This proposal is a follow up of GT observations of the galactic remnant RCW103 which were aimed at obtaining a template spectra of shock excited gas and which revealed very prominent lines (cf. Fig. 1). Here we concentrate on N49 which is the most luminous remnant in the Magellanic clouds, it is thoroughly studied at optical and UV wavelengths and is much more compact, isolated and less reddened than RCW103. N49 is therefore the ideal target to study the interaction of fast shocks with the interstellar medium. We propose SWS and LWS spectral observations of this luminous remnant with the following aims. 1) To derive the efficiency of dust destruction by measuring the gas phase abundance of refractory elements (Si, Fe) 2) Use fine structure lines of [OIV], [NeII,III,V,VI] and [SI,III,IV] to constrain the ionization level of the gas and infer information on the shock speed. 3) Verify if the above lines are emitted in the post-shock region, as in the classical shock models, or if they mainly arise from the pre-shock material photoionized by the X-ray precursor of a fast shock, as postulated by recent computations. 4) Study the physical conditions and excitation mechanism of molecular hydrogen.