Failure rate

This mod models the failure chance using a simplified real life model, where each part has an instantaneous chance of failure that depends on the following parameters.

Keep in mind that the failures are generated randomly: a new part has a low chance of failure, but it's not zero. Conversely, a very old part will have a higher chance, but you might still get lucky.

Mean Time Between Failures (MTBF)

This is the average number of hours between the occurrence of two failures. For example, if a component has a MTBF of 1 hour, you can expect to see it fail about 1000 times in 1000 hours.

Over time, the MTBF will decay, which effectively makes the part fail more frequently. The rate of decay is controlled by the second parameter, the life time.

Life time

The MTBF follows an exponential decay function: the life time of the part is simply the time constant of this decay function.

If you haven't encountered exponential decays in your studies, here is a quick cheat sheet for them:

• after (LifeTime) hours, the reliability has dropped by 63%;
• after (3*LifeTime) hours, the reliability has dropped by 95%: you are 20 times more likely to see a failure than when the part was new;
• after (5*LifeTime hours), the reliability has dropped by 99.3%.

End of Life

However, even near the end of the life time, your parts might still be somewhat usable: in fact, 0.7% of a very big initial MTBF might still be a reasonable number (especially if you are stranded with no life support and the odds are grim to begin with...).

For example, consider a part that has an original MTBF of 10 thousand hours: after 5 times its expected life time, the MTBF is still about 67 hours, which might be more than enough to attempt a return home (see the "no life support" scenario, above).

To help you estimate how far you can push your luck, in the editor you will also see an EOL value: this is the number of hours that it will take for the MTBF to drop below 1 hour. After this time, you can expect to see the part fail on average ever 60 minutes: at this point, I advise you to rage quit and close the game.

Multipliers

In addition to these, the instantaneous chance of failure is affected by two more multipliers:

• One based on temperature: if a part starts to overheat, it will be more likely to break;
• One based on the part's function. For example:
  • Engines are more prone to fail when run at low or high throttle, and are more reliable around 50% throttle;
  • The thicker the atmosphere, the more control surfaces are likely to get stuck.