Most of these are borrowed from a thread on NASA Space Flight (we’re big fans). We have just consolidated them for easier referencing.

Jump to the launch process explanation.

Q) Does the Commander of the shuttle (orbiter) control it during launch?
A) The Commander (CDR) doesn’t fly the orbiter.The Obiters are controlled by flight computers on the way to space, and the astronauts monitor the systems and make any required corrections on ascent. The two “important” astronauts on the flight deck are the CDR and PLT in the front two seats. They are used more on landing (after coming back through re-entry, which the orbiter navigates herself through with the flight computers). See Avionics.

Q; What are the sparks just before the main engines ignite? Is that what ignites the fuel?
A; Those are ROFI’s, or Radially Outward Firing Initiators, used to burn off excess hydrogen to prevent an explosion. They usually come to life at about T-10 seconds

Q; After MECO, the orbiter isn’t in an orbit yet, and isn’t fast enough to go into orbit. Is that what the Orbital Maneuvering System (OMS) is for? Are the OMS engines fired twice to get to the orbit?
A; The OMS is fired once to get into an elliptic orbit and as the distances between the orbiter and its destination close, the OMS is used for larger corrections to the elliptical orbit.

Q; If the orbiter has to go to a higher than usual orbit, a Hubble servicing mission for example, does it simply fire the OMS longer, or do the SSMEs burn longer as well and do the orbiter and ET SEP higher up?
A; No, the ET is always launched 100% full. The ECO’s (engine cut-off sensors) shut the engines down when they’ve drained the ET of 95% of its propellant, or at the prelanned orbit, whichever comes first.

Q; Is MECO (and ET SEP) always at the same altitude or does it vary?
A; It varies only slightly (couple miles).

Q; Is there a possibility that the MECO is so late that the ET goes into a low orbit itself and circles the earth a couple of times or can that never happen?
A; Can’t happen. The ET doesn’t have enough speed to keep out of the atmosphere and burns up in the atmosphere, with denser parts (pressure vessels, batteries, etc) falling harmlessly into the Indian Ocean.

Q;What happens if there’s an emergency on the orbiter well after SRB SEP but shortly before MECO?
A: The orbiter will do what’s called AOA, or Abort Once Around. If controls weren’t put into the computers, the orbiter would follow the ET into the Indian Ocean, but with the commands already in the computers, the OMS would fire for a bit, allowing the orbiter to reenter at about the same place as the ET, but it would have enough velocity to glide back to KSC for a landing.

Q; Has anyone ever stood in the orbiter during reentry? Is it safe?
A; Yes, Story Musgrave stood during a reentry and had absolutely no trouble doing so. He recorded the flight deck while standing to the aft of the PLT and CDR.

Q; What would happen if one SRB ignited and the other didn’t? Can it still make it up on one?
A; In short, no. The Shuttle stack would cartwheel off the pad, into the ocean if the left SRB ignited, possibly taking part of the pad with it. If the right SRB lit and the left failed, then it would likely cartwheel into the LLC and VAB area, killing everyone in the vicinity. The Range Safety Officer would not really have enough time to self destruct the Shuttle in time to try and stop either event. Either way, odds of crew surviving are extremely small.

Q; Aren’t the hold-downs strong enough to hold her down with the SRBs going? If you didn’t get positive ignition on both sides, could you cut SSMEs and wait it out?
A;Would make no difference. NASA techs release the hold down bolts before we send the signal (1/100ths of a second before) to the SRBs to fire. It’s a hugely unlikely event, but I think John Young once claimed you could get off the pad and try and ‘get light’ on a very slow and dangerous assent before a RTLS abort and called it a “sporty ride”. But that’s John Young for you.

Further, there have been a few times where one or two bolts did not fire. It just rips them out and carries them with it. The bolts are there to hold it to the pad during rollout and in high winds, mainly.

Q; Why does the Shuttle “Roll”? Why can’t it just arch on the way to Orbit?
A; The reason we do that roll is the launch pad had to be situated in such a way that the Orbiter could not be pointed in the correct direction when we launch. The launch pad was built for the Saturn Vs and it would have cost a huge amount of cash to re-situate them. Also it is more stable for the Orbiter to ascend in a heads-down configuration – its more stable that way and it also gives the astronauts a view out their window that gives them some orientation to where the earth’s horizon.

Q; How does the Shuttle actually roll? I see people say it doesn’t use its wings on lift off.
A; The SRBs nozzles pivot/gimbal causing the stack to twist into the roll, it allows line of sight of the communications (S-band) antenna which is located above the crew cabin, aerodynamic forces and weight balance are lessened a bit (stress is lessened) and the main engine thrust vector is balanced as well.

Q; How do they load the fuel into the SRBs? I know the pump the propellant into the ET, that makes sense, but the SRBs?
A; The propellant is poured into each segment, much like concrete or Jello into a mold. It is then cured and hardened into something that can be handled. The exact process is probably a closely guarded company secret. For at least the segments with the circular hole for the flame path, a plug is in the center of the casing ‘mold’ and that is later pulled out leaving the void. I have never read if the star-segments are molded that way or machined later. Perhaps someone else can say? To guard against inconsistencies in the batches of propellant, two cases are loaded in matched pairs so that the thrust is equal on both sides of the stack.

Q; How long does it take to completely open those huge high bay doors? Is it a matter of hours of 10′s of minutes?
A; A few minutes; the lower the panel the longer. The closing of the VAB doors (whatever part may be open) before launch is a part of the countdown. You can watch them close fairly quickly.

Q; The STS was designed to service space stations and fix satellites (I think…) why is there not a docking port internally within the crew quarters but only externally in the payload bay?
A; It was never intended primarily to be a ship that would be visiting space stations, even though many of the sales pitches had it on the side of a space station. When it was conceived, it was sold as a reusable and more economical alternative to the standard disposable stage rockets. The idea was to have a ship that could deliver payloads, in various ways, have multiple scenarios while delivering, be able to retrieve payloads that had malfunctions and basically have versatility on the side of them.

Q; What was the reasons for changing the STS mission # on the early fight after STS-9 to the, for example. STS-41D#
A; Because they thought there would be 40 flights a year and didn’t want to wind up with STS-400. First number is for the planned year of launch, second for the launch site (1 is KSC, 2 is Vandenburg AFB) and the letter was the planned mission of that year by number (A1, B2 etc).

Q; I’ve always been interested in how the orbiter stays on the side of the ET during launch. What are the attachments made of to keep the Orbiter from falling off? Separation seems so simple.
A; Short summary:
“The forward structural attachment consists of a shear bolt unit mounted in a spherical bearing. The bolt separates at a break area when two pressure cartridges are initiated. The pressure from one or both cartridges drives one of a pair of pistons to shear the bolt, “etc… The aft structural attachment consists of two special bolts and pyrotechnically actuated frangible nuts that attach the external tank strut hemisphere to the orbiter’s left- and right-side cavities. At separation the frangible nuts are split by a booster cartridge initiated by a detonator cartridge. ”

Q; On some of the launch videos you can see a flame tower to the side of the launch pad, like an oil rig. What is that and what does it do?
A; It’s called a flare stack. It burns off excess hydrogen in a controlled manner, to prevent any possibility of a powerful free-air explosion. Another fuel is added to the flame so it can be seen easily, confirming that it is working. When a shuttle is being fueled on the pad, some of the LH2 propellant warms up sufficiently to evaporate back into a gas inside the tank. This hydrogen gas is highly explosive so it needs to be removed and safely disposed of. The gas is vented from the External Tank, piped a fairly long way from the pad to the “Flare Stack”, and is then burned safely in the atmosphere.

Q;What is the white smoke that comes from the top of the ET?
A; Gaseous Oxygen (GOX)

Q; What does the External Tank do and what is “stable replenishment”?
A; “The technicians pump in more than half-million gallons of super-cold liquid oxygen (LOX) and liquid hydrogen (LH2) into two tanks inside the ET, separated by a intertank (not a tank itself, just part of the ET structure two thirds up in between the two tanks. 143,000 gallons of liquid oxygen chilled to -298 degrees Fahrenheit goes into the LOX tank (the top third) and 385,000 gallons of liquid hydrogen chilled to -423 degrees Fahrenheit into the LH2 tank. The tanking process takes three hours and is pumped from storage spheres at the pad, through feed lines to the mobile launcher platform, into into the orbiter’s aft compartment and finally into the ET.

Now you can’t leave it at that, because the cryogenic nature of the oxidizer and propellant means they boil off. It’s not a major boil off rate on the Shuttle ET as its highly insulated (the infamous foam shell) but you have to keep slowly pumping to keep the ETs topped off right up until the final couple of minutes before launch.

If something stops you from doing that, and you are delayed while you try and fix it, all the time you’re losing quality in your tank and the more you’ll need to top it up when you’ve fixed the problem. There’s also pressurization with Helium, which they also need to go and get more of, but to answer your question on “stable replenishment” that is how we do it with the Shuttles.

Q; How often do they repaint the orbiters? In some launch pad pics, they’re gleaming white; in others, they’re pretty dirty.
A; They’re not painted. To learn about it, the orbiters are 100% covered in tiles (all the black and gray; the gray is RCC panel) and thermal blankets (most of the white). They have replaced much of what used to be white tiling with white thermal blankets; and in addition, they replace tiles and blankets after missions if they took enough wear and tear such that they need to be.
Up close the orbiters always look pretty dirty from their many years of use and reentry heating. Only the newly added tiles and blankets may appear to gleam. New tiles have a hard semigloss coating on the outside which can shine in the sun.

Q; What is the meaning of the term “Press” as in “Press to MECO”, “Single engine Press 104″, etc?
A; “Press to MECO” is usually called up to crew in form of “Single engine press to MECO”. This means that in the event of two engine failures, the orbiter has gained sufficient energy to continue to the planned MECO trajectory on the single available engine. “2 engines TAL” means that the orbiter has gained sufficient energy to perform a Transoceanic Abort Landing using only two engines if needed. Another answer; 104% is the “nominal” engine setting used now (and why it’s > 100% is a story for another time…). “Press” is used as in “press on…” which for the non-english as a first language speakers is a phrase meaning “continue”.

STS Launch Countdown Sequence
Launching a Space Shuttle is an incredible and challenging process filled with tons of work. Here are the main milestones for the last 9 minutes before launch:

» Start automatic ground launch sequencer (T-9:00 minutes)
» Retract orbiter crew access arm (T-7:30)
» Start mission recorders (T-5:30)
» Start Auxiliary Power Units (T-5:00)
» Arm SRB and ET range safety safe and arm devices (T-5:00)
» Start liquid oxygen drainback (T-4:55)
» Start orbiter aerosurface profile test (T-3:55)
» Start main engine gimbal profile test (T-3:30)
» Pressurize liquid oxygen tank (T-2:55)
» Begin retraction of the gaseous oxygen vent arm (T-2:55)
» Fuel cells to internal reactants (T-2:35)
» Pressurize liquid hydrogen tank (T-1:57)
» Deactivate SRB joint heaters (T-1:00)
» Orbiter transfers from ground to internal power (T-0:50 seconds)
» Ground Launch Sequencer go for auto sequence start (T-0:31 seconds)
» SRB gimbal profile (T-0:21 seconds)
» Ignition of three Space Shuttle main engines (T-6.6 seconds)
» SRB ignition and liftoff (T-0)

The Space Shuttle launch countdown sequence is based on the S0007 document, available for download in Text file or PDF. Think you can launch a Shuttle? Enter the Firing Room and test your skills against the NASA countdown simulator.