The future of hypersonic flight

What if you could fly from Mumbai to New York in just 2.5 hours? If manned hypersonic flights were a reality, you wouldn’t have to spend about 18 long hours in a cramped aeroplane! Research works led by Professor Shripad P Mahulikar from the Aerospace Department, Indian Institute of Technology Bombay (IITB) is contributing to setting up a foundation to make hypersonic aeroplane design a reality. The researchers have proposed changes to the aircraft geometry to address the problem of heating of the body at extremely high speeds.

Hypersonic aeroplanes, meaning aeroplanes that can fly several times faster than sound, are the next big thing in space and military aerospace research across the globe. For example, SpaceX has announced its plans for building hypersonic reusable launch vehicles and China has successfully tested hypersonic missiles. “Reusable Hypersonic Vehicles (RHVs) are promising low-cost candidates for future space missions,” remarks Shripad, “The future of space missions will be based on the cost reduction for a kilogram of payload launched.” He adds that this can be achieved by the Single-Stage-to-Orbit RHVs. These vehicles are ‘fully-reusable’ as they reach the intended orbit without expending their hardware.

“The future of military missions will also get a boost with hypersonic attack vehicles that can achieve their offensive mission with an unprecedented surprise element due to the high speed,” says Shripad. The RHVs fly at enormous speeds (more than 6,000 kmph) which is greater than five times (known as Mach 5) the speed of sound and at a typical cruise altitude exceeding 35 km! In comparison, currently operational long-distance commercial aeroplanes such as the Boeing-747 are subsonic, and cruise at about Mach 0.8 (less than 1,000 kmph) at a cruise altitude of about 11 km.

Overcoming obstacles

However, there are unprecedented challenges in designing such RHVs due to the high speeds. To achieve hypersonic speeds, these planes need specialised engines, efficient management of the heating due to the air resistance, and changes to the configuration design of the aircraft. Shripad’s basic research findings provide clues and insights into cracking some of these challenges.

The air flow over an aircraft creates an aerodynamic drag force which resists the aircraft’s forward motion. Aircraft wings are designed to minimise the air-drag which reduces the fuel consumed. Aircraft designers modify the aircraft geometry to achieve this with insignificant repercussions to its weight. The aerodynamic drag also heats the aircraft’s body and is known as aerodynamic heating.

At hypersonic speeds, the aerodynamic heating can increase the vehicle temperatures to higher than 1,600°C. The designer’s focus then shifts from designing just for aerodynamic drag reduction to designing for managing the harsh aerothermal environment. Studying and understanding the aerothermal environment for RHVs enables designing a reliable Thermal Protection System (TPS). At speeds greater than sound, a different drag comes into play and designers address this by incorporating a ‘sweepback’ to the wings, that is, a wing that angles backward from where it is joined to the aircraft’s body.

The angle at which the wing is ‘sweptback’ varies from 0° (no sweep) for straight-winged low-speed aircraft to about 45° and beyond for supersonic aircraft like the fighter jets. The sweepback angle is designed to minimise aerodynamic drag and its value for minimum drag is the ‘drag-minimised sweepback’. Shripad proposed that due to aerothermal considerations, the sweepback angle of the lifting-body of hypersonic aircraft should be more than the drag-minimised value. He also proposed a modification to the geometry of the leading edge.

Modifying the designs

In a theoretical study published in the journal Aerospace Science & Technology in 2005, Shripad proposed a configuration design modification to the hypersonic aircraft’s lifting body. Based on mathematical derivations and through numerical simulations, he showed that due to aerothermal considerations, the sweepback angle of the lifting-body of hypersonic aircraft should be more than the drag-minimised value. For RHV cruising at a Mach number of 7 at an altitude of 35 km, this sweepback angle was calculated to be 79° to 80°. Further, the incremental aerodynamic drag for this newly proposed sweepback angle was found to be insignificant. The geometry of hypersonic aircrafts dramatically differs from commonly seen 
aircrafts. The lifting-body of RHV that provides the lift is shaped like a surf-board to reduce the wave drag.

In studies published in 2017, in the journals, Acta Astronautica and Journal of Aerospace Engineering, Shripad and his team computationally validated the earlier proposed theoretical concepts. They studied the aerothermal characteristics of the nose cap of RHV and Swept-Back Leading Edges (SBLEs) of the lifting body. Researchers found that the bi-curvature nose cap heats up lesser than an axisymmetric one.

The numerical simulations were run at a cruise Mach number of 7 and flight altitude of 35 km for different sweepback angles. They indicated that the surface temperature of RHV was high at 1335°C for a design with a sweepback angle of 40° and decreased to 914°C until a sweepback angle of about 79°. Increase in the sweepback angle beyond 79° was found to increase the surface temperatures. The sweepback at which the surface temperatures are the least is the ‘heat-transfer-minimised-sweepback’, which differs in concept and value from the ‘drag-minimised-sweepback’ of about 73°.

This ‘drag-minimised sweepback angle’ has so far been the consideration for the aerodynamic configuration design of RHVs. “The temperatures of the SBLE surface, as well as the upper and lower surfaces of the lifting-body, are significantly lower for the ‘heat-transfer-minimised-sweepback’ than for the ‘drag-minimised sweepback’,” says Shripad. These results have immediate practical implications in the selection of lightweight TPS-materials for RHV.

Researchers also studied how varying the radius of leading edge of RHVs lifting body can also reduce the temperatures. Conventionally, the leading edge is blunted to reduce the in-flight temperatures. Researchers computationally observed that for 80° sweepback, the leading edge with a smaller radius is at a lower temperature. For sweepback angles greater than about 60°, sharpening the leading edge reduces the temperatures along the SBLE surface.

These new findings have important implications for the configuration design of RHVs. But how far are we from actually realising a hypersonic flight? Quite far, perhaps a few decades, feel the researchers. Also, to have a full-fledged operational hypersonic vehicle, it is necessary that bulk manufacturing facilities are in place. Until then, we might still have to cramp ourselves on long-distance flights, hoping to soon fly at hypersonic speeds!

(The author is with Gubbi Labs, a Bengaluru-based research collective)