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With the Space X success at re-using rockets, my thought is that the second stage could be refueled at the ISS. Similarly, fuel could be sent to orbit Mars. My thought is it would allow for greater acceleration and therefore cut travel time.

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    $\begingroup$ why does time need to be saved? $\endgroup$ – JCRM Dec 13 '20 at 19:11
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    $\begingroup$ How does refueling at the ISS "allow for greater acceleration"? $\endgroup$ – Organic Marble Dec 13 '20 at 19:53
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    $\begingroup$ Can you explain what the benefit of refueling at the ISS is as opposed to refueling at a Tanker Starship as SpaceX is currently planning to do? $\endgroup$ – Jörg W Mittag Dec 13 '20 at 21:56
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SpaceX's intent is to use orbital refueling to allow Starship to reach destinations beyond low Earth orbit. However, these plans simply involve meeting up directly with the tanker to transfer propellant.

The ISS has no facilities for storing or transferring the liquid methane and oxygen Starship uses as propellant. If it did, you'd still need two separate rendezvous and docking operations when directly meeting with the tanker only requires one. And finally, the ISS is in a fairly highly inclined orbit. Not only would this limit the payload the Starships could carry, it wouldn't necessarily line up with your intended destination.

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    $\begingroup$ "The ISS has no facilities for storing or transferring propellant." – That is not technically correct. In fact, quite the opposite is true: the ISS is the closest real-world experience we have to what SpaceX is trying to do. The ISS is the only spacecraft in the world that has experience with regular on-orbit refueling. However, that's the "other direction" from what the OP is thinking of. $\endgroup$ – Jörg W Mittag Dec 13 '20 at 21:55
  • $\begingroup$ One case where a propellant depot might be worth having, even if it isn't the ISS: Staging large amounts of cryogens with enough solar panels, radiators, and cryocoolers to prevent it boiling off. $\endgroup$ – ikrase Dec 13 '20 at 22:34
  • $\begingroup$ @JörgWMittag clarified that it's LCH4/LOX propellants that the ISS can't handle. Though, while the ISS can be supplied with hypergolic propellants, I'm not aware of it being able to transfer them to a vehicle. $\endgroup$ – Christopher James Huff Dec 13 '20 at 23:03
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    $\begingroup$ @JörgWMittag I'm curious to hear more, so I've just asked What is the extent of on-orbit refueling experience at the ISS? Which fuels? which spacecraft? $\endgroup$ – uhoh Dec 13 '20 at 23:08
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The point from the Earth orbit from where we launch has little influence on the travel time. Currently all missions to Mars use the less energetic path. Basically, we accelerate the spacecraft until we reach an elliptical transfer orbit around the Sun with the periapsis (closes point to the sun) in Earth Orbit and the apoapsis (further point from the sun) intersecting Mars orbit. Once in that that trajectory the spacecraft coasts the rest of the way, braking once it reaches Mars's sphere of influence. This is the best you can get with current technologies and takes around 259 days, about 8.5 month, and most missions have barely enough fuel to perform this maneuvers, most Martian Rovers don't even use fuel to break, instead perform an aerobreak maneuver in Mars atmosphere. The only option to cut that time is to accelerate during extended periods of time during the first half of the trip and break during the same amount of time during the second half. We can't do that yet, and definitely not with chemical rockets, although some ion and plasma engines (some already in prototype and demonstration stage) have the theoretical capacity to reduce the trip to 2 months. Starship is designed to use the first approach and it will be limited to the less energetic path, so refueling in the ISS or refueling using a tanker makes little to no difference to the travel time, is completely limited to that specific transfer orbit. Added to this, the ISS has no infrastructure for refueling or storage and given the complexity of the structure it would be incredibly expensive to implement something like that, specially compared to use a reusable tanker from SpaceX.

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    $\begingroup$ A faster transit can be done using an elliptical orbit that goes past Mars orbit. Starship is intended to do this, reducing transit time to 3-4 months, and orbital refueling is what makes it possible using chemical engines without throwing away the majority of the vehicle. $\endgroup$ – Christopher James Huff Dec 14 '20 at 19:28

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