5 ms·

Weird, I didn't find the term "delta-V" anywhere in the article. Most on HN probably know this but delta-V is the change in velocity required to reach some goal

by cletus 6y ago

Weird, I didn't find the term "delta-V" anywhere in the article. Most on HN probably know this but delta-V is the change in velocity required to reach some goal. Fun fact: it requires more delta-V to hit the Sun than it does to leave the Solar System. For giggles, here's a delta-V map of the Solar System [1]. Of course there's the issue of getting things off Earth to begin with but even if you take that as (economically) solvable the detla-V problem remains. Site note: there are lots of proposals for cheaper LEO solutions (in $/kg terms). Probably the most famous is the space elevator but that's actually a bad solution for many reasons and possibly not even feasible since we'd have to invent a material strong enough. What could well reduce that from $thousands/kg to <$10/kg is an orbital ring [2] (and check out the rest of his Uplift series if you're interested at the other options). The beauty of this is it doesn't require any magical material (cough graphene cough) and requires no new physics. It's largely just an engineering problem. Granted it's a massive engineering problem. To be fair, the article does mention how much energy you'd have to lose but it strikes me as just being so much easier to explain in delta-V terms. [1]: https://external-preview.redd.it/U5iH7huE5qKth7ZFvipXt8vzaFOO99qHFh9o9_SkLLk.png?auto=webp&s=d145ac9ae496abe35fae86fc11a584d62fe42592 https://external-preview.redd.it/U5iH7huE5qKth7ZFvipXt8vzaFO... [2]: https://www.youtube.com/watch?v=LMbI6sk-62E https://www.youtube.com/watch?v=LMbI6sk-62E

twanvl 6y ago
> Fun fact: it requires more delta-V to hit the Sun than it does to leave the Solar System. Isn't that the delta-v needed to reach a stable orbit? That is, to slow down and counteract all the kinetic energy you gained from the sun's gravity. To just hit the sun (with high velocity) you wouldn't need to do that.
- me_me_me 6y ago
  Yes, moving towards sun/ any body that is inbeteen sun and earth you actually 'slowing down' (after reaching earth's orbit). When you reach zero V relative to sun you will just drop to its surface (same with any other planet). Anything on the outside you 'speedup'. That's why its called delta-V as its relative change in V needed to get somewhere. And its independent of size, type of craft etc.
robjan 6y ago
The article does mention it without using the specific words "delta-V"
me_me_me 6y ago
> Probably the most famous is the space elevator but that's actually a bad solution for many reasons and possibly not even feasible since we'd have to invent a material strong enough. That's true, after all we are still stuck trying to invent materials light enough to make areoplanes actually fly /s Sorry couldn't help it :) Pushing what is currently impossible is how we got where we are. We already have viable tech to capture asteroid that could be used as counterweight for space elevator.
- cletus 6y ago
  > Pushing what is currently impossible is how we got where we are. This is a tired cliche at this point. Yes the track record for what is "impossible" (flying, going to the Moon and so on) could lean one towards saying nothing is impossible but take that to the extreme and what's left? Is breaking the second law of thermodynamics "impossible"? Time travel? Wormholes? FTL? Engineering is ultimately limited by physics and the physics for a space elevator are dire. You need a material with a tensile strength such that it can withstand the centrifugal forces of being 35,000km+ long including having a counterweight pulling on it to counterbalance the force of gravity acting on the mass of that. At this point only theoretical materials come close to that. But even if you can solve the tensile strength problem (and that may well be solvable) you're left with a structure that isn't really as useful as you might think. Even getting between the geosynchronous point and the Earth requires speeding up and slowing down for a distance of 35,000km+. Average 1000km/h and you're still talking 35 hours. 1G of acceleration and deceleration could cover this distance a lot quicker but there are limits to how fast you can go when you need to stay attached to a column the whole way. But let's say you can solve that problem and get to geosynchronous orbit in reasonable time. Now what? Well if you want to go anywhere you need to accelerate. You need to bring the fuel for that. Now compare this to an orbital ring. An orbital ring: - doesn't require any magical materials. The core of it is nothing more than copper or iron cabling; - will get you get to and from different points on the Earth much faster; - provide LEO space to live and work on the stationary (relative to the surface of the Earth) platform. At 20 meters wide you're talking ~1 billion square meters of area, which is almost as large as the land area of Los Angeles; - allows for huge amounts of solar power to be generated and sent to the ground via transmission cables; - provides you with enough delta-V to escape the Solar System for almost no cost; and - has significantly more attractive failure modes. If an orbital ring severs it'll likely fly off into space based on inertia. If a space elevator severs part of it is likely to fall to the ground, which would be... bad.

by cletus 6y ago

twanvl 6y ago

> Fun fact: it requires more delta-V to hit the Sun than it does to leave the Solar System. Isn't that the delta-v needed to reach a stable orbit? That is, to slow down and counteract all the kinetic energy you gained from the sun's gravity. To just hit the sun (with high velocity) you wouldn't need to do that.

me_me_me 6y ago
Yes, moving towards sun/ any body that is inbeteen sun and earth you actually 'slowing down' (after reaching earth's orbit). When you reach zero V relative to sun you will just drop to its surface (same with any other planet). Anything on the outside you 'speedup'. That's why its called delta-V as its relative change in V needed to get somewhere. And its independent of size, type of craft etc.

robjan 6y ago

The article does mention it without using the specific words "delta-V"

me_me_me 6y ago

> Probably the most famous is the space elevator but that's actually a bad solution for many reasons and possibly not even feasible since we'd have to invent a material strong enough. That's true, after all we are still stuck trying to invent materials light enough to make areoplanes actually fly /s Sorry couldn't help it :) Pushing what is currently impossible is how we got where we are. We already have viable tech to capture asteroid that could be used as counterweight for space elevator.

cletus 6y ago
> Pushing what is currently impossible is how we got where we are. This is a tired cliche at this point. Yes the track record for what is "impossible" (flying, going to the Moon and so on) could lean one towards saying nothing is impossible but take that to the extreme and what's left? Is breaking the second law of thermodynamics "impossible"? Time travel? Wormholes? FTL? Engineering is ultimately limited by physics and the physics for a space elevator are dire. You need a material with a tensile strength such that it can withstand the centrifugal forces of being 35,000km+ long including having a counterweight pulling on it to counterbalance the force of gravity acting on the mass of that. At this point only theoretical materials come close to that. But even if you can solve the tensile strength problem (and that may well be solvable) you're left with a structure that isn't really as useful as you might think. Even getting between the geosynchronous point and the Earth requires speeding up and slowing down for a distance of 35,000km+. Average 1000km/h and you're still talking 35 hours. 1G of acceleration and deceleration could cover this distance a lot quicker but there are limits to how fast you can go when you need to stay attached to a column the whole way. But let's say you can solve that problem and get to geosynchronous orbit in reasonable time. Now what? Well if you want to go anywhere you need to accelerate. You need to bring the fuel for that. Now compare this to an orbital ring. An orbital ring: - doesn't require any magical materials. The core of it is nothing more than copper or iron cabling; - will get you get to and from different points on the Earth much faster; - provide LEO space to live and work on the stationary (relative to the surface of the Earth) platform. At 20 meters wide you're talking ~1 billion square meters of area, which is almost as large as the land area of Los Angeles; - allows for huge amounts of solar power to be generated and sent to the ground via transmission cables; - provides you with enough delta-V to escape the Solar System for almost no cost; and - has significantly more attractive failure modes. If an orbital ring severs it'll likely fly off into space based on inertia. If a space elevator severs part of it is likely to fall to the ground, which would be... bad.