Design and Manufacture of the Messenger Propellant Tank Assembly

An ultra-lightweight propellant tank was required for the MESSENGER spacecraft, and a new tank development program was conducted to design, fabricate, and test this tank. The development program was conducted in three phases: trade study, analysis and design, and hardware fabrication and test. A Phase 1 Trade Study was conducted to determine the most weight efficient tank design. This phase was done primarily by analysis with multiple iterations. Over 50 tank configurations were considered before a final selection was made. Phase 2, Design and Analysis, included efforts to design and analyze a vortex suppressor, anti-slosh baffle for nutation control during launch, and tank shell. The effort included subscale drop tower simulations to determine the number, size and location of anti-slosh baffle, analytical determination of the loads on the vortex suppressor and baffle, baffle structural analysis, and tank shell stress and fracture mechanics analyses. Phase 3 Fabrication produced a qualification tank and four flight tanks (3 flight and a spare). The tank shell components were fabricated from solution treated and aged (STA) 6AL-4V titanium alloy. The anti-slosh baffles were machined from annealed 6AL-4V titanium ring forgings, and the vortex suppressor was fabricated from 6AL-4V titanium sheets. The tank shell was assembled with 4 girth weldstwo of which were made to have STA properties, and the remaining two were annealed closure welds that were also baffle installation welds. All 5 tanks were fabricated using identical processes and procedures. The qualification tank must undergo a qualification test program that includes loaded sine and random vibration testing. The qualification test program concludes with a destructive burst pressure test. All flight tanks are protoflight tested prior to precision clean and delivery. The completed flight tank has a mass of less than 20 pounds, including attachment hardware. This ultra-lightweight tank will play a critical role toward the success of the MESSENGER Program. Copyright Q 2002 by Pressure Systems, Inc. Published by American Institute of Aeronautics 8 Astronautics with permission. Page 1 38th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit 7-10 July 2002, Indianapolis, Indiana IAA 2002-4139 Copyright © 2002 by the author(s). Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. INTRODUCTION In 2000 Pressure Systems, Inc. (PSI) was contracted to provide a full complement of propulsion system tanks for the MESSENGER spacecraft. These tanks include a 12-inch diameter elastomeric diaphragm tank, a 16-inch diameter by 30-inch long COPV helium pressurant tank, and three 24-inch diameter main propellant tanks-two fuel and one oxidizerof the same configuration. See Figure 1. Both the diaphragm and pressurant tanks are derivatives of existing PSI tank designs which required no development. However, the main propellant tank required significant effort for development and qualification. The MESSENGER propellant tank development program drew heritage from the NEAR program propulsion system tank development'. There were many similarities between the two programs, such as the initial trade study, the development of a vortex suppressor, and the propellant management philosophy of using a diaphragm tank to allow settling burns to settle propellant at the tank outlet prior to the start of main engine burns. However, there were also new challenges, such as the analysis and development of anti-slosh baffles, and the design, analysis, and test of a tank that's launched in an upside down orientation (outlet side on top). The main propellant tank development program was separated into three phases: (1) trade study, (2) analysis and design, and (3) fabrication and test. Figure 1, MESSENGER Propulsion System Tanks