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It is sometimes suggested that fuel produced on the Moon or on asteroids could be launched more cheaply to dock with and refuel for example communication satellites in GEO to extend their operating lifetimes (as in orbital station keeping). This has of course to compete with simply launching more fuel from Earth together with the satellite to begin with.

But isn't the economic lifetime in GEO limited more by the aging of the electronics than by the fuel consumption? Hubble Space Telescope was upgraded with new instruments several times and practically worked as a whole series of different telescopes in some respects. If com sats are designed to be upgraded robotically, wouldn't it require much less launch mass to do so than to refuel them in mid-life or launch a completely new satellite? An upgrade mission could even be launched as a secondary payload with another new satellite to GEO.

Isn't it in general more economical to launch the latest "smart mass" that in the end is producing the commercial service, than bulk mass like fuel, if one is to go through the hassle of docking anyway?

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  • $\begingroup$ There may be a trend to go towards all-electric propulsion (ion engines) - at least in the case of large communications satellites in GEO. The amount mass of propellant is a small fraction of what's needed for chemical thrusters, so the refueling may become even less of a question in the future. $\endgroup$
    – uhoh
    Commented Jan 11, 2017 at 15:41
  • $\begingroup$ At least part of it would depend on the satellite and its mission. E.G. a spy satellite in a low orbit would seem higher priority for upgrade than a weather satellite in geosynchronous orbit. $\endgroup$
    – Andrew Thompson
    Commented Jan 12, 2017 at 0:09
  • $\begingroup$ Is the sentence "If com sats are designed to be upgraded robotically, wouldn't it require much less launch mass to do so than to refuel them in mid-life or launch a completely new satellite?" the key part of your question? If so, then the part about making fuel on the moon is confusing me. (It may be cheaper only if one ignores the billions developing the technology to make it there and building and operating the plant to do so.) Overall I think this is a really interesting question! $\endgroup$
    – uhoh
    Commented Jan 12, 2017 at 3:54
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    $\begingroup$ @uhoh In general I think that resources from space will be bulk mass such as fuel and structure and will need a huge space program to supply in order to become economical. If satellite buses are reused in orbit and upgraded or even re-purposed by low mass electronics upgrades, then Earth will be the supplier also of fuel at the same time. $\endgroup$
    – LocalFluff
    Commented Jan 12, 2017 at 8:49
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    $\begingroup$ @Puffin and everyone. Here's a presentation made this week about NASA's tech demo of an orbital servicing mission. At least some real engineering is being done on that part of this topic. It could pay off by rescuing one single multi-billion space mission. But instead of transferring fuel, I think it's easier to just dock to the target and tug it with the tugging spacecraft's own engine. Which requires hardware made on Earth(R), not fuel from the Moon. $\endgroup$
    – LocalFluff
    Commented Jan 14, 2017 at 13:12

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Warning I've written this answer and there have been no opportunities for citations - that means you're just trusting this random guy on the Internet if you believe me!

I suspect the figure of merit for judging which is better (upgrading or refueling) is more a question of practicality than anything else.

We are extremely unlikely to develop a new significantly more effect fuel that can work with existing propulsion systems. We might be able to make small improvements on the fuel, or great new fuels that require different a propulsion system, but the key thing here is the old fuel worked well enough and is likely to do so for the foreseeable future (station keeping operations are reasonably secular).

We are extremely likely to develop new technology to make satellites harder/better/faster/stronger (or maybe more efficient/ faster down link/ lower power usage). This is great news if you run a satellite manufacturing/launch business because we can make a satellite that's 5 times better every 10 years (not correct numbers). But if you're the client/end user it sucks, for the same reason my old PC motherboards can't be used to play Kerbal Space Program:

To upgrade you need all interfaces to be forward compatible.

This means supplying the correct power (right voltages), having data transfer and storage that can handle increased loads, and even making sure the thermal system can keep your new hardware hot/cold enough. It might be a simple coefficient of thermal expansion problem or it might be that you have a square plug and a round hole. (For reference my old motherboards don't have the appropriate slots for graphics interfaces).

So although upgrading hardware sounds better in an ideal situation it's likely to be almost impossible to implement without a whole lot of tinkering.

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  • $\begingroup$ I think that's a good analogy. There's another idea that works with the same analogy which is that plenty of PC motherboard items do have mix and match properties (fans, memory, the motherboard itself w.r.t to the PC etc) and are also user serviceable (most important!) and so there is a potential business for "upgrade as you go a long". However it will also have end stops in the process because of the reasons you've mentioned. $\endgroup$
    – Puffin
    Commented Jan 14, 2017 at 12:25

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