Thruster technology shows promise

High-performing, compact swirl combustors offer significant volume- and weight-savings for air-breathing powerplants

BY ROBERT PEDERSON, STEPHEN SCHMOTOLOCHA AND RAYMOND EDELMAN

It's a technology with a tongue twister of a name, but its potential is clear. Boeing Rocketdyne of Canoga Park, Calif., and Simma Technologies of Thousand Oaks, Calif., have developed an advanced compact, swirl-augmented thruster (CoSAT) technology they can use in a broad range of air-breathing propulsion applications and energy conversion devices.

Engineers originally developed this technology as an auxiliary propulsion system to provide additional lift thrust augmentation (LTA) for the Boeing Joint Strike Fighter Short Take-Off Vertical Landing aircraft. The swirl combustion–based LTA system provided a compact thruster that has demonstrated nearly complete combustion across a broad range of operating conditions. Using jet propulsion fuels and air tapped from the main Joint Strike Fighter gas turbine engine, the tested LTA delivered fuel specific impulse of approximately 2,400 seconds, which was superior to all alternative approaches.

The Rocketdyne–Simma Technologies team had to tackle a number of challenges, including providing reliable performance and smooth combustion (with no instabilities) while dealing with tight equipment volume limitations that affected fuel injection, ignition and combustor length. Using swirl combustion technology allowed the team to overcome these issues.

The development team also was able to complete design, development and test demonstration in just six months.

"The reason the lift thrust augmentation program was successful in such a short time was due to three factors," explained Bob Pederson, lift thrust augmentation project engineer with Boeing Rocketdyne. "We have a committed swirl-combustion team with extensive experience in analysis, design, fabrication and testing; a large swirlcombustion test data base that allowed performance scaling; and advanced analysis capability to support design and testing."

The ability to provide reliable ignition, repeatable performance, wide throttle limits, with no moving components, in a compact, low-risk design underscores the viability and practicality of the tested CoSAT technology. These features provide a versatile combustor configuration that engineers can easily scale to meet the requirements of a wide range of air-breathing propulsion and energy conversion technology applications. To that end, Rocketdyne has identified a number of additional applications for the CoSAT technology, including

• Rocket-ramjet combined cycle missiles
• Ramjet missiles
• Ramjet projectiles
• Turbo-ramjet combined cycle engines for aircraft and missiles
• Enhanced military gas turbine engines
• Gas turbines for terrestrial power generation

For example, the performance of a ramjet-powered missile depends on the size, weight and performance of individual components and their integration into the overall system.

A typical long-range, ramjet powered, Mach 4 to 6 missile design has a high combustor length-to-diameter ratio of 5. Two factors drive the extended length: a conventional but inefficient flame stabilization and combustion propagation step-down mechanical flame holder, and the boost propellant packaging needed to accelerate the missile to ramjet takeover speed.

Introducing the CoSAT technology into the same missile results in a significantly shorter and lighter missile that retains high combustion efficiency, so that the booster can now be packaged separately and ejected as the ramjet takes over around Mach 2 to 3. This reduces both the weight and drag of the missile and results in much higher performance.

A turbo-ramjet combined cycle engine is another example of applying CoSAT technology to achieve compact packaging with high performance. The turbojet engine provides acceleration up to ramjet take-over in the Mach 3 to 4 range, while the swirl ramjets provide thrust from about Mach 3 to the cruise condition at speeds of up to approximately Mach 6. For flight speeds up to initiation of ramjet take-over, engineers can provide turbojet thrust augmentation by tapping turbojet compressor air and burning this air with additional fuel in the swirl ramjets.

With numerous potential applications for this CoSAT technology, Rocketdyne already has five patents on the technology either filed or pending.