Hacker News Reader

Comment by mytailorisrich | Hacker News Reader

> By extending a line, anchored on the moon, to deep within Earth's gravity well, we can construct a stable, traversable cable allowing free movement from the vicinity of Earth to the Moon's surface. With current materials, it is feasible to build a cable extending to close to the height of geostationary orbit, allowing easy traversal and construction between the Earth and the Moon.

If we still need rockets to reach the bottom of the cable 36,000 km above Earth's surface, doesn't that negate the point of a "space elevator"?

View on HN

You'd need to reach the height of 36,000 km, but not the velocity needed for geosynchronous orbit - only the velocity needed to match the angular motion of the moon, which I think would be 28 times less.

I’m not very good at physics but - intuitively- would not attaching a payload that have not reached geostationary velocity pull this space line thing to the side and down with the force for which it was not designed

Attaching the payload would indeed pull on the line, since the payload wouldn't have the velocity to maintain itself in that orbit, and so would need an upward force from the line to not fall down.

The line would of course need to be designed to handle that, as well as the increased load as the payload is hoisted up to the moon.

Why is it less energy expended to hoist a load on a line than to use rocket propulsion?

Well, whether it takes less energy would obviously depend on how the hoisting is done. But one advantage is that you can hoist slowly if that's advantageous, whereas you can't do things slowly with a rocket, at least until you've achieved orbit.

Also, if we start with a rocket+fuel on earth, and end with a rocket on the moon, there's an inevitable input of energy to move the rocket to a location of higher gravitational potential, that would have to be provided with hoisting as well, but there's also be a lot of rocket exhaust (formerly fuel) that's now zipping around the solar system, carrying away energy.

To attempt a generalized version of sibling comment from radford-neal, the energy that goes specifically into changing the position of the payload is the same, but how much of the total energy spent goes into that, depends on the method. For the rocket energy goes into so many things other than lifting the payload: kinetic energy of the exhaust, high temperature, sound, and - most importantly - lifting the fuel itself, quickly.

Or to think another way think of all the challenges with human powered helicopter where the record is reaching 3.3 meters in 64 second flight, whereas any moderately physically fit person can climb the same height on the rope much faster and hang out there indefinitely, because they did not have to lift the weight of the helicopter and send the big masses of swooshing air down from the blades just to combat the force of gravity.

You don't need to carry rocket fuel to do it. You can use solar energy.

So it's more like "use the moon as part of a non-rotating skyhook."

https://en.wikipedia.org/wiki/Skyhook_(structure)

Appendix B calculates a fuel savings of ~25% vs using only rockets to get to the moon.

The saturn 5 rocket weighed 3,000 tons and was 96% fuel. Even if we only save 10% fuel, that's an extra 300 tons we can get to the moon.

25% fuel saving seems like a small saving compared to the cost of building such cable. And of course, the saving up to geostationary orbit would be zero...

I don't think you have your maths correct, either.

Sure, but imagine that 25% repeated over and over again.

This got downvoted, but it is correct. This cable won’t be a net gain if you want to go to the moon once, twice, or even ten times, but _if_ you want to, say, build a moon colony housing thousands, and you need to launch thousands of rockets, it likely will be. A concern would be how fast this line would wear down, and what would happen if it broke. Does it destroy your lunar colony, take out a zillion satellites in earth orbit, or just float down onto the moon, and burn down in the earth’s atmosphere?

The Saturn V could transport about 50 tons to the moon. If it could transport another 300 tons, as GP said, it would be able to move 7 times as much payload.

The GP's value is for 10% savings, the article says it saves 25% of the fuel, what means 750 tons extra, or 15 times as much payload.

It's not about paying for the fuel, it's about not having to lift the fuel.

Going from a rocket that is 2880T fuel and 120T for everything else to a rocket that uses 25% less fuel gives you 2160T fuel and 840T everything else. Decreasing the fuel by 25% increased the rest of your budget by 7x!

It goes even further than that. Building bigger rocket engines is really hard because turbo-pumps and nozzles don't scale well. Smaller rockets are exponentially cheaper to design and produce. See the RD-170 program for example: it was easier to just use 4 nozzles instead of 1 to achieve the same output.

That's not how I interpret "using the spaceline can save around 25% of the cost per launch in fuel" (c.f. paper).

If the starting point is 2,880t fuel for 120t payload, so 24t of fuel per ton of payload, then 25% less fuel gives 18t of fuel per ton of payload and thus your 3,000t rocket can now carry 158t of payload, or 32% more.

Even "we can see that the spaceline cuts the fuel cost of such a mission by approximately two thirds" is not a game changer compared to the cost of building the cable.

Space elevators are touted as game changers because although building one would be extraordinarily expensive it would slash the cost of sending payload to space by orders of magnitude, making it worthwhile.

But amusingly enough, a spaceline costs orders of magnitude less than a space elevator. It might make sense as an intermediate step, where we launch a Spaceline using ol' fashioned rockets, and then use it to haul up the materials we're going to build an actual space elevator from.

A space elevator needs to be anchored in geostationary orbit at about 36,000k altitude, which isn't possible with available materials.

This article talks about a cable from geostationary orbit to the moon.

That still leave the ground-to-geostationary part for conventional lifters.

I must have misunderstood - I thought the Spaceline was going to actually reach into the atmosphere?

Dang, I totally misunderstood, I thought this thing was going to go much lower.

If you build the cable out of moon it would probably be a nice big cost savings.

Of course, it still probably isn't worth doing, there's not huge dollar reasons for people to be on the moon, mostly just fun science reasons.

But aren't there huge dollar reasons for wanting to get into orbit in general?

I guess not particularly if you are comparing them to the cost of the largest structure ever built.

And this thing doesn't even help with the orbit part, it helps with getting to a different part of the solar system once you get to orbit.

But it does! https://news.ycombinator.com/item?id=20896811

At that point it's better to build stuff on the moon and move them into geostationary orbit on earth via the cable the other way around.

It's not like the moon lacks resources, we won't be needing to push 300 tons of anything to the moon if we have the capability to build the cable from it.

A 10% reduction would mean you’d get approximately 5 more tons to the Moon (not 300.)

The Saturn V was capable of delivering 50 tons to lunar insertion. Reducing the fuel requirement improves the payload amount by that percentage. It’s not a direct trade off between the total mass of the vehicle and fuel for more payload.

25% fuel savings doesn’t mean you convert the excess into payload. It just means you need less fuel. The Saturn V could get ~50 tons to the moon using ~3,000 tons of fuel. A 25% savings means you get 50 tons to the moon using ~2,250 tons of fuel.

It still makes it easier to get raw materials to space; the difference is that in this case, the materials are mined on the Moon rather than Earth.

Couldn't we build another space elevator (or many) to orbit? Then we would just wait there, or move toward the moon's elevator?

I like Issac Author's "orbital ring" idea. He is on YouTube. If you like space and futurism you need to see this guy's ideas!

Also checkout my project: https://www.kickstarter.com/projects/poly-wants-a-cracker/po...

Reply on news.ycombinator.com