Starship On-orbit refueling - Options and Discussion

[Twark_Main]

Doing (roughly) the above math, I don't know where this 8,000 nanometer spectral cutoff comes from. By my curves, to minimize tank heating the surface should ideally be maximally reflective through the entire infrared band.

Well, there's this quote from the guys who came up with Solar White:

"First, assume that a perfect coating was developed with a cutoff at about 4 µm, so that only 1% of the Sun’s energy was absorbed, i.e. a ratio alpha/epsilon = 0.01. For a sphere the temperature now drops to 88 K, below the 90 K needed to maintain LOX, but still too high to run a superconductor. But if we can find a material with a transition wavelength at about 8 µm, where only 0.1% of the Sun’s energy is absorbed, then the sphere temperature will drop to about 50 K. This would not only allow superconductors to operate, but would allow LOX storage to occur at higher density and at lower pressure. Figure 7 shows the temperatures that could be achieved if a perfect material were available to coat a sphere at 1 AU from the Sun."
(Cryogenic Selective Surfaces: Final Report on a Phase I NIAC Study, p. 10, February 2016, Robert C. Youngquist and Mark A. Nurge)

That explains it.

Their 8 micron cutoff can only reach LOX temperatures in deep space. The authors don't say it's the "ideal" cutoff either (Figure 7).

We're doing calculations for LEO (significant IR heating), so 8 microns is not ideal here.

In LEO Solar White is also incapable of reaching LOX temperatures, which the authors themselves acknowledge (Section 2.3).

Phew! Mystery solved.

[Greg Hullender]

We're doing calculations for LEO (significant IR heating), so 8 microns is not ideal here.

In LEO Solar White is also incapable of reaching LOX temperatures, which the authors themselves acknowledge (Section 2.3).

What I would say is that, for a Starship in LEO, there is no completely passive ZBO solution. Not at the moment, anyway. There must be some cryocooler, with all the extra hardware that implies. The best that Solar White can do for you is reduce the mass of the cryocooler.

Are we in broad agreement?

[Paul451]

What I would say is that, for a Starship in LEO, there is no completely passive ZBO solution. Not at the moment, anyway. There must be some cryocooler, with all the extra hardware that implies. The best that Solar White can do for you is reduce the mass of the cryocooler.

Or use boil-off.

I did a back-of-the-envelope calc earlier in the thread (using someone else's thermal figures), and depending on how much heat you can reject, selectively boiling-off methane (boiling off LOx is less effective) can remove the remaining heat for sub-20tonnes of loss for a modestly slow refuelling cycle. Given the "propellant is cheap" model of operations, that's doable for a first generation depot.

But to make the decision, you have to know what kind of heat input you are dealing with.

[Twark_Main]

If there's no advantage to using a selective solar coating (which seems to be true), then we're back to MLI.

Boil-off can be made arbitrarily low by adding more MLI layers.

[TheRadicalModerate]

What I would say is that, for a Starship in LEO, there is no completely passive ZBO solution. Not at the moment, anyway. There must be some cryocooler, with all the extra hardware that implies. The best that Solar White can do for you is reduce the mass of the cryocooler.

Or use boil-off.

I did a back-of-the-envelope calc earlier in the thread (using someone else's thermal figures), and depending on how much heat you can reject, selectively boiling-off methane (boiling off LOx is less effective) can remove the remaining heat for sub-20tonnes of loss for a modestly slow refuelling cycle. Given the "propellant is cheap" model of operations, that's doable for a first generation depot.

But to make the decision, you have to know what kind of heat input you are dealing with.

I may be having a thermodynamic brain fart, but when you do this just from an energy balance standpoint, you've got a certain amount of heat seeping in, and in equilibrium you have to remove that heat by boiling prop and venting it, right?  So the heat rate that seeps in, divided by the enthalpy of vaporization, gives you the boiloff rate.

No cooling effects need to be considered.  Yeah, they're there, but the heat just warms stuff up until it vaporizes more prop.  If you know the heat balance, you know the boiloff rate.

[Greg Hullender]

If there's no advantage to using a selective solar coating (which seems to be true), then we're back to MLI.

I wouldn't say no advantage. At a minimum, you could use the paint (not the tiles) on the parts of the ship that don't have to be jet black and that would reject 90% of the heat from sun exposure at very little cost. But it definitely doesn't buy you ZBO. Not with a standard Starship, anyway.

At this point, I'd like to talk a little bit about exactly how the fuel is going to be transported. (Apologies if this has already been discussed. Extra apologies if it's already been discarded with good reason!) :-)

I know Elon doesn't like depos, but I'm starting to think it makes more sense to have a special "fuel depot" (for want of a better word) that a standard-issue Starship would carry as cargo rather than have a special tanker version of Starship. That would mean that two Starships would never dock to each other directly--not for purposes of fueling, anyway.

My reasoning is as follows:

First, that means you don't have an extra version of Starship with extra-large tanks. Second, the fuel depot would have the hardware required to mate to the QD ports on a Starship and do the refueling, so you wouldn't need a version of Starship with that equipment on it somewhere. (Or, worse, add that hardware to every Starship.)

The basic fuel depot would just be fuel tanks, the docking attachment, and pumps. It wouldn't ever leave the cargo bay of the Starship that brought it up to orbit. It's the sort of thing you'd use to refuel the Lunar Starship, assuming that only needs one extra load of fuel. You'd launch the two Starships at about the same time so boiloff wouldn't be a big concern.

A more complex fuel depot would be larger and have a cryocooler, radiator, extra insulation, and QD ports identical to those on a Starship. It would be put in orbit by itself and get filled up on multiple visits by Starships carrying basic depots full of fuel and LOX. (Note that you wouldn't need to develop this until after the lunar missions.)

That sort of assembly probably would benefit from extensive use of Solar White tiles. Yes, it would still need a cryocooler, but the tiles ought to reduce the mass of the cooler, the radiator, and the solar panels needed to power it.

You would cope with the orbital precession problem by timing the initial orbit and the refueling orbits so the depot is in the right place at the right time to refuel the Starship dedicated to the mission. After the mission Starship departed, you could send up another Starship to retrieve the empty depot.

If you wanted to use the same depot for multiple missions, you could use something like the RAAN-Agnostic 3-Burn Departure Methadology for Deep Space Missions from LEO Depots that has been talked about earlier.

Again, apologies if this has already been debated/discarded.

[Greg Hullender]

What I would say is that, for a Starship in LEO, there is no completely passive ZBO solution. Not at the moment, anyway. There must be some cryocooler, with all the extra hardware that implies. The best that Solar White can do for you is reduce the mass of the cryocooler.

Or use boil-off.

I did a back-of-the-envelope calc earlier in the thread (using someone else's thermal figures), and depending on how much heat you can reject, selectively boiling-off methane (boiling off LOx is less effective) can remove the remaining heat for sub-20tonnes of loss for a modestly slow refuelling cycle. Given the "propellant is cheap" model of operations, that's doable for a first generation depot.

But to make the decision, you have to know what kind of heat input you are dealing with.

Maybe I'm missing something, but how is this different from just doing nothing? (Nothing except venting gas as needed based on the pressure, that is.)

[Twark_Main]

If there's no advantage to using a selective solar coating (which seems to be true), then we're back to MLI.

I wouldn't say no advantage. [...]

I mean there's no advantage to the selectivity, ie the perfect paint would completely reject all wavelengths. There is no intentional cutoff.

Same principle, but in reverse: https://en.wikipedia.org/wiki/Selective_surface

The closest thing that exists in real life is MLI.

First, that means you don't have an extra version of Starship with extra-large tanks. Second, the fuel depot would have the hardware required to mate to the QD ports on a Starship and do the refueling, so you wouldn't need a version of Starship

Developing an entirely new vehicle from scratch is worse than reusing non-recurring engineering.

[Robotbeat]

If there's no advantage to using a selective solar coating (which seems to be true), then we're back to MLI.

I wouldn't say no advantage. [...]

I mean there's no advantage to the selectivity, ie the perfect paint would completely reject all wavelengths. There is no intentional cutoff.

Same principle, but in reverse: https://en.wikipedia.org/wiki/Selective_surface

The closest thing that exists in real life is MLI.

There is some advantage: a selective coating can reject the wavelengths of sunlight but still efficiently radiate heat to space in so-called thermal IR wavelengths.

There are thermodynamic limits to this, but it is possible.

[Greg Hullender]

Developing an entirely new vehicle from scratch is worse than reusing non-recurring engineering.

Sure, but I don't think what I'm describing is a vehicle. It's little more than two tanks, two pumps, and two hoses. In the basic form, it doesn't even leave the cargo bay of a Starship.

[oldAtlas_Eguy]

In thinking about the case of each tanker shipping to LEO ~200t of prop. And the HLS and all the tankers have same size tanks of 1500t. So that they can lift the 200t to LEO and the HLS can have the extra DV to not have any mass restrictions problems to met the mission requirements of leaving LEO going to NRHO, leaving HRHO to Surface, return from surface back to NRHO after having been on the surface for 30+ days and all of this time having boil off occur.

It is much more simple to manage the amount of prop than many other items. A side note is that since the collector has only 1500t tanks but the tankers bring up 1600t total there is 100t of prop that can boil away while waiting for all those tankers to dock to the collector so it can dock to the HLS to transfer prop.

Thus special measures to reduce boil off may not be applicable for the prop collector at LEO. But for the depot function at HRHO there is likely some reasoning for some level of additional boil off mitigation.

[Twark_Main]

If there's no advantage to using a selective solar coating (which seems to be true), then we're back to MLI.

I wouldn't say no advantage. [...]

I mean there's no advantage to the selectivity, ie the perfect paint would completely reject all wavelengths. There is no intentional cutoff.

Same principle, but in reverse: https://en.wikipedia.org/wiki/Selective_surface

The closest thing that exists in real life is MLI.

There is some advantage: a selective coating can reject the wavelengths of sunlight but still efficiently radiate heat to space in so-called thermal IR wavelengths.

There are thermodynamic limits to this, but it is possible.

Nope. That's exactly what I'm disclaiming.

The radiated heat into space is miniscule. It is totally swamped by the extra thermal IR absorbed from the Earth.

Note that we've all been using a "single particle" model here. If you have a surface that only ever points toward space and never has a view of the Earth, then the story changes, obviously.

[Robotbeat]

If there's no advantage to using a selective solar coating (which seems to be true), then we're back to MLI.

I wouldn't say no advantage. [...]

I mean there's no advantage to the selectivity, ie the perfect paint would completely reject all wavelengths. There is no intentional cutoff.

Same principle, but in reverse: https://en.wikipedia.org/wiki/Selective_surface

The closest thing that exists in real life is MLI.

There is some advantage: a selective coating can reject the wavelengths of sunlight but still efficiently radiate heat to space in so-called thermal IR wavelengths.

There are thermodynamic limits to this, but it is possible.

Nope. That's exactly what I'm disclaiming.

The radiated heat into space is miniscule. It is totally swamped by the extra thermal IR absorbed from the Earth.

Sure, it depends on how good the solar rejection is, and the orbit you’re in. If
You body is at human body temperature, you’re emitting 500W/m^2 and receiving 1350W/m^2 from the sun. I think it’s perfectly doable to make a coating that isn’t just insulating but actually allows heat to escape. At 6 bar, methane’s boiling point is 138K, so you emit a factor of 25 times less. But STILL feasible to cool if your emissivity is 100 times higher at thermal IR than sunshine wavelengths.

[Twark_Main]

Developing an entirely new vehicle from scratch is worse than reusing non-recurring engineering.

Sure, but I don't think what I'm describing is a vehicle. It's little more than two tanks, two pumps, and two hoses. In the basic form, it doesn't even leave the cargo bay of a Starship.

So... a box within a box?

Delete the tanks, use Starship's. Now you just two pumps and two hoses. That's a win.

(Now obviously it's not that simple since you need insulation too, but you didn't list that in your list.  )

[Twark_Main]

If there's no advantage to using a selective solar coating (which seems to be true), then we're back to MLI.

I wouldn't say no advantage. [...]

I mean there's no advantage to the selectivity, ie the perfect paint would completely reject all wavelengths. There is no intentional cutoff.

Same principle, but in reverse: https://en.wikipedia.org/wiki/Selective_surface

The closest thing that exists in real life is MLI.

There is some advantage: a selective coating can reject the wavelengths of sunlight but still efficiently radiate heat to space in so-called thermal IR wavelengths.

There are thermodynamic limits to this, but it is possible.

Nope. That's exactly what I'm disclaiming.

The radiated heat into space is miniscule. It is totally swamped by the extra thermal IR absorbed from the Earth.

Sure, it depends on how good the solar rejection is, and the orbit you’re in. If
You body is at human body temperature, you’re emitting 500W/m^2 and receiving 1350W/m^2 from the sun. I think it’s perfectly doable to make a coating that isn’t just insulating but actually allows heat to escape. At 6 bar, methane’s boiling point is 138K, so you emit a factor of 25 times less.

I ran numbers at 50 K, as requested. I can re-run them at this temperature. Give me a bit.

Edit: the boiling point of LOX at 6 bar is only 111.2 K. I'll run that too.

But STILL feasible to cool if your emissivity is 100 times higher at thermal IR than sunshine wavelengths.

That's the problem, it's not just the sunshine wavelengths. That's what was throwing us off before.

You also have to add in the incoming thermal IR from the Earth (if your surface has any non-zero view factor).

[Greg Hullender]

Delete the tanks, use Starship's. Now you just two pumps and two hoses. That's a win.

But is that a win? Now you've got another kind of Starship--one that can't be used for any other purpose. Is that desirable? Also, what kind of plumbing do you need to draw propellant from the bottoms of the LOX and CH4 tanks? That is, where do the attachments go and how does the docking work? Ideally, the attachments would be at the bottom of the tanks, since that's where the pipes will need to attach. But can you add a "male QD" at the bottom of a Starship without major changes? And if you try to put it anywhere else, don't you get unacceptable amounts of extra plumbing?

Putting it all in the cargo bay just seems a lot cleaner to me. (Not that I'm an expert, of course.)

[Robotbeat]

If there's no advantage to using a selective solar coating (which seems to be true), then we're back to MLI.

I wouldn't say no advantage. [...]

I mean there's no advantage to the selectivity, ie the perfect paint would completely reject all wavelengths. There is no intentional cutoff.

Same principle, but in reverse: https://en.wikipedia.org/wiki/Selective_surface

The closest thing that exists in real life is MLI.

There is some advantage: a selective coating can reject the wavelengths of sunlight but still efficiently radiate heat to space in so-called thermal IR wavelengths.

There are thermodynamic limits to this, but it is possible.

Nope. That's exactly what I'm disclaiming.

The radiated heat into space is miniscule. It is totally swamped by the extra thermal IR absorbed from the Earth.

Sure, it depends on how good the solar rejection is, and the orbit you’re in. If
You body is at human body temperature, you’re emitting 500W/m^2 and receiving 1350W/m^2 from the sun. I think it’s perfectly doable to make a coating that isn’t just insulating but actually allows heat to escape. At 6 bar, methane’s boiling point is 138K, so you emit a factor of 25 times less.

I ran numbers at 50 K, as requested. I can re-run them at this temperature. Give me a bit.

Edit: the boiling point of LOX at 6 bar is only 111.2 K. I'll run that too.

But STILL feasible to cool if your emissivity is 100 times higher at thermal IR than sunshine wavelengths.

That's the problem, it's not just the sunshine wavelengths. That's what was throwing us off before.

You also have to add in the incoming thermal IR from the Earth (if your surface has any non-zero view factor).

I already ran the numbers. That’s in my post. It’s just proportional to absolute temperature to the fourth power. 5.67e-8W/(m^2*Kelvin^4) is the Proportionality constant, the Stefan-Boltzmann constant.

Might be in an elliptical orbit or something so Earthshine isn’t bad, averaged overall the whole orbit.

[LMT]

...for a Starship in LEO, there is no completely passive ZBO solution. Not at the moment, anyway. There must be some cryocooler, with all the extra hardware that implies.

Well, for a dedicated depot Starship, you might use a passive V-Groove radiator, as on SPHEREx.  In fact, SPHEREx cools passively to LOX freezing point, past depot requirement.

--

Conceivably, three Mylar sheets might be stored in a tiny depot payload "cap".  In LEO, they're released down the hull, under slow rotation with small propulsive acceleration.

The deployed form requires little structure; e.g., a few magnets at the depot base, plus cables.  Cables could be wound on spools on the inner sheet edge.  As the sheet descends past magnets under propulsive acceleration, each cable's iron tip attaches to a magnet.  Then propulsion ends, rotation rate increases, and cables unwind, to deploy the inner sheet.  The other sheets deploy on further cables.  No cryocooler, coatings, or insulation required.

QD is exposed at depot base.  To initiate filling, the depot halts rotation for QD access.  (Inverted positioning for QD attachment?)

A simple formula to calculate V-Groove passive cryogenic cooling is given in Bhandari et al. 2020.

Refs.

Bhandari, P., Moore, B., Bolton, D. and Aboobaker, A., 2020, July. Design and Analysis of V-Groove Passive Cryogenic Radiators for Spaceborne Telescopes & Instruments. 2020 International Conference on Environmental Systems.

[Twark_Main]

Delete the tanks, use Starship's. Now you just two pumps and two hoses. That's a win.

But is that a win? Now you've got another kind of Starship--one that can't be used for any other purpose. Is that desirable?

Versus some completely new hardware of equal size that is also not usable for any other purpose? Yeah.

Also, what kind of plumbing do you need to draw propellant from the bottoms of the LOX and CH4 tanks? That is, where do the attachments go and how does the docking work? Ideally, the attachments would be at the bottom of the tanks, since that's where the pipes will need to attach. But can you add a "male QD" at the bottom of a Starship without major changes? And if you try to put it anywhere else, don't you get unacceptable amounts of extra plumbing?

Don't worry. It's just two tubes and two pumps! 

[Twark_Main]

I already ran the numbers. That’s in my post. It’s just proportional to absolute temperature to the fourth power. 5.67e-8W/(m^2*Kelvin^4) is the Proportionality constant, the Stefan-Boltzmann constant.

I'm doing something different, actually. See upthread.

Might be in an elliptical orbit or something so Earthshine isn’t bad, averaged overall the whole orbit.

Might be. Many won't be.

It would be a huge limitation if there was no good solution for LEO.