by Irvin Li
Most of us had the experience of skipping stones on a lake as a leisure activity, but did you know that the same principle can be applied to exo-atmospheric objects? Since the atmosphere is more of a gas while outer space is more of a vacuum, the relationship is analogous to that of the air and the water, as the dynamics involved are quite similar.
The mechanics of stone skipping can be explained roughly by the law of conservation of momentum – vertical momentum is pushed back by water while horizontal momentum more or less stays the same. However, without satisfying the proper conditions, it would just sink into the water. For a stone to skip, it needs to be relatively flat preferably disc-shaped and land on the water at optimally about 20 degrees tilted back and simultaneously maintain constant rotation for gyroscopic stability.
Modern technology has applied the principles of skipping stones on water to skipping crafts on the atmosphere, that is, if they are supersonic. The path taken by such objects is what we call a boost-glide trajectory.
The concept that you can skip objects on the atmosphere is not new. In fact, its phenomenon was observed by German artillerymen who manned the gigantic K5 railway gun in World War II. They found that when firing arrow shells at high altitudes, their range was unexpectedly extended. The anomaly was then studied, and over the years, became a key part of modern rocketry.
For spacecrafts, taking a boost-glide approach can extend the landing phase as well as alleviating the immense heat compared to more traditional means. The US, Russia, and China have all successively used it for their spacecrafts upon reentries.
Yet as space exploration is somehow falling out of favor these days, we see more applications of boost-glide trajectories to missiles, more specifically the maneuverable hypersonic glide vehicles that travel faster than Mach 5. In the endless between missile offense and defense, these gliders pose a challenge to contemporary defense systems: neither are they easy to predict like ballistic missiles, nor are they slow enough as most cruise missiles to be comfortably intercepted.
Unlike the “skip” we mentioned earlier, instead of bouncing off like a stone, these weaponized projectiles smoothly skim parallel to the Earth’s surface after being lifted outside and falling back into the atmosphere, which is said to sacrifice a bit of range for more stealth and accuracy. It works in the same way as skipping being that both movements relies on the difference in air pressure that generates lift.
The result? Hypersonic warheads so fast they cannot be intercepted and capable of considerable destruction by kinetic energy alone without carrying conventional or nuclear explosives. Russia’s Avangard and China’s DF-ZF are both HGVs of such type. (The US is also testing their hypersonic weapons, but those are rather aircraft/cruise missiles using compression lift, which I’m happy to explain in a different article.)
While boost-glide trajectories are only used for space exploration and missile systems so far, that doesn’t mean it will never be truly beneficial to humanity. Theoretically speaking, as infrastructure and technology in avionics, aeronautics, and rocketry become more advanced, who is to say that hypersonic propulsion, which could potentially exploit these trajectories with little resistance, would never become energy efficient and commercially viable?