'km per second' is correct and usable in written English.
You can use it when you want to express a rate of speed, such as when discussing the speed of light. For example, "Light travels at a rate of 300,000 km per second."
Sound has a velocity of about 0.33 km per second (0.2 mile per second) in air, 1.5 km per second in water, and 5 km per second in steel.
Moving through a vacuum, light travels at 300,000 km per second.
The gas is expanding at 1,100 km per second slower than the 10,000 20,000 km per second in the shells of new supernovae in other galaxies.
Radar waves travel through the atmosphere at roughly 300,000 km per second (the speed of light).
The spectrum showed hydrogen lines expanding at 12,000 km per second, followed by a long period of slow decline.
Very swift (71 km per second) meteors flash in all parts of the sky along tracks that point pack to this radiant.
Siraj says that with respect to the average velocity of stars in the solar neighborhood, this object was moving extremely fast, at around 60-km per second.
The energy of a CME depends on its velocity; CMEs are launched with a wide variety of speeds, from less than 10 km (6 miles) per second to more than 2,000 km (1,200 miles) per second.
This material, called plasma, moves through the interplanetary medium at speeds from less than 10 km (6 miles) per second to more than 2,000 km (1,200 miles) per second, so that the ejected material reaches Earth in approximately 21 hours.
In the Earth the speed of S waves increases from about 3.4 km (2.1 miles) per second at the surface to 7.2 km (4.5 miles) per second near the boundary of the core, which, being liquid, cannot transmit them; indeed, their observed absence is a compelling argument for the liquid nature of the outer core.
In the Earth, P waves travel at speeds from about 6 km (3.7 miles) per second in surface rock to about 10.4 km (6.5 miles) per second near the Earth's core some 2,900 km (1,800 miles) below the surface.
