Quote from: Req on 08/08/2017 12:49 amObviously RCS would be used, has the term "ullage thrusting" applied to main engines ever in anything?Why are no pumps needed? It'll either have to fill the tanks from the bottom or run up a tube along the side to fill from the top. Why doesn't the ullage thrusting cause head pressure which fights against the "passive filling" at some point in the transfer process?The bottom drilled overflows in my fish tanks won't push water through vinyl hoses if I grab one and hold it's end above the water level in the tank, so what gives?It does if you boil the tank. The transfer is driven by pressure differential. The ullage thrusting is measured in fractions of a micro-g. It's completely irrelevant for anything other than settling - it does not cause any significant flow.
Obviously RCS would be used, has the term "ullage thrusting" applied to main engines ever in anything?Why are no pumps needed? It'll either have to fill the tanks from the bottom or run up a tube along the side to fill from the top. Why doesn't the ullage thrusting cause head pressure which fights against the "passive filling" at some point in the transfer process?The bottom drilled overflows in my fish tanks won't push water through vinyl hoses if I grab one and hold it's end above the water level in the tank, so what gives?
Side by side, yes (as seen in video), but where do you get the rotating from? Sideways thrusting would work. (Spinning it also adds a complication of working against the spin gravity)
Also, no pumps are needed. Just pressure.
Of course you have to worry about where the center of rotation is. The stress is there exists no matter how the vehicles are docked.
... Is it completely apparent/proven that maintaining that pressure differential is preferable to some pumps in terms of mass/complexity/risk/cost/etc at the desired flow rates?
Quote from: KelvinZero on 08/07/2017 10:52 pmCan anyone give some numbers on the efficiency of using acceleration? What would be the delta v over the entire operation? Is this delta-v wasted, or a tiny nudge in the direction you were going anyway? Is it totally negligible?Until someone gives numbers, for all I know it could be as small as moving the length of the tank during the refueling operation. Some simple numbers might make discussion of spinning and so on obviously not worth the bother.According to this paper, it doesn't look that bad: http://www.ulalaunch.com/uploads/docs/Published_Papers/Extended_Duration/SettledCryogenicPropellantTransfer.pdf ~10 lb/hr with a 100 mT hydrogen stage at 10^-5 g. The BFS is a lot bigger and uses a different prop, but you are still looking at only hundreds of pounds per hour. Any sort of grappling system and spin up/down propellant is going to weigh more than that.
Can anyone give some numbers on the efficiency of using acceleration? What would be the delta v over the entire operation? Is this delta-v wasted, or a tiny nudge in the direction you were going anyway? Is it totally negligible?Until someone gives numbers, for all I know it could be as small as moving the length of the tank during the refueling operation. Some simple numbers might make discussion of spinning and so on obviously not worth the bother.
According to this paper, it doesn't look that bad: http://www.ulalaunch.com/uploads/docs/Published_Papers/Extended_Duration/SettledCryogenicPropellantTransfer.pdfThis 10^-5 g is sufficient for settling and pumping out liquid.Pumping out liquid infers almost no flow to the liquid in the vessel, pumping in liquid does.For settling pumped in liquid there is no data, and that's probably why they purpose to do an cryogenic transfer experiment.Suppose 10^-2 g is the limit for keeping pumped in cryogenic Methalox settled (viscosity is important for every pumped in liquid, to slow down flow).Than:Acceleration of 10^-2 g for 5 hours is a delta V of 1.8 km per second.if this is done by thrusters with low ISP the propellant loss is significantly more than with high ISP Raptor engines.Until the lower g limit for pumping-in cryogenic Methalox is comfimed, discussions about rotation vs linear acceleration are relevant.
Quote from: Req on 08/08/2017 02:28 am... Is it completely apparent/proven that maintaining that pressure differential is preferable to some pumps in terms of mass/complexity/risk/cost/etc at the desired flow rates?Yes. For two reasons:1) the tanks must be made to withstand a fair amount of pressure to survive launch. Just sitting on the pad, a 10 m tall water tank will have 1 atmosphere of hydro-static pressure at the bottom (i.e. the weight of the water), in addition to the pressure at the top. Rocket tanks are also pressure stabilized, which means that extra pressure is applied to add structural rigidity. I suspect the flight pressure is around 1 atmosphere, before adding the hydro-static pressure and the ambient outside pressure (the ambient pressure isn't felt on the pad since it's balanced, but the tank feels the lack of it in space). If you listen to the technical announcers during a launch, they'll usually call out tanks being at flight pressure, just minutes before ignition. 2) tanks pressure must be actively managed on-orbit, via venting, to control propellant temperature (conversely pressure becomes a side-effect of temperature control, if a cryo-cooler is used). For a liquid stored at it's boiling point, the boiling temperature is a direct function of the pressure (between the limits of T_freeze and T_critical). So in order to sub-cool the propellants, they must be stored at somewhere between 0 and 1 atmosphere of pressure. Of course larger pressures can be applied for a short time, but the propellant will gradually warm up (unless a cryo-cooler or really good passive cooling is used).So you've probably got over 1 atmosphere of pressure differential to work with, without adding any additional equipment or cost (just software). If your ullage acceleration is 1000 ugees, you can make the propellant flow uphill for about 29000 feet, with no pumps.Note that as the propellant is transferred from the higher pressure tanks into the receiving tank with lower pressure, some "ullage" gas must be vented in the receiving tank to make room for the incoming propellant. If you want to do no-vent propellant transfers, then you either have to use a cooler to condense the extra gas in the receiving tank, or pump it into the sending tank (thru more pipes, valves, and connectors). Remember that ullage gas is 200x less dense than LOX, so throwing away a tank of ullage gas for each transfer is really only wasting 0.5% of your propellant.
Gotcha. Is it completely apparent/proven that maintaining that pressure differential is preferable to some pumps in terms of mass/complexity/risk/cost/etc at the desired flow rates?
Quote from: Peter.Colin on 08/08/2017 06:51 amAccording to this paper, it doesn't look that bad: http://www.ulalaunch.com/uploads/docs/Published_Papers/Extended_Duration/SettledCryogenicPropellantTransfer.pdfThis 10^-5 g is sufficient for settling and pumping out liquid.Pumping out liquid infers almost no flow to the liquid in the vessel, pumping in liquid does.For settling pumped in liquid there is no data, and that's probably why they purpose to do an cryogenic transfer experiment.Suppose 10^-2 g is the limit for keeping pumped in cryogenic Methalox settled (viscosity is important for every pumped in liquid, to slow down flow).Than:Acceleration of 10^-2 g for 5 hours is a delta V of 1.8 km per second.if this is done by thrusters with low ISP the propellant loss is significantly more than with high ISP Raptor engines.Until the lower g limit for pumping-in cryogenic Methalox is comfimed, discussions about rotation vs linear acceleration are relevant.You are assuming that the ullage thrusters burn continuously for 5 hours. This is totally unnecessary. They only pulse as required to keep the prop settled. Note also, it's only necessary to keep the prop settled in the tanks of the tanker during the transfer to ensure that it remains over the outlet. It doesn't matter if the prop is not settled in the receiving tank. It can be settled later after the spaceship and tanker have separated.Also, cryogenic prop has already been kept settled for the order of five hours on Delta IV GSO missions. I think all the basic technology for the mass transfer of cryogenic prop is already in place. It requires development of course. But I don't see any obvious showstoppers. And I don't see the need for rotation.
How do you vent gas if the prop isn't settled in the receiving tank?
Quote from: Peter.Colin on 08/08/2017 03:46 pmHow do you vent gas if the prop isn't settled in the receiving tank?it is settled and vented
Will the prop be offloaded from the same tanks that provide fuel and oxidizer to the tanker's own engines (like siphoning gasoline from one car's tank to another) or would the prop which is to be delivered be in separate tanks? (E.g. when diesel fuel is delivered to a fuel station, the semi tractor draws from its own tanks and the diesel payload is separate in the trailer.)If the answer above is that it will be in a separate payload tank and if multiple tanker launches are already required, this begs a new question. Would it be more efficient to bring fuel and oxidizer on every flight (thus requiring two payload tanks, two sets of pumps/transfer lines, two pumping events, etc. on every flight, or would it be better to have specialized and separate fuel and oxidizer tankers, which would mean only one payload tank, one set of pumping equipment, one pumping event, etc. per flight?It seems that separate specialized tankers might require fewer separate transfer events and potentially be less risky. The tankers themselves would be a simpler design than a dual payload tanker. OTOH, it would mean different loading procedures on the ground and tankers which may be different sizes to accommodate liquids of differing densities.A tanker which delivers both fuel and oxidizer on each flight would mean only one method of loading for each launch, one tanker design, one docking method, etc., rather than two.Thoughts?
So Musk confirmed tanker and spaceship will mate end-to-end and use ullage thrusters to settle the propellant down into the empty tanks of the ship.He said the plan is to reuse the existing plumbing from the booster to transfer fuel. But in the image here, if say CH4 is on the left and O2 is on the right, when the ship and tanker dock end-to-end, then those connections don't line up. They might if the tanker rolled over, but that's not what's shown.Discuss.