Space shuttle HUD - flight director vs velocity vector

On the Space Shuttle HUD, in the final phases of TAEM (Terminal Area Energy Management), the flight director symbol (square with three dashes) changes to a velocity vector symbol (circle with three dashes).

I understand that the velocity vector represents the instantaneous velocity of the shuttle, but I don't understand what the flight director represents. In civil aviation, it seems to indicate what the computer wants to do, but is that not what the guidance diamond does?

I also don't understand how both the boresight (representing the orbiter's x-axis) and the flight director can be fixed in the display (which they are in cockpit videos), since there seems to be no reason why they should always have a fixed angle between them.

• I'm sure many readers like myself don't know what a "HUD, in the final phases of TAEM" is. If you use a term only once in a post, there's not much savings using the acronym instead of the actual words. Can you add a link or just spell it out? Thanks!
– uhoh
Feb 21 at 12:16

The angle between the boresight and flight director box was a fixed angle because the flight director box didn't represent anything in the real world. It was simply a box placed in the center of the HUD with no respect to any reference out the window. Its only purpose was to indicate how far off-guidance you were; if the diamond was in the box, then you were on target. Once the flight director turned into a velocity vector (circle), then the circle lined up with the outside world and indicated the direction of the shuttle.

I think the real question at the heart of your inquiry is, why didn't they use a velocity vector for all of TAEM? I don't have any authoritative source, but I believe there are two reasons:

1. The velocity vector isn't really helpful until you can see the visual approach references (PAPIs or triangle/bar, and the ball-bar later in the flare). Being able to line up the velocity vector with the visual references during final and flare greatly helps in keeping a smooth stable approach. But prior to pre-final, there are no visual references in sight. If you're not aiming at anything in the real world, then having a velocity vector isn't that helpful.
2. Someone should double-check this, but I believe if the velocity vector got to the edge of the HUD, it would blink, indicating that it wasn't truly aligned with the outside anymore. Because of high winds at certain altitudes and higher angle of attack early in TAEM, it's likely that the velocity vector would have been moving around and blinking for much of TAEM. If the velocity vector is blinking and moving around without obvious visual clues coming from outside, that's annoying at best, and potentially disorienting at worst.

So, the flight director was a nice way to keep things consistently in the middle of the HUD until it was useful to have a velocity vector instead.

• In case you don't follow this blog, this would probably be of interest to you waynehale.wordpress.com/2024/03/10/putting-atlantis-at-risk Mar 11 at 2:00
• @OrganicMarble I've read several of his posts, but that one is especially interesting. I knew STS-37 landed short, and have read Steven Nagel's account, but had no idea about the backstory in MCC! The last couple years, I've become friends with a former booster controller and love hearing his and his friends' old shuttle stories. Just wish I'd made these friends before the program was over and SMS was gone! I've at least gotten to sit in the commander's seat at SAIL and thoroughly enjoyed that. Mar 21 at 4:09

Shuttle HUD symbology (showing flight director):

From Entry, TAEM, and Approach/Landing Guidance Workbook 21002 USA005512 Page 4-4

Pre-final mode (showing velocity vector):

Prior to TAEM pre-final, the HUD has a flight director symbol fixed in the center of the FOV. This symbol can be used together with the guidance diamond to null out any guidance command errors. The guidance diamond depicts the direction to which the orbiter must be flown to satisfy the guidance solution. The diamond represents the intersection of the guidance needles on the ADI.

At TAEM pre-final the flight director symbol automatically releases and begins moving in the FOV as a velocity vector. A line-of-sight projection from the pilot's eye, through the velocity vector symbol, depicts the instantaneous flight path of the orbiter. Precise adjustments in flight path can be accomplished, as required, by overlaying the velocity vector symbol on the desired aim point.

From Entry Flight Procedures Handbook JSC-11542 page 5-119

Not a pilot, but from experience flying the Shuttle Mission Simulator, you "flew" the flight director towards the guidance diamond. When they were aligned you were pointed the right way. The flight director represented "where you are pointed", the guidance diamond "where you wanted to be pointed". Your task was to align them.