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NSF was insisting that there was no CH4 loaded because there was no sign of recondenser activity, but apparently their supposition was incorrect.
Well that news from Elon sounds hopeful. So now we know that there was CH4 onboard as well as LOX and the test involved starting up the pumps, igniting the pre-burners and spinning up the turbines on all engines at once but then not igniting the engines and immediately shutting down the pumps. Is that right?
But wouldn’t that mean that for a moment there would have been hot pre-burned CH4 and LOX flowing into the engine combustion chambers and then out the exhaust? And the videos showed a lot of gaseous LOX directly below the vehicle right before the test. Elons tweet shown just above by @HVM seems to point to that as the issue.
Is anyone else surprised that the testing program on the OLM didn’t start with static fires of subsets of the engines before proceeding to any test that involved all engines?
Obviously I am not a rocket engineer and fundamentally know nothing about this, just trying to understand the basics.
Well that news from Elon sounds hopeful. So now we know that there was CH4 onboard as well as LOX and the test involved starting up the pumps, igniting the pre-burners and spinning up the turbines on all engines at once but then not igniting the engines and immediately shutting down the pumps. Is that right?
But wouldn’t that mean that for a moment there would have been hot pre-burned CH4 and LOX flowing into the engine combustion chambers and then out the exhaust? And the videos showed a lot of gaseous LOX directly below the vehicle right before the test. Elons tweet shown just above by @HVM seems to point to that as the issue.
Is anyone else surprised that the testing program on the OLM didn’t start with static fires of subsets of the engines before proceeding to any test that involved all engines?
Obviously I am not a rocket engineer and fundamentally know nothing about this, just trying to understand the basics.
Guesstimation:
Individual engine lights don't give much data since the engines were already tested.
Difference from McGregor to Booster is the fuel feeds and external controls (whatever isn't built into the engine).
33 engines starting, drawing, and stopping flow will have interesting dynamics to verify.
To test the flow path at volume, they need to run the turbo pumps, meaning combustion.
Regarding gas temperature leaving the pumps. This site: The Insane Engineering Of the SpaceX Raptor Engine | Engineer Calcs calls out around 775K. At one bar, methane autoignition is about 875K, but the temp drops with pressure. That said, any external ignition source could light the mixture. Wrong pump mix maybe could allow flame propagation? Be great to have high speed footage.
Good point on testing the flow dynamics. This was called a "spin start" test... so I wonder if they were just testing the first portion of the sequence before actually lighting them all?
If so the large vapor cloud (LOX dump as it was assumed to be) from the engines spinning up could ostensibly be from the engines themselves during the initial start sequence, but I can't imagine that would include CH4, as purposely pumping a large amount of LOX/CH4 mix on to the pad would be bad (as we have seen). I wonder if this was a test that went too far/long, or if some engines failed to light, etc?
Herb Schaltegger posted this in an NSF forum:
Now I think I get it. So the pre-burners were not ignited. Just before the explosion there was a surge of cold CH4 (or something from the upper tank since it had a frost line) and LOX through the engines which was never meant to ignite. Somehow it did. You can see flames exiting the exhaust nozzles but it doesn’t look like a normal burn.
adiggs
Well-Known Member
With the caveat that a little knowledge is a dangerous thing…I know that basically a fire requires fuel, air (oxidizer) and a heat source for ignition. A mixture of cold O2 and CH4 at a relatively low pressure doesn’t seem like it could spontaneously ignite. In this instance there was a big dump of those cold gases below the OLM and then there was an explosion with flame apparently exiting from the engine exhaust nozzles. Whether the ignition first occurred below the nozzles or first started somewhere inside the engines doesn’t seem like a question that can be answered using the video available to us.
Feels like combustable mixtures in volume tend to have a nasty habit of finding ignition sources....
Doubt it. Any damage should be visible externally. Baring that, individual spin tests should confirm internal health. They are build to handle 4500-9000psi(though with reverse pressure differential).
Maybe control electronics, those would be more sensitive than the mechanicals.
Disclainer: I tend to be optimistic
Eric Berger’s analysis:
Odd how he refers to “ambient oxygen in the air” being the oxidizer for the fire. I thought that during a spin start or “spin prime” (as someone on NSF put it) CH4 and LOX would be used, for their respective pumps in each engine.
So he believes there was CH4 loaded in the tank and that the ignition event occurred somewhere “inside”.
Odd how he refers to “ambient oxygen in the air” being the oxidizer for the fire. I thought that during a spin start or “spin prime” (as someone on NSF put it) CH4 and LOX would be used, for their respective pumps in each engine.
Scott Manley’s perspective on the explosion. Summary; this was a “spin prime” test which involved flowing cold propellant through the engines and something ignited the methane present just below the engines. Explosion shockwave was probably not over the speed of sound indicating that the explosive force was equivalent to roughly 10kg of TNT. Damage to Stage Zero likely minor, possible damage to booster.
Scott showed the first video frame where the explosion can be seen and noted that the flame seems to be evenly distributed right at the base of the booster.
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