Apple recently lost three other engineers who were part of the company’s car project, according to Bloomberg.

The three engineers include Eric Rogers Apple’s chief radar systems engineer, who left the project for flying-text startup Joby Aviation; Alex Clarabut Apple’s engineering manager for the team’s battery systems group, who joined Archer Aviation; and Stephen Spiteri Apple’s hardware engineering manager, who also joined Archer.

Engineers leaving the project further underscore how difficult it is for Apple to succeed in a new industry. The three recent employees’ resignation further adds to the six employees who left the company this year.

After leaving the 7-year-old Apple project, the engineers were employed by car startups. Named “Project Titan” Apple’s car project looks to build self-driving electric cars. However, the technicality that comes with the production of such innovations, together with technology companies scouting Apple’s workers, has contributed to slowing the pace of the project.