Does Interstellar Trade Make Sense For Realistic Science Fiction?

[TGLS]

Originally Posted by Ulzgoroth

I wouldn't want to be using combustion rockets for this sort of thing, so the price of oxygen doesn't seem a leading concern...

Let's suppose we're using a thermal rocket of some design. For reaction mass, we need a fluid, and the obvious choices on Titan are Methane or Nitrogen. If you compare them to Hydrogen, Hydrogen is 2.8 times better than Methane and 3.7 times better than Nitrogen. Given that these both give you a comparable or worse performance than a chemical rocket, you'll want Hydrogen. How do you get Hydrogen from Methane? Steam reformation, which requires water. If you don't have water, you need Oxygen to make water, and if you do, you already have an easy supply of Oxygen.

Another reason why Earth is a superior source of Nitrogen (at least for Mars and Venus). You can forget all the work about building and fueling rockets like this and use solar sails to tug your Nitrogen around.

[AlexanderHowl]

Earth is the last place you want to use for Martian nitrogen and Venus has plenty of nitrogen (four times as much as Earth). The problem is that it is much harder to get off the surface of the Earth than to get off the surface of Titan. Even though Titan is very far away, high impulse drives allow nitrogen from Titan for a fifth the cost of nitrogen from Earth. In addition, solar sails such for moving stuff around, as their low acceleration means that you will spend years traveling per AU.

[ericthered]

Earth's gravity well is one of those pieces of nature that seems designed to mock us. Its just small enough that we can use chemical rockets to get things into orbit, if we use multiple stages and live with 90% of our weight being thrown away. Space elevators require materials that might just barely be possible. And so forth. If you get much smaller, transport gets a lot cheaper and a lot more efficient. Interplanetary travel does make sense... if you have people outside of earth-sized gravity wells. Though at that point it might not be interplanetary travel.

[Johnny1A.2]

Originally Posted by ericthered

Earth's gravity well is one of those pieces of nature that seems designed to mock us. Its just small enough that we can use chemical rockets to get things into orbit, if we use multiple stages and live with 90% of our weight being thrown away. Space elevators require materials that might just barely be possible. And so forth. If you get much smaller, transport gets a lot cheaper and a lot more efficient. Interplanetary travel does make sense... if you have people outside of earth-sized gravity wells. Though at that point it might not be interplanetary travel.

OTOH, there are some theorists who suspect Earth would not be readily habitable if it was much smaller than it is.

Seriously, there's a reason why SF in the old days used to tend to assume that interplanetary travel would be a post-20C thing, it needs energy sources and densities to do it well that are hard to achieve with near-term tech. Chemical rocketry is adequate for a bare-bones unmanned program, but not much more.

[whswhs]

Originally Posted by Johnny1A.2

Seriously, there's a reason why SF in the old days used to tend to assume that interplanetary travel would be a post-20C thing, it needs energy sources and densities to do it well that are hard to achieve with near-term tech. Chemical rocketry is adequate for a bare-bones unmanned program, but not much more.

Heinlein, at least, often assumed that interplanetary flight would use nuclear rockets that would take off from the surface of the Earth. This goes from the fission powered jet of vaporized zinc in Rocket Ship Galileo to the total conversion based "torchships" in Time for the Stars. Of course that approach was different realistic problems!

[Johnny1A.2]

Originally Posted by whswhs

Heinlein, at least, often assumed that interplanetary flight would use nuclear rockets that would take off from the surface of the Earth. This goes from the fission powered jet of vaporized zinc in Rocket Ship Galileo to the total conversion based "torchships" in Time for the Stars. Of course that approach was different realistic problems!

The old classic SF movie Forbidden Planet opens with a brief narration that tells the viewer that 'men and women' reached the Moon at the end of the 21C, with serious interplanetary travel not long after that, followed by FTL a little after that. That used to be a common trope of SF, because the writers who knew their stuff knew how hard space flight would be.

Heinlein used to propose that nuclear rockets would eventually made SSTO and escape from the ground, but he usually showed that coming later. Not always, as you note.

All the way back in the 30s, Jack Williamson's old space opera The Legion of Space turned on the invention of the geodyne drive which made interplanetary (and eventually interstellar) flight practical. He posited the earliest flights being made using fission rockets, but portrayed them as bulky and inefficient and uneconomical.