Starship On-orbit refueling - Options and Discussion

[eriblo]

[...]

Linking back to my actual question...

In any case, I was trying to recall whether we reached any consensus about whether we thought the 4-points-of-contact shown in the attached video (credit: SpaceX website) contained any kind of plumbing for prop transfer? And I see we have two camps for and against.

For a while I have been thinking that the 4 probe-drogue setup is purely a mechanical linkage such that the two ships' QD interfaces can be brought together for prop transfer, but in looking at this screen capture of what's on the SpaceX website currently, it's evident that there are two components to each of the 4 points. I could easily believe that the bulky longer one with the probe is there to handle the physical loads, and - once the ships are firmly linked - the narrow one adjacent in each set can extend for prop transfer.

Attaching a shortened version of the video for reference. It would be interesting if anyone had any of the older SpaceX renders to figure out how long the narrow "needle-like" component (present in each of 4 points) has been included in the renders, because when zoomed out, they just look like they could be part of the larger structure.

I've just interpreted those as strut/actuators reinforcing/positioning the four connecting struts/probes perpendicular to the main hinge direction. Somewhat similar to the stabilizers on the chopsticks. It makes little sense for any of the four docking points to be involved in propellant transfer as that would be a lot of extra plumbing for no obvious reason.

Reattaching the diagram from Roy_H's post showing separate pipes for equalising fuel and oxidiser ullage gas pressure across HLS and depot ships. Not sure if this type of setup is known or just speculated, but there have been other, similar diagrams elsewhere upthread. Seems to me that having those links closer to the forward part of the ships - through the 4 points - could actually minimise total plumbing pipe length, and keep the QD for launch site refuelling.

We have seen no sign that they intend to do any kind of ullage gas transfer and it does not make much sense to do so. In an autogenously pressurized system the ullage gas is just propellant vapor - any "excess" will eventually condense into the liquid unless it was already stored at the maximum pressure/temperature point of the tank.

While it should not be a factor for most transfers (or any with some propellant preconditioning) some combinations of transfer time and volumes can conceivably generate excessive back pressure through adiabatic compression. Some solutions I would expect before installing a lot of extra plumbing would be:

Take the loss and vent a fraction of the gas
Use the gas as RCS propellant.
"Bump" the liquid using the RCS to increase heat transfer.
Install a small sprinkler in the tank to collapse the ullage at will.

[rsdavis9]

[...]

Linking back to my actual question...

In any case, I was trying to recall whether we reached any consensus about whether we thought the 4-points-of-contact shown in the attached video (credit: SpaceX website) contained any kind of plumbing for prop transfer? And I see we have two camps for and against.

For a while I have been thinking that the 4 probe-drogue setup is purely a mechanical linkage such that the two ships' QD interfaces can be brought together for prop transfer, but in looking at this screen capture of what's on the SpaceX website currently, it's evident that there are two components to each of the 4 points. I could easily believe that the bulky longer one with the probe is there to handle the physical loads, and - once the ships are firmly linked - the narrow one adjacent in each set can extend for prop transfer.

Attaching a shortened version of the video for reference. It would be interesting if anyone had any of the older SpaceX renders to figure out how long the narrow "needle-like" component (present in each of 4 points) has been included in the renders, because when zoomed out, they just look like they could be part of the larger structure.

I've just interpreted those as strut/actuators reinforcing/positioning the four connecting struts/probes perpendicular to the main hinge direction. Somewhat similar to the stabilizers on the chopsticks. It makes little sense for any of the four docking points to be involved in propellant transfer as that would be a lot of extra plumbing for no obvious reason.

Reattaching the diagram from Roy_H's post showing separate pipes for equalising fuel and oxidiser ullage gas pressure across HLS and depot ships. Not sure if this type of setup is known or just speculated, but there have been other, similar diagrams elsewhere upthread. Seems to me that having those links closer to the forward part of the ships - through the 4 points - could actually minimise total plumbing pipe length, and keep the QD for launch site refuelling.

We have seen no sign that they intend to do any kind of ullage gas transfer and it does not make much sense to do so. In an autogenously pressurized system the ullage gas is just propellant vapor - any "excess" will eventually just condense into the liquid unless it was already stored at the maximum pressure/temperature point of the tank.

While it should not be a factor for most transfers (or any with some propellant preconditioning) some combinations of transfer time and volumes can conceivably generate excessive back pressure through adiabatic compression. Some solutions I would expect before installing a lot of extra plumbing would be:

Take the loss and vent a fraction of the gas
Use the gas as RCS propellant.
"Bump" the liquid using the RCS to increase heat transfer.
Install a small sprinkler in the tank to collapse the ullage at will.

What do they do on the pad? When they are fueling the methane tank do they capture the gas and re-liquify? Where are the lines? Or do they just vent methane to the atmosphere?

[Roy_H]

We have seen no sign that they intend to do any kind of ullage gas transfer and it does not make much sense to do so. In an autogenously pressurized system the ullage gas is just propellant vapor - any "excess" will eventually condense into the liquid unless it was already stored at the maximum pressure/temperature point of the tank.

While it should not be a factor for most transfers (or any with some propellant preconditioning) some combinations of transfer time and volumes can conceivably generate excessive back pressure through adiabatic compression. Some solutions I would expect before installing a lot of extra plumbing would be:

Take the loss and vent a fraction of the gas
Use the gas as RCS propellant.
"Bump" the liquid using the RCS to increase heat transfer.
Install a small sprinkler in the tank to collapse the ullage at will.

We are talking about 400,000 gallons of fuel. Seems like a lot for the proposed systems.
In your first suggestion, all of the gas would have to be expelled in the receiving tanks.
Yes, maybe using the gases as RCS propellant would be good use for the needed small acceleration.
Not quite sure how this would work, do you mean burn the mixture to create heat and boil off some liquid in the supply tanks?
The sprinkler would have to be sub-cooled liquid, so now adding active refrigeration. I am proposing that the ships have insulation and active cooling powered by solar panels, but only enough to stop boil off. It would have to be much larger for your scheme, or take a very long time.
Why do you think a mechanical pump is a bad idea?

[Skybert]

Could the theory that there is prop or gas transfere in the 4 contact points lign up with what we see happening at Masssey's? The new structure over the flametrench has two large mounting flanges on top that could potensial align with the docking points. Has the height ever been confirmed?

And any test happening at this test stand has to involve the Raptors in some extent. Makes no sense to me to do a structural test at this stand. So could it be that they will use a single raptor as a "pump" for prop transfere? They could start one preburner at the time to get the autogenous system going and that will drive the prop flow. No MCC ignition. First start the Oxygen preburner to pump the LOX and then the CH4 preburner to pump the Methane.

[DanClemmensen]

Could the theory that there is prop or gas transfere in the 4 contact points lign up with what we see happening at Masssey's? The new structure over the flametrench has two large mounting flanges on top that could potensial align with the docking points. Has the height ever been confirmed?

And any test happening at this test stand has to involve the Raptors in some extent. Makes no sense to me to do a structural test at this stand. So could it be that they will use a single raptor as a "pump" for prop transfere? They could start one preburner at the time to get the autogenous system going and that will drive the prop flow. No MCC ignition. First start the Oxygen preburner to pump the LOX and then the CH4 preburner to pump the Methane.

If Starship will use ullage methane for cold gas thrusters, they might want to reclaim it.

[OTV Booster]

Could the theory that there is prop or gas transfere in the 4 contact points lign up with what we see happening at Masssey's? The new structure over the flametrench has two large mounting flanges on top that could potensial align with the docking points. Has the height ever been confirmed?

And any test happening at this test stand has to involve the Raptors in some extent. Makes no sense to me to do a structural test at this stand. So could it be that they will use a single raptor as a "pump" for prop transfere? They could start one preburner at the time to get the autogenous system going and that will drive the prop flow. No MCC ignition. First start the Oxygen preburner to pump the LOX and then the CH4 preburner to pump the Methane.

If the turbo pumps are fired up a butt load of propellant will be lost through the main chamber.


Also, because each pump provides a small cross flow to the other to support preburner combustion, it's may be impossible to run only one. Ullage pressure alone may not be enough.

[OTV Booster]

Could the theory that there is prop or gas transfere in the 4 contact points lign up with what we see happening at Masssey's? The new structure over the flametrench has two large mounting flanges on top that could potensial align with the docking points. Has the height ever been confirmed?

And any test happening at this test stand has to involve the Raptors in some extent. Makes no sense to me to do a structural test at this stand. So could it be that they will use a single raptor as a "pump" for prop transfere? They could start one preburner at the time to get the autogenous system going and that will drive the prop flow. No MCC ignition. First start the Oxygen preburner to pump the LOX and then the CH4 preburner to pump the Methane.

If the turbo pumps are fired up a butt load of propellant will be lost through the main chamber.

Also, because each pump provides a small cross flow to the other to support preburner combustion, it's may be impossible to run only one. Ullage pressure alone may not be enough.

I read your post too quick and missed that you were talking about testing, not ops. Conceivably, at great effort, the propellant going through the MCC could be scavenged, but it still doesn't work out well. Better to test as you fly.


For a rude, crude test just hook up some pumps. For more refinement, assuming pressure feed is still the plan, use a pressure differential. Even will not be a very high fidelity test as gravity will have a strong influence.

[eriblo]

We have seen no sign that they intend to do any kind of ullage gas transfer and it does not make much sense to do so. In an autogenously pressurized system the ullage gas is just propellant vapor - any "excess" will eventually condense into the liquid unless it was already stored at the maximum pressure/temperature point of the tank.

While it should not be a factor for most transfers (or any with some propellant preconditioning) some combinations of transfer time and volumes can conceivably generate excessive back pressure through adiabatic compression. Some solutions I would expect before installing a lot of extra plumbing would be:

Take the loss and vent a fraction of the gas
Use the gas as RCS propellant.
"Bump" the liquid using the RCS to increase heat transfer.
Install a small sprinkler in the tank to collapse the ullage at will.

We are talking about 400,000 gallons of fuel. Seems like a lot for the proposed systems.
In your first suggestion, all of the gas would have to be expelled in the receiving tanks.
Yes, maybe using the gases as RCS propellant would be good use for the needed small acceleration.
Not quite sure how this would work, do you mean burn the mixture to create heat and boil off some liquid in the supply tanks?
The sprinkler would have to be sub-cooled liquid, so now adding active refrigeration. I am proposing that the ships have insulation and active cooling powered by solar panels, but only enough to stop boil off. It would have to be much larger for your scheme, or take a very long time.
Why do you think a mechanical pump is a bad idea?

Some numbers to illustrate my point (curtesy of WolframAlpha):

Oxygen:

6 bar (boiling point 111 K):
Liquid density: 1026 kg/m³
Gas density: 23.5 kg/m³
Ratio: 0.023
Specific heat of vaporization: 183 kJ/kg
Specific heat capacity (liquid): 1.8 kJ/kgK

80 K (vapor pressure 0.3 bar):
Liquid density: 1190 kg/m³
Gas density: 1.5 kg/m³
Ratio: 0.0013
Specific heat of vaporization: 225 kJ/kg
Specific heat capacity (liquid): 1.7 kJ/kgK


Methane:

6 bar (boiling point 139 K):
Liquid density: 379 kg/m³
Gas density: 9.7 kg/m³
Ratio: 0.027
Specific heat of vaporization: 434 kJ/kg
Specific heat capacity (liquid): 3.8 kJ/kgK

99 K (vapor pressure 0.3 bar):
Liquid density: 440 kg/m³
Gas density: 0.6 kg/m³
Ratio: 0.0014
Specific heat of vaporization: 541 kJ/kg
Specific heat capacity (liquid): 3.4 kJ/kgK


These suggest that:

1) In the worst case (densest possible gas at max pressure) the ullage gas being displaced is less than 3% of the transferred propellant mass and with colder propellant it can be <<1%. While not optimal if it all has to be vented it is far from deal breaking.

2) Using 3% in hot gas thrusters is less than 100 m/s of dv even under unreasonably optimistic assumptions. In many cases the excess gas might not be enough to supply the dv needed for RPOD and transfer.

3) If the gas condenses into the liquid that replaces it it will increase its temperature by a few K at most. Given the time to do so the liquid can absorb all excess ullage gas except if both are essentially at the maximum pressure equilibrium.


In some cases the propellant transfer flow, volume and condition may be such that the ullage gas is excessively heated by the adiabatic compression and you risk either exceeding maximum tank pressure or generate backpressure beyond the capabilities of the pressurization system and/or pumps. The following are the possible solutions I could think of ranked in order of complexity:

A) Vent it. Requires no extra modifications. Bonus if it can be used as useful RCS trust. Also remember that the losses are small enough that we can expect them to be covered by transfer quanta in all but exceptional cases.

B) Collapse it. You can increase the heat transfer from the gas into the liquid by sloshing with RCS pulsing (might require some retrograde thrust), although this could cost more in RCS propellant than it saves, especially as the source tanks will also be affected. No new hardware required.
You can also use a small internal line from the inlet to spray some of the transferred liquid into the ullage space. Can also include a pump for better control or the ability to use liquid from the receiving tank.

C) Transfer it. This requires plumbing connecting the ullage spaces of the source and receiving tanks and a gas compressor unless the transfer is done with mechanical pumping. I doubt that the autogenous pressurization lines could be used for this - as this is gas at tank pressure you would need piping with at least comparable diameter the much shorter liquid lines to not limit the transfer rate.


I personally expect SpaceX to keep things as simple as possible which would suggest that they would be looking hard on the need for hot gas RCS and propellant transfer pumps, much less extra fluid plumbing, QD's and compressors...

[eriblo]

My previous post assumes that autogenous pressurization can be made to work for orbital propellant transfers which is not a given. The trade-offs might of course change if a separate gas is used which would at least introduce the requirement for high pressure transfer along with the propellants.