3 ms·

It's a speed regime. Under Mach 1 is subsonic. From Mach 0.9 through 1.2 or so is transsonic. 1.0 and greater is supersonic. Above Mach 5ish is hypersonic. Mac

by dsr_ 5y ago

It's a speed regime. Under Mach 1 is subsonic. From Mach 0.9 through 1.2 or so is transsonic. 1.0 and greater is supersonic. Above Mach 5ish is hypersonic. Mach 25 is approximately orbital velocity, but since Mach is actually related to air pressure, it's not a very meaningful comparison except to note that anything moving that fast isn't going to be in-atmosphere for very long.

HPsquared 5y ago
In subsonic flight, the air behaves as if it's incompressible (like a liquid). In transonic flight, some parts of the airflow are starting to become supersonic, and compressibility starts to become a factor. Something called "wave drag" is important in this regime, as the compression wave is moving along with the aircraft. In supersonic flow, compressibility of the air becomes the main thing and the flow is defined by the positions of the various shock waves. Ramjet engines work in this regime (the aircraft is moving fast enough that the forward movement is enough to compress the incoming air to the engine, without compressor blades). In a conventional ramjet engine, the flow within the engine is subsonic (the compressed air, being more dense, moves more slowly within the engine), so combustion works the usual way. In hypersonic flow, temperatures are getting very high (air is heated by compression). Scramjet engines become a thing (the "SC" is for supersonic combustion: air flowing through the combustion stage at supersonic speed).
- ashtonkem 5y ago
  Fun, unrleated fact. Transsonic is very bad for rifle accuracy, as it tends to be somewhat unpredictable. For most rifle calibers this isn't a big deal, because they spend their entire flight time in the supersonic flight regime. But for 22lr, this is an issue because even the fastest 22lr around is going to be subsonic within 100 yards or so for reasons[0]. The result is a shot that’s laser accurate until 100 yards or so, and then suddenly becomes unpredictable beyond that. So "match" 22lr ammo tends to be subsonic even out of the barrel. 0 - The reasons are due to maximum allowable pressures, and barrel rifling. At max pressure for a 40gr bullet you’re going to get maybe Mach 1.1, maybe. If you want to go faster without increasing pressure above spec, the solution is a lighter bullet. But lighter bullets often like different rifling, and virtually all off the shelf 22lr barrels come with 1:16 rifling, unsuitable for extra light bullets. Also, lighter bullet get pushed around by the wind more; you’d often rather compensate for more drop than wind, since drop is more predictable.
  - nautilius 5y ago
    What kind of pressures do you have in a barrel?
    - ashtonkem 5y ago
      Lots! The maximum chamber pressure allowable for a given civilian caliber is set by one of two regulatory bodies, SAAMI (US) or CIP (EU). In some countries firearm barrels will be tested (“proofed”) at 125% of set pressure to ensure consumer safety, and then stamped with a proof house specific mark. My understanding is that the US doesn’t require this, but any imported barrels will be stamped. Specifically CIP sets the maximum pressure of 22lr at 24,646 psi. An extremely high number considering, but pretty pedestrian compared to other cartridges. For example NATO sets a maximum pressure of 62,366 psi for its primary rifle caliber, 5.56x45mm. It’s genuinely amazing that rifle barrels don’t regularly explode, to be honest.
      - nautilius 5y ago
        Oh wow, that is a lot! Thanks!
colechristensen 5y ago
The speed of sound in air (or mixtures of diatomic gasses) is a function of temperature only, independent of pressure and density. Or it depends on pressure and density in a way which cancels out and leaves only temperature as an independent variable for equivalent mixtures of gasses. (i.e. the speed of sound is the same at 20 atmospheres or 0.1 atmosphere as long as the temperature is the same) This becomes untrue when air is very very hot or very very cold, but generally those conditions aren't experienced on this planet outside of hot hypersonic flight. Water vapor content, not being a diatomic gas does very slightly change the speed of sound but usually to a degree which can be ignored. (people generally don't believe that the speed of sound in air depends only on temperature when you tell them)

by dsr_ 5y ago

HPsquared 5y ago

In subsonic flight, the air behaves as if it's incompressible (like a liquid). In transonic flight, some parts of the airflow are starting to become supersonic, and compressibility starts to become a factor. Something called "wave drag" is important in this regime, as the compression wave is moving along with the aircraft. In supersonic flow, compressibility of the air becomes the main thing and the flow is defined by the positions of the various shock waves. Ramjet engines work in this regime (the aircraft is moving fast enough that the forward movement is enough to compress the incoming air to the engine, without compressor blades). In a conventional ramjet engine, the flow within the engine is subsonic (the compressed air, being more dense, moves more slowly within the engine), so combustion works the usual way. In hypersonic flow, temperatures are getting very high (air is heated by compression). Scramjet engines become a thing (the "SC" is for supersonic combustion: air flowing through the combustion stage at supersonic speed).

ashtonkem 5y ago
Fun, unrleated fact. Transsonic is very bad for rifle accuracy, as it tends to be somewhat unpredictable. For most rifle calibers this isn't a big deal, because they spend their entire flight time in the supersonic flight regime. But for 22lr, this is an issue because even the fastest 22lr around is going to be subsonic within 100 yards or so for reasons[0]. The result is a shot that’s laser accurate until 100 yards or so, and then suddenly becomes unpredictable beyond that. So "match" 22lr ammo tends to be subsonic even out of the barrel. 0 - The reasons are due to maximum allowable pressures, and barrel rifling. At max pressure for a 40gr bullet you’re going to get maybe Mach 1.1, maybe. If you want to go faster without increasing pressure above spec, the solution is a lighter bullet. But lighter bullets often like different rifling, and virtually all off the shelf 22lr barrels come with 1:16 rifling, unsuitable for extra light bullets. Also, lighter bullet get pushed around by the wind more; you’d often rather compensate for more drop than wind, since drop is more predictable.
- nautilius 5y ago
  What kind of pressures do you have in a barrel?
  - ashtonkem 5y ago
    Lots! The maximum chamber pressure allowable for a given civilian caliber is set by one of two regulatory bodies, SAAMI (US) or CIP (EU). In some countries firearm barrels will be tested (“proofed”) at 125% of set pressure to ensure consumer safety, and then stamped with a proof house specific mark. My understanding is that the US doesn’t require this, but any imported barrels will be stamped. Specifically CIP sets the maximum pressure of 22lr at 24,646 psi. An extremely high number considering, but pretty pedestrian compared to other cartridges. For example NATO sets a maximum pressure of 62,366 psi for its primary rifle caliber, 5.56x45mm. It’s genuinely amazing that rifle barrels don’t regularly explode, to be honest.
    - nautilius 5y ago
      Oh wow, that is a lot! Thanks!

colechristensen 5y ago

The speed of sound in air (or mixtures of diatomic gasses) is a function of temperature only, independent of pressure and density. Or it depends on pressure and density in a way which cancels out and leaves only temperature as an independent variable for equivalent mixtures of gasses. (i.e. the speed of sound is the same at 20 atmospheres or 0.1 atmosphere as long as the temperature is the same) This becomes untrue when air is very very hot or very very cold, but generally those conditions aren't experienced on this planet outside of hot hypersonic flight. Water vapor content, not being a diatomic gas does very slightly change the speed of sound but usually to a degree which can be ignored. (people generally don't believe that the speed of sound in air depends only on temperature when you tell them)