Starship On-orbit refueling - Options and Discussion

[guckyfan]

Probably 5 or 6 with an optimized tanker, although filling up the ship in orbit isn’t required for Mars, so 4 is possible

What does Musk mean my this, that filling up the ship in orbit (LEO?) isn't required for Mars?  He's saying Starship doesn't need full tanks to get to Mars?
>

That's how I took it.

s

The travel time has increased somewhat. The fuel needs have come down. Looks to me like they need to reduce the load on the heat shield on EDL. Maybe with improvements of the heat shield they can go faster, or they can add payload.

[sebk]

I am not sure if this has been asked yet, but I could not find this talked about in this thread. Sorry if this has been already asked.

Does anybody have any idea about the technical details of actually doing In-Orbit refueling? How do you go about physically getting propellants in 0g to transfer from one vehicle to another? SpaceX renders show SS docking with a tanker from the aft end, does this make sense?

The consensus is that after docking reaction control thrusters produce small acceleration (about a milli-gee) to settle the propellants, then:

* receiving tank is vented to very low pressure (well below atmospheric)
* valves are opened and pressure difference pushes the propellants to the receiving tanks

Lowering pressure has one additional advantage: it ensures the remaining liquid is superchilled (lowering pressure lowers boiling point and boiloff then cools the stuff)

[Simbulation]

I am not sure if this has been asked yet, but I could not find this talked about in this thread. Sorry if this has been already asked.

Does anybody have any idea about the technical details of actually doing In-Orbit refueling? How do you go about physically getting propellants in 0g to transfer from one vehicle to another? SpaceX renders show SS docking with a tanker from the aft end, does this make sense?

The consensus is that after docking reaction control thrusters produce small acceleration (about a milli-gee) to settle the propellants, then:

* receiving tank is vented to very low pressure (well below atmospheric)
* valves are opened and pressure difference pushes the propellants to the receiving tanks

Lowering pressure has one additional advantage: it ensures the remaining liquid is superchilled (lowering pressure lowers boiling point and boiloff then cools the stuff)

I am not sure if this has been asked yet, but I could not find this talked about in this thread. Sorry if this has been already asked.

Does anybody have any idea about the technical details of actually doing In-Orbit refueling? How do you go about physically getting propellants in 0g to transfer from one vehicle to another? SpaceX renders show SS docking with a tanker from the aft end, does this make sense?

As I understand it...

SS is designed to be fuelled via feed lines from SH, through its skirt. To refuel in orbit you dock the two of them skirt-to-skirt and connect the same feed lines. Then they use RCS to accelerate slightly to settle the propellant, and pump it from the tanker to the other SS.

If you look at images/video of the refuelling you'll see that the two SSs are rotated 180 degrees with respect to each other, so the LOX feed line of one matches up with the LOX vent line of the other (and the same for Methane).

thank you very much for the answers

[Norm38]

Is there a need on orbit for the fuel to be superchilled?  If the tanks aren't full, then the higher density doesn't matter. 

And for lunar missions, assuming full tanks are needed, then how is the fuel maintained as superchilled?

[kkattula]

Is there a need on orbit for the fuel to be superchilled?  If the tanks aren't full, then the higher density doesn't matter. 

And for lunar missions, assuming full tanks are needed, then how is the fuel maintained as superchilled?

For a lunar mission from NRHO to surface and back, full tanks aren't needed.

Assuming an HLS SS with crew, cabin and all the equipment is 150 t and it takes 50 t cargo to the surface and only a few hundred kg of rocks back, it would need just over 600 t of propellant. Roughly 70% for the descent & landing and 30% for the ascent. And in this scenario the propellant need decreases by about 1.1 t for each 1 t decrease in cargo.

Of course, with or without cargo, a 150 t HLS SS can't get from LEO to NHRO with 600+ t of propellant remaining. It will need at one tanker flight for the first landing, and two for each subsequent landing.

[Twark_Main]

I am not sure if this has been asked yet, but I could not find this talked about in this thread. Sorry if this has been already asked.

Does anybody have any idea about the technical details of actually doing In-Orbit refueling? How do you go about physically getting propellants in 0g to transfer from one vehicle to another? SpaceX renders show SS docking with a tanker from the aft end, does this make sense?

The consensus is that after docking reaction control thrusters produce small acceleration (about a milli-gee) to settle the propellants, then:

* receiving tank is vented to very low pressure (well below atmospheric)
* valves are opened and pressure difference pushes the propellants to the receiving tanks

Lowering pressure has one additional advantage: it ensures the remaining liquid is superchilled (lowering pressure lowers boiling point and boiloff then cools the stuff)

The advantage of sub-chilling is that it gets more propellant in the tank. But venting would do the opposite. I expect they'll just pump liquid from A to B, while simultaneously letting ullage gas flow from B to A.

If this sub-chilling is intended to reduce boil-off on orbit, then it's counter-productive. You'd lose less propellant if you simply have an always-on "thermostat" that vents the tanker when it's over a set pressure (technically a barostat). This sort of "bang bang" pressure control is already used on Falcon 9, and presumably Starship. Standard PMDs allow gas venting without an ullage burn.

[eriblo]

I am not sure if this has been asked yet, but I could not find this talked about in this thread. Sorry if this has been already asked.

Does anybody have any idea about the technical details of actually doing In-Orbit refueling? How do you go about physically getting propellants in 0g to transfer from one vehicle to another? SpaceX renders show SS docking with a tanker from the aft end, does this make sense?

The consensus is that after docking reaction control thrusters produce small acceleration (about a milli-gee) to settle the propellants, then:

* receiving tank is vented to very low pressure (well below atmospheric)
* valves are opened and pressure difference pushes the propellants to the receiving tanks

Lowering pressure has one additional advantage: it ensures the remaining liquid is superchilled (lowering pressure lowers boiling point and boiloff then cools the stuff)

The advantage of sub-chilling is that it gets more propellant in the tank. But venting would do the opposite. I expect they'll just pump liquid from A to B, while simultaneously letting ullage gas flow from B to A.

If this sub-chilling is intended to reduce boil-off on orbit, then it's counter-productive. You'd lose less propellant if you simply have an always-on "thermostat" that vents the tanker when it's over a set pressure (technically a barostat). This sort of "bang bang" pressure control is already used on Falcon 9, and presumably Starship. Standard PMDs allow gas venting without an ullage burn.

Sub-chilling would still get more propellant into the tanks, it is just a question of whether it will be needed. If you need 5.x tanker loads to fully refuel in a specific orbit you get 1-x loads of propellant from tanker 6 to play with. You can use it to either raise the orbit (dragging the extra tanker mass up there as well) or you can use it to sub-chill the loaded propellant and squeeze in that last drop... It might even be that the Raptors needs/prefers sub-chilled propellants, I seem to recall that they have aborted static fires due to the propellant getting to hot.

BTW, sub-chilled technically just means below boiling point at the current pressure, the temperature can be both lower and higher than the boiling point at 1 atm. It is worth noting that the common bulkhead means that the methane will tend to always be sub-chilled and even then there will be a heat flow from the LCH4 tank to the LOX as the vapor pressure of oxygen at 90.86 K (the freezing point of methane) is still 1.07 bar (attempting to invert the bulkhead).

[Twark_Main]

I am not sure if this has been asked yet, but I could not find this talked about in this thread. Sorry if this has been already asked.

Does anybody have any idea about the technical details of actually doing In-Orbit refueling? How do you go about physically getting propellants in 0g to transfer from one vehicle to another? SpaceX renders show SS docking with a tanker from the aft end, does this make sense?

The consensus is that after docking reaction control thrusters produce small acceleration (about a milli-gee) to settle the propellants, then:

* receiving tank is vented to very low pressure (well below atmospheric)
* valves are opened and pressure difference pushes the propellants to the receiving tanks

Lowering pressure has one additional advantage: it ensures the remaining liquid is superchilled (lowering pressure lowers boiling point and boiloff then cools the stuff)

The advantage of sub-chilling is that it gets more propellant in the tank. But venting would do the opposite. I expect they'll just pump liquid from A to B, while simultaneously letting ullage gas flow from B to A.

If this sub-chilling is intended to reduce boil-off on orbit, then it's counter-productive. You'd lose less propellant if you simply have an always-on "thermostat" that vents the tanker when it's over a set pressure (technically a barostat). This sort of "bang bang" pressure control is already used on Falcon 9, and presumably Starship. Standard PMDs allow gas venting without an ullage burn.

Sub-chilling would still get more propellant into the tanks, it is just a question of whether it will be needed. If you need 5.x tanker loads to fully refuel in a specific orbit you get 1-x loads of propellant from tanker 6 to play with. You can use it to either raise the orbit (dragging the extra tanker mass up there as well) or you can use it to sub-chill the loaded propellant and squeeze in that last drop... It might even be that the Raptors needs/prefers sub-chilled propellants, I seem to recall that they have aborted static fires due to the propellant getting to hot.

BTW, sub-chilled technically just means below boiling point at the current pressure, the temperature can be both lower and higher than the boiling point at 1 atm. It is worth noting that the common bulkhead means that the methane will tend to always be sub-chilled and even then there will be a heat flow from the LCH4 tank to the LOX as the vapor pressure of oxygen at 90.86 K (the freezing point of methane) is still 1.07 bar (attempting to invert the bulkhead).

Good point. In the special case where it's the very last tanker load (which won't be used for Mars flights, per Elon), it might make sense to subchill the propellants. You gain between 0% and 7% vs raising orbit, depending on how much prop is left over in the tanker.

But I don't think they'll tailor the in-space refueling method for every flight just based on that one special case. Since Starship already has vents they can sub-chill in that special case (if the numbers shake out) without adding any additional hardware. But if venting is the only way to do on-orbit refueling, then they're forced to lose propellant always, whether or not it's the last tanker load.

[Thrustpuzzle]

Remember subchilling doesn't just increase storage capacity of tanks (by densification), it also increases both engine thrust and ISP by allowing the turbopumps to inject more mass per second at higher pressure. So even if the Starship doesn't need to subchill for extra capacity, it might still be worthwhile for performance, especially during a max-thrust injection burn.

With regards to in-orbit subchilling the cryogens by venting, I kind of cringe when it's proposed to subchill the cryogens by venting some into space to chill the remainder. Sure, that works, but you're literally throwing away precious mass. What are the energy economics comparing a solar-powered Stirling cryocooler versus venting?  I guess you'd start with computing how many kg of LOX or LCH4 you must vent to decrease temperature of the whole tank by say 20C, then compare to the wattage and mass of a Stirling cooler system (with solar and batteries) that could do the same thing in say 12 hours.

[FutureSpaceTourist]

https://twitter.com/spacex/status/1316417597257129985

NASA has selected Starship for a propellant transfer demonstration! Combining Starship’s rapid reusability with orbital refilling is critical to economically transporting large numbers of crew and cargo to the Moon and Mars go.nasa.gov/3jWLKpA

SpaceX of Hawthorne, California, $53.2 million
Large-scale flight demonstration to transfer 10 metric tons of cryogenic propellant, specifically liquid oxygen, between tanks on a Starship vehicle. SpaceX will collaborate with Glenn and Marshall.

https://www.nasa.gov/directorates/spacetech/solicitations/tipping_points/2020_selections/

[enzo]

The way it's worded, "between tanks on a Starship", is presumably a mistake, as there should be two Starships involved? Otherwise it would not be a "large-scale" test but a small one.

[maquinsa]

The way it's worded, "between tanks on a Starship", is presumably a mistake, as there should be two Starships involved? Otherwise it would not be a "large-scale" test but a small one.

It might be a fuel transfer between main LOX tank and header tank to test it, 10 tons is within the capacity of headers.

[RonM]

The way it's worded, "between tanks on a Starship", is presumably a mistake, as there should be two Starships involved? Otherwise it would not be a "large-scale" test but a small one.

It might be a fuel transfer between main LOX tank and header tank to test it, 10 tons is within the capacity of headers.

It would be a good idea to test the transfer method between tanks on a ship before testing transfer between two ships.

[eriblo]

The way it's worded, "between tanks on a Starship", is presumably a mistake, as there should be two Starships involved? Otherwise it would not be a "large-scale" test but a small one.

It might be a fuel transfer between main LOX tank and header tank to test it, 10 tons is within the capacity of headers.

It would be a good idea to test the transfer method between tanks on a ship before testing transfer between two ships.

And once you have docked the difference between two tanks on a single Starship or two different Starships is just slightly longer pipes and perhaps an extra valve or two.

[steveleach]

The way it's worded, "between tanks on a Starship", is presumably a mistake, as there should be two Starships involved? Otherwise it would not be a "large-scale" test but a small one.

It might be a fuel transfer between main LOX tank and header tank to test it, 10 tons is within the capacity of headers.

It would be a good idea to test the transfer method between tanks on a ship before testing transfer between two ships.

And once you have docked the difference between two tanks on a single Starship or two different Starships is just slightly longer pipes and perhaps an extra valve or two.

Even if the difference was that minor (which I don't think it is), there's the small matter of "once you have docked" :-)

SpaceX need to figure out the details of reliably transferring cryo propellants in micro-g, and starting with everything plumbed in together and shown to be working on the ground is a good first step.

That programme can happen in parallel with orbital rendezvous work, which would lead to docking, which would lead to connecting up the pipes and testing the valves, and then finally the two streams come together with actual orbital refuelling.

The award is likely just NASA paying SpaceX to do something that was an early step in their plan anyway, in order to share information / data.

[Nevyn72]

Cargo SS flies to orbit, opens chomper and drops a 'tank'.

Circle around and 'dock' with the tank and transfer fuel.

When finished, transfer fuel back and discard the 'tank' to burn up on re-entry.

[steveleach]

Cargo SS flies to orbit, opens chomper and drops a 'tank'.

Circle around and 'dock' with the tank and transfer fuel.

When finished, transfer fuel back and discard the 'tank' to burn up on re-entry.

But why?

It's practicing something they'll never need to do.

[rsdavis9]

Is it just me. It seems that inorbit fueling is pretty simple. No new physics. Nothing pushing physics to the limit.

Its just plumbing.

[steveleach]

Is it just me. It seems that inorbit fueling is pretty simple. No new physics. Nothing pushing physics to the limit.

Its just plumbing.

Safely, reliably and robotically transferring cryogenic liquids in microgravity while dealing with large changes in heat (sunlight/shadow) is presumably not trivial.

That said, these awards are for "tipping point" technologies that are on the brink of being considered viable, to nudge them across the line. NASA is quite a conservative organisation these days, and often needs to see something done before they will incorporate it into their plans.

[Jimmy Murdok]

NASA is covering the cost of launching a Starhip and testing the "baby steps" of cryogenic transfer. In large quantities. This is great news!

A sphere of 2.6m of diameter would have 10 tones of liquid.The header tanks fit the bill.
https://pbs.twimg.com/media/EOZKIjwUcAA96zr?format=jpg&name=small

I don´t think it´s between 2 Starships, that will come later. Neither there is need to shape it like a specialized payload.