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"DDT" stands for Deflagration to Detonation Transition and is the technical term for the little-understood phenomenon that occurs when the flame front inside a burning fuel/air mixture reaches and exceeds the (local) speed of sound. Note that the speed of sound changes with pressure, temperature, and even humidity, so the speed of sound inside the combustion chamber can be very different from the atmospheric speed of sound.

In a normal combustion situation, the flame front in a burning fuel/air mixture does not exceed the speed of sound, classifying it as a deflagration (see Deflagration on Wikipedia). The combustion inside a gasoline or diesel engine, gunpowder, and low-grade explosives are all deflagrations.

Under rare circumstances (generally in respectively long, thin combustion chambers containing pressurized fuel/oxidizer mixtures), the flame front can exceed the (local) speed of sound, as does the flame front of high explosives, classifying it as a Detonation. This transition from subsonic to supersonic can create an immense pressure spike that has damaged even well-constructed metal chambers, and is certain to cause a PVC chamber to fail. Usually pipe will fail at the point where DDT happens. This is because the point is subject to a localized rapid high pressure change.

It is the goal of anyone who constructs a hybrid launcher to avoid DDT. It is thought that a short, fat (theoretically closer to spherical) chamber is less likely to incur DDT than a long, thin chamber. Using oxygen enriched mixtures or exotic fuels such as acetylene increase the risk of DDT.

Here we can see the first normal pressure spike at 2510 µsec compressed the fuel and air and caused a DDT at 2555 µsec.

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For more information on DDT, try Google

GexCon article on DDT