VC-1: a vacuum-assisted hydrostatic and hot-gas launch accelerator for reusable single-stage-to-orbit vehicles. Concept and first-order performance model

The atmospheric phase of ascent dictates the size, staging and cost of every ground-launched vehicle. This paper describes VC-1, a fixed ground infrastructure that performs the first part of the ascent: a 10 m bore, 4.1 km vertical accelerator built into a coastal mountain and extended by a summit tower. The bore is evacuated before launch; a short channel admits seawater from 100 m depth to set the vehicle in motion; and 80 side chambers, spaced every 50 m, inject nitrogen at 900 °C and 50 bar behind a sealing buffer cylinder that carries the vehicle. A one-dimensional model combining the unsteady Bernoulli equation for the water column with the ideal unsteady-expansion base pressure of the gas stage is presented and solved numerically. For a 450 t vehicle limited to 30 g, the model gives an exit velocity of 1,042 m/s at 4,000 m altitude after 8.8 s, with energy closure better than 0.3%; with a 15 g limit the exit velocity is 995 m/s. Exit Mach number is 3.2 and dynamic pressure 405–445 kPa. The hydrostatic stage contributes less than 0.1% of the delivered energy; performance is governed by the gas stage. A rocket-equation analysis shows that the reduced velocity requirement makes a hydrogen–oxygen single-stage-to-orbit vehicle feasible over a range of dry-mass fractions where a ground-launched vehicle of equal mass is not, with payload between about 8 and 26 t depending on dry-mass fraction. The principal engineering uncertainties, including high-speed sealing, valve timing, exit closure, cyclic thermal loading of the lining and vehicle loads at exit, are identified as the subjects of a sub-scale demonstrator.