A combination launch system

is a launch system that consists of multiple launch technologies that work together to boost a payload into orbit for a small fraction of the cost of current launch vehicles.  It works by reducing the amount of velocity the rocket-powered components of the launch system need to achieve.  This reduces the propellant fraction and increasing the payload fraction of the launch vehicle to such a degree that airliner like operations to orbit with a fully reusable launch system becomes possible.

A combination launch system starts with either a

ground-assisted launch

or an air-assisted launch.

A ground-assisted launch can be performed using a horizontal track ground accelerator, as shown above,

with an inclined track/mountain slope ground accelerator,

with a vertically oriented ground accelerator,

or with a high-speed trackless winch-launch system 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,

and as was done with the Pegasus (rocket), 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 Concept.

An air-assisted launch can also be performed at supersonic speeds as was done with

the D-21 drone when it was launched from the back of the SR-71,

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 launch vehicle, or a fully reusable single stage launch vehicle.  The launch vehicles can be designed as vertical landers like the first stage of the Falcon 9, or as 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 can carry to zero when they go to orbit on their own.  Using either of these two types of 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 rocket-ramjet

or a combination flow path rocket-ramjet-scramjet.

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.

The non-rotating skyhook works by reducing the 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.

The idea of using a non-rotating skyhook as part of a space transportation system for Earth where suborbital reusable launch vehicles would fly to the bottom end of the tether, and spacecraft bound for higher orbit, or returning from higher orbit, would use the upper end of the tether, was first proposed by E. Sarmont in 1990, and later expanded on in a number of follow-on works.  Other scientists and engineers, as well as NASA, Lockheed Martin, former astronaut Bruce McCandless II, and Dr. Robert Zubrin, have also investigated, validated, and added to this concept.

Combination launch systems have been around for a long time.  The earliest occurred in 904 A.D. when the Chinese attached small 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 was used to add to the speed of the arrow and thereby increase its range.  They were called fire arrows.  Aircraft 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.  They have also been shown in movies and included in novels.

So why haven’t we built one?

We almost did.

Back in 1962, a proposal was made to attach a small expendable two-stage rocket to the underside of the X-15 for launching satellites into Earth orbit.  Unfortunately, in the rush to beat the Russians to the Moon, the Apollo program became our primary focus and high-speed aircraft programs such as the X-15 gradually came to a halt and were never restarted.


Further reading