Starship On-orbit refueling - Options and Discussion

[Twark_Main]

Do pressurized tanks like tapering at the end?

Ironic to fret over inefficiencies in a non-optimal dome shape....  and then immediately propose an entire (unnecessary as far as I can tell) crew habitat, using that same non-optimal dome as its pressure vessel. 

"I have an annoying splinter in my eye. Could I swap it for this plank instead?" 

Wasnt fretting.

Just asking questions

A fuel tank is 6-8 bar, the nose is 0-1. Not the same thing.

Somehow I anticipated this forest-missing.
 
You propose a habitat, so that's 1 bar (0 bar and your astronauts won't fare too well). Typical safey margins for a human pressure vessel are 2.5x, so that's 2.5 bar. So we're now at nearly half the equivalent internal pressure, not 1/6th.

And this is only a small fraction of the mass required for a hab. And on the gripping hand, you're adding risk of human lives, which makes the real world cost far higher than the mass penalty suggests.

Artemis III will need to use that volume and dock Orion at the nose. A depot may well need crew support occasionally. The two objectives align.

This isn't the 60s MOL. There's no reason to ever put people on a depot.

[DanClemmensen]

Artemis III will need to use that volume and dock Orion at the nose. A depot may well need crew support occasionally. The two objectives align.

You are discussing the Starship Depot variant. Orion docks to the Starship HLS variant. Depot and HLS are very different. I have never seen any speculative CONOPS in which Orion docks to Depot.

There is an alternative architecture that uses Crew Dragon for Earth-LEO and back, and the Crew Dragon may need to dock to a something in LEO to extend its loiter time. I supposed of could dock to Depot for this, but it's uncrewed while loitering.

[SpaceLizard]

Artemis III will need to use that volume and dock Orion at the nose. A depot may well need crew support occasionally. The two objectives align.

You are discussing the Starship Depot variant. Orion docks to the Starship HLS variant. Depot and HLS are very different. I have never seen any speculative CONOPS in which Orion docks to Depot.

There is an alternative architecture that uses Crew Dragon for Earth-LEO and back, and the Crew Dragon may need to dock to a something in LEO to extend its loiter time. I supposed of could dock to Depot for this, but it's uncrewed while loitering.

I think Norm has been trying to suggest that the, somewhat superficial, commonalities between HLS and a Starship Depot (white color + lack of re-entry hardware) implies to him that a depot should basically just be an HLS optimized for more tank space but retaining a small habitable volume and nose port.
Although I don't think a habitable volume on the depot is in any way desirable until after refueling without complication has been proven autonomously.

[Twark_Main]

Artemis III will need to use that volume and dock Orion at the nose. A depot may well need crew support occasionally. The two objectives align.

You are discussing the Starship Depot variant. Orion docks to the Starship HLS variant. Depot and HLS are very different. I have never seen any speculative CONOPS in which Orion docks to Depot.

There is an alternative architecture that uses Crew Dragon for Earth-LEO and back, and the Crew Dragon may need to dock to a something in LEO to extend its loiter time. I supposed of could dock to Depot for this, but it's uncrewed while loitering.

I think Norm has been trying to suggest that the, somewhat superficial, commonalities between HLS and a Starship Depot (white color + lack of re-entry hardware) implies to him that a depot should basically just be an HLS optimized for more tank space but retaining a small habitable volume and nose port.
Although I don't think a habitable volume on the depot is in any way desirable until after refueling without complication has been proven autonomously.

Before autonomous refilling is proven, you don't want people.

After autonomous refilling is proven, you don't need people.

[SpaceLizard]

Artemis III will need to use that volume and dock Orion at the nose. A depot may well need crew support occasionally. The two objectives align.

You are discussing the Starship Depot variant. Orion docks to the Starship HLS variant. Depot and HLS are very different. I have never seen any speculative CONOPS in which Orion docks to Depot.

There is an alternative architecture that uses Crew Dragon for Earth-LEO and back, and the Crew Dragon may need to dock to a something in LEO to extend its loiter time. I supposed of could dock to Depot for this, but it's uncrewed while loitering.

I think Norm has been trying to suggest that the, somewhat superficial, commonalities between HLS and a Starship Depot (white color + lack of re-entry hardware) implies to him that a depot should basically just be an HLS optimized for more tank space but retaining a small habitable volume and nose port.
Although I don't think a habitable volume on the depot is in any way desirable until after refueling without complication has been proven autonomously.

Before autonomous refilling is proven, you don't want people.

After autonomous refilling is proven, you don't need people.

Totally agree, was just trying to explain Norm's comments in the way I've understood them so far.

[Greg Hullender]

Before autonomous refilling is proven, you don't want people.

After autonomous refilling is proven, you don't need people.

It's possible you'll eventually want people if you're also going to try to test/refurbish Starships in orbit rather than landing them. That only makes sense if you're trying to reuse them though. You might also want something to support a repair team for the depot itself--assuming that's worth repairing rather than just replacing.

[OTV Booster]

Before autonomous refilling is proven, you don't want people.

After autonomous refilling is proven, you don't need people.

It's possible you'll eventually want people if you're also going to try to test/refurbish Starships in orbit rather than landing them. That only makes sense if you're trying to reuse them though. You might also want something to support a repair team for the depot itself--assuming that's worth repairing rather than just replacing.

IMO, a transshipment facility and repair depot could easily cohabit but a refueling depot needs maneuvering room for ullage settling. From recent discussion there could also be reasons for a fuel depot to hold different orbital height under varied circumstances. A fuel depot has no reason to haul around unnecessary mass.


If a fuel depot needs work it does like every other ship that needs work. It gets to the shop under its own power or on a hook, it takes a number and waits its turn. If any part of the process promises to cost more than it saves there's a place in the pacific where future archeologist will study the early space age.

[wannamoonbase]

Before autonomous refilling is proven, you don't want people.

After autonomous refilling is proven, you don't need people.

It's possible you'll eventually want people if you're also going to try to test/refurbish Starships in orbit rather than landing them. That only makes sense if you're trying to reuse them though. You might also want something to support a repair team for the depot itself--assuming that's worth repairing rather than just replacing.

IMO, a transshipment facility and repair depot could easily cohabit but a refueling depot needs maneuvering room for ullage settling. From recent discussion there could also be reasons for a fuel depot to hold different orbital height under varied circumstances. A fuel depot has no reason to haul around unnecessary mass.


If a fuel depot needs work it does like every other ship that needs work. It gets to the shop under its own power or on a hook, it takes a number and waits its turn. If any part of the process promises to cost more than it saves there's a place in the pacific where future archeologist will study the early space age.

I think depots need to be on their own simply because if you have 100's of tons of fuel and oxidizer near each other you have to be concerned about accidents.  That should apply in LEO as it does on Earth.

Also, depots should be in lower orbits so that any debris from a vehicle breakup will re-enter in short order.

[Asteroza]

Before autonomous refilling is proven, you don't want people.

After autonomous refilling is proven, you don't need people.

It's possible you'll eventually want people if you're also going to try to test/refurbish Starships in orbit rather than landing them. That only makes sense if you're trying to reuse them though. You might also want something to support a repair team for the depot itself--assuming that's worth repairing rather than just replacing.

IMO, a transshipment facility and repair depot could easily cohabit but a refueling depot needs maneuvering room for ullage settling. From recent discussion there could also be reasons for a fuel depot to hold different orbital height under varied circumstances. A fuel depot has no reason to haul around unnecessary mass.


If a fuel depot needs work it does like every other ship that needs work. It gets to the shop under its own power or on a hook, it takes a number and waits its turn. If any part of the process promises to cost more than it saves there's a place in the pacific where future archeologist will study the early space age.

Hrm, from a settling perspective, would being in a very low VLEO (ULEO) be enough to naturally settle? That also covers debris demise in case of an oopsie rather quickly...

Though if you had a VLEO depot, one wonders if there's merit in installing an ABEP thruster, but then you are just a step away from a PROFAC setup that only needs methane deliveries...

[Norm38]

A fuel tank is 6-8 bar, the nose is 0-1. Not the same thing.

Somehow I anticipated this forest-missing.
 
You propose a habitat, so that's 1 bar (0 bar and your astronauts won't fare too well). Typical safey margins for a human pressure vessel are 2.5x, so that's 2.5 bar. So we're now at nearly half the equivalent internal pressure, not 1/6th.

And this is only a small fraction of the mass required for a hab. And on the gripping hand, you're adding risk of human lives, which makes the real world cost far higher than the mass penalty suggests.

This isn't the 60s MOL. There's no reason to ever put people on a depot.

0 bar meaning the habitat could possibly be depressurized at some point.  Not that it has to, the point is that the hab structure doesn't require internal pressure for integrity like the fuel tanks do during launch.

Why worry about the mass penalty of a hab in the nose (that already exists pressurized or not) for a depot that is literally larger than the ISS?  This is a revolution, what are you saving mass for?  The hab has to be designed, it has to exist, it has to be tested.  I don't believe for one second that NASA would send astronauts to the moon in a craft that has not already been demonstrated to work.  This isn't the 60s as you say.


Finally, there's no reason to ever put people on the Moon or Mars, to risk their lives.  We can all just sit here on the ground and safely die of boredom.
We are either putting humans in space or we aren't.  And someone is going to have to explain to me how it's incredibly risky and dumb to have a depot able to house a repair/maintenance crew because it's a giant fuel tank, but it's okay for crew to fly to the Moon or Mars on top of a giant fuel tank.  It's the same system.

I'm not saying there must be a hab in the depot.  There could very well be a dedicated repair starship that goes around to the various depots.  That's fine and would accomplish all the testing goals.  But the dedicated repair starship is going look almost identical to the fuel depot and will also be a giant fuel tank to maneuver around where it needs to go.

[Twark_Main]

Before autonomous refilling is proven, you don't want people.

After autonomous refilling is proven, you don't need people.

It's possible you'll eventually want people if you're also going to try to test/refurbish Starships in orbit rather than landing them. That only makes sense if you're trying to reuse them though. You might also want something to support a repair team for the depot itself--assuming that's worth repairing rather than just replacing.

IMO, a transshipment facility and repair depot could easily cohabit but a refueling depot needs maneuvering room for ullage settling. From recent discussion there could also be reasons for a fuel depot to hold different orbital height under varied circumstances. A fuel depot has no reason to haul around unnecessary mass.


If a fuel depot needs work it does like every other ship that needs work. It gets to the shop under its own power or on a hook, it takes a number and waits its turn. If any part of the process promises to cost more than it saves there's a place in the pacific where future archeologist will study the early space age.

Hrm, from a settling perspective, would being in a very low VLEO (ULEO) be enough to naturally settle? That also covers debris demise in case of an oopsie rather quickly...

Though if you had a VLEO depot, one wonders if there's merit in installing an ABEP thruster, but then you are just a step away from a PROFAC setup that only needs methane deliveries...

If a depot is decelerating fast enough to provide a non-trivial settling effect, it won't be in ULEO for long.

Generally at those altitudes you assume need constant thrust for orbit maintenance. If you have thrusters that can counteract that much decceleration, you can go higher and use that same thruster hardware to settle the propellant directly.

[Norm38]

Yeah, my thought was that the thrust during propellant loading would either be used to reboost the depot, or to do plane/phase changes as needed.

[Narnianknight]

A fuel tank is 6-8 bar, the nose is 0-1. Not the same thing.

Somehow I anticipated this forest-missing.
 
You propose a habitat, so that's 1 bar (0 bar and your astronauts won't fare too well). Typical safey margins for a human pressure vessel are 2.5x, so that's 2.5 bar. So we're now at nearly half the equivalent internal pressure, not 1/6th.

And this is only a small fraction of the mass required for a hab. And on the gripping hand, you're adding risk of human lives, which makes the real world cost far higher than the mass penalty suggests.

This isn't the 60s MOL. There's no reason to ever put people on a depot.

0 bar meaning the habitat could possibly be depressurized at some point.  Not that it has to, the point is that the hab structure doesn't require internal pressure for integrity like the fuel tanks do during launch.

Why worry about the mass penalty of a hab in the nose (that already exists pressurized or not) for a depot that is literally larger than the ISS?  This is a revolution, what are you saving mass for?  The hab has to be designed, it has to exist, it has to be tested.  I don't believe for one second that NASA would send astronauts to the moon in a craft that has not already been demonstrated to work.  This isn't the 60s as you say.


Finally, there's no reason to ever put people on the Moon or Mars, to risk their lives.  We can all just sit here on the ground and safely die of boredom.
We are either putting humans in space or we aren't.  And someone is going to have to explain to me how it's incredibly risky and dumb to have a depot able to house a repair/maintenance crew because it's a giant fuel tank, but it's okay for crew to fly to the Moon or Mars on top of a giant fuel tank.  It's the same system.

I'm not saying there must be a hab in the depot.  There could very well be a dedicated repair starship that goes around to the various depots.  That's fine and would accomplish all the testing goals.  But the dedicated repair starship is going look almost identical to the fuel depot and will also be a giant fuel tank to maneuver around where it needs to go.

Why does there need to be repair done on a depot at all? Crewed depot maintenance is not only unnecessary; it is cost prohibitive. Even if a depot fails, a crewed mission will likely be at least an order of magnitude more expensive than constructing and launching a replacement depot.

[InterestedEngineer]

Before autonomous refilling is proven, you don't want people.

After autonomous refilling is proven, you don't need people.

It's possible you'll eventually want people if you're also going to try to test/refurbish Starships in orbit rather than landing them. That only makes sense if you're trying to reuse them though. You might also want something to support a repair team for the depot itself--assuming that's worth repairing rather than just replacing.

IMO, a transshipment facility and repair depot could easily cohabit but a refueling depot needs maneuvering room for ullage settling. From recent discussion there could also be reasons for a fuel depot to hold different orbital height under varied circumstances. A fuel depot has no reason to haul around unnecessary mass.


If a fuel depot needs work it does like every other ship that needs work. It gets to the shop under its own power or on a hook, it takes a number and waits its turn. If any part of the process promises to cost more than it saves there's a place in the pacific where future archeologist will study the early space age.

Hrm, from a settling perspective, would being in a very low VLEO (ULEO) be enough to naturally settle? That also covers debris demise in case of an oopsie rather quickly...

Though if you had a VLEO depot, one wonders if there's merit in installing an ABEP thruster, but then you are just a step away from a PROFAC setup that only needs methane deliveries...

If a depot is decelerating fast enough to provide a non-trivial settling effect, it won't be in ULEO for long.

Generally at those altitudes you assume need constant thrust for orbit maintenance. If you have thrusters that can counteract that much decceleration, you can go higher and use that same thruster hardware to settle the propellant directly.

I dunno, let's do some math 

Let's say the require acceleration to get the transfer going is 1mm/sec².  If it takes 4 hours (2 orbits) to make the transfer, that is a (deceleration) of 7200s * .001m/sec² = 7.2m/sec.

It turns out there's a calculator for EVERYTHING out there, including orbital height and velocity:  https://www.satsig.net/orbit-research/orbit-height-and-speed.htm

Plugging a 7.2m/sec drop with a starting orbit of 100km, I get about 89km.  That's a drop of 5km per orbit, so probably only survive one further orbit.  So there is a time limit here.  27kg/sec seems like a pretty small transfer rate, maybe they can transfer 100k/g sec which for 200t is 33 minutes, so one orbit at most.

The actual altitude where one gets deceleration of 1mm/sec² is something I've not calculated, as that calculation is harder. I think I know someone on this forum that has been messing about in air drag trajectories recently, maybe they can find the height where this occurs?

I'm starting to think that VLEO is a great place to do refueling.  Not only does it use acceleration that is "free", but making up the acceleration doesn't involve low ISP thrusters, but rather high Isp main engines, so it is more efficient.

To add further benefit to this idea, there's a very slight Oberth boost to starting a TMI from 100km vs 200km.

[InterestedEngineer]

Let's confirm whether 1mm/sec² (typo fixed) is enough to transfer the contents of fuel tanks from one Starship to another.

I note here I'm completely ignoring surface tension, while at the same time noting that surface tension based siphoning has already been demonstrated on the ISS, so surface tension could be a net positive or negative.

So that being said, how far does the mass of propellant need to move?  Worst case, it's 2 * depth of the tanks, which could either be 9m or 30m.  Let's take 30m worst case (tail to tail refuel scenario).  So that's 60m.

If we have a two hour window to move the propellant (about one orbit), then simply by s=1/2at² we know that acceleration needs to be 1.5mm/sec².  The total deltaV is thus .003 * 3600 = 11m/sec.   That's a bit more than the prior estimate but still within the realm of possibility.

A four hour window (2 orbits), is .76mm/sec, so even higher altitude could be used for longer transfer times.  DeltaV is the same. 

Now let's do back-back refueling, instead of tail to tail.  We have to move the fuel 18m worst case, so the deltaV involved is 6m/sec, which is easily doable in VLEO.

My prediction?  They will do refueling in a VLEO back to back, taking advantage of the atmospheric drag to move the fuel, and it'll take less than one orbit to do the transfer, with a total of 3 orbits.  1 to rendezvous, one to transfer, and one to get to the right position to do a burn for Luna, Mars, or EDL.

[Stan-1967]

Let's confirm whether 1mm/sec is enough to transfer the contents of fuel tanks from one Starship to another.

I think you erring in taking the 1mm/s^2 acceleration proposed for the settling of the propellant as meaning the velocity of the draining propellant out of the tanks occurs at 1mm/s.  Acceleration=/= velocity.

Don't conflate the settling acceleration with the rate of propellant transfer.  Once the propellant is settled, pressure differential between sending & receiving tanks determine the rate of transfer.


Also, if the mated vehicles are accelerating at 1mm/s, that accelleration vector can be used for lowering, raising, or even inclination adjustments.  I don't see why it must be assumed the orbit will be lowered during refuel operations.

[InterestedEngineer]

Let's confirm whether 1mm/sec is enough to transfer the contents of fuel tanks from one Starship to another.

I think you erring in taking the 1mm/s^2 acceleration proposed for the settling of the propellant as meaning the velocity of the draining propellant out of the tanks occurs at 1mm/s.  Acceleration=/= velocity.

I was relating them by s=1/2at² and v=at, aka the standard kinematic equations.  Did I have a typo where I forgot to put the square sign on the seconds? I don't see it. (edit:  found it thx)

Don't conflate the settling acceleration with the rate of propellant transfer.  Once the propellant is settled, pressure differential between sending & receiving tanks determine the rate of transfer.

I forgot to include the pressure differential, so I'm doing worst case to show that it's possible with atmospheric drag by itself.

Pressure diffeential AND surface tension can make the transfer go faster, which just shifts COM and not the whole attached pair of starships. (Atmospheric drag shifts both the same amount w/ opposite signs, which was what I was calculating)

Also, if the mated vehicles are accelerating at 1mm/s, that accelleration vector can be used for lowering, raising, or even inclination adjustments.  I don't see why it must be assumed the orbit will be lowered during refuel operations.

That's not how orbital mechanics works.  If you try to raise the orbit with drag, you are lowering the orbit on opposite side of the planet, and making the drag worse over there.

On purpose I calculated this over whole orbits, so that we get less funny elliptical orbit in thermosphere effects, which would require numerical integration to calculate.

[Narnianknight]

In October 2020 up-thread, the best guess was that the necessary settling acceleration is on the order of 1e-4 m/s². Also, keeping the depot pointing toward the sun to minimize illuminated surface area and therefore boil-off is probably important. Either way, using drag for the acceleration doesn't really make sense, because the ship would have to be brought up to altitude again using propulsion. Besides, boil-off will create ullage gas that will have to be vented anyway; they might as well use it for settling.

[eriblo]

Anyone suggesting to use drag to settle the propellant might want to take out an envelope and do a quick estimate of the associated heating...

[Twark_Main]

I dunno, let's do some math 

Let's say the require acceleration to get the transfer going is 1mm/sec².  If it takes 4 hours (2 orbits) to make the transfer, that is a (deceleration) of 7200s * .001m/sec² = 7.2m/sec.

It turns out there's a calculator for EVERYTHING out there, including orbital height and velocity:  https://www.satsig.net/orbit-research/orbit-height-and-speed.htm

Plugging a 7.2m/sec drop with a starting orbit of 100km, I get about 89km.  That's a drop of 5km per orbit, so probably only survive one further orbit.  So there is a time limit here.  27kg/sec seems like a pretty small transfer rate, maybe they can transfer 100k/g sec which for 200t is 33 minutes, so one orbit at most.

The actual altitude where one gets deceleration of 1mm/sec² is something I've not calculated, as that calculation is harder. I think I know someone on this forum that has been messing about in air drag trajectories recently, maybe they can find the height where this occurs?

I'm starting to think that VLEO is a great place to do refueling.  Not only does it use acceleration that is "free", but making up the acceleration doesn't involve low ISP thrusters, but rather high Isp main engines, so it is more efficient.

To add further benefit to this idea, there's a very slight Oberth boost to starting a TMI from 100km vs 200km.

I agree, logically VLEO is the best place for refilling, at least for the lowest "rung" of any refilling ladder. However I'm not convinced about using drag for propellant settling.

You may find this graph handy: https://space.stackexchange.com/questions/18223/where-can-i-find-data-for-atmospheric-density-vs-altitude

At hypersonic velocities you can assume a Cd of 2.2, so for a 9 m diameter 100 tonne vehicle you achieve 1 mm/s² at just over 100 km altitude, and for a 2,000 tonne vehicle it's almost exactly 100 km.