SpaceX Super Heavy/Starship (BFR/BFS) - Earth to Deep Space / Re: SpaceX Starship : Texas Prototype(s) Thread 24 : Discussion« Last post by eriblo on 01/30/2023 08:54 pm »
Yes I used the 4.9 bar max rounded to 6 bar absolute.Here is my attempt at some (mostly upper) bounding numbers for the Potentially World Ending Methane Vents of Doom:Six bar is the tank rating. At the tank bottom it is the sum of methane weight and ullage pressure. IIRC, John calculated the liquid methane to contribute 3.99 bar at full tank. That leaves ullage at ~2 bar max.
I am using WolframAlpha for physical properties/calculations and some rough pixel counting from the NSF livestream. Please feel free to find mistakes.
Two vents, total ~140 s.
IIRC temperature was 19 °C with a dew point of 13 °C. This means air is 1.2 kg/m3 with 10 g/kg of water.
Wind speed from the plume movement on the South/North views: 7m/s.
Average specific heat capacity of air is 1.0 kJ/kg.
Average specific heat capacity of methane (gas) is 2.2 kJ/kg.
Specific heat of vaporization of methane is 510 kJ/kg.
The vent has a plate welded on with a smaller hole that looks to be ~13 cm diameter. Assuming John's maximum ullage pressure of 6 bar absolute from above :
Venting gas, worst case: Choked flow at opening, methane at 6 bar, 150 K is 34 kg/s, total 5 t.
Venting liquid, worst case: Bernoulli equation for methane at 95 K and 5 bar pressure drop gives 280 kg/s, total 40 t.
The air flow across the initial vent plume is enough to mostly vaporize any liquid content (as we do not see the plume bending significantly downwards under gravity). Estimating the vent cross section to the wind is hard due to the rapid expansion downwind but I get range of 30 m2 - 70 m2. Note that most of the liquid in the plume would likely evaporate well before it starts to be deflected.
Using 0.5 kg methane per m3 of air and 7 m/s wind gets a range of 100 kg/s - 250 kg/s or a total of 15 t - 35 t
Any vented gas would at most be a few t.
Final plume size
Comparing to the stack the downwind plume expands to a diameter of ~50 m and then disperses (hard to tell because it interacts with the ground and extends beyond most video views).
At this point it has a flow of 12000 m3/s (neglecting wind gradient) and has warmed enough that all the water evaporates, i.e. 6 K below ambient.
Venting gas at boiling point (400 kJ/kg): 250 kg/s, total 35 t.
Venting liquid at boiling point (910 kJ/kg): 110kg/s, total 15 t.
This assumes a homogenous plume and should be an upper bound.
It looks like both plume appearance and size is inconsistent with the maximum gas vent rate, not to mention the amount of ullage gas available (even with rapid boiling).
My guess is a spray with significant liquid content totaling towards the lower range (i.e. on the order of 10 t).
This needs a sanity check as I can't find the referenced post and could easily have LOX and LCH4 switched.
Anyway, I did this screen grab from the NSF feed. It shows both tanks venting at the same time - something that SX should be allergic to. Either this vent was unplanned or had an inert component. I found this while searching for a video I saw showing the beginning of the vent. Didn't find it.
At vent initiation there were a few gobs spewed out that fell in a manner consistent with liquid. Not much, and for less than a second. This doesn't mean that there wasn't more liquid than initially apparent. More could have been hidden in the gaseous plume - or not.
We need Elon to step up and put us out of our misery.
Edit: found it and yes, I did booger the numbers. LCH4 ullage pressure should range between 2.4 and 4.9 bar.
While not optimal I do not think there is an absolute inhibit for venting both tanks simultaneously. You still want to control the pressures and that vent will not increase oxygen concentration but that much. If both vents were low velocity liquid it would be more of a concern