The Secret Of Supersonic Speed, Revealed

Illustration for article titled The Secret Of Supersonic Speed, Revealed

In the fall of 1997, two bands of engineers high on speed danced on the dry lake bed in Black Rock Desert, Nevada. One broke the sound barrier and the other didn't. We sat down with the team that didn't.

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This past week, land speed record hopefuls competed for titles at the Bonneville World Finals. Thirteen years ago to the date of this writing, Richard Noble and the Thrust SSC team set the pinnacle of land speed records: They broke the sound barrier.

Given the timing, I thought it apt to discuss land speed record attempts with some colleagues hoping to unearth some of their secrets. You see, these particular colleagues happen to have helped Craig Breedlove design and build a competitive project, the Spirit of America Formula Shell LSRV. They grossly underestimated their competitor's abilities that day, and their malfunctioning streamliner failed to travel faster than sound. However, the confounding Thrust SSC did reveal design secrets that remained unreported or deliberately misrepresented until now.

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Illustration for article titled The Secret Of Supersonic Speed, Revealed

Since much that has been published about the victor is a steaming pile of bullshit, let's let these speed-crazy mad men dispel three of the misconceptions.

Misconception: The supersonic shockwave would lift and destroy the wide vehicle.
Secret: Analyzing core samples taken from Black Rock, the Thrust SSC team correctly concluded that the porosity of the dry lake bed would actually absorb the shockwave and lessen its affect on the car. My colleague reported that the lake bed was initially hard as concrete, but soft as sand after the record setting pass.

Misconception: Rear wheel steering served an aerodynamic purpose.
Secret: Look at the SSC- it has the cross section of a barn door. They determined that the front wheels acted like gyros at speed and the stability depended on their directional plane. As a counterpoint, the Spirit of America tended to roll axially as the front wheels steered.

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Misconception: Engine airflow would suffer at supersonic speeds.
Secret: The long probe at the nose of the SSC broke up the shockwave enough so as not to disturb airflow to the engine inlets. This feature can be seen on Breedlove's latest machine, the Spirit of America — Sonic Arrow.

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DISCUSSION

The long probe at the nose of the SSC broke up the shockwave enough so as not to disturb airflow to the engine inlets. This feature can be seen on Breedlove's latest machine, the Spirit of America — Sonic Arrow.

This fundamentally misrepresents supersonic shockwave propagation.

The airspeed data boom would not "break up" the "shockwave," because there wouldn't be a pressure wave in front of it, as it was the front-most part of the car. Instead, the tip of the boom would create the leading pressure wave, slightly lessening the intensity of the pressure wave created by the nosecone.

The angle of the supersonic pressure wave depends on the speed of the object, as well as the shape; the faster and pointier, the more acute the wave angle. If the pressure wave intersects with the engine inlets, it will create a low-pressure zone that will prevent the engines from ingesting enough air to maintain combustion, thus causing a flameout.

My guess is that the air data boom was designed to be long enough that the leading pressure wave would remain outside the outer edge of the Thrust SSC's engine inlets.