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Hypersonic Aircraft: Breaking Mach-5 With Next-Generation Jets

For most of history, the only place that humans could travel at hyper-speed was in science fiction. Whether you prefer Star Trek or Star Wars, countless people have looked on in envy as their favorite characters blast off with unimaginable velocity, crossing the galaxy in a matter of seconds. But while humans may never reach the same speeds as Han Solo, hypersonic air travel may not be as far away as you might think.

On October 14th, 1947, humankind broke the sound barrier for the first time. The Bell X-1, flown by Major Charles E. Yeager of the US Air Force, reached supersonic speed in the skies above the United States. Yet, even before Yeager’s flight, engineers worldwide, urged by their governments, were looking into more ambitious velocities.

Hypersonic flight is achieved at Mach 5. That’s 6,170 km/h, or about 3,840 mph. Traveling at such speeds presents all sorts of difficult problems of physics, thermodynamics, and aerodynamics. Engineers have mostly cracked the code for unmanned spaceplanes or missiles to reach hypersonic speeds. But, reaching that speed in a crewed aircraft at lower altitudes is an entirely different beast of a challenge.

Today, hypersonic weaponry is on the brink of becoming mainstream, but hypersonic passenger planes have seemingly been just beyond the reach of humankind for the last half a century. In recent years, it’s begun to seem closer than ever. Several manufacturers have claimed that, by 2030 or 2040, we’ll be able to fly across the Pacific in just three hours. Today, we’re going to look at a few aircraft to successfully fly at hypersonic speeds and one or two that might do so in the future.

First Attempts

Before America broke the sound barrier, the Nazis and Soviets were already looking into hypersonic aircraft. In all likelihood, engineers had dreamt of such speeds since the earliest planes carried humans across wheatfields. But, perhaps the first detailed plans for such a vehicle came out of the Third Riech. In the 1930s, two German engineers named Eugen Sänger and Irene Bredt began work on the Silbervogel, or Silver Bird, a plane whose ambitions far outstrip current physics and material science. 

The Nazi Regime was interested in a bomber that could strike America, taking off from continental Europe, crossing the Atlantic quickly, delivering the weapon, and then continuing on to Japanese positions, all in a single day. Sanger and Bredt’s conception incorporated cutting-edge rocket technology with the principle of the lifting body to reach incredible speeds. In some ways, their idea was sound. One of the main problems with hypersonic flight is that an engine or aircraft that can reach supersonic speeds isn’t ideal for reaching hypersonic speeds. Hypersonic planes need one type of propulsion to get them to supersonic and another for hypersonic. Today, the most common propulsion is a rocket. In that way, Sanger and Brendt showed a sound understanding of the system used for later hypersonic projects like the X-20 Dyna-Soar and the Space Shuttle of the 1970s. 

Sanger’s 900-page report on the plan detailed a process of launching the plane from a 3-kilometer sled, ascending to over 140 kilometers, and reaching speeds of up to 21,800 km/h. The idea was considered too complex to pursue, and the report was too long to read, so the design never reached the mock-up phase.

After the fall of Nazi Germany, Stalin got his hands on the report. Soviet engineers began pursuing the design, calling it the Keldysh bomber. This aircraft carried many of the same principles as the Silver Bird, like the rocket-powered sled launch. But, the astronomical fuel costs rendered the project impossible. The Soviet Union would build some of the fastest supersonic aircraft of all time, but their work on hypersonic planes came to an end. 

Around this time, America first broke the sound barrier in a crewed aircraft, but they were pursuing much faster speeds. In 1949, the RTV-G-4 Bumper became the first manufactured object to go hypersonic. The rocket reached Mach 6.7, about 8,300 km/h. But that vehicle was unmanned. The first human to reach this speed was Yuri Gagarin, the first man in space. Gagarin went hypersonic during a piloted orbital flight. In May of 1961, Alan Shepherd eclipsed Mach 5 while his space capsule re-entered the atmosphere. But, the Americans sought that speed in an aircraft instead of a spacecraft falling from the thermosphere. In doing so, they would have to overcome some incredible challenges.

For instance, the heat generated by the friction between air and a high-speed plane is enough to combust most normal aircraft. This was even the case with the first hypersonic object, the Bumper, which burst into flames on atmospheric re-entry. Building usable hypersonic aircraft would require materials that could withstand up to 1,000 degrees celsius. One way to combat this issue is to fly at great altitudes, often exceeding 100 km above the earth’s surface. At this height, the laws of aerodynamics differ dramatically from those at 10-15 km. Despite all of these challenges, by late 1961, the Americans achieved the first crewed hypersonic flight.

The X-15

The X-15 was an experimental aircraft operated by NASA and the US Air Force. Designed in the late ’50s, the rocket-powered X-15 had a long, cylindrical body and prominent dorsal and ventral fins. The aircraft was built to be carried by a B-52 Stratofortress, then drop launched. Typically, the X-15 would be lifted to about 14 km and speeds of 800 km/h before it was released from its mothership. Other altitudes of launches were tested. Once dropped, the plane would activate its liquid-propellant rocket engine.

An early version of the plane used the XLR-11 engine, which could produce 16,000 pounds-force. That’s compared to the 6,000 pounds-force produced by the engine in the first supersonic plane. By 1960, though, Reaction Motors built the XLR-99, which generated 57,000 pounds-force of thrust. Now, instead of two XLR-11s, the X-15 was outfitted with a single XLR-99. Without this main engine running, the X-15 was incapable of maintaining altitude.

Aside from its advanced engine, the X-15 had a handful of other interesting features. For instance, given the incredible altitude of flight, there wasn’t enough air to use flight control surfaces, like a rudder, for steering. Instead, the plane was outfitted with a reaction control system using rocket thrusters to adjust direction. Most of the plane’s surface was covered with nickel alloy, a heat-resistant material, to sustain the temperatures at such high speeds. The plane even came with an ejection seat, though it only worked below Mach 4 and altitudes below 37 km.

On November 9th, 1961, the X-15 became the first human-crewed aircraft to break the hypersonic barrier. It reached 6,585 km/h, higher than Mach 6, at an altitude of 31 km. In July of 1963, the pilot Joe Walker brought the X-15 more than 100 km above the earth’s surface. Eight pilots flew 13 flights more than 80 km above the world, each one earning astronauts wings for their efforts. In 1967, William Knight flew the fastest flight, reaching Mach 6.7, or 7,270 km/h. Since then, no crewed, powered aircraft has ever exceeded this speed. 

Still, despite the impressive velocity, the plane could only maintain such extremes for short periods. On one occasion, the aircraft entered a hypersonic spin while descending. The airframe shattered at 18 km above the earth, killing the pilot and scattering wreckage across 130 square kilometers. Despite the crash, the X-15 is, to this day, the most successful crewed hypersonic plane.

Other Hypersonic Aircraft

In the 60 years since the X-15, the only vehicles to break its speed record have been unmanned. In fact, there have been too many crewless spaceplanes to discuss in this video. Most hypersonic aircraft have been built exclusively for military or scientific purposes, and only a select few qualify as airplanes instead of spaceplanes. For our purposes, we’re considering those that reached hypersonic beneath the Karman line—100 km above the earth—marking the outer limit of where space begins.

One example is the NASA X-43. Today, the X-43 is the fastest aircraft ever recorded at about Mach 9.6, or almost 12,000 km/h. The X-43 was, of course, unmanned. Like its ancestor, the X-15, the X-43 was dropped from a B-52 during its record-breaking flight in 2004. But, the X-43’s story is quite different from its predecessor. Three planes were built, and each one was flown once. The first test of the X-43A took place in 2001. After 10 seconds on the initial flight, the booster lost control, causing the plane to veer off path. It was destroyed by the Air Force immediately to limit collateral damage. The next test wasn’t until March of ’04, and the third was in November of the same year. The final test showed some promising results. Not only did the plane reach incredible speeds, but it did so at 33.5 kilometers. While flying at higher altitudes makes high velocities easier, future commercial uses will likely operate around 30 km. 

The most recent success in NASA’s X-series was the Boeing X-51 Waverider. The Waverider relies on a different type of propulsion than the X-15 or 43. It’s based on a scramjet engine. Scramjets are supersonic-combustion ramjets, an air-breathing engine in which combustion takes place in supersonic airflow. Today, scramjets are the favorite propulsion system for most hypersonic aircraft. In 2010, the X-51 completed its first successful hypersonic flight reaching Mach 5 at just 21,000 meters.

Most importantly, in 2013, the X-51 achieved the longest ever powered hypersonic flight, sustaining flight for more than six minutes, including 210 seconds above Mach 5. Like its predecessors, the X-51 is drop launched from 15 km by a B-52. While the X-51 has shown the unique ability to use its engines for an extended time, the next step will be managing a launch without the help of a B-52. This is where the scramjet may not suffice. As we mentioned, the engine requires supersonic airflow to function, possibly requiring a second propulsion system.

Of course, the United States isn’t the only country that’s integrated hypersonic aircraft into its arsenal. In fact, Russia and China are rumored to have more advanced high-speed technology. However, these countries utilize hypersonic rockets for ICBMs and other types of projectiles, many of which leave the earth’s atmosphere after launch. The most widely publicized of these secretive weapons is the Avangard, a kind of hypersonic glide vehicle that can deliver nuclear weapons. While the Avangard holds no future promise as a passenger vehicle, it’s supposedly capable of speeds over Mach 20. Whether the Russians have ever reached such velocity is unknown. China is said to be right alongside Russia in producing some of the highest speed vehicles in the world, though both countries seem to be unconcerned with any sort of crewed variations. If the world sees hypersonic planes carrying humans, it will most likely come from the United States.

What does the future hold?

Since the first hypersonic flight in 1961, aircraft manufacturers have claimed that hypersonic travel is just around the corner. Despite decades of advancements, it seems we still haven’t rounded that corner. However, recent developments in the industry have reinforced the belief that the day is fast approaching.

Most of these developments have been in the military industry. Lockheed-Martin has claimed for some time that they’re on the verge of early test flights with the SR-72. While the Son of Blackbird, which we’ve already done a video about, is an unmanned vehicle, it’s an excellent example of the recent advances that make hypersonic aircraft more likely than ever. According to the company’s vice president, the SR-72 would have been impossible without 3-D printing, which allows the company to embed a cooling system inside the engine. Advanced computer modeling has vastly reduced the price of testing possible designs, as have military-grade wind tunnels. 

As for more accessible examples, the company at the forefront is Boeing. In 2018, the company released renderings of crewed aircraft for military and commercial purposes. The models show a plane with a sleek, dramatic delta wing, a compact fuselage, a sharp nose, and two rear fins. 

Along with these designs, Boeing laid out some of the development plans. These plans show some crucial characteristics of hypersonic air travel that will likely permeate throughout other companies’ attempts to introduce hypersonic. The plane will be built to max out right around Mach 5. That’s because, at Mach 5, the temperatures that the jet will experience are significantly lower than those at Mach 6, 7, or 8. As such, Boeing’s plane can be made entirely of titanium instead of more expensive and labor-intensive materials. Boeing’s plans also reveal a different form of propulsion. Rather than just a scramjet engine, the plane would include a jet and a ramjet. The jet engine would get the aircraft to Mach 2 or 3, while the ramjet takes the plane the rest of the way.

As for dimensions, Boeing stated that they haven’t quite decided what size it will be, though they expect it to be smaller than a 737. The plane would fly at about 29 km, reducing turbulence and allowing the engines to operate efficiently. 

The experience for passengers would be considerably different from the typical airliner or even supersonic passenger jets like the Concorde. Not only could Boeing’s plane hypothetically cross the Atlantic in 2 hours, but the G-force at take-off would continue for 12 minutes. As you may know, on current airplanes, it typically lasts a couple seconds. However, thanks to the altitude, the cruising flight would be incomparably smooth. Window seats would go for a premium, as passengers would be able to see the earth’s curvature out their windows. The aircraft likely wouldn’t be particularly fuel-efficient, but with such incredible speeds and little air resistance, physicists believe that a hypersonic plane could coast for the final 800 km of a flight without using the engines.

Companies, including Boeing, are reluctant to provide concrete estimates as to when this type of plane could be ready. And understandably so. It’ll undoubtedly take decades to reach a point of widespread commercial use. Aggressive estimates believe that the earliest prototypes should be flying in the next 5 to 10 years. Perhaps, like Luke Skywalker and Han Solo, we will be able to fly at hyperspeed at some point during our lifetimes.

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