The solution was to pressurise the entire cooling system, thereby not only preventing the decrease in cooling performance with altitude, but in fact increasing the boiling point even on the ground. The Kestrel was built to maintain enough pressure to keep the boiling point at about 150 °C. The engine was first produced in 1927 at 450 hp (340 kW), which soon improved in the IB version to 525 hp (390 kW). This variant saw widespread use in the Hawker Hart family that was the mainstay of British air power during the early 1930s. However, it was not long before power output was increased dramatically; the V model provided 695 hp (520 kW) at 3,000 rpm with no basic change to the design, while the XVI used in the Miles Master delivered 670 hp (500 kW). Messerschmitt also tested its first Messerschmitt Bf 109 V1 prototype, bearing German civilian registration "D-IABI", with a Kestrel engine in 1935 as the German-designed intended engines were not yet ready. Junkers also used a Kestrel for the first prototype of the Ju 87 "Stuka" dive bomber. The Reich Air Ministry (RLM) acquired four Kestrel VI engines by trading Rolls-Royce a Heinkel He 70 "Blitz" as an engine testbed. Increased availability of higher octane aviation fuels in the late 1930s allowed the engine to be boosted to higher power levels without suffering from detonation, and the Kestrel eventually attained a power output of 720 hp (537 kW) in the XXX variant of 1940. Further developments of the Kestrel were the Goshawk and the Peregrine (and therefore the Vulture). In practice, development of the Peregrine and Vulture was troubled, and they were both cancelled with comparatively few built. The Kestrel was produced in 40 distinct variants which can be divided into three main groups: normally aspirated, medium supercharged, and fully supercharged. One variant, the Kestrel VIII, was configured as a 'pusher engine' for the Short Singapore flying boat. Apart from supercharging, the variant differences centred on varying compression ratios and propeller reduction gearing. "From Lumsden, the Kestrel may not be the main powerplant for these types." Several Rolls-Royce Kestrel engines remain in service, powering restored Hawker biplane types:  Preserved examples of the Rolls-Royce Kestrel engine are on public display at the: