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When the Germans projected the A4 missile, they discarded solid-fuel propulsion and went on to circumvent the enormous technical difficulties of building a liquid-fuel engine from scratch. Germany had, at the time, perhaps the best chemists in the world, but they were unable to formulate a good mixture for a solid rocket fuel. Is the present abundance of large solid-fuel rockets due to some recent discovery...?

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    $\begingroup$ You need not only the solid fuel, you also need the heat and pressure resistant container for the fuel and an ablative cooled nozzle for the rocket. Of course an attitude control is needed too. The mixtures developed some years later in the USA used asphalt or synthetic rubber as binder, just state of the art of chemistry. $\endgroup$ – Uwe Oct 8 '18 at 8:16
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    $\begingroup$ "... but they were unable to formulate a good mixture for a solid rocket fuel." Can you add to your question a link or cite a reference that supports that? Thanks! $\endgroup$ – uhoh Oct 8 '18 at 14:30
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    $\begingroup$ The A4 had predecessors, see wikipedia. There was a lot experience with liquid fueled rockets when the design of the A4 was started. No start from scratch but more than 10 years of work with liquid fuel. $\endgroup$ – Uwe Oct 9 '18 at 11:17
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    $\begingroup$ See this related history question with a good answer. $\endgroup$ – Uwe Oct 9 '18 at 11:22
  • $\begingroup$ @Uwe The predecessors of the A4 were very small, fuel and fuel and oxidizer were both gas-pressure-fed. The A3 had a mass of around 800 kg, and the A4 of 13000... $\endgroup$ – xxavier Oct 9 '18 at 12:07
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Contrary to what we learned in school, solid motors are not "dumb and simple". They are very, very hard to put together. Any small irregularity in the propellant grain can lead to a hot spot and failure of the pressure vessel.

To get an idea of the massive machines used for building the SLS motors take a look at this video:

I've seen the actual mixers. Imagine your standard Kitchen Aid mixer, but with a bowl the size of a small room! It's not just about the chemistry. Is the entire process.

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