# Why would sub-cooled LOX tanks need to “topped-off” until the last minute or so?

Here is my understanding, let me know if I've got it right or wrong:

Until recently, most cryogenic propellants were at or close to their boiling points. Each unit of heat leaking into the tank would then boil off a corresponding unit of propellant and the temperature would stay constant. This is roughly like adding near boiling "make-up" water to a pot of boiling water. It might be a little cooler, but not much.

But in the case of sub-cooled LOX, the temperature is way below the boiling point, somewhere between 10 and 30 degrees C depending on the case. When heat leaks into the tank, it will raise the temperature of the LOX which then expands. A full tank would then overflow - remaining full but with less mass of LOX due to the density change.

There would be no boil-off to "make up" by topping it off. It would remain full but steadily decrease in density as it warmed up.

So for a rocket with first stage tank filled with well sub-cooled LOX, why would it need to be "topped-off" until the last minute or so before launch?

This video at T -00:05:05, video time 06:52, my approximate transcription of the narration:

Right now we’re just coming up on the five minute mark here, we are concluding the loading of the RP-1 on to the first stage […] and we are topping-off liquid oxygen as well on that first stage, and we’re going to keep topping that off for about two more minutes.

The video is already queue'd up at the appropriate time code (in this case I think the video edit is final):

• Sure, the LOX gets warmer and less dense. But evaporation is happening as well - compare to a pot of water at 80°C, it won't stay full for a long time. – asdfex May 2 '17 at 8:18
• @asdfex very often (usually) in cryogenic systems, a 100% saturated vapor layer exists above the liquid within the cryostat. For example, if you add a simple cover over your 80°C water, or place a lid loosely over it, evaporation loss will cease. – uhoh May 2 '17 at 8:59
• there are 2 possible meanings of 'to top off'. 1. finish loading at a lower speed, or 2. make up for volume lost to evaporation. I'm not sure which meaning is used here. – Hobbes May 2 '17 at 9:06
• To be precisely, in the case of sub-cooled LOX, the temperature is way below the boiling point ( of LOX at a pressure of 1 bar ). But when the pressure is reduced, the boiling point drops. This is true not only for total pressure, but also for partial pressure of oxygen. – Uwe May 2 '17 at 19:20
• My comment was only for the time before launch. The temperature of sub cooled LOX is below the boiling point of LOX at 1 bar. But the process of sub cooling uses the fact the boiling point is lower for the bubbles of helium with a total pressure of 1 bar and a partial pressure of oxygen much lower than 1 bar. If sub cooled LOX is exposed to a lower ambient pressure than 1 bar after launch when the rocket is leaving the upper layers of the atmosphere, it will boil again. If this happens to a second stage while the first stage is still active, some of the oxygen is lost by venting. – Uwe May 2 '17 at 20:13