The Clock Started Eleven Hours Early — Java Bug Hunt

Briefing

Modelled on Boeing Starliner's Orbital Flight Test (20 December 2019): the capsule's flight computer read its mission elapsed time from the Atlas V launch vehicle at the wrong point in the countdown, so its clock was off by about 11 hours. After separation it believed it was in a different phase of the mission and fired its thrusters accordingly, burning so much propellant that the planned docking with the International Space Station was called off.

MissionClock.java is a reconstruction: it receives each launch-sequence event with the booster's clock reading and must latch the mission epoch at liftoff. FlightPhases maps elapsed time to the phase — and to the thruster plan — the software believes it is in.

Fix MissionClock so mission elapsed time is measured from liftoff and nothing else.

Bug report

BUG-OFT-MET · Priority: Critical · Reported by: flight software IV&V

onEvent(event, boosterTime) is called for every launch-sequence event, in order, with the booster's clock (seconds since booster power-on).

• the epoch is latched from the FIRST "LIFTOFF" event only; every other event, before or after it, and any repeated "LIFTOFF", leaves it alone

met(boosterTime)

• before liftoff has been latched -> -1
• after -> boosterTime - epoch

phase(boosterTime) -> FlightPhases.phaseAt(met(boosterTime))

Observed: with the countdown events delivered first, met is ~11 hours (39,600 s) too large and the capsule plans a burn for the wrong phase.

Logs

[fsw] event TERMINAL_COUNT booster_t=1000 -> MET epoch latched
[fsw] event LIFTOFF booster_t=40600
[fsw] T+300 MET=39900 phase=COAST — executing coast-phase attitude plan

The code as shipped

MissionClock.java (editable)

class MissionClock {
    private long epoch;
    private boolean latched;

    // Called for every launch-sequence event with the booster's clock
    // reading (seconds since the booster's own power-on).
    void onEvent(String event, long boosterTime) {
        if (!latched) {
            epoch = boosterTime;
            latched = true;
        }
    }

    long met(long boosterTime) {
        if (!latched) return -1;
        return boosterTime - epoch;
    }

    String phase(long boosterTime) {
        return FlightPhases.phaseAt(met(boosterTime));
    }
}

Read-only context: FlightPhases.java.