Ground tests of microwave electrothermal thrusters (METs) are conducted under 1 g, where buoyant forces distort the plasma morphology and bias performance data. We investigate these effects in a 2.45-GHz, TM \(^z_{011}\) cavity thruster operated with nitrogen over 200–2000 sccm and at 80–600 W. Three dimensionless surrogates—a modified Bond number \(B_\mathrm{o}\) (buoyancy), a swirl group \(\Theta\) , and an electromagnetic anchoring group \(\Pi_{\mathrm{EM}}\) —bring all experimental points onto a single, self-consistent scaling curve. Guided by these surrogates, we then recast the horizontal-nozzle data into a centered, reduced-order surrogate: a log-linear form in \(\dot{m}\) , \(P_{\mathrm{in}}\) , and the stagnation-pressure ratio \(P_{0,\mathrm{h}}/P_{0,\mathrm{c}}\) with a single exponential nonlinearity in \(P_{0,\mathrm{h}}/P_{0,\mathrm{c}}\) . The inferred elasticities show displacement grows with mass flow while remaining only weakly sensitive to small, local changes in \(P_{\mathrm{in}}\) or \(P_{0,\mathrm{h}}/P_{0,\mathrm{c}}\) ; the dominant response is a sharp departure once \(P_{0,\mathrm{h}}/P_{0,\mathrm{c}}\) moves away from its near-optimal neighborhood. Orientation controls performance: with the nozzle upward, buoyancy helps anchor the discharge at the throat and \(P_{0,\mathrm{h}}/P_{0,\mathrm{c}}\) often approaches \(\sim3{:}1\) (peaking near 3.3–3.5), whereas in horizontal/downward tests buoyancy displaces the bubble off-axis, reducing \(P_{0,\mathrm{h}}/P_{0,\mathrm{c}}\) by \(\sim20-40\%\) . Equal top–bottom injection maximizes swirl confinement and suppresses buoyant drift. The surrogate offers a compact path to correct 1-g measurements toward flight-like conditions and to couple directly with thrust and \(I_{\mathrm{sp}}\) models for MET design.
