Everything you never knew you wanted to know about the Mercury Project

From RixWiki

Angular References

The common way to talk about where things are on the spacecraft is in reference to three axes.

• The X axis runs parallel to the Astronaut's shoulders. It's also called the pitch axis. In a pitch maneuver the "nose" of the capsule moves up or down as the capsule rotates around the pitch axis.
• The Y axis runs parallel to the Astronaut's spine. It's also called the yaw axis when we are talking about attitude maneuvers. In a yaw maneuver the capsule rotates right or left around the Y or yaw axis.
• The Z axis runs through the center of the capsule from the retro package to the antenna fairing. It's also called the roll axis. In a roll maneuver the capsule rotates around this axis.

Some common terms used in these web pages:

top or TY 

The positive Y direction, the astronauts head is at the top

bottom or BY 

The negative x direction

left or LX 

To the astronaut's left

right or RX 

To the astronaut's right

front 

The escape tower is in the front

back 

The retro-package is in the back

In other words these terms are all defined from the astronaut's viewpoint.

The terms TY, BY, LX, and RX are commonly found in documentation about the Mercury spacecraft to describe positions around the circumference of the vehicle. For more precise location, I have adopted a convention of using degrees with top or Ty being 0° and right or RX being 90°. So bottom or BY is 180° and left or LX is 270°.

Linear References

X, and Y Axes

X,Y Station numbering

For the position along an axis the zero point, and the positive direction must be defined. The zero point for both the x and y axes is the center of the spacecraft, in other words where the X, Y, and Z axes intersect. This is natural because of the radial symmetry in the X-Y plane. The original NASA/McDonnell documentation used TX and LY

Z Axis

For the position along the Z dimension, the choice of zero point is less obvious. The NASA/McDonnell documents use a station numbering system with a unit of inches and assigning a station number of 104.5" for the bottom of the bottom shingle on the conical section of the spacecraft.

I haven't uncovered the reason why the 0" Z-station was picked to be 104.5" below the capsule, but that's what NASA and McDonnell used, and it's also what David Weeks uses in his 1/12 Mercury Drawings.