Combination launch system

A combination launch system is a launch system that consists of up to four different launch technologies working together to boost a payload into orbit for a small fraction of the cost of current launch vehicles. It works by reducing the propellant fraction and increasing the payload fraction of the launch vehicle to such a degree that airliner like operations to orbit become possible.
The components
The first component of a combination launch system consists of either a ground-assisted launch or an air-assisted launch. The ground-assisted launch can be performed using a horizontal track ground accelerator, an inclined track ground accelerator, a vertically oriented ground accelerator, or a trackless ground accelerator similar to the ones used to launch sailplanes. An air-assisted launch can be performed using a subsonic carrier aircraft as was done with the X-15 rocket plane, the Pegasus launch vehicle, and both SpaceShipOne and SpaceShipTwo. The Stratolaunch carrier aircraft is another example of subsonic air-assisted launch. Another type of subsonic air-assisted launch is the towed glider air-launch system. An air-assisted launch can also be performed at supersonic speeds as was done with the D-21 drone, and as was proposed for a follow-on X-15 program that would have used the XB-70 as a supersonic carrier aircraft.
The second component of a combination launch system is to make the launch vehicle reusable. This can be a reusable first stage with expendable upper stage launch vehicle, a fully reusable two stage to orbit launch vehicle, or a fully reusable single stage to orbit launch vehicle. They can be vertical landers like the Falcon 9, or horizontal landers like the Space Shuttle. Fully reusable two stage to orbit and single stage to orbit launch vehicles have not been possible in the past due to the increase in empty weight that comes with making them reusable. This increase in empty weight reduces the amount of useful payload they could carry to zero. Using these launch vehicles as part of a combination launch system reduces their propellant fraction enough that they can now carry a worthwhile payload.
The third component of a combination launch system is to include some sort of combination air-breathing and rocket motor propulsion system with the reusable launch vehicle. This can consist of separate ramjets mounted on the sides of a launch vehicle that also has conventional rocket motors mounted at the rear of the vehicle. It can also consist of a combination flow path ramjet/rocket, or a combination flow path ramjet/scramjet/rocket. All of these reduce the amount of oxidizer the launch vehicle needs to carry which allows it to carry a larger payload.
The fourth component of a combination launch system is a non-rotating skyhook, also known as a synchronous momentum exchange space tether. The non-rotating skyhook works by reducing the final velocity the launch vehicle needs to achieve to reach orbit as the lower end of the Skyhook is moving at less than orbital velocity for its altitude. Like air-assisted launch, ground assisted launch, and combination air-breathing and rocket motor propulsion systems, this reduction in velocity reduces the propellant fraction and increases the payload fraction of the launch vehicle which reduces the cost to orbit. Some proposals for combination launch systems that include a non-rotating skyhook start out with a skyhook that has an initial overall length of approximately 200-kilometers. This helps to keep the size of the initial investment down when the flight rate is low which also helps to keep the cost to orbit down. As demand for flights to the skyhook increases it is possible to Increase the length of the skyhook which also increases the amount of velocity reduction to the launch vehicle. This allows for an even lower propellant fraction and an additional increase in payload fraction on the launch vehicle which further reduces the cost of getting to orbit. Proponents of combination launch systems have claimed that a fully mature system has the potential of reducing the cost to orbit to $100 per pound or less.
History
Combination launch systems have been around for a long time. In 904 A.D., the Chinese attached gunpowder rocket motors to arrows as a way of extending the range of the arrows. In this case, the bow was the ground accelerator that gave the arrow its initial speed and direction, and the rocket motor was used to add to the speed of the arrow and thereby increase its range. Catapults/ground accelerators have also been used to accelerate aircraft up to flight speed as well as for launching the V-1 flying bomb in WW2. Other examples of combination launch systems were the air-launched reusable rocket planes of the 1940's, 1950's, and 1960's. The most well-known of these being the B-52 launched X-15 rocket plane.
Combination launch systems in literature and film
*Robert A. Heinlein used a mountain slope ground accelerator to launch a reusable rocket in his 1949 novel The Man Who Sold the Moon.
* used a mountain slope ground accelerator to launch the Ark, but the book did not.
*Fireball XL5, a 1960s children's television show, used a horizontal ground accelerator to launch a reusable rocket called the Fireball XL5.
*Dean Ing used a mountain slope ground accelerator with a reusable launch vehicle in his 1988 novel "The Big Lifters".

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