"X-minus" vs. "T-minus" in countdowns?

"X-minus" vs. "T-minus" in countdowns?

Post by James Wentwor » Thu, 16 May 1996 04:00:00



Subject: "X-minus" vs. "T-minus" in countdowns?

I have a few historical questions concerning countdown systems.  While
they are not strictly germane to model rocketry, the answers may be of
interest to scale modelers who would like to conduct "period authentic"
countdowns for their models.

Today, countdowns are conducted using the "T-minus" system, where T=0 is
the instant of liftoff (first motion).  However, up until the late 1950s
countdowns used the "X-minus" system, in which X=0 was the instant of
ignition.  The actual liftoff occurred at X+some figure.  The Viking
sounding rockets and early Explorer satellites, among other vehicles,
were launched using the "X-minus" system.  I believe that the "T-minus"
system was used for Alan Shepard's flight, but I am not certain of this.

Here are the questions.  Did the "X-minus" system originate with the
White Sands V-2 launches?  What and when were the last sounding rocket,
ballistic missile, cruise missile (Matador, Mace, Snark, etc.),
satellite, and manned space launches that used the "X-minus" system?  
What and when were the first of these launches to use the "T-minus"
system?  What was/were the reason/reasons for changing systems?
Which system do the Russians and other nations use?

Many thanks to you in advance for your help.

It's fun to use period authentic jargon in reference to scale models (I
often speak of "Brenschluss" <a German term literally meaning 'end of
burning'> to spectators when I fly scale models, especially my V-2).  I
guess it's akin to the fondness many hams have for older terminology like
"kilocycles" and "megacycles."

--
James J. Wentworth