It's not the fuel or even energy. Just money.
There's no shortage of the components of rocket fuel. It takes energy to convert them into rocket fuel but we have that in plenty. If money were no object we could build solar or nuclear power facilities to generate as much rocket fuel as we want.
The primary thing stopping our expansion into the solar system is the cost of launch. When NASA was launching shuttles it cost $10,000 to launch a pound of cargo into orbit.
Once you're in orbit it's pretty smooth sailing (it takes months to get anywhere but a gentle thrust will do and that does not take an insane amount of fuel)
there are all kinds of low reaction-mass propulsion systems
- Ion drives
- The new EM drive which nobody can explain but which seems
- Solar sails sound dumb but work fine
So lets work on the Cost of launch. My favorite quote from one of the scrappy new rocketry pioneers was:
Rockets are built by hand by people with phds
- Reusable rockets (Spacex has successfully landed first stage rockets 4 times)
- Cheaper rocket assembly (this should happen naturally now that commercial spaceflight is taking off)
- More seat of the pants design decisions (NASA's engineering is triple redundant which is great but we can accept more risk and move faster, cheaper and ultimately get better results)
Armadillo aerospace is the poster boy. I believe Carmack once diagnosed a problem wrote a fix and pushed the patch to a rocket in mid-flight.
Alternate launch methods
- High G launch with a space gun (bad for humans, fine for water dirt, steel rods)
- Of course space elevators
I suspect the first one will get blown to crap. I mean seriously imagine you're a terrorist that thing is going to look like the biggest target in the world.
Reduce the number and weight of things you need to launch:
Ultimately you'll want to bootstrap mining and manufacture in space. (this is probably the point when you know you can colonize the solar system)
Food production in space
Water harvesting in space
Waypoints would really help (imagine how boring the martian had been if there had been a habitable terrarium and emergency shuttle in orbit before anyone went down to the surface)