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 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.