The sixth and final definition of at once in the OED means "immediately, straightway" and is first documented in William Tyndale's Expos. & Notes way back in 1531:
The apostles were clear-eyed, and espied antichrist at once.
It comes from a meaning of "at one time" and other definitions of at once are of things happening at at the same time; or in one heap or body together; or at the same time or simultaneously.
So the "immediately" sense can be similarly thought of as things happening all at the same time, or right now.
Join the army at once & help to stop an air raid
Or:
Join the army right now & help to stop an air raid
You read the poster and then join the army all at the same time, or immediately.
The aeronautical sense of nominal derives from engineering where the nominal value is the specified dimension and the reference point for tolerances.
The Free Dictionary offers the following definition (from McGraw-Hill Dictionary of Scientific & Technical Terms, 6th edition) for tolerance:
(engineering)
A permissible deviation from a specified value, expressed in actual values or more often as a percentage of the nominal value.
Surprisingly, the sense of nominal used in the preceding definition of tolerance is missing from both the Free Dictionary (via AHD4) and OED3. But the requisite definition is found in the Oxford Dictionaries Online:
3 (Of a quantity or dimension) stated or expressed but not necessarily corresponding exactly to the real value:
'EU legislation allowed variation around the nominal weight (that printed on each packet)'
This usage in engineering long predates the space program. The following is from Douglas T. Hamilton's Gages, Gaging, and Inspection(1st edition; 1918; p.30):
It is, therefore, common practice to specify the "limit" -- that is the deviation from the true or nominal size which is permissible. The limit is generally stated by giving the amount that the dimension may be larger or smaller than the nominal size. The diameter of the shaft for example may be given as "one inch plus or minus 0.001 inch..."
A measurement corresponding to the nominal value is "according to plan or design." Similarly, when using bilateral tolerances the nominal value is by definition "within acceptable tolerances." The aeronautical usage is derived from engineering.
Best Answer
Etymonline has an entry for killing:
While its usage to mean "very funny" is partly covered in another question, its usage via idioms like to make a killing to indicate a "large profit" dates back to 1886 (as noted above). The American Heritage Dictionary of Idioms has the following to say about make a killing: