Starship On-orbit refueling - Options and Discussion

[InterestedEngineer]

If one applies an axial force so that the fuel settles at the base of the fuel tank, then one is left with "how does the corner opposite of the fuel transfer outlet get emptied" problem.

Diagram attached shows the problem.

The answer maybe "the force is 45 degrees to axial", which would solve the problem?

[Twark_Main]

If the acceleration is axial, then you've got to run a pipe to the bottom of the ship.

If they can drain the tank via GSE (which they can), then they already have a pipe running to the bottom of the tank.

[Twark_Main]

If one applies an axial force so that the fuel settles at the base of the fuel tank, then one is left with "how does the corner opposite of the fuel transfer outlet get emptied" problem.

Diagram attached shows the problem.

The answer maybe "the force is 45 degrees to axial", which would solve the problem?

This is why the bottom of a rocket tank is a sump (ie sloped toward the inlet), not a flat floor.

[xvel]

lol

[InterestedEngineer]

If one applies an axial force so that the fuel settles at the base of the fuel tank, then one is left with "how does the corner opposite of the fuel transfer outlet get emptied" problem.

Diagram attached shows the problem.

The answer maybe "the force is 45 degrees to axial", which would solve the problem?

This is why the bottom of a rocket tank is a sump (ie sloped toward the inlet), not a flat floor.

a sump at 1g sure.  at 0.00001g, is it it enough to deal with the surface tension?

[DanClemmensen]

If one applies an axial force so that the fuel settles at the base of the fuel tank, then one is left with "how does the corner opposite of the fuel transfer outlet get emptied" problem.

Diagram attached shows the problem.

The answer maybe "the force is 45 degrees to axial", which would solve the problem?

This is why the bottom of a rocket tank is a sump (ie sloped toward the inlet), not a flat floor.

a sump at 1g sure.  at 0.00001g, is it it enough to deal with the surface tension?

You don't ever completely drain the tanks. the Ship needs propellant to EDL or to control it's deorbit if expended. Even a Depot that will remain in orbit for awhile needs to keep some propellant for maneuvers.

[eriblo]

If one applies an axial force so that the fuel settles at the base of the fuel tank, then one is left with "how does the corner opposite of the fuel transfer outlet get emptied" problem.

Diagram attached shows the problem.

The answer maybe "the force is 45 degrees to axial", which would solve the problem?

This is why the bottom of a rocket tank is a sump (ie sloped toward the inlet), not a flat floor.

a sump at 1g sure.  at 0.00001g, is it it enough to deal with the surface tension?

You don't ever completely drain the tanks. the Ship needs propellant to EDL or to control it's deorbit if expended. Even a Depot that will remain in orbit for awhile needs to keep some propellant for maneuvers.

Although they might drain the main tanks completely and do all further manoeuvering from the header tanks (as they are much easier to settle and pressurize).

[Twark_Main]

If one applies an axial force so that the fuel settles at the base of the fuel tank, then one is left with "how does the corner opposite of the fuel transfer outlet get emptied" problem.

Diagram attached shows the problem.

The answer maybe "the force is 45 degrees to axial", which would solve the problem?

This is why the bottom of a rocket tank is a sump (ie sloped toward the inlet), not a flat floor.

a sump at 1g sure.  at 0.00001g, is it it enough to deal with the surface tension?

Generally you engineer it so surface tension helps you, eg https://en.wikipedia.org/wiki/Propellant_management_device

I'm not convinced that 1e-4 m/s² is sufficient for propellant transfer. The number comes from this post, but it refers to keeping propellant settled during coast, ie in the relative absence of inertial disturbances.

In your 10 cm pipe the flow rate is roughly 2 m/s. If we imagine a really really efficient diffusor on the outlet side, this might be reduced to an exit flow rate of 0.1 m/s (and moving 20x the mass). This flow velocity would still arc 50 meters in the air! Obviously it won't do exactly that because of fluid interactions, but this scaled value gives you an idea of how much inertia-driven "roiling" you can expect inside the tank at those low gravity levels.

[OTV Booster]

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)

Even with the fix, you seem to be assuming that prop transfer occurs via the hydrostatic pressure generated by settling, which is never going to be true.  Once the prop is settled, it can be either pressure-fed or pumped.  That will happen much, much faster than it would if it were tricking out via the hydrostatics.  So your time horizons will be much shorter.

The trade is always going to be based on whether the prop available in a tanker by launching into a VVLEO is increased enough to be a net improvement over the prop needed to be expended by the depot to maintain the orbit when the tankers and targets aren't there.  That needs to be carefully calculated, probably more carefully than we can calculate from the peanut gallery.  My intuition is that you're better off with a garden-variety VLEO of 300-350km, rather than hanging ten on the edge of an accidental deorbit disaster if your ops go wrong.

How much does the atmosphere expand on the daylight side of earth?

How much and how fast does the atmosphere expand when the sun gets angry?

When the weather people speak of a pressure dome, is this a real three dimensional event and if so, what is the impact on the upper fringes of the atmosphere?

What else might change VLEO atmospheric drag?

There is appeal in using what the orbital gods give for free but not at the risk of LoM. Do we know the atmosphere well enough to identify a minimum acceptable altitude that gives acceptable drag? How rapidly does the atmosphere thicken up and what options are available to work around density change?

If a single tanker transfer, final approach to disconnect, takes an arbitrary four hours can things go sideways fast enough to matter? Once a depot filling campaign has started is there enough operational flexibility to raise orbit between tankers?

The advantage that VLEO gives to filling the depot and the possibility of settling ullage with drag is intriguing. It would be a shame to back away from it because of undefined monsters in the dark. OTOH, if it really is too dicey...

[Twark_Main]

How much does the atmosphere expand on the daylight side of earth?

How much and how fast does the atmosphere expand when the sun gets angry?

When the weather people speak of a pressure dome, is this a real three dimensional event and if so, what is the impact on the upper fringes of the atmosphere?

What else might change VLEO atmospheric drag?

There is appeal in using what the orbital gods give for free but not at the risk of LoM. Do we know the atmosphere well enough to identify a minimum acceptable altitude that gives acceptable drag? How rapidly does the atmosphere thicken up and what options are available to work around density change?

If a single tanker transfer, final approach to disconnect, takes an arbitrary four hours can things go sideways fast enough to matter? Once a depot filling campaign has started is there enough operational flexibility to raise orbit between tankers?

The advantage that VLEO gives to filling the depot and the possibility of settling ullage with drag is intriguing. It would be a shame to back away from it because of undefined monsters in the dark. OTOH, if it really is too dicey...

Honestly I'm not too worried about that. If the drag is higher than expected you just do a quick burn to raise altitude.

The problem is that it's not "what the orbital gods give for free." You're just burning off delta-v that you spent earlier getting into (a higher than necessary) orbit.

[OTV Booster]

How much does the atmosphere expand on the daylight side of earth?

How much and how fast does the atmosphere expand when the sun gets angry?

When the weather people speak of a pressure dome, is this a real three dimensional event and if so, what is the impact on the upper fringes of the atmosphere?

What else might change VLEO atmospheric drag?

There is appeal in using what the orbital gods give for free but not at the risk of LoM. Do we know the atmosphere well enough to identify a minimum acceptable altitude that gives acceptable drag? How rapidly does the atmosphere thicken up and what options are available to work around density change?

If a single tanker transfer, final approach to disconnect, takes an arbitrary four hours can things go sideways fast enough to matter? Once a depot filling campaign has started is there enough operational flexibility to raise orbit between tankers?

The advantage that VLEO gives to filling the depot and the possibility of settling ullage with drag is intriguing. It would be a shame to back away from it because of undefined monsters in the dark. OTOH, if it really is too dicey...

Honestly I'm not too worried about that. If the drag is higher than expected you just do a quick burn to raise altitude.

The problem is that it's not "what the orbital gods give for free." You're just burning off delta-v that you spent earlier getting into (a higher than necessary) orbit.

Raising orbit between tankers is reasonable. It doesn't look like a good move while a tanker is hooked up. Can the atmosphere change fast enough to require higher orbit during a transfer op? The equation probably changes with the 11 year solar cycle. We're at maximum now.

[Twark_Main]

Raising orbit between tankers is reasonable. It doesn't look like a good move while a tanker is hooked up.

Using drag to lower the orbit means you still need to repay all that delta-v using thrusters anyway. The tanker has to reach the higher initial starting orbit, so there's no real savings here. You're just constraining your altitude regime and re-orienting into a high-drag attitude (wrecking any pre-settling) for no reason.


What happens if you add small methox thrusters that re-use the large vacuum bells as their expansion nozzle? In theory you should be able to reach extremely high Isps with a very minimal system. Naturally this helps a lot for both propellant transfer and long-term orbit maintenance at lower (safer) altitude.

https://ntrs.nasa.gov/citations/20160001041

https://ntrs.nasa.gov/citations/20110014049

[TheRadicalModerate]

If one applies an axial force so that the fuel settles at the base of the fuel tank, then one is left with "how does the corner opposite of the fuel transfer outlet get emptied" problem.

The bottom of the tank is an inverted dome--a bowl.  All prop collects at the bottom.  The inlet is also at the bottom.  All you have to do is prevent slosh, which can uncover the inlet.

All the plumbing for Starship is set up to deal with this architecture.  Why would you want to reinvent it?

I don't have any problem using drag for settling, as long as it's adequate and you can maintain constant axial acceleration.  If you can't, then it becomes one of those, "Doctor, Doctor, it hurts when I do this" kinds of problems.  Don't do that.  Do something else instead.

Remember that SpaceX is a company that lost a load of Starlinks for deploying them too low right after a pair of CMEs.  That loss is nothing compared to losing a depot because of a drag oopsie.  (The Starlinks burned up.  The depot won't; it'll be landing somewhere unknown.)  I'll be very surprised if they deploy the depot lower than 300km.  If that means they have a few fewer tonnes of prop to transfer from a tanker, so be it.

[TheRadicalModerate]

Raising orbit between tankers is reasonable. It doesn't look like a good move while a tanker is hooked up. Can the atmosphere change fast enough to require higher orbit during a transfer op? The equation probably changes with the 11 year solar cycle. We're at maximum now.

You're going to need a control system that keeps acceleration even for the two vehicles no matter what.  Drag isn't guaranteed to be even, any more than thruster accelerations will be perfectly even.

What happens if you add small methox thrusters that re-use the large vacuum bells as their expansion nozzle?

This is really freakin' complicated, unless there's a incredibly compelling reason.  The only reason I can think of is if prop transfer is really, really slow.

Average on-pad Starship prop load rate comes out to something like 450kg/s, and it takes a bit more than 45min.  Let's say that on-orbit prop transfer is only a tenth as fast, so we're at 9.3hr for a 1500t transfer.  Estimate the coupled system mass at...  2000t?¹  So, for 1E-4m/s², we need 200N of thrust, which, at Isp=250s, would be a mass flow of 0.08kg/s.  That's 2.7t of methalox.  That's a rounding error on any kind of sensible margins, at least to start.

_________
¹I'm still assuming a v2 form factor for the HLS-LSS.  Shorter is better, and 1500t of prop seems to be about right to get the job done.  If they want to re-do that for a new thrust puck and v3 engines, that doesn't sound particularly hard.  Removing a few ring segments from the tankage and barrel ought to be nearly trivial.

[Twark_Main]

This is really freakin' complicated, unless there's a incredibly compelling reason.  The only reason I can think of is if prop transfer is really, really slow.

Average on-pad Starship prop load rate comes out to something like 450kg/s, and it takes a bit more than 45min.  Let's say that on-orbit prop transfer is only a tenth as fast, so we're at 9.3hr for a 1500t transfer.  Estimate the coupled system mass at...  2000t?¹  So, for 1E-4m/s², we need 200N of thrust, which, at Isp=250s, would be a mass flow of 0.08kg/s.  That's 2.7t of methalox.  That's a rounding error on any kind of sensible margins, at least to start.

_________
¹I'm still assuming a v2 form factor for the HLS-LSS.  Shorter is better, and 1500t of prop seems to be about right to get the job done.  If they want to re-do that for a new thrust puck and v3 engines, that doesn't sound particularly hard.  Removing a few ring segments from the tankage and barrel ought to be nearly trivial.

1.) I didn't expect 1e-4 m/s² will be sufficient for much smaller flow rates, let alone something that energetic (and neither did the original source of that number). You're going to get a ton of sloshing which will couple to the draining tanker and expose the sump. I would be looking at an accelerations 10x that high, if not more.

2.) You're only accounting for the last tanker flight and RPOD. For a campaign requiring N refillings, the total mass penalty should be >N/2 times as large as you calculated.


So not 2.7 tonnes, but more like 145 tonnes out of 2,000 (7.2% penalty).

Hopefully the motivation for a lightweight, high ER ullage / reboost thruster is more clear?

[Stan-1967]

What happens if you add small methox thrusters that re-use the large vacuum bells as their expansion nozzle? In theory you should be able to reach extremely high Isps with a very minimal system.

You would need to have enough mass flow rate for choked flow in the nozzle, yes?  I'm assuming you are conceptualizing the small methox thruster discharging into the MCC?  If this worked you could just use  the ignition torches.

Basically this:  https://en.wikipedia.org/wiki/Choked_flow

I haven't run any conditions, but I suspect the mass flow will not be sufficient for such a mismatch in the exit plane area and low pressure this proposal will have.  If it fails to achieve choked flow, you haven't built a rocket engine.

[InterestedEngineer]

Average on-pad Starship prop load rate comes out to something like 450kg/s, and it takes a bit more than 45min.  Let's say that on-orbit prop transfer is only a tenth as fast, so we're at 9.3hr for a 1500t transfer.  Estimate the coupled system mass at...  2000t?¹  So, for 1E-4m/s², we need 200N of thrust, which, at Isp=250s, would be a mass flow of 0.08kg/s.  That's 2.7t of methalox.  That's a rounding error on any kind of sensible margins, at least to start.

where are you getting an Isp of 250s off a cold gas thruster?

100-125 range would be doing quite well for a cold gas thruster out of 5 bar.

[TheRadicalModerate]

Average on-pad Starship prop load rate comes out to something like 450kg/s, and it takes a bit more than 45min.  Let's say that on-orbit prop transfer is only a tenth as fast, so we're at 9.3hr for a 1500t transfer.  Estimate the coupled system mass at...  2000t?¹  So, for 1E-4m/s², we need 200N of thrust, which, at Isp=250s, would be a mass flow of 0.08kg/s.  That's 2.7t of methalox.  That's a rounding error on any kind of sensible margins, at least to start.

where are you getting an Isp of 250s off a cold gas thruster?

100-125 range would be doing quite well for a cold gas thruster out of 5 bar.

I'm assuming combusting gas.  Cold gas simply doesn't close for settling, unless transfer times are unreasonably short.  It especially doesn't close if you're doing pressure-fed transfers.

[InterestedEngineer]

Average on-pad Starship prop load rate comes out to something like 450kg/s, and it takes a bit more than 45min.  Let's say that on-orbit prop transfer is only a tenth as fast, so we're at 9.3hr for a 1500t transfer.  Estimate the coupled system mass at...  2000t?¹  So, for 1E-4m/s², we need 200N of thrust, which, at Isp=250s, would be a mass flow of 0.08kg/s.  That's 2.7t of methalox.  That's a rounding error on any kind of sensible margins, at least to start.

where are you getting an Isp of 250s off a cold gas thruster?

100-125 range would be doing quite well for a cold gas thruster out of 5 bar.

I'm assuming combusting gas.  Cold gas simply doesn't close for settling, unless transfer times are unreasonably short.  It especially doesn't close if you're doing pressure-fed transfers.

It would be interesting to pass all our proposals by the data we have on what SpaceX is declaring and building. As far as I can tell, none of these proposals fit that set of data:

1. Hot gas thrusters
2. Thermospheric surfing
3. axial coupling
4. Some strange method of taking advantage of surface tension (as demonstrated on the Space Station)

Which leaves us "how are they not violating the laws of physics?".  Which they aren't,  That's #5, they profess to always follow the laws of physics.

?Shrug?

[Twark_Main]

What happens if you add small methox thrusters that re-use the large vacuum bells as their expansion nozzle? In theory you should be able to reach extremely high Isps with a very minimal system.

You would need to have enough mass flow rate for choked flow in the nozzle, yes?

Yes.

I'm assuming you are conceptualizing the small methox thruster discharging into the MCC?

No, discharging into the nozzle after the throat.