NASA is currently exploring propellantless spacecraft propulsion technologies required to enable missions ranging from orbital debris capture and removal to station keeping in unstable locations in space to deep space exploration. In this paper we explore the stability of a particular propellantless spacecraft, a spinning, helicopter-like solar sail. We demonstrate the possibility of an instability due to the coupling between the structural dynamics and the solar radiation forcing using a nonlinear model. We show that our model reproduces previously published results and compares well with simulations conducted using a finite element simulation. In addition the paper explores the stability of a proposed spinning solar sail during deployment and identifies critical combinations of spin rate and lengths that increase the possibility of an instability arising. © 2014 by the American Helicopter Society International, Inc. All rights reserved.