Heading supersonic, aiming hypersonic

Imagine getting on a flight from Dhaka at 7am and landing all the way on the other side of the globe in New York before the hour's end. Seems impossible, especially when you know for a fact that it is going to take almost an hour or so to get to the airport from Gulshan during rush hour. 

When a 10-kilometre commute takes this long on an average day, it is justified to be sceptical about a 12,000-plus kilometre air commute that supposedly takes less than an hour. But this seemingly impossible commute is possible with hypersonic flights. A recent groundbreaking 19,000 kilometre per hour engine promises to cut such a 20-hour flight down to less than an hour.

The Beijing Power Machinery Institute has introduced a dual-mode engine capable of reaching 19,000 kmph. Published in the Chinese Journal of Propulsion Technology, this engine provides efficient propulsion at various speeds and could potentially reduce a 20-hour New York to Sydney flight to just 50 minutes.

But what exactly do hypersonic flights or hypersonic travel mean in aviation? The term hypersonic refers to the speed over five times the speed of sound, alternatively, Mach 5. The speed of sound is typically 1,235 kilometres per hour, also known as Mach 1. Any speed over Mach 1 or 1,235 kilometres per hour is called supersonic, and over Mach 5 or 6,175 kilometres per hour is known as hypersonic. So, flights that can cross the Mach 5 threshold are known as hypersonic flights. 

The engine designed by Beijing Power Machinery Institute reaches Mach 5 speed and surpasses that by a lot, reaching almost Mach 16. As for efficiency, unlike traditional engines that only achieve 20-30%, this hypersonic engine can convert up to 80% of the fuel into kinetic energy.

As of now, the average speed of big commercial flights is subsonic — around 800 kilometres per hour, slightly below Mach 1. But we did achieve above Mach 1 or supersonic speed and carried out supersonic flights almost five decades ago in 1976 with the Concorde. The legendary Concorde was capable of cruising at Mach 2 speed, cutting down the New York to London flight time to around three hours, which revolutionised transatlantic travel back in the day. But with supersonic speed comes a supersonic boom.

A supersonic boom, also known as a sonic boom is a loud explosive noise caused by an aircraft when it crosses the threshold of Mach 1. On top of the boom, Concorde was not as efficient and impacted the environment horribly, which led to its retirement in 2003.

Considering the operating cost and the environmental impact of supersonic flights, commercial aviation pretty much looked the other way after Concorde's dismissal. However, military applications of supersonic travel and even beyond remained a priority, and as a result, the US military's X-51 Waverider was born. 

Waverider can sustain flight speed at Mach 5 for more than 200 seconds straight, proving hypersonic flights are a real possibility. Even though most supersonic and hypersonic travel was experimental and primarily being worked on by the military, private investment and technological advancement have also brought commercial hypersonic travel into the foreground in recent years.

US-based Venus Aerospace is developing 'Stargazer', a Mach 9 capable aircraft. The feasibility of such an engine has already been proven in trials by the US Navy in Spaceport Houston. 

'Quarterhorse', a hypersonic aircraft capable of Mach 5 speed, is also being developed by another US-based company, Hermeus, that can seamlessly transition from turbojet for taking off to scramjet for sustaining hypersonic cruising speed.

However, the Chinese government's commitment to the advancement of hypersonic travel and willingness to invest in both military and civilian applications has resulted in the biggest leap in hypersonic aviation in the form of the aforementioned Mach 16, a dual-mode engine.

These advancements may seem like we are approaching hypersonic travel, but in reality, we are quite far from boarding commercial hypersonic flights. The biggest challenges in making hypersonic travel a reality are aerodynamic heating, lack of refined propulsion systems, and aviation regulatory problems.

An aircraft flying at hypersonic speed would generate temperatures around 2000 degrees Celsius, and there are not a lot of things that can be used to build aircraft that can also withstand that high of a temperature. 

Researchers are still working on different variations of advanced ceramics, heat-resistant compositions, and metal alloys that can withstand such conditions and not compromise with the structural integrity of the aircraft. 

The propulsion systems for hypersonic travel need further development to ensure efficiency, power, and safety. Scramjet engines can sustain hypersonic speeds but require supersonic airflow or assistance to accelerate from a standstill. 

While companies like Hermeus and Venus Aerospace are working on hybrid engines that transition between turbojet and scramjet, these technologies have not yet been validated for safe commercial air travel.

A significant concern is the noise generated by supersonic aircraft. While initiatives such as NASA's X-59 Quiet Supersonic Technology (QueSST) aim to address this by reducing the impact of a sonic boom to a softer "sonic thump," current aviation regulations are not yet prepared to handle the noise pollution caused by sonic booms from hypersonic engines.

Although we are not quite there yet for commercial hypersonic flights, we are definitely making excellent progress with supersonic travel. In fact, we can expect to see supersonic flights into commercial aviation as soon as a couple of years. 

Boom Supersonic, a US-based supersonic-focused company, is set to test its Mach 1.7-capable 'Overture' aircraft in 2026, which will use 100% sustainable aviation fuel to address the environmental impact. Airlines like American Airlines and United Airlines have already placed orders for these engines in anticipation that they can introduce a more sustainable form of supersonic air travel by 2029.

Although we are two-three decades shy of hypersonic travel, we are moving towards it at an excellent speed. But realistically, our next onboarding is with commercial supersonic flights, which could very possibly be in this decade.