Just some interesting variations (it's been a while, so these might not be exactly correct. For the Saturns, STA 0 was 100" behind the first stage engine gimball axis. For the Titan, it was 500 inches in front of the stage separation plane. NASA would print station diagrams for their individual sounding rocket rounds with two systems, one with STA 0 at the nose tip for whatever the payload configuration was, and one with STA 0 at the nozzle exit plane (NEP) of the first stage. The Atlas had its STA O forward of the nose of the original version, at a point that happened to be 1310 inches in front of the main engine nozzles. Black Brants usually had the STA O point af the front of the motor.

For some rockets, the STA numbers were always consistent, and for some, they shifted with configuration.

I ended up adopting STA numbers for all my drawings, even when the original designers didn't use STA numbers, for threeo reasns:

First, if there are a lot of dimensions to cover, point-to-point arrows become really cluttered.
Second, point-to-point arrows often conceal inconsistencies. Many times I've looked closely at drawings to discover the numbers don't add up, and I don't want that in my drawings.

Third, point-to-point dimensions conceal missing information. It's really frustrating when I find source drawings that at first glance seem to have everything I need, but there's no actual indication of where the fins lie along the length of the body, or where a transition lies along the length of the body. By working with station numbers, it keeps me honest,

By the way, when I draw, I almost always type in the station numbers first, then draw in the construction lines based on the numbers typed into the drawing. That way, if I mis-type, the drawing will look obviously wrong, and I'll catch the mistake. Sometimes I don't work that way, and it tends to cause embarrassing mistakes.

I know it's not ideal to force the modeler to subtract to numbers to get their model measurement, but truth be told, the point-to-point dimensions on a source drawing aren't always the ones you need, so you have to add or subtract anyway. At least with station numbers, the required information should always be there.

By the way, errors are still inevitable. For instance, the blueprints I'm working on now for the Astrobee 500 include an overall drawing with station numbers, and a bunch of smaller drawings of component parts, and even accounting for overlapping or telescoping parts, the numbers don't add up and there are fractional-inch errors. Yet they are better than what I worked with for the original RotW drawing (I learned that the Asp motor had a fiberglass insulating shell added that increased the diameter by a fraction of an inch).

Anyway, I wish all those modelers good luck making sense of my drawings!

Peter Alway