  Launching orbital and suborbital rockets from a high altitude is beneficial
because of e.g. nozzle optimisation and reduced drag. Aeroplanes and gas
balloons have been used for the purpose. Here we present a concept where a
balloon is filled with pure water vapour on ground so that it rises to the
launch altitude. The system resembles a gas balloon because no onboard energy
source is carried and no hard objects fall down. We simulate the ascent
behaviour of the balloon. In the baseline simulation, we consider a 10 tonne
rocket lifted to an altitude of 18 km. We model the trajectory of the balloon
by taking into account steam adiabatic cooling, surface cooling, water
condensation and balloon aerodynamic drag. The required steam mass proves to be
only 1.4 times the mass of the rocket stage and the ascent time is around 10
minutes. For small payloads, surface cooling increases the relative amount of
steam needed, unless insulation is applied to the balloon skin. The
ground-filled steam balloon seems to be an attractive and sustainable method of
lifting payloads such as rockets into high altitude.
