In this paper, we present analytical calculations and numerical simulations of the vertical component of the electric field close to a rocket-triggered lightning site during the ascent of the rocket and prior to the lightning initiation, taking into account the corona layer around the wire. In the analysis, the background electric field was modeled according to the available experimental data. The influence of the space charge layer extending upwards from the ground level was modeled and analyzed. The obtained values of the electric field reduction are shown to be consistent with experimental observations. The presented model can also be used to estimate the charge distribution in the rocket wire and the current flowing into the wire from the ground.