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DoD == Department of Defense
ELV == Expendable launch vehicle
FY == Fiscal year
GLOW == Gross liftoff weight
HTHL == Horizontal takeoff and landing
LACE == Liquid air-cycle engine
LEO == Low Earth orbit
LH2 == Liquid hydrogen propellant
LOX == Liquid oxygen propellant
NASA == National Aeronautics and Space Administration
RBCC == Rocket-based combined cycle (airbreathing) engine
RLV == Reusable launch vehicle
SSME == Space Shuttle Main Engine
SSTO == Single-stage-to-orbit
TSTO == Two-stage-to-orbit
USAF == United States Air Force
VTHL == Vertical takeoff and horizontal landing
VTVL == Vertical takeoff and landing