a lower orbit and continue the mission), but if 2 fail, the crew has a failsafe. Now a Transatlantic abort to Moron AFB is possible.

T+5 minutes, 50 seconds.

Discovery’s zooming along at 9000MPH. The GPCs command the engines to swivel and cause the entire shuttle to roll to a heads-up position, or right side up again. By 6:15, the roll is complete. This is done to have a better signal and use the NASA TDRS Network, way high above the Earth, for tracking.

T+6 minutes, 3 seconds.

Houston: “Discovery, Single Engine Zaragoza, 104.”
Discovery: “Single Engine Zaragoza, 104.”

The Transatlantic Abort site has been switched to Zaragoza AFB, and can be reached on one engine. If just one of the 3 engines fail, it still triggers an ATO. This will change in a few seconds…

T+6 minutes, 20 seconds.

Houston: “Discovery, Press to MECO.”
Discovery: “Press to MECO.”

Most would breathe a slight sigh of relief at this point. The ATO option is discarded. Discovery is well on its way to orbit, and has a little over a minute to go until Main Engine Cut Off. If an engine should fail at this point, the MECO point can still be reached. If 2 fail, Zaragoza is standing ready.

T+6:40. Discovery’s at 11,000MPH, 66 miles in altitude. Some callouts are done around this point for the MECO plan.

Houston: "Your shutdown plan is nominal. You are GO for the Plus X, GO for the Pitch."

Discovery: "Nominal Shutdown, Go plus X, Go Pitch."

T+7 minutes.

Houston: “Discovery, Single Engine Press 104, good readback.”
Discovery: “Single Engine Press 104.”

An even bigger sigh of relief from everyone. All the scary abort modes are gone, and now, even if 2 of the 3 engines fail, the correct MECO altitude and speed can be reached. The ‘good readback’ remark was to indicate that Discovery read the previous instructions correctly (the MECO plan).

T+7:35. As the ET gets lighter because the 3 main engines have eaten up almost all of the liquid hydrogen and oxygen, the shuttle begins to climb excessively, and accelerates faster than it was doing. The acceleration becomes uncomfortable as the astronauts are pressed like pancakes to their seats. The shuttle also has some structural limits that can easily be overshot if the engines remain at 104%. This is why, at this point, the engines are commanded to throttle down until the GPCs see that the acceleration stress is just under 3gs, that is, they’re feeling they’re just under 3 times heavier than usual. Go to your bathroom scale, and multiply that number by 3, and think about what that means for you.

Astronauts are effin’ hardcore.

T+7:43. Discovery’s massive acceleration boost has it now at 14,800MPH, 64 miles high (over 337 thousand feet, well above the thick part of the atmosphere), and 700 miles downrange.

T+8:30. Discovery’s engines are commanded to start throttling down even more, in preparation for MECO, 8 seconds later.

T+8 minutes, 38 seconds.

The GPCs send the shutdown command to the 3 engines, and cuts off the hydrogen and oxygen flow to them. The engines instantly die out, and the acceleration ceases. In a particular Youtube video, which I will post below, show the several milestones. However, at 10:09 they call the throttle down, and at 10:12, you see the seats slowly bend forward as the engines’ power is lessened. At 10:18, the seats bend all the way to an upright position because they reached MECO, and the acceleration is now 0. It’s fun to hear them whoop as they finally reach initial orbit. Who says they don’t have fun? Apparently this guy doesn’t know that orbiter is spelled orbiter, not orbitor…