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Instead of deorbiting the second stage (particularly with Falcon Heavy) can they be sent into a common parking orbit, there must be a lot of useful materials, not to mention engines, that can be used in the construction of space structure. Modern fabrication techniques would aid here, 3D print etc. The second stage could be boosted into a more stable orbit if it could be partially refuelled.

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That does not make much sense, unfortunately: Most payloads are launched into a Geostationary Transfer Orbit. Such an orbit is highly elliptic. If the second stage could reach the Geostationary Orbit, it would just go there directly and remain there with the payload.

So in order to reuse the second stages you would need to cluster (ideally dock) them in roughly the same spot on a highly elliptical but stable orbit. This will cost you money since parking (and carrying docking capabilities) requires more fuel. Hitting that spot constrains your launch window severely. Unfortunately your launch window for a geostationary satellite is already quite constrained (you want your apogee above your target, if you wait too long before raising your perigee your orbit will decay). So your orbital mechanics would be more complicated.

Finally, in order to do something with the spent stages, you would have to climb up to your parking orbit (i.e. a close-to GTO). This requires a lot of fuel and basically halves your payload compared to the Low Earth Orbit. As long as you do not need many used stages, you are better off launching your new engines into the LEO.

tl;dr: Too complicated, no real benefit

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There is a parking orbit that GTO launches often send their upper stages, that avoids the GEO belt.

However it is possible that you underestimate how big space is and how far apart everything is.

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  • $\begingroup$ Hi geoffc, I would propose something more complex, with the capacity of the falcon heavy I would place a docking station in that parking orbit with the capacity to hold multiple upper stages. It would not require complex architecture, something like a set of multiple rings that the upper stage slots into and is held. The docking station could have the ability to deploy small thruster packs to the upper stages to assist in retrieval. $\endgroup$ – FredK May 11 '16 at 7:15
  • $\begingroup$ With a GTO launch the second stage does not get into geostationary orbit and it reenters in few months to few years. It it had enough propelant and "stamina" (batteries etc.) it would probably insert the satellite directly into a GSO and then would be parked. The current Falcon second stage is not able to do that (limited by batteries and LOX boiloff) and would have to be redesigned if SpaceX ever wants to do direct GSO. $\endgroup$ – jkavalik May 11 '16 at 14:20
  • $\begingroup$ @FredK your docking station would be well-placed for a single GEO slot. But satellites are launched to all parts of the GEO orbit, so most launches would end up thousands of km away and the stages would need fuel-intensive maneuvering to end up at your docking station. $\endgroup$ – Hobbes May 12 '16 at 9:22

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