Comment by NortySpock | Hacker News Reader

Comment by NortySpock | Hacker News Reader

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To steelman the argument, maybe they meant "the monitoring system is good enough at catching defects that if none are reported, the engine will probably pass an all-up hotfire test of the engine".

Of course you could do a water or air-pressure leak test on the plumbing pretty easily, and you would likely do that on the first 30 engines...

But if you have confidence in your build process, maybe the juice isn't worth the squeeze on (say) a direct contact ultrasound void check on every square millimeter of the part.

It's all about "how expensive is it to run the test" vs "what is the likelihood the test catches an issue" vs "what's the cost of failing while everyone is watching?"

Same reason SpaceX went from dry-dress-rehersals to wet-dress-rehearsals to separate-static-fire-before-launch to hold-down-for-three-seconds-before-launch... The hold-down -before-launch is an integration test that covers everything the previous tests do, so eventually you can start removing redundant tests.

>> maybe the juice isn't worth the squeeze

In rockets/aerospace, where the failure of any one of a thousand different parts means instant disintegration, checks are always worth the squeeze. Everyone talks about building rockets on the cheap and accepting a slightly-higher failure rate, but in reality that doesn't work. Even a tiny increase in component fault rates translates to total mission failure once multiplied across thousands of vital parts. The answer isn't to not check but to find ways to more efficiently and more thoroughly check each part. This is only more true if one considers reusable rockets where components will be expected to participate in multiple launches.

> In rockets/aerospace, where the failure of any one of a thousand different parts means instant disintegration

To make matters worse the failure modes don't only affect the launch vehicle itself. A failure of a rocket likely means a total loss of the payload. It also runs the risk of damage/loss of the launch pad, support structures, and hapless down range victims.

Rockets contain a significant amount of stored chemical energy, enough to get the payload mass into a stable orbit of the Earth. If you release all of that energy at once as an explosion it will cause a significant amount of damage. Rockets aren't something to goof around with and make assumptions about safety.

My dad works in aerospace. In no field or world does anyone actually check every little part.

You create a process, you test that process so that you understand its limits, and then you make sure to follow that process.

Now I don't know if 3D printing rocket parts actually works and I have my doubts but this startup is currently testing the process and they will figure out its limits. That's the whole point of R&D.

For context, 3D printing rocket parts is incredibly common, even student teams often use printed nozzles and such. The linked article is about printing a whole engine as a single piece, which is a different beast.

Well, if you build it robust enough, you can test less. Not saying testing is worthless, but sometimes a one-piece that used to be 45 pieces held together by rivets is just, much much more resilient.

It’s a bit of chicken and egg to know if you built it robust enough without meticulous testing of your robustness

Robust enough to not need testing == too heavy to be cost-effective.

> checks are always worth the squeeze

This is simply inaccurate. You can spend months and hundreds of millions of dollars running scanning electron microscopes over all of your parts - no one does this, because it's not "worth the squeeze". The question is where to draw the line, and I have no idea what your opinion is there.

this might be 20th-century thinking. if you can build enough copies of a rocket cheap enough, maybe disintegrating a bunch of them isn't a showstopper

> if you can build enough copies of a rocket cheap enough, maybe disintegrating a bunch of them isn't a showstopper

The problem with that idea is that you won't be legally allowed to launch again until you root cause and fix the failure, which can take months (or years if you're Blue Origin). Also, your insurance rates tend to go up a lot when your rockets blow up regularly, which tends to push customers away.

In practice it doesn't work.

Notes: Astra said they were going to pursue this strategy. It was not well received by potential customers and they basically had to walk it back.

>The problem with that idea is that you won't be legally allowed to launch again until you root cause and fix the failure,

Only if you want to launch from US. I guess with a 160 countries there will some with the right area for a launch pad with way laxer requirements.

> I guess with a 160 countries there will some with the right area for a launch pad with way laxer requirements.

Good luck with that.

Exploding rockets are a very serious public safety risk since they're >90% propellant. Even Russia grounded Soyuz until they root caused its launch failure. From what I can gather China has the same policy, although they're very quiet about any launch failures that happen, so it harder to tell.

Doesn't work? It seems to be working great for the industry leader.

> Doesn't work? It seems to be working great for the industry leader.

What are you talking about? SpaceX's Falcon 9 is arguably the most reliable rocket ever made. They've launched hundreds of times in a row without failure.

If your point is that they're blowing up Starship prototypes, well... they're in the middle of a development program and they're not flying customer payloads.

They blow up a few of the early Falcons, and the explosions got a lot of news coverage.

Now they are launching an landing[1] safetly, but it's boring and the news show a cute puppy instead.

So for most people the rate of explosions vs launching is skewed.

[1] It's crazy that landing rockets is boring now.

Right, because as one of your rocket customers, I'm totally fine with you disintegrating my one-of-a-kind payload

The issue isn’t the rocket: it’s what you have affixed to it (the payload).

The rocket itself is purely a delivery system for a payload after all.

from the rocket perspective, sure, but not so much from the perspective of your cargo.

In many cases the cargo is more precious than the rocket so you need reliable rockets.

>> maybe disintegrating a bunch of them isn't a showstopper

But it isn't about destroying a bunch of them. Cut corners on checks and you very quickly blow up all of them. Any slight increase in the failure rate of individual parts, saving a few pennies, multiplies exponentially across the entire rocket into total system failure. So the money-saving approach is actually to test test and retest, to cram down the failure rates so low that the cumulative rate become acceptable (about 1%).

what about payload?

ready to build many Webb telescopes?

Who pays when the payload disintegrates?

So what you're saying is instead of inspecting the parts, you just try to launch the rockets and try again when they explode. Production is your testing ground. Statistically speaking, if a specific design succeeds a few hundred times in a row, it's probably sound? If it crashes, you just push a patch?

Day 1 patches for space travel?

Thanks, I hate it.

this works for munitions, but not for payloads that anyone cares about.

Not sure exploding rockets on their launch platform is such a good thing when they're carrying a bunch of highly explosive / fragmentary warheads (in addition to the rocket itself, which is plenty dangerous).

Unless this was something like a cruise missile dropped at altitude where a failure isn't a big deal.

>> cruise missile dropped at altitude where a failure isn't a big deal.

If failure isn't a big deal, then the weapon should no have been used. An ALCM costs millions. The destruction it causes is part of a larger battle plan. Should it not work properly then friendly forces may die. Should it work properly then enemy forces may die. The effectiveness of such a weapon is never not a big deal.

If it’s that critical then they aren’t launching one cruise missile.

If system A costs 1k quid but only works 50% of the time, while system B works 99% of the time but costs 10k quid, system A actually makes a lot more sense. On average you are going to spend a lot less money for the same outcome.

What's the price on weapons technology falling into enemy hands? A dud is a gold mine of data for an enemy's reverse-engineers.

I'm sure even in this case, risking the air-frame of the bomber or friendlies on the ground below is not ideal. Bear in mind cruise missiles are usually launched from friendly territory.

Also, failures might reduce the accuracy of the missile, leading to potential civilian causalities.

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