Comment by userbinator
10 hours ago
Does anyone else find it surprising that rockets are a century old[1] and yet still seem to fail spectacularly with amazing regularity, often due to some small flaw? Is it just that they're still relatively niche machines and thus haven't benefited from mass manufacturing improvements?
[1] https://en.wikipedia.org/wiki/File:Goddard_and_Rocket.jpg
https://web.archive.org/web/20120503175355/https://www.nasa....
> The percent propellant has huge implications on the ease of fabrication and robustness in achieving the engineering design (and cost). If a vehicle is less than 10% propellant, it is typically made from billets of steel. Changes to its structure are readily done without engineering analysis; you simple weld on another hunk of steel to reinforce the frame according to what your intuition might say. I can easily overload my ¾ ton pickup by a factor of two. It might be moving slowly but it is hauling the load.
> Once the vehicles become airborne, the engineering becomes more serious. Light weight structures made of aluminum, magnesium, titanium, epoxy-graphite composites are the norm. To alter the structure takes significant engineering; one does not simply weld on another chunk to your airframe if you want to live (or drill a hole through some convenient section). These vehicles cannot operate far from their designed limits; overloading an airplane by a factor of two results in disaster. Even though these vehicles are 30 to 40% propellant (60 to 70% structure and payload), there is room for engineering to comfortably operate thus there is a robust, safe, and cost effective aviation industry.
> Rockets at 85% propellant and 15% structure and payload are on the extreme edge of our engineering ability to even fabricate (and to pay for!). They require constant engineering to keep flying. The seemingly smallest modifications require monumental analysis and testing of prototypes in vacuum chambers, shaker tables, and sometimes test launches in desert regions. Typical margins in structural design are 40%. Often, testing and analysis are only taken to 10% above the designed limit. For a Space Shuttle launch, 3 g’s are the designed limit of acceleration. The stack has been certified (meaning tested to the point that we know it will keep working) to 3.3 g’s. This operation has a 10% envelope for error. Imagine driving your car at 60 mph and then drifting to 66 mph, only to have your car self-destruct. This is life riding rockets, compliments of the rocket equation.
Interesting post. I'd never thought of it that way. Not consciously anyway.
Might that make an air-launched system more reliable? Even if it's less efficient, the TCO would be lower using a winged system for the initial phases of launch.
It wouldn’t help much, sadly. Getting to orbit is about speed, not height — you need 27000 kph to get to orbit, and having an air launched platform would shave off 1k kph off it at most, perhaps 5k with some insane hypersonic engineering.
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There are some companies working on that and / or there have been some experiments with it, but there's two factors there; the one is of course how much weight an aircraft can carry. The other one is the altitude and / or angle; a big plane goes to about 10 kilometers (maybe more, idk), but that's a 'flat' flight, ideally you launch while angled upwards and that's a bit more involved.
But that's how a lot of the X projects were / are done.
That’s what Virgin Galactic was.
Nice and to the point.
Thanks for the link.
To add to this excellent explanation: Rockets have a fundamental problem. They need to go absurdly fast. If you have a rocket that can reach speed X, to go faster than X you need to reach X but also have fuel left over. However to get that fuel to speed X, you need even more fuel. This is the tyranny of the rocket equation.
Roughly put, the rocket equation is: change in speed = (engine efficiency) * log(mass of the rocket with fuel / mass of the rocket without fuel). So there's limited parameters to play with:
- The speed you need to reach is fixed.
- You can change the weight of the payload. Payload (eg, satellite) designers try to make things as light as possible, rocket designers try to give as much capacity as possible, and everyone prays they can meet in the middle.
- You want as little propellant as possible for cost and practicality, but mostly the other parameters fix how much you need. If the other parameters aren't good enough, you can easily get results like needing a rocket the size of Central Park. [1]
- You can make the engine more efficient. This means running it hotter with higher pressure, pushing the limits of material science. [2]
- You can make the non-payload static parts of the rocket lighter. This means removing structural integrity. It also means making the lightest parts to complete hard tasks like being a valve for cryogenically cooled, literally the smallest element, hydrogen.
Both the engine and non-payload static mass are essentially asking the question "How far can I push this without it breaking". Get your answer to that question even slightly wrong on any of the thousands parts in a rocket, and suddenly all of the fuel that you're using to go in one direction fast decide that you should instead go in every direction fast.
[1] https://what-if.xkcd.com/24/
[2] Or not using chemical propulsion. However things like ion engines don't have enough thrust to get through the atmosphere and into orbit, and things like nuclear propulsion spew fallout everywhere.
I've talked about this a few times before but – https://news.ycombinator.com/item?id=47726078 - to repeat myself;
It's because we're a very primitive species, and the forces involved here are genuinely new. It's physically not possible at our current level of technology to make this "safer" due to the distances and energies involved.
I will let John Young explain it his way;
As an aside, if you've never heard of John Young, I recommend learning a bit about him. He was an incredible person. And that statement is very funny in his voice; https://www.youtube.com/watch?v=KezwDfFcFhU
He test flew the shuttle. They put an ejection seat in the shuttle – which was obviously insane. And a reporter asks him about ejecting while the solid rocket motors were burning, https://www.youtube.com/watch?v=JLU4CK7UHd4
(I'm deeply saddened that I will never get to meet the man and ask him the secret to his magical heart rate.)
and the forces involved here are genuinely new
I remember growing up with things proudly advertised as "space-age technology"... which largely meant the 1950s and 1960s, and of course it's what got us to the moon, multiple times. Yet more than a half a century later, new rockets just don't seem that impressive in comparison.
> Yet more than a half a century later, new rockets just don't seem that impressive in comparison
We have 15x reduction in payload-to-orbit costs, 20x increase in launches/year, significantly increased reliability during missions (test explosions like this one are tests for a reason), and reliable vertical landings with reusable lower stages.
The current crop of rockets may not be as visually impressive as a Saturn 5, but they are well on their way to making orbital space flight a commodity rather than a risky experiment
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Our species is pretty young, around ~2 million years old, give or take a few million / hundred thousand years depending on whom you're talking to.
We've had this technology for ~70 years. That's 0.0035% of our species lifetime. That's pretty new.
We're used to thinking of things in human time scales, but it took us how long to master fire? And then smelt metals? And then learn mathematics...? These things take time for a species to master.
Personally, I'm impressed with just how unimpressed I am. Or rather, rocket launches feel like they really are becoming more and more commoditized. To the point that routine trips to the moon doesn't feel like a crazy future.
The contrast in my opinion comes more from the fact that 50 years prior to the space age people rode horses as a standard mean of transportation (more or less). It’s underwhelming to not see the line going from horse to rocket continue on the same path 50 years later.
We went to the moon in the late 60s as a massively expensive cold-war propaganda campaign, after the Soviet Union humiliating America for years when it came to firsts in space. It was a question of honor and showing that capitalism is better than communism.
Then it took roughly 50 years of progress to make space flight cheap enough that the economics make sense. With a couple setbacks a long the way that might have cost us a decade or two
"very primitive" - primitive in relation to who? As a species we control the planet, we rule every other species currently known to humans, how is that primitive?
We might well be the most advanced species in the universe. Seems unlikely, but we really don’t have anything else to measure against at the moment.
why did you preface this with how many times you've made your point to deaf ears in the past? Am I supposed to follow your opinions across the site?
It used to be a thing that people did when repeating a comment. I've used HN for a very long time.
It's a form of manners from those days so that people know that I'm not just spamming something. I think a lot of the people who used to write like that are gone. Most metaphorically, some physically. I'm trying to keep the tradition alive.
Did you know that we are a primitive species though? (up to anyone to guess what that means)
There are a number of ways of looking at this, which others have answered, but here's another:
The kinetic and potential energy of a 1 kg mass in orbit is around 33 MJ. The chemical energy of 1 kg of methane+oxygen propellant is only about 11 MJ.
Alternately, perfectly combusted methane-oxygen propellant has an exit velocity of around 3500 m/s. But you need about 7800 m/s to get into orbit.
Chemical energy is just very weak compared to the energy of things in orbit. It's really shocking that we can do it at all.
The result of this is that your vehicle is going to be almost entirely propellant. You simply can't just build a big, beefy rocket that's, say, only half propellant, with lots of extra safety margin for things that go wrong. Cars and bridges and things have immense margins. Airplanes, a bit less so, but still more than rockets. Rockets live right on the edge of what's possible, and as long as we use chemical thrust it'll always be that way.
Which isn't to say that rockets won't get more reliable. The Falcon 9 has had hundreds of flights since the last failure, and it isn't as optimized as it could be. But there will be a lot more failures before we get there.
Simplest explanation comes from Tory Bruno: they design with a factor of safety just above 1. 1.1 to 1.25. This is one of the reasons they wait for good weather to launch… they are trying to maximize payload. Also until recently, it’s been sort of a vicious cycle: rocket is very exquisite and expensive, so spacecraft needs to last longer and thus gets more exquisite and expensive, etc.
Have you seen how many issues race cars have? Same shit. It goes on and on.
One might make the same observations about software “bugginess” and complexity. The pace of improvement is such that everyone is riding the bleeding edge and, as such, the carpet inevitably gets a few spots of blood on it.
Software quality sucks because the consequences for getting it wrong are low for the people and organizations making it.
Which is often fine, but sometimes isn't.
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I believe the safety factors in Falcon 9 are 1.4. NASA credited SpaceX with using safety factors larger than normal for aerospace.
Surely reusability requires higher margins than ever needed before.
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> Does anyone else find it surprising that rockets are a century old[1] and yet still seem to fail spectacularly with amazing regularity, often due to some small flaw?
Not really. The performance metrics on rocket engines are utterly insane.
The jet kinetic power of a Merlin 1D engine at sea level is 1.3 GW. The work output of a nuclear power plant in a device weighing half a ton.
Well, rockets are more than a millennium old, but sure, solid fuel rockets tend to be less volatile by definition.
Honestly we’re really good at not prematurely combining tens to hundreds of tons of high-energy fuel and oxidizer put right next to each other and then combining them at several tons per second in a highly controlled way using a very complex system of plumbing and turbopumps powered by the same reagents.
Starting/igniting a liquid fueled rocket engine is an inherently complex process - everything has to be sequenced just right to get engines chilled, turbo pumps up to speed, any gaseous fuel vented and harmlessly ignited before it builds up, ignition of fuel, etc.
Here's a 1hr video from the Everyday Astronaut explaining the process and everything that can go wrong.
https://www.youtube.com/watch?v=bAUVCn_jw5I
> Does anyone else find it surprising that rockets are a century old[1] and yet still seem to fail spectacularly with amazing regularity, often due to some small flaw?
Not really. Rocketry is hard.
You deal with extremes in temperature (both high and low), extremes in speed and acceleration, and you're doing it all atop massive amounts of extremely explosive fuel. And, if you feel really crazy, you do it all while attempting to protect one or more fragile bags of meat and water as you travel into an environment that wants to kill them all.
Even when you think you've accounted for everything, something like a piece of foam insulation falling from an external tank is all it takes to produce a catastrophic failure later on during re-entry.
See: https://en.wikipedia.org/wiki/Space_Shuttle_Columbia_disaste...
This feels like more a matter of scale than anything else? We’re able as a species to do some absolutely insane wizard shit elsewhere (chip fab?), we just haven’t launched enough rockets yet to get there.
Less than perfect chips are printed every day but the consequences of a less than optimal chip is ending up in a cheaper laptop vs. causing a big boom. I think we are doing wizard shit in rocketry, mistakes are just very loud.
Definitely unexpected from BO, knowing that everyone is okay, I feel for their engineers right now.
A significant part of stuff like chip fabs is a controlled environment.
Which is tough with rockets.
Flagship chip yields are generally less than 50%. Over half the chips coming out are dead on arrival, and never leave the fab. Imagine if rockets had that sort of failure rate.
the differences is rocket failure is blow up while chip design is salvageable
I think more you’re just at the absolute margins of engineering to get to escape velocity. Those constraints haven’t changed, so until some major material or fuel advance happens things will continue to go wrong.
Probably the mistake is to keep relying on rockets and propellants. Need to think more revolutionary. But hard for a startup to do that, usually needs gov backing.
You know what they say, nature abhors colossal tanks of high-explosive.
Something like a bridge is easily possible with the gravity of our planet. If gravity were twice as strong, we would still have bridges. Orbital rockets are only barely possible (with practical, known chemical propellants). If gravity were twice as strong, we either wouldn’t have them or we would have to use very different methods of propulsion.
Given that it’s just barely possible, you can’t just make things twice as strong as you think you’d need to, just in case something unexpected happens. Anyhow when something moderately unexpected happens, that means you may get a giant fireball like we saw today.
Rockets are hard for sure but also almost nobody notices if there's a minor bug in your delivery app that causes it to crash every once in a while - but it can matter alot if there's a microscopic crack in a rocket engine that makes it blow up. Defect rate might be the same but the (literal) blast radius is much higher.
aerospace is operating at the absolute limit of what can be asked of known materials science
And an unexpected transient load or heat flux can easily exceed material limits, or materials can have flaws. The ability to qualify the materials, components and processes for aerospace use is an achievement in itself.
Exactly this. It is relatively easy to make something mechanical more safe provided you have have a significant material buffer. Rocket abhors additional material weight so everything has to run with a limited buffer space for safety.
The engines are seeing significant development. These engines are the most complex of their kind, they inject the fuel and oxidizer as hot gases. Google full flow staged combustion cycle
What you refer to as the rocket, meaning the tube itself isn't failing. It's just that a big explosion will treat it apart
BE-4, this rocket’s engine, is not a full flow engine.
The mass produced rockets explode very infrequently
I'm more surprised that they work at all.
Rockets are bombs. Rockets are big bombs. For a rocket to work correctly, you want it to explode a little more gently, and in one direction. The subtleties of making it explode a little more gently are where all of these failures are found.
> Is it just that they're still relatively niche machines and thus haven't benefited from mass manufacturing improvements?
Until very recently they were basically all custom with extreme tolerance requirements and absolute specifications. Nobody could have an "off day" on a single bolt, hose, nut, screw, wiring harness, etc.