# Why does the Falcon 9 require a helium pressurization system?

The Space Shuttle was designed in the seventies and did not require a helium pressurization system. Why does the Falcon 9 require one? What are the advantages/disadvantages?

• Comments are not for extended discussion; this conversation has been moved to chat. – called2voyage Jul 3 '18 at 12:38
• If you search this site for the falcon-9 tag and the word helium, it returns over fifty posts! space.stackexchange.com/search?q=%5Bfalcon-9%5D+helium – uhoh Aug 21 '20 at 23:34
• I believe that this is a very old question indeed... – mike Aug 23 '20 at 6:02

The shuttle propellant tanks required pressurization.

Prelaunch, the tanks were pressurized by helium, supplied from the ground support equipment.

After Space Shuttle Main Engine ignition, the system used autogenous pressurization. Hot gaseous hydrogen and oxygen were tapped from the engines and piped to their respective propellant tank to pressurize it. A sensor/valve system measured the tank ullage pressure and increased or decreased the pressurant flow as required. (Late in the program, the gaseous oxygen system was redesigned to remove the valves).

Helium pressurization may be simpler and cheaper (no sensor/valve control loop needed, just a regulator) and therefore more desirable for Falcon. Some failure modes are avoided. In addition, hot gaseous oxygen is fearsomely reactive and requires extreme cleanliness and material standards (this was the reason for the redesign of the system) and may be desirable to avoid. Helium is the very antithesis of reactive - although it requires extreme care to be taken to prevent leakage. The negatives of helium pressurization could include a weight penalty over an autogenous system, depending on the details of the particular systems.

Also see these related questions:

How does tank pressurization work?

Why Use Helium?

Also, although I realize it is not what you meant, the shuttle very much did have a helium pressurization system. Many of them, in fact. The main propulsion system had several dedicated helium tanks used to purge the system, actuate valves, and pressurize the plumbing for entry. The orbital maneuvering and attitude control systems used hypergolic propellants which were pressurized with helium. Other miscellanous systems also used helium pressurization such as the ammonia boilers in the environmental control system.

• Additionally, obtaining pressurized gaseous hydrogen from liquid hydrogen for the hydrogen tank pressurization appears easier than obtaining gaseous kerosene from liquid kerosene for the kerosene tank pressurization... – SF. Dec 19 '17 at 14:50

Thanks, that much is clear; the question is more why can they not make do with out the helium? Because the shuttle didn't need it and the MCT reportedly also won't need it, so why the falcon?

All rockets need to keep the tank pressurized during flight. Some use hot gases from the engines to do that. Others don't. The fuel used in the rocket is the main reason why many use helium. The the falcon 9 uses RP1 which is basically Kerosene. When you heat Kerosene you don't get enough hot gas to maintain tank pressure. So you need fuels that expand greatly in volume when heated. You could get enough hot oxygen from the engine to do that job but if you mix hot oxygens with kerosene you could have an internal tank fire and explosion. So oxygen is not compatible with any fuel. Helium and nitrogen are two good choices. I believe The Saturn V used nitrogen to maintain tank pressure in the first stage.

• Not sure why this was downvoted - it's the only answer mentioning the key point that you can't do autogenous pressurization with RP-1 (because its vapor pressure is too low). – pericynthion Oct 25 '17 at 7:07
• Yeah I don't think autogenous pressurization was ever on the table for the F9. It doesn't work with RP-1 and using it for the LOx side would mean dealing with hot gaseous oxygen which is highly reactive and difficult to handle safely. It's one of the major challenges of Oxygen rich staged combustion or full flow staged combustion. Since you have to have something for the RP-1 side of things might as well use it for the LOx too. The BFR changes fuel and will have autogenous pressurization. Going Full Flow they already have to deal with the oxygen issues and methane allows for it. – Evan Steinbrenner Jul 3 '18 at 18:19
• During flight the first stage of Saturn V tanks were pressurized with helium. Nitrogen was used for transport on the ground. – Uwe Oct 12 '20 at 20:21

The Merlin engines use Helium to purge before launch. At least in the Merlin 1C days, and you can see several aborts that were called because of Helium issues.

For the upper stage it is used to pressurize the tanks to make sure that the fuel and LOX flow into the engines properly. The pumps suck and the Helium pressurization push.

Helium is used since it is inert and compresses well. So it does not burn with the fuel or LOX (which would be very bad) and so it can be stored in smaller, denser packages.

• Thanks, that much is clear; the question is more why can they not make do with out the helium? Because the shuttle didn't need it and the MCT reportedly also won't need it, so why the falcon? – mike Apr 25 '17 at 17:10
• The shuttle did use helium for OMS and RCS systems. The shuttle main propulsion system also used hydrogen instead of RP-1, which doesn't lend itself well to autogenous pressurization the way cryogenic propellants do. – Tristan Apr 25 '17 at 17:30
• The first stage is pressurized to feed the pumps as well as for purge, isn't it? And I believe ITS also uses helium press. – Russell Borogove Apr 25 '17 at 17:35
• Helium is the only gas which is not liquified or even solidified at the temperature of liquid hydrogen. – Uwe Apr 25 '17 at 19:16
• For tanks with RP-1 or LOX, nitrogen may be used too for pressurisation. It is also inert and compresses well. It is much cheaper than helium, but also heavier. But if it is possible to increase the payload a little by using helium instead of nitrogen, the value of more payload weight may be bigger than the cost of the helium gas. – Uwe Dec 19 '17 at 21:54

The SSMEs were fed by four turbopumps; the fuel and oxidizer each passed through a low-pressure, then high-pressure turbopump. This allowed them to feed from a low-pressure fuel tank into a high-pressure (20.6 MPa) combustion chamber without cavitating. For the Falcon series, SpaceX made a design decision to use a single turbopump stage for the fuel and oxidizer. Pumping up to the 6.8 MPa chamber pressure in a single stage without cavitating is more than the pump can handle; incoming fuel and oxidizer must have a minimum pressure greater than that provided by propellant weight and acceleration. I speculate that they could dispense with the helium pressurization by adding a low-pressure turbopump stage similar to the SSME design, but weight and cost considerations favored helium pressurization. Also, pressurizing tanks adds mechanical strength by preventing buckling. The Shuttle tanks were fat and therefore less prone to this failure (buckling load is a function of slenderness ratio). The slender Falcon design may require a minimum internal pressure to survive Max-Q.

The advantage to helium in general is that it's literally a perfect gas, which makes it the perfect gas for pressurization. You can fit more helium into a tank of given size and weight (assuming it's built to take the pressure) than any other gas. Helium has been the preferred gas for pressurization since the '60's. The weight, simplicity, and reliability of high pressure helium bottles has so far won out over other proposed pressurization systems such as heating some of your fuel. Note that a Delta-IV launch failed because of cavitation in the turbopump inlet. When a rocket first launches it is full of fuel and the pressure at the bottom of the tank is very high; when it is nearing burnout then that fuel stack is only a few feet high so the pressure entering the turbopump is much lower.

• Shuttle main propulsion system propellant tanks required pressurization. – Organic Marble Apr 26 '17 at 1:47

The helium isolates Control circuits emissions from igniting the fuel mixtures and exhaust between the command module!They can use negative pressure to maintain system launch pressure!

• negative pressure? that's not possible. The lowest possible pressure is a vacuum, or 0 bar. – Hobbes Jul 23 '18 at 7:10