When a rocket takes off from the surface of the Earth and flies into orbit it increases its velocity by approximately 9,100 meters per second. That breaks down to 7,800 meters per second for the speed of orbit and 1,300 meters per second for drag and gravity losses. That is a lot of speed and it takes a lot of propellant to go that fast.
An example of just how much propellant is required is the Space Shuttle. Sitting on the launch pad waiting to take-off, the Space Shuttle was 85% propellant, 14% launch vehicle, and 1% payload. If Earth to orbit spaceflight is ever going to be affordable to everyone, the launch vehicle will need to be both fully reusable, and able to carry a large enough payload that it makes it worth all the trouble. Up to now, that has not been possible. To make the Space Shuttle fully reusable it would have been necessary to make it both larger and heavier which would have required a larger propellant fraction and that would have made the payload go to zero. Obviously, not a very workable solution.
This is where reducing the speed to orbit comes in.
There are a number of ways to do this. One is to use a ground accelerator that is located on the side of a tall mountain to boost the launch vehicle up to 600 MPH before starting its engines. This reduces the speed to orbit in two ways, by the speed added to the launch vehicle by the ground accelerator, and by reducing the drag and gravity losses that would have been incurred by the launch vehicle if it had accelerated to this speed and altitude on its own.
Another way to reduce the speed to orbit is to use a Skyhook at the upper end of the flight profile. Skyhooks can be short or long. The best way to use a Skyhook is to start small while the flight rate is low and gradually grow it into a longer and stronger version as demand increases. For this example, a short Skyhook, like the one shown in this video was selected.
The total velocity reduction made possible by the 600 MPH ground accelerator and 200-kilometer long basic Skyhook used in this example is 1,060 meters per second. This reduction in velocity will triple the amount of useful payload that can be delivered to the Skyhook compared to the same expendable launch vehicle flying to a space station without a ground accelerator or Skyhook. Increasing the amount of useful payload by a factor of three will reduce the cost to orbit to 1/3 of what it was without the ground accelerator and Skyhook. If it is assumed that the first stage of this launch vehicle is made reusable like the first stage of the Falcon 9, it then becomes reasonable to assume an additional 50% reduction in launch costs. This will reduce the cost to orbit to 1/6 of the cost of flying the expendable version of this launch vehicle without the ground accelerator and Skyhook.
And this is only the beginning. The longer the Skyhook becomes the lower the price becomes. Once the Skyhook is long enough it then becomes possible to use a fully reusable single stage launch vehicle that will reduce the cost even more. Best of all, the 600 MPH ground accelerator, the basic Skyhook, the reusable first stage launch vehicle, they can all be affordably built right now with existing materials and technology.
For more information about this and other related cost reducing concepts, read the book “Opening the High Frontier”.
Index of Articles
- Opening the High Frontier
- Skyhook, a Journey to Orbit and Beyond
- In the Beginning . . .
- Why do Rockets Cost so Much?
- Combination Launch Systems
- It’s All About Speed!
- Visions of the Future
- The Call of an Unlimited Future
- Combination Launch Systems, part 2
- Outward Bound: Beyond Low Earth Orbit
- and someday . . . Starships!
- Mars: how to get there
- Outpost Space Stations
- Dreams of Space
- The Moon or Mars?
- Skyhooks and Space Elevators
- Stratolaunch and the X-15
- Starship Congress
- Making Spaceflight Affordable
- How a Combination Launch System Works