totm nov 2023 SpaceX Discussion Thread

[Rakaydos]

1 minute ago, Xd the great said:

How about magnetic boots?

Not something I'd risk my life on- MILDLY magnetic, so a wrong move will send you back to needing a maneuvering unit.

You just program the rover not to make wrong moves.

[DDE]

13 minutes ago, Rakaydos said:

310 Steel is still mildly magnetic. An aft-cargo welding rover with magnetic wheels should be standard equipment.

This is not a movie I wanted to become a documentary.

[Xd the great]

19 minutes ago, Rakaydos said:

Not something I'd risk my life on- MILDLY magnetic, so a wrong move will send you back to needing a maneuvering unit.

How about an iron man suit?

[sevenperforce]

4 hours ago, Xd the great said:

Jokes aside, do you think the non ablative heat shield/meteoroid shield will work?

And i have seen sources where elon estimates the heat at 1750K. How do they do it?

I'm a little worried about steel's susceptibility to stress corrosion cracking. I worked in fuel transportation safety and steel pipelines are ridiculously vulnerable to SCC.

Micrometeoroids may prove an easier problem with steel than with composite. Notes from Apollo indicate that they felt impact vulnerability was far lower with steel pressure vessels, though there was no other description. Of particular importance is the possibility that the double-wall design could act as a Whipple shield, thoroughly insulating the tanks from damage in all but the largest impact events. If so, I could see SpaceX deciding to do the double-wall etched design across the entire skin of the vehicle, even the leeward side, when building man-rated Starships (as opposed to tankers and cargo launchers). 

 

2 hours ago, Xd the great said:

How about an iron man suit?

Obligatory: 

Spoiler

 

2 hours ago, DDE said:

This is not a movie I wanted to become a documentary.

YES this would actually be a thing, I love it.

[tater]

 

[Cunjo Carl]

19 hours ago, tater said:

I just redid my calc, I thought 5000 was what I used, but it was in fact 1500. I did 2Pirh (no need for the end caps), then checked 50 vs 55m, and rounded down to ~1500.

1500*116 (the apollo tps density/m^2)= 174,000kg (which I rounded up to 200 tonnes (fins, etc)) as an upper bound.

So I was off by a factor of 2, I meant to divide by 2 but forgot.

So upper limit on the order of 100 tonnes (Apollo TPS), and realistically PICA-X for this application is some fraction of that. 20% would be 20 tonnes.

PS--like your EV Nova avatar image.

 

Thanks! The game's pretty simple by today's standards, but the universe it builds is super fun to explore.

 

Don't worry about the nose cone everyone, they're just getting the RUDs out of their system early!

 

After that long post I made, I finally went and read what Musk had been saying, and I think I was a bit off base in one aspect. I still think the coolant will be pushed out in a single pass and not circulated. However, Musk was apparently mentioning that they're only intending to reduce the skin temperature by 300C using this system.

This makes a lot of sense, because having a skin temp of (apparently) 1200C will make even shiny stainless glow bright red hot and release a lot of heat by radiation,  which means we need to waste less fuel to transpiration. Meanwhile, I think my setup (being done with liquid methane) would wind up cooling the skin much more than 300C. To see if my idea is consistent (it isn't), we can estimate a lower bound on the Methane evaporation rate if there was 1200C on the outside of the skin, and the ~ -160C of liquid methane on the inside.  Let's consider just the conduction across the steel skin as a very low-ball estimate- a 4mm steel skin with the 1200C -> -160C temp drop and constant (estimated) properties gives us:  1360K * 20W/m^.K * 750m² /.004m = 5GW . Despite Stainless being a very poor heat conductor for a metal, it's still a ton of cooling this way! How much methane would this translate to? We add the heat of vaporization to the heat of heating it up ( 8200J/mol + 35.8J/molK * 1360K ) / (.016kg/mol) = 3.5MJ/kg = 3.5GJ/ton . Dividing through, my setup would evaporate 1.5+ ton/sec of methane if somehow the outer skin temp were maintained at 1200C! Ha! We could fiddle with the assumed values, but it still way too high.

So what instead?

I think the general idea is fine, but given this high skin conduction problem it seems smart to put an insulating thin film on the inner surface of the steel skin (like Calcium Silicate, enamel or a spray-on sinterable insulator). This will prevent heat conduction through the skin from being problematic, and will let us have the option to use many fewer pores (and less fuel) as a result. Given this extra layer though, we'd probably choose to evaporate the liquid methane before it enters the space between the steel layers. It sounds like a bad idea to allow some liquid drop to accidentally seep into the insulator, then boil inside and delaminate it off. It wouldn't be a loss-of-hull problem, because this would actually make a cool spot on the skin! It'd just a major nuisance. So, let's guess some numbers to see if they can be pushed into sounding halfway reasonable... Let's say we can boil the methane inside the rocket using heat that conducts in during the gentler early parts of reentry by running the liquid methane through narrow tubes around the inside like a regenerative cooling nozzle, and bring it up to room-ish temperature. Now, including the insulator (let's say something very non-space-age like 2mm at 0.2W/mK) the total heat flux through the skin being actively cooled would be 1200K * 750m² * ( 1 /  ((.004m / 20W/m^.K)+(.002m / 0.2W/m^.K)) ) = 88MW   . Meanwhile, the cooling of methane would come to ( 35.8J/molK * 1200K ) / (.016kg/mol) = 2.7MJ/kg = 2700MJ/ton . Dividing through, it comes to 2tons/min. Sounds nice! In this case, the steel skin would be almost entirely the same temperature throughout its thickness at 1200C, and the temperature drop would be almost entirely across the insulator. In this case, the steel would thermally just act as a coating, meanwhile the insulator would prevent the temperature from conducting into the rocket!

 

So it sounds nice at first blush but leads to tricky questions.

Does this ~100MW of cooling sound like a reasonable amount of heat that will need dissipating using methane during reentry? I based it on a random and out-of-context number I saw, but it's a pretty important part of the equation that I don't remotely have a handle on~!

Also, does anyone know how long reentry would last for Starship? I'd been assuming about 10 minutes like the shuttle, but don't really know.

Also, methane starts to pyrolize at ~1000C, forming H* and H2. Both of these are very damaging to steel due to Hydrogen embrittlement. I wonder if/how they plan to handle this? It's actually fairly standard to coat the inside of steel pores using various chemical and electrophoretic processes, but can any of these coatings handle the temp?

Now we're considering a different heat flow regime, what's the new guess for a number of pores? In effect, we're now on the opposite end of the biot number, so we can probably do an order or two less! 3-30M Maybe? A very non-zero issue will also be the pore diameter, which now we'd probably choose in order to get a nice pressure drop across the pores, maybe a couple psi? Too high of a pressure drop is annoying structurally for the steel, but too low is annoying to get uniform flow rates out the pores!

Well, they're all fun things to mull over.

[tater]

 

[magnemoe]

9 hours ago, DDE said:

This is not a movie I wanted to become a documentary.

Robots who walk magnetic is used to clean ships. Might also do welding but only saw them cleaning. Benefit of the cleaning one is that it works under water. 

[tater]

Static fire!

 

[sevenperforce]

1 hour ago, Cunjo Carl said:

After that long post I made, I finally went and read what Musk had been saying, and I think I was a bit off base in one aspect. I still think the coolant will be pushed out in a single pass and not circulated. However, Musk was apparently mentioning that they're only intending to reduce the skin temperature by 300C using this system.

I agree; single-pass, no circulation.

I wonder what the radiative absorption properties of liquid and gaseous methane are like. You have an outer skin (the one with pores) that receives primarily radiative heating from shock-compressed air during entry. That skin heats and radiates in every direction, heating the inner skin. The inner skin is also heated by conduction at the contact points. The inner skin will likely also cool radiatively, sending radiation back into the tank itself.

If liquid methane is fairly transparent to radiative heat but gaseous methane is more opaque, then the CH4 transpiring through the pores will be able to absorb both radiative heat from the shock-compressed air AND radiative heat from the outer layer of skin as the gas blows away. 

After a little digging, it looks like gaseous methane has a generally high transmission spectrum but has absorption peaks at 3500 nm and 7600 nm. Blackbody radiation at 1200C peaks at 2000 nm.

1 hour ago, tater said:

Flexibility on that bulkhead is so cool to see.

[CatastrophicFailure]

Seems there may be a problem...

 

 

[tater]

1 minute ago, CatastrophicFailure said:

Seems there may be a problem...

That's why they do static fires. Find anything they don't like, then fix it.

 

On the hopper front, there are pics at NSF of new, shiny cylinder rings being fabricated. New fairing already partway done. Maybe the second one will happen faster than the first. Clearly they need to make sure everything is braced up and ballasted better. Wonder what the plan is for moving it to fire the engines... Erect a crane towards launch site, swing hopper around. Move crane, repeat?

[CatastrophicFailure]

1 minute ago, tater said:

Wonder what the plan is for moving it to fire the engines... Erect a crane towards launch site, swing hopper around. Move crane, repeat?

Nah, just fire ‘em up and hop on over... 

I think there are cranes specifically for picking things like that up and moving with them, though. It’s big but not that big, in terms of cranes. 

Best they do it on a calm day.

[tater]

 

[Ultimate Steve]

5 minutes ago, tater said:

 

On the hopper front, there are pics at NSF of new, shiny cylinder rings being fabricated. New fairing already partway done. Maybe the second one will happen faster than the first. Clearly they need to make sure everything is braced up and ballasted better. Wonder what the plan is for moving it to fire the engines... Erect a crane towards launch site, swing hopper around. Move crane, repeat?

Where at? In the hopper thread all I can find is confirmation that the bulkhead is on:

Which is great progress, though. I wonder if they could fly it without the fairing if they really wanted to.

[tater]

1 minute ago, Ultimate Steve said:

Where at? In the hopper thread all I can find is confirmation that the bulkhead is on:

My bad, I got confused by the text, and reposting old images. The poster was showing that the shiny metal was straight over some thin framing (hence the super light nature of the fairing). Others had suggested that there was thicker steel under the cone (there was not).

[Xd the great]

9 hours ago, sevenperforce said:

Obligatory: 

  Reveal hidden contents

 

 

Sounds nice.

9 hours ago, sevenperforce said:

I'm a little worried about steel's susceptibility to stress corrosion cracking. I worked in fuel transportation safety and steel pipelines are ridiculously vulnerable to SCC.

Micrometeoroids may prove an easier problem with steel than with composite. Notes from Apollo indicate that they felt impact vulnerability was far lower with steel pressure vessels, though there was no other description. Of particular importance is the possibility that the double-wall design could act as a Whipple shield, thoroughly insulating the tanks from damage in all but the largest impact events. If so, I could see SpaceX deciding to do the double-wall etched design across the entire skin of the vehicle, even the leeward side, when building man-rated Starships (as opposed to tankers and cargo launchers). 

Space is very clean, so as long as they keep the vehicle in a clean room, it should be fine. Maybe they will add some paint to protect it.

19 minutes ago, Ultimate Steve said:

 

Which is great progress, though. I wonder if they could fly it without the fairing if they really wanted to.

Is it just me or is that a heat shield?

[Reactordrone]

49 minutes ago, Ultimate Steve said:

Where at? In the hopper thread all I can find is confirmation that the bulkhead is on:

Which is great progress, though. I wonder if they could fly it without the fairing if they really wanted to.

Going up probably wouldn't be too bad but coming down you want a nice light, draggy bit at the top to improve stability.

[sevenperforce]

1 hour ago, Xd the great said:

1 hour ago, Ultimate Steve said:

Where at? In the hopper thread all I can find is confirmation that the bulkhead is on:

Which is great progress, though. I wonder if they could fly it without the fairing if they really wanted to.

Is it just me or is that a heat shield?

Just you. That's the upper bulkhead.

Which means we know that my first cutaway from yesterday was broadly correct:

Spoiler

Should be able to firm up those internal dimensions nicely now.

[tater]

 

[CatastrophicFailure]

 

That’s hot. 

Also, do we have a launch day yet? Thought I heard 2/23 somewhere. 

[Mad Rocket Scientist]

[Ninja'd]

Is this slowed down after the first second, or is it sped up at the start?

EDIT: Ninja'd by @CatastrophicFailure, by literally one second:

Edited January 25, 2019 by Mad Rocket Scientist

[DDE]

15 minutes ago, CatastrophicFailure said:

Thought I heard 2/23 somewhere. 

Former Day of the Soviet Army?

[CatastrophicFailure]

3 hours ago, Mad Rocket Scientist said:

EDIT: Ninja'd by @CatastrophicFailure, by literally one second:

Spoiler

 

3 hours ago, Mad Rocket Scientist said:

Is this slowed down after the first second, or is it sped up at the start

Slowed, I think, looks like the slo-mo app from a run of the mill iPhone, honestly. 

[NSEP]

4 hours ago, CatastrophicFailure said:

 

That’s hot. 

Also, do we have a launch day yet? Thought I heard 2/23 somewhere. 

Is that a real actively cooled heatshield segment or are they just heating a sheet of stainless steel