What is a Skyhook . . .

and why build one?

Have you ever built a treehouse or seen one built?

It is a lot of work to carry every piece of wood and all the tools up the tree to build it, especially when it usually takes both hands and both feet to climb the tree.  Another important issue is how high up in the tree the treehouse is.  The higher up it is, the more work it takes to get there.  So most people usually take a rope up with them early in the construction process so they can use the rope to lift all the wood and tools to the treehouse.  Quite simply, it makes the job of building the treehouse much easier.

That is what a Skyhook is.  It is the rope that hangs down from the treehouse that is used to haul things up.  Only in this case the treehouse is a space station that orbits the Earth and we don’t have a rope that is long enough to reach all the way to the ground.  Even if the rope did reach all the way to the ground it would still be moving too fast to grab.  So instead we hang the rope down to just above the upper edge of the atmosphere and use a high-speed aircraft/spaceplane to fly to it.  It still isn’t easy to get to the lower end of the Skyhook, but it is much easier than going all the way to the space station without the Skyhook.

This is what a Skyhook is.  It is the rope that hangs down from the treehouse.  It is that simple.

How it works.

When a rocket is sitting on the launch pad ready to fly into orbit, 90% of its weight is the propellant it needs to fly to orbit, 9% of its weight is the rocket, and 1% of its weight is the payload.

While this picture does not show the weights of all the various components, it is pretty easy to see just how large the rocket is, and just how small the payload is by comparison.  In this case the payload is the satellite called GOES-N at the right side of the picture.  You also need to keep in mind that all the rocket parts shown in the picture represent only 9% of the take-off weight.  It is the propellant inside the LO2 tanks and the LH2 tanks that make up 90% of the take-off weight.  (LO2 stands for liquid oxygen and LH2 stands for liquid hydrogen.)

Imagine a car being built with these proportions.  If you weigh 150 lbs, the car will weigh 1,350 lbs, and it will need 13,500 lbs of propellant (approximately 1,875 gallons) to get to the space station.  At $4.00 per gallon that means it will cost $7,500 for the propellant.  This car is also the ultimate dragster.  It will accelerate from zero to 20,000 MPH in 8 minutes.  Not too many cars can do that.  This one does it by throwing off parts and pieces as it goes up.  That is why this car is only good for one trip.   Assuming you were able to buy an economy model on Amazon for $20,000,000, that means your trip to the space station will cost you a total of $20,007,500 dollars, give or take a few million.  It is a journey that not too many people can afford.

Now you know why spaceflight costs so much and why so few people do it.

The high cost of spaceflight is due to the speed of orbit.  The speed of orbit is approximately 5 mile per second.  In addition, the space station is approximately 200 to 300 miles up.  It is the sum of these two elements, the speed of orbit and the altitude, that represent the lion’s share of the work that a rocket has to do in order to reach the space station.  This is why the rocket has to carry so much propellant and has so little room for payload.

Another important detail: the speed for orbit goes down as you gain altitude.  This is due to the fact that the force of gravity becomes less as you get further away from the Earth.  Unfortunately, it takes more energy to climb to that higher altitude than the speed reduction saves you, so flying to the lower and faster orbit is still the lowest energy solution.  But it is this reduction in velocity with altitude that makes the Skyhook work so well.  By putting the space station higher up in a slower orbit and hanging the Skyhook down to just above the atmosphere, we get the advantage of the space station’s slower speed at a lower altitude.  This reduces the amount of propellant the rocket needs and allows it to carry a larger payload.  The end result is that our rocket can carry a much larger payload to the bottom of the Skyhook than it can when it goes directly to the space station.  Another advantage of this increase in payload capacity is that it allows us to build a rocket that is reusable and that operates like an airliner.  This combination of increased payload and airliner like operations makes for a drastic reduction in launch costs once the Skyhook becomes long enough.

This is what a spaceplane designed to fly to the bottom of the Skyhook looks like.  This particular spaceplane is powered by a combination propulsion system that consists of after-burning turbine engines, ducted rocket motors, a ramjet, and a scramjet.

This is the magic of a Skyhook.  It makes spaceflight affordable and gives us the solar system as a result.

For more information on Skyhooks go to:






Index of Additional 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. Non-rotating Skyhook for Earth