Space Shuttle Missions Background Information
What does "T" stand for?

The "T" in "T Minus 43 Hours" means "Time until launch". It means that there are 43 hours until the scheduled liftoff according to the countdown clock. There are numerous "built-in holds", where unscheduled problems can be corrected, and still launch "on-time". There is also the "L" (Local time) clock.

If it's 1:40 PM, and liftoff is scheduled for 2:40 PM, it is "L Minus 60 minutes", but "T Minus 20 Minutes, and holding". On the L, or Local clock, we're a whole hour away from launch, but we are in the midst of the built in hold at T-20 Minutes, and we still have a 10 minute built-in hold at T-9 Minutes to go. This means they plan to "pick up the count" at 2:10 PM at T-20 minutes. They will stop the count again at 2:21 PM at T-9 Minutes. When they come out of the hold at 2:31 PM, they plan to launch at 2:40.

You can order a copy of a NASA Space Shuttle Countdown Book directly from Amazon.Com.

What To Expect During A
Space Shuttle Countdown

by Robert Osband

Ho, hum. Another Space Shuttle launch. When you live here in Titusville, it's easy to get blasé. For our visitor's, however, it can be a once in a lifetime experience - and they don't know what to expect. Here's a little background on what goes on - and what to look for - during a Space Shuttle launch.

T Minus 43 hours. It's Monday. The launch will be on Thursday, and that's alot more than 43 hours away. But that's what the Countdown clock reads when they start going down the most complicated "checklist" in the world. There are a number of built-in "holds" in the countdown. They are there in case there is a small glitch along the way, and this "hold time" can be used to clear up small problems, and clear the way for the rest of the countdown.

T - 3 Hours, and holding. During this hold in the count, the Space Shuttle is fuelled up with liquid oxygen and liquid hydrogen. Combined with the solid rocket boosters, the Space Transportation System will produce 7 million pounds of thrust to lift 4 million pounds of spacecraft. Be sure to check the news reports, because if they've stopped fueling for any reason, you may as well head back to Orlando for the day.

T - 3 Hours. As NASA "picks up the count", the astronauts have been awakened, fed breakfast, presented with a cake with the mission emblem on it, and suited up in their "Launch & Entry suits", the flight suits they will wear when they launch, and again when they and their spacecraft returns to Earth. By now you want to be well on your way to Titusville.

T - 2-1/2 Hours. The astronauts will be strapped into their launch seats, and will begin a series of "Comm Checks" (communications tests) through a variety of frequencies available for air-to-ground communications. You can hear these over the NASA Select channel, if your "radio scanner" is set for 146.940 MHz, where the local ham radio operators broadcast it.

T - 20 Minutes. A built in hold begins. This is normal. On NASA Select, you'll hear a spiel from the Houston PAO (Public Affairs Office), who will take over from the KSC PAO (the commentator at the Kennedy Space Center who is explaining what the guys on the comm channels are talking about) shortly after lift-off. This tests the switch over procedures. Also, Program Managers will be "polled" at this time to make sure each of their departments is "Go" for launch.

T - 9 Minutes. The last built-in hold of the countdown. Sometimes they won't come out of the hold, and "work" the problem for a while. If it ain't fixed by now, they've got 4 more minutes to do something, or they have to hold - and maybe "scrub" the mission for the day. The 4 minutes I'm referring to is the time it takes to get to T - 5 minutes where the real Go/No-Go decision has to be made.

When the count does pick up at T-9 minutes and counting, it will be under the control of the Ground Launch Sequencer, a major mainframe computer that controls the final countdown. During the T - 9 minute hold, the guys on the consoles will be asked for their "go or no-go" decisions. Each console has displays and gauges showing each department's responsibility during the launch. They are looking for "out of specification" readings that could put a hold in the count. Safety is foremost in everyone's mind. These are their friends up there strapped into that spacecraft.

As you hear the TD (Test Director - a hold over from earlier days) call out the acronym of the guys on the consoles, each one yells back "GO!". Usually. The guys at T-20 were the bosses, who wanted their bosses to hear them over NASA Select to show how important they are. But these guys on the display consoles are the guys who count!

Watch out for the SRO (Supervisor of Range Operations). If he notices a ship or a plane in the "splash down" area of the solid rocket boosters, he's likely to hold the count. He's the most unpredictable guy on the consoles. Normally, when SRO is happy, everyone is happy. Weather is the only other console that can hold a countdown short at T-5 minutes, and still expect to launch today.

T - 5 Minutes. It's nail-biting time. If there is the slightest chance that the weather will change, they will "march it down" to T-5, and hold the count there. It is at the T-5 minute mark that the CDR (Commander of the Shuttle, and most of the crew acronyms are spelled out, so the Commander is Cee Dee Are) is given the "Go to start APU's". The Auxiliary Power Units provide the hydraulic power to move the rudder and the ellivons (combination ailerons and elevator on the Shuttle's delta wing), as well as gimbolling the Main Engines. It is at this time that they actually start using a major amount of "consumables" - the fuel of the APU's. They don't start using consumables unless they think they are going to get off the pad today. They can't test these systems until they power up the APU's, so this is another point where they might hit a new glitch that could halt the countdown. They have to have 3 good APU's to be allowed to proceed for launch.

T - 31 seconds. It is at this time when the Ground Launch Sequencer (GLC) "hands off" control of the countdown from itself, to the on-board computers in the orbiter. If there has been some minor glitch that the launch controllers told the GLC was "OK" by "patching" the software, then the on-board computers may stop the launch at this point, because no one told it that the out of spec parameter is OK. This kind of launch glitch hasn't happened in a while, but there's always a chance. If you get past T-31 seconds, you're at least going to "light the candle".

Photo by Ozzie@SpaceyIdeas.Com

STS-91 rises from Launch Pad 39-A
as seen from Space View Park.
T - 6 seconds. Things are starting to happen really quickly now. The computers are checking everything - much faster than humans could. If there is a glitch at this point, the computer will shut down the engines, and everyone in the control room goes to the red pages of their countdown book. Usually, everything is cooking just fine at this point. When the Space Shuttle Main Engines ignite, it causes the shuttle to lean over a bit in a phenomenon called "twang". As the shuttle comes back to the vertical, the countdown reaches...

T - 0. LIFT-OFF!! The Solid Rocket Boosters are ignited, explosive bolts that have kept the shuttle on the ground for the last 6 seconds are blown, and the Shuttle leaves the pad.

The Countdown is now over. We begin counting in Mission Elapsed Time, MET.

7 Seconds MET. The shuttle clears the tower of Launch Complex 39. Control of the mission switches from the Kennedy Space Center, to the Johnson Space Center in Houston TX as the astronauts travel "uphill". (Hey, that's what they call it. Who am I to argue?)

40 Seconds MET.The sound of the launch reaches Titusville - yes, we are that far away (12 miles) from the launch pad.

1 Minute MET. Max-Q - the shuttle goes through Maximum Dynamic Pressure where, as it passes through the sound barrier, the ship is "throttled down" until through Max-Q where the atmosphere is the thickest (and the atmospheric forces acting on the spacecraft are the greatest). It is then "throttled up" again, once the ship is past this point. To over simplify a bit, it's like slowing down to go over an atmospheric "speed bump". Once past Max-Q, we throttle up to 104%.

Why 104% ?

by Robert Osband,
Technical Editor,

When the designers of the Space Shuttle "locked in" their design for the Space Transportation System, the Space Shuttle Main Engines were given a specific power rating of 100%. When the engineers of the Rocketdyne company, the engines contractor, were finished, their engine actually ran at 109% of the established criteria. NASA decided that there would be a "safety margin" of 5% that would only be called upon in an emergency, so the Space Shuttle's main engines would only be "throttled up" to 104% of their 1972 paper-bound specification.

2 Minutes MET. The Solid Rocket Boosters drop off. Old timers still call this "staging", based on the way the old Saturn V used to drop stages (and unmanned rockets still do). If the shuttle is not lost from view in low clouds, this "staging" is visible from the ground with the naked eye, though binoculars are helpful. These boosters will deploy parachutes, and land in the ocean 140 nautical miles downrange, where two ships will retrieve them and return them through the Port Canaveral channel the next day. There are plans on the drawing boards to use liquid fueled boosters, link them with a large wing between them, and fly them back to the Shuttle Landing Facility as Remotely Piloted Vehicles saving lots of money in the process.

4 Minutes MET. On a day launch, this is as long as you can see the Shuttle without binoculars.

7 minutes MET. On a night launch, the STS is going over the horizon, and you lose sight of the spaceship in the haze of the horizon at this time (you'll lose it before this, unless you're keeping a careful watch during the entire ascent).

8 Minutes MET. MECO. Main engine Cutoff. With this call over the radio, the shuttle is in space.

45 minutes MET. The External Tank, which contained the fuel used by the Main Engines on the shuttle orbiter, is jettisoned. The orbiter, now half a world away (half an orbit, really), fires it's Orbital Maneuvering System engines to circularize it's orbit, and the crew is now safely orbiting the Earth. The external tank, without benefit of this rocket firing, re-enters the atmosphere, burning up over the Indian Ocean (it doesn't have heat resistant tiles, either).

The CDR (Commander), PLT (Pilot), MS (Mission Specialists), and PS (Payload Specialists) now begin their stay in space.

The Countdown Book

The countdown is really a "checklist" that operates on a timeline that counts backwards to the precise time that spacecraft controllers want to launch their spacecraft. At every console is a book with all the procedures that must be followed, in their precise order, and everyone keeps track of the status of the launch by following along in their copy. A "check mark" is placed at every procedure when it's reported completed.

And when they get to that all important poll of the Consoles, each console operator knows when to shout "GO!", because the TD will call off the Consoles by their acronym or title in the order provided in the Countdown Book. When the launch is over, everyone signs the covers of everyone else's book as a souvenir of a job well done.

You can order a copy of a NASA Space Shuttle Countdown Book directly from Amazon.Com.

Listening to the Countdown

You can hear the NASA Select Channel on a "radio scanner" of the type sold at Radio Shack. Tune to 146.940 MHz for the Merritt Island ham radio repeater where the LISATS (Launch Information Service and Amateur Television System) ham radio club has a line into the Space Center. A backup frequency is 147.135 MHz. The Titusville "chat" frequnecy is 146.910 MHz. If you're a ham operator yourself, call N4SCY on 146.970 MHz in Titusville, and I'll be happy to talk you into a good viewing site.

You can tune to WMEL on 1300 AM from Melbourne, which also carries unmanned launches. WMMB is also from Melbourne on both 1240 and 1350 KHz on AM. Local station WIXC on 1060 AM will sometimes carry the launch. On FM, National Public Radio member station WMFE on 90.7 MHz FM will pick up the count at T-5 minutes, and carry the launch until 2:30 Mission Elapsed Time (after the Solids drop off, and the most dangerous part of the mission is over). On most flights, local TV stations will also adhere to this schedule of launch coverage.

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