CINCINNATI — A Silicon Valley startup developing electric propulsion systems for satellites has raised $10 million and added the billionaire founder of LinkedIn to its board.

Apollo Fusion announced the $10 million Series B round July 11, bringing the total the company has raised to date to more than $18 million. The round was led by venture fund Greylock Partners, with one of the fund’s partners, Reid Hoffman, joining the board.

Apollo Fusion will use the funding to scale up manufacturing and testing facilities for its electric propulsion system, called the Apollo Constellation Engine (ACE). The company believes that ACE is well-suited to serve the growing demand for smallsats with onboard propulsion.

Several other companies, including Accion Systems, Enpulsion and Phase Four, are also working on a variety of electric propulsion systems intended for use on smallsats. Apollo Fusion argues, though, that ACE offers more performance in a smaller package.

“We believe that one of the most important things to measure systems on is the total impulse per unit mass of the propulsion system and total impulse per unit volume of the propulsion system,” said Mike Cassidy, chief executive of Apollo Fusion, in an interview. “For all the published stats we’ve looked at, Apollo is about three times better.”

Cassidy said that performance is due to several technical innovations in the ACE system, including a special cathode and a more efficient approach to magnetic lensing in the engine. ACE also uses an undisclosed proprietary propellant that he described as “very high performing.”

“This has helped us win many of the deals we have, because for a lot of satellite developers, volume and mass is the holy grail,” he said. “By being able to get lighter and smaller, we’re winning these contracts.”

Cassidy said that Apollo Fusion has one signed contract to date with ACE with an undisclosed customer, with two others “pretty far along.” The first ACE will be flown on a six-unit cubesat in the first quarter of next year.

The company sees several uses for the ACE system, from enabling remote sensing satellites to fly in lower orbits and thus generate higher-resolution images, to providing more cost-effective means to raise satellites to geostationary orbit. The company is focusing on microsatellite and other small satellites, versus cubesat-class spacecraft.

“We are a good fit for 30 to 50 kilograms up to 750 kilograms or so,” he said. “That’s a place where we really win hands-down.”

Besides its advanced technology, the company credits a more rapid testing cycle for the progress it has made. “Most propulsion labs can perform a handful of tests per week,” said Kevin McClellan, strategy director at Apollo Fusion. The company designed its testing facility to instead perform hundreds of tests per week. “That enables us to have a new generation of hardware designed over the course of three or four weeks. We’ve been able to iterate so rapidly we’re on our eighth-generation design.”

Apollo Fusion also plans to partner with an unnamed aerospace and defense manufacturing company, also based in Silicon Valley. McClellan said that will allow Apollo Fusion to focus on qualification and acceptance testing of its ACE thrusters while that partner handles the manufacturing, allowing for higher production rates.

In addition to the funding, Apollo Fusion is adding Hoffman, best known as the co-founder of the business-oriented social networking platform LinkedIn, as a board member. Cassidy, who previously led four other technology startups before Apollo Fusion, said he has known Hoffman for a couple of decades, meeting him and other entrepreneurs through a regular series of dinners.

“Reid, although he typically does more software stuff, is very interested in space and new technologies,” he said. “He’s a very excited and active advisor and board member for us.”

“Apollo Fusion is building the in-space propulsion system to deploy the next generation of satellites into space,” Hoffman wrote in a statement. “I am thrilled to have invested in the company and to have joined the board.”