track to the hulls of their Sherman tanks. U.S. tank crews often added sand bags in the hull and turrets on Sherman tanks, often in an elaborate cage made of girders. Some Sherman tanks were up-armoured in the field with glacis plates and other armour cut from knocked-out tanks to create Improvised Jumbos, named after the heavily armoured M4A3E2 assault tank. In the Vietnam War, U.S. "gun trucks" were armoured with sandbags and locally fabricated steel armour plate. More recently, U.S. troops in Iraq armoured Humvees and various military transport vehicles with scrap materials: this came to be known as "hillbilly armour" or "haji armour" by the Americans. Moreover, there was the Killdozer incident, with the modified bulldozer being armoured with steel and concrete composite, which proved to be highly resistant to small arms. Spaced Armour with two or more plates spaced a distance apart, called spaced armour, has been in use since the First World War, where it was used on the Schneider CA1 and Saint-Chamond tanks. Spaced armour can be advantageous in several situations. For example, it can reduce the effectiveness of kinetic energy penetrators because the interaction with each plate can cause the round to tumble, deflect, deform, or disintegrate. This effect can be enhanced when the armour is sloped. Spaced armour can also offer increased protection against HEAT projectiles. This occurs because the shaped charge warhead can detonate prematurely (at the first surface), so that the metal jet that is produced loses its coherence before reaching the main armour and impacting over a broader area. Sometimes the interior surfaces of these hollow cavities are sloped, presenting angles to the anticipated path of the shaped charge's jet in order to further dissipate its power. Taken to the extreme, relatively thin armour plates, metal mesh, or slatted plates, much lighter than fully protective armour, can be attached as side skirts or turret skirts to provide additional protection against such weapons. This can be seen in middle and late-World War II German tanks, as well as many modern AFVs. Taken as a whole, spaced armour can provide significantly increased protection while saving weight. The analogous Whipple shield uses the principle of spaced armour to protect spacecraft from the impacts of very fast micrometeoroids. The impact with the first wall melts or breaks up the incoming particle, causing fragments to be spread over a wider area when striking the subsequent