Hydrogen tube vehicle for supersonic transport: 5. Aerodynamic tunneling

•Aerodynamic tunneling increases Mach-1 speed or lowers vehicle drag.•Gas efficacy is a measure of how well a gas facilitates aerodynamic tunneling.•A theoretical relation for gas efficacy is based on a flat-plate aerodynamic probe.•Hydrogen has the highest efficacy, followed by ammonia, helium, and hydrocarbons.•Mixtures of gases for tube vehicles can lower cost or reduce flammability.

Aerodynamic tunneling is the process of transporting a vehicle from terrestrial point A to B via a closed tube containing an atmosphere more aerodynamically favorable than air. Equations are derived for “gas efficacy,” Γ, a measure of how well a gas increases Mach-1 speed or decreases drag of a vehicle. Theoretical results, Γp and its Mach-normalized form Γ1, based on reducing the vehicle to a flat plate, allows efficacy to be calculated ab initio as a function of only four gas parameters: ratio of specific heats, pressure, density, and viscosity. Hydrogen has the highest normalized gas efficacy (Γ1 = 48.5 s/kg). Ammonia, hydrocarbon gases, and helium have above-average efficacies. Xenon has the lowest (10.1 s/kg). Binary mixtures of hydrogen and methane (or natural gas) are proposed for lowering the upper flammability limit at a relatively small penalty in efficacy.