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.