During the livestream of the recent Vulcan-Centaur launch, the animation of the upper stage shows what appear to be extensive ullage burns (at least four minutes, at least three minutes, and at least 25 seconds) after each of the three upper-stage burns. SpaceX livestreams don't show any post-burn ullage, and the burns seem excessive as an anti-slosh measure -- a few seconds of tapering thrust would seem to be sufficient. What's going on?

I'm pretty sure the animations are correct, since the voiceover described the first one as a "settling burn, 85% duty cycle".


1 Answer 1



During Low-G coasting, pulsing its RCS thrusters at a reduced duty cycle reduces consumption for a system that also has a number of other jobs (vehicle control, roll). Settling acceleration has an impact on Centaur upper stage mission duration, so reduced duty cycle is basically using less fuel over a longer burn, enough to settle whilst conserving the fuel budget for the RCS. With less acceleration the longer burn can also be due to settling taking longer and for the sloshing to cease.

Seems related to the findings of tests such as this one:


Which was carried out during the 18 October 2009 Atlas V AV-017 mission.

It consisted of numerous on-orbit cryogenic-fluid-management demonstrations once the payload had been inserted, designed to improve understanding of propellant settling and slosh, pressure control, RL10 chill-down and RL10 two phase shutdown operations, on cryogenic upper stages, thereby improving performance and allowing longer future missions.


Low-G Settling Demonstration:

Centaur and other cryogenic space propulsion systems, such as the Delta IV second stage and Saturn V’s S-4B stage, use on-orbit settling to separate liquid from gas. This separation is
critical, enabling the stages to vent pure gas to control tank pressure. The required magnitude of settling acceleration directly affects the stage performance and maximum coast duration. For this demonstration, Centaur was settled by pulsing its four hydrazine thrusters at a reduced duty cycle to provide half of the acceleration that is typically used to support Centaur’s longer coast missions. Lower duty cycle settling reduces the hydrazine consumption rate, allowing longer mission durations.

The short sloshing period can be accommodated on future operational missions by inhibiting venting during the start of a coast. Ultimately, this lower acceleration level is shown to adequately support future long coast missions.


Solid-Body-Rotation Settling Demonstration:

For coasts longer than about 15 minutes, Centaur is rolled around its longitudinal axis to ensure uniform heating. Typically, the roll direction is regularly reversed to prevent solid body rotation of the propellant. For this demonstration, Centaur maintained a single roll direction to ensure solid body rotation. Following the low axial settling period, settling thruster firing was terminated with the objective to demonstrate that low level centrifugal acceleration could adequately retain liquid slosh. The liquid slosh must be kept sufficiently damped such that the hydrogen vent port, located on the forward door near the tank centerline, remains clear of liquid.

Other tests included pulsed chilldown prior to the second main engine start.

Other info:


Reaction Control System - A hydrazine reaction control system (RCS) performs vehicle control, thermal roll maneuvers and propellant settling.



Centaur V can also use its low thrust settling motors to place itself into a LEO, GTO, or GEO compliant disposal orbit.

Other info from the feed:


  • time: 10833
  • both engines show normal shutdown signatures we have settling established 85% duty cycle we are now in a 27 minute 51 second coast duration to Second burn of Centaur

This could be incorrect, or someone else might be able to put it in better terms than I have.

  • $\begingroup$ The very last burn is probably to get on a different trajectory from the payload. $\endgroup$ Jan 9 at 13:24
  • $\begingroup$ @OrganicMarble - second burn is the trans-lunar injection, followed by 20 sec burn, then shut off, and in 4 days will head for deep space. I read otherwise a heliocentric orbit to test long duration mission capabilities but I cant find the material i read originally atm. $\endgroup$ Jan 9 at 14:37

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