Skyhooks and Space Elevators

A skyhook is a proposed space transportation concept that will help make spaceflight affordable to everyone.  When used as part of a combination launch system it will make the building of a spacefaring civilization possible on a commercial basis.  There are two kinds of skyhooks, a rotating skyhook, and a non-rotating skyhook.

A non-rotating skyhook is a much shorter version of the Earth surface to geostationary orbit Space Elevator that does not reach down to the surface of the Earth.  It is much lighter in mass, can be affordably built with existing materials and technology, and in its mature form, is cost competitive with what is thought to be realistically achievable using a Space Elevator, assuming materials strong enough to build a Space Elevator ever become available.  It works by starting from a relatively low altitude orbit and hanging a cable down to just above the Earth’s atmosphere.  Since the lower end of the cable is moving at less than orbital velocity for its altitude, a launch vehicle flying to the bottom of the non-rotating skyhook can carry a larger payload than it could otherwise carry to orbit.  When the non-rotating skyhook is long enough, Single Stage To Skyhook flight with a reusable launch vehicle becomes possible at a price that is affordable to just about anyone.

Another way to understand the non-rotating skyhook is to think of it as a momentum exchange device that consists of a space station in a higher altitude, higher energy, elliptical orbit, with a cable that hangs down to just above the atmosphere.  When a suborbital spacecraft coming up from the Earth docks at the lower end of the cable, it pulls the space station down into a slightly lower more circular orbit.  In effect, the space station gives up some of its energy to the arriving spacecraft so that the arriving suborbital spacecraft can stay in orbit instead of falling back to Earth.  When the spacecraft lets go of the lower end of the cable to return to Earth, it gives that energy back which allows the space station to return to a higher altitude, higher energy, more elliptical orbit.  The end result is that the energy that is exchanged between the non-rotating skyhook and the arriving spacecraft and then returned to the skyhook when the spacecraft departs, gets used over and over again every time a spacecraft makes a trip to the skyhook.  This exchange and reuse of energy reduces the amount of propellant the launch vehicle needs to carry which allows it to carry more payload.  Less propellant also makes for a smaller, lighter, and more affordable launch vehicle.  More payload means that the cost of the launch can be spread out over a larger amount of cargo.  Both of these changes reduce the cost per pound of getting to orbit.  When the skyhook cable is long enough, airliner like operations to space become possible at airliner like prices.

History

The idea for a non-rotating skyhook evolved from the idea of an orbital tower which was first proposed by Konstantin Tsiolkovsky back in 1895.  The orbital tower consists of a really tall tower that goes from the surface of the Earth all the way to geostationary orbit.  Its purpose was to provide an economical way of getting to orbit so that the human race could start building a spacefaring civilization.  The reason for wanting to build a spacefaring civilization was to avoid the projected collapse of our civilization at some time in the near future due to overpopulation.  A collapse that is considered by many to be inevitable if we remain a single planet species.  So why not use rockets?  Konstantin Tsiolkovsky knew about rockets.  After all, he is the person who first worked out the mathematics for using rockets to travel through space to other planets.  As a result of that work, he knew just how uneconomical chemical powered rockets are, which is why he wanted to find a better way of getting into orbit.   He got the idea for the orbital tower as a result of a trip to Paris, France where he saw the Eiffel Tower.  While he knew that such an orbital tower could not be built, he felt certain that the existence of a theoretical solution to the rocket problem would eventually lead to a real world solution that could be built.  He was right.

The idea of the orbital tower led to the creation of the space elevator concept, another idea that cannot be built.  That led to the idea of a rotating skyhook, a type of rotating space elevator that rotates in the plane of its orbit like a two spoke wheel rolling across the top of the atmosphere as it orbits the planet.  While this idea can be built with existing materials, it also has three very significant operational problems that have yet to be solved.

The first of these is the very short amount of time that is available for an arriving spacecraft to hook up with the end of the cable.  A rendezvous window that is literally only three to five seconds long.  This is what engineers and scientists like to call a “non-trivial problem.”

The second problem is maintaining the synchronization between the rotation rate of the skyhook with its orbital period.  Since the rotating skyhook is in an elliptical orbit, the rotation rate of the cable needs to be in sync with the amount of time it takes to orbit the Earth so that the lower end of the cable will be at the bottom of its swing when the rotating skyhook is at the low point of its elliptical orbit.  When a spacecraft docks with the lower end of the rotating skyhook at the low point of its orbit, it pulls the skyhook down into a lower orbit with a shorter orbital period.  Since the rotation rate of the cable does not change when the rendezvous occurs, the rotation rate of the rotating skyhook is now out of sync with the new orbit.  The rotating cable will need to be brought back into sync with the orbit before another spacecraft can use the system.  This is another non-trivial problem.

The third problem with the rotating skyhook has to do with how the release orbit of the spacecraft occurs one-half a rotation after a spacecraft docks with the cable at the bottom of its swing.  This linkage of the release orbit to the time of arrival causes a problem in that only a very small percentage of the release orbits will be pointed in the right direction for a spacecraft that is going to the Moon and beyond.  The only solution to this is to limit the departure speed of the spacecraft to a speed that will take it to a higher altitude orbit where the spacecraft will use its onboard propellant to circularize its orbit and wait until it is in the correct position to boost for its final destination.  This noticeably limits the usefulness and cost advantage of the rotating skyhook for manned spaceflights to the Moon and beyond.

It was the search for a workable solution to all these problems that led to the creation of the non-rotating skyhook.  A skyhook that can be affordably built and operated with existing materials and technology and that doesn’t have the problems of the rotating skyhook.

For more detailed information about what a non-rotating skyhook is and how it works, go here.

A 200-kilometer long basic Non-rotating Skyhook configured to receive a suborbital spacecraft coming up from the Earth.

 

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

Outward Bound: Beyond Low Earth Orbit

Once a combination launch system that includes a Skyhook has been built and orbiting hotels and factories are in the process of being built, where do we go next?  Low Earth orbit is only two hundred to three hundred miles up, and some astronauts have described low Earth orbit spaceflight as “skimming the cloud tops.”  By comparison, the Moon is 240,000 miles away and Mars is measured in many millions of miles.  Based on this, low Earth orbit is the equivalent of standing on our doorstep.

So again, where do we go next in our outward bound journey?

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Some people want to return to the Moon to mine it for water, oxygen, minerals, helium 3, and as practice before going to Mars.

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Some people want to explore the near Earth asteriods with the idea of either mining them or moving them closer to the Earth where they could be processed.

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Others want to build an outpost space station near the Moon that would serve as a jumping of point for trips to the Moon, the near Earth asteroids, and eventually to Mars.

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Some people want to go directly to Mars,

and some want to build space colonies and satellite solar power stations.

The one thing that all of these ideas have in common is the need to go from low Earth orbit to escape velocity.  It takes a lot of velocity to do that. Velocity that requires an upper stage and a lot of propellant.  Building an upper stage, filling it with propellant and lifting it into low Earth orbit costs a lot of money even when using a combination launch system.  Even if you make the upper stage reusable you would still have to launch another load of propellant to refill it and use it again, and the propellant would still have to be contained in tanks.  Bottom line, making the upper stage reusable doesn’t save you very much.  So how do we make going from low Earth orbit to escape velocity as affordable as going from the surface of the Earth to orbit on a combination launch system?

The answer to this is the Skyhook that was built for the combination launch system.

In the same way that the lower end of the Skyhook cable is moving at less than orbital velocity for its altitude, the upper end of the Skyhook cable is moving faster than orbital velocity for its altitude.  Once the Skyhook is long enough, the upper end of the Skyhook cable will be moving at escape velocity.  That means that a spacecraft that releases from the upper end of the Skyhook cable can be placed on a free return orbit to the Moon, on a path to an outpost space station at L-2, or on an escape trajectory to a near Earth asteroid without the need to use an upper stage or any of its onboard propellant.  In other words, the upper end of the Skyhook will make low Earth orbit to escape velocity spaceflight affordable to everyone.

Once we have this we won’t need to choose between the Moon, Mars, or the asteroids as we will be able to afford all of them all on a commercial basis as profit making ventures.

 

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

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 concept, 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, air assisted launch can be either subsonic or supersonic, 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 track 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 increases even more, 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 it 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 beyween them are that the Skyhook is much shorter, it does not reach down to the surface of the Earth, and it can be affordable 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

It’s All About Speed!

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

  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