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Comment by std_throwaway | Hacker News Reader

I bet it didn't randomly lack fluid due to some stupid miscalculation. It's probably more like: "Due to unforeseen circumstances it needed more fluid than expected. But we adjust our numbers and next time we expect more (in the hopes that it will be enough, which it will probably be)."

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It says right there in the article that it used up the fuel while attempting to re-lights the engines (without success).

Sure, but that system was in place for all the Falcon 9 relights as well. Presumably there's something different about the FH - minor architectural differences, flight profile, etc. - that caused this previously reliable system to run out.

Core booster reaches higher hypersonic velocities.

The center core has a different structural design than the side boosters, which are stock F9 stages AFAIK.

Both of the side boosters were re-used for the Falcon Heavy launch. So stock F9s to some extent.

Elon mentioned that it’s heavier in the post launch press conference. But of course they would have used that information in their calculations...

> Musk said Monday that there wasn't enough ignition fluid to light the outer two engines of the booster "after several three engine relights."

I don't see a mention of whether the previous relights succeeded or not. I'm guessing they did succeed, but the center booster had to perform a higher number of relights than a Falcon 9 booster due to its higher speed.

It's the same amount of relights - boostback burn, re-entry burn, and landing burn. It's possible the re-entry burn used more TEA-TEB than usual because of the high speed, but it's also possible that non-boostback GTO missions re-entered at higher speeds (this is the case at least in flightclub.io's model of the mission). Remember that this wasn't a particularly challenging flight profile for Falcon Heavy, which allowed it to boost back to reduce speed and landing distance from shore.

The excessive TEA-TEB use may have been caused by an unrelated engine issue. In any case, Musk reported the fix is 'obvious'.

By my reading, the center core was using three engine burns for more efficient boost-back and re-entry burns. The side cores and typical F9 boosters (afaik) always use single engines for those and occasionally three-engine burns at landing, so three engine-ing all three burns would be 4 more relights than any previous core.

So, an equal number of "relights" of the entire rocket but more engine relights, and engine relights are the relevant unit for ignition fluid consumption.

Boostback and reentry burns are always 3 engines. Landing burns are often 2 engine and often 1-3-1, usually on the drone ship.

The unique part of the GovSat "failed to expend" mission a few weeks ago was not cutting off the outer two engines some time before hitting the surface, not that it used three engines at all.

No, three engine burns are typical for boostback/re-entry. In many of the webcasts you can see the initial round plume turn into a rectangle as the two outboard engines ignite.

This was the original tweet: https://twitter.com/elonmusk/status/963107229523038211 that the article referenced but not linked.

Yeah. I guess in space flight you can't just double the calculated requirement. That would be expensive.

Probably not as expensive as losing the entire booster, though.

Expensive as in, if you apply it to everything, the booster doesn't work.

True.

From what I understand, the problem was because the second burn was when the rocket was going tail-first through the atmosphere at hypersonic speed, and much faster than a Falcon 9 booster.

That's a problem because the amount of igniter fluid depends on the air pressure in the rocket chamber, and it was not possible to determine this exactly beforehand through either testing or modeling. So they took a guess and it turned out wrong, but they got lots of data on the rocket chamber pressures, and will adjust accordingly in the future.

Two big aerospace companies ran out of fluid recently. Southwest airlines cancelled basically all their flights from Chicago yesterday because they ran out of Deicing fluid. They can use your same excuse! Something tells me Southwest failed at a much easier task, though.

"Southwest has 'multiple glycol deliveries' scheduled for Monday and expects to resume 'close to normal operations' at Midway, she said."

https://www.usatoday.com/story/news/2018/02/11/de-icing-flui...

Interesting, I would have assumed that was a service they just paid the airport for, rather than one the airline handles itself.

SpaceX fluid issue is likely a calculation / estimation error.

Southwest fluid issue is likely a logistics failure.

I don't see them as related.

Well, it's probably a lot easier to store fluid at an airport than on a rocket

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