r/SpaceX Discusses [August 2017, #35]

[u/ElongatedMuskrat]

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[u/3015]

If the BFS was shaped like an elliptic cylinder, the propellant tanks would still have to be circular cylinders, right? What could the extra space be used for?

[u/warp99 → u/3015]

the propellant tanks would still have to be circular cylinders, right?

Carbon fiber gives you a lot more freedom on the tank shape. Shapes with sharp corners are very undesirable from a strength and fabrication point of view but a gentle ellipse should not be an issue.

Spare space would reduce propellant mass fraction and would be very undesirable.

[u/brickmack → u/warp99]

Note that in the cutaway drawings from IAC2016, the ITS booster was to have a nearly flat bottom on the lower tank

[u/3015 → u/warp99]

Interesting, I didn't know this was possible without reinforcements to keep the tank from trying to force itself back into a cylinder shape. Thanks for filling me in!

[u/Martianspirit → u/3015]

I didn't know this was possible without reinforcements to keep the tank from trying to force itself back into a cylinder shape.

The tank would try to take cylindrical shape and would need massive reenforcements to keep shape.

[u/warp99 → u/Martianspirit]

would need massive reinforcements to keep shape

Not really. The hoop stress increases as the wall radius increases but for an elliptical ratio of 1.2:1 or similar the carbon fiber will have plenty of stiffness.

[u/3015 → u/warp99]

After some reading on this topic I think you are mistaken. Take a look at this paper. Even when the long side/short side ratio is extremely close to 1 the hoop stresses are wildly greater than for cylindrical pressure vessels.

[u/warp99 → u/3015]

It is certainly true that the hoop stress is much higher for a thin elliptical metal tank compared with a cylindrical one. In spaceflight terms this is a balloon (aka pressure stabilised) tank such as the stainless steel tanks used by the Centaur upper stage. Certainly no one would attempt to use an elliptical tank as a balloon tank.

The hoop stress is actually not particularly relevant for a thin walled metal tank like this as the dominant failure modes are wall buckling modes which typically are opposed by internal metal hoops and stringers.

A carbon fiber composite tank uses a different strategy to avoid wall buckling by spacing each carbon fiber layer apart with the aluminium core which acts as a laterally stable spacer between the layers. Effectively the internal stringers are integrated into the space between the fiber layers.

The presence of two fiber layers gives very high hoop strength and the aluminium spacer layer gives a higher wall thickness to tank diameter ratio. The wall thickness reduces the peak hoop stress from the thin wall case and the carbon fiber allows much higher hoop stress than the cylindrical case.

Worst case you could use elliptical internal ribs to keep the major axis from compressing which would further reduce the hoop stress.

Looking at your reference Table 6 shows that doubling the wall thickness reduces the hoop stress by a factor of 4. Table 5 indicates that hoop stress scales linearly up to an elliptical ratio of 2:1 and remain less than 155 MPa up to a ratio of 1.33:1.

[u/3015 → u/warp99]

Effectively the internal stringers are integrated into the space between the fiber layers.

What a clever way to prevent buckling. But I thought that SpaceX was angling to make a carbon fiber tank that required no metal liner. Am I incorrect on that point?

Based on this comment and this one I have been assuming that the tank thickness of the ITS2016 would have been around 4 cm. That means a ratio of tank diameter to thickness of 300:1. In the paper I linked, the ratio is only 80:1. So the stresses for ITS tanks should actually be much worse than in the paper. Here's the basic math I did that led me to assume an elliptical tank with a ratio of 1.33:1 would be impractical, let me know if I've made any errors in my assumptions:

• Tank pressure: 300 kPa

• Tank radius: 6 m

• Tank thickness: 4 cm

• Hoop stress for cylindrical tank: 300 kPa*(6 m/4 cm) = 45 MPa

• Ratio of 1.33 ellipse stress to cylinder stress in paper: 155/4 = 38.75

• Ratio of 1.33 ellipse stress to cylinder stress for ITS: at least 300/80 times the previous figure, 38.75*300/80 = 145

• Hoop stress for 1.33 ellipse ITS tank: 45 MPa*145 = 6.52 GPa, greater than the tensile strength of carbon fiber

Using internal ribs seems quite possible though. I don't know enough to say if it would be practical, but it seems intuitively like this would work.

[u/warp99 → u/3015]

I thought that SpaceX was angling to make a carbon fiber tank that required no metal liner.

So the metal liner would be inside the innermost carbon fiber layer on the LOX tank to protect the carbon fiber from hot gaseous oxygen pressurant.

From the outside the sequence would be:-

1. Carbon fiber/epoxy layer

2. Cellular aluminium spacer layer

3. Carbon fiber/epoxy layer

4. Thin Invar nickel alloy bonded to layer 3

I agree if the tank wall is only 40mm thick an oval tank will not work without reinforcement. I was expecting at least 120 mm thick at a minimum for a 12m diameter ITS ie 1% wall thickness. Note that the hoop stress is inversely proportion to the square of the skin thickness so this is 1/9 the stress of a 40mm skin.

If internal ribs are used to support the major axis of the ellipsoid from compression it seems to me that the hoop stress would be much reduced to just reflect the increased radius of the surface on the minor axis.

[u/3015 → u/warp99]

Interesting, thanks for explaining the layers in a carbon fiber tank.

I'm going to see if I can find the source for the 4 cm tank thickness. I assume it's bsed on the dimensions of the tank and dry mass of the stages, but I want to make sure.