Combination launch systems are launch systems that use two or more different methods to accelerate a launch vehicle up to orbital velocity in order to reduce the cost of getting to orbit.
There are many different launch concepts that we can build today that can be combined together to do this: subsonic air-launch; supersonic air-launch; ground accelerators; combination air-breathing and rocket motor propulsion systems; reusable launch vehicles; and, Skyhooks. All of these concepts can be combined together to reduce launch costs.
There are also many ways to combine these concepts to make a combination launch system. They can be as simple as using a vertically oriented ground accelerator to boost an existing expendable launch vehicle up to 600 MPH prior to igniting the rocket motor. They can be as complex as combining air-launch with a reusable launch vehicle that uses a combination ramjet, scramjet, rocket motor propulsion system, that flies to the lower end of a non-rotating Skyhook. The hard part is finding the combination that has the lowest user cost to orbit for the lowest possible initial investment, that is sized for the existing launch market and has the potential to grow as the market grows.
Combination launch systems are not new. The Wright brothers used a catapult to boost their first airplane up to flight speed. The U. S. NAVY still uses catapults to launch fighter planes from the decks of aircraft carriers. The world’s first reusable rocket plane, the Bell X-1, was air-launched from a B-29. Probably the best-known combination lunch system was the X-15 rocket plane that was air-launched from a B-52. In 1967 they even test flew a mock-up of a scramjet on the X-15. There were also studies that examined the possibility of supersonic air-launching a rocket-ramjet-scramjet powered delta winged version of the X-15 from the back of the XB-70 at Mach 3 that had a small expendable upper stage rocket for carrying satellites into Earth orbit.
Unfortunately, all of these ideas fell by the wayside in our rush to beat the Russians to the Moon back in the 1960’s. It wasn’t until SpaceShipOne flew in 2004, the start of Virgin Galactic and Stratolaunch shortly thereafter, as well as SpaceX and Blue Origin developing reusable first stage launch vehicles, that we have seen a serious effort to develop combination launch systems again. The only negative about this is that none of these new combination lunch systems go far enough.
While all of these concepts, ground accelerators, air-launch, reusable first stage launch vehicles, air-breathing and rocket propulsion systems, and Skyhooks, will reduce the cost of getting to orbit, none of them, by themselves, will make spaceflight affordable to everyone. To do that we will need a launch system that combines almost all of these concepts together.
Once we have that, the solar system is ours.