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Using simple math or playing Kerbal SP inevitably leads to re-using first stages and refueling in orbit for missions far away. It seems, it is always the most economical and simple approach to refuel in orbit and/or assemble greater structures in space. It enables to circumvent rocket size constraints and it enhances the Delta-V. It is also cheaper and allows more launches.

What is the reason why we didn't do this for 70 years (despite ISS)?

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    $\begingroup$ Well, according to this blog post (caseyhandmer.wordpress.com/2019/10/02/…), it's because orbital refueling is harder than it sounds due to "orbit" not being one location. A fuel depot in one orbit doesn't help a spacecraft in a different orbit. $\endgroup$ – Pitto Sep 21 at 12:27
  • $\begingroup$ Manual assembling by astronauts wearing spacesuits is very expensive, takes a lot of time and requires a lot of underwater training. $\endgroup$ – Uwe Sep 21 at 12:54
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    $\begingroup$ There's a very large space station in orbit as I write this that was assembled there. It gets refueled by tankers. $\endgroup$ – Organic Marble Sep 21 at 13:20
  • $\begingroup$ As I understand it, the commercial moon landing bids include on orbit assembly/refuelling $\endgroup$ – JCRM Oct 3 at 0:08
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Several things that KSP does not model:

The challenges of actually docking two things mechanically together without a handy human such that the plumbing works (leaks have a tendency to go bang)

The extra instrumentation required to dock with enough reliability to not risk the mission (need duplicate equipment). Notably for the ISS many craft just take proximate position and a human uses the arm to actually do the final connection, making a much simpler design but requiring on orbit assistance.

The challenges of getting fuel from one tank to another in zero G

Stock KSP uses magic 'fuel' and 'oxidizer' that has infinite shelf life and good performance, Deep space missions normally use Hydrazine related compounds for good storage, high performance launchers generally cryogenics. You cannot top off one from the other.

Default KSP launches from the equator into a 0 inclination orbit, normally targeting low inclination targets. For real world launch sites and targets almost certainly launching the tanker will require some degree of inclination change to rendezvous, costing extra fuel.

KSP does not model the setup costs per launch (tracking, range safety, fire crews etc) that make launching two rockets more expensive than one larger rocket. KSP launches can also get onto the pad instantly and never have technical problems, weather or stray fishing boats causing launches to be scrubbed, all of which can add costs/complexity to a 'launch twice' mission.

KSP mechanical parts (antenna and solar panels) are 100% reliable and very light, real world physics start to matter if your first launch has to first deploy equipment to operate, then stow it again one or more times for refueling.

So in orbit assembly will always come with a weight penalty, and so far for all interplanetary missions it has seemed better to fly in a single stack and use that weight for other things. It could be technically claimed that Apollo did in orbit assembly (but not fueling), the various space stations are assembled and refueled in orbit, and the space shuttle program did several missions involving rendezvous with existing vehicles for various reasons

Things that potentially change the above:

If you have fuel/consumables coming from somewhere other than Earth to fuel an earth launched mission.

If you have a suitably situated manned space station with spare parts and assistance for docking. ISS is highly inclined to any likely moon or Mars departure trajectory.

If the docking vehicles are physically massive (required cameras/radars/docking hardware etc make up a lower % for a 100 ton mars mission than a 100kg satellite).

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  • $\begingroup$ +1 but this part of your answer confuses me "So in orbit assembly will always come with a weight penalty, and so far it has seemed better to fly missions in a single stack and use that weight for other things." Is this question and answer just about assembling boosters or stages in space? Because there's a very large space station in orbit right now that was assembled there. $\endgroup$ – Organic Marble Sep 21 at 13:18
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    $\begingroup$ @OrganicMarble Yep you are right - and that works because you are not trying to fly anywhere else afterwards. Will make an edit. $\endgroup$ – GremlinWranger Sep 21 at 13:28

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