Development is proposed of a novel refrigerator capable of achieving cryogenic temperatures to be used for densified (sub-cooled) propellants, specifically liquid hydrogen (LH2) and liquid oxygen (LOX). The refrigerator is a closed cycle two-stage pulse-tube refrigerator capable of providing, simultaneously, cooling at temperatures of 1) 10 K to densify liquid hydrogen (normal boiling point 20 K) and, 2) 80 K to densify liquid oxygen (normal boiling point 96 K). Alternatively the first stage cooling available at 80 K may be entirely or partially consumed to precool and shield the second stage so as to maximize the cooling capacity available at the lower temperature, 10 K. The compressor of the pulse-tube refrigerator consists of twin metal diaphragm compressors with hydraulic oil used to move the diaphragms to accomplish compression of the working fluid. One compressor acts as the pressure wave generator for the first stage. The other compressor acts as the pressure wave generator of the second stage. The second stage compressor operates at a lower speed than the first stage compressor. The system is designed to be a compact lightweight unit of high efficiency. There are no moving parts and no rubbing or wear surfaces so no detritus is generated and very long unattended service life is anticipated with no maintenance required. There is no mechanical vibration, no noise and no electromagnetic induction or static electricity generated. The propellant densifier uses conventional readily available materials with no exotic or strategically sensitive parts. Its operation has no deleterious impact on the environment.Potential Commercial ApplicationsIn the hydrogen economy predicted for the automotive future the proposed unit would find domestic and small commercial application as an overnight hydrogen liquefier. A more likely application is the use of a simpler, lower cost, single stage version used as a methane liquefier to refuel overnight liquid natural gas vehicles by liquefying pipeline gas supplied to homes and industrial companies.
