In the United Kingdom application of Chobham armour was delayed by the failure of several advanced tank projects: first that of a joint German-British main battle tank; then the purely British MBT-80 programme. A first directive to prepare Chobham armour technology for application in 1975 was already given in 1969. It was determined by a study of a possible Chobham-armour protected MICV that a completely new design using only Chobham armour for the most vulnerable front and side sectors (thus without an underlying steel main armour) could be 10% lighter for the same level of protection against KE-ammunition, but to limit costs it was decided to base the first design on the conventional Chieftain. The prototype, FV 4211 or the "Aluminium Chieftain", was fitted with a welded aluminium add-on armour, in essence a box on the front hull and front and side turret to contain the ceramic modules, of which box the fifty millimetre thick inner wall due to its relative softness could serve as their backing plate. The extra weight of the aluminium was limited to less than two tonnes and it was shown that it was not overly susceptible to cracking, as first feared. Ten test vehicles were ordered but only the original one had been built when the project was cancelled in favour of the more advanced programmes. However, the Iranian government ordered 1,225 vehicles of an upgraded Chieftain type, the "Shir-2" (FV 4030/3), using the same technology of adding Chobham armour to the main cast armour, bringing total weight to 62 metric tonnes. When this order was cancelled in February 1979 because of the Iranian Revolution, the British government, under pressure to modernise its tank fleet to maintain a qualitative superiority relative to the Soviet tank forces, decided to use the sudden surplus production capacity to procure a number of vehicles very close in design to the Shir-2, called the Challenger 1. On 12 April 1983 the first British tank protected by Chobham armour was delivered to the Royal Hussars.