Falcon 9 first stage use turbopumps as well as helium pressurisation system. Isn't the turbopumps enough to pump in the propellants to the combustion chamber?
Pumps are great at creating positive pressure, hundreds of bars. But with tank at ~1 bar, atmospheric pressure, they can only create a grand total of 1 bar of suction - can't go more vacuum than vacuum, can't create a negative density, can only go from original to zero - and 1 bar of pressure is a pretty meager amount, when this kind of flows is involved. And without a gas to replace the liquid already pumped out, the pressure would be dropping even more.
And not only does it limit the supply - As demand exceeds supply, vacuum bubbles start forming - aka cavitation, areas where new liquid can't flow in fast enough. And cavitation is bad. Liquids are very badly compressible, and such a vacuum bubble, when the flow catches up, vanishes completely as the liquids on its sides meet up - and does so quite violently. It causes a pressure wave that travels through the liquid at speed of sound (in that liquid) and can cause trouble wherever it hits, be it sending tank walls into vibration that can damage bearings, be it impacting turbine blades, bending them, or facilitating more cavitation where conditions were "almost but not quite there" and multiplying itself - never mind pump blades churning vacuum don't do anything useful, a lot of energy wasted on spinning pointlessly moving way less liquid than they could - if that liquid was there.
So you need that extra pressure to overcome the resistance of the plumbing, valves, acceleration of the liquid entering the pipes etc, so that the turbopumps never cause cavitation.
In a turbopump feed system it is still necessary to pressurize the tanks slightly (10 to 50 lb/in2) in order to prevent pump cavitation.
Rocket Propulsion Elements, Sutton, 4th edition, p. 223
Different vehicles have used stored onboard gas (typically helium) or autogenous propellants to provide this pressurization.
- Why does the Falcon 9 require a helium pressurization system?
- Why does the 2nd stage of Falcon 9 need compressed He tanks?
- Is presssuized gas absent in a turbopump-driven rocket engines?
- Why Use Helium?
- Disposition of the Oxidizer Tank in Rockets with Autogenous Pressurization
- How does tank pressurization work?