Steering a warp drive without exotic matter

Abstract

A useful warp drive must change its velocity, yet known positive-energy constructions are static or constant-velocity, while accelerating ones require exotic matter; Bobrick and Martire noted that warp drives ``do not have any natural way of changing their velocities.&#39;&#39; First, a conservation law settles the principle: for any asymptotically flat drive with a confined dominant-energy source and standard peeling, the Bondi--Sachs four-momentum changes only through radiation to null infinity, so the drive cannot steer without radiating. Second, we construct a positive-energy spacetime that saturates this bound. Prescribing the passenger worldtube, we take the exact Kinnersley photon rocket as exterior and solve for the matching timelike shell; the exterior and steering law are exact, while the shell is certified perturbatively and numerically. The mechanism is photon-rocket recoil on a tidally protected, exactly flat cavity; the warp drive is the decoupled flat interior, not a warp field. The exterior energy conditions reduce to $n^2\ge0$, and steering obeys $-\dot m\ge 3m|a|$, paid for by Bondi mass loss. The shell satisfies the surface dominant energy condition for $2m/R<24/25$; admissible accelerating shells exist near the Schwarzschild--Minkowski anchor and, for slow burns, as time-evolved spacetimes. A Buchdahl-type frontier $a_{\max}R\le g(2m/R)$ caps the acceleration between a numerical lower bound $\sim\!0.2$ and a closed-form ceiling ${1\over2}(1-2m/R)$. On the gravitational-wave--silent class, the optimal maneuver is the Damour dipole. The wall is marginally stable, with strict stability available at no energy-condition cost; fully dynamical flux-coupled stability remains open. The drive is causal, subluminal, and energetically costly but positive-energy: steering is a problem of energy budget, not exotic matter.