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I've been reading about some concerns about the densified (sub-cooled) propellants for Falcon 9 crewed missions in Spaceflight Now's article Lawmakers question safety, schedule for Boeing/SpaceX commercial crew ships.

Could this concern be sidestepped by launching with ambient temperature RP-1 and boiling point LOX? Taking a payload hit of course. If so, would doing so prevent reuse?

uhoh
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lijat
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    RP-1 needs to be pressurized, to provide uninterrupted supply to the turbopumps, otherwise you'd have a problem of cavitation, essentially bubbles of vacuum forming as supply from the tank can't catch up with suction of the pump. Obviously normal LO2 instead of supercooled could be used. I think safety-wise all it would change is appeasing the concerned individuals. – SF. Jan 18 '18 at 07:10
  • Sf: fixed typo, meant to write ambient temperature not ambient pressure. – lijat Jan 18 '18 at 07:14
  • Can you provide a link to something about these concerns? – Russell Borogove Jan 18 '18 at 16:30
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    It is mentioned in this article https://spaceflightnow.com/2018/01/17/lawmakers-question-safety-schedule-for-boeing-spacex-commercial-crew-ships/ – lijat Jan 18 '18 at 19:15
  • @lijat I've moved your link to the question itself. Comments are considered temporary, so it's always better to make the changes directly to your original post. – uhoh Jan 20 '18 at 14:56
  • I wonder about the umbilical cords that seem to have the possibility of topping off the fuel and oxygen present in every launch of modern rockets.. the problem posed is not about transferal of these materials, or when this occurs, but the amount transferred with men sitting in the capsule? – sean marshall May 07 '18 at 17:11
  • @seanmarshall Traditionally the rocket is filled with boiling point cryogenic fluid. After filling the tanks they go into "top off" mode. Gases are bled off the top of the tank and fresh fluid added to replace it. – Saiboogu May 08 '18 at 13:29
  • With densified propellants they're far below the boiling point, and the fluid warms and expands before turning to gas. This means there's a smaller window after loading before the propellant has warmed too much and the density dropped below their goals for flight. They need to drain and refill to get densified propellants in the tank again, after they warm. – Saiboogu May 08 '18 at 13:31

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enter link description hereThe LOX is densified by 9.9% and the RP-1 by 2.6%. This means that if you bring both back to regular temperatures, for the same flow rates, the engines will see a different fuel ratio (producing a different amount of thrust and possibly being less stable, or a much different temperature). You will also consume different ratios of propellant than your tankage - this means you carry extra dead weight, either in unused, empty tankage or unused, surplus propellant.

Warming the propellants and retaining the same fuel ratio could require changes to propellant feed valves in the engines, the turbopump itself, and the common bulkhead location between the LOX & RP-1 tanks.

These aren't small changes, the rocket would require redesign to a hybrid of the current model and the v1.1 model. You could bring the LOX densification up to match the RP-1 densification, but you wouldn't avoid having a small window after fueling to launch - you would just change the size of that window.

These changes would cost them payload mass on regular flights and reduce the number of missions that could recover, and flying these changes on only Dragon 2 flights would mean it wasn't of a common design and SpaceX would have to expend more flights proving the design's safety and reliability, rather than simply using paid for commercial flights to prove the design.

Saiboogu
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  • Sub cooling propellants is done on the launch pad, but not during the flight. So the propellants will warm up during flight and the engines should work the full period from ignition to shut off. Heating of LOX will be limited by boiling temperature. – Uwe May 07 '18 at 19:29
  • Stage one only operates for about 2 minutes - 10 if you count landing, the flight time isn't much compared to the waiting period between propellant load finishing and launch (seems like 20-30 minutes depending on mission). Stage two probably does need to handle some propellant warming between the initial burn and any later burns - but perhaps not, loitering in the vacuum of space. – Saiboogu May 07 '18 at 19:53
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    @Christoph It came from a Reddit comment, linked. I believe it's just a matter of collecting public statements about temperatures, and referencing the ratio the propellants shrink at that temperature. I just relied on someone else's lookup. – Saiboogu May 08 '18 at 13:26