totm nov 2023 SpaceX Discussion Thread

[RCgothic]

10 minutes ago, sevenperforce said:

They could gimbal quite close to each other for roll control, particularly if the center two engines were the only ones lit. But still probably not quite that close; you're right. And the angle would be offset anyway.

It's there anything stopping all the engines gimbaling even with only 2 lit?

[sevenperforce]

17 minutes ago, RCgothic said:

It's there anything stopping all the engines gimbaling even with only 2 lit?

Well, technically the engines need an operating hydraulic line to gimbal.

But in this particular case I was thinking of two central engines lit simultaneously and attempting to provide the maximum roll authority; they would point exactly opposite each other but at an angle.

[RCgothic]

20 minutes ago, sevenperforce said:

Well, technically the engines need an operating hydraulic line to gimbal.

But in this particular case I was thinking of two central engines lit simultaneously and attempting to provide the maximum roll authority; they would point exactly opposite each other but at an angle.

They couldn't interfere with each other that way, and any other TVC engines in the way could just gimbal out the way.

[sevenperforce]

6 minutes ago, RCgothic said:

They couldn't interfere with each other that way, and any other TVC engines in the way could just gimbal out the way.

I'm just not sure whether 0.51m would be sufficient clearance at that point.

Not concerned about other engines.

[magnemoe]

1 hour ago, mikegarrison said:

Car parts that are welded on are not meant to be replaced by mechanics. (We call replaceable parts "LRUs" in the airplane business -- "line replaceable units". LRUs are not welded in!) Sure, body shops might cut them off and weld a new one on after a crash, but that's usually never quite the same as when it was first built.

Mudguards are parts who you kind of expect to be replaced during an cars life time, rust or bumping into something. Main cost is the paint job. 
And they are not structural.
And yes people who want to fix their out car hate this. And an replacement is more expensive at an garage but again the paint job is more expensive. 
An reason to go for an standard color as they tend to be stock.  

Now for planes its other rules, you can not simply brake and pull over. 

[SOXBLOX]

I thought that with Starship, the hydraulic fluid was propellant or oxidiser? Am I wrong? Perhaps I misunderstood...

[sevenperforce]

3 minutes ago, SOXBLOX said:

I thought that with Starship, the hydraulic fluid was propellant or oxidiser? Am I wrong? Perhaps I misunderstood...

Both Raptor and Merlin use the fuel (CH4 and RP-1 respectively) for closed-loop hydraulic fluid. However that means the engine must be running in order to operate TVC for the engine.

It's possible, I suppose, that they have a redundant pressurization loop for hydraulics that runs between engines, but I feel like that would produce more failure modes than it would solve.

[SOXBLOX]

9 minutes ago, sevenperforce said:

Both Raptor and Merlin use the fuel (CH4 and RP-1 respectively) for closed-loop hydraulic fluid. However that means the engine must be running in order to operate TVC for the engine.

It's possible, I suppose, that they have a redundant pressurization loop for hydraulics that runs between engines, but I feel like that would produce more failure modes than it would solve.

Ah. So no gimballing for engines which are not running, then.

[mikegarrison]

1 hour ago, RCgothic said:

It's there anything stopping all the engines gimbaling even with only 2 lit?

I think that it only makes sense, for failure tolerance reasons, that the gimbal clearance should be unrestricted even if another engine is frozen in place. Let's say the engines that can gimbal all go hard over in one direction, and then one of them has a failure and it is locked in place. If that prevents the other engines from still having full authority, that means such a failure could be a single-failure loss of vehicle.

Obviously, this same scenario means that the engine pointing in the wrong direction would need to be immediately shut down and replaced by an engine that could still gimbal, or maybe the incorrect thrust vector would need to be compensated for by all the other engines, or ...

Bottom line is that I don't think I'm going to sign up to be inside a vehicle that is making a propulsive landing any time soon. There are so many ways this can go wrong, as we know from those "landing is hard" Falcon 9 montage videos.

Edited September 2, 2020 by mikegarrison

[Kartoffelkuchen]

What you have been forgetting in your discussion so far is that even if Thermal expansion of the nozzle is restricted due to cooling, it'll also have to cope with some sort of reentry heating where the engines will not be continously running, be cooled.

And yeah it just doesn't make any sense to weld the outer ones together from a reusablity and reliability standpoint.

[sevenperforce]

This is the densest packing I can find if we give each engine 51 cm clearance:

9.4 meter skirt with one engine set under each leg.

[mikegarrison]

1 hour ago, sevenperforce said:

This is the densest packing I can find if we give each engine 51 cm clearance:

9.4 meter skirt with one engine set under each leg.

This still has problems with the failure mode I mentioned, where an engine gets stuck in a full-gimbal position. You would need to make sure every one of those circles doesn't overlap.

It's very likely that they will have to end up accepting that there are some gimbal failure modes they can't overcome. This is for the uncrewed booster, right?

[zolotiyeruki]

I imagine if an engine has a gimbal failure, it would be shut off, so the other engines would not need to compensate for that engine's errant thrust.  Of course, they'd have to compensate for the loss of thrust from that engine.  Let's say the engine at 12 o'clock gets stuck with the bell towards the center, and it gets shut down.  The other center engines would have to push their bells toward the 6 o'clock direction to realign the center of thrust with the center of mass.  That's _away_ from the failed engine.

Alternatively, you could just shut down the engine that's radially opposed, and then the rest of the central engines don't have to compensate at all...

Realistically, the only time I'd expect any significant gimballing would be during the landing burn.  I imagine that the boostback and entry burns have a large-enough margin for error, and are long enough, that smaller gimbal angles would be required.

[tater]

13 minutes ago, zolotiyeruki said:

Realistically, the only time I'd expect any significant gimballing would be during the landing burn.  I imagine that the boostback and entry burns have a large-enough margin for error, and are long enough, that smaller gimbal angles would be required.

The entry burn on F9 is 3 engines. The center one, and 2 lateral engines canted outwards.

If they only needed one, they could have the center engines gimbal as a unit.

[magnemoe]

4 hours ago, sevenperforce said:

This is the densest packing I can find if we give each engine 51 cm clearance:

9.4 meter skirt with one engine set under each leg.

Why the 4+4 engines center rater than 1+6 engines? As in is this something official? 
1+6 is more compact. 

[tater]

4 minutes ago, magnemoe said:

Why the 4+4 engines center rater than 1+6 engines? As in is this something official? 
1+6 is more compact. 

Musk said 20 outside, 8 inside.

[tater]

 

 

 

 

[kerbiloid]

4 hours ago, tater said:

Musk said 20 outside, 8 inside.

FalcoN-1 aka Starship.

Spoiler

 

[RealKerbal3x]

SN10 parts already! They sure are moving fast!

 

[RealKerbal3x]

Starlink webcast is live:

 

Edited September 3, 2020 by RealKerbal3x
It didn't embed, weird.

[sevenperforce]

I never, ever get tired of that gorgeous lift off and hearing “the vehicle is pitching downrange.”

[tater]

Woot!

Nominal orbital insertion.

[RealKerbal3x]

I was like 'where's the landing video?', and just as I said that we got a fantastic view of the landing

[sevenperforce]

Just now, RealKerbal3x said:

I was like 'where's the landing video?', and just as I said that we got a fantastic view of the landing

Yep, it switched over just in time. Flawless as usual.

I would have to compare to prior starlink missions to be sure, but it felt like the time from entry burn shutdown to transonic envelope and landing was much, much shorter than usual. Did anyone else pick up on that?

[tater]