The US and Australia have been collaborating on technology for hypersonic weapons since 2007, but you might be forgiven for not knowing that. Until 2014, when China was seen to be testing the DF-17 boost-glide weapon, the standard line was that ‘hypersonics are the weapon of the future—and they always will be.’ The Hypersonic International Flight Research Experimentation (HIFiRE) program of Australia and the US was seen as a lab project.

Its successor, the Southern Cross Integrated Flight Research Experiment (SCIFiRE) program, is linked to a funded weapon: the US Air Force’s Hypersonic Attack Cruise Missile (HACM), to be carried by US Air Force F-15Es and, possibly, by Royal Australian Air Force F/A-18F Super Hornets.

HACM development won’t be complete until 2029, if it stays on schedule, and its costs are undefined. For now, US hypersonic weapons are confined to a different class of vehicle, similar to the systems that China is deploying. But due to geography, a technology that is useful to China may be less so to the Pacific alliance.

‘Hypersonic’ has a clear definition: Mach 5 and above. As a vehicle approaches five times the speed of sound (5300km/h at altitude), a layer of stagnant air forms at its nose and leading edges and becomes hot and compressed, subjecting the vehicle to much more extreme conditions than are encountered even at Mach 4.

Two hypersonic vehicle types are being developed: hypersonic glide vehicles (HGVs), launched by rockets but using aerodynamic lift to maneuver and extend their range; and cruise vehicles powered by supersonic combustion ramjets (scramjets), the only airbreathing engines that will function at such speeds. They fly as low as 30,000 meters, far lower than ballistic missiles, and maneuver at will. So the defenses get less warning and cannot predict their track as easily.

It’s not easy. Computational fluid dynamics (CFD) codes for lower speeds are known and reliable, and large-scale wind tunnels are available. Hypersonic CFD is less mature, because of the interaction of aerodynamics and heating and because of the changes in airflow at the hypersonic boundary. Wind tunnels for Mach 5 and above accommodate only small test specimens and are hard to operate: a hypersonic tunnel test has been compared to firing a large rocket engine.

Flight tests require large vehicles hurled towards hypersonic speed by powerful rockets. They must survive launch then separation from the rockets before gathering any data. Most programs have lost test craft in the boost phase.

That’s why the US has taken a low-risk approach to its first-generation hypersonic weapon. It comprises a Common Hypersonic Glide Body (C-HGB) mated to a two-stage solid rocket booster stack. The Army’s version is the Long-Range Hypersonic Weapon (LRHW) and the Navy weapon is called Conventional Prompt Strike system.

The C-HGB is based on a 40-year-old design by Sandia National Laboratories. It is cone-shaped with four delta fins that steer the body and orient it so that it generates lift.

The aerodynamics of the finned cone are not as hard to model as those of a wing or lifting body. It can rotate around its axis in flight: the windward side (the lower side) sees the highest temperature, so rotating the vehicle spreads the heating load over the entire skin. Even so, it needs a ceramic thermal protection system, using Sandia-developed zirconium diboride material.

But the finned cone generates more drag relative to lift than a flattened glider (which is more challenging to develop), so it needs more rocket boost to get the desired range. Result: the missile weighs a reported 7400kg to deliver a small 225 kg payload over 3000 km.

It is expected to cost $50 million per round, but most of the life-cycle cost of such a large weapon goes into the platform that moves and protects it. Because the Conventional Prompt Strike weapon is too large for standard vertical launch system tubes, the US Navy is putting it on its three Zumwalt-class destroyers, replacing the two 155 mm guns (for which the Navy never procured ammunition). Each of these 15,900-tonne ships would carry just 12 missiles. Nine of the ten Block V Virginia-class attack submarines, priced at $4.3 billion each, will also be able to carry 12.

But here’s one reason why hypersonic weapons may not be very useful for the US in the Western Pacific: the only ones that would be available for prompt strike would be those in vessels that were within range when a conflict starts. Not many.

China doesn’t have that problem. DF-17s are carried on trucks and most of the people operating them need about the same level of training as a mobile crane crew. Just about the whole stock can be available for firing from day one.

The US Army’s weapon would be carried in two-round packs on oversize semi-trailers. That’s similar in principle to how China deploys regional-range missiles. But the survival of the missile and launcher depends on mobility and dispersal, which is easier to achieve on mainland China than on Pacific islands. Guam has only 1050 km of highways.

The US Air Force has scrapped its HGV weapon, the AGM-183 Air-Launched Rapid Response Weapon (ARRW). Even with a glider with better aerodynamics than the army-navy configuration, ARRW would have delivered only a 110kg warhead. And the weapon was still so large that only the B-52H could carry it. Each bomber would have carried just four rounds.

The US Air Force’s 2025 budget included no funds for further development of ARRW but did fund the scramjet-powered HACM. Raytheon is the prime contractor on HACM, but Northrop Grumman is developing the engine, which comprises most of the missile.

Scramjets are more efficient than HGVs and are smaller for equivalent warhead size and range but are harder to develop. The air flowing through the engine is hot, compressed and undergoing thermochemical change, and the scramjet won’t light below Mach 4.

The scramjet engine creates almost as much drag as it does thrust. If either number is missed, the engine produces no net power. The only way to get good test data is to fly a large-scale vehicle.

But there’s one basic question about scramjet hypersonic missiles: how important is it to exceed Mach 5, compared with speeds of Mach 3.5 to 4 that can be attained with less risky ramjets? Weapon time of flight is less, but only by six to eight minutes for 1800 km range. A 2013 paper by Mark Maybury, then chief scientist of the US Air Force, shows that with modest stealth measures, a high-supersonic vehicle is as survivable as a hypersonic one. Moreover, the combination of high Mach and reduced radar cross section was demonstrated more than 60 years ago.

There’s no sign, in all the enthusiasm over hypersonics, that the question has ever been asked. Maybe it should be.