Starship 9/25 during hot staging on flight test 2.
Hot staging is a method of stage separation where a upper stage ignites its engines to push away from a lower stage, even while still attached to the lower stage and while the lower stage engine continues to fire. Starship is the first reusable rocket to use hot staging, with the procedure debuting on Starship Flight Test 2.
With Starship, the hot staging sequence begins with a staggered shutdown of all but the center 3 engines on the Super Heavy booster, followed by the ignition of the 3 Raptor Vacuum engines, followed shortly by the 3 sea level Raptor engines, where the clamps holding the stages together then release. With hot staging, the booster efficiency is improved, as leaving 3 engines on reduces velocity loss due to gravity and negative acceleration, and settles liquid propellants at the bottom of the ship tanks, reducing the risk of fuel slosh, which can lead to irregular supply to the engines. Furthermore, risk at stage separation is reduced by creating a passive staging system - physics is doing the work and no mechanical parts aside from the clamps are needed to push the vehicles apart. Ultimately, payload to orbit capacity could be increased by 10%.[1]
Prior to the implementation of hot staging, early Super Heavy boosters prior to Booster 9 (B9) (the first Booster to use hot staging) featured a different passive stage separation mechanism where the Booster engines would impart a slight rotation of the whole Starship stack immediately before engine shutdown, followed by releasing the clamps where the stages then separate from centrifugal forces. This method was never demonstrated in flight, as the only integrated stack to ever use this method, Starship 7/24, never made it to stage separation.
Hot staging is achieved by adding a hot staging ring between the booster and the ship. The hot staging ring has its own dome that acts as a heat shield to protect the booster from the exhaust of the Ship engines. A first hot staging ring test article was spotted on 25 May 2023 [2] and hot staging rings are now routinely used for Starship launches. Following issues on flight test 3, with the hot staging ring adding substantial mass to the forward section of the Booster not accounted for in the original design, on flight test 4 and all subsequent flights until the debut of the V3 Booster, the hot stage ring was jettisoned after the conclusion of the boostback burn. However, this is an off-nominal procedure used as a temporary way to reduce the weight of the booster for the water landing, and future tower catches. For the Block 3 Super Heavy, SpaceX will integrate a lightweight hot stage into the booster.[3] This new hot staging ring is a direct extension of the forward dome, rather than a retrofit, so it does not need to be jettisoned, allowing for true full reusability.
For Starship Flight Test 9, a modified hot staging ring was used with a number of vents blocked and welded shut, causing the exhaust gases to push the booster into a flip in a known direction, reducing the propellent needed for the flip.
Gallery[]
![Integrated Hot staging ring for a block 3 Superheavy Booster in Starfactory[4]](/wiki/File:Block3_HotStagingRing.jpg "Block3 HotStagingRing.jpg (177 KB)")