It takes pressure in the tank to suppress cavitation in the pumps for rocket engines. It is customary to use helium for most pressurization due to its’ low molecular weight. Unfortunately, it can take a lot of helium in some cases. Some propellants can self pressurize under the right conditions, though here is where the molecular weigh becomes important. O2 with a molecular weight of 32 is eight times the mass of helium at 4.
Heating up the pressurant gas helps considerably, though helium can be heated up as well. Sub-cooling the propellant helps suppress cavitation and allows a lower pressure in the tank to be effectively pumped. I am going to suggest an inducer pump in the tank instead.
If the required pressure in the tank can be reduced from 30 psi or more to 5 or so as the vehicle reaches altitude, the pressurant gas quantity can be reduced by a factor of 6. Â An electric inducer pump in the tank might make this possible. A pump that is bypassed early in the flight is gradually brought online with increasing power as the tank level drops and with it the head pressure.
The waste heat from the pump can be used on the pressurant gas to reduce the required mass. The pump power can be gradually increased to keep a constant 100 psi  or whatever the spec requires to the turbo pumps.
The objections I have had in the past to electric pumps have mostly to do with the mass penalties of the electric motors, and to a much greater extent the battery mass to reach engine pressure. A relatively low pressure pump used as an inducer gets around some of this. If pressurant gas is reduced along with the elimination of their tanks, that should compensate for the all of the motor weight and some of the battery weight. If the batteries are those of the satellite payload, then there might even be a mass savings. Many of the satellite payloads have a considerable amount of their mass in batteries which might as well help haul the freight during launch when they are probably bored anyway.
johnhare
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rather than electric pumps and battery, why not power pump with high pressure.
So if pumping Kerosene, small tank of Kerosene which pressurized to high pressure
with helium. So Helium pushes the kerosene which drives a pump which pumps kerosene
from larger tank, and the high pressure kerosene is added to the pumped kerosene.
So if had pipe say 6″ diameter with wall thickness of 1/2″, 40 K tensile strength [the small high pressure tank] it could withstand 6666.6 Psi, so could be pressurized from
5000 to 1000 psi by helium, the high pressure kerosene powers the pump, how powerful it pumps controlled by regulating amount of Helium into the tank. How much power you need controlled pipe size coming from the high pressurize tank and duration of power pump related pipe diameter, and length of 6 diameter high tank.
So high pressure tank could separate from main fuel tank, or be inside it, or run entire
length of larger tank and be structural component of larger tank.
Anyways, the mass left over is the high pressure tank and pump- which can simple and light weight.
This has me thinking of another configuration:
propellant gets a pressure boost from an “ejector” pump, then goes through the turbo pump for the major pressure rise. A small portion of the high pressure propellant is tapped to drive the ejector pump. For startup there could be provision for an initial high pressure source. Or the system could bootstrap, running the turbopumps at reduced pressure and ejector at higher drive flow until the system gets going.
Gbaike, I was going to offer an alternative but Peter beat me to it.
When first stages are being reused, just don’t vent the helium after the stage is recovered.
Another thought, for some propellants H2 is a good pressurant gas. Less expensive than He, but still requiring the tank and plumbing. … How well does H2 dissolve in kerosene, methane, etc? On the one hand, if it dissolves well it can save the extra pressure supply tank and valve. On the other, dissolved gas may agitate cavitation.
As I vaguely recall, hydrogen is not terribly soluble in hydrocarbon propellants, even cryogenic ones.
Here’s an old reference that indicates hydrogen is only sparingly soluble in liquefied methane, with solubility decreasing with temperature.
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19680020605.pdf