Tank Pressurization on Starship

Remember when David Spain bragged outrageously? That was Thursday:
On 2020-05-19 8:34 AM, Jeff Findley wrote:
I'm not 100% sure of the details, that's why I said they're "sources of
hot, gaseous, oxygen and hot, gaseous, methane". They may use the
respective exhaust coming out of the turbines of the turbopumps. One
side would be mostly hot oxygen and the other hot methane. But,
obviously, there would be some combustion products mixed in if they go
that route.

The other route would be to have some sort of heat exchanger and just
gasify the LOX into gaseous oxygen and liquid methane into gaseous
methane.

Honestly, I don't know which way they've chosen.

Jeff


I did a little digging and found this "unofficial" block diagram of a Raptor
with tank pressurant feed-lines shown.

https://upload.wikimedia.org/wikiped...Scheme.svg.png

Nifty; black type on a dark grey background. WiPi's picture of the
Merlin engine didn't have that feature.

If this is correct, it is as I believed, they are bleeding off a small amount
of *uncombusted* pressurized *gaseous* methane and oxygen to pressurize the
tanks. Thus there should be no combustion product involved here.

This diagram doesn't show any heat exchangers unless they are positioned on
the tanks themselves, but I suspect this drawing may be fairly accurate,
because they are feeding pressurized gaseous material. I suspect directly
back into the tanks. They'd get some temperature drop when transitioning to
the gaseous phase. But the key is pressure and just enough to keep the tanks
pressurized during operation.

HTH.

Thanks. That's well worth studying despite the color problems.

/dps

--
"I am not given to exaggeration, and when I say a thing I mean it"
_Roughing It_, Mark Twain

On 2020-06-07 9:13 PM, JF Mezei wrote:
Not sure of attriobutions anymore, sorry.

https://upload.wikimedia.org/wikiped...Scheme.svg.png

Is it corect to state that "Methane preburner" is the yellow area with
the text "512 bar, 811K?

Yes

Or does pre-burner include the plumbing to take some liquid methane for
a trip aroiund the engine bell and then back to the bottom of that area
labeled 512bar 811K ?

No. That is for bell and nozzle cooling and pre-heating the methane.

When liquid methane the turbopump, does it exit either to the left,
going to the engine bell to get warm or to the right, going to the input
of the LOX turbopump? Or does some flow straight down to what I assume
is the pre-burner combustion chamber?

The diagram suggests that the flow is around the nozzle and bell housing
before entering the pre-burner. Exit to the left.


In that scmatic, would it be correct to state that the "business end"
from a thrust point of view is the narrow liquid methane pipe from the
methan turbopump that goes to the input of LOX turbopump, the two get
mixed together in turbopump, then go through a heat excanger (warmed up
by the pre burner) and into the main engine combustion to produce thrust?

No. The "business ends" are the yellow and purple parts. The yellow
preburner output of hot gas labeled 321 bar (774K) is methane rich
exhaust from the methane preburner that is capable of further combustion
with additional oxidizer. The purple part labeled 377 bar (748K) is
oxygen rich hot gas from the preburner output of the oxygen preburner.
These are 'partially' combusted gases that get mix at the top of the
nozzle (in pink) are ignited at 300 bar and pass on through the nozzle
pinch to the expansion bell housing. You can visualize the flow by
knowing that gas flows from higher pressure to lower pressure, which
will give you a feel for the flow through the engine, AFTER the
turbopumps. The width of the pipes is also a clue.

The schematic seems to put a lot of emphasis on the pre-burner so not
sure how much of the fuel destined for actual thrust flows through
pre-burner vs going directly to the LOX turbopump.

It all goes through the pre-burners, it is a closed cycle. See this
awesome tutorial about the Raptor and other rocket engine design he

https://everydayastronaut.com/raptor-engine/


Also, silly question, but where do turbopumps get the mechanical energy
to turn and pus liquid to freat pressures?
In the case of mechane, if I read the graph right, would the preburner
output spin a turbine as it travels up and then right to engine and that
turbine then spins the turbp pump that suck liquid methane down?

Yes

In the case of the turbopump for LOX where does it gets it mechanical
energy?

In the same fashion. See the link above for a complete explanation. One
turbopump pre-burner is running methane rich and is powering the liquid
methane turbopump, while the second turbopump preburner is running
oxygen rich and powering the liquid oxygen turbopump. The above link
does a pretty decent job of explaining these differences. By 'rich' the
meaning here is not fully combusted, only partially so. Leaving plenty
of fuel and oxidizer to be combined and combusted in the main engine
chamber.

There are two turbopumps. One for liquid methane and one for liquid
oxygen. If you look closely at the diagram you can see it tries to
depict them simplistically at two white shafts with black lines
depicting the base of the turbopump rotors. For the methane turbopump
it actually looks like there are two axial pump compressors one feeding
the other, the other is the oxygen turbopump with one axial compressor.
Without getting into too much detail, here is a Wikipedia article that
describes how a turbopump works:

https://en.wikipedia.org/wiki/Turbopump

To crudely summarize: A turbopump consists of two axial turbines
connected to a common rotor shaft. One turbine is a compressor and is
used to compress the input fluid (the pump part) and the other turbine
is a true turbine and is using the expansion of a combusted gas to turn
the rotor that powers the pump turbine.

The flow of hot gas out of the preburners is passed through the turbine
part of each turbo pump to power the pumps. Note that the pressure drops
from the input of the preburner to the output, part of that drop is
because the hot gas is doing work to turn the pump rotors.
This is how turbopumps work. Whether it be in a SpaceX Raptor rocket
engine or a "turbocharged" car.

Hope This Helps,
Dave

In article ,
says...

Not sure of attriobutions anymore, sorry.

https://upload.wikimedia.org/wikiped...Scheme.svg.png

Is it corect to state that "Methane preburner" is the yellow area with
the text "512 bar, 811K?

Or does pre-burner include the plumbing to take some liquid methane for
a trip aroiund the engine bell and then back to the bottom of that area
labeled 512bar 811K ?

isWhen liquid methane the turbopump, does it exit either to the left,
going to the engine bell to get warm or to the right, going to the input
of the LOX turbopump? Or does some flow straight down to what I assume
is the pre-burner combustion chamber?


In that scmatic, would it be correct to state that the "business end"
from a thrust point of view is the narrow liquid methane pipe from the
methan turbopump that goes to the input of LOX turbopump, the two get
mixed together in turbopump, then go through a heat excanger (warmed up
by the pre burner) and into the main engine combustion to produce thrust?

The schematic seems to put a lot of emphasis on the pre-burner so not
sure how much of the fuel destined for actual thrust flows through
pre-burner vs going directly to the LOX turbopump.

Raptor is a *full flow* staged combustion engine. All of the LOX and
liquid methane flows through their respective preburners. Some very
tiny amount of oxygen and methane are used for other purposes, like
pressurizing the tanks, but *everything* that goes into the combustion
chamber goes through the preburners. That's why it's called "full
flow".

Also, silly question, but where do turbopumps get the mechanical energy
to turn and pus liquid to freat pressures?

From the preburners. The hot gas from the preburners expands and drives
a turbine. The turbine is connected by a shaft to the pump. Hence the
name turbopump. This is not at all dissimilar from the operation of a
turbojet engine.

In the case of mechane, if I read the graph right, would the preburner
output spin a turbine as it travels up and then right to engine and that
turbine then spins the turbp pump that suck liquid methane down?

Yes.

In the case of the turbopump for LOX where does it gets it mechanical
energy?

Yes.

Jeff
--
All opinions posted by me on Usenet News are mine, and mine alone.
These posts do not reflect the opinions of my family, friends,
employer, or any organization that I am a member of.

On 6/8/2020 6:24 PM, JF Mezei wrote:

But this brings a new question (sorry :-)

The SN4 test firings includded intially only the pre-burners (allegedly,
from the commentators).

That seems possible to me yes.


How can they run only the pre-burners? Wouldn't that run turbopumps that
would push a lot of unburned methane and oxygen out of the engine bell ,
risking ignotion when it sees a spark or a falme at the landing site?

Yes I would assume so. There is a risk of ignition outside the engine
until the methane gas dissipates to below about a 6% methane/air ratio.
Below that it isn't methane rich enough to ignite.

Or can they adjust mixes so that they have full burn of methane in both
the pre-burners so the the exchaust no longer contains combustible gases?


They have a full burn of methane in the LOX turbopump preburner,
remember it's output is oxygen rich, no methane left.

Theoretically you could do a run on the oxygen preburner only with no
risk of explosive gases being ejected out the engine nozzle. Only
oxygen. Not sure the engine is designed to do that however.

They are probably limited on what kind of fuel mixtures they can pass
through the pre-burners. Tinkering with the fuel and oxidizer ratios can
drastically alter the temperatures inside the pre-burner and can effect
the turbines in the turbopumps in highly negative ways. For years it was
thought among US rocket companies and NASA that the effort to design and
operate an oxygen-rich preburner was just too difficult to justify
because of the high temperatures involved.

I believe The SpaceX Raptor was the first serious US attempt at it on a
commercial rocket engine, followed by Blue Origins BE-4.

Yes, this Wikipedia article gives this detail:

https://en.wikipedia.org/wiki/Staged_combustion_cycle

The Russians came out with first with oxygen rich preburners starting
with the S1.5400 and the NK-33, then the RD-170 of which the RD-180 used
in the Atlas-V is a derivative.

Dave

Suggest you watch Tim Dodd's video to also help answer some of your
questions.

On 6/9/2020 10:07 AM, David Spain wrote:
On 6/8/2020 6:24 PM, JF Mezei wrote:
How can they run only the pre-burners? Wouldn't that run turbopumps that
would push a lot of unburned methane and oxygen out of the engine bell ,
risking ignotion when it sees a spark or a falme at the landing site?

Yes I would assume so. There is a risk of ignition outside the engine
until the methane gas dissipates to below about a 6% methane/air ratio.
Below that it isn't methane rich enough to ignite.

Or can they adjust mixes so that they have full burn of methane in both
the pre-burners so the the exchaust no longer contains combustible gases?


Another thought, all the test firings I've seen so far of SN4 have been
very very short. So if they did run a preburner only test, it was for so
short a period of time there was very little volume of gas ejected from
the engine to be that hazardous. Pressure can also be controlled by
limited tank venting as well. Stay below 6% ambient mix and nothing will
happen.

Dave

On 6/9/2020 7:53 AM, Jeff Findley wrote:
Raptor is a *full flow* staged combustion engine. All of the LOX and
liquid methane flows through their respective preburners. Some very
tiny amount of oxygen and methane are used for other purposes, like
pressurizing the tanks, but *everything* that goes into the combustion
chamber goes through the preburners. That's why it's called "full
flow".


Yep. I should have been more clear about that in my answer as well.

Dave