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

Dreams of Space

What are your dreams of space?

People have dreamed about building cities on the Moon and Mars for years.

Some people even dream of making Mars into an earthlike planet – a true second home for mankind.

Others want to go asteroid mining so as to bring home the wealth of the solar system.

There are others who want to build cities in the stars, O’Neill style space colonies scattered throughout the solar system, to develop a truly space based civilization.

They are all worthy and wonderful dreams.  Yet none of them will happen until a way is found to make space travel affordable to everyone.  Affordable to the individual spaceflight is the foundation of all these dreams if any of them are ever to be made real.

Affordable to everyone spaceflight is not just about the big dreams, it is also about the personal dreams of every person who has the courage to dream.  For some, that personal dream might be to spend a week or two in an orbiting hotel watching the Earth pass by underneath.

For others, it might be to get a job in one of the orbiting factories or research stations so as to become part of the new frontier.

Then there are those who dream of starting their own business in space such as a repair and refueling service for satellites that orbit the Earth or to build a farm module where they can make a living growing food for the people who live and work in space.

Other still might dream of getting a job in space so they can use their spare time to build a small spaceship that will allow them to homestead an asteroid.

The possibilities are endless.  The only limitation is your imagination and how hard you are willing to work.

Whatever your dreams of space are, affordable to everyone spaceflight is what will make them possible.  Without it, all these dreams, both large and small, will remain forever unattainable.

Isn’t it time we built a combination launch system that will make all this possible?

 

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

Outpost Space Stations

The idea of using a network of outpost space stations (also known as deep space habitats) scattered around the solar system to assist in making manned spaceflight between the planets affordable has been around for at least 25 years.  Who came up with the idea originally I don’t know, but it is an idea that has its roots in the network of coaling stations that were built all around the world for refueling steamships back in the 1800s.  The steam engines that powered the early steamships were not very efficient and they burned a lot of coal.  So much coal that if a ship had to carry the coal for an entire voyage there wasn’t much room left for cargo to pay for the voyage.  To solve this problem coaling stations were built at strategic locations all around the world so that the steamships would only have to carry enough coal for the journey to the next station.  This reduced the steamship’s propellant fraction and increased its payload fraction, which reduced the cost of shipping people and cargo all around the world to an amount that just about anyone could afford.

This idea also applies to automobiles.  The average automobile only carries enough fuel to give it a range of about 300 miles.  Not a lot of range if you are planning a cross-country trip.  If it was necessary to design a car so that it could go across the country and back on a single tank of fuel, the car would be much larger and much more expensive to build, buy, and operate.  So much more expensive that very few people could afford them.  To solve this problem, a network of gas stations was built around the country.  It is the existence of this network of gas stations that makes automobiles so effective and affordable.  They decrease the car’s propellant fraction and increase its payload fraction.

This is what outpost space stations are about.  They are the refueling stations on the trade routes to the planets and asteroids.  They allow the spaceships traveling between the planets and asteroids around the solar system to only carry the propellant that is needed to travel to the next outpost space station along the route.  This reduces the amount of propellant the spaceship needs to carry and allows it to carry a much larger payload.  This reduces the cost of space travel to an amount that just about anyone can afford.

An example of this are the spaceships that were designed by Wernher von Braun for going to Mars in 1948.  These ships were designed for four major propulsive events: first, to accelerate from Earth orbit velocity to Mars transfer orbit velocity; second, to decelerate from Mars transfer orbit velocity to Mars orbit velocity; third, to accelerate from Mars orbit velocity to Earth transfer orbit velocity; and fourth, to decelerate from Earth transfer orbit velocity to Earth orbit velocity.  The total amount of change in velocity required for these four maneuvers is 11.48 kilometers per second.  The amount of propellant required to do this using the low-performance rocket motors of the day required that the spaceships be over 98 percent propellant when departing from Earth orbit.  This meant the empty weight of the spaceship, plus the weight of the crew and all their supplies and equipment, had to be less than 2 percent of the departure mass.  In all of human history, no one has ever come close to building any type of vehicle with a propellant fraction that high and with an empty weight fraction that low.  Even if it were possible to build such a vehicle with existing technology, it would not be affordable on a commercial basis due to the extremely low payload fraction.

Now assume a non-rotating Skyhook in Earth orbit that has an upper endpoint velocity of just short of Earth escape velocity, an outpost space station at the Earth-Moon L2 Lagrange Point with a local source of propellant (either the Moon or a near-Earth asteroid that has been moved to L2), and a non-rotating Skyhook in Mars orbit that has an upper endpoint velocity of just short of Mars escape velocity as well as a local source of propellant (either Mars or one of the Martian moons), and look at what happens to the change in velocity requirement and the propellant fraction of an Earth-Mars spaceship.

Passengers and cargo bound for Mars, fly to the lower end of the Skyhook that is in Earth orbit using the fully reusable combination launch system described in previous posts.  From there they transfer to the upper end of the Skyhook where they transfer to the spaceship that will take them to the outpost space station at the Earth-Moon L2 Lagrange Point where they board the Earth-Mars spaceship.

On the day of departure, the Earth-Mars spaceship leaves the halo orbit at L2 and heads toward Earth where it will perform a gravitational slingshot maneuver as it accelerates to Mars transfer orbit velocity.  The change in velocity for these two maneuvers is approximately 1,500 meters per second.  When the Earth-Mars spaceship gets to Mars it slows down to just under Mars escape velocity and enters a high Mars orbit.  The change in velocity required for this maneuver is approximately 900 meters per second.  The total change in velocity required for this voyage is 2.4 kilometers per second.  That is 1/5th the amount of change in velocity required by the spaceships used in Wernher von Braun’s original Mars fleet.  This difference is due to both the Skyhooks and the outpost space stations with local sources of propellant at both Earth and Mars.

The transfer of passengers and cargo from the Earth-Mars spaceship to Mars will be performed by smaller spacecraft operating from the upper end of the Martian Skyhook using locally supplied propellant.  These spacecraft will also be used to carry locally supplied propellant to the Earth-Mars spaceship for its return trip to Earth.  If it is assumed that the Earth-Mars spaceship uses existing LOX/LH2 chemical rocket motors with oversized expansion nozzles (a specific impulse of 480 seconds), the propellant fraction for the journey to Mars will be 40 percent.  This will leave plenty of room in the mass budget for the Earth-Mars spaceship to make it completely reusable (no drop-off propellant tanks or stages), allow for a spinning section with artificial gravity for the crew and passengers, a hydroponics garden for fresh vegetables, plus plenty of radiation shielding.  If it is assumed that the empty weight fraction for this Earth-Mars spaceship is 40 percent of the departure mass, that will leave 20 percent for the payload.

If a nuclear thermal rocket motor that uses water for reaction mass is used for the propulsion system for the Earth-Mars spaceship, the propellant fraction for the trip to Mars drops to 25 percent and the payload fraction increases to 35 percent.  When used with a combination launch system that includes an escape velocity capable Skyhook, either of these Earth-Mars spaceships would make a trip to Mars mass market affordable on a commercial basis and would operate at a profit for the owners.

Either of these spaceships would also be capable of making trips to the asteroid belt, and to dwarf planets like Ceres, both affordable and possible.  Put an outpost space station in orbit around Ceres with a reusable lander that has the ability to lift water from the surface of Ceres into orbit, and these spaceships would have the ability to take a manned expedition to the moons of Jupiter and to explore the Greek and Trojan asteroids.

As hard as this might be to believe, we really are on the cusp of another Great Age of Exploration.  Only this time, instead of exploring a single planet, we will have people exploring the entire solar system.  All that is required is a combination launch system with a Skyhook and a couple of outpost space stations.  It truly is the beginning of our reaching out to the stars.

 

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