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Watching the recent SpaceX Starship tests got me to thinking:

Could you actually make a rocket using grain as the propellant? Starship SN5 Image from https://www.teslarati.com/spacex-starship-sn5-second-hop-plans/

Grain, especially dry grain is quite flammable, and grain explosions are a thing that can happen when the grain is finely dispersed.

The rocket would either have to be a solid-fuel or hybrid rocket. Salami rockets have been successfully made and tested.

Given the right binder and oxidiser, what are some estimates of how well a grain rocket would perform, in terms of Isp and likely max altitude?

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  • $\begingroup$ maybe you could achieve a higher isp if you turned the grain into a sort of water/grain powder solution similar to how diesel is sometimes made, and combust it with liquid oxygen instead? This would no doubt cause coking issues but there are some ways around that. $\endgroup$ Sep 4 '20 at 3:57
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    $\begingroup$ It is not the grain that may explode, it is the grain dust. Small particles are necessary. $\endgroup$
    – Uwe
    Sep 4 '20 at 10:42
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    $\begingroup$ If you turn grain into a mixture with water, why not heat it and afterwards distil it? If you do it right, it would be a waste to burn it though... $\endgroup$
    – oerkelens
    Sep 4 '20 at 16:35
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    $\begingroup$ @oerkelens The V-2 and Redstone rockets used 75% ethanol, 25% water as fuel -- close enough to Everclear 151. $\endgroup$ Sep 4 '20 at 20:29
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Since the term "grain" is already in use in the solid rocket context, I'm favoring the term "cereal".

Cereals contain about 66–76% carbohydrates -- mostly starch (55–70%) plus some sugars and cellulose.

Cellulose combusted with gaseous oxygen yields surprisingly good specific impulse, up in the 240 sec range; sugars with potassium nitrate oxidizer (used in DIY amateur rocketry) get figures in the 100-130 sec range.

There's also some fats and proteins, which burn well, and some moisture which wouldn't contribute to combustion (but would provide reaction mass for thrust at least).

All things considered it seems likely that you could get a credible hybrid or solid rocket motor going on cereal flour and binder.

Max altitude depends entirely on the overall scale and design of the rocket. Four-stage orbital rockets using solids with Isp in the 240s exist (Minotaur) -- a cereal rocket certainly wouldn't be a practical orbital launcher, but it should be theoretically possible.

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    $\begingroup$ So beware if the kids raid the kitchen for cereal. They might build a rocket. $\endgroup$
    – Polygnome
    Sep 4 '20 at 14:23
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    $\begingroup$ ^ Only if they also raid the kitchen for oxygen tanks $\endgroup$
    – IronEagle
    Sep 4 '20 at 15:15
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Cereals are largely composed of complex sugars: starch and cellulose. Simpler sugars combined with an oxidizer are sometimes used in rockets, and there seems no fundamental reason that more complex sugars wouldn't work.

A solid fuel rocket is, however, a serious pressure vessel, unlike a silo.

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