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> After that, they still have a fair way to go on Raptor reliability

I suspect the debris from blowing a whole in the ground was the biggest contributing factor for the engines out. It looked like at lease 4 we're gone before it left the pad.

Isn't it pretty unlikely that this debris, flying against the exhaust, can reach the engines with significant energy?

Considering the energy they have comes from precisely that exhaust, I don't think so. I'm pretty sure some pieces would make it through.

Rocket "engines" aren't like engines in your car. The bell and the plumbing for it is the engine. There's not too many moving parts besides a pump. Any sort of damage to the bell can cause a failure.

The bells are cooled by pumping super cold fuel over them, a rock hitting those thin lines of fuel could rupture a line, or divert on causing a failure in the bell which eventually (seconds) ends in an engine rich fuel mixture.

That's my thinking, but ignition is a staggered 6 second duration, so there's at least a window for adverse directional flow.

But looking at that pad and the damage to it- without deluge dampening, I bet the RC spalling might have been a bigger issue than just ground blowback.

Put a blowtorch to some concrete a and watch how quickly trapped moisture starts causing miniature explosions and flying shrapnel.

But that's just my first uninformed impression.

They have special concrete now which shouldn't explode from moisture. But apparently it didn't hold up at all. I don't think anyone expected this.

The output power of a rocket is different than a blow torch. The sound pressure alone is near or above atmospheric pressure. That concrete is being exposed to vacuum and more than 1atm pressure fairly rapidly. The heat on top of that and if the concrete has any moisture, the water inside is going to rapidly sublimate and condense several times a second. That is on top of the concrete being slammed into by an entire atmosphere of pressure in the meanwhile.

There is a reason that NASA dumps a few million liters of water under their rockets.

My understanding is that one of the Starship prototypes were damaged by flying debris during a static fire. I think the available evidence points towards it being possible.

https://www.youtube.com/watch?v=jM61ZkUoO4U

> About 2 secs after starting engines, martyte covering concrete below shattered, sending blades of hardened rock into engine bay. One rock blade severed avionics cable, causing bad shutdown of Raptor.

https://twitter.com/elonmusk/status/1328742122107904000

Assuming that this photo

https://twitter.com/LabPadre/status/1649062784167030785

is real, I guess not. What used to be in that crater couldn't just move sideways, it had to go up to get out of the hole. Engine exhaust hit the center of the crater, dislodged rocks, pushed them away radially and up the sides of the hole.

Four of the six engines later seen to have gone dark were in the outer ring. Coincidence?

> but having that gut feeling that maybe there's more to a situation than the numbers would have you believe is still something that humans can do better

In my life experience so far, humans having gut feelings it 99% of the time complete nonsense. But people forget the 99 times and that one time their gut was right they tell that story forever about how smart they were.

Like survivor bias.

Correct gut feelings are just your subconscious running the numbers without bothering you about it. A human having a (correct) gut feeling that the numbers are wrong just means that the human has not chosen the correct numbers to look at, or possibly that a human has made an error in telling the computer how to calculate the numbers.

Yes, computers are really good at making calculations, but we have to tell them which calculations to make, so there's still (will always be?) a weak link in the chain.

> I was a bit disappointed with the number of engine outs

Who knows if they were controlled tests, you need failures to understand the reliability domain. Engineering is about handling failure, not success. You learn very little from success, esp with so many free variables.

Reminds me of the flash memory cells SSD's are made of - overprovision them because some are going to become/or is bum anyways.

The Raptor issues may well be just a symptom of the concrete issue; large chunks flying up into complicated plumbing isn't ideal.

This will be more of an issue on landing (the moon, mars, mercury, the bricks in red square).

Not necessarily; low gravity plus lighter weight means much, much lower thrust on landing in those locations. F9 lands on the drone ships with one engine, for example. It also looked like today's launch involved a hold-down period while they lit them all; that wouldn't be the case for landing either.

That makes sense, but on mars? Full payload, very little atmospheric drag, little atmosphere to keep the blastolith from scrubbing the bottom of the vehicle. I am talking generically since presumably the first stage won't be landing on mars.

Lower pressure could help help the exhaust spread more.

Mars is far off enough it's entirely speculative on how they'll handle this. Drop some robot bulldozers first? Specialized landers for landing pad construction crews? Bring the heavy cargo via a separate non-atmospheric craft built in Earth orbit? Hover from height for a while to blow debris off a rocky flat spot?

I don't have the answer, but IMO cheap mass to Earth orbit is the biggie that enables a whole bunch of options.

There was an idea of adding aluminum powder to exhaust and hovering a little over the pad, so that you deposit a solid metal surface where you intend to land.

You missed the moon solution - use engines high off the ground (most of the way up the ship) for landing.

Another idea (not starship specific) is to introduce cement (or similar) into the engine burn to build a landing pad from middair.

I was under the impression that there's no hold-down or clamping of the booster for this vehicle. The clamps are disengaged around T-10 (maybe T-15?).

Might just be the thrust-to-weight being under 1 while they light the engines, but it visibly sits there for a signifcant amount of time, much more than you'd get with a landing suicide burn.

Then what holds the rocket down while the engines are spooling up?

Could possibly this explain why there was so much lateral movement of the booster before it cleared the tower?

The booster is securely clamped to the launch stand. However the starship is only resting on top of the booster - the two parts of the ship are not clamped together.

Launch clamps are released on command of the engine computer - i.e when it detects that sufficient engines are running correctly it commands a clamp release.

Note that Starship Booster's outer engines are actually started using feeds from the launch pad. They need power and pressurized nitrogen from ground equipment to start. The outer engines are not able to restart once off the ground.

So the clamps are required to keep the vehicle in place until all the engines that will start have had a chance to do so. They're also useful if there is an abort during engine start so the vehicle remains secure.

> However the starship is only resting on top of the booster - the two parts of the ship are not clamped together.

That isn't what it looked like to me when it made what, 3 go arounds?

I may be incorrect and they are now using explosive bolts. I've seen those mentioned elsewhere.

However the engines were on the whole time, producing >1G of thrust along the direction of the booster. So that force is going to hold the StarShip and the booster together. But you would expect some lateral force from air friction to push the stack apart at some point.

I hadn't known that. Is that a center engine?

Intuitively it seems like more would be better so that in the case of that engine failing you could use the other engines to push up and maybe light another engine.

Of course, if there's two engines and one fails, it's going to tilt. Same with 3, though to a lesser degree. 4 engines lessen it even more, etc. etc.

Or maybe they can detect a bad center engine on descent (it doesn't light or flame is detectably not the right) and then go to outer engines. Or drop it in the ocean. Hope there aren't people on that one.

> Intuitively it seems like more would be better so that in the case of that engine failing you could use the other engines to push up and maybe light another engine.

The landing F9 first stage is so light even one engine at minimum throttle has a thrust-to-weight ratio greater than one; they can't throttle it down enough to hover. Three lit engines would send it shooting back up rapidly.

they’re not going to land with all the 30+ engines turned on and they won’t need the same amount of thrust since the first stage is gonna be mostly empty. Plus they won’t land the first stage on the moon or on mars.

And the plan is to grab the first stage before it touches ground anyway. So this is mostly an issue on liftoff.

> clearly the engines have not arrived.

I think Raptor2 as an engine has definitely arrived. Igniting and running this many at the same time has some work remaining however.

Pretty sweet that they got to test 33 Raptors at a time. You can find failure modes approximately 33 times faster that way.

Plus the combinations.

There was a time where people questioned if 33 of them packed in like this would work at all. Pretty impressive that it does and survives multiple engine failures.

> After that, they still have a fair way to go on Raptor reliability

They seem to have worked fine individually and in smaller groups. There could be issues with the plumbing, which they seem to be iterating on quite a lot with the newer boosters.