totm nov 2023 SpaceX Discussion Thread

[DDE]

6 hours ago, tater said:

The best thing about this whole situation is that they are just building the %$#@ thing.

For my entire life being interested in spaceflight, stuff like this has been "artist conceptions" and "paper rockets," then "powerpoint" and renders. It's like the ULA/Bigelow video about B330 around the moon. Cool render, bros, but that ending—"for only X billion tax $ we can start making this a reality!" (paraphrase)

That is what makes the whole affair compelling... But it is a (secondary) factor driving the skeptics.

Whoah, we just made it 1001.

[kerbiloid]

9 hours ago, sevenperforce said:

Also, Starship likely will not be able to dock directly to LOP-G due to transverse loading limits on the docking port. After the ISS was constructed, the Shuttle could only dock to the end node for this reason.

If I get this right, the IDSS standard allows up to 375 t heavy things to attach.(pg 1-1)

https://www.internationaldockingstandard.com/download/IDSS_IDD_Revision_E_TAGGED.pdf

[RCgothic]

16 hours ago, sevenperforce said:

Starship is 9m standard and I believe the official renders show a 10m skirt. 

He says the outer ring are fixed and do not gimbal at all. So you only need space for TVC in the middle. They want a LOT of control authority on Starship, so we could conceivably be looking at 10 degrees or more in any direction, and they wouldn't do different engines on the booster.

Do you mean centimeters? Or are you talking about something different?

Yes, I meant centimeters. I keep getting the diameter confused with the orange one. 9m works fine!

 

15 hours ago, tater said:

Center engines can hang down for gimbal room...

Plus this completely fixes any residual space issues.

[RealKerbal3x]

That's a lot of damage steel rings! Presumably at least some of them are for Super Heavy...

 

[tater]

4 hours ago, RCgothic said:

Yes, I meant centimeters. I keep getting the diameter confused with the orange one. 9m works fine!

 

Plus this completely fixes any residual space issues.

Musk said outer 20. How far out do you have to bump them for a circle of 20?

 

[AVaughan]

23 minutes ago, tater said:

Musk said outer 20. How far out do you have to bump them for a circle of 20?

Assuming 1.3 metre engine bells in contact with each other, I make it a circle of 9.61 metres.  That should be the diameter of the circle touching the outside edge of the engine bells, so also add any gap between the engine bells and superheavy's outer skin/skirt, plus the thickness if that outer skin/skirt (if any).   (Note it's been a long times since I've done any trig, so take that figure with a grain of salt, especially if someone else disagrees).

[AVaughan]

Any theories as to why such a big discrepancy between the thrust of the outer 20 and the central 8?  Surely thrust vector control wouldn't affect max thrust that much?  Is this because they want those central engines to deep throttle for landing, and they don't need deep throttling on the outer 20?   Or maybe because they don't want to stress engines that need to relight for landing as hard as the the outer 20?

 

Also welding engine bells together and/or to the skirt worries me.  It seems like a recipe for extra stress when the engine bells expand during launch.  I hope they get their models right during design, and that this doesn't come back to bite them later. 

[SOXBLOX]

Welding is ridiculous! The vibrations and thermal expansion will tear them apart! Are they really doing that!?!?

[sevenperforce]

2 hours ago, tater said:

Musk said outer 20. How far out do you have to bump them for a circle of 20?

A "ring" of outer 20 doesn't necessarily mean that they would be in a perfect ring. But if they are, that's a minimum diameter of 9.6 meters.

2 hours ago, AVaughan said:

Any theories as to why such a big discrepancy between the thrust of the outer 20 and the central 8?  Surely thrust vector control wouldn't affect max thrust that much?  Is this because they want those central engines to deep throttle for landing, and they don't need deep throttling on the outer 20?   Or maybe because they don't want to stress engines that need to relight for landing as hard as the the outer 20?

It's entirely a matter of deep throttling. Raptor is a gas-gas engine and so combustion efficiency (which is the usual problem with downthrottled engines) is a non-issue. It also has extremely high chamber pressures so unstart (where the supersonic transition moves inside the throat) isn't a problem either. Elon has said that for Raptor, the limitation on downthrottle happens in the preburners. If you design a preburner that doesn't need to downthrottle (or only needs to downthrottle to, say, 90%), it can be MUCH more robust and produce much more thrust than a preburner which needs to handle high throttle transients.

Raptor also uses its turbopump to drive hydraulics for the TVC, so it's possible that going too high on thrust would overstress the TVC, but I doubt it.

1 hour ago, SOXBLOX said:

2 hours ago, AVaughan said:

Also welding engine bells together and/or to the skirt worries me.  It seems like a recipe for extra stress when the engine bells expand during launch.  I hope they get their models right during design, and that this doesn't come back to bite them later. 

Welding is ridiculous! The vibrations and thermal expansion will tear them apart! Are they really doing that!?!?

Elon has already said that they will weld the RVacs to Starship's skirt, so we know that's the plan.

These are regeneratively-cooled engine bells so they should not experience thermal expansion. They are also much more rigid than the radiatively cooled engine bell we're used to seeing on the MVac.

[RCgothic]

1 hour ago, SOXBLOX said:

Welding is ridiculous! The vibrations and thermal expansion will tear them apart! Are they really doing that!?!?

No positive confirmation for booster, but it has seriously been suggested by Elon for RVac on the Starship (plus double bell) as a way of stabilising the bells against turbulent flow separation for sea level aborts.

[tater]

1 hour ago, sevenperforce said:

A "ring" of outer 20 doesn't necessarily mean that they would be in a perfect ring. But if they are, that's a minimum diameter of 9.6 meters.

Renders have shown a flair, so...

Also, 9.6m is the outside circle that they are inscribed within, right? (skirt diameter) That would put the engine centers well inside 9m. The bells sticking out are no big deal, and fairing (skirt) could be a thing.

[mikegarrison]

1 hour ago, sevenperforce said:

These are regeneratively-cooled engine bells so they should not experience thermal expansion. They are also much more rigid than the radiatively cooled engine bell we're used to seeing on the MVac.

Everything experiences thermal expansion if it gets hot, and they will get hot.

They are unconstrained for growth in the Z direction, so that's fine. They will experience hoop stresses, though. (Welded or not.)

3 hours ago, SOXBLOX said:

Welding is ridiculous! The vibrations and thermal expansion will tear them apart! Are they really doing that!?!?

Welding the engines together is ridiculous for the started goal of a rocket that is as reusable as an airplane. When an engine fails (and it will), what then? How do you swap in a spare if it's welded to all the other engines or welded to the ship? Any part that is welded to any other part needs to be considered an entire assembly.

Edited September 2, 2020 by mikegarrison

[sevenperforce]

22 minutes ago, tater said:

Renders have shown a flair, so...

Also, 9.6m is the outside circle that they are inscribed within, right? (skirt diameter) That would put the engine centers well inside 9m. The bells sticking out are no big deal, and fairing (skirt) could be a thing.

Raptor is a really big engine though.

The above image is a early dev model, so it has a lot of wiring and such that is not present in flight articles, but the overall footprint/cross-section is going to be very similar. They will want to put the engine itself in a frag shield, so the cross-sectional footprint of the engine is not actually going to be significantly smaller than the cross-sectional footprint of the bell itself. The Merlin 1D, which is GG, has a much smaller single turbopump and a much smaller upper footprint.

While we're on the subject of dimensions...we know from the video of the Starship hop test that the entire engine is actuated via TVC rather than the nozzle alone. We know we need to have 10 degrees of gimbal in any direction, so we can calculate the gimbal footprint:

Spoiler

The sine of ten degrees is 0.174, so the horizontal displacement is 0.51 meters.

You need 0.51 meters of clearance between a gimballed Raptor and any fixed engine, and 1.02 meters of clearance between any two gimballed Raptors.

10 minutes ago, mikegarrison said:

Everything experiences thermal expansion if it gets hot, and they will get hot.

Will they though?

Spoiler

 

 

[mikegarrison]

2 minutes ago, sevenperforce said:

Will they though [get hot]?

OK, I should have spoken better. "Any solid will experience thermal stress if it is constrained to move when it changes temperature."

Whether that change is "getting hot" or "getting cold" doesn't matter. It will want to grow or shrink, and if it constrained from doing either then it will experience a stress.

Some materials are less prone to thermal strain than others, and therefore develop less thermal stress than others, but most metals are among the materials with the highest thermal strain.

[sevenperforce]

Just now, mikegarrison said:

OK, I should have spoken better. "Any solid will experience thermal stress if it is constrained to move when it changes temperature."

Whether that change is "getting hot" or "getting cold" doesn't matter. It will want to grow or shrink, and if it constrained from doing either then it will experience a stress.

Some materials are less prone to thermal strain than others, and therefore develop less thermal stress than others, but most metals are among the materials with the highest thermal strain.

Sorry, not trying to be a smartass, although I think I probably was. My point is that regenerative cooling will limit the thermal range of the outer edge of the engine bell and therefore will all but eliminate significant expansion or shrinking. 

[mikegarrison]

17 minutes ago, sevenperforce said:

Sorry, not trying to be a smartass, although I think I probably was. My point is that regenerative cooling will limit the thermal range of the outer edge of the engine bell and therefore will all but eliminate significant expansion or shrinking. 

And I'm skeptical about that. Very much so.

Let's say it's sitting on the ground at ambient temperature. Then you flow a bunch of fluid through it at 100 kelvin. Then you put a hot fire in the middle of it. It's going to go through some thermal cycling there.

The only way around that is if the cooling is actively controlled, and they only flow enough coolant to counter the heat flux, but I'm guessing that's not what is happening here.

[magnemoe]

46 minutes ago, mikegarrison said:

Everything experiences thermal expansion if it gets hot, and they will get hot.

They are unconstrained for growth in the Z direction, so that's fine. They will experience hoop stresses, though. (Welded or not.)

Welding the engines together is ridiculous for the started goal of a rocket that is as reusable as an airplane. When an engine fails (and it will), what then? How do you swap in a spare if it's welded to all the other engines or welded to the ship? Any part that is welded to any other part needs to be considered an entire assembly.

Pretty sure it was a bit literal, now they could have some flanges who are weld together and can be easy cut. 
Yes you could use bolts here, perhaps they do or the bolts become an maintenance check point. Flanges also handles the thermal gradient well. 

Many car parts like mudguard tended to be welded on cars today, on old cars they tended to be bolted on but its easier for robots to weld also an 2-3 mm error will not cause issues. 

[mikegarrison]

17 minutes ago, magnemoe said:

Many car parts like mudguard tended to be welded on cars today, on old cars they tended to be bolted on but its easier for robots to weld also an 2-3 mm error will not cause issues. 

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.

Edited September 2, 2020 by mikegarrison

[RCgothic]

1 hour ago, sevenperforce said:

Raptor is a really big engine though.

The above image is a early dev model, so it has a lot of wiring and such that is not present in flight articles, but the overall footprint/cross-section is going to be very similar. They will want to put the engine itself in a frag shield, so the cross-sectional footprint of the engine is not actually going to be significantly smaller than the cross-sectional footprint of the bell itself. The Merlin 1D, which is GG, has a much smaller single turbopump and a much smaller upper footprint.

While we're on the subject of dimensions...we know from the video of the Starship hop test that the entire engine is actuated via TVC rather than the nozzle alone. We know we need to have 10 degrees of gimbal in any direction, so we can calculate the gimbal footprint:

  Hide contents

The sine of ten degrees is 0.174, so the horizontal displacement is 0.51 meters.

You need 0.51 meters of clearance between a gimballed Raptor and any fixed engine, and 1.02 meters of clearance between any two gimballed Raptors.

Will they though?

  Hide contents

 

 

Pretty sure there's no situation where you'd want Raptors to Gimbal towards each other. They'd be fighting each other's control.

[sevenperforce]

23 minutes ago, mikegarrison said:

And I'm skeptical about that. Very much so.

Let's say it's sitting on the ground at ambient temperature. Then you flow a bunch of fluid through it at 100 kelvin. Then you put a hot fire in the middle of it. It's going to go through some thermal cycling there.

The only way around that is if the cooling is actively controlled, and they only flow enough coolant to counter the heat flux, but I'm guessing that's not what is happening here.

This analysis of a regenerative cooling sleeve in a methalox engine arrived at thermal minimums and maximums of 120-295 K. I'm not entirely sure what the engine bells are made of, but let's assume that they are copper (it has the most extreme thermal expansion of any metal that would likely be used). Cooper's fractional thermal expansion coefficient at 293.15 K is 1.7e-5 per degree or 0.0000017, so we're looking at something on the order of 0.298% expansion over the entire possible thermal gradient.

[mikegarrison]

1 minute ago, RCgothic said:

Pretty sure there's no situation where you'd want Raptors to Gimbal towards each other. They'd be fighting each other's control.

Yeah, that would be a failure mode, not anything intentional.

[sevenperforce]

3 minutes ago, RCgothic said:

1 hour ago, sevenperforce said:

You need 0.51 meters of clearance between a gimballed Raptor and any fixed engine, and 1.02 meters of clearance between any two gimballed Raptors.

Pretty sure there's no situation where you'd want Raptors to Gimbal towards each other. They'd be fighting each other's control.

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.

[SOXBLOX]

1 minute ago, sevenperforce said:

This analysis of a regenerative cooling sleeve in a methalox engine arrived at thermal minimums and maximums of 120-295 K. I'm not entirely sure what the engine bells are made of, but let's assume that they are copper (it has the most extreme thermal expansion of any metal that would likely be used). Cooper's fractional thermal expansion coefficient at 293.15 K is 1.7e-5 per degree or 0.0000017, so we're looking at something on the order of 0.298% expansion over the entire possible thermal gradient.

Sure. But the vibrations? And replacement concerns, as voiced by @mikegarrison above. 

[mikegarrison]

Just now, sevenperforce said:

This analysis of a regenerative cooling sleeve in a methalox engine arrived at thermal minimums and maximums of 120-295 K. I'm not entirely sure what the engine bells are made of, but let's assume that they are copper (it has the most extreme thermal expansion of any metal that would likely be used). Cooper's fractional thermal expansion coefficient at 293.15 K is 1.7e-5 per degree or 0.0000017, so we're looking at something on the order of 0.298% expansion over the entire possible thermal gradient.

What's important is the stress, not the theoretical strain. Obviously more "desired" strain means higher stress in the same material, but since the stress/strain modulus varies and so does the strength, it's not just a matter of looking at which material expands the most.

A range of 200 kelvin isn't a huge range of temperatures for an application like this, so yeah, it's probably manageable. But it's still thermal cycling.

[SOXBLOX]

Also, spot welds aren't known for being extraordinarily heat resistant.