What is Kinetic Space Launch?

“Dr. Hoenikker used to say that any scientist who couldn't explain to an eight-year-old what he was doing was a charlatan.”

– Cat's Cradle, Kurt Vonnegut

A rocket is a boat full of rocks with no paddles or engine. It’s propelled by a guy, Mr. Combustion, whose job is to throw rocks in the opposite direction as hard as he can. To go faster, or farther, he needs more rocks to throw. But the more rocks piled onto the boat, the bigger the boat needs to be and the more effort he uses just pushing the rock pile and the bigger, heavier boat. Eventually, for every ‘useful’ extra rock adding speed, many rocks are needed to make it pay off. A bigger boat is more efficient at carrying rocks than a smaller boat, so to go fast, big boats are good.

To go really fast, a huge boat has to be loaded with many tons of rocks. Eventually, people get clever and start launching smaller boats from the big boat once it runs out of rocks. This makes things more complicated but means that Mr. Combustion is not pushing the weight of the big boat once it’s empty. That’s why big rockets have two, three, or even four stages.

Constantine Tsiolkovsky who first wrote the rocket equation coined the boat analogy. Rocket science over the past 100 years has been focused on improving Mr. Combustion’s throwing arm, making lighter boats, or finding better rocks. However, the underlying constraints of having to carry a lot of rocks remains.

Over the past 15 years, SpaceX has drastically improved things with the Falcon 9, a mostly reusable rocket (can you believe they used to just throw the boats away?). They are now working to combine 100% reusability with the advantages of mega-hugeness in the upcoming Starship which, when it flies, will be the largest rocket ever built. That’s super cool. However, it’s still a boat throwing a lot of rocks. Starship will only move about 1 to 2% of its total wet mass (boat + rocks) to orbit.

Throwing rocks is not actually very hard. The hard part is that they have to travel with the boat. If you remove that, things become super easy. In fact, if Mr. Combustion and the rocks he’s chucking don’t have to move at all, this turns into an easy problem.

Instead of piling rocks up on the boat, the boat is floating in a canal lined on either bank by guys holding a single rock. Each guy throws his rock at the back of the boat as it passes. The rock bounces off, pushing the boat faster. The boat itself is empty. No rocks and no Mr. Combustion along for the ride. When the boat exits the canal, it has all the speed it needs to coast to its destination.

There are a lot of different ways to build that canal. Some might look like a gun, others like a trebuchet, or a magnetic rail. Some may be better than others. But they all leave the hard work on the ground and just send the bit that actually matters to space. A rocket, or something that is as energetic as a rocket, is not only a lot cheaper to build if it doesn't have to fly, but it also spends a lot less energy pushing rocks.

In a nutshell, that's the difference in physics between rockets and kinetic space launch.