Is it possible to make an engine using only Blender?

[Random Annoying Guy]

That is without Unity or other stuff

Edited March 12, 2020 by Random Annoying Guy

[Shadowmage]

7 minutes ago, Random Annoying Guy said:

That is without Unity or other stuff

No. You have to use PartTools (in the Unity Editor) to compile the model to be compatible with and loaded by KSP.  KSP will not load the standard .fbx, .obj, .dae, etc. model formats, and will only load its special .mu model format (incompatible with everything, and only used by KSP; binary encoded model with no published spec sheet for the encoding/decoding standard).

In the past there was a '.mu importer/exporter' plugin for Blender (it still around), but I don't think the 'export' functions are compatible with newer KSP versions.  You could give it a try (might work), but the real answer is still 'You have to use Unity, cannot be avoided'.

(or if you are a c# wizard, you can whip up your own model-loader KSP plugin, and then use whatever model format you want...)