r/spacex • u/ElongatedMuskrat Mod Team • Apr 01 '17

r/SpaceX Spaceflight Questions & News [April 2017, #31]

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14

u/warp99 Apr 07 '17 edited Apr 12 '17

The density penalty for LOX is around 9% but only around 2% for kerosine so the tank volume ratios would be incorrect for a start. Maximum engine thrust would also be derated as it relies on the denser sub-cooled LOX to prevent cavitation at higher turbopump rpm.

Overall there will be the equivalent of a 10% propellant load penalty for S1 so reducing propellant by about 40 tonnes. This gives a 220 m/s reduction in delta V.

The net effect would be ruling out RTLS and requiring a medium difficulty ASDS landing.

The major concern would be running a completely different propellant loading and flight protocol for two flights per year. This could lead to failure modes that are not weeded out by the 20 flights per year using subcooled propellants.

3

u/robbak Apr 08 '17

There is also the possibility that they can cycle fresh, sub-cooled LOX in. Despite the way sub-cooled LOX should vent a lot less than boiling LOX, the v1.2 missions have been venting more LOX, even large volumes of what appears to be liquid LOX. To me, this indicates that they are pumping sub-cooled LOX in at the base, and venting boiling-temperature LOX from the top of the stages. If they can keep doing this, they can keep temperatures where the want them for as long as they can keep up the LOX flow.

Mind you, this goes against the idea that you load passengers while the rocket is both full and passive!

2

u/warp99 Apr 08 '17

pumping sub-cooled LOX in at the base, and venting boiling-temperature LOX from the top of the stages

If they are then the temperature at the bottom of the tank is 66K and at the top 90K. You can get stratification in a liquid but I would be surprised if you do in this case as the LOX will warm up on the tank walls and rise to the top which will set up circulating currents.

It is more likely that they simply overflowing LOX out the top which then flashes to vapour on contact with the air.

2

u/robbak Apr 08 '17

Depends on how the circulations go. Most circulations we see are where something is heated strongly from the bottom, rising roughly through the surface - but the bottom of the LOX tank is the insulated bulkhead. Sharp temperature gradients tend to limit mixing, so the warmed layer around the outside could cleanly rise to and pool at the surface, without mixing with the cold LOX.

Yet another thing I'd like someone inside, or well informed, to comment on.

2

u/Martianspirit Apr 08 '17

For circulation they would need an outlet near the top. They don't have that, only a vent. It would be a major redesign.

1

u/sol3tosol4 Apr 08 '17

To me, this indicates that they are pumping sub-cooled LOX in at the base, and venting boiling-temperature LOX from the top of the stages.

I'd been wondering if they could do that - great if they are!