We did some flow tests today to see if we can run the pumps under atmospheric pressure in the fuel tanks. And the answer is theoretically yes.
Im saying theoretical because it is a number of factors that is involved here.
The pump can theoretically not run at full power at startup under atmospheric pressure in the tanks, and that is due to the low hydrostatic pressure which is about 2.8 psi at full tank. The hydrostatic pressure will also decrease when the fuel level drops.
But what makes this work is the volume of fuel that is admitted in each cycle. The pump intake is deliberately oversized to both reduce bubbles and ensure the flow rate.
But there are also many other factors we must keep in mind here. One of them is the increased G-force when the rocket accelerates.
We will do more flow tests and mathematical calculations before we can come up with a final decision whether we will use pressurized fuel tanks or not.
For those not familiar with the pistonless pump technology, it has been developed by a company called Flometrics and can replace the conventional turbopump as used on many rockets.
A full explanation of the system can be found in this paper here, and seen in action in the video below.