In parallel with the specification to which the La-168 was to be developed, a requirement was formulated for a lighter "frontal fighter" powered by the Rolls-Royce Derwent turbojet. To meet the latter demand, the Lavochkin bureau evolved two designs, the La-172 and the La-174TK. The former was effectively a scaleddown version of the La-168 tailored for the lower-powered, smaller engine and the latter featured exceptionally thin, unswept wings and a configuration generally similar to that of the earlier La-152 series of fighters. The La-172 was powered by an NII-1 turbojet (as the pre-series Soviet version of the Derwent was designated) rated at 1600kg, and entered flight test early in 1948, with armament comprising three 23-mm NS-23 cannon. Early in the test programme, while being flown by I Ye Fedorov, the La-172 suffered uncontrollable flutter at 8000m and entered a flat spin. Recovery was effected at 3000m, but the prototype was heavily damaged in a subsequent crash landing. Nevertheless, development was continued via the La-174D to result in the La-15.