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Aerospace engineering

Hypersonic wind

A tunnel to test airplanes that are much faster than the speed of sound

ieavA blast of air produces a plasma layer on the surface of the satellite modelieav

We do not even notice it, but when we are traveling in airplanes manufactured by Boeing, Airbus or Embraer, whose equipment is used by most of the world’s airlines, we are flying on average at 800 km per hour (km/h). This speed was only beaten by Concorde, the supersonic commercial aircraft that was withdrawn from service in 2003 after having flown since 1976. It flew faster than the speed of sound, at 2,170 km/h. Currently only military fighter aircraft fly supersonically, but the technological evolution of hybrid airplanes, which could fly both in and above the earth’s atmosphere, continues to fly at  high speeds. Current technological research is aiming at producing aircraft that are even faster, more economic, more comfortable and emit fewer pollutants. In Brazil, an important instrument for this type of research, a hypersonic wind-tunnel, is already being tested at the headquarters of the Institute of Advanced Studies (IEAv) of the Aerospace Technology General Command (CTA), which is linked to the Brazilian Air Force in the Sao Paulo State city of São José dos Campos.

Inaugurated in December, 2006, the wind-tunnel also has other functions, such as testing satellite capsules that will go through re-entry into the earth’s atmosphere when they are subjected to both high speeds and extremely high temperatures. Within the analysis chamber of the tunnel, called the T3, replicas of the micro-satellite capsule, Sara, (the acronym for atmospheric re-entry satellite in Portuguese), a reusable platform that is being studied by the Brazilian Space Agency, have already been installed.

The IEAv hypersonic wind-tunnel works with a continuous flow of air, as in tunnels used for testing airplanes, automobiles or building structures. This type of tunnel works with pulses. “The test is a pulse at an incredibly high speed burst of air that lasts for between 100 micro-seconds to 10 thousandths of a second”, explains mechanical engineer Paulo Toro, researcher at the IEAV’s Aero-thermodynamic and Hypersonic Division. Using a high speed movie camera capable of taking 2 million frames a second, one can see the precise moment at which the layer of plasma forms around the model of the satellite capsule as a result of the pulse of air and its high temperature (nearly 2,000°C). “This layer is called the shock-wave, which results from the hypersonic flow of atmospheric air as it interacts with the surface of the model in the section of the test tunnel”, says Toro.

The pulse is produced in a storage system that alternates high and low pressure mechanisms and releases the air at an extremely high speed over a prototype installed in the chamber of the test tunnel. The movement of air is hypersonic, because it travels at a minimum of five times the speed of sound, which is nearly 1,155 km/h at sea level. The maximum speed is close to 25,000 km/h, the equivalent of Mach 25, a measure used to identify the speed of aircraft in flight. In comparison, Concorde flew at Mach 2.

These extremely high speeds are associated with satellites and spacecraft when re-entering the earth’s atmosphere, and with future aircraft that will use different types of fuel to become viable. An example of the latter technology occurred in 2004, when Nasa, the American space agency, kept a prototype of an aircraft flying at Mach 10 (around 11,500 km/h) in the air for 10 seconds. The propulsion system of this aircraft, called the X-43, works in a different way from traditional jet turbines, in which the air is pulled into the engine and turns the blades that throw the air into a chamber into which fuel is injected, producing combustion and the consequent exhaustion of hot air at the back of the equipment, thus thrusting the aircraft forward. In the scramjet (supersonic combustion ramjet) system, used by the X-43 that was launched from another aircraft, the idea is to have no moving parts such as fan blades.

In the scramjet the air is compressed by the vehicles own geometry and speed and it is sent to a chamber underneath the aircraft, which also contains pulverized hydrogen gas, which cause combustion and propels the aircraft. This system of combustion should be tested in the T3 this year, in preliminary tests using a model developed in Brazil by the IEAv. “It will be called the 14-X as a tribute to the 14-Bis of Santos- Dumont”, says Lt Colonel Marco Antônio Sala Minucci, one of those responsible for developing the T3 and the previous hypersonic wind-tunnels, T2 and T1, that were smaller.

The 14-X will be nearly 1.5 meters long and 80 centimeters wide. The forecast is that it will be launched from a Brazilian rocket in 2010. Another study possibility that will start this year is laser-assisted supersonic combustion. “With the laser we shall also be able to test propulsion for spacecraft in the T3 and, in the future, possible nano-satellites. We have entered into an agreement with the US Airforce Research Laboratory (AFRL) that is going to provide us with two sources of laser radiation to enable us  to develop our research together as partners”, says Sala. All experiments that involve lasers in the combustion and propulsion phases are still in their early stages, even in the United States. Even if they prove feasible, they will not be commercially available for between 20 to 50 years. Therefore, the T3 will be fundamental for these experiments. The equipment was fully developed by IEAv and received funding from FAPESP, while four metallurgical and boiler-making industries from the State of São Paulo and from the South of Brazil were involved in its manufacture.

The Project
Preliminary experimental investigation into supersonic combustion
Normal Line of Research Help
Paulo Toro – IEAv-CTA
R$ 1,755,353.81 and US$ 235,000.00 (FAPESP)