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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 on

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.

mytailorisrich 7y ago
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.
- pavel_lishin 7y ago
  Sure, but imagine that 25% repeated over and over again.
  - Someone 7y ago
    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?
- juliangoldsmith 7y ago
  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.
  - marcosdumay 7y ago
    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.
- jefftk 7y ago
  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!
  - api_or_ipa 7y ago
    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.
  - mytailorisrich 7y ago
    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.
    - pavel_lishin 7y ago
      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.
      - TheSpiceIsLife 7y ago
        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.
- maxerickson 7y ago
  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.
  - pavel_lishin 7y ago
    But aren't there huge dollar reasons for wanting to get into orbit in general?
    - maxerickson 7y ago
      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.
      - pavel_lishin 7y ago
        But it does! https://news.ycombinator.com/item?id=20896811 https://news.ycombinator.com/item?id=20896811
dogma1138 7y ago
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.
amayne 7y ago
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.
mikeash 7y ago
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.

mytailorisrich 7y ago

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.

pavel_lishin 7y ago
Sure, but imagine that 25% repeated over and over again.
- Someone 7y ago
  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?
juliangoldsmith 7y ago
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.
- marcosdumay 7y ago
  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.
jefftk 7y ago
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!
- api_or_ipa 7y ago
  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.
- mytailorisrich 7y ago
  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.
  - pavel_lishin 7y ago
    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.
    - TheSpiceIsLife 7y ago
      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.
maxerickson 7y ago
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.
- pavel_lishin 7y ago
  But aren't there huge dollar reasons for wanting to get into orbit in general?
  - maxerickson 7y ago
    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.
    - pavel_lishin 7y ago
      But it does! https://news.ycombinator.com/item?id=20896811 https://news.ycombinator.com/item?id=20896811

dogma1138 7y ago

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.

amayne 7y ago

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.

mikeash 7y ago

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.