Hypersonic travel for passengers

The “Jet Age” — this is often a reference to the time we live in. The phrase challenges us, especially specialists in the aviation community. Today when we speak of passenger aircraft, it is difficult to imagine an aircraft flying at speeds of ... 7 000-8 000 km/h! Currently, however, scientists from the European Union, Russia and Australia are working on this very task.

“The ability to quickly cover long distances is a natural human need in the 21^st century. For example, nowadays it takes more than 24 hours of ‘net flight’ from Moscow to Sydney. And, believe me, it is very difficult; I have travelled to Australia several times. It is rather tough on the body. So the idea to create an aircraft that would cover that distance in, say, three hours is very tempting,” explains Nina Voevodenko, head of sector of Aerodynamics Department of TsAGI.

Nina Vladimirovna is Leader of the Russian side work of the Work Package 6 — Simulation Support and Verification of the international project HEXAFLY-INT (High-speed EXperimental FLY Vehicles-INTernational). ESA-ESTEC (Netherlands) is the coordinator of HEXAFLY-INT. Together with TsAGI, in this work Russia is represented by Central Institute of Aviation Motors (CIAM) named after P.I. Baranov, Flight Research Institute (LII) named after M.M. Gromov and Department of Aeromechanics and Flight Engineering (DAFE) of Moscow Institute of Physics and Technology (MIPT).

The aim of the project, started in 2014, is to explore the concept of high-speed civilian aircraft. The aircraft is supposed to cruise at Mach numbers 7-8. “There are no such devices now,” emphasizes Nina Voevodenko. “I don’t mean missiles rather, passenger planes that can fly at Mach number 5 or even 3; they simply do not exist.” So the challenge facing the project participants seems to sound truly fantastic. Nevertheless, the acronym for HEXAFLY-INT is the result of work of a number of specialists from the Netherlands, Germany, France, Italy, Belgium, the UK, Russia and Australia. Together, they seek to create a flying machine of “high aerodynamic efficiency” (Lift-to-Drag ratio — L/D about of 6.5). But it is important to understand that the concept of “high aerodynamic efficiency” for a hypersonic aircraft differs considerably from the same parameter of a subsonic passenger aircraft in use today (for them the L/D is about 18) [editor’s note].

I wonder what the futuristic flying machine will look like. The lower surface of the aircraft has the configuration of a waverider. This layout has a triangular wing with a low-bluntness leading edge and negative transversal V-shape that helps reduce air resistance. A distinctive feature is the completely integrated air inlet. Together, these features augment lift and increase the Lift-to-Drag ratio at hypersonic speeds. Elevons at the wing tips are used to control the aircraft. Hydrogen is the preferred fuel. A natural question is what the expected dimensions will be. Nina Vladimirovna specifies: “If we are talking about the passenger version for 300PAX, it will be 90 meters in length and the total weight — 400 tons.”

There are a number of fundamental issues that remain open to the participants of HEXAFLY-INT. Primarily, creation of an engine to match the characteristics of the aircraft at its flight regimes. Perhaps, along with thermal protection, it is the main “Achilles heel.” Some parts of the aircraft can “warm up” up to 2,000°C at such speeds. Hence, it will be necessary to develop special thermotolerant materials for the aircraft skin. All this provides motivation to acquire new knowledge and technology.

TsAGI performs a significant amount in the project work in the field of aerodynamics: the production of models for experimental work, wind tunnel tests, calculations, etc. “HEXAFLY-INT proved once again the uniqueness of our experimental base. In particular, the T-116 wind tunnel, which provides the real experimental conditions for hypersonic regimes, allowed identifying the problematic locations of the layout being studied. It would be simply impossible to achieve such results at the facilities of other research centers involved in the project. Foreign partners rate high the quality of the experiment in our wind tunnel,”- explains Nina Vladimirovna.

Also, she stresses that the work through such cooperation is a source of replenishment of the TsAGI’s technology advance. “We are engaged in research in a field that is extremely important and interesting for us. This is an opportunity to explore the laminar-to-turbulent transition, to verify our methods, to compare with our colleagues’ data and the results of the experiment. This is a huge layer of material; its value for the advancement of science cannot be overestimated.” As an aside, the Institute’s creation of the model of high speed aircraft for wind tunnels tests was a part of TsAGI’s exposition at MAKS 2017 which sparked the interest of many visitors.

In 2019, the participants of HEXAFLY-INT are going to perform a flight experiment at a rocket range in Brazil. TsAGI’s task is to manufacture the model for this flight. It will be a test of a glider — an unpowered airplane. It will be 3 meters long and weigh about 400 kilos. We will need 10 tons of titanium to create such a flight model. “This is a critical part of the work,” says Nina Voevodenko. “The flight is planned to have a complex trajectory using a sounding rocket that will lift the model on a sub-orbital trajectory (maximum height is about 90 km). Next (after separation from the carrier), the model will accelerate due to the acquired potential energy to the speed range corresponding to Mach numbers 7-8, at an altitude of about 30 kilometers.”

Today, hypersonic passenger aviation is one of the important areas of scientific and engineering thinking. Many scientists around the world are working at conceptions of super-speed aircraft to move passengers quickly over long distances in a short time. But the HEXAFLY-INT project is unique as it joins the efforts of specialists from different countries. There has been no similar research cooperation on this subject. Ongoing exchange of experience and best practices is very important to create this, without exaggeration, fantastic aircraft.

It is difficult to imagine now what it feels like to be a passenger of a hypersonic airplane. “This is a far future project”, ponders Nina Voevodenko. "It has not been a long time between the first aircraft and the present jet airliners. It has been a path of tests, errors, and search. And in the early 20^th century aircraft seemed no more fantastic and unimaginable than the one being developed within the HEXAFLY-INT project. I think the hypersonic aircraft will come to fruition and eventually will take its rightful place in the world of passenger aviation. And it is important that promising materials and engines will be created in the course of this work. This all will give impetus to the development of various technologies and air transport as a whole.

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