Hot staging

Igniting a rocket's upper-stage engines while it is still attached to the booster, so their thrust pushes the stages apart.

• In conventional staging the booster shuts down and separates, and the upper stage ignites seconds later, a gap with no thrust.
• During that coast the vehicle loses speed to gravity, and propellant can float away from the engine inlets.
• Hot staging removes the gap; SpaceX estimates it adds about 10% payload to Starship, and Soviet-designed rockets have used it for decades.
• The booster needs a vented ring on top to let the upper stage's exhaust escape, which adds weight and must withstand the blast.