Flying into the future of speed
20 years after Concorde’s demise, is a return to commercial supersonic air travel an unsustainable dream? And what will hypersonic technology mean for global defence and security? In episode seven of Zero Pressure's second season, host Dr Helen Sharman seeks answers to these and other questions from Conrad Banks, Chief Engineer at Rolls-Royce.
Commercial supersonic flights; ramjets and scramjets; cooled gas turbines; air-breathing hypersonic missiles; synthetic and sustainable aviation fuels: the future of high-speed air technology is filled with thrilling possibilities, but it also poses challenges in terms of sustainability, usability, and security.
So finds episode seven, series two of the Zero Pressure podcast, presented by Imperial College London and Saab, as host Helen Sharman discusses ‘Flying into the future of speed’ with Conrad Banks, the Chief Engineer at Rolls-Royce.
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Going supersonic again?
Helen and Conrad begin by looking at whether we will again see supersonic commercial flights. The demise of Concorde in 2003, following the deadly air crash in Paris in 2000, ended the dream of supersonic travel due to cost efficiency and safety.
“The industry lost confidence,” says Conrad Banks, but adds: “There’s a real demand for it again now.” While there are huge benefits in terms of the convenience of long-distance travel, Conrad says “We cannot do it in such a way that we risk the sustainability and environmental concerns of net zero emissions.”
Because, he explains, “The higher and faster you fly, the more fuel you burn per mile. It’s far more efficient to fly people in an A350 than to fly them in a new supersonic aircraft when they are developed.” And, while the likes of Rolls-Royce are working on electric propulsion, hybrid-electric propulsion, and hydrogen-fuelled applications, none are suitable for long-distance, high-speed flights because of the energy and power density that are needed.
According to Conrad, the answer is SAFs: synthetic and sustainable aviation fuels. “For commercial high-speed travel we have to look at conventional air-breathing with synthetic aviation fuel,” he says.
Nonetheless, Conrad estimates that the days of three-to-four-hour trips across the Atlantic are perhaps just a decade away, with planes flying at Mach 1.5 speed, using normal gas turbines running on SAFs. Aeroplane manufacturers are working on that now.
How can we fly even faster?
Aerospace industry horizons stretch much further than Mach 1.5. Transatlantic flights at Mach 4 or 5 that take just 90 minutes are also an ambition. However, that depends on the development of a smooth and safe way to transition to ‘ramjet’ propulsion, which uses the ram intake to compress air and increase pressure, which is then mixed with fuel for the reaction to propel the vehicle forward.
“When you go above Mach 2 to 2.5, you’re at the gas turbine limit, but ramjet is only possible at Mach 3. There’s a window in between where it’s too hot to get enough thrust,” says Conrad.
The obvious way, rocket-based propulsion, is both too expensive and environmentally unsustainable. Instead, the answer lies with using conventional aero gas turbines for the transition to supersonic speeds.
The alternatives here are an advanced gas turbine or an affordable gas turbine that’s been pre-cooled. Conrad says it’s the latter that’s most promising, involving the rapid cooling of the air into the gas turbine through heat exchange that uses thousands of small tubes filled with coolant.
“The turbine goes from 1000 degrees Celsius to 0 degrees in the blink of an eye,” he says.
What hypersonic technology means for defence and security.
When something goes through the air at a speed above Mach 5, it becomes hypersonic, and is known as a ‘scramjet’ (a supersonic combustion ramjet). There are already hypersonic missiles travelling through the air at incredible speeds, and which are controllable and directable mid-flight in a way that far outdoes intercontinental ballistic missiles (ICBMs).
This has become a key area of concern of the defence and security industries, as Helen hears from Dr Kelly Stephani, Associate Professor of the Department of Mechanical Science and Engineering at the University of Illinois Urbana-Champaign.
“A platform capable of travelling at such high speeds challenges the way we think about responding to those systems in terms of defence. It also changes how we think about leveraging the capability -the response time required to defend against these systems. If they’re operating outside the typical performance envelope, then shorter response times are needed,” says Dr Stephani.
It’s hugely challenging. No wonder that Dr Stephani was keen to encourage young scientists and engineers to get involved in this research area.
Conrad Banks says the prospect of a projectile or vehicle travelling at Mach 10 is less than a decade away; the Russians already claim to have a missile that reaches Mach 9.7.
A whole host of developing technologies is already being developed to counteract the threats. It’s not just high speed, then, but the speed of response to it that’s vital, too.
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