I see! However my opinion is still that they are too weak. For example, many fighter jets easily break the sound barrier at sea level, while I cant get more than mach 0.5 with the FAR Slothful Hummingbird. Also to note, 150kN thrust is very similar to the engine in sr-71 blackbird, so the value itself isn't unrealistic. Regardless of actual values, two engines and a lightweight airframe should do mach 3.2 like the blackbird, in my opinion A question, does ksp simulate the exhaust velocity of the jet engines? Because a "basic" engine could have more static thrust, but too low exhaust velocity to go much above mach 1, compared to a ramjet style engine that has less static thrust but keeps going past mach 3. Actually to me, 3 types of engines are needed: Turbofan (airliner engine), for heavy planes that are subsonic. High static thrust but low exhaust speed. "normal" jet engine perhaps with afterburner, for lighter "fighter jet" style planes. Lower thrust but higher exhaust speed. Still viable in a wide range of operation from sea level to "pretty high" (unsure if the ksp atmospheric model is comparable to the earths, but like 16 000 m.) Then the ramjet, which plain sucks a low altitudes and speeds, but has a very high exhaust velocity enabling speeds above mach 3. Soo... One more suggestion Was flying along trying to push the mach numbers ever higher, and realized all I did was trying to find the best altitude with full throttle all the time. Found that it was very sensitive and I didn't think it was all that fun. So my suggestion here to make things more interesting is adding overheating as a element. Basically trying to take of at 100% will blow your plane up, so you have to dial it back a bit and beware of the temperature and wait for some more speed for cooling. Only when in the correct altitude/speed range can you really floor it! But then if you are going too fast (over 3.5 or something) you start to heat up again, so that the speed limit isn't really how much thrust you have but your cooling capacity. In my mind, this will make pushing ever greater speeds more exiting because if you're not on top of it, the plane will blow to bits. Of course this is mostly only relevant to the ramjet category and perhaps a bit for the "normal" engines. The turbofans should be easy to fly. Anyway just my 2 cents, and I'm no jet engine expert anyhow so I might just be plain wrong Thanks!