Harry J. Kazianis at National Interest had an interview with senior fellows Bryan Clark and Mark Gunzinger of the Center for Strategic and Budgetary Assessments.
The biggest technical challenge is the fact you essentially need two different motors to power the missile: one to get the missile up to supersonic speeds (over Mach 1, or about 750 mph), and another to then take the missile up to hypersonic speeds (over Mach 5, or about 3,750 mph). The kinds of motors that work at lower speeds, such as turbojets and turbofans, will not work at higher speeds, which require scramjet or ramjet motors. Conversely, the higher-speed motors don't work at lower speeds. This can increase the cost of the weapon significantly, because you need to boost the missile to high speed and then use a scramjet or ramjet to attain and maintain hypersonic speeds.
There are essentially two ways to get a missile up to the speed where a scramjet or ramjet will work. One is a "boost-glide" weapon in which a large rocket boosts the missile to high altitudes and speeds before its ramjet or scramjet ignites and powers the missile at hypersonic speed to a target. The other is an air-launched weapon that uses the launch aircraft to get the missile to a high altitude and speed; after launch, a small booster rocket takes the missile to high enough speeds for the ramjet or scramjet to take over.
Air Launched are cheaper and small 3D printed scramjets motors could be cheaper
Air launched weapons could be far less expensive than a surface-launched weapon. Some U.S. manufacturers are looking at reducing the cost of air-launched hypersonic weapons by using inexpensive rocket boosters to accelerate it to high speeds, and using additive manufacturing to build the hypersonic ramjet/scramjet motor. Additive manufacturing enables them to produce the motor less expensively because it eliminates the need for extensive machining to create the motor’s combustion chambers and fuel systems.
Raytheon is in the process of increasing the use of 3D printing extensively and their Missile Systems division has already 3D printed almost every part of a guided weapon, including rocket engines, guidance and control systems components, fins, and more. They are now proceeding to look at the possibility of 3D printing conductive circuits, as well as housings for their gallium nitride transmitters and fins for guided artillery shells. Not only would 3D printing help reduce costs, by eliminating the need to machine parts, but it might help them print missile components in the field of war.
Defending against hypersonic weapons
It is very difficult to defend against hypersonic weapons using our traditional "layered" approach. Since they are going very fast, it will be hard for area air-defense interceptors such as the Navy SM-6 or Army PAC-2 / PAC-3 to catch them unless they are launched from the target's location. Because they travel so fast, an electronic warfare effect may not cause them to fail or go off-course soon enough for the missile to miss its target. The best defenses against them will likely be high-capacity point defenses such as Rolling Airframe Missile, CIWS and possibly rail guns that are co-located with a target.
China's Hypersonic weapons
China will likely employ hypersonic weapons as part of their A2/AD (area denial) strategies, but in limited ways. While "boost-glide" weapons will have long ranges and be highly survivable, but they will also be very expensive. China could use them as a "silver bullet" weapon to hit high-value targets, or do so in conjunction with less-expensive weapons that reduce the defender's capacity first.
When they develop them, China will likely also employ air-launched hypersonic weapons to attack U.S. and allied bases with a high probability of being able to circumvent U.S. defenses
Conclusion hypersonics are an evolutionary improvement, rather than a revolutionary one
Hypersonic weapons are faster, and more survivable, but have limited payload, less maneuverability and less-sophisticated sensors and logic.
SOURCE - Harry J. Kazianis at National Interest, 3dprintingindustry