67

In addition to the answers that have been given, it should be noted that there is a window of opportunity for a launch to occur, known as a launch window. This certainly occurs when trying to get somewhere such as the International Space Station, but even earth orbiting satellites have a window of time where the rocket can meet its performance. Bottom line ...


54

T minus zero appears to generally indicate the moment of booster liftoff, but can also indicate something different. To borrow Cort Ammon's wording, it's an action or event that results in a substantial change in the amount of control you have over the situation. Taking the countdown for the Falcon 9 as an example, T-00:00:00 is the instant of lift-off; in ...


31

There is a long list of things that have to happen to launch a rocket after it gets to the pad. Fueling, engine gimbal tests, sensor checks, computer start ups, etc, etc. There's a certain order these have to go in. A countdown provides the backbone for the planning of all this. Don't think of it so much as "time until launch," but rather "progress through ...


26

Hydrogen-oxygen engines produce a relatively faint blue flame, with visible blue-white shock discs or diamonds under certain circumstances, particularly low altitudes where air pressure confines the plume. At high altitude the plume is nearly invisible. You can look at a number of shuttle launch photos and see the different appearances the main engine plume ...


24

Chilldown is needed for engines using cryogenic propellants -- liquid oxygen as the oxidizer most commonly, sometimes in conjunction with liquid hydrogen as the fuel. Pumping large amounts of very cold fluids into an engine resting at ambient temperature causes a variety of problems, so the plumbing and pumps need to be brought down to a working temperature ...


23

No, they were never off. Here is my explanation: have you ever noticed that the top engine is tilted down? This is us to keep the center of thrust more or less aligned with center of mass of the space shuttle. As the SRBs produce 83% of the thrust at lift-off, a counter-force was needed to go straight-up. With shuttle engines off, I am not sure the whole ...


21

Imagine a colonoscopy for a rocket engine. You go in with a flexible light pipe that has a camera on the end. (Engine is nicely sedated, usually). It looks around, cuts out any polyps it sees, oh I mean obstructions. So if a valve failed to open/close, they can see that without taking it all apart.


17

This article might offer some clarity. The Saturn V was held down to the pad until T=0. The engines started up several seconds before. The engines took a few seconds to build up to and stabilize at lift-off thrust. During that time, if an anomaly is detected, the engines could be shut down, leaving the vehicle still safely anchored to the pad. Once the hold-...


14

The go/no go poll is the last major hurdle before the launch. If one of the polled stations says 'no go', the launch will be delayed. If the rocket is full of subcooled propellant, that delay means the propellant will warm up and start to expand. They want to avoid that, so they decided to move propellant loading to after the go/no go poll, which means ...


14

Countdown is more than the "60", "30 seconds", 10,9...1 we see on TV, or hear on the PA adjacent to the launch site. Countdown commences well before the launch date with regular checkpoints for various systems and parts on the schedule as Thomas Tarrants mentions in his answer. It is also a process that may be forked - more on that below. In addition to ...


13

The report via Tweet from Elon Musk initially is that a slower than usual ramp up of thrust caused the flight computers to abort. Elon Musk via Twitter: Launch aborted by autosequence due to slower than expected thrust ramp. Seems ok on closer inspection. Cycling countdown. There is reported leeway in relaxing some of the sensitivity on this issue ...


12

A planned hold adds flexibility to the schedule. The hold at T-4 was planned to be 10 minutes long if everything went without a hitch. If a problem had cropped up (e.g. weather suddenly deteriorates and goes outside the launch limits), the hold would have been longer. If there were delays earlier in the countdown, the T-4 hold might have been shorter. The ...


12

To add to @aramis' excellent explanation on clarity and ability to discern the go/no-go launch status check polling, these seem to have been introduced to NASA's (and U.S. in general) launch terminology during the first manned spaceflights of Project Mercury. I wasn't able to find a good example for Project Mercury launches, but I did find this video of ...


12

Having worked mission communications in CAP for live Search and Rescue operations, on the radio, go/no-go is VERY clear. It's one syllable versus two. (Three is the interrogative.) The longer "affirmative" "negative" are 4 and 3, and not as clear, and further, subject to more interference due to length. Note that this usage parallels the morse use of C (-.-...


10

I imagine that borescoping allows them to get a look at the inside of the engine's combustion chamber without having to disassemble it. If the problem was physical damage or obstruction to the injectors, for example, this could be visually detected. In this case, it likely wouldn't have shown the contamination of the ignition fluid; it would rule out some ...


10

When the rocket is in design phase the designers usually design the the graph known as thrust-time graph. Nowadays the flight computers integrated to the rockets measures the thrust build up during lift off (which is important phase) if the thrust build up is not satisfactory the flight computers close the fuel and oxidizer inlet valve and shut downs ...


10

The awkward pause is mostly due to the commentator. He counts along with an on-screen counter until T-0. Then he waits until he sees the rocket move, or he hears ground control confirm liftoff, before he says 'liftoff'. Generally, at T-0, the hold-down clamps are released and the rocket begins moving, but it may be a few seconds before that's visible to the ...


10

The phrase 'with the nose to the east' is a shorthand. The direction depended on the orbital inclination chosen for the mission. Missions to the ISS have to match the 51.6° inclination of the ISS the possible inclinations are limited by the launch location: from Cape Canaveral, you have to launch over water, so in a direction roughly between North-East and ...


8

How else would you do it? Choose a random time? Really, the purpose of a countdown is to make sure all the engineers know exactly when liftoff should be, and how much time they have to make last minute adjustments. There are a surprising number of things that have to work together, and a countdown allows engineers to start those things working at the right ...


8

"CLG INIT" stands for "closed loop guidance initiation", and means that the launcher is switching into a mode where it actively steers based on its current trajectory rather than a fixed, unconditional program (referred to as "open loop guidance"). In the early part of flight, it is safer to use a "pitch-versus-time" or similar simple logic to guide the ...


8

Offered as a supplement to the other answers - here's a frame from an SRB mounted camera (post-separation) showing the SSMEs running in the absence of SRB plumes. As stated by others, there's no real visible SSME plume at this altitude, just a glow inside the nozzles.


8

It was filled through the "T-0 umbilicals" (referring to the time of disconnect). LO2 through an umbilical on the Orbiter boattail, LH2 through one in the midbody. This schematic shows the plumbing from the umbilicals to the Centaur through the CISS (Centaur Integrated Support System). Source


7

From Orbcomm OG2 Mission 1 Update (June 20, 2014): During today’s countdown, engineers noticed fluctuations in pressure on the Falcon 9 vehicle’s second stage and are taking additional time to evaluate. The next available launch opportunity is tomorrow evening, Saturday, June 21st. Additional updates will be posted as schedules are confirmed. ...


7

Yes, there was at least one. Well, we're pretty sure he didn't make it.


7

Everything I can find says the crew would have died. Depending on which one lit, the space shuttle would have either ended up in the ocean, or smacking the Launch Control Center. Of course, before either of those events would happen, the Flight Termination System would have been activated, which would have caused all 3 of the rocket stages to explode. Even ...


6

I'm not sure where you're getting your 30-40 minute figure; initial orbital entry generally takes about 10. Getting into the exact orbit you want takes longer, of course, but getting from ground to a reasonably safe and stable orbit is pretty quick. Apollo 11: 12 minutes Soyuz: 9 minutes Long March/Shenzhou: 10 minutes Getting to orbit substantially faster ...


6

Borescoping is nothing other than optical inspection of the engines and plumbing from inside. The purpose is usually to look for debris and/or damage in the fluid ducts. I don't think "how long" can be answered because there are too many variables: what is the purpose, does it succeed, what is the configuration, what is the equipment. On STS it was a non-...


6

For Space Shuttle: In the flight software, "tower clear" was defined as the end of the vertical rise phase when it was OK to start the Single Axis Rotation (aka "roll program"). This happened when the vehicle center of mass was at 376 feet above the Fischer Ellipsoid. Source - Ascent Nominal I-Loads Definition and Verification (not online) However, "...


5

There are several factors at work here. Edit: Noise canceling is not used in Mission Control. It is used in the astronauts' headsets, and has been since the 1960s. Headset. Businesses typically use $20 headsets with lousy microphones and no noise cancelling. They have a non-adjustable microphone boom so the mic is always too far away, and tiny, low-...


5

As discussed in the related questions TildalWave posted in a comment it really depends on the rocket (because I have it here's an article about a Soyuz launch in a snowstorm) and what the range they're launching from supports. Considering you asked on April 27th you were probably watching the SpaceX TurkmenÄlem52E/MonacoSat (Thales) launch. For that ...


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