We present the first 3D radiation-hydrodynamic simulations on the formation and evolution of born-again planetary nebulae (PNe), with particular emphasis to the case of HuBi1, the inside-out PN. We use the extensively-tested GUACHO code to simulate the formation of HuBi1 adopting mass-loss and stellar wind terminal velocity estimates obtained from observations presented by our group. We found that, if the inner shell of HuBi1 was formed by an explosive very late thermal pulse (VLTP) ejecting material with velocities of $sim$300 km s$^{-1}$, the age of this structure is consistent with that of $simeq$200 yr derived from multi-epoch narrow-band imaging. Our simulations predict that, as a consequence of the dramatic reduction of the stellar wind velocity and photon ionizing flux during the VLTP, the velocity and pressure structure of the outer H-rich nebula are affected creating turbulent ionized structures surrounding the inner shell. These are indeed detected in Gran Telescopio Canarias MEGARA optical observations. Furthermore, we demonstrate that the current relatively low ionizing photon flux from the central star of HuBi1 is not able to completely ionize the inner shell, which favors previous suggestions that its excitation is dominated by shocks. Our simulations suggest that the kinetic energy of the H-poor ejecta of HuBi1 is at least 30 times that of the clumps and filaments in the evolved born-again PNe A30 and A78, making it a truly unique VLTP event.
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