The Advanced Cryogenic Evolved Stage (ACES)-A Low-Cost, Low-Risk Approach to Space Exploration Launch

Abstract

Space exploration top-level objectives have been defined with the United States first returning to the moon as a precursor to missions to Mars and beyond. System architecture studies are being conducted to develop the overall approach and define requirements for the various system elements, both Earth-to-orbit and in-space. One way of minimizing cost and risk is through the use of proven systems and/or multiple-use elements. Use of a Delta IV second stage derivative as a long duration in-space transportation stage offers cost, reliability, and performance advantages over earth-storable propellants and/or all new stages. The Delta IV second stage mission currently is measured in hours, and the various vehicle and propellant systems have been designed for these durations. In order for the ACES to have sufficient life to be useful as an Earth Departure Stage (EDS), many systems must be modified for long duration missions. One of the highest risk subsystems is the propellant storage Thermal Control System (TCS). The ACES effort concentrated on a lower risk passive TCS, the RL10 engine, and the other subsystems. An active TCS incorporating a cryocoolers was also studied. In addition, a number of computational models were developed to aid in the subsystem studies. The high performance TCS developed under ACES was simulated within the Delta IV thermal model and long-duration mission stage performance assessed. Pratt & Whitney Rocketdyne studied the effects of long-duration missions on the RL10 engine, and found that, with few exceptions which could be dealt with by design, the RL10 was suitable for the long EDS missions. The high performance TCS, when used in orbit, requires a thermodynamic vent system (TVS) and vapor cooled shields (VCS). The MLI/TVS/VCS architecture was optimized using the Boeing Design Sheet tool for two configurations. The first was “independent”, where both the LH2 and the LO2 tanks vented, and the vent gas used in a fuel cell to produce onboard power. The second was “integrated” where only the LH2 tank vented and the H2 vent gas used in the LO2 tank VCS to keep the LO2 tank vent free. All other affected EDS subsystems were studied for operation over long-duration missions. Most of the subsystems are technology extensions of currently operational and flying hardware with relatively low risk. The higher risk subsystems include the TCS, the engine packaging and performance, the autonomous rendezvous and capture mechanisms, the low heat leak skirt structure and micrometeorite-orbital debris protection. As a result of this study, a design was developed for an EDS suitable for long duration space exploration missions