# How does a thin metal rocket engine not get crushed by the weight of the flying rocket?

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?

• It's still not clear what your question is, sharptooth. Are you asking about the design requirements on the rocket engine itself that are needed to ensure the structural integrity of the engine, or the design requirements on the rocket engine support structure that are needed to ensure that the rocket as a whole maintains structural integrity? Mar 26, 2014 at 14:16
• @DavidHammen: The engine itself. I edited the question once again and hope it's clearer now. Mar 26, 2014 at 14:27

Nothing personal, but it looks like it's made of several thin metal cans connected with gazillion of pipes.

Those "gazillion of pipes" are the mechanism via which force is transferred from the rocket engines to the structure of the rocket.

Here's a simple experiment: Get a couple of empty aluminum soda cans. Stand them upright on a flat surface. Now stand on them, one can per foot. Do it carefully and those two soda cans will support all of your weight. Think about that. All of your weight is supported on two very thin cylindrical structures.

Next, notice how those "gazillion of pipes" form lots of triangles. Triangles make for very sound structures. Look at a bridge. Look in your attic. You'll see lots and lots of triangles.

• Now that the question has been edited, it might be worth pointing out that this answer's interpretation of "gazillion of pipes" no longer meshes with the question's wording thereof. That's not to say this answer is incorrect, just that the phrase now refers specifically to the various feed lines throughout the engine's plumbing. Mar 26, 2014 at 17:51
• @Tristan - I took that "gazillion of pipes" as meaning the long, thin, tube-shaped structural members that connect the engines to the body of the rocket rather than the plumbing. The plumbing contributes very little to the structural integrity. Those "gazillion of pipes" do. Mar 26, 2014 at 18:52
• Sure -- just pointing out that the question was edited after this answer -- didn't want anyone to get confused and think you weren't answering his question :-) Mar 26, 2014 at 20:20

The forces that act on the walls of a rocket engine look like this:

The propellant is ignited in the chamber, this causes the combustion products to expand, and the pressure in the chamber and nozzle rises above the external atmospheric pressure.

The only force vectors in this diagram that contribute to pushing the rocket forward, are the ones that point in the direction of travel or less than 90° off that direction. I.e. the net thrust is created by the force on the top of the chamber (usually the injector plate) and to some degree by the vertical component of the forces on the nozzle.

So yes, you should be able to cut off the chambers and nozzles, and place the fully-fueled rocket on the injector plates.

You can also see that the sides of the chamber are under the same amount of force as the injector, so the chamber walls need to be strong enough to withstand this pressure. From the throat down, the pressure decreases, so a smaller force acts on the end of the nozzle, and this section doesn't have to be as strong. So the bottom end of the nozzle is usually thinner than the walls of the combustion chamber.

More details here (this is also where I found the schematic)

The weight is supported quite evenly by the pressure of the exhaust gases in the nozzle. Spread the weight over a wide enough area, and the resulting stresses are quite manageable.

• as the gas flows out its pressure decreases.Does pressure of exhaust gases is evenly distributed over the nozzle?
– Hash
Mar 25, 2014 at 14:54
• @Hash -- I'm not quite sure what you are asking. Mar 25, 2014 at 15:55
• @Hash If I understand you correctly, then no. Even assuming combustion products as ideal gas and the whole expansion isentropic (meaning no expansion waves, no shear layering, no supersonic shock regions, perfect exhaust products w/ constant temperature,... big assumptions!), the pressure ratio changes with volume over surface area of a specific section of the nozzle. In an axially symmetric De Laval nozzle with a circular cross section this means (or rather it would mean, if only it was that simple) that pressure ratio changes by one half the change in its radius over the length of it. Sep 28, 2014 at 5:06

pressure though out a generic rocket thrust chamber and nozzle while throttling through different ranges. https://www.youtube.com/watch?v=SlPHYc0mXEw this demonstrates that on a vast majority of rocket designs that are used in optimum power ranges often there is a significant pressure decrease around the throat and pressure is drasticly lower in the nozzle section. it does not have to be this way and you could design a rocket with near equal pressure distribution in the chamber and nozzle however this usually would result in a grossly under expanded nozzle and other drawbacks. i hope for the sake of credibility of this site you reconsider your answers. the primary section that transmits thrust to the rocket structure is the injector plate and machinary ahead of it this is commonly known as the power head and is the reason most heavy lifting engines are connected from here. further in no case does any rocket transmit thrust from the nozzle alone. also if a significant portion of thrust was generated from the nozzle, this would imply that the forces on all of the nozzle before the throat would sum up and need to pass through the bottle neck of the throat, the throat being the cross section of least material area density and thus higher stress per unit. this would be poor design in most cases.

• "i hope for the sake of credibility of this site you reconsider your answers." I think the credibility of the site is just fine. You might want to revisit this answer and consider if you still stand behind it (and it's peculiar punctuation) almost four years later.
– uhoh
Jul 9, 2018 at 11:40
• So punctuation effects credibility? As well 4 years time, the physics of rocket engines. The new York times very elegantly wordedly insulted Goddard when he tried to stand behind Newton's laws that a rocket engine would work in vacuum. Ironically here it took nearly 60 years for new York times to issue an apology, even though it wasnt needed by that time. I doubt Goddard needed the apology or approval of idiots and the general masses. Jul 10, 2018 at 14:23
• I can look aty words from deleted answer which is heavily down voted then at the accepted answers to know popular opinion on this site is failing. Jul 10, 2018 at 14:27