The flight software team did a reddit AMA a while back [0]. They use C++ for writing the flight software (what flies falcon, dragon, etc) running on a customized RT Linux.

[0] https://www.reddit.com/r/IAmA/comments/1853ap/we_are_spacex_...

Thanks for this! ”We’re looking to start crew trials as early as 2015.” :)

And FreeRTOS. RTLinux is only barely soft realtime.

> RTLinux is only barely soft realtime.

Linux used to run as a non-privileged process in hard real-time kernel, which taken together was guaranteeing responses in nanoseconds range.

If this is still the case (along with what Wikipedia states) , the whole system is by definition hard real-time.

Wow, that's really silly that someone went out, made a hard realtime system using Linux as the lowest priority level, and then called it RTLinux. RTLinux for years had meant Linux with the PREEMPT_RT patches.

In any case, SpaceX doesn't use Linux for hard realtime systems.

> They do not have hard realtime requirements

https://lwn.net/Articles/540368/

And I probably wouldn't use the RTLinux for a real hard realtime system if I were making the choice. Putting all of the realtime code in the same memory domain as the rest of the Linux kernel is a huge surface area for corruption. I'd probably use sel4 with a virtualized Linux to get the same semantics.

But only the underlying kernel part is actually real-time, the Linux process wouldn't be, right? That leads me to the question: Shouldn't all things in a rocket be real time? Are they "post-poning" stuff like telemetry and other communications?

All critical processes can run outside of Linux, in hard real-time fashion. Linux is used for non-critical things, like networking... The whole RTLinux includes Linux (non-RT part) and a hard RT part, for which you can write your code.

From what I understand, they are pushing the boundaries on the hardware front, i.e. relatively recent, typical Linux computers onboard, and just relying on having a few redundant ones and short flight times, rather than very radiation hardened systems like those used in satellites or real-time operating systems.

However on the SpaceX software side I think they’re fairly conservative and “boring”, so I’d guess C and similar guidelines to what you’ve mentioned.

I'd be surprised if they were able to be human flight certified without all the redundancy and hardening that any other manned spacecraft has, though.

For example, my understanding is that the redundancy approach is basically "do everything on 3 computers, vote on the outcome".

Sometimes you even have different firmware/software teams write code for each of the three different configurations (often targeting different processors/architectures, etc). Pretty cool

You can "skimp" on hardening if you use more redundancy (which is generally cheaper, and often lighter (which is the real cost savings)).

You can also skimp on hardening if the operational lifetime in space is minutes to days, instead of months to years like most satellites.

They build everything in-house - software and hardware. You can read about it in detail in Musk's biography - https://www.amazon.com/Elon-Musk-SpaceX-Fantastic-Future-ebo...