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

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