I don't understand anything, did I do something wrong?
Well, not really. It's an artistic supersonic missile thingy, hardly a crime to to bend some physics. And it's mostly a reaction to what the other user said.
It needs no heat to work? So what? Real life missiles are made of sturdier stuff than SFS rockets.
Engines are shrunk? That's alright.
Clipped engines and fuel tanks? For this? I can think of much worse applications. At least it has a reasonable TWR for a missile.
Sure, if you could make it without those physics rule breaking things then by all means do so and we'll acclaim you for it, but in this case it seems rather restrained and hardly worthy of an overreaction.