Quote from: Greg Hullender on 08/04/2015 02:47 pmQuote from: CameronD on 08/04/2015 03:46 amThanks for the clarification. I would not have thought it was worth it for such a small gain, but I guess every little bit counts when aiming for max performance.Reading Kabloona's links, it looks like there's a 10% gain from chilling the liquid oxygen and a 5% gain from the RP-1. Theyre also stretching the RP-1 tank.Just to be clear, we don't really know the actual % gain that SpaceX will shoot for in the RP-1 density. All we know is that Gwynne said they would "slightly" chill the RP-1, which implies something less than the 5% gain mentioned by Green Shrike, which I believe is a theoretical maximum. So the word "slightly" suggests something less than that.
Quote from: CameronD on 08/04/2015 03:46 amThanks for the clarification. I would not have thought it was worth it for such a small gain, but I guess every little bit counts when aiming for max performance.Reading Kabloona's links, it looks like there's a 10% gain from chilling the liquid oxygen and a 5% gain from the RP-1. Theyre also stretching the RP-1 tank.
Thanks for the clarification. I would not have thought it was worth it for such a small gain, but I guess every little bit counts when aiming for max performance.
Quote from: Kabloona on 08/04/2015 03:02 pmQuote from: Greg Hullender on 08/04/2015 02:47 pmQuote from: CameronD on 08/04/2015 03:46 amThanks for the clarification. I would not have thought it was worth it for such a small gain, but I guess every little bit counts when aiming for max performance.Reading Kabloona's links, it looks like there's a 10% gain from chilling the liquid oxygen and a 5% gain from the RP-1. Theyre also stretching the RP-1 tank.Just to be clear, we don't really know the actual % gain that SpaceX will shoot for in the RP-1 density. All we know is that Gwynne said they would "slightly" chill the RP-1, which implies something less than the 5% gain mentioned by Green Shrike, which I believe is a theoretical maximum. So the word "slightly" suggests something less than that...and "slightly" is all that makes sense to me - keeping the RP-1 above or not much below 0degC. Especially given (a) the structural changes involved in modifying tank volumes to suit, (b) changes to fuel characteristics if they chill the RP-1 too far and (c) the cost of modifying pad equipment to suit the new "cryogenic" liquid.Sometimes the benefits aren't worth the risks.. but, like everything else we've seen from SpX, no doubt they know what they're doing and have good reasons for doing it.
Whatever the RP-1 temperature, it is cold enough to freeze out water vapor condensation from the atmosphere on the fuel tank skin of the rocket. Seen on the Inflight Abort Test Booster doing tanking and propellant densification tests at Vandenberg this spring.
From experience flying aircraft at altitude and chilling the wing fuel to the point it frosts the aircraft skin, that fuel temperature is probably somewhere below -30 degrees C
Whatever the RP-1 temperature, it is cold enough to freeze out water vapor condensation from the atmosphere on the fuel tank skin of the rocket. Seen on the Inflight Abort Test Booster doing tanking and propellant densification tests at Vandenberg this spring. From experience flying aircraft at altitude and chilling the wing fuel to the point it frosts the aircraft skin, that fuel temperature is probably somewhere below -30 degrees C
We won't really know the temperature of the RP-1 until someone at SpaceX tells us, but in an aircraft wing, if the fuel is above -20 degrees C, the wing will be merely wet with nonfreezing condensation with no frost forming unless it is very cold outside, or very high humidity, or there is no sun shining, neither which was the case for RP-1 tank on the test booster that day at Vandenberg. That tank frosted completely on all sides, even the sun facing, on a 24 degree C day.
Quote from: Helodriver on 08/05/2015 12:34 amWe won't really know the temperature of the RP-1 until someone at SpaceX tells us, but in an aircraft wing, if the fuel is above -20 degrees C, the wing will be merely wet with nonfreezing condensation with no frost forming unless it is very cold outside, or very high humidity, or there is no sun shining, neither which was the case for RP-1 tank on the test booster that day at Vandenberg. That tank frosted completely on all sides, even the sun facing, on a 24 degree C day.Interesting. So they may be closer to "freezing" RP-1 than I would have guessed.
Quote from: Kabloona on 08/05/2015 02:11 amQuote from: Helodriver on 08/05/2015 12:34 amWe won't really know the temperature of the RP-1 until someone at SpaceX tells us, but in an aircraft wing, if the fuel is above -20 degrees C, the wing will be merely wet with nonfreezing condensation with no frost forming unless it is very cold outside, or very high humidity, or there is no sun shining, neither which was the case for RP-1 tank on the test booster that day at Vandenberg. That tank frosted completely on all sides, even the sun facing, on a 24 degree C day.Interesting. So they may be closer to "freezing" RP-1 than I would have guessed.As Helo notes, we won't really know until someone at SpaceX tells us. As has been touched on before, RP-1 is comprised of a rather interesting mix of hydrocarbons (better defined than for jet fuel, but not by much) plus entrained water (it's hygroscopic) and as the temperature drops below zero, some of it will start to freeze, some will go waxy and some of it won't.One way around this is to use fuel line heaters to bring the temperature back up to the point where fuel flow is predictable again - but this adds weight and if you're only gaining a few percent extra volume, any performance gain might be offset by the extra weight.In summary: AIUI, densifying LOX/LH2 makes some sense to me, but densifying RP-1 is a fine line and it would be interesting to know where SpaceX plan theirs.
Musk mentioned in the preliminary findings presser that one change we can expect to see is the move from Dragon v1 to Dragon v2 for CRS flights. The statement, ISTR, was in Musk-speak which made it sound like the changeover would be immediate, that all future cargo vehicles to ISS would be Dragon v2's.I just wanted to sort of reality-check that, here -- are we agreed that CRS-8 will almost definitely still be a Dragon v1? And, if that is so (and it seems almost inevitable), are we talking about a changeover to v2 only after the current scheduling of development for the crewed v2?(Oh, and as a complete aside -- it would be nice if the text formatting and smiley controls could be fixed sometime soon. It's a huge pain in the butt to have to type in the frikkin' text commands by hand, just to italicize something.)
This isn't exactly on topic for this thread, but I don't see any reason why they couldn't have the same basic structure for Dragon V2 Cargo as for Dragon V2 Manned. If the cargo version gets the same new parachute arrangement and SuperDracos as the passenger version, then the only part that needs to be different is the very top. The passenger version gets a docking adapter and nosecone. The cargo version gets a berthing ring, and would probably not retain the nosecone.And if the top portion is bolted on, rather than welded, then the two versions could be interchanged rather quickly. Unless doing it that way compromises the pressure vessel....
Musk mentioned in the preliminary findings presser that one change we can expect to see is the move from Dragon v1 to Dragon v2 for CRS flights. The statement, ISTR, was in Musk-speak which made it sound like the changeover would be immediate, that all future cargo vehicles to ISS would be Dragon v2's.
I thought I have seen every significant public statement by Elon Musk in the last few years. I have not seen him saying they will switch cargo to Dragon 2. I recall statements that they may use Dragon 2 some time in the future. I have heard him say they may do a Dragon 2 or two with cargo to prove out the system. I think NASA would agree with that. They don't need the wide berthing port on every cargo flight so why not allow SpaceX to use Dragon 2 to prove it out before crew is flying? But that is not the same as an instant switch to Dragon 2 for cargo.
I'd bet a nickel that all of the currently contracted CRS flights will be on Dragon v1.