Abstract
NASA is currently in development of a vertical test bed vehicle demonstrating new green propellant propulsion systems and autonomous landing and hazard detection technology. The vehicle is required to land on the Earth’s surface without the aid of parachutes using a single gimbaled LOX/Methane engine to control its descent. One of the many challenges of this system is the ability to control the descent while the vehicle is exposed to external and internal disturbances, and under changing mass properties (propellant consumption). In order to develop a suitable control system, the maximum expected magnitude of the disturbance inputs must be predicted. One such disturbance is the propellant slosh created during vehicle accelerations. This paper analyzes the effects of propellant slosh on the rigid body motion of the vehicle for both liquid methane (fuel) and liquid oxygen (oxidizer) residing in spherical propellant tanks containing cruciform and rigid ring baffles. This was accomplished by analysis and software simulation. The results are compared with the NASA models as well as existing historical data. With a validated analysis model, the software can be used to run multiple scenarios to gain information on vehicle response prior to testing using expensive vehicle hardware. The software used for the simulation is SimWise 4D by Design Simulation Technologies, Inc. Modeling of the rigid body vehicle was done using Pro ENGINEER and Siemens NX8 CAD software. Simulation in SimWise 4D showed that the induced vehicle rotation rates due to propellant slosh in the spherical tanks are dependent on the propellant fuel height, slosh frequency, slosh amplitude, and propellant mass. Induced angular rates and accelerations decreased with a decrease of propellant loading beginning at the 2000 lb load case. Higher load cases (at 4000 lb for example) exhibited decreased induced velocity due to decreased sloshing as the liquid free surface decreased within the spherical tank.