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Updated press kit capture with OCR
John Livingston did some ullage calcs that are a bit disturbing. They show a full LOX tank having a bottom pressure of 6.664 bar at ignition. This is over the 6bar tank rating.

If that's at ignition (at 1 g) then what is it immediately after the clamps release (at 1.5 g)?

On Tuesday, Jan. 31, Demo-2 astronauts Douglas Hurley and Robert Behnken will receive the Congressional Space Medal of Honor for bravery from @VP Kamala Harris. Live coverage is scheduled to begin at 4:15pm ET (2115 UTC) on social media and NASA TV:

Jan 30, 2023

Former NASA Astronauts to Receive Congressional Space Medal of Honor

Vice President Kamala Harris will award former NASA astronauts Douglas Hurley and Robert Behnken the Congressional Space Medal of Honor at 4:15 p.m. EST on Tuesday, Jan. 31. Hurley and Behnken will receive the award for bravery in NASA’s SpaceX Demonstration Mission-2 (Demo-2) to the International Space Station in 2020.

The ceremony will air on NASA Television, the agency’s website, and the NASA app, as well as the agency’s flagship Twitter, Facebook, and YouTube channels.

Limited pre-credentialed media will join pooled press for the event.

On May 30, 2020, a SpaceX Falcon 9 rocket carrying the company’s Crew Dragon spacecraft launched to the space station, marking the first mission to launch with astronauts as part of NASA’s Commercial Crew Program.

To learn more about NASA’s Commercial Crew Program visit:


Photo caption:

NASA astronauts Douglas Hurley (left) and Robert Behnken (right) participate in a dress rehearsal for launch at the agency’s Kennedy Space Center in Florida on May 23, 2020, ahead of NASA’s SpaceX Demo-2 mission to the International Space Station. Demo-2 served as an end-to-end flight test of SpaceX’s crew transportation system, providing valuable data toward NASA certifying the system for regular, crewed missions to the orbiting laboratory under the agency’s Commercial Crew Program.
Credits: NASA
John Livingston did some ullage calcs that are a bit disturbing. They show a full LOX tank having a bottom pressure of 6.664 bar at ignition. This is over the 6bar tank rating.

The 6bar rating was for SS, IIRC. Is SH rated higher? If it's only rated to 6bar the tank would have to be 100% full with zero ullage space, or the common dome is going to blow out.

A related question. How sensitive is a turbopump to variations in inlet pressure (assuming it stays above minimum necessary pressure). With an FFSC engine this question gets really interesting.
There might be some convoluted scenario where it is better to vent liquid rather than gas due to the higher mass flow.

There is a not-very-convoluted scenario where it's better to vent liquid:  A fuel dump as part of an emergency abort/reentry.  You need to get the wet down-mass down to the point of viability.

But that would likely be a vent near the bottom of the tanks, not the top.  I'd guess that you'd want to vent directly from the prop distribution manifold.  Near the top would be useless after getting to orbit, because the liquid level will be many meters below it.

Now I am wondering about connecting some of the vents to the dome apex and some directly into the tank... ???

Mod the prop dump scenario, I can't think of a reason why you'd nominally vent liquid--especially on the pad.  If they actually did vent liquid during the WDR, I'm sure the EPA is on its way with the proctoscope.

All that said, I also can't think of a reason why they wouldn't have assembled the vent inlet and its piping down to the cylindrical portion before doing the dome-flip, so a recent good photo of the inside of a methane dome that shows the interior of the dome but doesn't show the requisite plumbing (either Starship or SuperHeavy should be largely the same in this regard) would be fairly strong evidence of a straight-through vent, rather than one with an inlet near the top of the dome.
Pumps will not cavitate as long as the ullage pressure is large enough.

Good point.  So you can still have equalized pressure between the two ullage spaces, as long as the absolute pressure is a few bar, correct?
If you already have high ullage pressures, then as calculated above you can omit the pumps entirely and move fluids by pressure difference along. Can't cavitate pumps if you don't have any pumps.

There's a fundamental difference between high ullage pressures and high ullage pressure differences.  The first will eliminate cavitation.  The second will move propellant.  If you're going to pressure-feed, you need to manage the ullage pressures in both tanks, via venting (on the receiver) and heating (on the sender).

I'm still struggling a bit trying to figure out pump power requirements, so really big numbers there could change my mind.  But unless that happens, pressure-feeding sounds insanely more complicated than equalizing pressures, especially since the QD has pre-press plumbing built into it.
Equalising pressures during pumping requires continuous ullage gas generation for the sender tank, and continuous venting for the receiver tank. This requires continuous closed-loop control of both gas generation and tank venting throughout the entire transfer process.

Pressure transfer requires pressurising the sender tank once at the start of transfer, venting the receiver tank once at the start of transfer, then opening the inter-tank valve(s) to allow fluid to flow. No additional venting or pressurisation is required during transfer. In the event of an extreme fluid volume transfer (e.g. completely full sender to completely empty receiver) then once transfer ceases (pressure equalised) the tanks can be isolated again, the sender repressurised, the receiver vented, and the inter-tank valve opened again to repeat the process. Venting and pressurisation are "run to completion" processes with no direct constraints on pressurisation/venting time or rate.
ISTM that continuous venting of the receiver during pressure powered transfer would simplify the operation. If parasitic losses cause things to stall out before completion the COPV's on the supply side could add more ullage pressure.

That said, has anyone looked at how this would line up with cold or hot gas consumption for settling thrust? The receiver ullage would be lost anyway whether continuous or pulsed. Might as well make use of it if it adds enough thrust to make the added complexity worth while. I've the echo of a memory of this being asked but no memory of a quantitative answer.
Here is my attempt at some (mostly upper) bounding numbers for the Potentially World Ending Methane Vents of Doom:

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.

Vent opening
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.

Vent size
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).

A few comments:

1) The condition you're describing is one where liquid isn't completely covering the vent, but rather one where the flow across the top of the liquid is enough to loft spray into the flow, correct?

But that doesn't account for the position of the frost line, which is well above the vent.  So you should see a pure liquid flow (which should look like a stream, with a narrowly expanding cloud of water condensation), followed by mixed vapor-spray (which should look like what you're describing), followed by pure vapor (which should look like a rapidly expanding cloud). 

Note that edzieba thinks that the frost line and the liquid level aren't one and the same.  I don't know if he's right or not, but that could be a conceivable explanation for why you're getting vapor+spray, rather than pure liquid.

It's freakin' complicated!

2) I think that gsa is right above, and the transient on the beginning of the vent looks like it's the flow being directed upward as a butterfly-like valve is opening.  That's a partially throttled flow, which probably isn't choked.  If it truly is a butterfly valve (which seems... unreliable?), then flow ought to become choked as the valve opens fully and becomes parallel with the flow.

3) You calculated 10g/kg of water content, but you didn't seem to factor in its enthalpy of vaporization (2.26kJ/g).  So you're adding roughly 23kJ of heat to each kg of air as the water condenses.

4) ISTM that you really need to be figuring out how much volume of ambient air you need to mix with a volume of vapor+liquid methane (possibly with inert gas mixed in) before the temperature of the mixture rises above the dew point.  I don't think the mass helps you very much.

5) Don't forget that there were probably water ice chunks blown off of the area around the vent when it opened up.  Those could easily be mistaken for LCH4 blobs.

6) I searched in vain for a photo of a dome-flip that yielded any interesting information.  But, in addition to the argument that SpaceX would be extremely conservative with their air/methane mixtures, to avoid blowing up their multi-billion-dollar hunk of infrastructure, there's another argument:  If the vent goes straight through and SpaceX just screwed up on the liquid level, then there's way too much ullage space at the top of the tank.  They need just enough of an ullage bubble for two purposes:

a) Fine-tuning of the pressure control.
b) Keeping ullage pressure high enough until the autogenous hot gas can take over.

If the vent is nominally above the fill level, they'll have the whole dome as ullage, plus maybe half a meter of cylinder.  That's maybe... 50m³?  That doesn't make sense.
Advanced Concepts / Re: Realistic, near-term, rotating Space Station
« Last post by LMT on Today at 09:20 pm »
NN GNC in the field

2D navigation in crowded conditions is not relevant to realistic, near-term, rotating space station.

And if you look at the chart of how much human intervention Tesla FSD needs (starting at 2:20), you'll see that it is still nowhere close to be reliable.

But again, Tesla 2D driving software is not relevant to this topic, because EVERYONE knows that computer systems can do navigation tasks in the right conditions - commercial airliners can take off, fly a route, and land on their own with no human intervention. So this is not new, or news.

Tell us how those novel FSD v11 GNC NNs work, and why they're needed, Coastal Ron -- if you know what they are now.
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