An analytical model for the energy gain of a spherical shell target is developed. Realistic ignition profiles are calculated in a consistent way, taking into account the implosion phase. The restrictions on the hot spot size posed by the symmetry requirements are included in the model, and scaling relations for the fuel and the implosion parameters are obtained. The target gain is found to be strongly dependent on the ratio between the hot spot size and the initial target radius, whereas the preheating effects by radiation or hot electrons are seen to be less deleterious in this model than in previous models The presence of a relatively heavy layer around the fuel, which acts as a pusher during the implosion phase, has multiple effects on the energy gain