Complex sentence: In fact, Lavochkin, Gorbunov and Gudkov had originally designed their prototype for the powerful Klimov M-106 engine, but it proved to be unreliable, so they were obliged to install the relatively weak Klimov M-105P.
Split into simpler sentences: In fact, Lavochkin, Gorbunov and Gudkov had originally designed their prototype for the powerful Klimov M-106 engine. So they were obliged to install the relatively weak Klimov M-105P.