Hypersonic weapons: What’s the difference between Boost-glide and Scramjet?

If you’re familiar with the old addage, “there’s more than one way to skin a cat,” it certainly applies to hypersonic flight, but most weapons capable of exceeding Mach 5 tend to come in one of two forms: boost-glide vehicles, and scramjet-powered cruise missiles.

Hypersonic, while sounding like a term invented for a kid’s TV show about superheroes, relates specifically to traveling at speeds in excess of Mach 5. At such high speeds, even the most modern air defense systems in the world pose little threat to these weapons as they close with their targets.

In fact, hypersonic weapons are currently considered all but indefensible at scale, thanks not only to their high velocities, but because of the maneuverability allowed by some modern hypersonic designs.

Hypersonic flight is not a new thing, despite its recent launch into the limelight. Even the Nazi V-2 rocket could break the Mach 5 barrier, and as we’ve discussed before, the United States had a hypersonic bomber program in the works before the Soviets launched Sputnik. What has changed, however, is the ability to control flight at this rate of speed to a high degree of accuracy through onboard hardware and advanced software.

There are multiple efforts underway to modify existing systems or devise new ones to effectively counter the hypersonic threat, but to date, no single solution has emerged. Instead, nations like the United States, Russa, and China have resorted to the time-tested approach of devoloping and fielding ever-more advanced weapon systems to serve as a deterrent. Even in missile development, the old adage, “the best defense is a good offense” sometimes still applies.

Today, there are at least 15 formally disclosed hypersonic weapons in service or development around the globe (more if you count hypersonic programs not overtly tied to weapons), with a number of others hinted at or unintentionally disclosed through leaks. To date, only Russia and China have operational hypersonic weapons in service, with the United States still at least a year away from fielding its first fully operational weapon. These three nations make up the brunt of of the hypersonic list, but other nations, including India, Australia, France, Germany, Brazil, Japan, and even North Korea have all begun developing hypersonic technologies for weapons applications in one form or another.

Two Kinds of Hypersonic Weapons

When people talk about hypersonic weapons, they’re usually referring to one of two kinds: hypersonic glide vehicles and hypersonic cruise missiles.

Hypersonic Boost-Glide Vehicles

Hypersonic Glide Vehicles (HGVs) aren’t all that different than the warheads on traditional long range ballistic missiles, at least in the early stages of their flight path. They are carried into the upper atmosphere via high velocity boosters just like traditional ICBMs. The missile then deploys one or more glide vehicles that rely on momentum and control surfaces to manage their high-speed descent as they close with their targets.

Hypersonic Scramjet Cruise Missiles

Hypersonic cruise missiles, on the other hand, often rely on an advanced propulsion system called a scramjet. A scramjet, or supersonic combusting ramjet, is a variation on tried and true ramjet technology that allows combustion to take place with supersonic airflow. Because scramjets are really only efficient at high rates of speed, these missiles are often deployed from fast moving aircraft or rely on a different form of propulsion to get them to these speeds.

From there, hypersonic cruise missiles operate much like traditional cruise missiles–at least in theory. In practice, these platforms are far more difficult and expensive to build than traditional cruise missiles.