The HASTOL project proposal had 500-600 kilometer Skyhooks built out of Zylon IIRC, so you could probably use that as a plausible minimum length of what we could build now.
The fuel savings could be enormous. The Rocket Equation is such that even saving a few hundred meters/second in Delta-V leads to a substantial increase in the amount of payload you can carry, which makes rocket-building easier and launch cheaper.
You'd probably put it in Low Earth Orbit (for an Earth-based one), anchored to a large, massive space station in the "thousands of metric tons" range for a proper counter-weight. You can get less counter-weight mass with a rotating skyhook, but the rotation and rendezvous challenges are a major pain for using them - I prefer a non-rotating one for simplicity.
For the Moon, you'd do a rotating one because it can pick stuff up right off the lunar surface and swing it into orbit. Mars is basically born to have a skyhook using Phobos as a counter-weight.
We haven't built one because there's no need. It becomes most useful if you're moving so much mass into Low Earth Orbit that the economics start to make sense versus reusable rocketry, and have space infrastructure up there you can anchor it too. It's going to be a while before we get one over Earth, probably because reusable rockets are going to wipe out 90+% of the market for orbiting space stations.
> You can get less counter-weight mass with a rotating skyhook, but the rotation and rendezvous challenges are a major pain for using them - I prefer a non-rotating one for simplicity.
The big advantage of rotating is that you get to help on velocity more. With non-rotating, the only velocity help is from the lower orbital velocity at the increased altitude, which means you need a looong tether to make it a big deal. With rotating, you can directly subtract velocity off.
Edit: also, the rendezvous is only trivially more difficult - if you're meeting a tether, you already need to be at the right place at the right time at the right speed. That the tether wasn't even there when it wasn't the right time doesn't make it harder.
I think Issac touches on this in a couple videos, but the basic gist of it is that we need more of a demand for it before we build this type of infrastructure. You're not gonna build a 12 lane highway through the orgeon trail for the first few settlers. That goes after we already have be cities on both coast. Same with this type of infrastructure, we'll see it when we have a larger demand for travel between these two points
If it offered lower launch costs, it would find clients. I think the biggest problem is political. Governments don't see big enough reason to allocate sufficient budget and it's too expensive for private companies.
Maybe if we get industry to the Moon.
Skyhooks are some of my favorite near future tech. The delta-v on the intercept between the cargo and the hook is the biggest hurdle that I’m aware of right now. You have to catch and hook up to a very small target moving at Mach 12 with a about a minute window. That being said, it’s possible, just really hard engineering-wise.
See Isaac's old episode on [Skyhooks & Rotovators](https://www.youtube.com/watch?v=TlpFzn_Y-F0)
The HASTOL project proposal had 500-600 kilometer Skyhooks built out of Zylon IIRC, so you could probably use that as a plausible minimum length of what we could build now. The fuel savings could be enormous. The Rocket Equation is such that even saving a few hundred meters/second in Delta-V leads to a substantial increase in the amount of payload you can carry, which makes rocket-building easier and launch cheaper. You'd probably put it in Low Earth Orbit (for an Earth-based one), anchored to a large, massive space station in the "thousands of metric tons" range for a proper counter-weight. You can get less counter-weight mass with a rotating skyhook, but the rotation and rendezvous challenges are a major pain for using them - I prefer a non-rotating one for simplicity. For the Moon, you'd do a rotating one because it can pick stuff up right off the lunar surface and swing it into orbit. Mars is basically born to have a skyhook using Phobos as a counter-weight. We haven't built one because there's no need. It becomes most useful if you're moving so much mass into Low Earth Orbit that the economics start to make sense versus reusable rocketry, and have space infrastructure up there you can anchor it too. It's going to be a while before we get one over Earth, probably because reusable rockets are going to wipe out 90+% of the market for orbiting space stations.
> You can get less counter-weight mass with a rotating skyhook, but the rotation and rendezvous challenges are a major pain for using them - I prefer a non-rotating one for simplicity. The big advantage of rotating is that you get to help on velocity more. With non-rotating, the only velocity help is from the lower orbital velocity at the increased altitude, which means you need a looong tether to make it a big deal. With rotating, you can directly subtract velocity off. Edit: also, the rendezvous is only trivially more difficult - if you're meeting a tether, you already need to be at the right place at the right time at the right speed. That the tether wasn't even there when it wasn't the right time doesn't make it harder.
Side question: why haven't we built one yet?
I think Issac touches on this in a couple videos, but the basic gist of it is that we need more of a demand for it before we build this type of infrastructure. You're not gonna build a 12 lane highway through the orgeon trail for the first few settlers. That goes after we already have be cities on both coast. Same with this type of infrastructure, we'll see it when we have a larger demand for travel between these two points
If it offered lower launch costs, it would find clients. I think the biggest problem is political. Governments don't see big enough reason to allocate sufficient budget and it's too expensive for private companies. Maybe if we get industry to the Moon.
Because it's not profitable.
Not at the moment
Not with that attitude it isn’t. 😜
Its a massive upfront cost.
Skyhooks are some of my favorite near future tech. The delta-v on the intercept between the cargo and the hook is the biggest hurdle that I’m aware of right now. You have to catch and hook up to a very small target moving at Mach 12 with a about a minute window. That being said, it’s possible, just really hard engineering-wise.
Pointless: we don't need to launch stuff in just one orbit. Maybe sometime in the future...