Somewhat ironically for a vehicle designed to reach orbital speed, the space shuttle’s flight control system was only certified for airspeeds up to 333 KIAS, and exceeding 470 KIAS (the maximum allowable in dire emergency) would have led to an immediate loss of control. This constraint was the reason for at least two, and possibly three, of the low-survivability zones in the event of a triple engine failure (go to page 14 of the linked PDF and look at the number of times it says “excessive EAS during pullout”1).

In contrast, essentially all modern jets have flight controls certified to well over 500 KIAS, and capable of safely controlling the aircraft at airspeeds considerably higher still.

Given the lethal effects of an overspeed-induced loss of control during an emergency situation, why were the shuttle’s flight controls certified with such a ridiculously low maximum airspeed?

1: Technically, it uses KEAS (knots equivalent airspeed) instead of KIAS (knots indicated airspeed), but, assuming that your airspeed indicator is properly calibrated and the pitot-static system on your aircraft is working properly, KEAS=KIAS.

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    $\begingroup$ Where do you get 500kt? Vmo on most jets is in the low 300s. Speed limits are generally related to Mach related issues or flutter margins. $\endgroup$ – John K Feb 24 '19 at 4:55
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    $\begingroup$ This seems a bit like asking "why wouldn't the brakes on my truck work at F1 car speeds?" You're comparing two very different aircraft designed for completely different purposes and flight parameters The comparison may simply not make sense. $\endgroup$ – Pondlife Feb 24 '19 at 4:57
  • $\begingroup$ @JohnK: The CRJ700 has a maximum cruise speed of 470 KIAS, and I would imagine other jetliners would be similar. $\endgroup$ – Vikki - formerly Sean Feb 25 '19 at 3:56
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    $\begingroup$ @Sean You have a basic misunderstanding. That's true airspeed not KIAS. What matters is indicated/calibrated airspeed for certification limits, what shows on the speed tape or indicator in the cockpit. The RJ's Vmo is 315, 330 and 335 KIAS at various altitudes, or at higher altitude, an Mmo of Mach .8 up above FL250 and .85 above FL320, (where Mach # is limiting). The Shuttle's Vmo is actually comparable to other jets. If the Shuttle is indicating 330 kt at say 80000 ft, the TAS is gonna be over 700 or 800 kts or something like that. $\endgroup$ – John K Feb 25 '19 at 5:08
  • $\begingroup$ @Pondlife: Your truck has no prospect of ever reaching Formula 1 speeds, whereas the space shuttle could easily have exceeded 470 KIAS if something went wrong. $\endgroup$ – Vikki - formerly Sean Feb 26 '19 at 3:48

From what I understand the limits seem to be related to contingency maneuvers such as RTLS or TAL where the external tank was still attached. These were only possible after the solid rocket boosters had been jettisoned, but while the external tank was still attached.

I couldn't find any altitude references for RTLS, but according to this TAL required the shuttle to fly to around 350.000 feet. RTLS would have been at a lower altitude and required a 180 degree turnaround. Doing these maneuvers with the ET places a higher stress on the vehicle resulting in black zones. Wayne Hale also mentions this in an old blog post, but doesn't go into specifics.


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