How a Combination Launch System Works

A combination launch system works by reducing the total speed a launch vehicle needs to achieve in order to reach low Earth orbit.  For a launch vehicle to reach low Earth orbit on its own without the assistance of a combination launch system, it needs to accelerate to a very high apparent velocity.  In addition to achieving the actual speed of low Earth orbit (7,800 m/s), it also needs additional speed to overcome the force of gravity that is working to pull it down as it boosts for orbit and even more speed for overcoming the aerodynamic drag that wants to slow it down.  There are also other issues that require additional speed to overcome but these are the three largest.  It is the total of all these speeds that are known as the apparent velocity, or the total speed required for orbit.  When all of these speeds are combined together, the total amount of speed required to reach low Earth orbit when launching due east from Cape Kennedy is approximately 9,100 m/s.  This is the total speed required for orbit.

For a launch vehicle to reach orbit and do it affordably it also needs to be single stage, completely reusable, and carry a large enough payload to make it all worthwhile.  Unfortunately, it is not possible to build such a launch vehicle with current technology.  The reason for this is the amount of propellant that it takes to get to low Earth orbit using chemical rocket motors.  Using the highest performance chemical rocket motors available, a Single Stage To Orbit launch vehicle would need to be 87% propellant when it leaves the launch pad.  That leaves only 13% for the launch vehicle and payload.  That is not enough to make a Single Stage To Orbit launch vehicle completely reusable and still carry enough payload to make the cost per pound to orbit mass market affordable.

The closest we have come to building such a launch vehicle was the partially reusable Two Stage To Orbit Space Shuttle.  It had a propellant fraction of 87.2%, an empty weight fraction of 12%, and a maximum payload fraction of 0.8% when flying to the International Space Station.  It was also significantly more expensive to fly than any expendable launch vehicle.

This is where the combination launch system with non-rotating skyhook comes in.  The first step of a combination launch system consists of either

an air-assisted launch

or a ground assisted launch.

A subsonic air-assisted launch using a carrier aircraft like Stratolaunch will reduce the total speed required for orbit by approximately 1,100 m/s.  A 600 MPH ground assisted launch from a mountain top will reduce the required speed for orbit by approximately 900 m/s.  An air-assisted launch will require the addition of wings to the launch vehicle.  This will increase the empty weight of the launch vehicle as well as the drag.  A ground assisted launch does not require this.  The end result is they are both about the same when it comes to reducing the total speed required for orbit.  Reducing the total speed required for orbit by 900 m/s will reduce the necessary propellant fraction from 87% to 84%.

If it is assumed that a fully reusable single stage launch vehicle requires an empty weight fraction of 15% to be built (3% more than the empty weight fraction of the Space Shuttle, or a 25% increase in empty weight), then either of these assisted launch concepts will make this kind of launch vehicle possible but the payload fraction will still be only 1%.

If it is assumed that a fully reusable single stage launch vehicle will reduce the cost to orbit by 90% compared to existing expendable launch vehicles, then this system will reduce the cost to orbit to approximately 1/10th of what it is today.

If this reduction to the propellant fraction is used to increase the payload fraction from 1% to 4% and the launch vehicle is left unchanged, then this system will reduce the cost to orbit to approximately 1/4th of what it is today.

Either approach will work, but neither of them by themselves will make spaceflight affordable to everyone.

Now add the non-rotating skyhook to the launch system.  One design for a mature non-rotating skyhook has an overall length of 2,200 km and a lower endpoint velocity of 80% of orbital velocity for its altitude.  This will reduce the total speed required for the launch vehicle by 1,560 m/s.  Combine this with the 900 m/s velocity reduction that comes with a ground assisted launch and the total speed required for flying to the lower end of the skyhook becomes 6,640 m/s.  If it is assumed that the same high-performance LOX/LH2 rocket motors are used, the required propellant fraction will drop from the original 87% to 77%.  If it is also assumed that the fully reusable Single Stage To Skyhook launch vehicle can be built with an empty weight fraction of 15%, the payload fraction becomes 8%.  This is 10 times the payload fraction of the Space Shuttle when it flew to the International Space Station.  Now, keep in mind that this launch vehicle is a fully reusable Single Stage To Skyhook vehicle that is expected to cost 1/10th the amount to fly as any expendable launch vehicle and combine this number with the 10 fold increase in payload.  That works out to 1/10th divided by 10 = 1/100th the cost in dollars per pound to orbit of any expendable launch vehicle flying to orbit without the assistance of a combination launch system.  In other words, if the original expendable launch vehicle cost $10,000 per pound to orbit, the fully reusable Single Stage To Skyhook vehicle will cost somewhere in the neighborhood of $100 per pound.

If an empty weight fraction of 15% is not enough to build a fully reusable Single Stage To Skyhook launch vehicle, increase the empty weight fraction to 18%.  That is 50% more than the empty weight fraction of the Space Shuttle.  This will reduce the payload fraction to 5% and increase the cost of flying to the skyhook to $160 per pound.  To get back to the $100 per pound launch cost it will be necessary to increase the length of the skyhook until the lower endpoint velocity is moving at 73% or orbital velocity for its altitude.  This will reduce the propellant fraction of the launch vehicle to 74% and increase the payload fraction back to 8%.

This $100 per pound launch cost is what most people think of as affordable to everyone spaceflight.  Think about that.  If the seat weight per passenger is 200 pounds, that means the price for a ticket to the lower end of the skyhook will be in the neighborhood of $20,000.  How many people would buy a ticket to fly to the lower end of the skyhook for that price?  Would you?

What the view from the lower end of a skyhook will look like.

Impact on Lift-off Weight

Another issue that doesn’t get discussed very often is how the combination launch system and non-rotating skyhook affect the lift off weight of the launch vehicle.  Existing launch vehicles are large and that adds to their cost.  The Space Shuttle had a lift off weight of over 4.4 million pounds.  The Falcon 9 has a lift off weight of over 1.2 million pounds.  The largest Delta launch vehicle has a lift off weight of 1.6 million pounds.  By comparison, the Boeing 737 MAX airliner has a take off weight of 195,000 pounds.  As the old saying goes, size matters, and it has a direct impact on cost.

What determines the lift off weight of a launch vehicle is its payload fraction and the size of the payload that needs to be delivered.  If the design payload size for a conventional launch vehicle along the lines of the Atlas or the Delta is 12,000 pounds, and the payload fraction for the launch vehicle is 1%, then the lift off weight will be 1.2 million pounds.  If the design payload size for a fully reusable Single Stage To Skyhook launch vehicle using the previously mentioned ground accelerator and skyhook is also 12,000 pounds and it has a payload fraction of 8%, its lift off weight will be 150,000 pounds.  That is 1/8th the lift off weight of the conventional expendable launch vehicle and on top of that, it is fully reusable.  It is also worth noting that its lift off weight is less than the take off weight of the Boeing 737 MAX.

Now notice the empty weight.  If the empty weight fraction is 15% its empty weight will be 22,500 lbs.  If the empty weight fraction is 18% its empty weight will be 27,000 lbs.  By comparison, the empty weight of the Space Shuttle Orbiter was 172,000 pounds.  Either way, imagine how much easier it will be to move the Single Stage To Skyhook vehicle around, to service it, and to prep it for flight.

When it comes to cost, smaller is definitely better.

What an air launched fully reusable Single Stage To Skyhook launch vehicle might look like.

 

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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