7 ms·

What does hypersonic mean? I thought all missiles were largely supersonic, except for maybe cruise missiles.

heydabop 5y ago
Reading the article or simply typing "hypersonic" into Google would've gotten you an immediate answer, but it means faster than Mach 5.
mchristen 5y ago
> In aerodynamics, a hypersonic speed is one that greatly exceeds the speed of sound, often stated as starting at speeds of Mach 5 and above.
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)
credit_guy 5y ago
A good starting point is this Congressional report on hypersonic weapons, updated one month ago: https://sgp.fas.org/crs/weapons/R45811.pdf https://sgp.fas.org/crs/weapons/R45811.pdf
- x86_64Ubuntu 5y ago
  Thanks, I've started reading it and will continue later, the material seems to be perfect for a non-Aerospace professional.
- nielsbot 5y ago
  OMG "The Pentagon’s FY2022 budget request for hypersonic research is $3.8 billion"
  - credit_guy 5y ago
    Is this a lot? Missiles are quite expensive in general. $3.8 BN is not the budget for this single program, but for several, 8 per this source [1] (although, now it might be 1 or 2 less). For this particular weapon, a somewhat similar weapon I can think of is the European Meteor [2], which is ramjet (rather than scramjet) powered, and reaches Mach 4, rather than over Mach 5 as this one is supposed to do. Meteor comes at a cool cost of EUR 2 MM per piece. Think about it, you fire one missile like that, and it's like destroying 10 Ferraris. [1] https://www.flightglobal.com/flight-international/8-key-hypersonic-missile-efforts-for-the-us-department-of-defense/138052.article https://www.flightglobal.com/flight-international/8-key-hype... [2] https://en.wikipedia.org/wiki/Meteor_(missile) https://en.wikipedia.org/wiki/Meteor_(missile)
    - R0b0t1 5y ago
      > it's like destroying 10 Ferraris. But, like, really cool.
    - nielsbot 5y ago
      Just saying that's a lot of money in absolute terms. I'd hope that money could be spent in other ways.
mzs 5y ago
Though it means mach 5+ in terms of weapons systems it indicates a capability for a long highly maneuverable period at those speeds. https://www.gao.gov/blog/faster-speed-sound-u.s.-efforts-develop-hypersonic-weapons https://www.gao.gov/blog/faster-speed-sound-u.s.-efforts-dev...
- lumost 5y ago
  It's worth noting that at these speeds a "long" period need only be ~1 hour to reach a target from launch. A flight from Hawaii to Taipei would be 1.3 hours at Mach 5, and 54 minutes at Mach 7. These weapons represent an ability for both the US and China to conventionally intervene from their mainlands to any location within the Pacific.
  - gonzo41 5y ago
    It's an expensive asset though. You'd think planners would only considered large missiles like this for nuclear weapons. There's got to be a risk that using a conventional warhead would get mistaken for a nuclear attack. I think hypersonic weapons will be largely stranded assets.
    - lumost 5y ago
      A ballistic missile at Mach 25 is an easily detected threat that the other side wouldn't be able to determine was inbound for the mainland or some fleet off the coast. Odds are inbound hypersonic weapon would avoid detection until it's directly within radar range. This would likely mean the weapon hits its non-mainland target prior to the other side taking notice for a retaliatory strike. In the event the other side did notice - it's likely they wouldn't treat it as a nuclear incursion unless the weapon was heading inland towards missile silos or other infrastructure that would motivate a nuclear retaliation. To your point on stranded assets - these weapons make performing any large military action in the pacific much riskier with greater probability for missteps or stalemates. Assuming a cost of 20-100 million dollars per missile I wouldn't be surprised if they are cost competitive with high risk carrier based F-35 operations in a hypothetical Pacific conflict. It's probably cheaper to stock pile such missiles than maintain additional carrier strike groups for the sole task of participating in a great power conflict.
colechristensen 5y ago
There aren't exact thresholds, but hypersonic generally refers to speeds fast enough that fluid heating can no longer be approximated away and chemical reactions and ionization in the air start to dominate. Aerodynamics is a practice of which physical phenomena you can ignore. At low speeds you can pretend the speed of sound is infinite / air is incompressible (these are equivalent), as you get faster you have to start paying attention to compression, then shock waves, heating, ionization, and the mechanics of individual particles.
- volta83 5y ago
  And to give some examples, while you could maybe ignore ionization for the aerodynamics of an F22 travelling at Mach 3, you probably could not ignore them for a school bus travelling at Mach 3.
rektide 5y ago
One key consideration that I only got recently is the difference between a cruise & ballistic missile. A ballistic missile launches at great velocity, then more or less falls at it's target (over an extended distance) using a ballistic trajectory. A cruise missile is under power for a large part of it's journey, typically flying much lower, & able to navigate/direct itself. A ballistic missile is often quite supersonic. But cruise missiles being hypersonic is definitely a big shift.
- foobarian 5y ago
  I was pretty surprised by how slow cruise missiles are when I read up on them recently - similar to big airliner speeds. I always had a mental image of missiles as these "fastest in the sky" things that you could only dodge but not outrun. It's possibly true for Sidewinders but even they top out around Mach 2.5, so the hypersonic developments are a huge step forward. Still kinda surprised there aren't fast missiles that use rocket engines. It works for ICBMs? I don't see a reason they couldn't maneuver as well. Maybe the hypersonic regime makes things novel not just for propulsion but control too.
  - ethbr0 5y ago
    Rocket engines require onboard oxidizer (or equivalent), so you have to carry a substantial additional amount of mass along with you. Air-breathing engines are able to save all that mass by using atmospheric oxygen. This matters, because it's the rocket equation in reverse: no onboard oxidizer = lighter vehicle = less fuel needed = even lighter vehicle = longer range / more payload
    - rektide 5y ago
      this is a good queue-in to start talking about mass ratio: the ratio of weight that is dedicated to payload versus the weight of the delivery platform and it's fuel (slightly simplified take). this device is almost certainly air breathing. there's simply too big an oxygen-breathing-demand at speed to do anything else for sustained high speed flight, where you need to be generating a lot of motive force.
anovikov 5y ago
For a powered flight, it refers to the state where combustion in the engine happens at supersonic speeds (for a "normal" supersonic planes like fighters, it's subsonic because speed of sound in a heated air is much higher). For an unpowered flight, it is a rather meaningless term, but normally refers to maneuvering warheads on ballistic missiles
- sandworm101 5y ago
  That is the difference between ramjets and scramjets, not necessarily the definition of hypersonic. And some "hypersonic" missiles don't breath air but rather conventional rocket motors to muscle their way to higher speeds. The oldest definition of hypersonic I have heard is those speeds where compression heating of the atmosphere ahead of the craft generates plasma. A space capsule on reentry hits such speeds, causing the classic radio blackout. So hypersonic aircraft are those that must deal with plasma-related issues that are irrelevant to slower supersonic aircraft.

Reading the article or simply typing "hypersonic" into Google would've gotten you an immediate answer, but it means faster than Mach 5.

mchristen 5y ago

> In aerodynamics, a hypersonic speed is one that greatly exceeds the speed of sound, often stated as starting at speeds of Mach 5 and above.

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)

credit_guy 5y ago

A good starting point is this Congressional report on hypersonic weapons, updated one month ago: https://sgp.fas.org/crs/weapons/R45811.pdf https://sgp.fas.org/crs/weapons/R45811.pdf

x86_64Ubuntu 5y ago
Thanks, I've started reading it and will continue later, the material seems to be perfect for a non-Aerospace professional.
nielsbot 5y ago
OMG "The Pentagon’s FY2022 budget request for hypersonic research is $3.8 billion"
- credit_guy 5y ago
  Is this a lot? Missiles are quite expensive in general. $3.8 BN is not the budget for this single program, but for several, 8 per this source [1] (although, now it might be 1 or 2 less). For this particular weapon, a somewhat similar weapon I can think of is the European Meteor [2], which is ramjet (rather than scramjet) powered, and reaches Mach 4, rather than over Mach 5 as this one is supposed to do. Meteor comes at a cool cost of EUR 2 MM per piece. Think about it, you fire one missile like that, and it's like destroying 10 Ferraris. [1] https://www.flightglobal.com/flight-international/8-key-hypersonic-missile-efforts-for-the-us-department-of-defense/138052.article https://www.flightglobal.com/flight-international/8-key-hype... [2] https://en.wikipedia.org/wiki/Meteor_(missile) https://en.wikipedia.org/wiki/Meteor_(missile)
  - R0b0t1 5y ago
    > it's like destroying 10 Ferraris. But, like, really cool.
  - nielsbot 5y ago
    Just saying that's a lot of money in absolute terms. I'd hope that money could be spent in other ways.

mzs 5y ago

Though it means mach 5+ in terms of weapons systems it indicates a capability for a long highly maneuverable period at those speeds. https://www.gao.gov/blog/faster-speed-sound-u.s.-efforts-develop-hypersonic-weapons https://www.gao.gov/blog/faster-speed-sound-u.s.-efforts-dev...

lumost 5y ago
It's worth noting that at these speeds a "long" period need only be ~1 hour to reach a target from launch. A flight from Hawaii to Taipei would be 1.3 hours at Mach 5, and 54 minutes at Mach 7. These weapons represent an ability for both the US and China to conventionally intervene from their mainlands to any location within the Pacific.
- gonzo41 5y ago
  It's an expensive asset though. You'd think planners would only considered large missiles like this for nuclear weapons. There's got to be a risk that using a conventional warhead would get mistaken for a nuclear attack. I think hypersonic weapons will be largely stranded assets.
  - lumost 5y ago
    A ballistic missile at Mach 25 is an easily detected threat that the other side wouldn't be able to determine was inbound for the mainland or some fleet off the coast. Odds are inbound hypersonic weapon would avoid detection until it's directly within radar range. This would likely mean the weapon hits its non-mainland target prior to the other side taking notice for a retaliatory strike. In the event the other side did notice - it's likely they wouldn't treat it as a nuclear incursion unless the weapon was heading inland towards missile silos or other infrastructure that would motivate a nuclear retaliation. To your point on stranded assets - these weapons make performing any large military action in the pacific much riskier with greater probability for missteps or stalemates. Assuming a cost of 20-100 million dollars per missile I wouldn't be surprised if they are cost competitive with high risk carrier based F-35 operations in a hypothetical Pacific conflict. It's probably cheaper to stock pile such missiles than maintain additional carrier strike groups for the sole task of participating in a great power conflict.

colechristensen 5y ago

There aren't exact thresholds, but hypersonic generally refers to speeds fast enough that fluid heating can no longer be approximated away and chemical reactions and ionization in the air start to dominate. Aerodynamics is a practice of which physical phenomena you can ignore. At low speeds you can pretend the speed of sound is infinite / air is incompressible (these are equivalent), as you get faster you have to start paying attention to compression, then shock waves, heating, ionization, and the mechanics of individual particles.

volta83 5y ago
And to give some examples, while you could maybe ignore ionization for the aerodynamics of an F22 travelling at Mach 3, you probably could not ignore them for a school bus travelling at Mach 3.

rektide 5y ago

One key consideration that I only got recently is the difference between a cruise & ballistic missile. A ballistic missile launches at great velocity, then more or less falls at it's target (over an extended distance) using a ballistic trajectory. A cruise missile is under power for a large part of it's journey, typically flying much lower, & able to navigate/direct itself. A ballistic missile is often quite supersonic. But cruise missiles being hypersonic is definitely a big shift.

foobarian 5y ago
I was pretty surprised by how slow cruise missiles are when I read up on them recently - similar to big airliner speeds. I always had a mental image of missiles as these "fastest in the sky" things that you could only dodge but not outrun. It's possibly true for Sidewinders but even they top out around Mach 2.5, so the hypersonic developments are a huge step forward. Still kinda surprised there aren't fast missiles that use rocket engines. It works for ICBMs? I don't see a reason they couldn't maneuver as well. Maybe the hypersonic regime makes things novel not just for propulsion but control too.
- ethbr0 5y ago
  Rocket engines require onboard oxidizer (or equivalent), so you have to carry a substantial additional amount of mass along with you. Air-breathing engines are able to save all that mass by using atmospheric oxygen. This matters, because it's the rocket equation in reverse: no onboard oxidizer = lighter vehicle = less fuel needed = even lighter vehicle = longer range / more payload
  - rektide 5y ago
    this is a good queue-in to start talking about mass ratio: the ratio of weight that is dedicated to payload versus the weight of the delivery platform and it's fuel (slightly simplified take). this device is almost certainly air breathing. there's simply too big an oxygen-breathing-demand at speed to do anything else for sustained high speed flight, where you need to be generating a lot of motive force.

anovikov 5y ago

For a powered flight, it refers to the state where combustion in the engine happens at supersonic speeds (for a "normal" supersonic planes like fighters, it's subsonic because speed of sound in a heated air is much higher). For an unpowered flight, it is a rather meaningless term, but normally refers to maneuvering warheads on ballistic missiles

sandworm101 5y ago
That is the difference between ramjets and scramjets, not necessarily the definition of hypersonic. And some "hypersonic" missiles don't breath air but rather conventional rocket motors to muscle their way to higher speeds. The oldest definition of hypersonic I have heard is those speeds where compression heating of the atmosphere ahead of the craft generates plasma. A space capsule on reentry hits such speeds, causing the classic radio blackout. So hypersonic aircraft are those that must deal with plasma-related issues that are irrelevant to slower supersonic aircraft.