T-20 minute vent

[gsa]

I am sure this was discussed long ago, but I couldn't find it.
The Falcon 9 second stage is filled separately: RP-1 first, LOX second. Why is that so? I have heard several times on NSF streams that it's because RP-1 and LOX use the same lines in the TE. Is it really that way, or is it just a common misconception?
It's hard to see how the TE connects to the second stage in Florida because it reclines only at T-0. However, in California, it reclines earlier (see screen grabs from the latest Starlink launch thread attached), and we can see four connections: one at the bottom of the fairing, two at the bottom of the second stage, and one at the bottom of the interstage.
The topmost connection is most likely for payload conditioning. But what are the other three for? Apart from the two propellants, there is a need for GHe for the second stage and GN for the first stage's reaction control. If both are supplied through these connections, it actually leaves only one connection for both propellants.
I would greatly appreciate it if someone could shed some light on this topic.

[Alexphysics]

The interstage's quick disconnect is mainly for conditioning. It supplies helium that pushes away gaseous oxygen from building up inside of it since it's an enclosed environment where you have the oxygen from the S1 LOX tank and then the one that may be coming off from the MVac engine. There are also other functions for this such as raceway purge, temperature control, and things like that. The second stage also has a single QD but multiple lines connect to it, the oxygen line needs to be chilled down before flowing liquid oxygen into the second stage, this is what the T-20 minute vent is... just gaseous oxygen being vented overboard similar to engine chill. We see the same thing happen at Starbase with the OLM and tower vents and they serve the same purpose.

[gsa]

The second stage also has a single QD but multiple lines connect to it...

Yeah, at least two lines are seen. Guess, others are just smaller or inside those two. So, you're saying that RP-1 has its own line? Then why is it filled earlier? Because the lines are too close for warm RP-1 and cold LOX to flow simultaneously?

...the oxygen line needs to be chilled down before flowing liquid oxygen into the second stage...

This part I understand, thank you.

[Jim]

I am sure this was discussed long ago, but I couldn't find it.
The Falcon 9 second stage is filled separately: RP-1 first, LOX second. Why is that so? I have heard several times on NSF streams that it's because RP-1 and LOX use the same lines in the TE. Is it really that way, or is it just a common misconception?
It's hard to see how the TE connects to the second stage in Florida because it reclines only at T-0. However, in California, it reclines earlier (see screen grabs from the latest Starlink launch thread attached), and we can see four connections: one at the bottom of the fairing, two at the bottom of the second stage, and one at the bottom of the interstage.
The topmost connection is most likely for payload conditioning. But what are the other three for? Apart from the two propellants, there is a need for GHe for the second stage and GN for the first stage's reaction control. If both are supplied through these connections, it actually leaves only one connection for both propellants.
I would greatly appreciate it if someone could shed some light on this topic.

There is one umbilical to the second stage with many separate lines:  LOX, RP-1, GN2, Ghe, power, data, conditioned air, etc

Fuel and oxidizer can not share common lines.

The top umbilical is payload ECS.

The bottom is likely to the booster.

[Jim]

Then why is it filled earlier? Because the lines are too close for warm RP-1 and cold LOX to flow simultaneously?

So the sub cooled LOX is not sitting in the tanks warming up.  And they share a common bulkhead and in addition to tanks walls and the outside, there is heat flow from the RP-1 to the LOX

[Alexphysics]

The bottom is likely to the booster.

Yeah that's the interstage QD which, before someone gets confused, has no role on loading the first stage. For that there are two quick disconnects on the base of the transporter erector that provide the fluids and power needed to the first stage to work, including supplying the ground-fed fluids for ground-only operations (engine startup prior to liftoff, gimbal checks, etc)

[gsa]

Then why is it filled earlier? Because the lines are too close for warm RP-1 and cold LOX to flow simultaneously?

So the sub cooled LOX is not sitting in the tanks warming up.  And they share a common bulkhead and in addition to tanks walls and the outside, there is heat flow from the RP-1 to the LOX

Yeah, you want to load LOX as late as possible, I understand that. The question is - why not load RP-1 later too? Is there any particular reason (such as the one I mentioned - filling lines are too close, LOX would be warmed up by RP-1), or is it just because they can?

[Jim]

Yeah, you want to load LOX as late as possible, I understand that. The question is - why not load RP-1 later too? Is there any particular reason (such as the one I mentioned - filling lines are too close, LOX would be warmed up by RP-1), or is it just because they can?

they don't want both flowing at the same time.  They have to manage the loads and pressures on the common bulkheads.