By Paul Evancoe

Better is the enemy of good enough. While this statement remains valid for most everything that costs money, it is not so when it comes to weapons and warfighting. Continual upgrades to existing weapons along with next generation weapon development is necessary to win future conflicts. The development and fielding of hypersonic weapons is a prime example.

The media tells us that China and Russia, even Iran and North Korea, are ahead of the U.S. in the development and fielding of hypersonic weapon technology. We’re told hypersonic technology is also being developed by India, Japan, France, and Australia. We are led to believe that the U.S. Navy’s aircraft carriers are vulnerable to hypersonic weapons as is Guam, Hawaii, and coastal regions of the continental United States itself. All the media’s hypersonic hype leads many of us to reason, if hypersonic missiles and, perhaps, hypersonic aircraft, both manned and unmanned, are an unbridled threat to us, then development of antiaircraft guns firing hypersonic munitions, or super hyper-fast interceptor missiles must be a necessity. The media conveniently neglects discussion about the physical and material limitations involved in hypersonic weapons; leaving the reader fearing our certain demise should, say, a conflict over Taiwan or free passage of the South China Sea arise. China will fire thousands of hypersonic weapons at our forces and outlying territories like Guam and we’ll be at their mercy – or so the media would have us believe. As you will read below, our competitors are not ahead of us. In fact, their hypersonic technology pales in comparison.

UNDERSTANDING A MACH NUMBER

Named after the Austrian physicist Ernst Mach, a Mach number is a dimensionless quantity in fluid dynamics representing the ratio of flow velocity past a boundary to the local speed of sound. The speed of sound (Mach 1) is approximately 1,125 feet per second (FPS) which translates to 767 miles per hour (MPH) or, one mile in 4.69 seconds. These velocities assume a constant temperature of 69F at sea level. As temperature and elevation vary, so does the speed of sound.

HYPERSONIC WEAPONS TRAVEL FIVE OR MORE TIMES THE SPEED OF SOUND

At five times the speed of sound (Mach 5), a vehicle travels at approximately 3,835 MPH which is a little faster than a mile a second. In comparison, the fastest performing small arms bullet can achieve bullet velocities approaching the 4,000 FPS range. At velocities beyond 4,000 FPS, average off-the-shelf bullets literally disintegrate from heat generated by air friction.

It is one thing to have a hypersonic weapon that can only fly in a straight line to a preprogramed static target. It is quite another to have steerable hypersonic weapons that can maneuver inflight to engage a moving target hundreds of miles, even thousands of miles away. Reportedly, the U.S. has steerable hypersonic vehicles that are capable of Mach 22 (or faster). We’re talking a speed of 16,874 MPH. That speed translates to faster than 1 mile every 1/18th of a second, or a speed over ground of 18-plus miles a second. So how do hypersonic weapons survive hypersonic velocities without vaporizing, and how are they steered?

HYPERSONIC CRUISE MISSILES AND HYPERSONIC GLIDE VEHICLES

There are two principal types of hypersonic vehicles: hypersonic cruise missiles and hypersonic glide vehicles.

Hypersonic cruise missiles are powered by airbreathing scramjets, and are limited, because of air density necessary for fuel combustion, to flight below 100,000 feet. While airbreathing like a slower flying conventional fanjet engine, a scramjet significantly differs. Like a turbocharger on a car engine, a conventional fan jet engine uses a series of turbine fans to compress the air that feeds into its combustion chamber. This is necessary to richen the oxygen ratio for the air-fuel mixture entering the combustion chamber where it is ignited. The rapidly expanding gases resulting from combustion provide thrust from the rear of the engine. Conventional jet engines cannot be used at hypersonic speeds because they “choke” on the air that gets backed up in front of the engine’s compressor blades and air intake.

Scramjets don’t use compressor turbines because the speed of the vehicle is sufficient to compress the air entering the combustion chamber. Think of a scramjet as more of a pipe with a venturi combustion chamber inside. Scramjets can propel a vehicle well into the Mach speed range but still require air (oxygen) for fuel combustion; thus, capping their usable altitude at around 100,000 feet where there is still enough air for compression and combustion.

A hypersonic glide vehicle travels at much higher altitudes (outer atmosphere) and at greater speeds than their hypersonic cruise missile cousins. A hypersonic glide vehicle is usually launched atop a ballistic missile first stage called the boost stage. Upon reaching Earth’s outer atmosphere, the glide vehicle separates and transitions to hypersonic flight as it re-enters the lower atmosphere. This is called the glide stage and it is this super-fast maneuverable hypersonic glide stage that allows it to evade most existing nuclear missile defense systems, which were designed to counter ballistic missiles during a warhead’s ballistic re-entry stage. Even so, hypersonic vehicles are generally slower than ballistic missiles (i.e., sub-orbital or fractional orbital), because hypersonic vehicles travel in the atmosphere, and ballistic missiles travel in the vacuum of space outside Earth’s atmosphere.

Reaching hypersonic speed is not a significant engineering challenge. We have been achieving hypersonic speeds since the V-2 rocket was fielded by Germany during WWII. The piloted X-15 experimental rocket plane, flown in the late 1950s into the early 1960s, was hypersonic. The engineering challenges in hypersonic flight involves building vehicle bodies from exotic composite materials that can survive sustained frictional temperatures exceeding 3,632 degrees Fahrenheit. These same materials must also withstand the extreme air pressures of hypersonic flight without compromising vehicle strength and payload.

While hypersonic vehicle material construction is both demanding and expensive when compared to conventional cruise missiles or aircraft, the greatest challenge is directional control of the vehicle at hypersonic velocity, e.g., being able to correct the vehicle’s flight path to hit a target during, say, a 1,500-mile flight. Conventional winged or finned aerodynamic control surfaces like airplanes and cruise missiles have, don’t effectively work at hypersonic velocities. The physics speak for themselves; the faster a hypersonic vehicle goes, the less conventional aerodynamic control surfaces work. In fact, conventional airframe designs create unwanted turbulence and drag and serve to destabilize a hypersonic vehicle rather than facilitate its flight.

STEERING A HYPERSONIC VEHICLE

The U.S. has developed several unique means of steering a hypersonic vehicle that may already be incorporated into operational hypersonic vehicles. Operational status of this technology remains classified. Being able to make course changes in-flight at hypersonic speed is essential, especially if the target is moving, e.g., an aircraft carrier battle group. It is also essential to vary the hypersonic vehicle’s attack profile (altitude and track) making it less vulnerable to countermeasures.

One of these steering methods uses high-pressure air jets (thrusters) mounted along the vehicle’s body and flight surfaces which act as steering thrusters. These thrusters nudge the vehicle left, right, up, and down. Another steering method for ultra-high Mach speeds uses multiple pulsed lasers mounted along the vehicle’s leading aerodynamic edges and body. The lasers are aimed a short distance ahead and to the side of the vehicle. When the lasers are selectively pulsed, they create a low-pressure air plasma that reduces the drag over the lasered area. The unlasered counter pressure effectively nudges the craft in the direction of the lasered low drag area; thus, maneuvering at ultra-high Mach speeds is achieved. These same lasers, when pulsed ahead of the vehicle, create a low-pressure area reducing drag (less frictional heat) and further provide a radar-defeating stealth plasma envelope around the vehicle.

COMMUNICATING COURSE CORRECTION COMMANDS TO HYPERSONIC VEHICLES

Hypersonic vehicles must be maneuverable (steerable), either by means of an on-board GPS-based steering command system (likely AI-based), or a satellite link, or both. At hypersonic speeds antennas don’t function, or if they do, they marginally function. This means receiving GPS satellite navigation data can be sketchy, especially if flying inside a plasma envelope (described previously). Hypersonic vehicles employ internal guidance systems that are continually updated via satellite downlink. But steering a hypersonic vehicle has another set of issues that result from its speed. Course corrections and altitude changes require adequate standoff (distance-to-target) to be realized.

A marriage of satellite-based target detection, hypersonic vehicle tracking, and steering commands is necessary. This requires robust satellite capabilities that can understand and predict motion, and can perform data association, relative position determination, and maneuver detection. This must all be achieved real-time and then communicated to the hypersonic vehicle while it’s in-flight so the appropriate course changes can be achieved.

Let’s say, for example, a hypersonic glide vehicle has separated from its booster and is now traveling in-bound at Mach 20. Its target is an aircraft carrier 900 miles away traveling at 20 knots on a perpendicular heading to the glide vehicle’s course. The carrier is being tracked by satellite and the satellite is providing constant position updates to the hypersonic glide vehicle. Computation of the intended rendezvous attack point is constantly updated, and small course corrections are made to the glide vehicle’s flight path. The glide vehicle is less than 25 seconds out when the carrier battle group detects it and makes a radical course and speed change to evade the incoming hypersonic glide vehicle. The glide vehicle, because of the relative speeds involved, has virtually no time window to correct its course and still hit the carrier. The attack fails.

The lessons in the preceding scenario are this. Faster is not always better, and an agile target will usually always result in a miss. To up the odds of a hit in the carrier scenario, numerous hypersonic missiles would need to be fired in swarms (close sequence) – and that’s exactly the strategy our potential enemies intend to use. But this strategy can also fail if the carrier battle group has the appropriate countermeasures (decoys, counter missile batteries, and guns), and in that case, it leaves the attacker vulnerable with a depleted magazine of hypersonic weapons as expenditures outpace resupply.

COUNTERMEASURES

Are there effective countermeasures against hypersonic weapons?

The short answer is “yes,” but current detection and fire control systems are mostly inadequate no matter who’s flag you’re flying. The longer answer is countermeasure success depends on detection range, incoming speed, and flight profile at which the hypersonic weapon is traveling. Countering hypersonic weapons during the glide phase (at maximum speed) requires powerful sensor data fusion consisting of long-range radar(s), as well as dedicated space-based infrared sensor tracking capabilities to capture hypersonic signatures in the atmosphere, and fire control systems for tracking and aiming directed energy weapons and interceptor missiles. Much in the same way that anti-ballistic missiles were developed as countermeasures to ballistic missiles, directed energy weapons and ultra-fast hypersonic interceptor missiles are under development as countermeasures to hypersonic weapons.

SPACE-BASED FOO FIGHTER TRACKING SATELLITES ARE NECESSARY

A cohesive “FOO Fighter” satellite constellation capable of detecting hypersonic missiles and then directing interceptor countermeasures to destroy them is under development by the U.S. This highly classified program’s name is borrowed from the mysterious balls of light that plagued Allied aviators during WWII. Many pilots reported that luminous orbs appeared to be able to out-maneuver their aircraft and couldn’t be detected by radar. The pilots nicknamed these mysterious balls of light “Foo Fighters.” Hypersonic vehicles can out-fly modern military fighter aircraft and evade conventional missile tracking systems and that is what’s prompting the development of the FOO Fighter satellite constellation and other related hypersonic countermeasure programs.

CONVENTIONAL GUN AND HYPERSONIC BULLETS

As mentioned previously, off-the-shelf bullets disintegrate around velocities exceeding 4,000 FPS largely because of air friction-generated heat and conventional bullet material construction. Bullets could certainly be made from exotic metals, or ceramics, and specifically designed to survive hypersonic velocities, but exotic material construction is expensive and would solve only part of the problem. Bullet aerodynamic design would also have to radically change from the shapes we currently see; like spitzers or hollow-point boat-tails. A hypersonic round would more likely resemble a dart-like anti-armor sabot light armor penetrator (SLAP) round. Use of a sabot to carry the projectile down the bore would likely be necessary to gain hypersonic velocity. A SLAP-like projectile would be necessary to reduce drag and eliminate excessive mass. And last, the gun’s bore would likely have to be smooth since a rifled bore would burn out from hypersonic velocities and bullet spin would necessarily not improve projectile accuracy at hypersonic speed. This would all come with an excessive price tag and no guarantee of increased range or accuracy.

Directed energy weapons seem to offer the most effective counter to hypersonic vehicles. The U.S. military has already developed land-based, seaborne, and airborne versions capable of neutralizing hypersonic missiles. Even so, some missiles may get through if fired in a swarm.

Currently, the last line of defense against hypersonic weapons is the Navy’s 20 millimeter close-in anti-missile weapons system Phalanx C-WIZ and Army’s C-RAM land-based mobile version that fire at a blazing rate of 4,500 rounds per minute. The gun is automatically aimed using an automated radar and infrared optical fire control system. The gun’s high rate of fire literally puts a dense wall of 20mm bullets in the direct path of the incoming target that shreds it. While 20mm bullets are far from hypersonic, projectile quantity, from the gun’s high rate of fire, makes the destructive difference. That said, the gun has a limited effective range of about two miles and can only be used as a last line of defense. If it misses, there is no second chance.

Another shipboard countermeasure weapon being experimented with is a linear explosive resembling a hose that is projected by rocket a few hundred feet from a ship onto the sea surface perpendicular to the incoming path of a hypersonic missile. When the missile is on its final approach with limited maneuverability and closing on the target ship; the seaborne explosive charge is detonated, raising a curtain of water in the path of the oncoming missile. When the missile hits the water at hypersonic velocity it either disintegrates or veers off course.

A DEVELOPMENTAL PRIORITY IS NECESSARY

Which should be fully developed first: hypersonic weapons or hypersonic countermeasures? The price tag to develop both offense and defensive hypersonic weapons simultaneously is too costly. Both offensive and defensive weapon technologies require a dedicated satellite constellation for sophisticated detection, tracking and (fire control) targeting. Many believe that defense is principal, considering our competitors claimed technical maturity of their offensive hypersonic weapons. However, capabilities-based thinking has two consequences. First, it assumes that no weapon is ever “good enough” so capabilities must be continually improved, requiring more research, development, and lots of money. Second, because in the future anything is possible, weapon concepts advance using borderline-theoretical technologies. This approach bets that the technologies used will have ample time to mature and that time is on our side. This thinking seconds the possibility that our adversaries will likewise be striving to acquire the same capabilities and may succeed before we do. It further assumes our potential enemies are far advanced compared to us. This has recently been touted by the media and in adversaries’ claims about military application of autonomous systems, artificial intelligence, and machine learning into unstoppable hypersonic weapons. Such claims are largely deceptive, but we are meant to be afraid.

Fortunately, our competitors’ share the same issues we have. Without a strong defense against hypersonic weapons, and the industrial base to readily restock expended swarms, one must be very selective of the fight one picks.