totm nov 2023 SpaceX Discussion Thread

[AckSed]

Are the water sprinklers on the drone ship a new thing?

[darthgently]

1 hour ago, AckSed said:

Are the water sprinklers on the drone ship a new thing?

Apparently not given some discussions on X but I didn’t notice until the recent landing myself.  I wonder if they went from merely spraying the deck to the integrated bubblers recently?  But honestly I don’t recall seeing any water until about now

[tater]

 

[tater]

 

[StrandedonEarth]

Looks like more of a matte finish instead of polished. Possibly thermal reasons?

I noticed on one of the clips of IFT5 returning, not only the engines were glowing yellow, and unsurprisingly the grid fins, but even the intertank section appeared to be glowing yellow, before fading to orange and red…

Edited December 6 by StrandedonEarth

[JoeSchmuckatelli]

2 hours ago, StrandedonEarth said:

Looks like more of a matte finish instead of polished. Possibly thermal reasons?

I noticed on one of the clips of IFT5 returning, not only the engines were glowing yellow, and unsurprisingly the grid fins, but even the intertank section appeared to be glowing yellow, before fading to orange and red…

matte finish,,, Yeah - that's interesting; gotta be a reason.

glowing... - Now I'm wishing for a thermal camera view of SH Booster returning.

... ... ...

So - I looked at the image @tater posted above - and all the vertical lines/dots on the segments (where I'm guessing there are stringers) are those weld lines?  Surely not rivets.  Also - are there MORE of them than usual?  i.e. are there more stringers now than previous iterations - or have I just not seen such a good view for a while and did not notice?

[darthgently]

1 hour ago, JoeSchmuckatelli said:

matte finish,,, Yeah - that's interesting; gotta be a reason.

glowing... - Now I'm wishing for a thermal camera view of SH Booster returning.

... ... ...

So - I looked at the image @tater posted above - and all the vertical lines/dots on the segments (where I'm guessing there are stringers) are those weld lines?  Surely not rivets.  Also - are there MORE of them than usual?  i.e. are there more stringers now than previous iterations - or have I just not seen such a good view for a while and did not notice?

Something about polished vs rough surfaces and radiative cooling?  Idk

[Ultimate Steve]

It is very clean looking. Quite a stark contrast to the early days when they thought they could get away with significantly more than they ended up getting away with.

[darthgently]

1 hour ago, Ultimate Steve said:

It is very clean looking. Quite a stark contrast to the early days when they thought they could get away with significantly more than they ended up getting away with.

In hindsight, I’m seeing a lot of the earlier work more as SpaceX mostly growing a skilled workforce for things that hadn’t been tried from scratch.  Multiple things were going on in parallel:

Assessing current skill levels, shaking out who would be leading and who would be following on various teams, shaking out build techniques and material variations.  But perhaps mostly keeping people busy and moving forward to keep attitude up.  It’s ok if it is messy at first as the goal was not to land on Mars with those products, but to learn from honest appraisals of failure

[tater]

 

[Ultimate Steve]

Sooooooo, remember that little drama from a few months back about how SpaceX was using preburner exhaust to directly pressurize the tanks?

Some people: "This would explain the engine issues with Super Heavy on flights 2 and 3 and possibly the Starship RCS issues on flight 3, and would also explain why the heat exchanger hardware went missing"

Some other people: "There's no way they would be that mind numbingly stupid to send several tons of ice into their tanks? How the hell did these idiots think that was a good idea if they actually did do that"

Elon: "We are using the preburner exhaust to pressurize the tanks" (from Everyday Astronaut video, not exactly said but highly strongly implied)

Some people: "This probably saves on preburner mass"

Some other people: "So they really are that stupid and are doubling down with several tons of ice filters" (I have not seen confirmation of this but I have heard they have a giant metal mesh strung across the bottom of the booster to catch ice chunks and sheets)

 

Well, in arguing against a separate argument, I checked the numbers for Super Heavy's ullage gas mass (or tank gas mass, or whatever you want to call it), and I think I figured out the actual reason why SpaceX ditched the heat exchangers and is having to deal with tank ice!

 

Assumptions: V1 capacity, ideal gas law, Super Heavy's tank is entirely liquid oxygen (I can't be bothered to do the math on methane as well), Super Heavy is pressurized to 7 Bar (6 Bar above ambient, not sure if that is the correct number but I've heard it thrown around).

 

Disclaimers: I keep accidentally writing "degrees Kelvin" despite a Kelvin degree not being a thing, it is just called a Kelvin. Deal with it. I'm not exactly sure what is traditionally done for autogenous pressurization and I'm not exactly well versed on heat exchanger design, so I may get a few things wrong.

 

But say you want to pressurize Super Heavy. It has a volume of roughly 4400 cubic meters (we don't know exactly but I rounded to a nice number near what a 70x9m cylinder would be). Traditionally you would want to be able to hold your tank in equilibrium before launch. Liquid Oxygen is roughly 90K (though SpaceX subcools theirs, I'm not gonna get into that right now), and you would want to pressurize with a gas also at 90K. There's not much that is a gas at that temperature. Let's try Helium. At 90K, that 4400 cubic meters is about 4,167,000 moles. At Helium's molar mass of 4g/mol, the ullage gas in Super Heavy would come in at about 16.6 tons.

If we have had a little bit too much to drink, we could instead try Hydrogen, which would work, and would get the mass down to 4.2 tons with a molar mass of about 1g/mol. However having a tank with both hydrogen and oxygen inside of it is not advisable.

This is great and all, but helium is mega expensive. It costs more than the liquid oxygen does on Falcon 9. That works fine for small to moderate fight rates from Earth with small expendable or partially reusable rockets. But, at Super Heavy scales, planning for a future in which the fuel cost becomes significant, the cost becomes very significant. And helium is a fossil gas. We have a limited amount of it, and the flight rate necessary for even a small Mars colony would vastly outstrip supply - The United States produced about 14,000 tons of helium in 2023. All of the United States' helium put towards Super Heavy launches (without even talking about Starship itself) would be enough for 850 a year. Which is a lot. But the rest of the US, and the rest of the world, needs helium too.

It would be possible to have some sort of reclamation system to recycle the helium after each launch. But I digress. That was a long tangent for something that is not in any way related to my main point.

 

Plus you can't easily get helium on Mars. TLDR: You don't want to use helium, and hydrogen is a bad option. So, that brings us into the wonderful world of autogenous pressurization.

At its most basic, you take some of the liquid propellant and boil it into a gas, and if you keep it at the same temperature (providing only the latent heat of vaporization), you get a nice mixture in thermodynamic equilibrium. However, you do have to take care and control it - with subcooled propellants, you cannot have a mixture in equilibrium, and the gas will gradually cool and, without care, condense back into a liquid, and your tank pressure drops, and your tank collapses.

Hydrogen on the surface seems like a wonderful fuel to do this with. Its absurdly low molar mass means that in theory it should be great at this. However, remember our good old friend PV=nRT? Hydrogen would be in equilibrium at 20K, 4.5x less than the 90K we were talking about earlier, and a hypothetical Hydrogen fueled autogenously pressurized Super Heavy would have 18.7 tons of ullage gas. Still alright. But that 20 Kelvin, that's really killing us.

Back to liquid oxygen. Super Heavy with equilibrium oxygen ullage gas, with it's comparatively extreme 32 grams per mole molar mass, at 90 Kelvin, well, you'd need ONE HUNDRED AND THIRTY THREE TONS of it to fill Super Heavy's tanks!!!

That's absurd! That's a really large percentage of the entire dry mass of the Super Heavy booster! That is completely untenable from a rocket equation perspective to be pushing around 133 tons of extra mass. Note: Methane is 16g/mol so these numbers are artificially more extreme than the true numbers through only considering the oxygen, but I'm lazy so. You do the math if you want to see more accurate numbers.

Looking at PV=nRT, and m=n*molar_mass, we can't really use a different gas, so molar mass is fixed. Thus we must minimize n to minimize n, and in order to do that, we can do 4 things. 1: Alter the laws of the universe to change R (quite unrealistic). 2: Reduce pressure. This means more structural mass to avoid collapsing the booster. Not good. 3: Reduce volume. You can't really do this without carrying less fuel, though subcooling does help to an extent. And then there's option 4... Raise the temperature.

(Note: While writing the previous section I completely forgot that liquid nitrogen is colder than liquid oxygen, so nitrogen is a valid ullage gas, though its molar mass is not much better at 28g/mol, but it doesn't flow as well if I include N2 and I've already tangented too much so I'm leaving it out)

(SECOND NOTE: On Falcon 9, SpaceX subcools the oxygen to 66.5K, lower than the point where nitrogen liquifies. Starship is likely similar, so I accidentally was right but for the wrong reason, nitrogen is not an option)

 

Welcome to the world of, there's probably a technical term for this that I'm not aware of, "Non-equilibrium autogenous pressurization." Maybe you start out with the tanks in equilibrium so you can have better chances at making it through a launch hold, but at launch, you then start flooding the tanks with warmer gas. The same mass of gas at the same pressure will take up more space, so you need less of it! The catch is that now your fuel tanks are dynamic and heat will transfer between your fuel and your ullage gas throughout the flight. Not very much as it is only happening for a few minutes, but the temperature of your gas will change, and your fuel levels will change as, depending on the temperatures, some propellant may convert to ullage gas or some ullage gas may convert to propellant.

TANGENT: This could be a small part of the motivation for continuous acceleration during Super Heavy staging. With an engine cutoff, the liquid and gas would mix and more readily exchange heat, resulting in either a pressure spike due to propellant boiling, or a pressure drop due to ullage gas condensing, depending on how things are set up. This effect would probably by my intuition be minor, but every little bit adds up, and continuous acceleration keeps the surface area between the ullage gas and the propellant fairly constant.

 

The space shuttle did this for its hydrogen. I'm having trouble finding the exact temperature, google isn't turning up much, but one source claims that the hydrogen tank gas averaged around 250K. This is 12.5x hotter, and therefore lighter, than if it was done at equilibrium. If we do the same, and put the Oxygen at 250K, the ullage gas mass goes down to 48 tons! This is amazing progress!

 

Now you're probably thinking "Duh, hotter gas = less mass, that's why they are using the preburner exhaust" and no! That's not quite it. There's more to it than that.

Typically you would use a heat exchanger. Let's say you are heat exchanging between the preburner exhaust and some liquid oxygen in the tank. The issue is that it gets a lot harder to heat exchange with something once it turns from a liquid into a gas. Heat transfer through a wall in Watts is proportional to the surface area. However, once you boil a liquid, its volume dramatically increases, and its speed through the piping would increase as well, unless you did some weird trickery with choked flow to keep it "only" at the local speed of sound (which would still be increasing as you heated it up). You would need more surface area to keep in contact with the same mass for the same amount of time as the density is so drastically different, though presumably the density is still rather high at this point.

That gets worse the hotter you get the gas as it will take up more and more space, so you either have to increase the pressure or keep making the heat exchanger larger and larger with diminishing returns for each extra Kelvin you want to raise the temperature.

Tangent: I know some space missions keep fluids supercritical to avoid stuff like this from being a problem, it might be possible here to avoid much of the heat exchanger scaling issues by keeping the oxygen as a supercritical fluid.

You know what else has diminishing returns? Heat transfer rate is also proportional to the difference in temperature between the two sides of the wall. As the temperature of the oxygen gets closer to the preburner exhaust, you need more heat exchanger are per degree  for each degree closer you get.

AND, the preburner exhaust is also cooling down throughout all this. I have no idea how hot Raptor's preburner exhaust is. But let's say 400K because it is a round number that is close ish to a very old number I had on a spreadsheet. It is probably off but the exact number isn't important. Exactly how cold it gets depends on how the heat exchanger is set up and the ratios between the gases. But the oxygen by definition cannot get up to 400K by cooling something at 400K unless the ratio is infinite. So your 400K preburner exhaust may only get the oxygen up to like 350K (random number, could be 395, could be 300, who knows). And then, that's in ideal conditions. As I stated above, you get diminishing returns for each additional kg of heat exchanger you add, each extra one raises the temperature even less. So you're gonna get a lower temperature than even that.

 

Okay, but how much does that lower temperature affect us?

At 300K, you would need 39,989kg of oxygen. At 301K, you would need 39,857kg of oxygen. That is a 132 kilogram difference.

The difference between, say, 400K and 350K is 4.3 tons!

 

Again, I'm not quite sure what the preburner exhaust temperature is, it could be 600, it could be 250, who knows. And I don't know how close SpaceX can reasonably get to that preburner temperature with a heat exchanger alone. The difference between ONE Kelvin at high temperatures (which are worse for my point) is over 100 kilograms and the benefit is almost certainly several degrees at a minimum. Taking it to the extremes, the difference between what the shuttle got to (250K) and 400K is nearly 18 tons!

 

And the point that I have been building to: This isn't about the mass of the heat exchangers, it is about the mass of the ullage gas! The mass of the heat exchangers is purely an afterthought! This isn't even about the difference between equilibrium temperature and preburner exhaust temperature, this is about the difference between heat exchanged exhaust temperatures and direct exhaust temperatures, which is significant even if it is only a few degrees!

Hundreds or thousands of SpaceX employees labored day and night to save 38 tons in vehicle mass with the gap between Raptor 2 and Raptor 3, or ~3.5 tons counting just the engine side stuff (vehicle side commodities is most of the difference). In a world in which they spent countless hours wizarding even more performance out of Raptor for arguably 3.5 tons of weight shaving, shaving several tons (I'm not claiming to know an exact number but even ten degrees at high temperature is over a ton) of ullage gas mass simply by deleting a part to get higher tank temperatures is a no brainer!

Of course that does come with the costs of ice filters, and possibly having to wait for the ice to melt between flights if they do get to the point of multiple flights per day. But unless they actually have several tons of filters on board it is well worth it.

A tangential benefit is slightly better cold gas thruster performance as the total temperature and therefore exhaust velocity is higher.

I wouldn't be surprised if they intentionally run Raptor's preburner hotter than they would otherwise just to get tank temperatures higher.

 

TLDR: I believe that Starship uses the preburner exhaust directly as tank pressurant because this allows them to get the tank hotter than they would have using a heat exchanger exchanging from the very same gas, without the additional mass of a heat exchanger, but with the additional mass of ice filters. The exact numbers don't matter because the effect is significant at only a few degrees of difference at high temperature, but the effect could plausibly be comically large with a few tens of degrees of difference at moderate temperature.

[DAL59]

Why not use a bladder/piston/diaphragm inside the Starship fuel tanks instead of using gas?

 

[darthgently]

25 minutes ago, DAL59 said:

Why not use a bladder/piston/diaphragm inside the Starship fuel tanks instead of using gas?

 

Mass

[Ultimate Steve]

20 hours ago, DAL59 said:

 

Why not use a bladder/piston/diaphragm inside the Starship fuel tanks instead of using gas?

In both of your scenarios there you still need gas, it is just that there is something between the gas and the propellant. I think bladders are more for zero g fluid management and are useful if you don't have/can't have ullage/settling thrusters.

Going back to the case of Super Heavy, the gas isn't there to help with fluid management, it is there primarily for two reasons, firstly, to take up space. If all that propellant that the engines used was not replaced with something, the declining propellant level would pull a vacuum to an extent inside the tank, and the atmospheric pressure would crush the booster from the outside. Secondly, to provide structural strength. Think about how strong an opened soda can is vs a sealed, pressurized soda can. The walls of the booster are made enormously more resistant against buckling and crippling failures when the booster is pressurized. For an example of why this is important, refer to the SN3 failure. 6 Bar seems a bit excessive for that but hey what do I know.

In a vacuum, you can get away with lower pressures, the limiting factors are going to be the boiling point of your propellant as that changes with pressure, and needed structural strength during maneuvers and re-entry. On the surface it seems like Starship should be able operate at a lower pressure than Super Heavy, but it has to take a lot of sideways loads during re-entry, so I would think it remains highly pressurized for structural reasons. I can't really think of a use for a bladder in this case besides eliminating the need for settling thrusters, which Starship has anyway for reaction control.

[darthgently]

What materials are there for a flexible bladder at cryogenic temperatures?  I’m drawing a blank

[tater]

 

[AckSed]

1 hour ago, darthgently said:

What materials are there for a flexible bladder at cryogenic temperatures?  I’m drawing a blank

Your three-letter acronym to search for is PMD or propellant management devices.

I also found "CRYOGENIC POSITIVE EXPULSION BLADDERS" by Raymond F. Lark, Lewis Research Centre in 1968:

Quote

SUMMARY
Lewis Research Center in-house and contractual experimental programs were conducted to determine the cryogenic flexibility of single and multiple plies of thin polymeric films at -320° and -423° F (77° and 20° K). Results of cryogenic flexibility or "twist-flex" testing showed that thin Mylar films (that is, 0. 5 mil (0. 0013 cm) and less) are the most flexible of the films tested."?The relations between cryogenic cycle-life and film thickness and also for the number of plies of Mylar film are reported at -320 and -423 F (77° and 20° K). "Twist-flex" cycle life of 0.25-mil (0. 00063-cm) Mylar C samples at
-423° F (20° K) range from 20 to 400 cycles, for 1 and 10 plies respectively.
Results of contractual testing of spherical multi-ply Mylar, Kapton, and polyethylene film bladders, 12 inches outside diameter (30. 5 cm), are summarized. Bladder material combinations showing the lowest helium gas porosity after 25 liquid hydrogen positive expulsion cycles are:
(1) Twelve plies of 0. 15-mil (0. 00038-cm) Mylar C in combination with 20 plies of 0.1-mil (0.00025-cm) polyethylene film
(2) Ten plies of 0. 5-mil Kapton in combination with three plies of 2-mil (0. 005-cm) Nomex-Nylon paper.

 

[darthgently]

46 minutes ago, AckSed said:

Your three-letter acronym to search for is PMD or propellant management devices.

I also found "CRYOGENIC POSITIVE EXPULSION BLADDERS" by Raymond F. Lark, Lewis Research Centre in 1968:

 

For disposable rockets this all seems fine, I suppose.  It just seems like an “O-ring” waiting to happen in reusable tanks.  Best part = no part

If the water ice issue ends up reoccurring in nasty ways something like this could be useful to look at

[Ultimate Steve]

Ohhhhh silly me. The bladder is to prevent the ice from making it to the propellant. My bad.

[darthgently]

25 minutes ago, Ultimate Steve said:

Ohhhhh silly me. The bladder is to prevent the ice from making it to the propellant. My bad.

I understood that, but they seem to have that solved for now unless I missed something

[tater]

 

[Minmus Taster]

Just now, tater said:

 

If they don't fill the cargo bay with banana's and release them into space I will be deeply disappointed.

[darthgently]

3 hours ago, Minmus Taster said:

If they don't fill the cargo bay with banana's and release them into space I will be deeply disappointed.

An excellent opportunity to do fruit fly research in orbit

[darthgently]

 

[JoeSchmuckatelli]

You work sixteen hours, whaddaya get? 

... Another day older and deeper in debt
St. Peter, don't you call me 'cause I can't go
I owe my soul to the company store