Startup Tries to Revive Interest in Aerospike Rocket Engines

Space fever is spreading on Earth. Plans for lunar habitats are underway, China recently completed a biological growth experiment on the moon, and the first all-woman space walk took place earlier this year.

NextAero, a startup based in Melbourne, Australia, wants to capitalize on this energy with its 3D-printed aerospike engines. Compared to conventional rocket engines with bell-shaped nozzles, aerospike engines have conical nozzles shaped like funnels that protrude from the engine-hence the spike.

Unlike Relativity Space, which can 3D print an entire rocket, NextAero's focus is on building propulsion systems for small satellite launch vehicles. The small satellite market is set to become a US $15 billion industry by 2026, with growing demand for CubeSats in sectors such as agriculture, energy, defense, and communications.

To build its first rocket engine within four months in 2017, NextAero turned to additive manufacturing, also known as 3D printing. ProjectX, an aerospike combustion engine, relies on compressed natural gas and gaseous oxygen for fuel. The proof-of-concept engine has water channels below its surface for active cooling and multiple combustion chambers separated by fins to hold the spike. It produces 4 kilonewtons of thrust at sea level-small compared to the 7,607 kilonewtons of thrust from SpaceX's Falcon 9 rocket, which the aerospace company described as "thrust greater than five [Boeing] 747s at full power," or the 2 meganewtons of thrust of the Raptor engine that will power SpaceX's in-development Starship spacecraft.

The founder's motivation for starting the company hits close to home. "There seems to be a lack of high-value engineering done in Australia, so we're passionate about building a company that's headquartered in the country," says founder and CEO Graham Bell.

Designing the engine involved building 3D models and developing a software toolkit. To achieve the aerospike geometry of its engine nozzle, NextAero turned the traditional bell shape inside out and added a spike. "We challenged ourselves to think not just [about] how we build it, but if we could make the optimum fluid surface, what would it look like?" says Bell.

The aerospike design comes with the benefit of increased engine efficiency. As the rocket travels through the atmosphere, the spike allows it to expand the plume, giving it extra thrust-about 25 to 35 percent more than conventional rocket engines of the same size, according to Bell.

Photo: NextAero These 3D-printed parts are for NextAero's ProjectX. On the left is the aerospike nozzle and on the right is the rocket engine body.

Moreover, aerospike nozzles perform well at different altitudes compared to a bell nozzle-which is designed for one particular altitude-because the aerospike design can allow gases to expand and match the current atmospheric pressure, thereby producing maximum thrust at any altitude.

Despite their advantages, aerospike engines have been slow to catch on. NASA's X-33 launch vehicle was supposed to feature an aerospike rocket engine, but the project ran out of funding in 2001. In 2014, Firefly intended its Alpha rocket to have an aerospike engine, but the technology curve was too steep and development proved too costly, so the rocket company resorted to a conventional bell-shaped nozzle.

The only successful flight involving the design so far is NASA's Dryden Aerospike Rocket Test in 2004, which saw the launch of two solid-fuel rockets with aerospike nozzles. As NASA wrote in its report about the flight: "Although the advantages of the aerospike nozzles are well understood through analysis and ground test data, a shortage of actual published flight test data has precluded use of these nozzles in current as well as next generation space launch vehicles. In addition, the configuration of an aerospike nozzle presents unique challenges to the designer and fabricator."

NextAero aims to change the course of aerospike engines with its efforts on Project19, a 10-kilonewton prototype engine. Similar to ProjectX, this aerospike engine operates on a pressure-fed combustion cycle, but runs on high-purity ethanol and liquid oxygen. A throttling mechanism powers its solid-state thrust vector control. "It's more of a commercial engine, something closer to what the industry is expecting using the correct fuel, the correct pressure, and all that," says Bell.

The startup is in discussions with pilot partners and hopes to get some actual flight data after the engine completes on-the-ground testing. "The big thing missing with this engine type is the lack of flight data-how it behaves when air that's twice the speed of sound rushes past it," Bell says. "Our major goal is to get a flight by the end of next year."