“I feel the need, the need for speed!”

So, what does hypersonic actually mean? Aside from the nuances around what the speed of sound is, simply put, hypersonic is the term used when discussing speeds of, or greater than five times the speed of sound.

However, the huge engineering challenges for designing aircraft at even half that speed are not so simple. Travelling at high supersonic speeds requires hugely powerful engines, a lot of fuel and a structure that can withstand massive temperature changes during flight. 

The high temperatures experienced during supersonic flight famously caused Concordes fuselage to grow around 300mm, with its max speed limited to Mach 2.02 due to the 127 degree Celsius limitation of some of the materials. Hypersonic air vehicles are expected to see temperatures due to friction as high as 1000 degrees Celsius when operating at low altitude. 

So, with numerous complex challenges, why all the interest in going so fast? Again, the answer is actually quite simple:

Imagine a bowler in cricket has to deliver a ball from point A to a batter at point B. The faster the ball is moving, the less time the batter has to react and therefore the harder it is to hit. Translated into aerospace applications, this makes a hypersonic aircraft or missile very difficult to shoot down.

If everything is stealth, what is the worry?

Stealth is an often-misunderstood term when it comes to combat aircraft. There is no single definition or feature that makes something “stealth”. It’s more a collection of technologies and tactics that when combined, attempt to stop an aircraft being detected by radar. 

Radar works by shooting electromagnetic energy at a target and measuring the waves which bounce back. Reducing the amount of energy that bounces off an aircraft makes it harder to detect by radar. What is often overlooked however is range.

The misconception is that stealth makes aircraft like the F-22 and F-35 invisible to radar at any range, this is understandable given how effective they are in reducing the energy that they return to the source, but they still return some energy. In the past, that small amount of energy may have been indistinguishable from the background noise, but if you got too close you would still be detected. 

Over the last decade radar systems have improved vastly, increasing the range at which even stealth aircraft can be detected. These improvements are chipping away at the advantage those platforms hold and there are many that are worried that soon stealth alone will not be enough to keep an aircraft safe.

The SR-72 (pictured to the left) in ‘Top Gun: Maverick’ exists only in Hollywood, but it’s co-creators, Lockheed Martin’s Skunk Works published a near identical concept in 2016, and developed the iconic SR-71 blackbird in the 1960s.

The SR-71 is a very important example in understanding the current interest in hypersonics. Over its service life the SR-71 Blackbird reportedly outran over 4,000 missiles fired at it by enemy air defence systems, without losing a single aircraft in combat.

The modern hypersonic train of thought then follows that targeting and hitting a highly manoeuvrable aircraft, travelling at hypersonic speed (above Mach 5) and at high altitude, would be almost impossible to track and hit, even with the most sophisticated air systems in service today and in the near future.

What would a hypersonic aircraft actually do?

No SR-71 was ever shot down, however 12 of the 32 made were lost in non-combat accidents. The aircraft was hugely expensive to build and maintain and came from an era before the wide use of satellites for reconnaissance. In its heyday it provided invaluable intelligence, but it was ultimately replaced by drones and satellites that were cheaper and offered more time over the target. So, the question must be asked, what would a hypersonic aircraft actually provide?

While the US, Russia and China are all at various stages of developing hypersonic missiles, the UK announced development of a hypersonic unmanned air vehicle that could deliver payloads at great distance and then return to be used again.

Ed Gower, who leads the HVX programme at Reaction Engines told the BBC:

“…what we’re doing here is … not a missile; it’s something that is able to return sub-sonically. Because of that it will need to operate at high speed and low speed, which is obviously what distinguishes it from something like a ramjet on a missile…”

Image: Concept art of the HVX “Concept V” Hypersonic Air Vehicle

Speed will improve reaction times for roles like air defence interceptors or ground attack close air support. However the aerodynamic realities required to achieve hypersonic speeds would severely limit the effectiveness of the aircraft in its intended role once it reached its target. The survivability that high speed would give you could be really valuable in a “Wild Weasel” suppression of enemy air defence role, but would a hypersonic aircraft be better than a swarm of cheap, attritable drones?

Air Vice-Marshal Linc Taylor, the RAF’s Chief of Staff Air Capability recently shared:

“…the UK has some novel technologies that could allow it do things differently to those seen elsewhere in the world, and at much lower cost… One of the things we believe will have value in our future way of war-fighting, changing the way we fight, is in reusable hypersonics… At the moment we’re exploring the technologies. If they do come to fruition, we will then go, ‘Okay, this does have utility, we have the evidence behind it’, and then we’ll develop it… One of the things we’re looking at is how can we do this for a fraction of the cost that our adversaries might spend in this area.”

For the UK, cost is key, but as with other areas it really seems like the MoD, DSTL and the RAF RCO have a new approach to technology development: all of which means an exciting time for the UK aerospace sector.

Written by: Principal Engineer, Chris Summers