When the player uses one shell to complete a round within 50 seconds, it vanishes forever. At the end of two successful rounds, for instance, the player has only two shells to pick from during docking.

When life had labour'd up her midmoſt ſtage, / And, weary with her mortal pilgrimage, / Stood in ſuſpenſe upon the point of Prime; …

1993, Don Munday, Tin Dog, Damper and Dust, autobiography.

“The Covenant don’t ‘miss’ anything when they glass a planet,” the Master Chief replied.