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I deemphasized upper stages referred to as space tug to increase emphasis on reusable vehicles, which I agree is the more proper use of the term. But I don't think it can be claimed that the term originated with the STS proposal, and it should be noted that expendable upper stages are sometimes (granted, much less often) referred to as "space tugs," especially expendable stages used primarily to insert payloads into precise orbits, like Fregat and Sherpa. Is that acceptable? A(Ch) 02:54, 26 July 2014 (UTC)[reply]
You're quite right; the term had to exist in the 1950s, as Murray Leinster's novel proves. I tried to restructure the article, maintaining an historical progression. I agree they should be split between reusable and expendable, but I'm not familiar with the details of the newer proposals. Are they all mostly reusable? JustinTime55 (talk) 19:33, 3 October 2014 (UTC)[reply]
The modern replacement for an expendable space tug is just a relightable upper stage. 2nd or 3rd, usually. For instance, a Russian Proton-M second stage relights multiple times when placing an object into GTO, placing it closer to a circular orbit than, say, a Falcon 9v1.1 GTO launch. (The difference in delta-V is about 900m/s between the GTO placements for launchers, in favour of the Proton-M.) That's really close to what might have been called an expendable space tug in the 50s, with the notable difference that the Proton-M upper stage doesn't launch separately and then dock with the payload of a second launch.
That's really the key (and only) difference. An expendable space tug would launch on one vehicle, and payload would launch on another. Today this concept has been replaced with the more practical idea of bigger launch vehicles (Atlas V, Delta 4 Heavy, and especially the Proton-M and Ariane 5), with larger, more capable upper stages.
The "modern" idea of a space tug is a tug that is launched and remains in orbit permanently, usually in conjunction with an orbital fuel depot. Rather than just tug until it runs out of fuel, such a tug would move sats around into different orbits, then return to the fuel depot until needed (like a Roomba!). The depot itself would need to be refueled at regular intervals, especially if it used Hydrogen/LOX fuel, as the Hydrogen would slowly boil away even if it wasn't used. Methane/LOX would probably be a better system, as they're stored at similar temperatures, and don't have to be anywhere near as cold as Hydrogen. — Gopher65talk23:18, 3 October 2014 (UTC)[reply]
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This article by Space News does a good job of covering the broad space with a wide look on what's happening with OTVs and with true space tugs (can dock and/or service an existing satellite). Space Tugs as a Service: In-orbit service providers are bracing for consolidation, 6 July 2021. The market is clearly expanding, and more economic actors are entering the space. I don't have time just now to improve this article, but this source will help when/as anyone gets to it. N2e (talk) 11:23, 6 July 2021 (UTC)[reply]
Should this be considered a kick stage or a space tug or both? Include in this article? Should the article say more about differentiating a kick stage from a space tug? C-randles (talk) 01:01, 13 January 2023 (UTC)[reply]
