Stratolaunch and the X-15

Stratolaunch is the big, beautiful carrier aircraft that is being built by Vulcan Inc. for launching rockets into space.  It has an empty weight of approximately 500,000 pounds, holds up to 250,000 pounds of aviation fuel, and is designed to launch a rocket that weighs up to 550,000 pounds.  It is one monster sized incredible achievement on the part of Paul Allen and Vulcan Inc.  Hopefully, it will become the first step of a combination launch system that will make spaceflight affordable to everyone.  A fully reusable combination launch system that could eventually consist of Stratolaunch, an X-15 style first stage, a vertical landing suborbital upper stage, a suborbital spacecraft with a built-in upper stage rocket motor for carrying passengers and cargo, and a space station equipped with a basic 200 to 400-kilometer long non-rotating skyhook for the suborbital spacecraft to dock with.

While that might sound like a lot of parts and complexity in order to get to orbit, keep in mind it is a 100% fully reusable system that is less than half the size of existing rockets, that can be affordably built on a step by step basis using existing technology, and that will make spaceflight affordable to just about everyone.  Also keep in mind, that as the skyhook is made longer, it will become possible to eliminate the vertical landing suborbital upper stage from the system.  In addition, as the skyhook continues to be made longer, it will also become possible to add a Rocket Based Combined Cycle (RBCC) propulsion system to the X-15 style first stage so that it can fly directly to the bottom of the non-rotating skyhook without the need of any upper stages.

An example of how this step by step development might work is as follows.

In order for Stratolaunch to get started as a low-cost Earth to orbit satellite launch system, it needs two things.  It needs a winged reusable first stage launch vehicle similar to the X-15, and it will need a small expendable two-stage solid propellant rocket.  This is exactly what was proposed back in 1962 for making the X-15 into a low-cost satellite launch system.

The X-15

In June of 1952, the National Advisory Committee for Aeronautics (the predecessor to NASA) decided to expand its research aircraft program to include aircraft designs capable of speeds between Mach 4 and 10, and altitudes of 12 to 50 miles.  This led to a number of paper studies that resulted in a joint program between NACA/NASA, the U.S. Air Force, and the U.S. Navy, to build a hypersonic research aircraft.  In late 1954 it was decided that this aircraft was to be capable of Mach 6.6+ and be able to reach 250,000 feet or more of altitude.  As with previous rocket-powered research aircraft, it was to be air launched.  It was the beginning of the incredibly successful X-15 program that resulted in 199 flights, speeds in excess of Mach 6.6 (4,500 MPH), and altitudes in excess of 350,000 feet (66 miles).  It was this program that also led to the idea of attaching a small expendable rocket to the underside of the X-15 for launching small satellites into orbit.  If this had been done it would have become the world’s first combination launch system.  Pictures of what this would have looked like can be seen here.

The X-15 was designed for a maximum dynamic pressure of 2,500 pounds per square foot, a positive load factor of 7.33 g’s, and a maximum temperature of 1,200 degrees F.  The skin of the X-15 was made of a nickel alloy called Inconel X, and the internal structure was mostly titanium.  Sixty-five percent of the structure was welded.

The propulsion system used on the X-15 was the XLR-99 rocket motor which had a vacuum thrust of 57,000 pounds, a vacuum specific impulse of 279 seconds, a propellant flow rate of 213.8 pounds per second, and weighed 910 pounds.  It was a variable thrust motor that could be throttled between 50% and 100% thrust and was restartable in flight.  The propellants for the motor were anhydrous (water-free) ammonia and liquid oxygen.

The X-15 carried 8,400 pounds of fuel and 10,400 pounds of LOX in its internal tanks and had a useful burn time of 85 seconds.  The empty weight of the X-15 was 15,000 pounds (of which 1,300 pounds was instrumentation) and the launch weight was 33,800 pounds.  The top speed of the X-15 using internal propellant was 4,150 MPH (Mach 6).  This speed was also the maximum it could achieve without supplemental thermal protection for the airframe.

An interesting proposed follow-on program to the X-15 that did not get built was the addition of a highly swept delta wing that was expected to increase its top speed from Mach 6 to Mach 8.

Additional pictures of a model of this concept can be seen here.

The Expendable Rocket

The small expendable rocket that was proposed for launching satellites from the underside of the X-15 was called the Blue Scout.  It was a two-stage solid propellant rocket that was made from the 2nd and 3rd stages of the Scout rocket.  The 1st stage of the Blue Scout had a launch weight of 4,424 kg and an empty weight of 695 kg.  It had a vacuum specific impulse of 262 seconds.  The 2nd stage had a launch weight of 1,400 kg and an empty weight of 300 kg.  It had a vacuum specific impulse of 311 seconds.  The pylon for attaching the Blue Scout to the underside of the X-15 had an estimated weight of 500 pounds.  The amount of payload that could be delivered to low Earth orbit using this system was estimated to be 150 pounds.  The total weight of the Blue Scout with payload and pylon came out to approximately 13,500 pounds.  The top speed of the X-15 when carrying this additional weight was estimated to be 2,280 MPH.

So what is the purpose of all this information?

To rough out the design of a reusable first stage launch vehicle for Stratolaunch.

Use the highly swept delta wing version of the X-15 for the basic configuration.  Build it with Inconel-X skins and a titanium interior.  Build them two at a time, as prototypes, using soft tooling, 3D printing, and welding as much as possible.  Plan on refining the design based on lessons learned every time a new pair is made.  Make them as unmanned, computer-controlled remotely piloted vehicles.  Use a modern LOX/Methane rocket motor.  Make this vehicle large enough in proportion to the expendable rocket that it can reach a speed of 4,150 MPH before launching the rocket.  Consider using carbon-carbon for the leading edges and nose of the vehicle in order to increase the launch velocity.  This will increase the size of the reusable parts of the launch system while reducing the sizes of the expendable parts which will reduce the cost of getting to orbit.  In order to keep development costs to a minimum, build the expendable two-stage rocket using existing solid-propellant rocket motors and hardware as much as possible.

Once the reusable delta wing X-15 style first stage vehicle and the two-stage expendable rocket are operational, start working on a vertical landing, LOX/Methane powered reusable upper stage rocket to replace the first stage of the expendable rocket.

Once that is done, start working on the reusable spacecraft with built-in rocket motor for carrying passengers and cargo to the bottom of the non-rotating skyhook.  This built-in rocket motor to supply the remaining velocity for matching speed with the lower end of the skyhook and for landing when it returns to Earth if it is a vertical lander.  The amount of propellant that it will need to carry will depend on the length of the non-rotating skyhook and how it lands.  This spacecraft could be a vertical landing spacecraft like the Dragon V2 being developed by SpaceX or a horizontal lander like Dream Chaser.

 

Why the Delta Wing X-15?

Making spaceflight affordable to everyone is all about cost.  The X-15 is flight proven concept that worked that was affordable to build and operate.  That minimizes both risk and development cost when building a new vehicle.  Both of which are necessary if launch costs are to be kept to a minimum.  As for delta wings, they are lower in drag, don’t get as hot at high speeds, are lighter in weight, structurally redundant, simple to build, and have been used on just about every high-speed aircraft ever built.  The Avro Arrow, the F-102, the F-106, the SR-71, the B-58, the XB-70, the X-24B lifting body, the Concorde, and the Space Shuttle, to name just a few.  There were even a number of proposed space shuttle designs from the 1950’s that had delta wings.

Wernher von Braun’s “XR-1”, 1955.

Darrel Romick’s “Meteor”, 1956.

Boeing X-20, “Dyna-Soar”, 1957.

Another more modern delta winged version of the X-15 with expendable upper stage rocket that could be air-launched by Stratolaunch is the XS-1.

 

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

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

Combination Launch Systems

Combination launch systems are launch systems that use two or more different methods to accelerate a launch vehicle up to orbital velocity in order to reduce the cost of getting to orbit.

There are many different launch concepts that we can build today that can be combined together to do this: subsonic air-launch; supersonic air-launch; ground accelerators; combination air-breathing and rocket motor propulsion systems; reusable launch vehicles; and, Skyhooks.  All of these concepts can be combined together to reduce launch costs.

There are also many ways to combine these concepts to make a combination launch system.  They can be as simple as using a vertically oriented ground accelerator to boost an existing expendable launch vehicle up to 600 MPH prior to igniting the rocket motor.  They can be as complex as combining air-launch with a reusable launch vehicle that uses a combination ramjet, scramjet, rocket motor propulsion system, that flies to the lower end of a non-rotating Skyhook.  The hard part is finding the combination that has the lowest user cost to orbit for the lowest possible initial investment, that is sized for the existing launch market and has the potential to grow as the market grows.

Combination launch systems are not new.  The Wright brothers used a catapult to boost their first airplane up to flight speed.  The U. S. NAVY still uses catapults to launch fighter planes from the decks of aircraft carriers.  The world’s first reusable rocket plane, the Bell X-1, was air-launched from a B-29.  Probably the best-known combination lunch system was the X-15 rocket plane that was air-launched from a B-52.  In 1967 they even test flew a mock-up of a scramjet on the X-15.  There were also studies that examined the possibility of supersonic air-launching a rocket-ramjet-scramjet powered delta winged version of the X-15 from the back of the XB-70 at Mach 3 that had a small expendable upper stage rocket for carrying satellites into Earth orbit.

Unfortunately, all of these ideas fell by the wayside in our rush to beat the Russians to the Moon back in the 1960’s.  It wasn’t until SpaceShipOne flew in 2004, the start of Virgin Galactic and Stratolaunch shortly thereafter, as well as SpaceX and Blue Origin developing reusable first stage launch vehicles, that we have seen a serious effort to develop combination launch systems again.  The only negative about this is that none of these new combination lunch systems go far enough.

While all of these concepts, ground accelerators, air-launch, reusable first stage launch vehicles, air-breathing and rocket propulsion systems, and Skyhooks, will reduce the cost of getting to orbit, none of them, by themselves, will make spaceflight affordable to everyone.  To do that we will need a launch system that combines almost all of these concepts together.

Once we have that, the solar system is ours.

 

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