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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 tha

by 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.
    - pavel_lishin 7y ago
      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.

by jefftk 7y ago

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.
  - pavel_lishin 7y ago
    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.