MODANE, France — In a narrow Alpine valley, a stone’s throw from the border with Italy, sprawls a complex of wind tunnels owned by ONERA (Office National d’Etudes et de Recherches Aérospatiales), the French Aerospace Lab. The star here is undoubtedly the “Wind Cathedral,” aka S1MA, the world’s biggest supersonic wind tunnel that stretches more than 1,300 feet and has a max diameter of 79 feet — big enough to stand a tennis court up on its end.
ONERA claims it is also the “greenest,” given that its two giant fans are powered with water from two reservoir lakes in the mountains above.
The Modane-Avrieux complex has just celebrated its 70th anniversary so it’s hard to believe that these vast structures, construction of which started in 1942 by German and Austrian engineers in the Austrian Tyrol before they were dismantled and brought here by train as war booty, are still today the best way of simulating how air passes around an object flying at speeds of up to Mach 12.
Despite advances in digital design and development, decades-old wind tunnels like these are experiencing something of a renaissance in defense tech research as, for example, nations around the world race to develop next-generation warplanes and hypersonic weapons. In 2021, the Government Accountability Office warned that America’s wind tunnel infrastructure “is aging and may be unable to meet demand” for airframe testing.
“There’s only so much you can do digitally. At some point you need to have a model and see how it behaves in real wind with the right pressure and the correct Reynolds numbers,” Pascal Crozier, director of the site explained last week to visiting journalists. Reynolds numbers refer to key aerodynamic metrics, as explained in slightly more detail by NASA.
Likewise, Bruno Sainjon, director general of ONERA, remarked that in the United States the aerospace industry has come back to wind tunnels after having dismissed them as old-fashioned “because there are certain effects of physics which cannot be modeled digitally. You cannot [with computers] achieve the same level of precision,” he said.
The four wind tunnels on the site, together with the two at Le Fauga-Mauzac in southwest France and the aeroacoustic one in Saclay, southwest of Paris, enable ONERA to offer test services for flying objects that range from military to commercial aircraft, missiles or space-bound platforms. Here, the testing area for each wind tunnel is actually just one section of what amounts to a large rectangular track system for the air to flow through in loops at varying speeds.
In Modane the S1MA and its sister tunnel S2MA are continuous-flow wind tunnels, the first being atmospheric that allows tests for speeds between Mach .05 and Mach 1. The second providing variable pressure to test from speeds of Mach 0.1 to Mach 3.
The S3MA is what’s known as a blowdown wind tunnel that can test in gusts of wind that last between 10 seconds and 15 minutes, creating simulated flight speeds of between Mach 0.1 and 5.5. The fastest tunnel, S4MA, is a blowdown hypersonic wind tunnel that was used to test a space shuttle. Tests in there last between 25 to 90 seconds in speeds from Mach 6 to 12. Journalists were not shown these latter two systems due to their sensitive nature. (Hypersonic speeds are generally defined as five times the speed of sound, or Mach 5.)
Back at S1MA, walking through the innards is like being in a science fiction movie where humans have been shrunk to ant size.
When the Germans designed the first tunnel in the early 1940s, they’d planned on putting a full-sized Messerschmitt aircraft in there with the pilot. Today no human would ever be in the system when it’s switched on. Still, its size means that full, large-scale models can be tested, even those that are powered. Military aircraft are generally tested at one-fifth scale to validate release of bombs, missiles or other ordnance. Air intake is tested up to a very high angle of attack and IR signatures are measured with jet engine simulation.
Full-scale missile tests with real engines firing can take place in S1MA. And all sorts of optical and acoustical tests also can be undertaken.
The wind in S1MA is created by two 15-meter (50-foot) diameter drive fans, one of which has 10 blades and the other 12 blades, each weighing 900 kilograms, nearly a ton. The fans are driven by two 27 metric tonne (30 US tons), 25 bucket Pelton turbines, which are 16 feet in diameter.
The wind is blown through a rectangular frame (509 feet by 131 feet) of large pipes known as diffusers. The pipes get bigger and bigger in the direction the wind is blowing to reach a maximum diameter of 79 feet. The wind is then funneled into the 26-foot diameter test unit.
Three mobile test units, much like stage scenery in major theaters, can be prepared off-stage and then pushed along permanent rails in the floor into the wind funnel. This allows for two of the test units to be set up — given that it can take up to three weeks to prepare — while the third is being used for a test.
The S2MA, in comparison, is smaller. Its rectangular frame is 318 feet by 84 feet, and walking through it was like being in the tight confines of a submarine, stepping through doors that, when closed, are absolutely airtight.
The four 1.5 metric tonne (1.65 US tons) Pelton turbines that drive the fans there are five feet in diameter with 22 buckets. They are powered by the 2,624-foot down flow of water from the lake reservoirs up the mountain. And once the water has done its job, it empties into the nearby Arc river.
On one of the long sides of the rectangle, a section of the diffuser can be adjusted to attach either to the transonic nozzle or to the supersonic nozzle to adjust the wind speed. The transonic nozzle is just under six feet high and wide and is used to test with winds from Mach 0.1 to 1.5. The supersonic nozzle is the same width but a bit taller, at over six feet, and can test speeds from Mach 1.5 to 3.0.
This is the wind tunnel in which military aircraft handling quality tests are undertaken, as well as separation tests, because, as this video attests, when an aircraft attempts to drop a missile, it’s not a guarantee that aerodynamics will allow it to actually fall. Air intake performance, static and dynamic flow distortion measurements and jet plume tests are also undertaken here.
Missiles themselves are tested to take global and local load measurements, to test air intakes, to simulate jet controls.
Space launchers, meanwhile, are also tested here to take global load measurements, to test booster separation, to simulate nozzle flow and to distribute loads.
On average, wind is created in these tunnels for between 800 to 1,000 hours a year. Sainjon announced that in 2022 the tunnels accounted for €29.9 million ($32.2 million USD) of the record €169.2 million ($175.7 million USD) orders taken by ONERA, of which about one-third comes from foreign clients.
“And we’re going to be really busy in the next four or five years because of all the testing that will need to be done for the Future Combat Air System,” he said, referring to the Franco/German/Spanish project that will be centered on a New Generation Fighter.