ExoMars Mission Analysis and Design from Mars Arrival to Landing
Luis Cano, Juan
Haya Ramos, Rodrigo
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ExoMars is ESA's next mission to planet Mars. The project is currently undergoing Phase B2 studies under ESA management. In that context, DEIMOS is responsible for the Mission Analysis support to Thales Alenia Space Italy as ExoMars Prime Contractor, covering all mission phases, including launch, interplanetary, Mars orbit injection and orbiting, down to entry, descent and landing (EDL). Tessella (previously under the name Analyticon) supports DEIMOS in the area of Descent and Landing Analyses. The current mission baseline is based on an Ariane 5 launch in 2013 of a spacecraft Composite made up of a Carrier Module (CM) and a Descent Module (DM). The trajectory profile is characterised by: a) direct transfer to Mars with an intermediate deep space manoeuvre (DSM), b) insertion into a 4-sol Mars waiting orbit (WO) and c) Composite descent from orbit, initiated by the Carrier, which then separates and burns up in the Martian atmosphere, while the DM goes on to complete the descent and landing. The role of the EDL Mission Analysis and Design is to provide support to the System Design Team for preparing the specification of the EDL System (EDLS) components, namely the heat shield, the two-stage parachute system, the throttleable retrorocket system and the vented airbags. At the beginning of Phase B2, a complete design loop of the DM is available, which means that the feasibility of the EDLS specification has been assessed through a detailed design of its components. Now, the Mission Analysis and Design activity has to provide support to the next design loop of the DM in which some of the mission requirements have been revised. In addition, the greater knowledge of the system after the first complete design loop allows a refinement of the EDL mission design methods and tools. The objective of this paper is to present the current mission design of the EDL phase of the ExoMars mission, focusing on the Mars Orbit Insertion (MOI) targeting up to the touchdown. Some specific analyses that are addressed in the paper are outlined in the following paragraphs. The first one is related to the connection between the arrival and the EDL phase. Landing from orbit rather than from infinity allows optimisation of the landing epoch w.r.t. solar conjunctions, the Mars Global Dust Storm Season, and other criteria. The evolution of the WO over several months in orbit before descent is a function of the targeting parameters (landing site and entry angle) and the arrival epoch. The consequence is that the EDL Phase is tightly linked, via the Waiting Orbit, to the Mars orbit insertion conditions. In other words, once the target landing site (and particularly the landing latitude) is selected, the target point in the B-plane of the arrival hyperbola will be fixed. After achieving the injection conditions in the proposed 4-sol orbit, the spacecraft will be required to wait in orbit until the foreseen Mars solar longitude is achieved for landing. In order to tackle this problem, the EDL mission design has been extended up to the Mars Orbit Insertion (MOI) in such a way that an end-to-end, i.e. continuous, mission profile from the MOI to touchdown is obtained. The orbit strategy prior to arrival to the Entry Interface Point (EIP, 120 km altitude) is based on the reduction of the trajectory dispersions at EIP. This is achieved in several ways, among others minimising the errors in the implementation of the DM targeting manoeuvre (DMTM) and reducing the size of the DMTM. The first effect is optimised by reducing as much as possible the errors in implementation of the actuator and achieving the best possible orbit knowledge through Orbit Determination (OD). The second effect is reduced by staging the reduction in perigee height, first reducing the perigee height to 250 km and in the next orbit finally targeting to the expected EIP. The result is that the dispersion at the EIP significantly reduces and the landing accuracy is below 25 km for the steepest entries compatible with the entry constraints (heat flux, heat load, load factor, drogue deployment window). These values have been verified by detailed end-to-end 3-DOF Monte Carlo simulations from the DMTM up to landing. The Global Entry Corridor (GEC) method has been extensively applied to identify the sizing trajectories for the design and to evaluate the feasibility of selected landing sites. Combination of the GEC with the Engineering Constraints (terrain slopes and footprint size) and Scientific Requirements allows the identification of feasible landing sites. The above mentioned link between the B-plane targeting and the EDL phase is considered in the GEC analyses. This paper also addresses the detailed evaluation of the performance of the mission in selected landing sites through detailed end-to-end Monte Carlo analyses (from DMTM to touchdown). Multibody simulations for the descent phase have been also run to verify the stability of the system and the sensitivity to perturbations (gust). These results serve not only as a verification of the mission design but also as a validation of the design methods and tools. The use of a parametric DLS sizing tool is foreseen to support further optimisation of the descent and landing system, in particular the allocation between the two stages of the parachute system and the retrorockets system.