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SRBs and missiles use grain to regulate thrust over time, as only exposed surface of the propellant burns.

But what causes propellant to burn only on the surface, and regulates the speed at which the surface burns? Surely not atmospheric oxygen supply (which is what causes similar property of free-burning solids) as it has oxidizer distributed throughout the volume; there's no apparently clear reason why the solid fuel couldn't just burn all at once throughout the volume; something puts it apart from common explosives which are effectively just that, a kind of fuel and oxidizer mix that burns all at once all throughout the volume, in enclosed space, releasing all the combustion products at once.

So - what chemical additives or properties set solid propellants apart from explosives?

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    $\begingroup$ Something like this? Difference between deflagration and explosion I think the answer will depend on the exact propellant and explosive being compared though. (Some evolve a lot of gas which causes the actual explosion.) $\endgroup$ – Andy Jun 27 '16 at 15:13
  • $\begingroup$ Pretty much this; in most cases deflagration is limited by oxygen accessibility, and most oxidizers release oxygen by being heated. In our case we have the combustion front propagating at scarce millimeters per second, while the temperatures exceed decomposition of oxidizer and combustion of the fuel by many orders of magnitude, suggesting the combustion should proceed much faster. Also, what's so special about the surface of the propellant that it can contain the combustion, not allowing it inwards? $\endgroup$ – SF. Jun 27 '16 at 15:59
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    $\begingroup$ On a different note - I watched an instructable video on construction of an amateur rocket engine; the engine has a short "delay" segment which burns at a rate much slower than the main propellant, leading the ignition to a small gunpowder charge that ejects the parachute. The "delay" was the same propellant, but with a small amount of baking soda added; it would burn much, much slower as result. But the effect was not explained. $\endgroup$ – SF. Jun 27 '16 at 16:05
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    $\begingroup$ Sounds like adding inert materials to solid fuels to change the burn rate. By the way even custard powder explodes violently under the right conditions - safety warning for any engineers out there, it has happened... By the way I think the Chemistry stackexchange might be a good place to ask if a good answer doesn't emerge here. $\endgroup$ – Andy Jun 27 '16 at 16:12
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    $\begingroup$ @Andy: In that case the reduction in burn rate was totally disproportionate to the addition of soda. I believe it was something like 2% of soda, for a slowdown to something that is encountered in a slow-burning fuse, so it definitely wasn't mere dilution. (unless it was dilution of oxygen by carbon dioxide from the soda maybe?) $\endgroup$ – SF. Jun 27 '16 at 16:16
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If we have a pressure shock front travelling very fast through the material it is an explosion. The reaction inside the explosive is started by the sudden pressure rise, not by a temperature rise. But when the solid rocket fuel burns, we have a nearly constant pressure inside the rocket and no travelling shock front. The thermal conductivity of the fuel is much slower, the reaction in a deeper level of the propellant only starts when the temperature is high enough there. The outer layers of the propellant stay cool and protect the walls of the rocket from high temperatures. If the solid rocket is reuseable, the walls should not be damaged in the last seconds of the burn when the reaction zone comes close to the walls.

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A propellant deflagrates, rather than explodes. The difference between them is that deflagration relies on the thermal energy of the flame front, rather than the energy of the shockwave caused by detonation.

The key to this process is a balance between two factors, the activation energy of the propellant and the temperature at which the propellant burns. This balance is set against the thermal diffusion of the propellant. If the heat cannot diffuse fast enough into the propellant, it won't stay lit. If it diffuses too fast... well... you wondered what keeps them from going boom?

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I've asked the same question on Chemistry.SE and got some more factors than mentioned in the answers here.

  • flexible binding agent (rubber), that prevents forming cracks under pressure, is essential to maintaining stable, low deflagration rate vastly lower than in blasting charges of the same composition.

  • modifying ratio of oxidizers (perchlorate, ammonium nitrate) regulates the speed; perchlorate burns faster.

  • catalysts like carbon and metals can increase the deflagration rate.

  • MIL-STD-286C. defines a method of determining linear burning rate of propellants.

So, essentially, first - applying a rubber-like binder is the critical part that reduces deflagration rate to "propellant speeds". Then, through "trial and error" the rate can be fine-tuned - adapted to requirements of construction, geometry and purpose of specific SRB/missile - through modifying oxidizer composition and catalysts. The final part is the grain shape inside the rocket, which regulates how area of deflagration changes over time, regulating thrust of the rocket over time - and the nozzle area, limiting the pressure (and deflagration rate) inside the SRB.

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Surface area, pure and simple. The more surface area visible, the faster the burning will occur.

As to what separates them from explosives, it is that the propellant is so tightly bound that only the surface can burn. If there are cracks, then solid rockets can in fact explode, so they are very carefully manufactured.

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    $\begingroup$ How does the tight binding prevent the heat (and combustion) from penetrating deeper across the volume, ignoring the surface? Or what limits the rate the surface ablates away - exposing deeper layers at given speed, the volume of the propellant taking around a minute to burn through, instead of all burning away in scarce seconds or less? $\endgroup$ – SF. Jun 27 '16 at 12:30
  • $\begingroup$ The second is correct, only the upper layer of the propellant burns. As the surface burns away, the part underneath can start burning. $\endgroup$ – PearsonArtPhoto Jun 27 '16 at 14:58
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    $\begingroup$ But why? In normal burning process, the inner area of material is protected from burning by lack of oxygen, as it all burns away on, or near the surface. This is not the case with propellant mixed with oxidizer where only heat is needed to release oxygen; and I'm fairly sure explosives exist, that are packed just as tightly as solid propellant. $\endgroup$ – SF. Jun 27 '16 at 15:06
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    $\begingroup$ @SF. I'm going to guess before I try to look it up. To burn, you need two kinds of molecules touching, AND a high temperature, all at the same time. In a car engine, you need gas and air mixed as vapor, AND a spark or heat. They can mix the solid propellant so that they are not uniformly mxed, but instead form little alternating pockets. The solid conducts the heat, so only the surface can reach ignition temperature. Possibly they need to be vaporized before they can react. OK now it's time to go look it up :) $\endgroup$ – uhoh Jun 27 '16 at 15:18
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    $\begingroup$ Your answer suggests that a block of C4 should not explode then, since it is a tightly bound piece of a homogeneous substance. $\endgroup$ – Austin Jun 29 '16 at 8:31
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Looking through the description of APCP, I noticed that it contains metal powder. That means your assumption about the oxidizer isn't correct. At microscopic level, the oxidizer is not distributed throughout the volume of the metal. The distance between oxidizer molecules and metal atoms is measured in micrometers, not nanometers. Of course, a micrometer still isn't that much, which explains why the reaction is very quick once the propellant is vaporized. The atoms only need to travel those few micrometers before they can react. But that's not instant, and it limits the burn rate.

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  • $\begingroup$ The propellants are classified into 4 kinds: Composite, Single-, Double- and Triple-base. Composite, like APCP has macroscopic separation; the others are chemical substances or atomic-level mixtures. E.g. Nitrocellulose (used e.g. in RS-82) is a single-base propellant, and it's a single substance, not a mixture; a monopropellant - not even inter-particle separation, fuel and oxidizer the same substance. Stabilizers and other additives are used to control the chemical stability and enhance the propellant’s properties. $\endgroup$ – SF. Jul 6 '16 at 10:44
  • $\begingroup$ There's some confusion here as to whether the "metal" is a fuel or oxidizer. APCP consists of ammonium perchlorate and a binder, plus some metal additives like aluminum, zinc, or magnesium. Those last three additives are considered fuels. The main fuel, though, is the ammonium (NH4). XLR99 engine on the X-15 space plane, for example, ran on ammonium and LOX. The perchlorate (ClO4) is the main oxidizer. Metal-oxide additives are also oxidizers, and catalysts. The catalysts are a big factor in the burn rate, so you may want to focus research on them in your quest. $\endgroup$ – DrZ214 Nov 17 '16 at 4:21
  • $\begingroup$ @DrZ214: Clarified, a metal is unambiguously a reductor (electron donor). Once it's donated those electrons, the associated ion can become an oxidizer (taking back its electrons) but it won't be a strong oxidizer. $\endgroup$ – MSalters Nov 17 '16 at 13:27

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