conversion of the Sherman to take a 17-pounder (as the Sherman Firefly) proved effective in providing more 17-pounder-gun tanks. The Firefly accompanied Churchills, Shermans and Cromwells generally at a ratio of 1:4. The production of Fireflies greatly outpaced that of the Challenger but in Cromwell-equipped units, the Challenger was generally preferred as the Sherman had a slower road speed and inferior cross-country mobility. The culmination of British efforts was the Comet tank with a cut down 17-pounder design, the 77 mm HV. The Comet was a further development of the Cromwell, a "heavy" cruiser tank, which sought to remove the need for 17-pounder armoured vehicles, such as the Challenger or Firefly. The Comet reduced its road speed in comparison to the Cromwell to , in favour of better armour protection and a weapon able to penetrate the armour of the heavier German tanks whilst not sacrificing HE capability. The tank had a short service life as design for the Centurion was already well underway, with the first prototype arriving in 1945. Despite the emphasis on mobility, most early cruiser designs were plagued by mechanical unreliability, notably in the hot and gritty desert of the North Africa Campaign. This problem was usually caused by rushed development and introduction into service. After the debacle in France of 1940, cruiser tank designs were ordered "off the drawing board", particularly given the urgent need for tanks. The Liberty engine which also powered early Cruiser tanks was beginning to show its age and was being pushed to its limit in tanks such as the Crusader. This problem was not fully solved until the débût of the Cromwell in 1944, with its powerful and reliable Rolls-Royce Meteor engine. History Interwar In 1936, the War Office decided on a light tank for the cavalry, a cruiser tank, a medium tank and an infantry or assault tank. By 1938, the medium tank had stagnated as a research project, in favour of heavier cruiser and infantry tanks and after the outbreak of war, the move towards heavy infantry tanks capable of breaking through the Siegfried Line (Westwall) on the German border. In 1934, Sir John Carden of Vickers-Armstrongs had produced a "Woolworth" medium tank to a 1934 specification (General Staff number A.9) for a close support tank, using elements of the Medium Mk III design (which had been abandoned due to financial reasons) but lighter and using a commercial