Stage separation — the ascent sequence as a proven order

stage_separation.cell

// ============================================================================
// STAGE SEPARATION — the ascent sequence as a proven order
// ============================================================================
//
// Launch vehicle staging has exactly one correct order: first-stage
// cutoff, THEN separation, THEN second-stage ignition. Every permutation
// is a lost vehicle. Igniting stage 2 while still bolted to stage 1
// fires an engine into a full propellant tank. Separating under thrust
// drives the stages into each other (recontact has ended real flights).
// Lighting stage 2 before separation completes burns the interstage.
//
// In flight software these are timer-sequenced events that a dropped
// packet or a stuck sensor can reorder. Here the sequence is a LINEAR
// state machine: each event is reachable only from its predecessor, and
// orbit_insertion is terminal — soma verify proves there is no path that
// skips, repeats, or reverses a staging event.
// ============================================================================

cell StageSequencer {
    face {
        signal main_engine_cutoff(id: String) -> Map
        signal separate_stage(id: String) -> Map
        signal ignite_stage2(id: String) -> Map
        signal confirm_orbit(id: String) -> Map
        signal phase(id: String) -> String
    }

    memory {
        // mission elapsed time of each event, seconds — strictly ordered
        // bookkeeping that can never record a negative or absurd time
        met_s: Map<String, Int> [persistent, consistent]
        invariant met_s >= 0 && met_s <= 3600
    }

    // A straight line. ABSENT: every shortcut.
    //   stage1_burn -/-> stage2_burn        (ignition while mated)
    //   stage1_burn -/-> separated          (separation under thrust)
    //   separated   -/-> orbit_insertion    (orbit without a burn)
    state ascent {
        initial: stage1_burn
        stage1_burn -> stage1_cutoff     // MECO: thrust at zero
        stage1_cutoff -> separated       // pyros fire only after cutoff
        separated -> stage2_burn         // ignition only with clear gap
        stage2_burn -> orbit_insertion   // terminal: the mission is up
    }

    on step_event(id: String, target: String, t: Int) {
        let r = try { transition(id, target) }
        if r.error != () { return map("ok", false, "reason", "{r.error}") }
        met_s.set("{id}:{target}", t)
        return map("ok", true, "phase", get_status(id), "met", t)
    }

    on main_engine_cutoff(id: String) { return step_event(id, "stage1_cutoff", 162) }
    on separate_stage(id: String)     { return step_event(id, "separated", 165) }
    on ignite_stage2(id: String)      { return step_event(id, "stage2_burn", 171) }
    on confirm_orbit(id: String)      { return step_event(id, "orbit_insertion", 540) }

    on phase(id: String) { return get_status(id) }
}

cell test SequenceTests {
    rules {
        // every shortcut is unrepresentable from the pad
        assert_fails transition("v1", "stage2_burn")      // ignite while mated
        assert_fails transition("v1", "separated")        // separate under thrust
        assert_fails transition("v1", "orbit_insertion")

        // the one legal order works, step by step
        assert main_engine_cutoff("v1").ok == true
        assert_fails transition("v1", "stage2_burn")      // still mated
        assert separate_stage("v1").ok == true
        assert ignite_stage2("v1").ok == true
        assert confirm_orbit("v1").phase == "orbit_insertion"

        // orbit_insertion is TERMINAL: no event can rewind the mission
        assert_fails transition("v1", "stage1_burn")
        assert_fails transition("v1", "separated")

        // a second vehicle cannot replay an event out of order either
        assert main_engine_cutoff("v2").ok == true
        assert ignite_stage2("v2").ok == false            // not yet separated
        assert phase("v2") == "stage1_cutoff"

        // event times can never be recorded outside the mission window
        assert_fails met_s.set("v1:bogus", -3)
        assert_fails met_s.set("v1:bogus", 90000)
    }
}