Ground prepressurization by helium bubbling for cryogenic upper stages

Abstract

This paper presents a method to prepressurize a cryogenic upper stage by routing ambient helium through the fill and drain lines, feedlines, and propellant tanks. Ambient helium supplied in this manner is cooled to the propellant temperature when reaching the tank ullage. The cold helium prevents ullage pressure collapse caused by liquid sloshing during first stage ascent and prior to upper stage ignition. It eliminates the need for dedicated prepressurization lines, extra onboard helium to pressurize the tank during ascent, or a ground heat exchanger to condition the ambient helium. It also reduces the upper stage weight and cost. A tank pressurization analysis was performed to determine the feasibility and benefits of this method over conventional methods. The analysis shows that the higher mass accumulation of cold helium during prepressurization slows down the ullage pressure decay. NOMENCLATURE Am = ullage-liquid interface surface area h.n = inlet enthalpy houl outlet enthalpy hUL = average heat-transfer coefficient between ullage and liquid Mf = fluid (helium, vapor, or liquid) mass MHe helium mass ML = liquid mass Mv = vapor mass nine helium mass flow rate nim inlet mass flow rate rriout = outlet mass flow rate po = total pressure at flow control orifice inlet pu = ullage pressure pVL = liquid vapor pressure Qn = net heat-transfer rate to the node QUL = heat-transfer rate between ullage and liquid RHe = helium gas constant Rv = vapor gas constant t = time TL = bulk liquid temperature T0 = total temperature at flow control orifice inlet Tv = ullage temperature Un = internal energy of node n Vu = ullage volume VT = tank volume W n = power input to node n pL = liquid density INTRODUCTION The propellant tanks of a cryogenic upper stage are normally prepressurized on the ground with ambient helium. The problem with this ground prepressurization method is that, during ascent and prior to upper stage engine ignition, liquid sloshing can cool the ullage and lead to ullage pressure collapse in the propellant tanks. This paper presents a method of prepressurizing a cryogenic upper stage on the ground that can prevent ullage pressure collapse during ascent while saving vehicle weight and cost. HELIUM BUBBLING METHOD OF PREPRESSURIZATION Description This method consists of flowing ambient helium gas at a controlled rate from the ground supply through the fill and drain lines, feedlines, and propellant tanks (Fig. 1). Cooled along its path by the propellant, the helium emerges from the liquid surface as a cold gas to * Principal Engineer/Scientist. Associate Fellow AIAA. Copyright © 2001 by The Boeing Company. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. 1 American Institute of Aeronautics and Astronautics (c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. AIAA-2001-3833 pressurize the tank and prevent ullage pressure collapse during ascent. The ambient helium also vaporizes a small amount of propellant, thus enhancing the tank pressurization.