Airbag-Based Crew Impact Attenuation Systems for the Orion Vehicle – Single Airbag Optimization Anonymous MIT Students Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, MA, 02139 Airbag-based methods for crew impact attenuation have been highlighted as a potential means of easing the mass constraints currently affecting the design of the Orion Crew Exploration Vehicle. This paper focuses on both the single and multi- objective optimization of a simplified version of such a system using both gradient-based and heuristic methods. From this, it is found that for systems implemented with pressure relief values, the optimal design is one with the minimum geometry such that bottoming-out does not occur. Moreover, maintaining this condition while varying the burst pressure of the valve was found to correspond to moving along points on the Pareto front of impact attenuation performance and system mass. I. Introduction Traditionally, manned Earth reentry vehicles have used rigid structures supported by shock-absorber type systems to protect astronauts from the impact loads incurred upon landing. These systems have consistently proven to be reliable and capable for their intended function on vehicles of the past. However, the advantages of their impact attenuation performance over their mass penalty have been questioned for more capable and inherently complex spacecraft systems. This is particularly the case for the recently redirected Orion Crew Exploration Vehicle program, where the mass budget was constantly under strain due to highly demanding performance requirements In response to this, airbag-based systems have been identified as a potential alternative to this concept, as shown in Figure 1. Figure 1– (a). Orion CEV Baseline Configuration (b). Orion’s Baseline Crew Impact Attenuation System (c). Airbag-Based Crew Impact Attenuation System Concept to be Investigated The main advantage of such a configuration is its significantly lower mass relative to traditional crew impact attenuation system (CIAS) designs, whilst having potentially comparable or even improved performance. An additional benefit includes the ability of airbag systems to be deflated and stowed away, hence providing additional in-cabin volume once the spacecraft is in orbit. Initial estimates have found that compared to the baseline design of the Orion CEV, these savings equate to a potential 36% reduction in CIAS mass without the crew, and an increase of 26% in in-orbit habitable volume1. * Graduate Research Assistant, Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, MA, 02139, and AIAA Student Member 1 American Institute of Aeronautics and AstronauticsII. System Modeling Airbags attenuate impact loads by converting the kinetic energy of an impacting event into the potential energy of a compressed gas, and then venting this gas to remove energy from the system. In order to model this phenomenon, several disciplines and their interactions need to be captured. These include thermodynamics, fluid mechanics, and structural dynamics. For the current study, these disciplines were combined to model the impact dynamics of a single airbag, rather than that of a multi-airbag system (see Figure 2). This decision was made after a failed attempt at modeling the dynamics of a multi-airbag system in a robust and reliable manner within the time constraints imposed on this study. Thus, it is intended for the framework developed in this study to be incorporated with a multi-airbag model once it is developed in the future. (a). (b). Figure 2 – (a). Original Airbag-based Crew Impact Attenuation System Concept (b). Simplified System Model (Inset – Flapper-type Pressure Relief Valve) As can be seen in Figure 2, the system modeled consists of a cylindrically shaped fabric bladder, filled with a gas at a given pressure and loaded with a test mass. In addition, the concept also consists of a venting mechanism, which facilitates the expulsion of gas from the system. Currently, the design of this venting mechanism is fixed to be a flapper-type pressure relief valve of fixed venting area, but with adjustable burst pressure, as shown in Figure 2(b). Also fixed are the operating medium, being air, the impact velocity of 7.62m/s, which corresponds to the nominal impact velocity of the Orion vehicle, and the environmental parameters of gravitational acceleration and standard atmospheric pressure. These are summarized below in Table 1. Table 1 – System Fixed Parameters Fixed Parameter Value Comments Airbag Geometry Cylindrical Selected for ease of manufacture Venting Mechanism Concept Flapper-type pressure relief valve Design fixed before this study Venting Area Equivalent to a 2 x Ø2” area Fixed before this study. May be revisited as a design variable in the future Operating Medium (γ) Air (1.4) Compatible with spacecraft cabin atmosphere Impact Velocity 7.62m/s Nominal impact velocity of Orion CEV Gravitational Acceleration 9.81m/s2 Landing on Earth’s surface Atmospheric Pressure 101.325kPa Assumed landing in standard atmospheric conditions Test Mass 2.5kg Corresponds to mass of occupant head. This will be the test condition for this study Based on the information given in Table 1, the design variables can be defined as the remaining values required to characterize the impact dynamics of the system. These, along with their bounds are given in Table 2. Table 2 – System Design Variables Design Variable Range Comments Radius [R] (m) 0.1 ≤ R ≤ 0.5 Lower geometry bounds correspond to geometric requirements of the human body, whilst upper bounds correspond to space constraints within the Orion CEV cabin Length [L] (m) 0.3 ≤ L ≤ 0.85 Inflation Pressure [PbagI] (Pa) PbagI ≥ 101325 Airbag internal pressure must be ≥ atmospheric pressure in order to maintain its shape Valve Burst Pressure [∆Pburst] (Pa) ∆Pburst > 0 Measured as pressure in addition to inflation pressure. This value must be positive to allow for the airbag to inflate initially (ie. initial pressure < pressure at burst) 2 American Institute of Aeronautics and AstronauticsIn regards to the system metrics, impact attenuation capability and mass have been selected as the basis for the optimization framework developed. Here, impact attenuation capability is measured by a metric known as the Brinkley Direct Response Index. This index measures the