THIS is a Slide Rule!

This simple device is what equated to a modern scientific pocket calculator when I first entered the aerospace defense workforce (see the photo).  This is a slide rule,  the "calculator" used by engineers and scientists for 300 years before there were any electronic calculators at all!  This very slide rule is what I designed my first airplane with,  and my first half dozen supersonic missile propulsion systems with. 

For problems you could not handle pencil-and-paper with a slide rule,  there was the mainframe computer.  These were devices that filled a room the size of a small house,  air-conditioned to about 60-65 F,   so that the magnetic iron cores and wiring would not try to melt down!  You loaded your data and programs into the mainframe "card batch" in trays,  up to 2000 cards at a time,  using paper punch card technology.  Job turnaround time was measured in hours,  sometimes days. 

As for modern spreadsheet technology,  when I first entered the workforce,  repetitive calculations were manually laid out on a big piece of paper in a matrix format.  You ran the actual calculations yourself,  using a slide rule,  or a bit later,  a hand-held electronic calculator.  You filled in the matrix slowly,  literally doing each and every calculation yourself,  and finding out "up-close-and-personal" what could go wrong with the processing of the data.  That is where today's spreadsheet software came from!

For a given manual spreadsheet job,  this experience taught you exactly how to program the calculations into a scientific programming language (in those days,  something like an early FORTRAN or BASIC),  complete with all the processing logic and error-trapping.   That required punching the program statements onto cards,  for card-batch load and debugging (again,  job turnaround time was hours-to-days between each run).  Once you did this,  you could do similar jobs,  requiring the exact same analysis,  far more quickly.  (Or you could modify your program to handle other jobs that were similar,  but slightly different in a few details.)

Most people today do not realize this,  but NASA mission control in Houston did not have any real computer consoles until the Space Shuttle first flew!  During the earlier Mercury,  Gemini,  and Apollo programs,  those flight controller consoles were only keyboard-controlled communication displays,  each slaved to a counterpart in a back room outside the mission control room.  There was a team of people (both men and women!) in each back room,  who answered the flight controller's question with slide rule calculations,  and typed in the answers,  so that their numbers appeared on-screen in mission control.  That,  plus analog instrument readouts converted to digital format on-screen,  is literally all the mission flight controllers had to work with!

With the exception of a mainframe-computed figure-eight orbit between Earth and moon,  NASA literally sent men to the moon during Apollo with slide rules (just like the one in the photo)!  And,  the record-breaking X-15 rocket plane (and all its earlier progenitors),  plus the SR-71 jet aircraft,  and all the early supersonic jet fighters,  were designed with nothing but slide rules.  Same for all their rocket and gas turbine engines!  And their heat protection schemes.

My slide rule still works.  I still use it when the electronics conk out,  which they inevitably do,  occasionally.  The slide rule never conks out like that.  It's just slower.