Tag Archives: friction

Base Bleed Artillery

base-bleed-patentMost of the drag on an artillery shell comes from friction between the nose of the shell and the air, as the shell pushes air out of the way at very high speeds. But some of the drag on a shell comes from the sucking effect of the vacuum left behind the shell as it pushes air in front out of the way faster than air can move to fill the space left behind.

To combat this, many artillery shells employ a system called base bleed in which the shell produces gas at its rear to fill this vacuum. This gas produces very little thrust, but by reducing the effect of the vacuum it increases the range of the shell enormously, typically by around 30%. On the diagram on the right (taken from this patent) the top image shows a view from below, with the gas generator’s exhaust labelled “5”. The housing of the gas generator is labelled “1” and the casing of the shell “2”. The igniter that starts the gas generator is labelled “4” and the fuel charge that produces the gas is labelled “6”.

Triboelectric envelope

The triboelectric effect (the prefix tribo- comes from the Greek τρίβω for “rubbing” or “friction”) results in the creation of a charge difference between two surfaces: one becomes positive and the other negative. The difference in charge is neutralised when a spark jumps between the two surfaces.

Opening “self-stick” envelopes quite often results in a noticeable triboelectric effect, as I discovered when opening this week’s copy of The Economist.

Kopp-Etchells Effect

The Kopp-Etchells effect is the name given by Michael Yon to a phenomenon he witnessed in Afghanistan.


As a helicopter comes in to land a great cloud of dust is stirred up; the impact of this dust on the rotor blades would quickly wear the blades down, making them unsafe. The rotors are coated in titanium to resist this damage; the sparks you see are this titanium coating being worn away (a process physicists call ablation).