Don’t push that button: Exploring the software that flies SpaceX rockets and Starships

Charles Martin and Ben Popper:

The actual work of software development by vehicle engineers such as Gerding is largely done using C++, which has been the mainstay of the company’s code since its early days. The software reads text-based configuration files. “We invented simple domain specific languages to express those things, such that other engineers in the company who are not software engineers can maybe configure it.”

Flight software for rockets at SpaceX is structured around the concept of a control cycle. “You read all of your inputs: sensors that we read in through an ADC, packets from the network, data from an IMU, updates from a star tracker or guidance sensor, commands from the ground,” explains Gerding. “You do some processing of those to determine your state, like where you are in the world or the status of the life support system. That determines your outputs – you write those, wait until the next tick of the clock, and then do the whole thing over again.”

The control cycle highlights some of the performance requirements of the software. “On Dragon, some computers run [the control cycle] at 50 Hertz and some run at 10 Hertz. The main flight computer runs at 10 Hertz. That’s managing the overall mission and sending commands to the other computers. Some of those need to react faster to certain events, so those run at 50 Hertz.”

There is a wide variety of machines talking to the central flight system. “We have inputs from sensors all over the vehicle, all kinds of different sensors.” Many are measuring internal values critical to the health of the ship and crew. “Temperatures are important. For crewed vehicles, we have oxygen and carbon dioxide sensors, cabin pressure sensors and things like that.” 

Another set of sensors looks externally to aid in navigation and telemetry. “That would be like the IMU, GPS, and star trackers.” Once they are close enough to the space station, they also use laser range finders.