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5 hours ago, YNM said:

Yeah, they could just make a good amount of spare and roll them through the inspection-repair-recertification process.

Well, how fast are they turning around F9/FH Block 5 boosters right now ?

 

Honestly, when I think again about why are they suspending a tank in the air, there's one advantage to this than supporting it from the bottom : the walls of the tank are under tension when suspended vs. under compression when supported from the bottom. Steel under tension cannot undergo buckling, unlike steel under compression...

Does this mean that to some extent they're doing a balloon tank sort of thing ? Centaur stages have stiffeners next to them to resist compression when unfueled, here they just keep them hanging.

All rockets uses pressurization for strength, not at the level as the old atlas upper stages as they can stand empty without issues. 
However they definitively need the added strength during max q and you get this strengthening for free. 
As for the centaur, I guess the stiffeners are to support the payload on top before fueling. 

Now I'm a bit surprised starship don't have any stiffening at all with its huge volume and thin walls. The nose has stiffing because of max q and the skirt who need to hold the weight of the fully fueled ship.
The tanks will be pressurized during decent to for strength, no idea of pressure, might be 6 bar or lower. 

Edited by magnemoe
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3 hours ago, magnemoe said:

The tanks will be pressurized during decent to for strength, no idea of pressure, might be 6 bar or lower. 

And they'd lose all this pressure upon touching down and depressurization...

Hanging them down sounds like a pretty good plan if they really want to make it as balloon-ish as possible. You can train the PID controls, you can change the grid fin size and strength, you can even build windbreaks (idk if they're considering that). But the thickness of the tank is a basic feature of the forces acting on a rocket...

Also, perhaps just to be clear if anyone is wondering, tank pressurization doesn't increase the strength of the material per se - what it does is that it ensures the material remain in tension and not compression due to ring stresses from the pressure inside. Doing this means that buckling of the material is avoided - and buckling is not a good thing, it lowers the usable strength of your material down because it happens without any yielding happening beforehand.

Edited by YNM
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3 hours ago, JoeSchmuckatelli said:

Why do we pressurize with helium and not nitrogen? 

It would depend on the propellants being used I think.  Nitrogen liquifies at -195.8 Celsius, so it’s no good for pressurizing liquid hydrogen, for example. Nitrogen also dissolves in oxygen so it’s not the best choice for pressurizing LOX.  For other propellants it’s probably fine.

Helium is inert, very low density (i.e. doesn't tend to mix with the propellants) and low boiling (good with cryogenic propellants), so it’s a good all rounder if you don’t mind the cost.

I think.

 

Edited by KSK
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17 hours ago, tater said:

Grid fin catch? Use cables.

Use 2 V shapes, open ends overlapping, but like <> where there are arms at the "top" of the V and the base, and the actual V is cables. By changing the position of the arms you control the spacing. Cables can deal with the loads like arrestor cables for carrier aircraft.

Arrestor cables are a good place to start but there's a degrees-of-freedom problem. For a carrier tailhook system, gravity and friction with the deck damp any motion perpendicular to the direction of travel. But with a vertical "midair" catch with motion in the z-axis, you can have motion in the x and y axes that remains undamped.

If the wires provide vertical damping in the same way as the arrestor cable on a carrier, then Superheavy is going to be hanging from rather long cable loops. Any slight timing difference in which cable catches first is going to result in a tremendous torque/rotation, which is amplified by the length of the cables.  I have a mental image of Superheavy being caught by the wires and then swinging back and forth like a 22-story explosive pendulum until the lower end smashes into one of the towers and kablooey.

14 hours ago, YNM said:

Honestly, when I think again about why are they suspending a tank in the air, there's one advantage to this than supporting it from the bottom : the walls of the tank are under tension when suspended vs. under compression when supported from the bottom. Steel under tension cannot undergo buckling, unlike steel under compression...

Superheavy still has to have the heaviest loads during boost when the tanks are under compression, so it has to be strong enough to support itself from the base for that reason.

3 hours ago, JoeSchmuckatelli said:

Why do we pressurize with helium and not nitrogen? 

Helium remains a gas at much higher densities and pressures than nitrogen, which allows it to be stored in a much smaller space and makes the tanks much less heavy.

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1 hour ago, sevenperforce said:

Superheavy still has to have the heaviest loads during boost when the tanks are under compression, so it has to be strong enough to support itself from the base for that reason.

Yeah but you get all the extra ring stresses from the tank pressurization, which generates tension, which negates the compression. Pressurization and compression-tension is a different thing since we're talking about a tank skin that's used to store pressures inside larger than outside.

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18 hours ago, JoeSchmuckatelli said:

Why do we pressurize with helium and not nitrogen? 

[edit] I didn't see that somebody else already provided the answer I was going to provide, about helium being a gas at much lower temperatures than nitrogen.

Edited by mikegarrison
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In addition to the things other about helium people have mentioned, (low density, resistance to being a liquid, inert), probably the most important thing is that helium provides exceptional pressure per kg.

The ideal gas law is PV=nRT.

R is the ideal gas constant, so at similar T and V, P depends solely on "n", the number of atoms/molecules in the volume. Because helium gas has very low atomic weight, it has a very high "n" per kg, better than any other substance except hydrogen (which due to having diatomic molecules is not as ideal a gas). This means you need less mass of helium to pressurise a tank compared with pressurising it with another gas.

High pressure per kg is such a key feature of rocket propellant pressurant. 

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12 hours ago, Flying dutchman said:

Imho this could Just be posted in the SpaceX discussion thread..

And so it is.

Overlapping threads have been merged

 

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