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Old 06-07-2020, 09:40 PM   #11  
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IAS starts at about zero on reentry, and gradually goes up from there
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Old 06-20-2020, 12:49 PM   #12  
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Quote:
Originally Posted by Barnstormer View Post
Max Q is probably the most import factor.
The max q condition is the point when an aerospace vehicle's atmospheric flight reaches maximum dynamic pressure. This is a significant factor in the design of such vehicles because the aerodynamic structural load on them is proportional to dynamic pressure. This may impose limits on the vehicle's flight envelope.
To add on what this person said you'll hear them say when the vehicle is supersonic and when they reach max q (max dynamic pressure). Mach number is important until you hear them say "Vehicle is supersonic." and sort of important until you hear "Max Q". The mach effects vary gradually and abruptly. Abruptly because a large drag rise and stability change happens as you cross the speed of sound. There's more about shockwave propagation and reflection which I typed about below, but that's me self-satisfying my inner nerd. So I deleted it. The gradual variation from there has to do with the gradual way the atmosphere thins out with altitude.

Last edited by Elevation; 06-20-2020 at 01:02 PM.
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Old 07-09-2020, 07:56 PM   #13  
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Originally Posted by Bahamasflyer View Post
Just watched the launch of SpaceX and it got me wondering? What altitude does one have to reach in order for mach # to become meaningless? Is it a sudden thing, or gradual?
It never becomes "meaningless" per se at a certain *altitude*. Once you enter LEO, you need to be at a certain speed to maintain orbit. Once you accelerate to this speed, you no longer need to add thrust - around Mach 25, or 17500 mph, and the astronauts earn their "Mach 25" patch. At this point, it doesn't really have any meaning anymore. For the astronauts who serviced Hubble, which is further from the Earth than normal LEO, they actually reach Mach 26 on their return to Earth, so they get "Mach 26" patches. To re-enter, they only need to reduce their orbital velocity by about 200mph, so I guess their exact speed technically matters at this point as well.

Last edited by PlaneS; 07-09-2020 at 08:11 PM. Reason: Clarity
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