Military Embedded Systems

Hypersonic vehicles and the kill web


September 30, 2019

Ray Alderman

VITA Standards Organization

WARFARE EVOLUTION BLOG: The U.S., China, and Russia are spending a ton of money on research and development for hypersonic vehicles these days, so it?s time to explore what the aeronautical engineers are doing and why.

First, we must define what hypersonic means. Previously, the primitive speed spectrum for missiles and aircraft was divided into three segments: subsonic flight (below Mach 1), supersonic flight (Mach 1 to Mach 5) , and hypersonic flight (above Mach 5). Mach 1 is the speed of sound, 767 miles per hour (MPH) or 0.213 miles per second (MPS), through air at a temperature of 68 degrees (F).

Recent research demands a new and improved speed spectrum definition containing six segments. Subsonic speeds go as fast as Mach 0.8 (609 MPH or 0.169 MPS). The flight is smooth and the aircraft is well behaved in this region. Next, transonic speeds go from Mach 0.8 to as fast as Mach 1.3 (915 MPH or 0.254 MPS). This is where the ride gets bumpy and the aircraft gets harder to control. Then, we have supersonic speeds that go from Mach 1.3 up to Mach 5 (3,806 MPH or 1.057 MPS). In this region, the aircraft skin starts heating and the vehicle starts bouncing around from the shock waves.

Hypersonic speeds go from Mach 5 to as fast as Mach 10 (7,680 MPH or 2.133 MPS). Here, the wings and nose turn red from the heat, begin to deform, and vibrations start tearing the aircraft apart. High-hypersonic speeds go from Mach 10 up to Mach 25 (19,031 MPH or 5.28 MPS). At this point, if the aircraft has not melted, the air molecules surrounding the vehicle break apart, form into oxidizers (dissociation) and charged particles (ionization), and start eating the skin. Finally, reentry speeds for space vehicles are above Mach 25 (19,031 MPH/5.28 MPS) and conditions get even worse. The reason for this expanded spectrum definition is obvious: the aerodynamics, fluid dynamics, thermodynamics, and aero-chemistry all start doing strange things in the transonic, hypersonic, and high-hypersonic regions.

There are two types of hypersonic aircraft being developed. The Hypersonic Glide Vehicle (HGV), also called Hypersonic Boost Vehicle (HBV), is launched from earth up to the Karman Line, the edge of the earth’s atmosphere 62 miles high, by a rocket booster. Once released from the booster, the vehicle can skim along the Karman Line, reenter the atmosphere, and theoretically hit a target anywhere on the planet in less than an hour, powered only by gravity. The Russians reported in 2018 that their Avangard HGV reached speeds of Mach 27 (20,716 MPH/5.75 MPS), hit a target at their Kura Missile Testing Facility, and has a range of more than 3,700 miles. China has tested their DF-ZF (WU-14) HGV at least seven times. It reached speeds of Mach 10 (7,680 MPH/2.133 MPS) and has a range of 7,500 miles. Dynetics Technical Solutions is building the first version of the hypersonic glide body for the U.S. military. It will fly at Mach 5. The scientific reason for the big speed difference of the Russian HPV is probably due to faulty measurements or propaganda.

The Hypersonic Air-breathing Weapon Concept (HAWC) is basically a Tomahawk cruise missile that flies at Mach 5 or faster. It can be launched by a rocket booster or dropped from an airplane, and then it flies to its target using jet engines. While the HGV has to fight the shock waves at hypersonic speeds, the air-breathing hypersonic vehicles ride on top of them. In 2013, the Boeing X-51 Waverider reached Mach 5.1 (3,913 MPH/1.087 MPS). This aircraft is 25 feet long, weighs only 2 tons, and has a range of 460 miles. HWAC vehicles must fly below 19 miles in altitude, since they need atmospheric oxygen for the engines. Russia is working on their Zircon (3M22) HAWC with a range of 600 miles and speed of between Mach 8 and Mach 9 (about 6,521 MPH/1.81 MPS). China is working on their Starry Sky-2 HAWC, that reaches speeds of Mach 6 (4,603 MPH/1.28 MPS) and has a range of a few hundred miles. What you can see here is that HGVs can fly thousands of miles while the HAWC weapons have a range of 400 to 600 miles.

Hypersonic flight is very complicated so let’s examine the major engineering problems: heat, vehicle structure, and propulsion. Aerodynamic heating of the aircraft skin, from air friction and the air being compressed by the nose and wings in flight, start showing up in the supersonic range. That’s why the B-2 bomber flies at subsonic speeds and relies on its stealthy flying-wing design for survival. If it flew at Mach 1.3 or higher, the skin would heat up and enemy infrared sensors could detect it. Once you get above Mach 5, the temperature on the nose of the hypersonic aircraft can reach 3,000 to 5,000 degrees ( F), and the leading edge of the wings can reach 2,000 to 3,000 degrees (F). The melting point of aluminum, the material used for most aircraft, is 1,221 degrees (F). So, a hypervelocity vehicle must be constructed with special materials like graphite and ceramic composites, chrome-nickel metals, and refractory metals like titanium and molybdenum. Otherwise, they will melt during flight.

Next is vehicle structure. The Space Shuttle has a blunt nose, to push the shockwave out in front of the aircraft into a curved pattern, to slow it down on reentry. Hypersonic vehicles have sharp pointed noses to reduce drag. That makes the shockwave look like an inverted “V” and brings it closer to the wings. The tradeoff here is that by reducing drag overall, air friction on the nose and wings increases. That, in turn, increases the heating on those surfaces.

What you get is an aircraft that looks like a flying wedge (the Waverider) or a flying triangle (the Avangard), but they still exhibit a lot of drag at high speeds. The control surfaces are placed on the tips of the stubby wings, instead of the middle like most aircraft. Small changes in these control surfaces must be executed by high-speed computers for maneuvering (turns, climbs, and descents) since the airflow around the vehicle is very turbulent. The guidance computers, control electronics, and explosive warhead must be protected from the heat mentioned above. Heating also causes the materials on the body and wings to warp and buckle, so the aircraft must be rigidly reinforced internally to avoid what air force pilots call the RUD phenomenon (Rapid Unscheduled Disassembly).

Then, there’s the propulsion system problem. We already know that HGVs are launched to the edge of space with rocket boosters and they gain their speed through gravity. Air-breathing missiles have a different set of launch problems. We can’t use conventional turbofan jet engines on these aircraft because those engines can only go to about Mach 3 (2,301 MPH/0.639 MPS). After Mach 3 we need a ramjet engine, but they can only get us to Mach 6 (4,603 MPH/1.279 MPS). To go from Mach 6 to as fast as Mach 14 (10,742 MPH/2.98 MPS), we need a scramjet engine. After that, we’re probably talking about rocket engines again.

Turbofan jet engines use fan blades to compress the subsonic air coming into the engine. Jet fuel is squirted in and ignited, and you get thrust from the exhaust gases. A ramjet engine has no moving parts, no fans. The air coming into the engine must already be traveling at Mach 3, it is compressed by a cone-shaped diffuser, fuel is squirted in and ignited, and the expanding exhaust gases create thrust. Scramjet engines (supersonic combustion ramjet engines) have no moving parts either. Air coming into the engine must already be traveling at Mach 6. Again, the air is compressed by a cone-shaped diffuser and reaches a temperature of 3,600 degrees (F). Fuel is squirted into the combustion chamber and ignited, and the exhaust gases produce thrust.

While the turbofan and ramjet engines can use jet fuel, advanced scramjet engines use liquid hydrogen for speeds above Mach 6. There are several reasons for this. Air coming into the engine at Mach 5 is about 1,800 degrees (F). By using the liquid hydrogen fuel in a heat exchanger as a pre-cooler, the incoming air temperature can be reduced to about 500 degrees (F). Temperatures inside a scramjet engine can reach thousands of degrees due to the high temperature of the incoming compressed air and the heat created by the burning hydrogen. Secondly, jet fuel is hard to ignite above Mach 5, but hydrogen will burn easily.

How did we launch the Waverider to reach Mach-5.1? It was carried on a B-52 bomber up to 50,000 feet, at a speed of about 550 MPH, and then dropped. A rocket booster then took the aircraft to Mach 4.5 (3,453 MPH/0.96 MPS). At that point, the rocket booster fell away and a small scramjet engine took the aircraft up to Mach 5.1 (3,913 MPH/ 1.087 MPS). However, the Waverider used a special jet fuel instead of liquid hydrogen. It could only carry about 270 pounds of JP-7, the fuel used, and the scramjet engine only ran for 210 seconds. That’s a fuel consumption rate of roughly 10 pounds of fuel per second. JP-7 weighs about 6.67 pounds per gallon, so the scramjet engine was consuming about 1.5 gallons of fuel per second.

What we learned here is that an air-breathing hypersonic missile will need to be launched with a rocket booster, up to Mach 3. Then, a ramjet engine takes it to Mach 6. At that point, a scramjet engine takes it to Mach 10 and above. But, there might be better way to do this: the combined cycle engine. A turbofan engine, a ramjet engine, and a scramjet engine are all aligned inside the fuselage of the aircraft. That way, the turbofan engine can take off from a runway and get the vehicle up to Mach 3. The ramjet can take it to Mach 6, and the scramjet can go up from there. The U.S. Defense Advanced Research Projects Agency (DARPA) has a program on this concept. And, there is also a combined turbofan-ramjet-rocket engine in testing, called the SABRE reaction engine. It can fly from zero to Mach 5.4 (4,143 MPH/1.15 MPS). As you can see, a lot of work is being done to integrate turbofan, ramjet, scramjet, and rocket engines together.

Why do we need hypersonic weapons? In August 1998, intelligence sources reported that Usama Bin Ladan was visiting an al Qaeda training camp near Khost, Afghanistan. This was shortly after al Qaeda terrorists bombed the U.S. embassies in Kenya and Tanzania. The U.S. Navy fired 66 Tomahawk cruise missiles at the training camp from the Arabian Sea, about 1,100 miles away. The speed of a Tomahawk missile is 550 MPH, so they took two hours of flight time to reach the target. Bin Laden left an hour before they hit. If those missiles had been hypersonic, at Mach 5 (3,836 MPH/1.065 MPS), they would have hit Bin Laden in 17 minutes. Another benefit is that enemy anti-aircraft defense systems cannot react fast enough to shoot-down missiles flying at hypersonic speeds.

Air Force General John Jumper said that we must be able to hit targets in 10 minutes or less on the battlefield. At Mach 5, that means our weapons must launch inside a 639 mile radius from the target. Air Force Lt General Robert Kehler (U.S. Strategic Command) said that once we go to war, we must hit enemy targets anywhere in the war zone in one hour or less. At Mach 5, that means our weapons must launch inside a 3,836 mile radius from the target. General Jumper was talking about tactical distances, and Lt General Kehler was talking about strategic distances. Either way, moving the speed of the missile to as fast as Mach 10 changes the kill radius to 1,279 miles and 7,673 miles respectively. We know how to hit targets accurately. We know how to hit targets at long distance. What we need to do at this point is hit targets in less time. When we can do that with hypersonic missiles, everything is tactical and the kill web covers the entire planet.

Would it make sense to design and build manned hypersonic fighter planes? No, not when you consider the heating problems, the structural instability, the propulsion issues, and how many Gs (gravitational forces) a human can survive. A trained pilot can pull about 8 to 9 Gs for about 8 to 10 seconds before passing out. This condition is called G-LOC (Gravitational Loss Of Consciousness). Hypersonic aircraft will be pulling 50 Gs or more from acceleration and when making tight-radius turns. That would kill the pilot in a few seconds. However in 1967, Air Force Col William Knight flew the X-15 experimental plane at Mach 6.7 (5,140 MPH/1.42 MPS) about 50 miles above the earth. He survived because he got to Mach 6.7 slowly, and didn’t make any sharp turns. That record still still stands as the fastest manned aircraft flight in history. What’s the fastest object ever observed in our galaxy? In early August, astronomers clocked star S5-HVS1 zooming across the Milky Way at 1,056 miles per second (MPS). That’s 3,801,600 MPH, or Mach 4.955.

If you didn’t suffer G-LOC while reading this article, you learned a little about how hypersonic vehicles work, the problems that must be solved, and what they do in the kill web. Now that you are acclimated to these extreme speeds and G forces, let’s move up to Mach 25, explore the newly commissioned Space Command (and the Space Force), and investigate how that fits into the kill web. Have no fear, because you are already flying through space at 66,627 MPH (18.5 MPS or Mach 87) with no ill effects. That’s how fast the earth is traveling around the sun.


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