alt Hacker NewsDoes 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
Replies
I've talked about this a few times before but – https://news.ycombinator.com/item?id=47726133 / 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;
> ‘You put some people on top of four million pounds of high explosives, you light the fuse, and in eight and a half minutes they are going eight times faster than a rifle bullet. What part of that sounds safe to you?’
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.)
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.
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.
> 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...
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.
> 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 and costing maybe $400K.
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
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
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.
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.