Propulsion concepts under study may lead to a radical change in the way in which space is accessed

In the 1951 science fiction classic When World's Collide, a rocket-powered spaceship hurtles down a ramp loaded with hapless escapees from Earth, gathering speed before making a boosted take-off to escape the clutches of gravity. Almost 50 years later, NASA is leading another escape attempt from Earth by exploring propulsion concepts that could end up bearing an uncanny resemblance to this "B movie" spacecraft.

The concepts involve ramjets, scramjets (supersonic-combustion ramjets), gas turbines and rockets. While none of these are particularly new as study concepts or real engines, the idea of combining them into a single propulsion system is in its infancy. Studies at NASA's Glenn Research Center are grouped under the Hybrid Hyperspeed Propulsion (HHP) project, which forms part of the Aerospace Propulsion and Power programme headed by Frank Berkopec.

Rocket-based cycle

Formerly known as the Trailblazer project, after the reference vehicle designed to "drive out the requirements" for a rocket-based combined cycle (RBCC) propulsion system, the project is focused on meeting the outline needs of a recently defined third-generation reusable launch vehicle.

The objective of HHP is to reduce the risk associated with air-breathing launch vehicles, and to set the stage for aircraft-like operations for space access. "It is one of those bridging technologies that will open the door to space," says NASA Glenn director Dr Carol Russo.

"Perfecting air-breathing technology as a stepping stone to space, and reducing the cost of access to space, has got to be a fundamental keystone of the future. In this project, we see the beginning of a merging between the aircraft and spacecraft. They are not distinct classes any more."

Concepts receiving most interest include an unconventional propulsion system known as the pulse detonation engine (PDE), and a host of other air-breathing engine powerplants including the RBCC and a turbine-based combined cycle (TBCC) engine.

Pulse detonation

The PDE has been in early development for several years as part of a project that has targeted 2002 as the date by which NASA aims to establish the feasibility of using pulse detonation-based hybrid cycle and combined cycle engines for space access. In 1999, the PDE was selected as one of three projects to be taken to a demonstration stage under NASA's new Revcon (for revolutionary concepts) programme. A small-scale PDE will be flown on NASA's SR-71 and F-15B to collect data, and could be followed "in several years" by a larger-scale demonstrator, says Berkopec.

The tests will evaluate the operability of the PDE which, theoretically, could be activated from a standing start. The PDE operates on a basic five-phase wave cycle which starts with the mixing of fuel with the air passing through the tube-like engine. Detonation is initiated and a combustion wave begins running through the engine. As the flame front moves down the tube, it compresses the air/fuel mixture in front of it, which then explodes (rather than burns) as the pressure reaches a critical level.

Fresh fuel, meanwhile, is drawn in by pressure reduction behind the passing flame front, while the departure of the detonation wave from the exhaust helps reduce pressure even further, which draws fresh air into the intake. The entire cycle is repeated in microseconds, and a full-scale PDE would comprise an array of combustion tubes to provide constant thrust.

The RBCC blends the performance of the rocket, ramjet and scramjet into one system for seamless ground to orbit operation. Take-off to around Mach 2.25 is achieved using power from the rocket, augmented with air from the inlet which mixes with fuel to provide afterburner power. Above Mach 2.25 the rocket, which is mounted on a pylon inside the flame tube of the engine, is shut off and the powerplant goes into ramjet mode. Fuel for the ramjet continues to be provided by injectors at the exhaust.

As the vehicle accelerates beyond Mach 5, the engine transitions to scramjet mode. New fuel injectors are activated midway along the flame tube, while the aft injectors are shut down. Beyond Mach 10.5, the vehicle is leaving the atmosphere and can no longer take advantage of its oxygen. The inlet is therefore closed off and the rocket re-ignited. The TBCC is similar in concept, but uses a turbine-based compression system for the low-speed flight regimes.

Flight demonstration

NASA's Marshall Space Flight Center is studying several options for flight demonstration of the RBCC concept. They include the wedge-shaped Stargaze small launch vehicle, which could be powered by four scramjet RBCCs; the simpler Starsaber, with two ramjet RBCCs; and the DRACO, a Lockheed D-21 drone modified to demonstrate RBCC/ramjet propulsion integration and mode transition. Both Stargazer and Starsaber would go all the way to orbit, while the modified D-21, originally designed for launch from the Lockheed SR-71, would stay in the atmosphere.

Although it will be some time before a human-rated HHP engine takes to the air, these relatively low-cost concepts are already attracting the interest of the US military for possible application in hypersonic missile projects. The prospect of low-cost access to space remains a real possibility. "Access to space may go through a complete metamorphosis with Generation III [of reusable launch vehicles]. We may start with RBCC and either single- or two-stage-to-orbit, and ultimately include lightweight turbines in them for efficiency and scramjets to have enough safety margin to take tourists into space. I'd go in a second," adds Berkopec.

Source: Flight International