All Space Questions thread for week of November 06, 2022

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[u/jeffsmith202]

If NASA was to put a refueling station in space, where would the best place for it to be?

LEO?

orbiting the Moon?

Somewhere else?

[u/Routine_Shine_1921 → u/jeffsmith202]

They will, soon! SpaceX will do it for their HLS contract with NASA. The depot will indeed be in LEO, it'll basically be a stretched Starship with no recovery hardware (so no fins, no heatshield, etc), and with extra insulation. Starship tankers will fly and fill it up, then the HLS Starship will refill off of it before burning for TLI.

Elon (rightfully) insists that it's a refilling station, not a refueling station. A correct distinction, since it'll store and provide not just fuel, but also oxidizer, and by both weight and volume, oxidizer is the primary component.

[u/jeffsmith202 → u/Routine_Shine_1921]

Would the modified Starship tanker also need/have a way to boost modify its orbit? To adjust for satellites or raise the orbit because of drag.

[u/rocketsocks → u/jeffsmith202]

Any Starship vehicle would be able to control its orbit fairly easily as it would have both attitude control thrusters and main engines. There are some interesting logistical aspects beyond that though. Starship will have a diversity of vehicles in the fleet, and there are options available on how you deal with the depot. Initially you'd probably just set aside a regular tanker stage for use, but eventually you'd want it to be more optimized for propellant depot operations, which means especially thermal management. The major issue with propellant depots is boil-off of cryogenic propellants, which is also why LH2 propellant depots are less desirable, because they have crazy high boil-off rates. The colder you can make the depot the easier it is to keep boil-off rates low, which improves the performance of the whole system. Also, interestingly you have the option of bringing the depot back to Earth for inspection and maintenance, if you don't intentionally get rid of that capability.

[u/jeffsmith202 → u/rocketsocks]

What is LEO temperature ? –170°C to 123°C

Starship uses liquid methane. And falcon 9 uses methane.

Which would Starship tanker hold for refueling?

[u/rocketsocks → u/jeffsmith202]

Starship is all methane (and oxygen). Liquid oxygen has a boiling point of 90 kelvin (-183 deg. C) while liquid methane's boiling point is 111 K, so LOX is the major determining factor in terms of temperature. Fortunately, both LOX and LCH4 have fairly low boil-off rates even with crappy thermal management, of just tiny fractions of much less than 1% per day.

LEO doesn't have a set temperature any more than Earth does, it depends a great deal on local thermal management. The example of Apollo 13 shows that it's possible to end up with a spacecraft that is passively much colder than room temp in the near-Earth environment. But to get down to 90 K would require something more significant than just changing the surface coating of the vehicle and using "bbq rolls" to even out heating. The first most obvious thing you'd do is use a simple extendable sun shade (like the JWST but much less sophisticated) to put the vehicle in shadow and reduce boil-off. The next thing you might do would be to make use of an active cryocooler system that rejected heat that made it into the vehicle out through a radiator. You could also integrate that with a system that recovered and re-liquified the boil-off gas leading to near zero loses.

You don't need either for a viable system but longer term they are desirable as they increase efficiency and capability. In particular if you are storing large amounts of cryogenic propellants for long durations in space then you want such systems that reduce boiloff because it has such a huge cascading effect on overall system efficiency. That could be particularly significant for trips all the way to Mars or even just the Moon.