Harry Kazianis, a non-resident Senior Fellow at the China Policy Institute (University of Nottingham) and Managing Editor of the Washington, DC-based international affairs publication The National Interest interviewed John Stillion, a Senior Fellow at the Center for Strategic and Budgetary Assessments (CSBA).

Q: Please describe what exactly a hypersonic weapon is, for our readers.

A: NASA defines the hypersonic regime as speeds greater than Mach 5 but less than Mach 25. It further divides this speed regime into two parts. One is the 'high-hypersonic' speed range between Mach 10 and Mach 25. The other is the range between Mach 5 and Mach 10 referred to simply as the hypersonic speed range (this is about 5300 to 10,600 kmh). The latter is the speed regime where most of the recent discussion of hypersonic weapons has been focused.

Ballistic missiles with ranges between about 300 and 1000 km travel in this speed range, but they generally don't travel long distances through the atmosphere at these speeds. Usually when hypersonic weapons are discussed people are referring to machines that can sustain flight in the Mach 5 to 10 speed range for a significant distance and period of time measured in minutes. For perspective, the Concorde supersonic transport cruised at Mach 2.

Q: What nations have the strongest hypersonic weapons programs? How advanced is American technology in this regard?

A: Press reports indicate there are only three nations with hypersonic weapons programs: the US, Russia and China.

In November 2011 the US Army conducted a successful test of the Advanced Hypersonic Weapon (AHW) demonstrator. This is a hypersonic glide vehicle similar in concept to the reported Chinese system. A hypersonic glide vehicle couples the high speeds of ballistic missiles with the maneuverability of an aircraft. The goal of the AHW test was to collect data on hypersonic glide vehicle technologies to inform possible future designs. The test used a three-stage missile booster system to power the test vehicle to hypersonic speed and evaluated its performance on a flight over the Pacific Ocean.

A second US approach to hypersonic weapons made a similar advance on 1 May 2013 when the US successfully tested the Boeing X-51 hypersonic research vehicle (video above).

It is powered by a supersonic combustion ramjet or 'scramjet' engine and flew about 306 km in three and a half minutes at just over Mach 5. This was the first successful test of a scramjet-powered vehicle. The scramjet is efficient at hypersonic speeds, but as the name implies, the air flowing through the engine is traveling at supersonic speed, so the fuel must be precisely measured, injected into the air flow and ignited with extreme speed. Work on what eventually became the X-51 began in the early 1990s.

These successful tests indicate the US is well along the path to solving many of the problems associated with sustained hypersonic flight. These include the high drag and temperatures generated by vehicles traveling at hypersonic speed and developing an efficient powerplant.

Q: There have been reports that America is considering building such weapons for deployment on submarines. How challenging would this be and is it practical?

A: The X-51 had to be boosted to high speed (Mach 4+) by a rocket before it could start its scramjet engine. So, any weapon employing a similar propulsion system would probably initially be launched like a missile. The US has been launching missiles from submarines for decades and is familiar with, and has overcome, the technical challenges likely to arise in that part of the development program. Alternatively, launching a missile with an AHW-derived weapon might be equally feasible.

Q: China's various hypersonic glide vehicle tests have garnered a lot of attention. How advanced might Beijing's hypersonic program be compared to the US?

A: Not much is really known publicly about the Chinese program. What has been reported indicates that their initial investments might be focused on building vehicles that can replace the re-entry vehicles usually carried by ballistic missiles. These 'hypersonic glide vehicles', as the name implies, are carried by ballistic missiles, but once they descend into the upper atmosphere, their shape gives them much greater range and maneuverability than 'normal' cone-shaped re-entry vehicles. So, based on press reports, the Chinese AHW programs might be characterised as working to improve the capabilities of ballistic missiles while the X-51 program is focused on making weapons that behave more like very fast cruise missiles.

Q: Many have stated Chinese hypersonic technology could be used as a new form of anti-ship weapon like the DF-21D. Would you say this is possible?

A: Again, based on press reports, the DF-21D seems to rely on a maneuverable cone-shaped re-entry vehicle. Replacing this with a hypersonic glide vehicle might give the existing weapon greater ability to maneuver to attack targets and avoid defences as well as greater range.

Q: How long would it take for such Chinese tests to move towards a weaponised system?

A: Our track record in predicting when new Chinese weapons will come on line is not very good. The DF-21D and J-20 stealth fighter both materialised more quickly than most outside observers thought they would. If the Chinese tests are as far along as they seem to be from press reports, it might be possible to see operational systems with this technology in the field within a decade.

Q: Can US missile-defence platforms such as AEGIS defend against such weapons? If not, what options would America have?

A: Defensive missiles have very limited time and a finite amount of energy available to position themselves to intercept an incoming offensive missile. Like most guided weapons they constantly compute and re-compute the point in space where they will intercept the incoming missile and fly toward that point. If the incoming missile is truly a ballistic missile, then its trajectory is essentially fixed and the interceptor will not need to maneuver much because the calculated intercept point will be quite stable. However, if the incoming missile can maneuver, the interceptor will need to maneuver as well. Given the high speeds and short timelines involved, hypersonic glide vehicles have the potential to make defensive missiles less effective than they might be against non-maneuvering targets.

Options for bolstering defences include the electromagnetic railgun and directed-energy technologies currently under development. Other possible countermeasures include using jammers or other electronic countermeasure techniques to deny targeting data to the attacker or to confuse the hypersonic glide vehicle's own sensors as it attempts to hit its target. Disrupting communication links between sensing, command-and-control, and missile units is another possible means of decreasing the effectiveness of such weapons. Over the long run, developing long-range, survivable strike systems that would allow our ships to operate beyond the effective reach of weapons like the DF-21D may be the most robust defensive approach.