It looks like a liquid fuel rocket engine is made of rather thin metal. Here's photo of RD-107 from Wikipedia
Nothing personal, but it looks like it's made of several shiny thin metal cans (connected with gazillion of pipes for feeding fuel and oxidizer). When the rocket is flying with the engine working, a jet of burning fuel is going down and pushing the rocket up which means that all the rocket weight (dozens of tons and btw the engine accelerates the rocket so that weight should be multiplied by some number larger than one) is resting on the engine (and the engine is resting on the jet being produced) so all the weight is in fact resting on those shiny thin metal cans.
Why isn't the engine crushed by the rocket weight?