Friday, November 27, 2009


Another pause for the mundane but necessary: coordinates.

Coordinate axes provide a convenient method for communicating the orientation of a flight vehicle and describing the location of key features. We followed traditional missile/aircraft conventions when establishing the MLAS coordinate system. Visualize a set of 3-dimensional coordinates with the X-axis running vertically, "Y" 90-degrees to that, horizontally, and "Z" coming out of the page. The MLAS X-axis is the longitudinal axis of the vehicle .... the -X direction is towards the nose and the +X direction is towards the tail. "Y" and "Z" are a little more arbitrary. We based them on the vehicle orientation at the pad .... an observer facing the ship as shown to the left would be looking straight down the Z-axis in the -Z direction with the +Z axis coming directly towards him. In other words, the -Z side of the vehicle would face the ocean and +Z the land. The -Y-axis would be to the observer's left and +Y to the right.

The "zero" reference depends on the axis. We set Y-zero and Z-zero at the vehicle centerline, where those axes intersect X. We based the X-zero reference on the Crew Module apex: if you extend lines down the sides of the CM, the point at the top where they intersect is X-zero. The rest of the ship is referenced from there. That's why the tip of the nose is at X minus 26.8" and why you have to add 26.8" to all dimensions if you wish to measure from the Forward Fairing nose cap instead of from Xo as shown in the configuration drawings. We labeled directions from the point of view of an observer looking up from below, the ocean at his feet. The -Z (ocean) side is thus the "bottom;" +Z (land) is the "top;" +Y (the observer's right) is "right;" and -Y (the observer's left) is "left." This all looks odd on drawings done from the perspective of one looking down from the nose of the vehicle, but makes perfect sense to one working on the hardware from underneath.

The roll patterns on the fairing and CM correspond to the vehicle axes. The +Z face is the one with two black bars as shown, -Z (the ocean side) has a single long black bar, +Y (right per our convention and from the point of view of the picture above) has a single short black bar, and -Y (left per convention) a pattern of three bars.

You'll find all this in the configuration drawings ... I carried it through to µMLAS with the added twist of angles numbered in a counterclockwise direction looking up from the bottom with -Z at 0°, -Y at 90°, and so forth. Naturally, that's differs from the MLAS convention which calls that angle omega and sets 0 at the +Z axis. The confusion just adds to the challenge.

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