Opening the High Frontier is about making spaceflight affordable to everyone so we can start building a spacefaring civilization.
We stopped going to the Moon because of cost and we have yet to go to Mars because of cost.
Once the cost of spaceflight is made affordable to everyone, the entire solar system becomes our domain and it will become possible to build a spacefaring civilization.
People have been trying to figure out how to make spaceflight affordable to everyone ever since Konstantin Tsiolkovsky figured out the rocket equation in 1897. It is this equation that tells us how much propellant a rocket needs to carry to get to a certain velocity. If we want to build an airliner to orbit that uses chemical propellants that anyone can afford, this equation shows that our launch vehicle will need to be 87% propellant at take-off. That leaves only 13% of the take-off weight for the launch vehicle and payload. The problem with this is that in the 50 plus years people have been going to Earth orbit, no one has been able to build a single stage to orbit reusable launch vehicle that has these figures. The Space Shuttle, which was only partially reusable and used staging, was 85% propellant at take-off, its empty weight was 14% and its payload was 1%. The reason its propellant fraction was 85% instead of 87% was because it dropped the solid rocket boosters and the external tank when they used up all their propellant. This reduced the amount of weight that needed to be carried to low Earth orbit which reduced the amount of propellant required. If the solid rocket boosters and the external tank had been made a fixed part of the Space Shuttle so as to make it into a more affordable single stage vehicle, the propellant required would have increased to 87% and the empty weight of the launch vehicle would have gone up due to the need to make the heat shield larger to contain the additional parts. The end result would have been a single stage reusable rocket that couldn’t make it to orbit even if it had no payload onboard. Quite simply, it is not possible to build an affordable to use single stage to orbit reusable launch vehicle that uses chemical propellant with our current technology. The only way we can get to orbit is with staging and that will always keep the cost too high to be affordable for the average person.
So how do we make spaceflight affordable to everyone using existing technology?
As best as I can tell, there is no one single answer. The best answer I have found is a combination system that starts with either a ground accelerator launched or an air-launched launch vehicle. The launch vehicle uses either a reusable rocket powered first stage or a combination rocket, ramjet, scramjet powered reusable first stage. The upper stage of the launch vehicle is a reusable suborbital spacecraft that is designed to fly to the lower end of a Skyhook.
Obviously, this isn’t a simple system and it doesn’t look like anything you have seen or read about in a science fiction movie or book. It doesn’t have anti-gravity which we don’t know how to build, it doesn’t zoom into space like an X-wing fighter and then jump to hyperspace, it doesn’t have a matter transmitter or a warp drive, it doesn’t use a nuclear powered rocket motor, and it doesn’t need some super strong yet to be invented material like carbon nanotube fibers. The one overwhelming advantage of this system is that it can be affordably built right now with existing technology.
Combination launch systems work by reducing the velocity that the launch vehicle needs to achieve to reach orbit. This reduces the amount of propellant the launch vehicle needs to carry which allows it to carry more payload. The more the velocity is reduced, the greater the payload, and the lower the cost becomes. Add a 600 MPH ground accelerator to the launch system and the cost to orbit goes down by half. Add in a 200-kilometer long basic Skyhook and the payload size is tripled. Now the cost is 1/3 of what we started with. Make the first stage of the launch vehicle reusable and the cost of the rocket goes down by an additional half. Now the cost is 1/6 of what we started with. Make the Skyhook longer and the price goes down again. Add in air-breathing propulsion and the cost goes down even further. This is the magic of combination launch systems. They work, we know how to build them, we can afford to build them, and they can be built in incremental steps. They work so well that 20 years from now when a fully mature system is in place, the cost of a ticket to orbit will be $20,000 per person.
If you could buy a ticket to orbit for $20,000 would you go? If you had a job offer to work on a Skyhook, a Space Colony, an orbital factory, an outpost space station, on the Moon or on Mars, would you take it? These are the kind of opportunities that will become available once affordable to everyone spaceflight becomes a reality.