Update: After over six thousand views and 100 upvotes, SpaceX went ahead and did a “Zero, Ignition, Liftoff” call-out on the DM-1 mission this morning. Apparently just to prove me wrong… (added – March 2, 2019 03:08 AM Eastern)
They say “Zero” because the countdown reaches zero. Ignition does not occur at zero, although, technically liftoff should.
All those terms have meaning in the parlance of launching a rocket.
The “T-minus-ten” countdown means that there are ten seconds remaining until you reach “T”, where “T” is launch, or “Takeoff” time. The clock doesn’t stop at zero, mind, you, just the sign on the number changes.
So the count is:
Most of the time, the “T-Minus” gets dropped during the last ten seconds, and only the numbers get counted, so the SpaceX commentator is completely right in calling out Zero at “T-Zero.” There’s nothing special going on at T-Zero on a Falcon 9 flight that isn’t expected to happen. Also, it’s difficult to visually ensure that liftoff has taken place.
On the Falcon 9, main engine start begins around T-3 seconds, with all 9 engines starting in a slightly staggered pattern over about 1 second. By T Zero, they should all have reached 100% throttle, and be ready to release.
在Falcon 9上，主引擎在T-3左右启动，所有9个引擎会在随后一秒陆续启动。在T Zero时，它们的节流阀已经被推到100%，随时准备起飞。
So ignition has occurred well before zero. The only thing that actually happens at T Zero on the Falcon 9 is that the pad hold-downs release the rocket and it begins to move. This could be called “liftoff”, but SpaceX seems to avoid calling out “liftoff” until they can visually confirm rocket movement. They do usually do a call-out at around T+2 or T+3 seconds. On rare occasions they’ll call out tower clearance at T+7 or thereabouts.
(Note the callout in the box — there was audio call-out of liftoff at the same time.)
At any time up to when the hold-downs actually release the rocket, the computers can abort the liftoff of the Falcon 9. So, there’s nothing special about zero — it’s just the time when the math said the rocket should leave the ground to correctly hit its desired orbit.
And this calling out of “zero” actually isn’t unusual. Watch launches of the Delta, or the Atlas, and you’ll hear the same lack of “ignition and liftoff” on the countdown. Again, sometimes they’ll do a liftoff call-out once the rocket has visually begun accelerating upward.
So, why do we associate “Ignition and liftoff” with T Zero?
Probably because of the Space Shuttle.
Much like the Falcon 9, the main engines on the shuttle were actually ignited well before liftoff. The SSME startup sequence began at about T-6.6 seconds, and by T-3 seconds, all three main engines had to be running at 90% or more, or the computer would shut down the launch. This was critical because the computer needed to decide whether to abort the launch before the countdown clock reached T-1.
就像猎鹰9一样，航天飞机上的主要引擎实际上在起飞前就点燃了。 SSME启动程序大约在T-6.6秒开始，到T-3秒，所有三个主引擎的节流阀已经到了90%或更高，否则计算机将终止发射。 这很重要，因为计算机需要在倒数时钟达到T-1之前决定是否终止发射。
(Main engine start on Discovery. Note how they stagger the starts, and then allow them all to come up to even throttle long before the boosters ignite.)
That’s because the Space Shuttle used two massive solid rocket boosters to actually lift the shuttle off the pad. Those engines were ignited by a flare system that was activated right about the time T-1 rolled around. These electrical igniters started a small solid rocket sort of like a hobbyist model rocket engine, that fired a long flame down into the main propellant grains of the SRBs.
At about T-0.25 seconds, the top of the SRBs began to ignite. By T-0.06 seconds, the computer would decide whether the boosters had actually ignited, and, if so, they would cut the restraining bolts, because by T Zero, those SRBs would be generating about 6 million pounds of thrust that would shear the restraining bolts right off if they weren’t released.
Then the shuttle would leap off the pad and head for space. It didn’t take long to confirm movement visually. Usually the shuttle was visibly climbing before you could finish the word, “ignition”. Sometimes the announcer would hang on the word “and” before confirming it.
So, the Shuttle countdown often sounded like…
“10, 9, 8, Go for main engine start, 4, 3, 2, 1, Ignition aaaand Liftoff of the Space Shuttle [insert name here]…”
The thing is, from the moment the electrical signal went to the igniters at the top of the SRBs, there were no more decisions to be made. The Shuttle was going to launch from that moment on. Even if all three shuttle main engines failed at T-0.01 seconds, it didn’t matter, the stack was leaving the launch pad.
Solid Rockets do not have an “Off” switch.
So, for the space shuttle, “Ignition and Liftoff” were a pretty connected event, and it’s what we heard from NASA commentators from 1981 until 2011, so I can’t blame you for expecting that in the countdown. They probably emphasized it so strongly because, at least from the standpoint of the astronauts and the engineers, it is the ultimate, “Hold on to your butts,” moment in the launch countdown.