为什么SpaceX的指挥员在倒计时中总要说到“0”,而不是说“点火,发射”来代替呢?Why do SpaceX commentators keep saying ‘zero’ when counting down, instead of ‘ignition, liftoff’?

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)

更新:在得到6000观看和100推荐后,SpaceX在今天早上的DM-1任务的倒计时中说“0,点火,发射”。显然是想证明我错了。(添加于2019,5月2号,03:08)

They say “Zero” because the countdown reaches zero. Ignition does not occur at zero, although, technically liftoff should.

他们会数到“0”是因为他们的倒计时就是数到0的。并不会在数到0时才点火,但是,发射确实在那个时候。

All those terms have meaning in the parlance of launching a rocket.

所有这些术语在发射火箭的用语中都具有特别含义。

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.

这个“T-minus-ten”倒计时意味着离时间“T”还有10秒,而”T”就是发射的时刻。但时钟不会因为到了“T”而停止,而会改变符号继续。

So the count is:

  • T-minus-3
  • T-minus-2
  • T-minus-1
  • T-Zero
  • T-plus-1
  • T-plus-2
  • T-plus-3
  • etc.

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.

大多数时候,只有在发射前最后十秒才会说“T-Minus”,但在倒计时的时候,SpaceX的指挥员说“T-Zero”并没有错。在Falcon 9上数到0的那个时刻没什么特别的事会发生。当然,您很难凭肉眼看到火箭是多久起飞的。

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.

在数到0之前就已经点火了,在T Zero时刻发生的事就是发射台上牵制释放装置被放下,火箭准备发射。这应该就被叫做“发射”,但SpaceX似乎在看到火箭移动前就说出“发射“。【后面两句看不懂?】

(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.

在牵制释放装置释放火箭之前的任何时候,计算机都能终止Falcon 9发射。所以在数到0的时候没什么特别的——这只是在数学意义上来说应该在这个时间离开地面。

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.

倒计时数到0并不罕见。观看Delta和Atlas的发射,你同意会发现在倒计时时缺少了“点火,发射”。再说一次,只有当他们观察到火箭移动时,才会喊出“发射”。

So, why do we associate “Ignition and liftoff” with T Zero?

所以,为什么我们偏要认为只有在T Zero的时候火箭才点火发射呢?

Probably because of the Space Shuttle.

可能是因为航天飞机。

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.)

(探索者号的主引擎启动。注意它们是如何交错启动的,这让它们能够在助推器点火之前就让节流到达很高的水平)

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.

这是因为航天飞机使用两个巨大的固体火箭助推器,来帮助航天飞机离开发射台。大约在T-1的时候,这些引擎将被用火炬系统【?】点燃。这些电子点火器将点燃一个小型固体火箭,有点像火箭引擎的业余爱好者模型。这些小固体火箭将长长的火焰喷向SRBs的主推进剂颗粒。

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.

大约在T-0.25秒时,SRBs的顶部开始点火。在T-0.06秒时,电脑将决定助推器是否应该点火。如果是的,牵制螺栓将被切断,因为在T Zero时,这些SRBs将产生大约600万磅的推力,就算牵制螺栓还没被松开,巨大的推力也能轻松将它们扯断。

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.

然后航天飞机将离开发射台并进入太空。要确定肉眼可见的火箭运动并不用太久。通常来说,航天飞机在指挥员说出“点火”这个词时已经开始爬升了。不过有时候指挥员会在确定要说出下个词之前先说“和”。

So, the Shuttle countdown often sounded like…

所以,航天飞机倒计时听起来通常是…

“10, 9, 8, Go for main engine start, 4, 3, 2, 1, Ignition aaaand Liftoff of the Space Shuttle [insert name here]…”

“10,9,8,主引擎启动,4,3,2,1,点火和~~~发射航天飞机”

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.

这就是说,当电信号到达SRBs的顶部的点火器时,已经做不了其他决定了。航天飞机将在那一刻发射。就算航天飞机主引擎在T-0.01秒时失效了,但暂时也不会造成多大影响,航天飞机仍然会离开发射台。

Solid Rockets do not have an “Off” switch.

固体燃料火箭一旦启动,就没有“暂停”选项。

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.

因此,对于航天飞机来说,“点火和发射”密切相关,这是我们从1981年到2011年一直在NASA指挥员那里听到的,所以我不能怪您在其他火箭的倒计时中期望在听到它。 他们如此强调这一点,因为至少从宇航员和工程师的角度来看,这是发射倒计时中最终的“激动人心的时刻就要到来了”的时候。

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