Fifty years and one day after Chuck Yeager became the first man to break the sound barrier in an airplane, Andy Green did it on land.
Yeager’s air speed? 700 mph.
Green’s land speed? 763.035 mph.
Although Green and Thrust SSC set the still current World Land Speed Record, Yeager’s accomplishment has long since been eclipsed. On 16 November 2004, NASA’s X-43A airplane reached Mach 9.6 or almost 7,000 mph. More than nine times the speed of Yeager’s Bell X-1 in 1947, that 2004 flight still holds the record for a jet-powered aircraft.
The Reno Gazette has a photo story of that day’s events.
The jet car, explained
The record run happened at Black Rock Desert, Nevada, the largest dry lake bed in the country. To set a record, a driver must make two runs on the, one to the north and the other to the south. Green broke the sound barrier in both directions.
Green was driving for the British team run by Richard Noble, who set the record in 1983 (633 mph).
Driver Green sits in a cockpit placed at midsection between the two engines, allowing him to gain a better feel for side-to-side movement of the car. Green steers the two rear wheels, which are tucked toward the back of the body’s underside to avoid interference with the jet exhausts. One rear wheel is placed slightly behind and to one side of the other, avoiding a drag-producing bulge in the rear section that would have occurred if the wheels had been placed parallel to each other. The front wheels are fixed in place: avoiding a front steering mechanism reduces drag.
He set that land speed record in a vehicle most of us would not consider “a car.”
- Fuel consumption: 4.8 US gallons (18 liters) of jet fuel per second
- Power: two Rolls-Royce engines that were designed to power a McDonnell-Douglas jet fighter (1968-1992)
- Size: 54 feet (16.5 meters) long and 12 feet (3.7 meters) wide
- Weight: about 10 tons (about 20,000 pounds if US tons)
Compare those specs with Yeager’s X-1.
- Fuel consumption: 4,680 pounds of fuel = 2.5 minutes of flight
- Power: one four-chamber rocket engine
- Size: 30 feet 11 inches (9.42 meters) long and 28 feet (8.53 meters) wide
- Weight: gross 12,250 pounds; empty 7,000 pounds
Slip inside and it looks like an airplane cockpit!
About the science
Airplanes and jets moving through the atmosphere produce “air-pressure waves similar to the water waves caused by a ship’s bow.”
When one cruises at less than 250 mph, air density remains constant. But as its speed increases, the plane changes the density of the air surrounding it. (For more about what happens to the air, see this Wired story from 17 October 2012.)
In the 1940s, scientists identified a “sharp rise in aerodynamic drag” (the resistance an airplane encounters as it moves through the air) as planes flew faster. They dubbed this the “sound barrier.” Passing through it leads to a “sonic boom.”
In 1947, engineers worried that the “buffeting produced by supersonic shock waves might tear apart [Yeager’s] sleek craft.”
Flying at the speed of sound is called flying at sonic speed.
When airplanes fly at “somewhat less than sonic speed,” sound waves spread out and do not affect the plane. However, at sonic speed (Mach 1), sound waves form a barrier. That barrier can cause the plane to become dangerously unstable.
What is the speed of sound?
There is no constant speed of sound. It depends upon the temperature of the air that sound waves pass through. Sound moves faster through warm air than cold air.
On Earth, the speed of sound at sea level — assuming an air temperature of 59 degrees Fahrenheit (15 degrees Celsius) — is 761.2 mph (1,225 km/h).
Because gas molecules move more slowly at colder temperatures, that slows the speed of sound; sound moves faster through warmer air. Therefore, the speed required to break the sound barrier decreases higher in the atmosphere, where temperatures are colder.
Mach 1 means that an object is traveling at the speed of sound. Mach 0.8 reflects 80% of the speed of sound. Conversely, Mach 3 is three times the speed of sound.
- subsonic: a vehicle traveling slower than the speed of sound (M < 1)
- supersonic: a vehicle traveling faster than the speed of sound (M > 1)
- hypersonic: a vehicle traveling more than five times the speed of sound (M > 5).
According to CNN, 15 October 1997 was a bit cooler than normal in the Nevada desert:
The speed of sound, which varies according to weather and altitude, was calculated Wednesday morning at 748.111 mph. Temperatures were cooler than normal in the Black Rock Desert, 125 miles north of Reno, making the vast region more conducive to faster speeds.
For context, the Concorde could fly up to five times the speed of sound. It was the first, and only, supersonic commercial passenger plane. Jointly owned by Great Britain and France, the Concorde first crossed the Atlantic Ocean on 26 September 1973. The last flight was 24 October 2003.
Why is there a “boom”?
A sonic boom is an “impulsive noise” that sounds lot like thunder. The video explains: