Mdthbs

I've been wondering how a rocket fuselage can support all the weight of the upper stages when it is only made of a cylinder of very thin sheet metal. (Especially considering acceleration, vibration and aerodynamic force.) A few rockets have relied on internal pressure for strength, but these are not the ones I am talking about. In general I would appreciate any insights on the engineering principals used.

My specific question is:
Why aren't rockets built with truss structures inside their fuel & oxidizer tanks to increase structural strength?

There's almost nothing to be gained by a truss. The load being applied is along the axis of the tank. A simple hoop of material is very strong in this orientation. (Try it with a piece of paper, you'll be surprised at how much it can hold--just keep the weight even!) A truss in the tank would only help against loads off axis--and you don't want those in the first place!

Because they don't need to be.

Clearly the current design of rockets can be successful. So adding truss structures to the current design would add weight for no reason and take away from the payload capacity.

Most modern rockets do rely to some extent on tank pressure for strength. The tankage needs to be pressurized in any case to drive the turbopumps without risk of cavitation, so the structural strength benefits come for free or nearly so.

I'm not certain what you're envisioning when you say "truss structures". There are usually strengthening ribs along the interior walls of the propellant tanks -- welded in in rockets like Zenit and Falcon 9, milled "isogrids" in Atlas V and Delta IV. This provides enough strength to handle the g-loads encountered in the ascent (often as high as 6-g depending on the launcher and mission details), so there's no need for any cross-tank support structures.

Because it would be an inefficient way to handle the loads.

Let's say your rocket is a simple cylinder with engines at the bottom (no strap-on boosters or fins that might actually need extra structure to attach to and transfer the loads). Such a rocket will be subject to two main kinds of loads:

• axial compression (engine thrust vs dynamic pressure of ramming into air head-on)


• bending/shear by aerodynamic forces (flying at nonzero angle of attack causes the body to generate some lift)

As hinted in other answers, compression is easy to handle with what you already have: the skin of the cylinder. You just need something with a sufficient cross-section that won't buckle easily, and a big metal pipe is a good match for that requirement. And guess what, you already need that pipe to keep your propellants in.

The bending is a bit more tricky (and it also comprises vibrations of various frequencies), but a truss won't help very much with that. For a truss to resist bending, you need to make it wide. A single rod on the axis of the rocket won't help. And as you make it wider and wider, it will become stiffer against bending, until it finally is as wide as the entire cylinder. That means you've found the optimal arrangement: strengthening the walls.

I worked on the Atlas rockets which had pressurized tanks (balloon tanks) for structural rigidity/strength. For transport, the Atlas went into a truss to hold the rocket in tension to keep it from collapsing. On the pad, it needed to be pressurized. I once saw a pic of a retired Atlas on display, but the air pump had failed and it crumpled like a soda can. (I searched the internet but can't find it now.)

Also, we were warned not to touch the stainless steel skin with our bare hands, because the skin oils could cause a weak spot and mission failure. Not sure if that was true, but when you consider it gets filled with cryogenics, who knows.

A problem with trusses inside a liquid is that you have the possibility to set up currents and cavitations in the fluid that can cause it to move in ways you don't want. Without the trusses, the fluid moves (moderately) smoothly in the direction of the nozzle. With the trusses, it can set up eddy currents and (in extreme cases) voids in the liquid.

When you have a pressurized vessel, you don't really want any extra pressures that you can't expect. The collapse of a void/cavitation can set up pressure waves that may cause over-pressure in a weak spot in the hull or, in this case, uneven burning of the fuel.

In an extreme case of many trusses, you might end up with an unintended baffle system, where it restricts the fuel flow so much that it slows the burn, potentially causing the engine to stall. Semi-tankers use baffles to help prevent sloshing so they can brake and accelerate easier. In the case of a rocket, you aren't stopping and starting a lot, so sloshing shouldn't be a problem, especially with essentially a single force acting on it in the direction the fluid needs to go.

Also, rockets have a problem named after them: The Tyranny of the Rocket Equation. It says that the more weight you have, the more fuel you need and the more fuel you need, the more fuel you need to lift the more fuel you need to lift the weight of the fuel you need to lift the other parts of your rocket. Simple, right? Trusses add weight to the rocket, so if you can do without them, you're better off. As other people mention, the skin of the tank and ship are more than enough to deal with the pressures and forces involved. If they weren't, the engineers would have added the necessary trusses. ;-) Or used a stronger material.

When you have a reusable tank that is recovered after a mission, you have to inspect it inside and out. With more surfaces and joints/seams/welds to inspect, you increase the time, effort, and cost to do the inspection. If there are any coatings to prevent the fluids from eating away the tank, you increase the cost of originally applying and any need for reapplying them by adding structure to the inside of the tank.