Comment by kamray23 | Hacker News Reader

Comment by kamray23 | Hacker News Reader

Parent Full thread

Starship weighs 1300+ metric tonnes. Accelerating to earth gravity, the cable will have a tension of 1.3 MN. On the other end the counterweight has to be of roughly equal mass by way of not being able to launch anything heavier (so prob. another starship), meaing that the barycentre will be roughly in the middle, and the counterweight applies a second 1.3 MN. The total tension the cable is under is going to be 2.6 MN, well outside any single cable today. Taking a comfortable velocity of 1 rpm, and calculating a radius, we get a cable 894 meters long. This is right in the comfort zone as far as difference in force over your body and disorientation from spinning are concerned. Our strongest cables, made of Aramid (which degrades too quickly because of radiation, but ignoring that) is going to be 58 cm and have a weight of 2.2 metric tonnes. Since we actually need twice the cable, it's actually 4.4 tonnes. A similar cable made of carbon fibre or another such element will give us a launch weight of 430+ tonnes (close to the mass of the ISS), which rules basically anything but polymer fibres out. The R&D funding that would have to go into developing the cabling and methods of shielding it and preventing MMOD, solar, or GCR degradation is astronomical. It's way cheaper to just make a special sleeping bag or two.

I agree it's impractical, but I believe you're double counting the force on the cable. Each end of the cable pulls with 1.3 MN, which means that the cable itself can be in static equilibrium (in the rotating frame of reference). The same is true if you hang a 1.3 MN space station from the ceiling. The space station pulls down with a force of 1.3 MN, while the ceiling pulls up with a force of 1.3 MN.

That's probably a reasonable conclusion. It does happen to be totally overshadowed by my bigger mistake, which is an order of magnitude. Repeating the calculations using my physics CAS today:

Starship mass: 1.3·10³ kg Starship weight: 1.3·10⁷ Newtons

Converting that into something reasonable, it's 13 MN, not 1.3. A Kevlar fibre cable will be 22 tonnes, a PE one 9000 tonnes, and a carbon one 2200 tonnes. I wouldn't like to even imagine steel. And these are just with a breaking strength of 13 MN.

Not the most reasonable, though obviously we've held up things like Arecibo in the past. Using multiple smaller cables would be the optimal solution and what we would go with, since a single cable is >1 meter in diameter in any case. The mass doesn't get any smaller like that, though, so it's not a consideration we'd need to make.

I don't think you'd need full earth graffiti to minimise the negative health effects of microgravity. Probably half or so would be enough. Just make sure people don't try to jump lol

Low gravity will always have health effects. If you plan on returning to the Earth and have a very short stay, 0.5 G is probably a massive improvement over 1 G, especially if you keep doing exercise. For a martian transfer, you'd acutally only need 0.38 G as they'll be losing their bone structure to Martian gravity either way.

Funny thing actually, a person born on Mars could probably never walk on the Earth without years of intensive physiotherapy.

Can't we use multiple cables then? There are ships that can lift over 10,000 tons with their cranes. Some of these are decades old.

Yeah, we can. The issue is that tensile strength is measured as pressure, as force per area. Splitting a one meter cable into 10 cm cables is certainly possible. Those are half the length of your normal school ruler. You will need 100 of them, though, and since the area and length are the same, the volume, and by extension the mass are the same. It's certainly easier to manufacture, but it doesn't change the calculation. Cranes capable of lifting metric tonnes are giant. Just the cabling alone can weigh as much as a small skyscraper. That just isn't going to space.

Reply on news.ycombinator.com