So I was researching on another technical musing, related to navigation in the solar system. And I really enjoyed this map I came across by Reddit user ucarion which shows the delta-v's required to get to different orbits of interest in the solar system. And its format is very intuitive, being the same as the globally popular subway map style.
What I've been thinking about is a way for a vehicle to be piloted around the solar system with impunity. i.e. not having to be restricted to the hohmann transfers and planetary momentum captures commonly used by NASA to get to Mars and other planetary bodies.
Right now, the limiting factor as I see it is the fact that we are very propellant-poor in our current means of pushing ourselves around the solar system. By contrast, energy (for adding kinetic energy to propellant) is less of a concern, as it can be generated from a variety of sources.
My motivation to get this solved? Exploration, but I recognize that our economy won't expend real currency at the problem unless there's the potential for financial return. So to that I suggest asteroid capture e.g. like Planetary Resources advocates for.
What do we do with a big asteroid? No doubt you can think of your own application from them. From my perspective, precious metals aside, lets say people living on Earth want to start spreading out across it - e.g. for lower rent or more agricultural area. Then you can envision massive tropical colonies in the Pacific Ocean's international waters as advocated for by seasteading companies! Where is the material going to come from to build floating landforms that covers millions of square kilometers? Most countries aren't going to let you displace from land from theirs, and environmental advocates are going to make it politically difficult for you to dredge up enough material from the sea floor. How about material harvested from asteroids instead?
But back to the propellant problem. Where are we going to get all the propellant we need to navigate with impunity around the asteroid belt and bring massive material mined from the asteroids back over and down into Earth's low-earth-orbit (LEO) for eventual transport down to the Earth's surface?
I bring you:
Jupiter!
Yes, the Jovian monstrosity can be monstrously helpful in our quest to zip around the solar system for economic and exploration gain!
How? Simple! Jupiter's atmosphere is packed with gaseous hydrogen. We just need to capture *lots* of it on an ongoing basis to fuel our spacefaring adventures! So how do we get it?
First, we need to get away from the incumbent paradigm of chemical rocketry. It's too expensive to bring fuel and oxidizer to orbit. Enter nuclear technology. Normally I'm bearish on nuclear for terrestrial activity for its hazard potential, but in space, its a wonderful thing. According to wikipedia (yes I'm citing wikipedia, deal with it) fission-heated hydrogen gas rockets can achieve twice the exhaust velocity (twice the specific impulse for rocket aficionados) than hydrogen in a chemical rocket. That's basically more bang for our propellant buck. But even more importantly, we don't have to bring any oxidizer on a fission rocket, we only need fissile rods for replacing every once in a while, (and they are much easier transport for their small size than chemical fuel). We are just heating the propellant gas to high pressure and then releasing it through a rocket nozzle. So, we don't need to find and harvest oxidizer somewhere or send up expensively from Earth.
To first order, I advocate for having many such automated fission-powered rockets making round-trips to Jupiter, equipped with a very wide scoop to harvest Jovian hydrogen as propellant for itself and for other vessels it will transfer to. The scooper craft doesn't need to descend far down into Jupiter stratosphere at all - because the craft is traveling so fast and Jupiter is so vast. The craft will naturally slow as it gathers gas, but it can still even wildly overshoot Jupiter afterward, because it can use the propellant it just gathered to change to the opposite direction, only to gather more propellant for its return leg for delivering onward.
Then we'll want to set up propellant "coaling stations" at key Lagrange points like Jupiter's L1 as well as in sun-centric orbits that allow for easy interception - effectively a supply chain in space. Our asteroid-mining craft can then transact at these coaling stations to collect propellant and travel where they may. No more propellant-hoarding, no more strict Hohmann transfers. Just get to where you need to go and come back whenever you're ready.
Going back to the Solar System Subway Map image at top, there are three primary phases of travel from one planet to another that apply here. Surface-to-LEO, LEO-to-Earth-intercept, and Earth-intercept-to-Jupiter-intercept. The nuclear (fission) powered gas scoopers lend themselves to Jupiter-intercepting transfers. Origin: perhaps Earth intercept only initially, but before long one of the coaling stations.
Then what about the other two transit phases? Maybe I'll save the detail for another technical musing since I have to go, but I advocate for maglev / hyperloop-esque mass drivers to get from the Earth's surface to LEO (massively reduced cost to orbit - no expended propellant required!) and yes, chemical rockets to go from LEO to intercept. Why chemical rockets and not fissile ones? Because I don't think people on Earth will stand for having a nuclear reactor in LEO that could come down in the event of an accident and create a Fukushima-like incident. So there still is a place for the chemical rocket industry (chemical rocket industry lobbyists rejoice!), but luckily now over the long term this industry only has to bring its own oxidizer from Earth, since the hydrogen is already going to be there courtesy of Jupiter. :)
