When I heard the early reports about the Columbia accident, there were several reports on TV saying that the shuttle was traveling 12,500 mph or Mach 6. When I heard that, my first thought was that Mach 6 is nowhere close to 12,500 mph. My second thought was that the speed of sound varies with atmospheric density, so that perhaps the shuttle was traveling at Mach 6 at its altitude of over 200,000 feet.
Later reports said that the shuttle was traveling at Mach 17 or 18. NASA has also used these numbers in their press releases and during their press briefings. This appears wrong and it seems that the actual speed at that altitude was somewhere around Mach 6.
Why even report a Mach number at all? I can see that perhaps what is being reported is shorthand for “equivalent to Mach 18 at sea level”, but it still seems confusing to me.
I can understand why engineers would use Mach numbers since they take into account the actual dynamics acting on the vehicle. From my limited research though, it appears that there are only two Mach numbers that have particular significance. That would be Mach 1 (speed of sound) and Mach 5 (hypersonic).
If this is correct, this leads me to my main question. Is Mach 6 the maximum speed of the shuttle? (Since it has been reported that the shuttle was at its maximum aerodynamic and thermal loading at 12,500 mph @ 200,000 feet.)
Would it be possible to design a future space shuttle / space plane that would stay below just below Mach 5 during reentry? Since Mach 5 is the point that the air around the shuttle becomes a plasma is that not the point where the temperature makes a great jump? I am assuming this from my microwave plasma experiment (which was referenced in another thread), since the top of the bowl in contact with the plasma became quite hot very quickly.
I’m sure that I got a lot of things wrong here, so feel free to correct any of my misconceptions.
I think I can answer one question. The shuttle travels faster than 12.5 thousand MPH when it is escaping earth, and was travelling at least 17,000 mph on take off. (hope I don’t sound like a dope, but when talking such big numbers, one loses confidence)
If you think about it, if there are no forces going against the shuttle while it is in space, a vacuum, how could there be a terminal velocity? What could keep it from maxing out at 25,000?
You are working off some myths here.
You still need energy, you still have gravity, mass, inertia. The shuttle uses escape velocity to get into orbit, and that orbit is limited by the speed generated and fuel carried while making it’s take off blast.
For example, the shuttle could not have attained an orbit 50 miles higher because it didn’t rocket long enough. It was set in orbit (free fall) and could not have stayed there indefinitley, nor attained more speed.
No, it doesn’t, because the Shuttle doesn’t escape the Earth. It just stays in a nice low orbit, which takes about half the energy needed to escape. And even if you do want to get to escape speed (the Apollo missions reached a fair approximation, as does anything that goes significantly beyond the planet) you usually start by going into low Earth orbit, and then boosting to escape speed from there, once you don’t have to worry about air resistance any more. Note that if you have enough fuel, you can theoretically get any distance from the Earth at any speed; it’s just less efficient to do it slowly.
Mach number is typically cited instead of velocity because it is very important to the operating characteristics of the vehicle. Machs 1 and 5 are not the only significant values. The angle of attached shockwaves, the standoff distance of an oblique shock, the pressure jump across the shock and a lot of other factors depend on the Mach number, not the velocity, and all these things dramatically affect the vehicle.
Also, Mach 5 is not terribly significant as the boundary of hypersonic flow. The hypersonic regime does not have a precise boundary like the subsonic/supersonic regimes. The word hypersonic is simply used to define flows “where certain physical flow phenomena become progressively more important as the Mach number is increased”[sup]*[/sup]. In general, it refers to flows where shock deflection angles are very small, boundary layers very thin and temperature effects become much more important. In some cases, this can happen as low as Mach 2 or 3. In other cases, the effects may not become important until much higher Mach numbers. Mach 5 is only a typical value and serves as a rule of thumb, not a precise boundary on flow characteristics.
from Hypersonic and High Temperature Gas Dynamics by John D. Anderson.
So micco (or anyone else), are you saying that an aerodynamically designed aircraft can ionize the air at speeds significantly below Mach 5? The cite seems to be a general statement about hypersonic flows and I’m not sure if it is meant to apply to aerodynamically designed vehicles.
The speed of sound varies with the square root of temperature. At 200,000 feet the temperature has risen back to near sea level temperatures and Mach 1 is approximately the same as sea level, therefore, around 1,000 ft/s.
The Mach number is reported because the shuttle, in a sense, is starting to fly again, and so a more “aerodynamic” measurement is used. When it is up in orbit speeds are reported in feet per second.
The term “hypersonic” does not refer strictly to flows in which the gas ionizes. As I mentioned above, it encompasses a whole range of effects which are present in supersonic flows, and the term “hypersonic” is used to indicate that certain effects start to dominate. This includes but is not limited to flows in which the temperature is high enough to cause ionization. I don’t have specific examples to cite, but I believe you could find cases below Mach 5 which showed important dissociation or ionization effects, and I’m sure you could find examples above Mach 5 which did not.
Note that your specification of an aerodynamically designed aircraft is important here. You’re much more likely to see these effects in a blunt body with a detached shock than in a thin body with attached shocks because of the temperature and pressure characteristics of the two shocks. Also note that the shuttle is definitely NOT a very aerodynamic body, at least as far as reentry is concerned. It pitches to a very high angle of attack specifically to be “non-aerodynamic”, in as much as that term means high drag. In this configuration, it is a blunt body with a detached shock, which leads to very high temperatures.
Thanks MonkeyMensch, that was very helpful. Looks like the temperature starts rising at about 83,000 feet. So it seems that the correct Mach number is 17 or 18 as reported. Apparently the Mach 6 number was just another one of those common errors that get reported early in a breaking story. So as for the OP…