I see a lot of doubt and uncertainty around SpaceX orbital refilling plans (once they get it working). Esp around the unknowns of boil off rates, capacity, operational tempo, insulation, etc.
I want to provide a bit of context here to help interpret what we see 1/
I want to provide a bit of context here to help interpret what we see 1/
First, regarding the question of whether the depot would be filled just in time for a particular mission or maintained partially filled in between missions:
the main reason to fill just for a particular mission is to avoid boil off, but if you're trying your best to avoid 2/
the main reason to fill just for a particular mission is to avoid boil off, but if you're trying your best to avoid 2/
boil off by not leaving excess prop in the depot, you have no need for capacity much larger than the largest set of tanks you're going to refill for one mission. It's very much counterproductive to go larger, since that increases your tank surface area and your boil off rate. 3/
Here, we've gotten some conflicting hints: on the one hand, Elon talking about high cadence tanker flights minimizing boil off in the depot; otoh renders of a depot that is longer than a standard Starship (which already has >1.5x the internal volume of its tankage)
This might 4/
This might 4/
be a matter of two different approaches SpaceX has considered. And maybe they've decided on one, or maybe they haven't that.
If they do go for extended capacity, they're almost certainly going to want some highly effective insulation, b/c w/o it the boil off rates would be 5/
If they do go for extended capacity, they're almost certainly going to want some highly effective insulation, b/c w/o it the boil off rates would be 5/
prohibitively high to justify the operational advantages of a large tanker. The incident sunlight and IR from the Earth would average in the neighborhood of 400kW; most can be reflected, but you're at least going to have 10s of kW absorbed (polished stainless has an 6/
impressively low emissivity of 0.075 per engineeringtoolbox.com/emissivity-coe…, so they might be able to go as low as ~30kW w/o any coating)
W/o insulation, the skin temp will be low enough that most is not radiated back out, so that's tens of kW of heating, enough to boil off on the order 7/
W/o insulation, the skin temp will be low enough that most is not radiated back out, so that's tens of kW of heating, enough to boil off on the order 7/
10t of methalox a day (rough order of magnitude here).
(oxygen has a heat of vaporization of ~210kJ/kg and methane ~510kJ/kg, and a bit over half the heat would be absorbed by oxygen, so you can easily end up w/ a kg of boil off in less than 10 seconds)
8/
(oxygen has a heat of vaporization of ~210kJ/kg and methane ~510kJ/kg, and a bit over half the heat would be absorbed by oxygen, so you can easily end up w/ a kg of boil off in less than 10 seconds)
8/
This would be barely an inconvenience if you're launching tankers daily (and you could get it to around half that rate by using only ~1200t of tankage), not even necessitating an extra tanker launch for most missions, but if you're wanting to keep your tanks filled, it can add 9/
a couple dozen tanker flights a year just for depot maintenance.
And at these power levels, it's also impractical to add a recondenser without first adding insulation -- you'd be looking at power consumption on the order of hundreds of kW.
You could use tricks like 10/
And at these power levels, it's also impractical to add a recondenser without first adding insulation -- you'd be looking at power consumption on the order of hundreds of kW.
You could use tricks like 10/
powering a recondenser with vaporized propellant -- (you can even avoid a radiator by dumping all of your waste heat into the propellant that's fueling your pumps, at the cost of performance), and recondensing only oxygen. You can get prop loss down to a fraction of what it 11/
would otherwise be this way, but it's still not enough to justify an XL depot (or even a full Starship volume depot) w/o insulation.
12/
12/
So two viable paths:
a) No insulation, shiny exterior, ~1.2kt tanks, high cadence tanker launches, depot left empty between missions.
b) Highly effective insulation, potentially larger tanks, depot filled whenever it's convenient, left partially filled between missions.
13/
a) No insulation, shiny exterior, ~1.2kt tanks, high cadence tanker launches, depot left empty between missions.
b) Highly effective insulation, potentially larger tanks, depot filled whenever it's convenient, left partially filled between missions.
13/
But what would that insulation look like?
If you go with something like typical 25mm SOFI at ~8mW/m/K of thermal conductivity (in vacuum) (ntrs.nasa.gov/citations/2011…)
you get on the order (rough approximation again) of 50 Kelvin of temperature drop across the foam, which isn't 14/
If you go with something like typical 25mm SOFI at ~8mW/m/K of thermal conductivity (in vacuum) (ntrs.nasa.gov/citations/2011…)
you get on the order (rough approximation again) of 50 Kelvin of temperature drop across the foam, which isn't 14/
enough to raise the outer temperature enough to raise the radiated heat enough (proportional to fourth power of temperature) to substantially reduce the heat that'll soak into the tanks.
You can go with thicker foam or a better (and more expensive) insulator like aerogel, but 15/
You can go with thicker foam or a better (and more expensive) insulator like aerogel, but 15/
it's a low return approach. You'd be better of just going to a higher reflectivity coating but even that's not going to be enough (and the lower your absorbed heat, btw, the more insulation you need to make a difference).
What will work wonders is multi layer insulation /16
What will work wonders is multi layer insulation /16
layers of vac-separated sheets that (ideally) minimize conductivity and insulate radiatively. Esp. w/ low emissivity coatings fine-tuned to the expected temperature of the layers, the sky is the limit for thermal resistance, but 1-2 orders of magnitude reduction over what 17/
you'd get with an uninsulated but polished tank is feasible without a lot of effort. That would bring the boil off rate low enough that it'd make sense to add a modestly sized recondenser, radiator, and solar panels to get you to zero boil-off.
Arbitrarily low leakage would 18/
Arbitrarily low leakage would 18/
be the only source of loss, aside from orbital maintenance (easily under a few tons /yr depending on altitude) and milligee provision during transfers.
/19
/19
One more note on ZBO. For option (a) (high tempo, no insulation), it's still possible to get zero boil off. 150t of fresh subcooled propellant can have a lot of heat capacity before it gets to boiling (depending on how cold it was when filled and how much it warmed up during 20/
launch).
At 30kW of heating, 150t of methalox will gain around 15 degrees Kelvin in a day...
All that to say, they've got options, but the size of depot will say a lot about how they want to use it, which will say a lot about what kind of equipment it'll likely need.
21/21
At 30kW of heating, 150t of methalox will gain around 15 degrees Kelvin in a day...
All that to say, they've got options, but the size of depot will say a lot about how they want to use it, which will say a lot about what kind of equipment it'll likely need.
21/21
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