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The Ascender rocket plane, an X-Prize contender.


Ascender Series
Tech Level: 11-13

Kelly Eclipse
Tech Level: 11
Tech Level: 11
VentureStar Descendants
Tech Level: 12

Rocket planes were developed in the late 1940s and were flown extensively into the late 1960s. They reached the very edge of space, over 60 miles up, and if the US had not hastily switched over to vertically launched rockets in the wake of Sputnik 1, they would have probably been the West’s first true space vehicles.

The X-33, which until recently NASA was developing as a replacement for the Space Shuttle, was a modern, highly advanced version of the rocket plane. A number of private interests have revived the rocket plane concept in order to capture the X-Prize.


Rocket planes are basically what the words imply: an airplane built around a rocket engine instead of a jet or propeller engines. The earliest rocket planes were little more than missiles kitbashed with wings and a cockpit. Yet they were the first manned vehicles to break the sound barrier, reach the edge of space, and push the limits of hypersonic flight.

The X-15 rocket plane

Rocket plane technology was basically abandoned after the X-15 program ended in 1968 as vertical launch vehicles came to completely dominate space travel. The Space Shuttle is only tangentially connected to the rocket planes; it is vertically launched and glides unpowered back to Earth.

However, rocket planes were revived somewhat in the 1990s when NASA launched its X-33 program to develop a replacement for its aging Space Shuttle system. The X-33 was the designation given to the half-sized test vehicle(s); the full sized, fully functional orbiters were to be called VentureStars.

The centerpiece of the X-33 project were its linear aerospike engines. Its plumbing and accessories are very similar to a normal rocket engine. The major difference is the absence of a bell-shaped nozzle. For optimum engine performance and efficiency, the atmosphere serves as part of the aerospike’s nozzle with the rocket's exhaust plume contained in the surrounding airflow, allowing it to compensate for atmospheric changes, unlike traditional rocket engines.

The linear aerospike’s 75 percent shorter length means less engine weight and less engine support structure required, allowing for a lighter vehicle and lower launch costs than conventional rockets.

The aerospike engine concept was developed in the 1960s and 1970s, but was never actually flight tested until 1998, when a SR-71 Blackbird was outfitted with one and flight tested as part of the X-33 development program. Linear aerospike engines were extensively tested between then and the termination of the X-33 program in 2001.

Each of the two aerospike engines aboard the full-scale X-33 was to have 20 seperate combustion chambers arrayed in a 4 x 5 grid. The fuel could be directed at one, all, or any combination of combustion chambers anywhere along the grid, allowing the X-33 unprecedented precision control of its flight.

Combined with a computer designed, aerodynamically optimized lifting body, the X-33 represented a quantum leap forward in orbital interface travel technology. Though NASA has killed the project because of budget concerns, it has a solid foundation of proven engineering that it may be revived one day as the aging shuttle fleet needs to be replaced.


Tech Level: 11

The jewel in the conceptual crown of the Kelly Space & Technology corporation. Exact specifications for this design have proven sparse, but it appears to be a conservative design that borrows heavily from proven NASA shuttle designs and off-the-shelf technology. The Eclipse would be tow-launched by a 747, then boost into LEO with a single powerful, hydrogen-fueled rocket, carrying up to 5 tons of cargo. It would then glide back to earth unpowered much like the Space Shuttle.


Tech Level: 11

Another proposed rocket plane design from the Pioneer Rocketplanes company that uses mostly existing technology. The Pathfinder would take off from a runway exactly like a conventional jet, but with almost no liquid oxygen in its tanks. Once at its conventional flight ceiling, it would be refueled using well-proven mid-air refueling techniques, and essentially "take off" fully fueled at high altitude with its rocket engine.


Tech Level 11-13

A more direct X-15 descendant under development is the Ascender (TL-11) project, being constructed by Bristol Spaceplanes Limited. Using already existing and well-proven technology, the Ascender craft looks like a small, sleek passenger jet. And that’s exactly its purpose; its designed to help get a burgeoning space tourism industry off the ground by taking passengers to the edge of space. A conventional jet would take the Ascender up to a height of 8 km, where the rocket plane would then seperate and engage its rocket engines in a vertical parabola, peaking at a height of about 100 km. It then glides back to earth unpowered, much like the Space Shuttle. Currently under development, it may be operational within a few years if its private funding holds out. Its builders hope to capture the X-Prize.

The makers of the Ascender are already envisioning upgrades. The first would be the Spacecab (TL-12), a two-stage system about the size of a concorde. The first stage is a full-scale hypersonic craft capable of achieving Mach 4; riding on its back would be the actual orbiter, a souped-up second-generation version of Ascender. The first stage aircraft gives the orbiter, riding piggy-back, a velocity as well as altitude boost before they seperate, allowing the second stage to achieve actual orbit.

The Spacecab Ascender upgrade

The next stage of development for this class of vehicle would be the Spacebus (TL-13), similar in design to the Spacecab but bigger, more powerful, and more advanced. Using a combination of turbo-ramjet engines and rocket engines, the first stage concorde-sized "booster" could achieve a height of of 24 km and a top speed of Mach 6. The orbiter then seperates and boosts to orbit, able to carry an estimated 5.4 tons into Low Earth Orbit (LEO). The creators of the Ascender project see the Spacebus becoming the first true space tourism vehicle accessible by common people (estimated cost for a trip would be about $10,000 per person, less than half the cost of a new car nowadays) and would open up not only true space tourism but perhaps the advent of "space hotel" stations.

The Spacebus Ascender upgrade


Tech Level: 12

The X-33 went through more than a decade of development before it was axed, and proved that its core technologies were workable (though, of course, not without their problems.) When the world space agencies get serious about upgrading beyond the Space Shuttle for a heavy orbital cargo lifter, something very similar to the X-33/VentureStar may be revived. Barring any significantly new propulsion breakthroughs between then and now, something like the VentureStar or a close cousin may fly into orbit sometime in the next 25 years.

One of the main advantages of the X-33 that promised to greatly reduce costs was the fact that it could take off and land using conventional runways. Theoretically, it could operate out of any major airport in the world if it had to. This would also greatly reduce theoretical minimal turnaround times to a few days instead of the 12 weeks the Space Shuttle needs.


The X-15 Homepage

The X-33/Venturestar Homepage

The Ascender Homepage

Kelly Space & Technology Homepage

Pioneer Rocketplane Homepage (Pathfinder)

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