The inner surfaces of rocket engines can be subjected to extremely high heat. Temperatures as high as 3227°C (5800°F) can result from the powerful, thrust-generating exothermal reaction between fuel and oxidizer, which runs hot enough to melt or damage the walls of the combustion chamber.
That's why the cores of hot-running rocket engines are made of temperature-resistant materials that often incorporate networks of cooling passages. In these engine designs, vein-like ducts run with low-temperature (sometimes cryogenic) liquid propellants to carry off the heat and thus maintain structural integrity despite the violent conflagration just adjacent.
Rocket scientists and engineers at a small space-tech company in Madison, WI, think that they have developed a better way to burn rocket fuel. Orbital Technologies (Orbitec) has developed a system that keeps the hot burning gases away from the chamber walls. Orbitec's vortex-cooled liquid engines inject liquid oxygen into the combustion chamber in such a way to generate a stable, tornado-like cyclonic flow that confines the combustion to the central region of the chamber, which protects the surfaces.
In October, Orbitec flight-tested its new rocket engine in the Mojave desert. The engine successfully powered a 25-ft (8-m) long, 800-lb (360-kg) Prospector P-15 sounding rocket built by Garvey Spacecraft to 600 mph (270 m/s). California State University at Long Beach also took part as did subcontractors Moog, Barber Nichols, Concept NREC, and Boeing.
The motor was a version of the 30,000-lb (13,600-kg) thrust liquid engine that Orbitec is developing for the U.S. Air Force's Advanced Upper Stage Engine Program and for several NASA in-space and planetary propulsion systems including the Space Launch System.
The patented vortex engine technology, funding for which has been provided by the USAF Research Laboratory and other DOD agencies, is mature and ready for teaming partners and investment to help it enter applications in space and boost propulsion, said a company spokesman. Orbitec, which has about 70 employees, also builds fire-suppression units as well as life-support and environmental control systems, such as kits that will enable astronauts on the International Space Station to grow fresh food next year.
"Our vortex generator eliminates the high temperatures at the inner surfaces of the engine," said Martin Chiaverini, Principal Propulsion Engineer.
By carefully tweaking the oxidizer-injection parameters, he said, the vortex flow establishes radial gradients of pressure and density that cause the lower-density hot combustion products to be confined near the central axis of the combustion chamber while cold gases yet to be burned are spun out to the walls.
The new method, which enhances general burning efficiency, could be smaller and lighter-weight as well as significantly simpler and cheaper to build, by requiring less, more regular materials.
"It's very expensive to manufacture the internal regenerative-cooling channels and ducts that reject combustion heat" by circulating unburned fuel through the chamber walls, Chiaverini continued. The passages are often leak-prone as well.
Because the vortex engines are not subject to severe thermal fatigue, they can be reused.
Orbitec's engines might find use in the upper stages of launch vehicles that deliver satellites to orbit; upper stage technologies tend to exert considerable leverage on overall mission costs.
Advanced upper stages could cut costs by allowing a given payload to be delivered using a smaller launch vehicle or by boosting the payload capacity of current launchers. The same fundamental technology could also be used on smaller systems for maneuvering spacecraft and to assist larger systems to make it off the launch pad, paving the way for significantly safer, low-cost space access.
The vortex, or swirl, generator technology, Chiaverini explained, relies on oxidizer injectors at the base of the combustion chamber that are aimed tangentially to the inner surface of the curving walls.
"The angle sets up a circulating force that forms a vortex" that keeps the hot-burning reaction gases confined in the tame, central tornado, he added. The circumferential component of the injection flow forms a free vortex that spirals forward along the walls to the head end, where it turns inward to form a second vortex, concentrated along the axis, that flows in the reverse direction out of the chamber at the aft end.
Orbitec, established in 1988, has mounted a long RD&E effort aimed at perfecting the technology.
"In 1998, we were working on a small hybrid [part solid/part gas or liquid] rocket for NASA Marshall," he recalled, and were "trying to get the burning rate to increase so we tried using a swirl injector." The team found that under the right conditions the vortex flow could be used to prevent flame spreading along solid fuel surfaces, rather than enhancing the burning. "Next we tried the technology on a liquid-fuel rocket."
Since then, with USAF, NASA, and some DARPA funding support, the company has tested its patented concept using several liquid and gaseous propellants including (gaseous and liquid) hydrogen, kerosene, propane and gaseous methane in rocket motors that produce thrusts ranging from 10 to 7500 lb (4.5 to 2400 kg). Chiaverini does not see any fundamental barriers to additional scale-up.
The recent flight test also demonstrated a new igniter and nozzle. The novel igniter, which is the size of a thumbnail, employs acoustic resonance heating to spark off the fuel, he said. When a valve opens, oxygen gas flows into a resonant cavity to cause shockwave compression-heating that ignites the fuel in milliseconds. The highly reliable device, also USAF-funded, "doesn't take much energy to light off the fuel."
The flight test also demonstrated the extreme heat-resistance of a new lightweight carbon-carbon composite nozzle extension from Alliant Techsystems. The uncooled nozzle extension, which glowed white hot in tests, was attached to the cooled metal housing using that company's dissimilar materials-joining technology.