It seems like this system is crucially dependent on the balance of departing and returning vessels to conserve momentum. While that balance might be maintained over long timescales, it seems unlikely over short ones, especially the first years of operation at earth, which are likely to have many departures and few returns. And anyone who's seen a UHaul lot in a college town on the first week of school knows that departures and returns are often quite out of balance for short periods.
It also seems like there is a conservation of energy issue here that I don't understand. In a state of balanced departures and returns, the energy used to launch departing vessels would be captured from returning vessels, but in the Earth-Mars loop, the returning vessels were themselves launched from the surface of Mars and accelerated by tethers there. It feels like a perpetual motion machine.
“ And of course, none of these deltaV savings are for ‘free’. Accelerating payloads means the tether will slow down. If it slows down too much, it will de-orbit itself. The momentum lost with each catch-and-release operation must be recovered either by absorbing momentum from payloads being slowed down, or by using its own propulsion system.
A major advantage of an orbital tether is that you do not have to immediately recover that momentum - it gives time for slower but more efficient propulsion systems like a solar-electric thruster to gradually accelerate the tether. A chemical propulsion system limited to 450s of Isp is not needed as the acceleration can be done over time with something that has thousands of seconds of Isp. The propellant needed to run the tether’s engines is greatly reduced. Even more interesting is the possibility of propellantless propulsion, such as electrodynamic tethers that push off the magnetic fields around a planet.”
One option available: add momentum very efficiently on the tether.
Rockets have to balance their specific impulse (efficiency) against their need for massive amounts of thrust immediately. You can't take off from the ground using an ion thruster, despite what Kerbal Space Program tells us.
But a tether system in orbit could be using a very efficient engine to gain momentum over a very long period of time (months, years, etc). Give it large solar arrays or a small nuclear plant on-board. It's getting a much better exchange rate on that delta-V than the rocket, which needs to fight earth's gravity and air resistance.
The momentum transfer is never perfect, so every tether station will need to have a propulsion system to boost it back to the base orbit. But since the tether is already in orbit, its station keeping system doesn't have to be chemical rockets. On top of momentum exchange, it can use various form of electric propulsion that are slower but much more efficient.
RE balance of incoming/outgoing boosts, that's why the article mentions the option of having the ship carry some extra fuel as payload to refuel the tether - because even with that, tethers still get you energetically and economically ahead over having the rocket fly itself all the way to destination, due to exponential nature of the rocket equation.
There are 7 million seconds in three months. That's a lot of time for a Hall Effect thruster running off of solar power at 1 AU to boost the momentum of a tether.
It also seems like there is a conservation of energy issue here that I don't understand. In a state of balanced departures and returns, the energy used to launch departing vessels would be captured from returning vessels, but in the Earth-Mars loop, the returning vessels were themselves launched from the surface of Mars and accelerated by tethers there. It feels like a perpetual motion machine.