Scaling of the ignition energy threshold Eig with theimplosion velocity vim and isentrope parameter α ofimploding spherical DT shells is investigated byperforming one dimensional (1-D) hydrodynamic simulations of the implosion andhot spot formation dynamics. It is found that the a and b exponentsin the power law approximation Eig ∝ αavim-b depend crucially on the subset of initial configurationschosen to establish the scaling law. When the initial states are generatedin the same way as in the Livermore study (W.K. Levedahl, J.D. Lindl,Nucl. Fusion 37 (1997) 165), the same scaling,Eig ∝ α1.7 vim-5.5, is recovered. If, however, the initialstates are generated by rescaling the parent configuration according tothe hydrodynamic similarity laws, a different scaling is obtained, Eig ∝ α3.0 vim-9.1, which is very close to the α3vim-10 dependence predicted by the simple isobaric model forassembled fuel states. The latter is more favourable than the Livermorescaling when rescaling the fusion capsules to higher implosionvelocities, but requires the peak drive pressure to be increased asP ∝ vim5.