# Dynamic Modeling and Simulation of a Robotic Lander Based on Variable Radius Drums
## 6 Conclusions  
The use of a passive shock absorber, based on the use of VRDs, cables, and preloaded springs, in landers for space applications has been investigated in this paper. The design of a possible lander based on this mechanism has been proposed. In order to prove the effectiveness and the soundness of the analytical model of the dynamics of the lander the behavior of the lander has been investigated numerically, first by implementing the analytical model, and then by mean of the multi-body simulator ADAMS on a physical 3D CAD model of the proposed lander. Seen the similarities between the results obtained using the analytical model and the results obtained in ADAMS we can truly say that the analytical model describes well the behavior of the lander for the case of soft-landing on a flat surface. However, it has been observed that, with respect to ADAMS, the analytical model overestimates the contact force and the acceleration the lander is subject to during impact. This discrepancy might be due to several reasons. The main reasons are the fact that the landing pad shape causes the contact point to change during the leg rotation and that the ADAMS model includes the three ratchets, contrary to the analytical one. Moreover, the contact models are different, i.e., in the analytical model a point to point contact model is used, while in ADAMS it is a solid-to-solid contact model; furthermore, in ADAMS the damper used in the contact is nonlinear while in the analytical model a liner damper is used. Finally, in the simulations it has been observed that this kind of passive landing system guarantees a certain degree of control on the forces and accelerations acting on the lander structure during soft-landing.  
Future works foresee the design and fabrication of a real prototype based on the one that has been studied in this paper. Considerations on the ground characteristics will be the basis for the design of the main components of the lander, e.g., the landing pads. Experimental tests will be carried out to further validate the models and to gain insight on the mechanics of the soil. For example, drop tests and deceleration tests will be tailored on those described in this paper and performed using regolith simulants. The results obtained will be compared with the numeric results obtained in this paper.  
Moreover, a detailed research about implementing an active control system acting on the preloaded springs will be considered and implemented. Adjusting the preload allows to reshape the VRD response in order to adapt lander to land on different scenarios, or when the ground is at an incline. In this specific situation, a softer action on some legs would allow the lander to avoid toppling or even experience lower deceleration. Other uses of the proposed technology will be explored in the future, especially in fields unrelated to space exploration.  
Conceptualization, M.C. and S.S.; methodology, S.S., L.S., and P.G.; software, M.C. and S.S.; numerical validation, M.C.; experimental investigation, M.C.; resources, S.S. and P.G.; writing--original draft preparation, M.C.; writing--review and editing, S.S., P.G., and L.S.; supervision, funding acquisition S.S. and P.G. All authors have read and agreed to the published version of the manuscript.  
This work has been partially supported by the PRIN project \"SEDUCE\" n. 2017 TWRCNB, and by the internal funding program \"Microgrants 2020\" of the University of Trieste.  
The authors declare no conflict of interest.