Perhaps some rocket scientist can explain why this seemingly-obvious error even happened? Shouldn't it be possible to calculate the precise amount of fuel used, and therefore required?
It's because there are very high penalties for carrying excess weight in aerospace, and the environment is uncontrolled. Imagine trying to calculate how much gasoline you need to drive from A to B which is 1,000 miles away, but for every extra fluid ounce of fuel you have at the end, you have to pay $10,000. You have to deal with varying weather, wind speeds, traffic and so on. How would you do this "precisely"? Let's remind ourselves that this rocket did 99% of its mission successfully, but didn't seal the deal.
They probably used a simple model (and experience) to determine the needed amount. But the model was wrong because, well, it encountered the real world. So they'll adjust in the future. Probably they could have analyzed the heck out of it for years, and gotten it right, but then that would take years and cost buckets.
I’m not a rocket scientist, but the Navier-Stokes equations are nasty; the best we can do is model what happens, assuming that we can exactly predict what will happen, which we can’t. Temperature, pressure and humidity of the air the rockets will fall through will not be exactly known, and all will affect how the rocket moves.
And even the best of modeling will not guarantee that reality will behave as predicted. Suspension bridges, for example, often need some ‘tuning’ to remove resonance after construction.
Bridge engineers, knowing that, often won’t even try to spend time and money to exactly predict bridge behavior. This may be similar: they may just have thought it would be faster or easier, maybe even cheaper to do the experiment than to work on exactly modeling their engine.
(The center engine is different from the others, but it also could just be statistics, with the amount they used giving a probability slightly below 1 of igniting one of these engines, and this being their unlucky day (given that they already have quite a bit of data from earlier launches, that is not too likely, but it is something I would let an engineer look at)
I'm not a rocket scientist, but I think I can explain why this happened in a more general probabilistic sense. This error may have been "obvious", but successfully landing a supersonic rocket onto a floating platform probably requires hundreds or thousands of "obvious" things to go exactly right, and if any one of them doesn't, then it explodes. Even assigning a very small probability to each obvious failure mode, compounded together it gives a non-trivial chance that the landing will fail, especially in the maiden launch when there are still lots of "unknown unknowns". If it hadn't been the igniter fluid, it could just as easily have some other "obvious" thing.
Anyway, that's why they do these tests: to make the "unknown unknowns" known.
It's not "fuel" in this case, it's the highly reactive compounds that are used to ignite the engines (TEA/TEB are pyrophoric, so they ignite spontaneously when exposed to air).
The precise amount they'll need depends on how quickly and reliably the engines restart. If the engines take a couple extra shots to light, then you may run out.
No, they have used this profile several times in the past. The past couple droneship landing have used three engines for the landing burn (the outer engines shut down before landing).
The recent "stage that lived" was testing three engines all the way to "0" (which worked fine, apparently).
My guess is the hotter than normal reentry meant the engines were a bit tougher to relight, and they burned through more starter loads than they expected.
I'm not sure what your sequences mean there, almost every boostback and re-entry burn has used three engines (they always light the center engine first, then the outer two).
Most recent droneship landings recently have used three engines for the 'middle' part of the burn, shutting down the other two before actually landing.
The recent GovSat launch tested a three engine burn all the way down (the first time they have tried that, AFAIK).
There was nothing unusual about the number of relights. My guess is the issue was with the difficulty of the relights.
It's not quite that simple. So many factors, as well as random chance, affect the quantity of TEA/TEB used that it's virtually impossible to create an accurate prediction. Other comments in this thread mention that they could probably have modeled and tested on the ground until they had a better idea, but that takes a long time and is expensive. Actually launching the thing and learning from that is a much more effective usage of time and money. As someone else ITT quoted George Box "all models are wrong, but some are useful.".
