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Auto-rotation in Martian Descend and landing

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Auto-rotation in Martian Descend and landing

The Entry, Descent and Landing System (EDLS) is one of the main system drivers for an interplanetary mission aiming at landing a payload on a planetary surface. The ultimate goal is to land safely the payload onto the planet surface. Towards that end, different constraints must be fulfilled in order to achieve a successful landing.

In this sense, autorotation appears to be a promising conceptual approach to decelerating a probe in the atmosphere of a planetary body. A rotor would offer greater control over the descent of a probe than a traditional disk-gap-band parachute. Future missions that require visiting a specific location in a small area, or landing at a location with relatively hazardous terrain may well require such early and precise control capabilities. The ARMADA project investigates the feasibility of using an autorotation system to replace all elements of a traditional EDL system for application at Mars, except for the aeroshell (that is, parachutes, retrorockets and/or airbags).

The ARMADA study approach consists of three main themes that are mutually interrelated:

  • General design of the layout of the autorotation system within the lander, including the deployment of the
  • rotor
  • Evaluation of the performance of the autorotation system by means of a software program designed for this
  • task,
  • Windtunnel testing of a model of the autorotation system, with a focus on the deployment system.

In general a strategy is followed that aims to keep the final design as close to current technological capabilities as possible, for the purpose of reducing to a minimum the amount of research and development needed to further mature the concept.

The confidence in the results obtained during the design of the ARMADA system is considered quite high, due to the very different methods used to obtain the results (that is, for example, the equilibrium descent velocity, the rotor drag coefficient, etc.). Some limitations have been exposed that are mainly due to the mechanics of the concepts and the maturity of the required technologies. These problems can be summed up as follows:

  • High mass fraction of the rotor, High mechanical complexity leading to reliability problems,
  • Too high equilibrium descent velocity,
  • Potential problems with vibration, and mechanical loads.

As mentioned, most of the problems are of a mechanical nature, and should be solvable in the future. Candidate solutions can be found in various fields.

  • Improvements in structures and materials could lead to lighter rotors, while control mechanisms integrated into the blade (possibly combined with ‘smart’ materials) could substantially reduce the mass of the control systems.
  • Improvements in materials may also mitigate the problem with vibration and mechanical loads, for example by means of active damping, and through mass-reduction.

The problem with the descent velocity can be resolved by relaxing the constraints on the system: although the aim of the system has been to replace all components of the traditional EDL systems, some additional braking systems could still be considered: tip rockets, airbags, or retrorockets. The optimal mix of these landing system ingredients could be investigated in future studies. In the near-term the problems outweigh the advantages presented by the ARMADA system, namely early trajectory control and high cross-range capability. The situation changes for applications at Titan and Venus; in this case, the required size of the rotor is quite small, so rotor blades can consist of a single piece, and the rotor can be constructed using current technology (made space-qualified). Compared with traditional EDL systems, the performance of ARMADA is not good enough to justify incorporating such a system in the short term. However, in the mid-term the concept may be a feasible option if an effective flare manoeuvre or the use of tip rockets is taken into account. It must also be remarked that in the mid-term scenario the value of the velocity at about 1-2 km AGL is comparable to the velocity of traditional EDL systems before the retrorockets ignition.

The results presented here do not include either a flare manoeuvre or tip rockets:

  • A flare manoeuvre, which is inherent to rotary systems, that converts 20 % of the energy stored in the rotor into a change in the velocity would allow an equilibrium descent velocity of up to 35 m/s and reduce the velocity to 20 m/s. This manoeuvre would work better for proportionally heavier rotors, meaning that they would work best for heavy landers.
  • Tip rockets could potentially allow the descent velocity to be reduced to 0 m/s. Moreover, tip rockets add mass to the tips, increasing the stored kinetic energy and also increasing the potential effectiveness of a flare.

Neither solution was sufficiently verified due to the limited amount of time available within the present study; both deserve more attention in the future. Further studies should be focused in this direction. Summarizing, in the mid- to long-term, and for application on Earth, Venus or Titan in the near-term, the concept may be quite an interesting and promising alternative to traditional EDL systems.

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Executive summary