But I think not having the ability to have infrastructure like satellites, fuel dumps, stations, skyhooks, or whatever the heck else you feel like putting there is going to be a problem and again make the whole operation a lot less efficient.
Are you open to updating that assumption?
Well, the best way to get an idea of how actual spaceflight physics work is to play kerbal space program. But, to summarize:
(1) kessler doesn’t apply to both very high and very low orbits, leaving room for satellites (insufficient energy for the debris to reach very high orbits, very low orbits have drag that cleans them)
(2) fuel dumps do not work (and no, spacex doesn’t plan to use them) as ‘the rocket equation doesn’t work that way’
The tyranny of the rocket equation means that we’re going to really struggle to make it worth it. For the same reason that we can’t just make fuel tanks bigger, it is very inefficient to send fuel out from the same gravity well as you want to refuel from—orders of magnitude.
The thing to remember when we talk about “kg of fuel per kg of cargo” is that the vast majority of that fuel is burned in the lower atmosphere. The majority of the work of shooting a rocket off to space is just getting it moving. So if you want to ship enough rocket fuel up to form a fuel dump with something like hydrogen rocket fuel, then you need to expend vastly more fuel than you end up storing.
Well yeah, but that’s just an argument for looking for extraterrestrial fuel sources. Insofar as your fuel comes from earth, putting it in a depot isn’t obviously worse than launching it on an as-needed basis, and arguably it’s better. And insofar as your fuel doesn’t come from earth, then similar considerations could weigh in favor of a depot. (The tanker from the Belt comes once a year with a buttload of fuel, puts it in an earth-orbit depot and leaves)
This is air launch—using a balloon is just one variant. All of the schemes I’ve seen seem to have fairly small payloads, I assume the trade-offs don’t work well above some threshold.
If we’re talking about orbital propellant depots, the individual launches of fuel don’t have to be very big, as long as the price per kilogram to LEO is favorable. Rockoons are but one method of circumventing the rocket equation. Many others are known, with some being more realistic than others in the short term.
The reason is that there is no need for crews or for a staff at the dump to have a place to “stretch their legs”. Nor is there any advantage to have a vast stockpile of fuel ‘just in case’, you might as well launch it as needed. Most of the reasons to have a dump irl do not appear to be true in space. And the rocket equation means you pay in mass fractions regardless of scale.
Hmm, without knowing what the reasons for fuel dumps are, I can’t tell how convincing those points are. I am not very convinced myself. Off the top of my head here are some reasons you might want an orbital fuel depot:
--You are limited to certain windows of opportunity for launches, or perhaps you are pad-constrained, or both. So you can’t just launch a fleet of tankers alongside your regular ships; it pays to spread out the launches, and have tankers go up when regular ships don’t need to, and then regular ships can go up and have fuel waiting for them already, anytime. (I guess this is an objection to your “might as well launch as needed” claim.) (I think this is the biggest reason; currently it takes several tankers to refill one starship I think, which means you either have to launch all of them at around the same time as the starship, or you have to have the starship wait around for one tanker to fly up and down and up and down. Might not be feasible given launch pad and launch window constraints.)
--Your tankers aren’t optimized for holding onto fuel in vacuum or for long periods of time, or maybe they aren’t optimized for transferring it quickly to other kinds of ships. So you build a depot that is.
The second part—you basically need a solar power gas recondensor. Or to have one onboard. I take your point, fuel refrigeration is a role that you might not have ships do and you could keep a module in orbit to do.
But the hassle of docking and maintenance—and possibly zero net cost savings—might make it cheaper to just launch fuel condensing modules on be vehicle for the mission itself.
Hello Gerald! For sure. To be honest the Kessler syndrome was an afterthought and I may be overestimating its impact. I think the far more relevant danger is active measures against a launching or landing craft. Things like fuel dumps (you are totally right in that it doesn’t make sense to take fuel from Earth up into LEO, I was more thinking about bringing fuel from some other much-lower-energy gravity well like the moon) would probably be better placed in Lagrange points.
I mean you can replace all this with “airline travel”. Already nations can shoot down airliners that travel within hundreds of kilometers of top end air defense systems like the S-400. So if a one of the central European nations decided to make anywhere they can hit a ‘no fly zone’, and they had nukes so no one can invade, they could de facto ban air travel in Europe.
This is true for a lot of technologies—ultimately this planet is small.
But I think not having the ability to have infrastructure like satellites, fuel dumps, stations, skyhooks, or whatever the heck else you feel like putting there is going to be a problem and again make the whole operation a lot less efficient.
Are you open to updating that assumption?
Well, the best way to get an idea of how actual spaceflight physics work is to play kerbal space program. But, to summarize:
(1) kessler doesn’t apply to both very high and very low orbits, leaving room for satellites (insufficient energy for the debris to reach very high orbits, very low orbits have drag that cleans them)
(2) fuel dumps do not work (and no, spacex doesn’t plan to use them) as ‘the rocket equation doesn’t work that way’
SpaceX doesn’t plan to use fuel dumps?
Can’t argue with a tweet but it doesn’t change the rocket equation.
It does change my opinion about whether the rocket equation renders fuel dumps a bad idea though.
What’s the argument that the rocket equation makes fuel dumps a bad idea?
The tyranny of the rocket equation means that we’re going to really struggle to make it worth it. For the same reason that we can’t just make fuel tanks bigger, it is very inefficient to send fuel out from the same gravity well as you want to refuel from—orders of magnitude.
The thing to remember when we talk about “kg of fuel per kg of cargo” is that the vast majority of that fuel is burned in the lower atmosphere. The majority of the work of shooting a rocket off to space is just getting it moving. So if you want to ship enough rocket fuel up to form a fuel dump with something like hydrogen rocket fuel, then you need to expend vastly more fuel than you end up storing.
Well yeah, but that’s just an argument for looking for extraterrestrial fuel sources. Insofar as your fuel comes from earth, putting it in a depot isn’t obviously worse than launching it on an as-needed basis, and arguably it’s better. And insofar as your fuel doesn’t come from earth, then similar considerations could weigh in favor of a depot. (The tanker from the Belt comes once a year with a buttload of fuel, puts it in an earth-orbit depot and leaves)
Then why not launch from high-altitude balloons?
This is air launch—using a balloon is just one variant. All of the schemes I’ve seen seem to have fairly small payloads, I assume the trade-offs don’t work well above some threshold.
If we’re talking about orbital propellant depots, the individual launches of fuel don’t have to be very big, as long as the price per kilogram to LEO is favorable. Rockoons are but one method of circumventing the rocket equation. Many others are known, with some being more realistic than others in the short term.
The reason is that there is no need for crews or for a staff at the dump to have a place to “stretch their legs”. Nor is there any advantage to have a vast stockpile of fuel ‘just in case’, you might as well launch it as needed. Most of the reasons to have a dump irl do not appear to be true in space. And the rocket equation means you pay in mass fractions regardless of scale.
Hmm, without knowing what the reasons for fuel dumps are, I can’t tell how convincing those points are. I am not very convinced myself. Off the top of my head here are some reasons you might want an orbital fuel depot:
--You are limited to certain windows of opportunity for launches, or perhaps you are pad-constrained, or both. So you can’t just launch a fleet of tankers alongside your regular ships; it pays to spread out the launches, and have tankers go up when regular ships don’t need to, and then regular ships can go up and have fuel waiting for them already, anytime. (I guess this is an objection to your “might as well launch as needed” claim.) (I think this is the biggest reason; currently it takes several tankers to refill one starship I think, which means you either have to launch all of them at around the same time as the starship, or you have to have the starship wait around for one tanker to fly up and down and up and down. Might not be feasible given launch pad and launch window constraints.)
--Your tankers aren’t optimized for holding onto fuel in vacuum or for long periods of time, or maybe they aren’t optimized for transferring it quickly to other kinds of ships. So you build a depot that is.
The second part—you basically need a solar power gas recondensor. Or to have one onboard. I take your point, fuel refrigeration is a role that you might not have ships do and you could keep a module in orbit to do.
But the hassle of docking and maintenance—and possibly zero net cost savings—might make it cheaper to just launch fuel condensing modules on be vehicle for the mission itself.
Hello Gerald! For sure. To be honest the Kessler syndrome was an afterthought and I may be overestimating its impact. I think the far more relevant danger is active measures against a launching or landing craft. Things like fuel dumps (you are totally right in that it doesn’t make sense to take fuel from Earth up into LEO, I was more thinking about bringing fuel from some other much-lower-energy gravity well like the moon) would probably be better placed in Lagrange points.
I mean you can replace all this with “airline travel”. Already nations can shoot down airliners that travel within hundreds of kilometers of top end air defense systems like the S-400. So if a one of the central European nations decided to make anywhere they can hit a ‘no fly zone’, and they had nukes so no one can invade, they could de facto ban air travel in Europe.
This is true for a lot of technologies—ultimately this planet is small.