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Rendezvousing with another orbiting object requires a Hohmann[1] or bi-elliptic[2] transfer maneuver in order to "catch up" to the target object. This requires

Rendezvousing with another orbiting object requires a Hohmann[1] or bi-elliptic[2] transfer maneuver in order to "catch up" to the target object. This requires multiple engine burns, and thus lots of delta-v. If the two objects are in different orbital inclinations, an orbital inclination change[3] maneuver is also required. This requires very long, possibly multiple engine burns, and thus lots of delta-v. In short, "glomming" a station onto another requires quite literally astronomical amounts of delta-v. It's not that launching new stations into orbit is cheap, it's that moving objects in orbit is astronomically expensive as to be downright impractical or even impossible. [1]: https://en.wikipedia.org/wiki/Hohmann_transfer_orbit https://en.wikipedia.org/wiki/Hohmann_transfer_orbit [2]: https://en.wikipedia.org/wiki/Bi-elliptic_transfer https://en.wikipedia.org/wiki/Bi-elliptic_transfer [3]: https://en.wikipedia.org/wiki/Orbital_inclination_change https://en.wikipedia.org/wiki/Orbital_inclination_change

yellowapple 4y ago
Hopefully as ion engines and such catch on for these exact sorts of maneuvers we'll see more efforts to glom stations together rather than plunge them back to Earth.
cornholio 4y ago
You are perhaps exaggerating the "astronomical" delta-V required. Space stations are generally in a low ~400km prograde circular orbit, at an inclination efficient for an northern hemisphere launch. For example, you could push the last stable Mir into the orbit of the ISS using only a few hundred m/s delta-v, regardless if it was a good idea or not. Of course, you could construct other scenarios where docking is not possible with current available thrust options.
- Dalewyn 4y ago
  Delta-v is a change in velocity, so "only a few hundred m/s of delta-v" isn't painting the entire picture. The more massive an object is, the more energy (aka fuel) it takes to impart a given amount of velocity change (aka delta-v) upon it. Both a small satellite and a big station module need the same "only a few hundred m/s of delta-v", but the amount of fuel needed to accomplish that maneuver will be astronomically different between them because the station module is so much more massive.
  - cornholio 4y ago
    You can move the goalposts, but still aren't getting to "astronomical" energy, just linear with the mass. If a Dragon or Soyuz craft has fuel for 400m/s in orbit maneuvering, that's about 40m/s for a 10x more massive structure. So definitely plausible, could even happen over a large number of crew rotations. Both crafts are automated so you could hypothetically use the payload mass and volume to deliver extra fuel tanks, with plumbing to the onboard engines. So a full orbit change could happen in a few such flights.

yellowapple 4y ago

Hopefully as ion engines and such catch on for these exact sorts of maneuvers we'll see more efforts to glom stations together rather than plunge them back to Earth.

cornholio 4y ago

You are perhaps exaggerating the "astronomical" delta-V required. Space stations are generally in a low ~400km prograde circular orbit, at an inclination efficient for an northern hemisphere launch. For example, you could push the last stable Mir into the orbit of the ISS using only a few hundred m/s delta-v, regardless if it was a good idea or not. Of course, you could construct other scenarios where docking is not possible with current available thrust options.

Dalewyn 4y ago
Delta-v is a change in velocity, so "only a few hundred m/s of delta-v" isn't painting the entire picture. The more massive an object is, the more energy (aka fuel) it takes to impart a given amount of velocity change (aka delta-v) upon it. Both a small satellite and a big station module need the same "only a few hundred m/s of delta-v", but the amount of fuel needed to accomplish that maneuver will be astronomically different between them because the station module is so much more massive.
- cornholio 4y ago
  You can move the goalposts, but still aren't getting to "astronomical" energy, just linear with the mass. If a Dragon or Soyuz craft has fuel for 400m/s in orbit maneuvering, that's about 40m/s for a 10x more massive structure. So definitely plausible, could even happen over a large number of crew rotations. Both crafts are automated so you could hypothetically use the payload mass and volume to deliver extra fuel tanks, with plumbing to the onboard engines. So a full orbit change could happen in a few such flights.