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.

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.

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.

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).

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.

What kind of pressures do you have in a barrel?

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.

Oh wow, that is a lot! Thanks!

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)

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-dev...

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.

> 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.

Is that the kind of range these have? Almost 9,000 miles from LA to the South Western most corner of the Pacific. If you meant the North Pacific then even the western edge of the US to the yellow sea is over 5,500 miles.

Ballistic missiles of course have these kind of ranges and can carry conventional payloads.

>Ballistic missiles of course have these kind of ranges and can carry conventional payloads.

Just call while the missile is on the way to let the people it's going to kill know its not a nuclear bomb headed their way so they shouldn't fire a nuclear bomb back.

On the same call you could also tell them not to worry about the nuclear capable hypersonic intercontinental cruise missile that's on the way.

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.

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.

> 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.

Why not? If it was carrying a nuclear payload and was destined for strategic sites on the mainland, it could be too late to retaliate if they waited to see where it is going, particularly if it was aimed inside the first island chain.

Isn’t this why some countries have ICBM carrying submarines, so a retaliatory strike can be performed even if the mainland was hit?

I don't know exactly how the game goes. I'm sure there is no one single solution to it.

If China detected nuclear capable missiles heading toward their country then they would have to consider the possibility their second strike capability is not a sufficient deterrent (including being compromised in various ways such as their subs being all located) so there seems like a large incentive to retaliate before missiles land.

Would there be enough time for them to watch and wait it out and see if it approached their missile installations? If it was a very stealthy missile, could they be sure they had detected them all?

No, armies do not have their finger on nuclear trigger with a time horizon of 30-ish minutes. We are not in the 1960s when at times it was literally true.

The concept of nuclear triad is that, yes, armies very strongly depend on their second-strike capability. So, the game is for the subs to not to become compromised.

> No, armies do not have their finger on nuclear trigger with a time horizon of 30-ish minutes.

> We are not in the 1960s when at times it was literally true.

You are claiming that not only China, but the US and Russia don't have the capability of launching strikes from 30 minutes? (i.e., well beyond the time to respond to ballistic missile launch detection)? Do you have a source for this?

This doesn't appear to agree with you:

https://www.mitpressjournals.org/doi/pdf/10.1162/ISEC_a_0021...

  Launch-On-Warning
  One source of ambiguity in the implementation of China's no-first-use policy is the ongoing debate in China regarding the pursuit of a launch-on-warning posture. Recent doctrinal publications and Chinese interlocutors indicated that the debate has yet to be resolved. On one side, some members of China’s strategic community argued that a launch-on-warning posture would ensure the survivability of its nuclear deterrent if its opponent has robust targeting intelligence. The Science  of  Military Strategy states that a launch-on-warning posture "is in accordance with China's long-standing no-first-use policy, and may effectively protect China's nuclear forces from sustaining even greater losses, improving the survivable nuclear counterstrike capability of China’s nuclear missile forces." China's first priority should be to prevent its adversary from precisely locating its missile launch positions. If China is able to reliably ascertain that an adversary has already launched nuclear missiles at China, however, China could quickly launch its nuclear missiles for a counterstrike "before the enemy has been able to actually inflict nuclear destruction." Launch-on-warning would therefore provide China with an option for nuclear retaliation if a nuclear adversary were able to overcome Chinese efforts at concealment, deception, and mobility to ensure that its forces survived a first strike.

This touches on what I was talking about too. The next few paragraphs interestingly provide a counter-point that disagrees China would take this posture, but not because they don't have the capability to retaliate quickly enough, but for other reasons.

Yes, I was writing about launch-on-warning posture although I can now see I could have been more clear. As far as I know no country is presently assuming that it is able to detect, interpret, and authoritatively decide to enter full-scale nuclear war within ~25 minutes of enemy ICBMs' launch. (Leaving say ~5 minutes to implement the decision.)

What you cited is sad, but firstly it is still an "ongoing debate", and secondly the road from declarations to the actual ability is long with this one. And may it never materialize again.

> Yes, I was writing about launch-on-warning posture although I can now see I could have been more clear. As far as I know no country is presently assuming that it is able to detect, interpret, and authoritatively decide to enter full-scale nuclear war within ~25 minutes of enemy ICBMs' launch. (Leaving say ~5 minutes to implement the decision.)

Do you have a source for that?

The sibling comment says Russia and USA have a prompt alert which can launch in under 15 minutes. Other articles say the US president could be contacted and approve in under 30 seconds, not sure how the Russian chain of command goes.

The decision process does not begin when nukes are detected of course, that's the end of the process. Pre agreed criteria have already been met. Quite likely there is no decision about entering a full scale nuclear war because they are already in one.

Pretty much this, both sides know that a large surprise attack against the entire triad would be effective. That’s why monitoring of military exercises and other precursors is critical. It’s very hard to kill all nuclear capable aircraft when they are already in the air. Placing all submarines on an alert posture also helps prevent the possibility that they’ve been intercepted. An around the clock war room helps ensure the president makes the decision to launch.

Actually, since the invention of ballistic missiles we are permanently 30 minutes from death. There are around 900 flight ready warheads both in US and Russia.

https://uploads.fas.org/2014/05/Brief2017_GWU_2s.pdf

A good starting point is this Congressional report on hypersonic weapons, updated one month ago:

https://sgp.fas.org/crs/weapons/R45811.pdf

Thanks, I've started reading it and will continue later, the material seems to be perfect for a non-Aerospace professional.

OMG "The Pentagon’s FY2022 budget request for hypersonic research is $3.8 billion"

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-hype...

[2] https://en.wikipedia.org/wiki/Meteor_(missile)

Just saying that's a lot of money in absolute terms. I'd hope that money could be spent in other ways.

> it's like destroying 10 Ferraris.

But, like, really cool.

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.

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.

Most missiles do use rocket engines. Solid rockets are much easier to store and use than liquid fueled engines, whether rocket or turbojet. They can get pretty fast, too: the AMRAAM is supposed to go mach 4. The downside of any rocket engine is that the fuel and oxidizer run out quickly; the AMRAAM goes that fast in order to have a long range, as the motor burns out fairly soon. A few newer air to air missiles, like the British Meteor, have air breathing liquid fueled turbojets in order to increase range. Cruise missiles with their air breathing engines are the odd one out in missile tech.

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

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.

In German the name is Marschflugkörper = marching flight body.

People imagine how drones are going to change everything. Cruise missiles are already bomb shaped drones...

And existing bombs get "drone" capability...

https://en.wikipedia.org/wiki/GBU-53/B_StormBreaker

The Standard-Missile family from the US can all go faster than Mach 2.5. But land attack missiles have never needed to be particularly fast until recently.

Yes, extreme ends of the spectrum.

All ballistic missiles are hypersonic. The V-2 was hypersonic. That's because range is directly dependent on speed and you wouldn't have basically any range if you had a mere supersonic speed.

> 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.

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

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.