In October, a small team of rocket scientists gathered in the middle of the Mojave Desert to watch a launch they hoped would help change space flight forever.
Poised in front of them on the flat, open pan was a slender, 7.5-m (25-ft) tall, Garvey Prospector P-15 sounding rocket. To the untrained eye, the rocket was nothing special. But inside was a radical new engine technology that promises to cut the size, weight and therefore the cost of putting a rocket – and payload – into space.
When the countdown clock ticked zero, the rocket fizzed into a roar and rose rapidly from its launch pad; its blazing engine pushing the rocket to 270 m/s (600 mph). After 30 seconds, the engine cut out and the first of the rocket’s red parachutes popped from their casing. The flight test was a success.
For the crew from Orbital Technologies Corp (Orbitec) that built the test motor, it was another step in its decade-plus-long journey to prove its technology and show it to be an attractive alternative to a technology that’s been around since the birth of rocketry and one that is still used in most big rockets today.
“Orbitec is ready and excited to compete for any future rocket engine and propulsion application,” Paul Zamprelli, business director at the firm said at the time. He and his colleagues believe that their “game-changing technology” could have a major impact on lowering the cost of space access. “We look forward to supplying the Air Force, Nasa and commercial markets with all of our affordable advanced engines and technologies.”
To understand why Orbitec’s engines are different, you must first understand how larger liquid-fuel rocket engines – the ones that power astronauts and satellites into orbit – work. At their most basic these rockets have a combustion chamber that’s fed by two pressurised tanks – one of a rocket fuel and one of an oxidiser. When these two are forced into the chamber they mix, ignite and the exhausts are sent at high speed through a nozzle at the end of the rocket, propelling it forward.
At full thrust, these engines get incredibly hot, reaching temperatures upwards of 3,000C (5,400F) or more, hot enough to melt the metal chamber in which the rocket fuel mixes with oxygen and burns. At these extremes, even rockets with sidewalls made of heat-resistant superalloys would fail catastrophically.
To solve this problem, rocket scientists usually incorporate vein-like networks of cooling tubes through the sides of the combustion chamber which contain heat-absorbing liquid fuels that carry off excess thermal energy. The arrangement is like a car’s radiator system with internal coolant ducts arrayed around the outside of the engine core. It is only through this so-called "regenerative cooling" system that the rocket is able to maintain its structural integrity. Although the system works it adds considerable weight, cost and complexity to the engine.
Orbitec’s alternative approach keeps the hot burning gases away from the chamber surfaces altogether. The company’s patented designs create a cyclonic swirl, or vortex, of fuel and oxygen that holds the searing gases and fumes in the very centre of the cylindrical combustion chamber, away from the vulnerable sidewalls.
“Our vortex generator eliminates the high temperatures at the inner surfaces of the engine,” says Martin Chiaverini, principal propulsion engineer at the firm. “You can touch the exterior during lab-test firings and not get burned.”
The vortex, or swirl, is produced by placing the oxidiser nozzles at the base of the combustion chamber and aiming them tangentially to the inner surface of its curving walls. This produces an outer vortex of cool gases that spiral up the walls forming a protective, cooling barrier. When this meets the top of the chamber it is mixed with rocket fuel and forced inward and down, forming a second, inner, descending vortex in the centre of the chamber that is concentrated like a tornado. The escaping downward stream of hot, high-pressure gases are then forced through the nozzle at the back of the chamber, producing thrust.
As well as keeping the exterior of the system cool, the vortex also works to burn the rocket fuel more efficiently by promoting more complete mixing of the fuel and air in a confined area. In addition, the longer path of the spinning vortices give the fuel more opportunity to burn, meaning the chamber height can be reduced, making for a significant weight savings – and therefore cost – savings.
Vortex rockets can also be considerably simpler and cheaper to build than conventional engines. And because vortex engines are less subject to severe heat and wear they last longer, which may make them suitable for reuse, cutting mission costs even further.
Such potential benefits explain why work on the unique power plants has been funded by the US Air Force Research Laboratory and other defence department agencies for more than a decade.
“It’s a nice, simple solution to the rocket chamber-cooling problem,” says Michael Micci, a rocket propulsion specialist and professor of aerospace engineering at Pennsylvania State University.
Orbitec’s research team now thinks that their rocket is ready for bigger and better things. Its October test in the Mojave used a version of a 13,600-kg-thrust (30,000-lb) liquid engine that it is developing for the US Air Force’s Advanced Upper Stage Engine Program, which aims to find a more capable and affordable upper-stage motor to sit atop the Atlas 5 and Delta 4 rockets it uses to launch most national security payloads.
The upper-stage of a rocket is the last to fire – and is usually the smallest segment of the vehicle. It operates at high altitudes once the lower stages have fallen away and pushes the payload into its final orbit. Current upper-stage technologies tend to account for a significant – and seemingly disproportionate – amount of the overall mission costs, according to the US space agency Nasa. Better-performing upper-stage engines would reduce costs because a given payload could be delivered using a smaller launch vehicle.
Nasa is particularly interested in the Orbitec’s vortex/cold-wall engine technology as a possible candidate to power the upper stage of its Space Launch System, the agency’s next-generation heavy-launch vehicle that will take the place of the space shuttle and may be used to kick-start manned missions deeper into the solar system.
The same technology could be used for manoeuvering spacecraft and propelling planetary probes. It could also help booster rockets get off the launch pad, paving the way for significantly safer, low-cost space access, Orbitec says.
Not bad for a firm that began in 1988 as a tiny university spinoff with “three engineers in a garage” in Madison, Wisconsin, says an admiring industry insider.