Thunderbirds are go

The title is the name of a British science-fiction TV serial dating back to 1966! I wonder how I should understand the meaning of the sentence. You all know that the correct or the usual sentence had to be written either (1) Thunderbirds go or (2) Thunderbirds are going based on the standard grammmer. I am not neither a native speaker nor a grammer police and I just want to know the logic behind this strange usage of British English. Is there any native speaker who can explain the problem in plain English?

Ben Waggoner
·

It’s not standard English, but it’s jargon that originated (as far as I know) in the US space program. “We are go for launch” means “We are cleared for launch”, or in other words, “Everything is working as it should, and there is nothing that would force us to delay launch”.
So “Thunderbirds are go!” means “Thunderbirds are ready to launch on their mission!”

On a related note, the expression "good to go" has the same meaning without the military connotations and has become trendy and common in recent years.