Racing for Land Speed Records
Running fast is an old occupation, and sometimes a risky one. One of the most
fascinating pursuits is the land speed record by vehicles that now resemble rockets
on wheels, rather than cars. This record has now surpassed Mach 1 (i.e. the sonic
speed).
We report here a few basic aerodynamic considerations. Torda and Morel (1971) pointed
out that at high subsonic to low supersonic speed the vehicle generates shock waves
that are reflected from the ground.
These shock reflections cause a pressure build-up below the vehicle that have an
effect dependent on the speed: at subsonic speed shock wave interference produces
additional downforce; at transonic speeds the problem is further complicated by the
vehicle clearance; at supersonic speeds the interaction may result in reversal of
direction of the lift force.
Since these vehicles are rockets, transonic drag rise can be reduced by increasing
the finess ratio. Nose design is also critical. For example, a von Karman ogive is
known for having minimum wave drag at transonic speeds.
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