totm nov 2023 SpaceX Discussion Thread

[tater]

If the current Dorian guidance holds, this sketchy facility dodged a bullet.

 

[Ultimate Steve]

5 minutes ago, tater said:

If the current Dorian guidance holds, this sketchy facility dodged a bullet.

Oh, I never knew that Tory Bruno made hurricanes.

[Aegolius13]

2 hours ago, sevenperforce said:

There's no extra fuel/oxidizer; that's why it's a Full Flow Staged Combustion cycle. Something like 90% of the LOX and 10% of the CH4 runs into the oxidizer preburner; something like 10% of the LOX and 90% of the CH4 runs into the fuel preburner. Then 100% of both exhaust flows is routed into the combustion chamber, so you have a purely gas-gas reaction in the chamber, leading to very high efficiency. All of the propellant goes through either one preburner or the other.

I think we're saying the same thing, me not very artfully.  "Extra" was supposed to mean the majority of the fuel / oxidizer that passes through but doesn't get burned in the respective preburner.  Which, presumably, would mix with the hot preburner exhaust such that the mixture is not very hot by the time it reaches the combustion chamber.

[sevenperforce]

5 hours ago, Aegolius13 said:

I think we're saying the same thing, me not very artfully.  "Extra" was supposed to mean the majority of the fuel / oxidizer that passes through but doesn't get burned in the respective preburner.  Which, presumably, would mix with the hot preburner exhaust such that the mixture is not very hot by the time it reaches the combustion chamber.

Hmm. I'm still not sure what you're intimating.

There is no "mixing" with "hot preburner exhaust" because hot preburner exhaust is the only thing that enters the combustion chamber. 

The methane preburner exhaust comes out at 774 K; the oxygen preburner exhaust comes out at 748 K. Autoignition temperature of methane is well over 800 K.

Edited August 31, 2019 by sevenperforce

[Aegolius13]

2 hours ago, sevenperforce said:

Hmm. I'm still not sure what you're intimating.

There is no "mixing" with "hot preburner exhaust" because hot preburner exhaust is the only thing that enters the combustion chamber. 

I mean that the "hot preburner exhaust" is itself a mixture of the C_O2 / H₂0 formed by preburner combustion, and the remainder of the 90% of the methane/oxygen flowing through each respective pump.   The "mixing" (probably a poor choice of words by me) occurs at the point of combustion in the preburner -  I didn't mean a separate stream of propellant was being added later.

What I was trying to say was that in full-flow staged combustion, the propellant entering the combustion chamber would presumably be cooler than whichever propellant was being routed through the turbo in a single-turbo staged combustion engine, since in FFSC all of the propellant is available to absorb some of the preburner heat.  (Hence the similar exhaust temperatures you cite). Whereas in an SSME for example, the fuel-rich gas entering the combustion chamber would be pretty hot, and the oxygen would be pretty cold.  

But anyway, your numbers on exhaust temperature were what I was looking for.

[sh1pman]

24 minutes ago, Aegolius13 said:

I mean that the "hot preburner exhaust" is itself a mixture of the C_O2 / H₂0 formed by preburner combustion, and the remainder of the 90% of the methane/oxygen flowing through each respective pump.   The "mixing" (probably a poor choice of words by me) occurs at the point of combustion in the preburner -  I didn't mean a separate stream of propellant was being added later.

Side note: CO2 and H2O will only form in the LOX preburner. I’m fairly certain that in the fuel preburner there’s a slew of partially oxidized products like CO, decomposing and/or ionized methanol, formaledhyde, formic acid, hydrogen, acetylene, various free radicals and polymerization products, even carbon and higher hydrocarbons. 

[mikegarrison]

32 minutes ago, sh1pman said:

Side note: CO2 and H2O will only form in the LOX preburner. I’m fairly certain that in the fuel preburner there’s a slew of partially oxidized products like CO, decomposing and/or ionized methanol, formaledhyde, formic acid, hydrogen, acetylene, various free radicals and polymerization products, even carbon and higher hydrocarbons. 

I would expect to find all those when running rich with a complex hydrocarbon, but I'm not sure you will get very much if the fuel is just methane. Some, yeah, but I wouldn't think a lot.

[sh1pman]

1 minute ago, mikegarrison said:

I would expect to find all those when running rich with a complex hydrocarbon, but I'm not sure you will get very much if the fuel is just methane. Some, yeah, but I wouldn't think a lot.

A lot less than with kerosene, that’s for sure. Still, it’s an incomplete oxidation with great excess of methane at ~500 bar pressure, so even my chemistry degree can’t help me predict how many intermediate products would be there. It’s probably mostly CO and a small amount of other stuff. 

[Xd the great]

2 hours ago, sh1pman said:

A lot less than with kerosene, that’s for sure. Still, it’s an incomplete oxidation with great excess of methane at ~500 bar pressure, so even my chemistry degree can’t help me predict how many intermediate products would be there. It’s probably mostly CO and a small amount of other stuff. 

An assortment of molecules and radicals formed from C, O, and H. 

In short, a mess.

[kerbiloid]

According to wiki, Raptor's component ratio is 3.8.

As Both CH₄ and O are 16, this means CH₄ + 4 O, so they optimistically expect stoichiometry.

[RCgothic]

What do you mean, optimistically expect stochiometry?

3.8 is deliberately non-stochiometric for ISP reasons.

[kerbiloid]

2 hours ago, RCgothic said:

What do you mean, optimistically expect stochiometry?

3.8 is deliberately non-stochiometric for ISP reasons.

I mean, CH₄ + 2O₂ → CO₂ + 2H₂O

[Xd the great]

2 hours ago, kerbiloid said:

I mean, CH₄ + 2O₂ → CO₂ + 2H₂O

In the world of rocket science, 3.8 and 4 is vastly different. The temperature, material compatability of engine.

Edited September 1, 2019 by Xd the great

[kerbiloid]

7 minutes ago, Xd the great said:

In the world of rocket science, 3.8 and 4 is vastly different. The temperature, material compatability of engine.

That's why I've written 'optimistically'.

[Xd the great]

1 hour ago, kerbiloid said:

That's why I've written 'optimistically'.

More like "a 4.0 will be bad, damage the engine, or may even blow stuff up".

[MinimumSky5]

Yeah, they're not trying to increase the stochiometric rating of the engine up to the exact figure, that would lower the Isp and make the engine even more of a technical marvel (for no benefit). 

[Jacke]

Not to mention that in any engine, the reactions don't go to the stochiometric reactants.  Based on the temperature and pressure of the combustion chamber, the reactants proportions can be predicted (and it ain't 100% stochiometric), but then the exhaust expands through the nozzle.  Everything I read says to take an average of the frozen flow (no further proportion changes) and perfect adiabatic reactions to make a prediction.  It's why rocket engines are tested, to see what they actually produce.

[ThatGuyWithALongUsername]

Holy cow, check this out: THEY HAVE MARS LANDING SITES ALREADY.

A post on the NASASpaceFlight forums pointed to this, where someone found newly released MRO imagery labeled as "Candidate Landing Site for SpaceX Starship." Then they found more.

The images found show 6 landing sites. I added the numbering, original pictures do not have numbering. There are pairs of images for each site as a stereo pair.

Arcadia Planitia area:

• Landing site 1: 38.083°N, 189.793°E
  • https://www.uahirise.org/ESP_060416_2195
  • https://www.uahirise.org/ESP_060706_2195
• Landing Site 2: 39.822°N, 192.058°E
  • https://www.uahirise.org/ESP_060429_2200
  • https://www.uahirise.org/ESP_060363_2200
• Landing Site 3: 39.809°N, 195.615°E
  • https://www.uahirise.org/ESP_060719_2200
  • https://www.uahirise.org/ESP_060785_2200
• Landing Site 4: 39.094°N, 196.706°E
  • https://www.uahirise.org/ESP_060376_2195
  • https://www.uahirise.org/ESP_060732_2195
• Landing Site 5: 40.016°N, 203.330°E
  • https://www.uahirise.org/ESP_060323_2205
  • https://www.uahirise.org/ESP_060811_2205

Phlegra Montes area:

• Landing Site 6: 35.493°N, 163.619°E
  • https://www.uahirise.org/ESP_060707_2160
  • https://www.uahirise.org/ESP_060773_2160

That is crazy, but also an interesting detail to decide this early on. I mean, we don't even know anything about what the base is supposed to look like. They have 4 6 alternatives, but they're all in the same area all but one are in the same area.

 

And it's pretty easy to see what all these sites have in common: they all have water ice right nearby, hidden under some kinda weird Mars structures. Personally, I think the Phlegra Montes area site they chose looks cooler with the large hills (also, come to think of it, if their ultra-ultra-long-term plan is to terraform Mars, the other landing sites would be underwater), but clearly they've given more thought to the Arcadia Planitia area. Some neat stuff over there, too, and it's "only" 2000-ish km from Olympus Mons. Great tourist destination if you ask me.

 

But yeah, if (and only if) everything goes to plan over the next decade or so... one of these locations is about to become a lot more well-known...

 

[EDITED TO ADD MORE INFO ON 2 EXTRA LANDING SITES]

Edited September 2, 2019 by ThatGuyWithALongUsername

[RCgothic]

11 hours ago, kerbiloid said:

I mean, CH₄ + 2O₂ → CO₂ + 2H₂O

This isn't something that they aim for and would be damaging and inefficient if achieved.

Note that the fuel is lighter (CH4 - 16) than the products. As you approach stochiometry the exhaust therefore gets heavier and therefore slower on average. This is inefficient for rocket engines. It also releases more energy which is more difficult to manage, and there's a higher proportion of free oxidiser for longer, which also does bad things to engine internals.

3.8 is the design ratio for good reasons.

[Ultimate Steve]

Just now, RCgothic said:

This isn't something that they aim for and would be damaging and inefficient if achieved.

Note that the fuel is lighter (CH4 - 16) than the products. As you approach stochiometry the exhaust therefore gets heavier and therefore slower on average. This is inefficient for rocket engines. It also releases more energy which is more difficult to manage, and there's a higher proportion of free oxidiser for longer, which also does bad things to engine internals.

3.8 is the design ratio for good reasons.

Also the unburned fuel acts as a coolant, keeping engine temperatures a bit lower.

[mikegarrison]

49 minutes ago, ThatGuyWithALongUsername said:

Holy cow, check this out: THEY HAVE MARS LANDING SITES ALREADY.

https://behindtheblack.com/behind-the-black/essays-and-commentaries/spacex-begins-hunt-for-starship-landing-sites-on-mars/

That is crazy, but also an interesting detail to decide this early on. I mean, we don't even know anything bout what the base is supposed to look like. They have 4 alternatives, but they're all in the same area.

Kim Stanley Robinson had plenty of Mars landing sites. Doesn't mean he ever actually landed anyone.

[ThatGuyWithALongUsername]

1 hour ago, mikegarrison said:

Kim Stanley Robinson had plenty of Mars landing sites. Doesn't mean he ever actually landed anyone.

Didn't say it does... again, it's an oddly small step. I never claimed this was evidence that SpaceX is any closer to anywhere, I just get excited at neat little details like this. Makes it seem more real. And they have been moving at an incredible pace, so...

Also, it's one thing to pick out a bunch of different possible random landing sites. It's another to have been evaluating this long enough that you have it narrowed down to four in the same geographic area [EDIT: 6 in 2 geographic areas, but still-ish]. Still doesn't mean too much, but it does mean a bit more.

It's just... sometimes it's easy to forget that Mars is a whole dang planet. Yes, it doesn't have the added variety of a biosphere like Earth does, but if you sand on two completely different places on Mars, are they going to look the same? No. So I guess this is good for imagining details of what their plans will look like.

And yeah, I already mapped out all 4 [EDIT: 6] locations on Google Earth.

Edited September 1, 2019 by ThatGuyWithALongUsername

[kerbiloid]

6 hours ago, Ultimate Steve said:

Also the unburned fuel acts as a coolant, keeping engine temperatures a bit lower.

6 hours ago, RCgothic said:

This isn't something that they aim for and would be damaging and inefficient if achieved.

Note that the fuel is lighter (CH4 - 16) than the products. As you approach stochiometry the exhaust therefore gets heavier and therefore slower on average. This is inefficient for rocket engines. It also releases more energy which is more difficult to manage, and there's a higher proportion of free oxidiser for longer, which also does bad things to engine internals.

3.8 is the design ratio for good reasons.

Everything listed is just several percents of propellant, and it doesn't matter if it is exactly 3.8 or 4.2.
Unless Musk innovations in chemistry let them change valences of elements.

So, as they have O:CH₄ ~ 4:1 they obviously plan that the carbon gets oxidized into CO₂.

If it was oxidized into CO, the basic reaction would be: CH₄ + 3 O → CO + H₂O , and the component mass ratio ~3:1.

As well, the more the engine was producing methanol and other by-products, the less oxygen would be spent.

So, the component ratio ~4:1 obviously demonstrates that they are presuming that the engine burns the fuel to the most degree of oxidation, with minimum amounts of by-products or CO.

***

For comparison, kerolox Merlins have component ratio ~2.36 (2.17 for early models, up to 2.38 for later ones).

Kerosene is basically (CH₂)_n , unless Musk innovation let them have their own innovative kerosene with fur-tree structure with a ring on top.

n O : CH₂ = 2.36:1 = 16 * n : 12+1 * 2 = 16 * n : 14
16 n / 14 = 2.38
n = 2.08

I.e. the basic reaction of Merlin is ~2 O + CH2 → CO + H₂O

So, Merlin (like most of other kerolox engines) doesn't burn fuel to the highest degree of oxidation, and exhausts mostly CO (carbon monoxide).

***

So Raptor exhausts mostly greenhouse CO₂, while Merlin exhausts toxic CO later either getting partially oxidized by atmospheric oxygen into greenhouse CO₂, or bound with atmospheric water and turning into toxic methanol, formaldehyde, etc, raining into the ocean.

***

Now please tell me moar about the special corporative Space-X chemistry with its own stoichiometry differing from the muggle world, and healing vitamines falling from their exhausts.
As environment-friendly as other rockets.

Btw, the atomic ratio numbers close to integers (~4 O, ~2 O) obviously show that yes, they use that simple stoichiometry from the school classes, later adding plus/minus 5% for losses and cooling.
Not a rocket science, even in innovative Space-X.

If the propellant is lighter than by products or not means absolutely nothing here.

Edited September 2, 2019 by kerbiloid

[kerbiloid]

8 hours ago, ThatGuyWithALongUsername said:

THEY HAVE MARS LANDING SITES ALREADY.

(Claiming Skadi Mons on Venus for my permanent hot ground base.
All, hands off Skadi Mons!)

[Jacke]

3 hours ago, kerbiloid said:

So, as they have O:CH₄ ~ 4:1 they obviously plan that the carbon gets oxidized into CO₂.

I remember one rule-of-thumb from old rocket design texts I read a while ago: assume CO, not CO₂, in the exhaust.  The exact proportions of what reacts to what depends on the temperature and pressure and changes somewhat in the gas as it expands in the nozzle.