![]() Heritage airbag landing systems include early Soviet lunar landers, where airbags were used after landing to surround and cushion an ejected payload module, and more recently the NASA Mars Exploration Rover Mission operated a more advanced system of lobed Vectran airbags arranged in a tetrahedral structure around the rovers due to engine plumes, could cause the lander to become unstable ( Crisp2003MarsMission). A strong cleaner alternative with similar, if not improved, propulsive properties would be hydroxylammonium nitrate ( Spores2014GPIMSystem). Other alternative fuels are under consideration in the event that ESA moves to limit the use of hydrazine and other hazardous fuels. ![]() The resulting decomposition is highly exothermic and can be harnessed to produce up to 400N of thrust at over 200s ISP in existing motors. The most suitable and means-tested fuel mixture consists of liquid hydrazine passed over a granulated iridium-coated alumina catalyst prior to combustion ( Harden1965ThermodynamicHydrazine). Monopropellant engines also add additional flexibility in the form of being reignitable and offering a high degree of throttleability, making them ideally suited for descent engines. The monopropellant option has more precedents when it comes to landers such as Phoenix and Schiaparelli, as well as the Curiosity and Perseverance skycranes, simplifying the ignition process and reducing the likelihood of a hard start which would be mission-fatal at Triton. ![]() Hydrazine and its derivatives can be utilised in both monopropellant and bipropellant systems. The wealth of operational time and experience (TRL 9) of similar systems makes them an attractive choice. However, the low gravity of Triton means that this is acceptable as the minimum required thrust for safe deceleration remains low.Ī precedent established by the unmanned interplanetary landers to date is to utilise hydrazine-fuelled engines. The need to optimise propellant pressure against added fuel tank mass means there are practical limits on the combustion chamber pressure. This offers a high specific impulse and a comparatively low associated ‘cost’ in dry mass, due to the lack of required plumbing. ![]() As such, Bingham employs a pressure-fed system. To date, even advanced storage concepts would suit mission durations of 60-90 days, which is far too short for this mission and would require substantial mass gains ( Doherty2013CryogenicTDM). The expander cycle offers a high specific impulse but is hampered by the need for cryogenic fuel, the management of which would add significant operational complexity to the spacecraft: to prevent boil-off, the fuel must be shielded from even minimal temperature fluctuations. Looking to minimise complexity and simplify the construction of the spacecraft, while also limiting the potential for mechanical failure to arise during the coast phase of the mission, a pressure-fed or expander cycle system would appear most attractive. With the desire to maximise the lander mass available for instruments whilst minimising the total wet mass, a staged combustion engine cycle or similar would not be suitable. It is likely that a thruster-based system will be utilised for all or part of the terminal descent of the Bingham Lander.
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