Combination Launch Systems, part 2

Component One    The first component in a combination launch system is a choice between using an air assisted launch as shown in this video

or a ground assisted launch as shown in this picture.

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As with any competing concepts, there are advantages and disadvantages to both of these systems.  Some of the advantages and disadvantages are technical, some are operational, some are financial, some of them vary depending on the other components of the combination launch system, and some are political.  There are also a number of design variations to both of these systems, the selection of which is usually determined by the specific needs and goals of the developer.  For example, an air launch can be either subsonic or supersonic, and a ground assisted launch can be on rails, with maglev, or trackless.  Ground assisted launch can also be enclosed in a tunnel as shown in the picture, out in the open on a mountainside, or on a vertical tower.

Component Two    The second key component in a combination launch system is making the launch vehicle reusable.  There are a number of ways this can be done with the best choice being determined by the total amount of velocity reduction that is made possible by the air assist/ground assist launch, the non-rotating Skyhook (component three), and the type of propulsion system being used on the launch vehicle (component four).  If it is an all-rocket powered launch vehicle with an entry level length Skyhook, the best choice for the initial launch vehicle will be a reusable first stage/expendable upper stage configuration.  If air-launched, the launch vehicle will use a winged horizontal landing first stage, if ground accelerator launched on a steep enough slope so that wings are not needed, a vertical landing first stage like the ones being developed by SpaceX and Blue Origin will be best.

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As the length of the Skyhook is increased and the velocity reduction for the launch vehicle increases, it will become possible to combine the expendable upper stage of the launch vehicle with the spacecraft.  This will make the launch vehicle into a fully reusable two stage to Skyhook launch system which will further reduce the cost of getting to orbit.  As the Skyhook continues to be made longer and the velocity reduction continues to increase, it will eventually become possible to make the launch vehicle into a single stage to Skyhook vehicle.  This will reduce the cost of getting into space even more.

Component Three    The third key component in a combination launch system is a non-rotating Skyhook (see section 3.2.1 on page 7).  A non-rotating Skyhook is a vertically oriented cable that is attached to a space station.  Since the speed of orbit goes down with increasing altitude, the lower end of the cable is moving at less than orbital velocity for its altitude, and the upper end of the cable is moving faster than orbital velocity for its altitude.  The end result is that a launch vehicle arriving at the lower end of the cable does not have to go as fast as it would need to without the Skyhook.  This reduced velocity requirement allows the launch vehicle to carry a larger payload which reduces the cost of getting to orbit.

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A non-rotating Skyhook works on the same principles as an Earth surface to geostationary orbit Space Elevator.  The main differences between them are that the Skyhook is much shorter, it does not reach down to the surface of the Earth, and it can be affordably built with currently existing materials and technology.

Component Four   The fourth key component in a combination launch system is a combination air-breathing and rocket motor propulsion system.  This works by reducing the amount of oxidizer the launch vehicle needs to carry which makes for a smaller and more affordable launch vehicle.  The reduced propellant requirement also increases the payload fraction and thereby reduces the cost to orbit.

There are many ways to make a combination air-breathing and rocket motor propulsion system.  They can be rocket/ramjet combinations, ducted rocket/ramjet combinations, ducted rocket/ramjet/scramjet combinations, and so on.  All of them have different costs, different weights, and different performance advantages.  Some of them will require extensive development effort, some will not.  The one that reduces the cost to orbit the most will depend on the details of all the other components of the combination launch system, the amount of development work required, and the flight rate.

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Index of Articles

  1. Opening the High Frontier
  2. Skyhook, a Journey to Orbit and Beyond
  3. In the Beginning . . .
  4. Why do Rockets Cost so Much?
  5. Combination Launch Systems
  6. It’s All About Speed!
  7. Visions of the Future
  8. The Call of an Unlimited Future
  9. Combination Launch Systems, part 2
  10. Outward Bound: Beyond Low Earth Orbit
  11. and someday . . . Starships!
  12. Mars: how to get there
  13. Outpost Space Stations
  14. Dreams of Space
  15. The Moon or Mars?
  16. Skyhooks and Space Elevators
  17. Stratolaunch and the X-15
  18. Starship Congress
  19. Making Spaceflight Affordable
  20. How a Combination Launch System Works

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