Cool concept. I’m a bit puzzled by one thing though—presumably every time you use a tether, it slows down and drops to a lower orbit. How do you handle that? Is the idea that it’s so much more massive than the rockets its boosting that its slowdown is negligible? Or do we have to go spin it back up every so often?
One way to regain energy is to run the tether in reverse—drop something from a faster orbit back into the atmosphere, siphoning off some of its energy along the way. If every time you sent one spacecraft up another was lined up to come back down, that would save a lot of trouble.
But you’ll still need to do orbital corrections, offset atmospheric drag, and allow for imbalances, so yeah, it would seem like you still need a pretty beefy means of propulsion on this thing, which is oddly unmentioned for being key to the whole design.
Tethers can theoretically use more efficient propulsion because their thrust requirements are lower. The argon Hall effect thrusters on Starlink satellites have around 7x the specific impulse (fuel efficiency) of Starship engines while needing 7x the energy due to KE=mv^2/2 and having a tiny fraction of the thrust. This energy could come from a giant solar panel rather than the fuel, and every once in a while it could be refueled with a big tanker of liquid argon.
After looking into this a little more because it didn’t seem like ion thrusters would have the requisite thrust, it looks like this whitepaper from boeing explores using electrodynamic interaction with Earth’s magnetic field for thrust (see p. 34) - but they don’t find great numbers on how fast you can correct the orbit using that either, unfortunately.
The numbers for ion don’t seem crazy. To get the impulse to catch a 1,000 ton object every 2 weeks you would need 10,000 Starlink thrusters massing 21 tons, plus 42 MW of power, which is 14 hectares of solar panels at an average of 300 W/m^2. That’s only a couple dozen times the ISS.
You can use the tether to catch payloads on the way down and boost the tether back up while also reducing the payload’s need for heat shielding
You can use more efficient engines with low thrust/weight ratios to reboost the tether
There are some propellent-free options that use the magnetic field to reboost the tether in exchange for energy (I’m unsure if the energy needs are practical or not)
If you had a way to catch them, I think you could just throw rocks down the gravity well and catch them for a boost too.
you can recover lost momentum by decelerating things to land. OP mentions that briefly
And they need a regular supply of falling mass to counter the momentum lost from boosting rockets. These considerations mean that tethers have to constantly adapt to their conditions, frequently repositioning and doing maintenance.
If every launch returns and lands on earth, that would recover some but not all lost momentum, because of fuel spent on the trip. it’s probably more complicted than that though
Yeah, my overall sense is that using falling mass to spin the tether back up is the most practical. But solar sails and ion drives might contribute too, these are just much slower which hurts launch cadence and costs.
The fact that you need a regular supply of falling mass from e.g. the moon is yet another reason why tethers need a mature space industry to become viable!
Cool concept. I’m a bit puzzled by one thing though—presumably every time you use a tether, it slows down and drops to a lower orbit. How do you handle that? Is the idea that it’s so much more massive than the rockets its boosting that its slowdown is negligible? Or do we have to go spin it back up every so often?
One way to regain energy is to run the tether in reverse—drop something from a faster orbit back into the atmosphere, siphoning off some of its energy along the way. If every time you sent one spacecraft up another was lined up to come back down, that would save a lot of trouble.
But you’ll still need to do orbital corrections, offset atmospheric drag, and allow for imbalances, so yeah, it would seem like you still need a pretty beefy means of propulsion on this thing, which is oddly unmentioned for being key to the whole design.
Tethers can theoretically use more efficient propulsion because their thrust requirements are lower. The argon Hall effect thrusters on Starlink satellites have around 7x the specific impulse (fuel efficiency) of Starship engines while needing 7x the energy due to KE=mv^2/2 and having a tiny fraction of the thrust. This energy could come from a giant solar panel rather than the fuel, and every once in a while it could be refueled with a big tanker of liquid argon.
After looking into this a little more because it didn’t seem like ion thrusters would have the requisite thrust, it looks like this whitepaper from boeing explores using electrodynamic interaction with Earth’s magnetic field for thrust (see p. 34) - but they don’t find great numbers on how fast you can correct the orbit using that either, unfortunately.
This reminds me of a Brin short story which I think exactly discusses what you’re talking about: https://www.davidbrin.com/tankfarm.htm
The numbers for ion don’t seem crazy. To get the impulse to catch a 1,000 ton object every 2 weeks you would need 10,000 Starlink thrusters massing 21 tons, plus 42 MW of power, which is 14 hectares of solar panels at an average of 300 W/m^2. That’s only a couple dozen times the ISS.
This linked article goes into some options for that: https://toughsf.blogspot.com/2020/07/tethers-all-way.html
You can use the tether to catch payloads on the way down and boost the tether back up while also reducing the payload’s need for heat shielding
You can use more efficient engines with low thrust/weight ratios to reboost the tether
There are some propellent-free options that use the magnetic field to reboost the tether in exchange for energy (I’m unsure if the energy needs are practical or not)
If you had a way to catch them, I think you could just throw rocks down the gravity well and catch them for a boost too.
you can recover lost momentum by decelerating things to land. OP mentions that briefly
If every launch returns and lands on earth, that would recover some but not all lost momentum, because of fuel spent on the trip. it’s probably more complicted than that though
Yeah, my overall sense is that using falling mass to spin the tether back up is the most practical. But solar sails and ion drives might contribute too, these are just much slower which hurts launch cadence and costs.
The fact that you need a regular supply of falling mass from e.g. the moon is yet another reason why tethers need a mature space industry to become viable!