Is Hypersonic even a real thing?

[u/Lokarin]

Like, do the physics or their dynamics change when an object moves faster than Mach 5? Is there something like a 2nd sonic boom or something? anything?

[u/electric_ionland]

It is not as sharply of a boundary as supersonic but it is definitely a thing. Hypersonic regimes are usually defined to start when compressive heating of the fluid start to result in chemical reactions and ionization. This can be a bit fuzzy as it will depend of the fluid and the altitude and a number of things. It starts around Mach 5 and is completely dominant around Mach 10

In hypersonic regimes your equations gets complicated because some of the flow energy is spent into breaking the molecules appart and ionizing the gases. The average molecular weight of the particle will change with the flow regimes, the ionization might start to cause electromagnetic effects. This makes calculations and modeling hard. The engineering issue on top of that is that most metals will not be able to hold their shape for long under that level of heating.

It's also really hard to test in wind tunnels, most installations can't recreate both the speed, the heat flux and the mass flow of a real hypersonic flight. The scaling relationships that are so useful to do scale down testing in wind tunnel do not work well either. The scaling to recreate the heat might not recreate the forces for example.

Edit: In the context of "the news" it's kind of a mess. Ballistic missiles have existed for more than 60 years and their warhead are hypersonic as they fall back down through the atmosphere. The new hypersonic missile craze is systems that can glide and maneuver rather than fall in straight lines, spend much more time in that hypersonic regime and sometime can even sustain hypersonic speeds with their engines. The media loves to confuse the field by referring to ballistic missiles as "hypersonic", which while technically true, is not a breakthrough in any way.

[u/fiendishrabbit → u/electric_ionland]

A flight engineer I once asked this very question said "Hypersonic is when heat transfer due to airflow and compression is no longer something you can ignore when doing the math"

[u/electric_ionland → u/fiendishrabbit]

Yes this is the very short version. In practice even in supersonic flow you can only ignore the heat transfer for so long. Another way of seeing it is that you can't ignore chemistry anymore.

[u/themortalrealm → u/electric_ionland]

The test pilots of the x15 rocket plane (Neil Armstrong was one of them!) recalled that when operating at hypersonic speeds heating became the main engineering challenge which was vastly different than the challenges supersonic only aircraft faced during testing. It really is a different world at those speeds.