True Airspeed (TAS) is the vehicle speed relative to the surrounding air. My assumption is that if I am in orbit say at 120 km altitude with an instantaneous velocity of 8 km/s my TAS would be fairly close to 8 km/s even though my Indicated Airspeed (IAS) shows zero.

Assuming the above holds true, the "ram rise" (RR) formulas I find are all based on Mach (which, in turn, is based on TAS) or TAS directly. They do not appear to take into account the static pressure or atmospheric composition, only the adiabatic index. Therefore, my TAT (OAT plus ram rise) is in the thousands of degrees Kelvin ... which makes no sense because there are practically no molecules of air to hit!

So are these RR=TAS²/87 formulas assuming to be used below a specific altitude? I can't find anything for orbit or reentry. I also can't find any space shuttle telemetry to corroborate the TAS assumptions I made above. I am aware there is also a Mach-based RR formula and it has the same problem.

Your help is sure appreciated!

  • $\begingroup$ So we figured out a Mach formula that works above 90km which takes into account the change of atmospheric composition. So the problem still is TAT calculation whether it is based on TAS or Mach (we are leaning towards Mach now). $\endgroup$ – Jason Reskin May 25 '18 at 19:35
  • 1
    $\begingroup$ A practical problem is the temperature rise will trigger chemical reactions which involve heat. So it turns out to be very geometry dependent in that it takes time for the reactions to occur. This is basically the definition of hypersonic speed - the speed at which current instruments are useless. The entire process of stall, temp rise, chemical reactions and eventual thermal load during reentry was modeled as a semi-closed integral by a couple of American mathematicians. Not only did we get there first (unusual in engineering math), but we kept it a secret for decades (even more unusual). $\endgroup$ – Phil Sweet May 26 '18 at 2:41
  • $\begingroup$ Very insightful! When you say "we kept" you mean that these formulas are available in some form somewhere? Or do you mean "we've kept"? $\endgroup$ – Jason Reskin May 26 '18 at 15:36

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.