[annie123 (OP)]

I often hear the countdown for space take offs starting 2 minus 10 . . .What does that mean? Why does the cou ntdown begin like that?

[evan_au]

2 minus 10

I think you misheard - the phrase is "T minus 10".
- where "T" is a scheduled milestone in preparation of a rocket for launch
- The dramatic milestone that most people hear about is the actual liftoff.
- I recall that the first attempt to launch a space shuttle was put on hold at T minus a few minutes due to a computer lockup, and eventually scrubbed.

Apparently, the launch countdown was originally used as as device to increase suspense in a movie, and then adopted by NASA.
See: https://en.wikipedia.org/wiki/Countdown

[alancalverd]

I'm fairly certain that some films of V1, V2 and Komet test launches show a mechanical timer superimposed in one corner of the frame. Presumably this device, or whatever was driving it, initiated the pre-launch sequence of pumps and igniters so it was a countdown device  even if the absolute time of launch wasn't critical.

Whilst the newsreels generally only show the bit from T - 10, an intercept mission might start from T minus several months or years, and will include critical points for delivery and testing of subsystems for a vehicle that only exists on paper at the beginning of the count.

[Petrochemicals]

It also includes T plus time post launch, ie t plus 60 whilst the rocket it ignighted. "ROCKET FAILURE AT T PLUS 45 SECONDS"

[annie123 (OP)]

What happens at all the letters before T? Are there 20 checks that come before that that are recorded?

[Bored chemist]

What happens at all the letters before T? Are there 20 checks that come before that that are recorded?

All the other letters  are not the first letter of the word "time".

[alancalverd]

Like D-day: there was no A B or C day. Or Zulu time.

[annie123 (OP)]

where "T" is a scheduled milestone in preparation of a rocket for launch

As per above quote I wasn't thinking of T as being only time, just one in a long list of check points.
As for D and D day, D stood for Dunkirk.

[Bored chemist]

As for D and D day, D stood for Dunkirk.

Nope
https://en.wikipedia.org/wiki/D-Day_(military_term)

[alancalverd]

Dunkirk: 1940, evacuation of 300,000 Allied troops from France with surprisingly few casualties

D-day: 1944, and several million deaths later, invasion of France by 156,000 Allied troops..

.....unless you are the President of the United States or Tony Blair, in which case history is anything you would like it to be, in the future.

[Bored chemist]

President of the United States or Tony Blair,

... other dishonest politicians are also available.

[annie123 (OP)]

To "God" and "King" - I stand corrected re accuracy - I was always given to understand by my wartime family it meant  Dunkirk. That was just a coincidence I suppose.  So - is it generally accepted that the T is for time - and it is not in the context of all the other letters?

[alancalverd]

That's affirmative, good buddy. Or as we say over here, "yes"!

Coincidentally I saw a clip last evening of the Lawrence Livermore Ignition Facility with a test run starting at T-297. I'm pretty certain it took several weeks to assemble the test rig, so the count may well have begun at T- umpteen thousand.

Strongly recommend "Darkest Hour" and "Dunkirk", preferably seen in that order. I was taught to fly by WWII veterans and I'm convinced that my parents' generation were the last heroes.

Also worth looking out for a new play entitled "Pressure" about the meteorologist who, against a powerful consensus,  persuaded Eisenhower to delay the D-day landings by one day. The storm he predicted turned up exactly as forecast, so the Germans battened down their gun hatches and Rommel went home for a family party on 5 June. 6 June, as predicted, was calm and sunny and the invaders almost caught the defenders napping.

[annie123 (OP)]

I think I've seen Darkest Hour - and I saw another D movie yesterday on tv PBS, mostly about the role of the RAF and the use of Spitfires. But most memorable in many ways was the recent movie Dunkirk which I saw in IMAX, louder than the real thing i think. Not really to be recommended for those with good hearing.

[alancalverd]

Not really to be recommended for those with good hearing.

The military understatement of the year. Almost British.

Sadly, the "real thing" was louder, colder, wetter (though not a drop to drink), and stank of bowel contents and rotting flesh (mostly but not exclusively other people's).

[chris]

Loved reading through this thread; Alan, you crack me up!