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    • Well, obviously the pogo stick method is the best, although probably will not be happening ever. But the space elevator is the most nice. The most plausible is one that’s partially happened like reusable rockets. So we dug into this field of why is it really expensive to put stuff in space - the rough metric we used was about $10,000 a pound, which means if you see an astronaut holding an apple seed in space, that’s a $10 apple seed. The reasons for the cost are counter-intuitive. If you get in an airplane, a 737 from NYC to LA, the major cost is maintenance and fuel, the staff, equipment and fuel. The plane is already made, it has a big upfront cost, but it’s reused. But with a rocket, that’s what you do - you burn up your fuel but you also burn the spaceship itself. It’s incredibly expensive -  imagine if you had to explode a 737 every time you flew. I think they cost $400 million, and if it seats 400 people, that’s $1 million per passenger. Then with physics stuff, if you look at a rocket, only 3% of it is stuff going into space, it’s about 80% propellant, and 16.5% the ship - for the Apollo trips, it was only 1.5% stuff going into space. So it’s not just that you’re burning up your vehicle, you’re also not transporting much - most of it is machine and propellant. So that’s the basic deal. Between those two things, you end up with the enormous price you have. So there are a couple options to get around that. The reason the pogo stick method is interesting - if you could just increase your efficiency SLIGHTLY, that takes you from 3% to 4% with a just slight efficiency improvement, that’s 33% more stuff. If 80% of a rocket is propellant, if you can just get to 77% propellant, you’ve effectively doubled your cargo space. So for the pogo stick method, you lift up your ship, you drop it, it bounces and then it takes off - you get a little lift going upwards and then save 1% of your fuel. It’s like having a skyscraper you have to drop so it bounces.

      But the general issue in all the cases we describe is you can get better results by making the system more complicated and dangerous. One method that’s really obvious, everyone probably thinks of it, is that propellant is 2 things: fuel and oxidizer. Just like a campfire, you have to have wood and oxygen, the same with spaceships. Very roughly speaking it’s half oxidizer, half fuel on a spaceship, but with this you're using lisquid oxygen as an oxidizer as the spaceship goes through an atmosphere of oxygen. So it’s another level of complexity that introduces another level of danger. You should read the book to understand why!