A recent "Anomaly Update" dated
September 23, 1:00pm EDT by SpaceX, includes the sentence:
"At this stage of the investigation, preliminary review of the data and debris suggests that a large breach in the cryogenic helium system of the second stage liquid oxygen tank took place."
This has been repeated in several news outlets, but spaceflightnow adds:
The Falcon 9 rocket’s upper stage liquid oxygen tank contains several composite helium vessels, each pressurized to about 5,500 pounds per square inch in flight. The helium is routed through the second stage’s Merlin engine, where the helium warms up and is injected into the rocket’s propellant tanks to pressurize the stage as the launcher burns fuel, keeping the tanks structurally sound.
While cryogenic helium was aboard the Falcon 9 at the time of the explosion, the mishap occurred around eight minutes before the rocket’s main engines were scheduled to ignite for the on-the-pad “static fire” test Sept. 1.
At that point in the countdown, the propellant tanks are normally not pressurized for launch.
I'm trying to understand the implications behind: "The Falcon 9 rocket’s upper stage liquid oxygen tank contains several composite helium vessels...". According to this answer "In the move to Falcon 9 v1.1 and the Merlin 1D engine they moved them into the LOX tanks on both stages. This de-cluttered the engine area a great deal." - mentioned in this answer as well - I can see that the helium tanks have been relocated inside the LOX tanks for several practical reasons including layout/arrangement and thermal management - inside the LOX tanks the temperature is much colder than ambient.
At a given maximum pressure, a lower temperature allows for more helium to be stored in the tanks, or a smaller/lighter tanks can be used for a fixed amount of helium, although I am not sure the actual reason. Boiling LOX is about 90K (only about one third of ambient absolute temperature), and SpaceX is (often/usually?) subcooling below that to pack even more LOX into the tanks.
- Is the helium now substantially colder by moving the tanks inside the LOX tank, or is it simply easier to keep them cold - avoiding coolers and insulation?
- If the helium tanks are pressurized only in the last minutes before launch, doesn't compressing this gas to roughly 400 atmospheres generate a huge amount of heat? If the tanks are fixed size, the pressure will increase steadily as it is filled, and that makes a heat source inside the LOX tank just before launch. I can understand adding the helium beforehand, letting it cool a bit after compression, and then filling with LOX, but wouldn't compressing the helium in the tank at the last minute be incompatible with sub-cooled LOX?
- Or, can "...each pressurized to about 5,500 pounds per square inch in flight." be taken literally - the tank becomes pressurized in flight? The pressurization occurs in flight?
- And does "At that point in the countdown, the propellant tanks are normally not pressurized for launch." refer to LOX and RP-1 but not helium? Is the helium pressurized but not the propellants, or does this also refer to the helium being not pressurized?
Working inside the high bay of the Space Station Processing Facility at Kennedy, about a dozen people perform the operations to fill a single tank following two to three days of inspections and work stand preparations. They work about 40 feet from the stand with the tank and leave it alone for a day so it can cool down after the pressurization. (my emphasis)
The team performed several fills on other tanks to make sure the process could be handled safely and performed as planned.
"There's really no other place in the world that operates up to 10,000 pounds per square inch like this in an operational environment," Bigos said. "We've had to be exceptionally cautious. It's not yet normal day-to-day operations but we're getting there."
It doesn't sound like you'd want to pressurize a 6000 psi tank submersed inside a sub-cooled LOX tank just minutes before liftoff - due to the heat generation if for no other reason. The helium tanks may or may not be smaller, but this is not a location where you'd want to be generating much heat.
above: NORS oxygen tank from here. Credit: NASA, cropped/rotated.
I know it's just the color and general shape, but just looking at that makes me nervous - I think it's reminding me of these for some reason.
You can always top-off a boiling LOX tank at the last minute, but I don't think you can really re-sub-cool a LOX tank inside of a rocket. Heat introduced into sub-cooled LOX will raise its temperature and the LOX will expand and its density decrease, unless there are additional "chillers" in there as well.
However in any case I now understand that the pressure of the helium is still used outside of the LOX tank and that they were moved inside for other reasons.