Last updated January 9, 2018 at 4:24 pm
Sometimes time flies past incredibly quickly – it’s been five years since Austrian daredevil Felix Baumgartner skydived from the edge of space. During his record breaking jump he not only completed the highest skydive ever attempted, but also broke the sound barrier on his way down.
It’s taken until now for scientists to catch up to him and analyse the aerodynamics of his jump, and they have found a pretty amazing result – Baumgartner, with his irregularly shaped equipment, fell faster than a smooth object would have.
On 14 October 2012 Felix Baumgartner opened the door of his capsule hanging beneath a gigantic helium balloon, stepped onto the edge, and jumped back towards Earth. From a height of nearly 39 kilometres Baumgartner accelerated to a terminal speed of Mach 1.25 (1357.6km/h), the first and so far only person to have broken the speed of sound without the aid of an engine.
Professor Ullrich Walter from the Technical University of Munich was the scientific advisor to the Red Bull Stratos jump project and admits he was surprised by the huge speed Baumgartner achieved. “Our calculations (before the jump), based on the fluid dynamics of a smooth body, indicated that Baumgartner would need to jump from an altitude of about 37 kilometres in order to break through the sound barrier, i.e. to fall faster than Mach 1 or about 1200 kilometres per hour.”
This left him intrigued – why would Baumgartner, wearing his bulky pressurised suit and parachute backpack fall faster than a smooth object with its low drag?
Using the data he and Red Bull collected during the jump, including atmospheric conditions, Baumgartner’s speed and his shape at every point of freefall, Walter has been able to set another first for the Stratos jump – the first ever study into irregular shaped objects travelling at extreme speeds.
“The results really surprised us,” said Markus Gurster, an astronautics engineer who did the analysis with Walter. “While the drag coefficient of a smooth cube increases continuously from Mach 0.6 to Mach 1.1, according to our results, the coefficient remained almost unchanged during Baumgartner’s flight – that means the sound barrier hardly generated any additional drag at all.”
Usually shockwaves created by an object travelling near the speed of sound create a force called wave drag, which causes a sudden increase in the amount of drag on the object. The researchers claim that the rough surface of Baumgartner’s suit changed the formation of the boundary layer – a thin layer of air immediately surrounding an object – and that in turn reduced the effects of wave drag.
Professor Richard Kelso, a bluff-body aerodynamics expert from the University of Adelaide, disagreed with this finding.
He did agree that the shape and roughness of Baumgartner’s suit would have affected the boundary layer, but says this would have only led to a small change in the wave drag.
Compared with a smooth objects, at low speeds an irregular or rough object’s drag coefficient, and thus their aerodynamic drag, is often cut almost in half. This is the same effect as dimples on a golf ball, which create a thin turbulent boundary layer on the ball’s surface. This allows the smoothly flowing air outside the boundary layer to follow the ball’s surface a little further around the back side of the ball, which in turn decreases the size of the wake trailing the ball. Smaller wake, less drag.
However the idea that this change in boundary layer was responsible for the changes in wave drag, he says, are incorrect. Instead he suggests there was another effect which would have had a much larger bearing on the change.
“Whilst the described boundary layer effects do occur to some degree, they have very little influence over the overall flow pattern and the wave drag.”
Kelso points to the changing of Baumgartner’s body angle.
“A more plausible explanation for the small drag change is that when Baumgartner was falling at supersonic speed, he was diving head-first with his body axis at 30 degrees from vertical.
“Later in his descent he was falling at subsonic speed with his body angle varying in the range of 50 degrees (steep glide) to 80 degrees (flat glide). The flow patterns and shock wave patterns around Baumgartner’s body at each of these angles would be substantially different.
“Diving head-first would lead to a weaker shock wave pattern, less wave drag and a higher terminal velocity than would a flat glide. Therefore, I suggest that the smaller than expected rise in drag coefficient through the transonic regime is an effect of changes in body orientation rather than roughness and shock wave-boundary layer interactions.”
Future modelling will hopefully reveal more about the exact forces which Baumgartner was experiencing, and what was causing them. But until then, if a daredevil lines up to try to break Baumgartner’s records, aerodynamicists around the world will surely be watching intently.
The research was published in PLoS One
Images courtesy of Red Bull Stratos / Red Bull Content Pool