Spacecraft waste an incredible amount of energy getting into orbit, partially because most of what they're lifting is useless fuel. This is why a space elevator is such an appealing idea, but there's no reason why the same basic concept wouldn't be beneficial for regular old airplanes.
Here's the idea: there's a super strong cable attached to an electric aircraft. The aircraft lifts one end of the cable up to 30,000 feet or so, and then orbits up there in a big circle. Back on the ground, the other end of the cable is attached to a hub that slowly spins to keep the cable straight. When an aircraft wants to take off, it's hooked onto the cable, and then an electric winch hoists it up to cruising altitude. The aircraft detaches, and off it goes.
Sounds simple, but what's the point? Well, take your average Boeing 737. Climbing to altitude is a 20 minute process that burns 5,000 pounds of jet fuel. Using a cable system, the 737 could make it to the same spot in the sky in under three minutes using only $650 worth of (climate friendly) electricity. In fact, this system is so efficient that projections suggest it would bring the cost of air freight down to cost less than shipping something by diesel truck. It would be much better for the environment, too.
This all does seem like a crazy idea, but fundamentally, there's no reason why it can't be done. We can produce electricity cheaply and efficiently using renewable resources. Cables strong enough to support the weight of jet aircraft already exist. And the cable tow plane itself could be unmanned and powered by electricity from the ground, allowing it to remain on-station indefinitely.
Once we have a system like this working, the sky isn't even the limit anymore. Heavy freight could move efficiently without clogging up roads. Gliders could take passengers hundreds of miles without using any fuel at all. And even space travel becomes cheaper, since several stages of tow ropes could be used to hoist payloads up to 100,000 feet, making low Earth orbit a breeze, and going even further a real possibility.