The launch video had a long discussion about the upper stage and its ullage motor system. The tanks are pressurised by hot gases which must not be allowed to go too cold during the coast phase or else the tank pressures will drop dramatically. So during the coast phase auxiliary power unit power up to continue to generate gas, some of this gas is tapped and feed into several small ullage thrusters which keep the rocket under low thrust rather than allow the rocket to go completely into free fall. This ensures the liquids remain settled on the bottom of the tanks for the reignition - if you let the liquid flow away from the feed and you suck in gases instead that would be bad news for the rocket.
I wanted to look up this some more and the
had this to say about the upper stage:
- hypergolic propellants
- 1 × YF-50D engine
Common guys fix up your shit. It's clearly a LH2/LOX cryogenic stage with two engines, the video also specified that the upper stage is using expander cycle (when the engines are running the fuel is used for regenerative cooling, then the hot gas that result is use to pressurise the tanks).
The upper stage's odd shape us because the smaller oxygen tank uses a 3.35m tank while the hydrogen tank uses 5m diameter tank. 3.35m being the common diameter for other Long March rockets prior to Long March 5.
