On 14 October 1947, over the Mojave Desert, Chuck Yeager took a rocket plane to Mach 1.06 and became the first person confirmed to have flown faster than sound in level flight.
Bullets had been doing it since the 1880s. The tip of a stockman’s whip does it every time it cracks, which is what the crack is. Ernst Mach and Peter Salcher had photographed the shock wave off a supersonic projectile in 1887, sixty years earlier.
So the sound barrier was never a barrier to speed. It was a barrier to keeping control, and the physics of why is worth getting right.
Why the Air Stops Getting Out of the Way
An aircraft in flight is constantly announcing its arrival. As it pushes air aside, the pressure disturbance travels outwards in all directions at the speed of sound, which means that at subsonic speeds the air ahead has already been warned. It starts moving before the wing arrives, and the flow slides smoothly around.
The speed of sound is the speed at which that message travels. Approach it and the message can no longer get far ahead. Reach it and the aircraft arrives at the same moment as its own announcement.
The air ahead has no notice at all, so instead of flowing gradually it is compressed almost instantaneously. That is a shock wave: a layer a fraction of a millimetre thick across which pressure, temperature and density jump nearly discontinuously. It is the most violent thing that happens in ordinary fluid mechanics, and the process is irreversible, which means entropy increases across it. A shock wave is not just a loud noise. It is a thermodynamic one way door, and the energy it costs shows up as drag.
What Killed the Pilots Who Tried
The trouble began before the war and had nothing to do with supersonic flight, because no aircraft of the 1930s could reach Mach 1. It appeared in dives.
An aircraft flying at Mach 0.75 has air moving much faster than that over the upper surface of the wing, where the curvature accelerates it. Long before the aircraft itself is supersonic, patches of the flow over the wing are, and shock waves form in those patches.
The consequences were bewildering to the pilots who met them:
- Violent buffeting, as the shock waves made the airflow separate from the wing surface and then reattach, shaking the airframe hard enough to break it
- Controls going solid. Control surfaces work by changing the pressure distribution behind them, and a shock wave sitting in front of a hinge stops that change from propagating. The stick simply stopped doing anything
- Mach tuck. As the shock pattern shifted, the centre of lift moved rearward and the nose pitched down, steepening the dive, which increased the Mach number, which made it worse
- A steep rise in drag near Mach 1, so that engines with plenty of thrust at Mach 0.8 could not push through
Put together, those effects looked exactly like a wall in the sky. Pilots in P-38s and Spitfires and Typhoons hit it in dives, some of them fatally, and the phrase sound barrier came from the reasonable inference that something out there was solid.
The drag rise is the part that most deserves correcting. It is not a wall, it is a hump. Drag climbs steeply as you approach Mach 1, peaks slightly above it, and then falls again. Get through and it gets easier. The difficulty is that you must have enough thrust to climb the hump and enough control authority to stay pointed the right way while you do it.
A Bullet With a Pilot In It
The Bell X-1 was designed around the one thing everybody knew went supersonic reliably. Its fuselage was shaped like a .50 calibre machine gun bullet, because bullets of that shape were known to be stable above Mach 1 and there was a great deal of ballistic data on them.
Three decisions made it work:
Very thin wings. Eight per cent thickness to chord on one wing set, ten on another. A thin wing accelerates the flow over it less, so the local shock waves form later and are weaker.
A rocket engine. The XLR-11 burned alcohol and liquid oxygen in four separate chambers that could be lit individually, giving crude throttling by switching chambers on and off. A rocket does not care about intake conditions at high Mach numbers, which is a problem that would occupy jet designers for another decade. This was the same propellant combination Robert Goddard had first flown in a cabbage field in 1926.
The flying tail. This is the decisive one. Instead of a fixed horizontal stabiliser with a hinged elevator on the back, the whole horizontal tail surface could be pivoted to change its angle of incidence. A shock wave on a hinged elevator makes the elevator useless. Moving the entire surface changes the flow around all of it and keeps working transonically.
The flying tail came out of British work on the Miles M.52, a supersonic research aircraft cancelled in 1946, whose data was passed to Bell under an exchange agreement. The British had done the aerodynamics and then abandoned the aeroplane, and the argument about that decision is still going in the aviation press.
The Flight
The X-1 could not take off under its own power with enough fuel to be useful, so it was carried to altitude in the bomb bay of a B-29 and dropped.
Two nights before the attempt Yeager fell off a horse and broke two ribs. He had them strapped by a civilian doctor off base, said nothing to the flight surgeon, and could not reach across to pull the cockpit hatch closed. A colleague cut down a length of broom handle so that he could lever it shut one handed.
Dropped at around 20,000 feet, he lit the chambers, climbed to about 43,000 feet, and the Machmeter went off the scale at 1.06, roughly 700 miles per hour at that altitude. On the ground, observers heard a bang like distant thunder.
The ride through the transonic region was rough, and then it went quiet. Past Mach 1 the shock waves trail behind and the buffeting stops. Yeager reported afterwards that the flight was so smooth his grandmother could have flown it, which is both a good line and true.
It stayed secret for months. Aviation Week leaked it in December 1947 and the Air Force confirmed it in June 1948.
The Boom Is Not a Moment
The most widespread misunderstanding about all of this is what the bang is.
The sonic boom is not a single event that happens when an aircraft crosses Mach 1. It is a continuous cone of shock waves dragged along behind the aircraft for as long as it remains supersonic. Anybody the cone sweeps over hears a boom, one after another, all along the flight path. In the aircraft, nobody hears anything, because the shocks are always behind.
Which means an aircraft cruising supersonically over land leaves a corridor of noise tens of kilometres wide beneath it for the whole flight. This turned out to be the thing that limited supersonic aviation, not engineering. The United States banned civil supersonic flight over land in 1973, and Concorde spent its career subsonic over populated ground and supersonic only over water, which is a large part of why the economics never worked.
Current work is aimed at the shape of the boom rather than its existence: design the aircraft so that the shocks from nose, wings and tail do not coalesce into two sharp pressure jumps, and what reaches the ground is a soft thump instead of a crack. NASA’s X-59 exists to find out whether people on the ground will accept it.
Seventy Nine Years On
The physics has not changed since 1947 and neither has the essential trick. Every supersonic aircraft since has used thin wings, an all moving or carefully placed tail, and enough thrust to climb the drag hump.
What has gone is the fear. In 1946 a great many competent engineers believed that something fundamental might prevent a piloted aircraft ever exceeding Mach 1, and the belief was not stupid: the evidence from dive accidents pointed that way, and the wind tunnels of the time choked at exactly the speeds where the answers were.
Air had been understood as a substance with weight and pressure since Torricelli put a tube of mercury upside down in a dish in 1643. It took three hundred years to discover that pushing through it fast enough changes the rules, and one man with broken ribs and a piece of broom handle to demonstrate that the change was survivable.
No products in the cart. 