totm nov 2023 SpaceX Discussion Thread

[tater]

 

[sevenperforce]

On 1/8/2021 at 5:51 PM, JoeSchmuckatelli said:

The problem I see with a lot of these 'catch the grid fins' designs is that it requires the spacecraft to land exactly on the x. 

We need a solution that can catch a craft that lands a bit off center - and I like your cable thinking - but I don't think it's long enough. 

You need arms that can swing into positions relatively quickly and cables to snap up and take the strain / stabilize the craft - but I don't think you can do it if it's a glorified jack stand and the craft has to thread the needle to get to the pad. 

The design I proposed can actually handle a considerable amount of displacement by adjusting the swing angle and the platform height. For example:

This will handle off-center displacement in any single axis, and the semicircles are wide enough that the grid fins should be able to "catch" in the corresponding arms. They're also wide enough to handle a little bit of off-center rotation.

On 1/8/2021 at 8:10 PM, mikegarrison said:

People are pointing out that the grid fins are designed to take very large aerodynamic loads, which is true. But those are distributed loads, not point loads. Which could be another issue.

The grid fins are welded steel and should be able to handle a wire snag. There's a point load at the booster attachment point anyway. 

[JoeSchmuckatelli]

27 minutes ago, sevenperforce said:

The design I proposed can actually handle a considerable amount of displacement by adjusting the swing angle and the platform height. For example:

This will handle off-center displacement in any single axis, and the semicircles are wide enough that the grid fins should be able to "catch" in the corresponding arms. They're also wide enough to handle a little bit of off-center rotation.

The grid fins are welded steel and should be able to handle a wire snag. There's a point load at the booster attachment point anyway. 

What if you put the bases of your pillars on tracks?  That way as the craft descends, they are well out of the way, and then can be moved in to stabilize the craft once its on the final 'hover'?  I don't think you have to engineer the towers to be able to take up the whole weight of the craft - rather assume the base will rest on the ground and the towers are there to keep the ship from tipping once the rockets are cut.  Making them lighter should allow you to be able to move them fairly rapidly, and with greater precision.

[sevenperforce]

3 minutes ago, JoeSchmuckatelli said:

What if you put the bases of your pillars on tracks?  That way as the craft descends, they are well out of the way, and then can be moved in to stabilize the craft once its on the final 'hover'?  I don't think you have to engineer the towers to be able to take up the whole weight of the craft - rather assume the base will rest on the ground and the towers are there to keep the ship from tipping once the rockets are cut.  Making them lighter should allow you to be able to move them fairly rapidly, and with greater precision.

Strength would seem to be the biggest problem here. Towers large enough to hold all this actuated machinery, which in turn must be robust enough to catch and hold Superheavy, are going to be quite heavy themselves. Trying to put THAT on tracks seems like it would be really tough.

That being said, this is my favorite design so far: 

Presumably one of the arms could rotate less and one of the arms could rotate more if it was coming in off-center. The only things I'm concerned about here are the bending moment on the tower attachment point and the inability to catch it if it's a little further from the tower. 

[JoeSchmuckatelli]

2 hours ago, sevenperforce said:

Strength would seem to be the biggest problem here. Towers large enough to hold all this actuated machinery, which in turn must be robust enough to catch and hold Superheavy, are going to be quite heavy themselves. Trying to put THAT on tracks seems like it would be really tough.

That being said, this is my favorite design so far: 

Presumably one of the arms could rotate less and one of the arms could rotate more if it was coming in off-center. The only things I'm concerned about here are the bending moment on the tower attachment point and the inability to catch it if it's a little further from the tower. 

I look at those kinds of designs, and cannot help but see an errant gust of wind or something else causing the rocket to slam into the tower.  On the other hand, if you had widely spaced towers with a pair (or quad) of slack cables between them that can be drawn in and up after the craft passes a critical point, you can stabilize the craft.  Then all its weight is on a pad, and it doesn't go anywhere.

 

It's the idea of trying to catch the thing and then fully support the weight that I don't think is reasonable - or rather the list of things that has to go just right is too long.  I think it better to go with the idea that the rocket is going to land and have it's legs or skirt be the primary load bearing structure - with whatever design is used being for stabilization (not catching) just to keep it upright - is far more conservative.

[sevenperforce]

8 minutes ago, JoeSchmuckatelli said:

I look at those kinds of designs, and cannot help but see an errant gust of wind or something else causing the rocket to slam into the tower. 

A few pages back I did the math and just two hot-gas thrusters will be able to handle gusts of up to 50 mph. Plus, Superheavy is HUGE so even without thrusters firing, even a tremendous wind gust would only produce a tiny amount of acceleration on the booster (f = ma, so when m is very big, a is very small).

[mikegarrison]

3 hours ago, sevenperforce said:

The grid fins are welded steel and should be able to handle a wire snag. There's a point load at the booster attachment point anyway. 

Bridges are made of welded steel, and yet they can fail if overloaded. I swear sometimes people in this forum seem to ascribe mythical abilities to steel.

I'm just pointing out here that point loads and distributed loads are not the same thing, so it's not automatically true that if the grid fins can take the aerodynamic loads of re-entry then that means they could take the loads of catching and holding the whole booster. It's a different loads problem so it needs a separate analysis.

Edited January 11, 2021 by mikegarrison

[JoeSchmuckatelli]

15 minutes ago, sevenperforce said:

A few pages back I did the math and just two hot-gas thrusters will be able to handle gusts of up to 50 mph. Plus, Superheavy is HUGE so even without thrusters firing, even a tremendous wind gust would only produce a tiny amount of acceleration on the booster (f = ma, so when m is very big, a is very small).

I missed that.  What about turbulence during the final landing?  With an open pad, the exhaust spreads out uniformly as it hits the ground.  Is there any danger from having blowback from the tower? 

[sevenperforce]

2 hours ago, mikegarrison said:

Bridges are made of welded steel, and yet they can fail if overloaded. I swear sometimes people in this forum seem to ascribe mythical abilities to steel.

I'm not ascribing any mythical abilities to steel, nor would I. My point is that aircraft tailhooks are made out of welded steel, too. If you are designing the grid fins to take a wire catch point load, steel is a perfectly fine material to work with. 

2 hours ago, JoeSchmuckatelli said:

I missed that.  What about turbulence during the final landing?  With an open pad, the exhaust spreads out uniformly as it hits the ground.  Is there any danger from having blowback from the tower? 

They could design for that -- whatever the impact of tower blowback would be, you just aim slightly in the opposite direction.

[xebx]

Catching SH ? why not a 27m damped heavy frame, with two rapidly moving parallel parts with heavy airbags on the sides and some gum (tire-like) where large grid fins make contact ?

2 towers rather than 1 can help a lot like others said.

 

* the heavy airbags will deal with the peak impact while the damping (hydraulic) will further extend energy absorption. (SH dry weight 210t and speed <1 or 2 m/s??)

[tater]

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.

[mikegarrison]

2 hours ago, sevenperforce said:

I'm not ascribing any mythical abilities to steel, nor would I. My point is that aircraft tailhooks are made out of welded steel, too. If you are designing the grid fins to take a wire catch point load, steel is a perfectly fine material to work with.

Steel is a fine material, yes, but all I said before is that you can't just assume that since the grid fins are designed to take one kind of high load (aero) that they can automatically take another kind of high load (this catch business). An analysis would have to be done to look at critical load scenarios in both cases (and for any other load scenarios too).

I never said it can't be made to work. All I was doing was responding to a claim that because the grid fins were already designed to take large aerodynamic loads, then it was somehow obvious that they would also be able to withstand this completely different load scenario.

Edited January 12, 2021 by mikegarrison

[YNM]

6 hours ago, JoeSchmuckatelli said:

It's the idea of trying to catch the thing and then fully support the weight that I don't think is reasonable - or rather the list of things that has to go just right is too long.

F9 and FH as-is requires very clear weather both on the launch site and the landing site (well it is one site already if they do the LZ that's closer to the launch pad).

Landing back at the launch site is actually one less parameter to worry about - weather.

But we'll never see this kind of business done in the middle of  a blizzard, like Soyuz do.

5 hours ago, mikegarrison said:

Bridges are made of welded steel, and yet they can fail if overloaded.

They can also fail merely from fatigue. The loads they see may never reach anywhere near the yielding point, but there's a reason why we count lifespan in load cycles and ASD uses merely half the yield point to determine failure.

Then again you'd repair this machine everytime it's done flying... right ?

[mikegarrison]

3 minutes ago, YNM said:

Then again you'd repair this machine everytime it's done flying... right ?

That is not the vision. More like a quick walk-around (or maybe drone-around) for a visual inspection, then refuel and fly again. This would obviously also be dependent on built-in test equipment (BITE) reporting any faults that need to get flagged up for attention.

This leads to the concept of what in airplanes is called an LRU (line replaceable unit). That is, if some system has a hardware failure, can you swap it out right there on the spot or do you need to take the vehicle back to some kind of maintenance facility? Many parts of an airplane can be replaced while the plane is sitting at the airport gate, even when passengers are boarding it.

[paul_c]

There's a scale of how closely you inspect something, though. For a Cessna 172, it might be a 5 minute walk-around. For an SR71, there's thousands of man-hours inspection/repair/other put in per flight. I imagine SuperHeavy will be closer to the SR71 end of the scale than the 172. Not as bad as the Space Shuttle was....but not just a cursory check. Sure, it has complexities a plane doesn't have too, but its all part of the fun.

Regarding steel as suitable/unsuitable, its a well-understood material and I imagine there's some clever people at SpaceX who can work out things like forces and point loadings.

[YNM]

57 minutes ago, mikegarrison said:

That is, if some system has a hardware failure, can you swap it out right there on the spot or do you need to take the vehicle back to some kind of maintenance facility?

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

40 minutes ago, paul_c said:

I imagine SuperHeavy will be closer to the SR71 end of the scale than the 172.

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.

Edited January 12, 2021 by YNM

[magnemoe]

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 January 12, 2021 by magnemoe

[YNM]

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 January 12, 2021 by YNM

[JoeSchmuckatelli]

Why do we pressurize with helium and not nitrogen? 

[kerbiloid]

The nitrogen would solve in blood, while helium would solve but we don't care?

[RealKerbal3x]

Cargo Dragon is away:

 

[KSK]

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 January 12, 2021 by KSK

[wumpus]

3 hours ago, JoeSchmuckatelli said:

Why do we pressurize with helium and not nitrogen? 

I'd have to assume mass would be a big reason.

[kerbiloid]

Maybe because melting points of nitro and oxy are almost equal, and we want something gaseous at that temperature?

[sevenperforce]

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.