Touchdown stably is a precondition for exploring a small celestial physique in situ. The floor of a small celestial physique incessantly has weak gravity and is irregular, and the floor setting is unknown and unsure. The touchdown mechanism tends to rebound and switch over, and the touchdown stability time is lengthy. Nonetheless, whereas most touchdown efficiency analysis has centered on lunar touchdown, there are variations between the surfaces of the moon and Mars.
Due to this fact, it necessary to check touchdown efficiency in several circumstances with the intention to analyze the touchdown stability boundary, and to suggest cheap touchdown ideas to assist China’s small celestial physique exploration.
In a analysis article lately printed in House: Science & Know-how, researchers from Beijing Institute of Spacecraft System Engineering, Harbin Institute of Know-how, and Polytechnic College of Milan have established a simulation mannequin of a touchdown mechanism underneath completely different touchdown circumstances, analyzed the sensitivity of the important thing parameters affecting the touchdown efficiency, and verified correctness of the simulation by way of experimental assessments, which might present steerage for a touchdown mechanism to land stably on a small celestial physique.
First, the authors briefly reproduce the touchdown mechanism and the touchdown simulation. The small celestial physique touchdown mechanism used within the simulation comprises a touchdown foot, touchdown legs, cardan aspect, damping aspect, tools base, and extra. In simulation, two eventualities are considered: the touchdown mechanism touchdown towards the touchdown slope with Vx > 0; and the touchdown mechanism touchdown away from the touchdown slope with Vx < 0.
In every situation, three touchdown modes are categorized in accordance with the contact order between the touchdown foot and the touchdown slope, i.e. (a) 1-2 touchdown mode, (b) 2-1 touchdown mode, and (c) 1-1-1 touchdown mode (with 30° yaw angle). For all touchdown modes in each simulation eventualities, the touchdown mechanism turnover is prevented by the retro-rocket, and there’s no sliding of the touchdown toes.
The utmost overloading acceleration of the tools base is lower than 10 g, and the touchdown stability time is lower than 4 seconds. This reveals that the touchdown mechanism can land safely in several touchdown circumstances. Moreover, when Vx > 0, the analysis reveals that the 2-1 mode has the most effective touchdown efficiency amongst three modes, and 1-2 and 1-1-1 modes’ touchdown performances are related. When Vx < 0, the touchdown efficiency of the 2-1 mode is the most effective, the 1-2 mode is normally, and the 1-1-1 mode is the worst.
Secondly, key elements affecting the touchdown efficiency are analyzed.
Cardan aspect damping (c2)
The touchdown stabilization time is considerably shortened and the overloading acceleration is weakened when c2 is variable compared to fixed c2. The touchdown mechanism has higher touchdown efficiency when c2 is variable.
The foot anchors have an effect on the friction coefficient between touchdown toes and the touchdown floor. Slipping induces the touchdown mechanism distant from the touchdown level, which might have an effect on the anchorage of the anchoring system. Friction between the touchdown mechanism and the touchdown floor ought to be excessive to keep away from sliding of the touchdown mechanism. Overturning of touchdown mechanism because of excessive friction might be eradicated by retro-rocket thrust. Due to this fact, it’s useful to design foot anchors on the touchdown mechanism, as it could penetrate the touchdown floor and forestall or weaken sliding of the touchdown mechanism.
Retro-rocket thrust can stop the touchdown mechanism from bouncing or turning; thus the retro-rocket thrust is useful for touchdown efficiently.
The bigger the slope angle is, the upper the turning angular velocity of touchdown legs is, and the longer the touchdown stabilization time is. The affect of slope angle on tools base overloading acceleration just isn’t apparent. Due to this fact, the touchdown floor with smaller slope angle ought to be chosen to cut back the touchdown stabilization time.
When the touchdown mechanism lands in several touchdown attitudes inside the allowable touchdown velocity, the utmost overloading acceleration is lower than 10g and the touchdown stabilization time is lower than 5 seconds. Touchdown efficiency is nice. When the yaw angle is 60° (that’s, the 2-1 touchdown mode), the touchdown mechanism experiences the minimal overloading acceleration and the shortest touchdown stability time, and the touchdown efficiency is the most effective.
Then, the validity of the simulation mannequin is verified by assessments. These assessments are carried out on the air-floating platform. The touchdown accelerations are measured by acceleration sensors. Assessments of touchdown on a 30° slope within the 1-2 mode, the 1-2 mode, and the 1-1-1 mode are carried out individually. These touchdown modes and velocities are imported into the simulation mannequin. Touchdown performances between take a look at and simulation are in contrast.
The overloading acceleration of the tools base obtained by simulation is near that obtained by take a look at, and the simulation result’s barely bigger than the take a look at. That is as a result of mechanical flexibility of the touchdown mechanism, which is able to produce versatile deformation within the take a look at and take up a part of the influence load. The modifications of touchdown leg turnover angular velocity and turnover angle in simulation and take a look at are comparatively constant.
However between about 0.7 seconds and a pair of.5 seconds within the 1-2 mode, about 0.5 and a pair of seconds within the 2-1 mode, and for the entire length of the 1-1-1 mode, the touchdown leg turnover angle in take a look at is lower than that in simulation. The reason being that touchdown floor in take a look at is tough wooden and the foot anchors fail to penetrate the arduous wooden, which leads to a slight slip of the touchdown mechanism. As well as, it’s discovered that the 2-1 touchdown mode has the shortest stability time, and there’s no apparent relationship between the overloading acceleration and the touchdown mode.
Lastly, authors come to the conclusion that the next strategies are useful to enhance touchdown efficiency:
- A 3-leg touchdown mechanism ought to preferentially select the 2-1 touchdown mode.
- Adjustable damping comparable to touchdown circumstances is useful to enhance touchdown stability.
- Foot anchors can cut back touchdown slip and shorten touchdown stabilization time. A retro-rocket on high of the touchdown mechanism can weaken or stop rebounding when touchdown.
- The touchdown mechanism ought to preferentially land on flat areas.
Zhijun Zhao et al, A Legged Small Celestial Physique Touchdown Mechanism: Touchdown Simulation and Experimental Take a look at, House: Science & Know-how (2023). DOI: 10.34133/area.0066
Beijing Institute of Know-how Press Co.