DDT is a two stage process, and for primary explosives the exact mechanism is still kinda sketchy, and there are a couple schools of thought. (bowden et al, 1930 something)
The mechanism I remember involves the gasses generated by the initial deflagration pressurize the explosive above of the combustion front, and the deflagration turns into a detonation when the compression heats adiabatically to the decomposition point. Before the DDT, rxn propagation is by thermal conduction/convection of the hot gasses, whereas after it goes by a shock wave propagated mechanism.
If done in vacuum, you reduce the pressure of gasses available to compress the unreacted explosive, so the DDT time is substantially increased.
The above is for shitty primary explosives such as tetrazene, styphnates, Hg fulminate etc. The quality primaries that have no noticeable DDT lag time (ie touch a single crystal or grain of material with a match and instant detonation with no deflagration time) such as Ag nitrotetrazolate, many heavy metal azides, etc. For these quality initiators, I am unsure what would happen in vacuum, I suspect that the longer the DDT lag time, the greater the influence of vacuum, so these would still detonate, but I am unsure.
DDT is best understood for FAEs as gaseous modeling is easier than solid state.
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