327 An Approach to Designing Passive Sel f-Leveling Landing Gear with Application  to the Lunar Lander 

Troy B. Rippere * and Gloria J. Wiens* 

Abstract 

Once the lunar lander has touched down on the moon problems can occur if the crew module is not level.  To mitigate, compliant landing gear provide a solution  that would allow the module to be leveled once it  has landed on some ground slope. The work presented here uses compliant joints, or flexures, for each  leg of the module and optimizes the mechanics of th ese flexures such that the module can be passively  leveled over a range of landing slopes. Preliminary re sults suggest that for landing on a slope of up to 12  deg the effective slope of the module can be reduced to a maximum of 2.5 deg. 

Introduction 

During the next lunar mission one of the challenges t hat will be faced is the possibility of landing on a  slope of up to twelve degrees. Among other potential issues caused by landing on a slope, such as difficulties while offloading cargo, is the concern of “fly-out” problems duri ng ascent from the lunar  surface. By considering the lunar module and its four  deployable legs as a single spatial mechanism, the  legs can be designed in a novel manner with t he objective of passive self-leveling in mind.    Additionally, to avoid issues associated with lubricating joints this problem can be approached using compliant joints, or flexures, which are thin memb ers that provide the relative rotation between two  adjacent rigid members through bending. Previous work  has shown how flexures  can be approximated as  linear torsion springs with stiffness k [1]. From this, a variety of techniques can be applied to find  appropriate flexure stiffness of each jo int so that the combination of the weight of the lunar lander and the  resistance to deflection from the flexures will effectively reduce the relative angle of the module with respect to the horizontal plane over t he range of potential ground slopes. 

Background   It should be noted that the approaches presented here model the module with four identical equally spaced legs. Furthermore, each leg is designed to have a joint connecting it to the module, a joint connecting it to the landing pad at the bottom of the leg, and a third joint spaced somewhere between the first and second joints.  Assumptions

For these analyses, the following initial assumptions were made:  1. The landing area of the lunar surface can be mode led as a plane. In other words, there are no  craters or boulders that would affect a single leg.  2. There is a uniform probability of landing on any  ground slope between zero and twelve degrees.  3. There is a uniform probability of landing at any twist angle. Here twist angle is defined as rotation  of the module in the ground plane. A 0 deg twist angle means that the line between one set of two opposing pads is orthogonal to the local gravity field, or that those two pads are level. A 45 deg twist angle means that line between two adjacent pads is orthogonal to the local gravity field, or  that they are level. It follows that the other set of two adjacent pads will also  be level with respect  to each other.                                                    * University of Florida, Gainesville, FL 

Proceedings of the 40th Aerospace Mechanisms Symposium, NASA Kennedy Space Center, May 12-14, 2010  NASA/CP-2010-216272