Author Topic: The best method to transfer Methalox Fuel in space?  (Read 42860 times)

Offline Kenp51d

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Re: The best method to transfer Methalox Fuel in space?
« Reply #60 on: 08/08/2017 02:53 am »
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?

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.
Thanks for the correction. Micro g= micro flow. Duh the flow needs a bit of help.
I got super busy with multiple things at once and did not think it all the way through. Got hung up attempting to describe alignment.

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Offline Coastal Ron

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Re: The best method to transfer Methalox Fuel in space?
« Reply #61 on: 08/08/2017 05:39 am »
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)

You have a point, I may have inferred that. In which case using ullage as many have suggested may be the what SpaceX is planning.

Also, no pumps are needed.  Just pressure.

For a simple system I can see how this would be an advantage, but it would appear to require venting gas from the receiving tank so that the gas in the supply tank can pump liquid into the receiving tank. Is that what you mean?

However if you connect the supply tank to the receiver tank with two connections (liquid-liquid, gas-gas), couldn't that be a closed loop system where the liquid and gas could be moved between tanks via pump? Maybe it would require active cooling, but it would seem to reduce gas loss. Not a lot of fuel stations in space, so just trying to see if there is a less wasteful way of refueling.

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.

I could be wrong (see top), but if the two bodies are not rotating on a fixed point, meaning the center of rotation changes as mass shifts between the two bodies, then my assumption was that because the total mass is the same that the rotational forces at the docking points should be the same too.
If we don't continuously lower the cost to access space, how are we ever going to afford to expand humanity out into space?

Offline Nathan2go

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Re: The best method to transfer Methalox Fuel in space?
« Reply #62 on: 08/08/2017 06:19 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.
« Last Edit: 08/08/2017 06:40 am by Nathan2go »

Offline Peter.Colin

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Re: The best method to transfer Methalox Fuel in space?
« Reply #63 on: 08/08/2017 06:51 am »
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.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.

This 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.
 
« Last Edit: 08/08/2017 06:59 am by Peter.Colin »

Offline douglas100

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Re: The best method to transfer Methalox Fuel in space?
« Reply #64 on: 08/08/2017 08:05 am »

According to this paper, it doesn't look that bad:
http://www.ulalaunch.com/uploads/docs/Published_Papers/Extended_Duration/SettledCryogenicPropellantTransfer.pdf

This 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.
Douglas Clark

Offline Req

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Re: The best method to transfer Methalox Fuel in space?
« Reply #65 on: 08/08/2017 08:37 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.

Great post.  Thanks for going to the trouble of laying it all out.

Offline john smith 19

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Re: The best method to transfer Methalox Fuel in space?
« Reply #66 on: 08/08/2017 10:14 am »
A few notes. There's quite a lot of history around this.

Propellant settling was done on Apollo on the LH2/LO2 upper stages. Later tests by ULA on Centaur stages confirmed you could get settling with thrust in the 10 micro g range and it didn't need to be constant, but could be pulsed. 

IIRC they said settled propellant could cut propellant venting 50%, greatly increasing on orbit life, even without improved sun shielding.

Propellant transfer by pressure difference was extensively discussed when SX were planning to do cross feeding for FH. I think thre are a good few posts on the subject.

John Carnack's team at Armadillo had trouble with this when they had 4 pressurized tanks on one of their vehicles. They found even quite small differences (<  a few psi) were enough that instead of all tanks emptying evenly one would flow into another, instead of the engine.
MCT ITS BFR SS. The worlds first Methane fueled FFSC engined CFRP SS structure A380 sized aerospaceplane tail sitter capable of Earth & Mars atmospheric flight.First flight to Mars by end of 2022 TBC. T&C apply. Trust nothing. Run your own #s "Extraordinary claims require extraordinary proof" R. Simberg."Competitve" means cheaper ¬cheap SCramjet proposed 1956. First +ve thrust 2004. US R&D spend to date > $10Bn. #deployed designs. Zero.

Offline Jim

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Re: The best method to transfer Methalox Fuel in space?
« Reply #67 on: 08/08/2017 02:11 pm »

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?

At launch complexes, LH2 is only moved using pressure (the tanks are loaded only by pressure).  There are no pumps involved.

Offline Peter.Colin

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Re: The best method to transfer Methalox Fuel in space?
« Reply #68 on: 08/08/2017 03:46 pm »

According to this paper, it doesn't look that bad:
http://www.ulalaunch.com/uploads/docs/Published_Papers/Extended_Duration/SettledCryogenicPropellantTransfer.pdf

This 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?

Offline Jim

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Re: The best method to transfer Methalox Fuel in space?
« Reply #69 on: 08/08/2017 03:58 pm »


How do you vent gas if the prop isn't settled in the receiving tank?

it is settled and vented

Offline tdperk

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Re: The best method to transfer Methalox Fuel in space?
« Reply #70 on: 08/08/2017 04:30 pm »


How do you vent gas if the prop isn't settled in the receiving tank?

it is settled and vented

Or, a screened off volume within the has a small heat source and the vent aperture inlet inside the screened volume.  With thermal equilibrium in the tank as a whole, there will be no liquid within the screened off volume.

Offline MP99

Re: The best method to transfer Methalox Fuel in space?
« Reply #71 on: 08/08/2017 08:33 pm »
ISTR it's possible to use a liquid / gas separator - a rotor flings any liquid outwards, and any gas which might be left within the chamber can be released. I'd guess this relies on waiting until there is sufficient gas inside the chamber to be vented.

Cheers, Martin

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Offline TomH

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Re: The best method to transfer Methalox Fuel in space?
« Reply #72 on: 08/13/2017 02:55 am »
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?

Offline guckyfan

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Re: The best method to transfer Methalox Fuel in space?
« Reply #73 on: 08/13/2017 06:09 am »
A tanker that uses its main tanks is more flexible.

It needs only one tanking procedure on the ground.

It can be used as a depot. Fill it up in orbit and then transfer the propellant to a departing ship.

It can be sent to lunar orbit to refuel a moon lander.

Offline Peter.Colin

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Re: The best method to transfer Methalox Fuel in space?
« Reply #74 on: 08/13/2017 11:28 am »
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?

I see your point, about specialized tankers. it might be that a tanker with a disproportionally large 
LOX tank and another with large methane tank would simplify things. Also a specialized tanker for water only could be possible (water can also be used for shielding, so you might need a lot of it)

I do not see the need for a separate tank for Methalox, because the regular tanks already have a separate tank inside them, for slosh free landing.
Also using a separate and thus smaller tank eliminates the possibility of one tanker being completely filled up by multiple other tankers. (= Depot option stated in above post(s))
If you do this, you can have all (but one) tankers completely fueled up in space, before all the spaceships with people are launched. Having all fuel already in space, saves a lot of waiting time for the passengers, and also less risk to the spaceship because there would be only one fueling event instead of 5 or 6 (per liquid).
« Last Edit: 08/13/2017 02:12 pm by Peter.Colin »

Offline Norm38

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Re: The best method to transfer Methalox Fuel in space?
« Reply #75 on: 09/29/2017 03:15 pm »
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.

Offline RonM

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Re: The best method to transfer Methalox Fuel in space?
« Reply #76 on: 09/29/2017 03:26 pm »
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.

The plumbing on the tanker can be on opposite sides compared to the spaceship. It would match when docked end-to-end and when rotated 180 degrees on the booster.

Offline Norm38

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Re: The best method to transfer Methalox Fuel in space?
« Reply #77 on: 09/29/2017 03:38 pm »
That's a good point.  If the booster has rotational symmetry, then the tanker could go on 180.  That's different from the 12m booster which had a flange at the top and would not allow the ship to rotate.  I didn't see anything that showed the tanker could not rotate.

Offline DanielW

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Re: The best method to transfer Methalox Fuel in space?
« Reply #78 on: 09/29/2017 03:40 pm »
Isn't it easier to assume that the picture is wrong for the sake of illustration and that the spacecraft will be rotated with respect to each other in reality?

Offline John Alan

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Re: The best method to transfer Methalox Fuel in space?
« Reply #79 on: 09/29/2017 03:43 pm »
Bottom line for me... was SpaceX took all the ideas put forth before yesterday and showed a simple concept that uses existing plumbing and existing thrusters... and can be done unmanned... Brilliant.. 

No pumps... no tethers... KISS at it's finest...  :o  8)

On edit..
I am assuming there is really 4 lines... a gas and liquid line for each tank... a top and bottom feed...
IF you cross them over by backing up back to back like that... the routing is correct...
Draw yourself a sketch and think on it a bit...  ;)

Attached pic down below later in the day...  :)
« Last Edit: 09/29/2017 05:58 pm by John Alan »

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