Question about units on Vert-Sit 1 and 2

[Lieste]

On the real world Shuttle, which units are the 'velocity' targets defined?

My first assumption was they are in KEAS, but it is possible that they are in qbar units which are based on the square of the ft/s speed and the relative density.

These are reasonably close to each other, but there is a small difference between them, with qbar giving a slightly higher speed for the same numeric value.

While most of the flight instrumentation in particular the HUD gives KEAS values, the engineering limits are usually defined in terms of qbar, and this is included on the VertSit display IIRC.

[Nephi]

KEAS (source : DPS dictionnary)

[Lieste]

Yes, I read the same thing there too, but I am wondering if this is an error.

The reason I query it is that the moderate weight Shuttle has a best L/D speed considerably above 214KEAS (as shown in the associated diagrams), which would imply a failure to stretch the glide if this schedule was followed. This is from the listed qbarmin values from the TM104744 TAEM document (for a light weight shuttle - extrapolated to a higher weight).

On the other hand there is a reasonable safety margin between the Vert-sit values and the qbarmin values if VertSit does indeed use qbar - there is also a marginal but 'real' buffer between the qbarmax and 'upper speed limit' values (275 vs ~270). The 270 value is pretty meaningless if it is KEAS, as the supersonic 275 qbarmax used by guidance commands gives ~313KEAS.

Doesn't definitely mean the DPS is wrong, but the DPS seems to raise more questions than it answers in this case...

[Lieste]

Actually, on reflection, I can't see anywhere where qbar is displayed on the instrumentation - I had thought I had seen this on one of the CDR instruments in one of the NASA documents but cannot find it referred now..

Without a reference instrument a display requiring knowledge of qbar would be useless.

[Lieste]

And again, checking the GFR, I get a different conversion from qbar to KEAS than if I use the described formulae for qbar from other NASA documents... These limits make much more sense, except that I don't get the same values out from my conversion.

qbar 350 = KEAS 321
qbar 375 = KEAS 333

The relationship between qbar and KEAS^2 is not perfectly uniform, but it is within +/-0.5 unit error tolerance so is probably ok.

Why these quoted values are so much higher than the 1991 subsonic qbarmax of 340 isn't immediately clear, but perhaps the data from flights to date have broadened the flight envelope here?