Introduction

The Apollo flights during the 1960's proved man's ability to reach the surface of the moon. Now, the goal is the production of oxygen on the lunar surface. Oxygen produced on the moon could be used for Earth-return fuel-oxidizer or for life support of a permanent lunar colony. Tests on Earth using soil returned from the moon have revealed that significant amounts of oxygen can be extracted from the material by several different processes. The proof of oxygen production on the lunar surface amid its known resources and still unknown features is the next step in the quest of a permanent colony on the moon.

The Texas Space Grant Consortium has taken on this challenge by creating the Interdisciplinary Design Project. The main objective of this project for the student teams is to develop a preliminary design for a small lunar lander to demonstrate the production of oxygen on the surface of the moon. This unmanned mission will be the first of its kind to test this technology on the moon. The main goals of this project have been developed to be:

1. Successful launch of the spacecraft,
2. Successful trip to the moon,
3. Land the oxygen plant on the lunar surface,
4. Perform oxygen experiments in-situ and report the results back to earth,
5. Perform other scientific experiments as mass permits,
6. Generate public interest and awareness in the current space program.

Upon completion of this project, a design proposal will be submitted to NASA in the hopes of obtaining government funding and industrial support to fly the mission. The final spacecraft project goal of $75 million US dollars is projected in order to make this mission financially feasible.

The proposed mission description is as follows:

1. Launch the spacecraft using a small, inexpensive booster
2. Spacecraft follows a ballistic trajectory to the Moon,
3. Spacecraft may orbit the Moon,
4. Spacecraft lands at lunar dawn on the near side of the lunar surface,
5. Lunar soil is collected as the raw material for oxygen production,
6. Oxygen production experiment is performed,
7. Experiment results are transmitted back to Earth,
8. Secondary experiments are performed (mass permitting),
9. Survive the 14 day lunar night and
10. Repeat steps 5-9.

The major systems of the project have been divided among eleven Texas universities. Graduate students at the University of Texas will coordinated the project and act as the systems integrators. Each team is responsible for the design of their system in accordance with specifications provided by UT and in conjunction with other systems that it interfaces with. Communications be will primarily over the Internet via e-mail and a World Wide Web home page with teleconferences and other communications planned as the need arises. Each team is responsible for weekly progress reports, this mid-fall Conceptual Design Review, and an end-fall Design Report and oral Design Review. The task assigned to this group of students at Lamar University is the Landing Gear Sub-System.

Scope of Effort

The scope of the landing gear subsystem team for this semester, fall of 1995, is to begin the process of designing a feasible landing system for the lander. In specific, several conceptual designs will be partially developed, a preferred conceptual design will be selected, a detailed evaluation of this preferred design will be performed, and our design and conclusions will be documented.

Design Approach / Methodology

DESIGN SPECIFICATIONS

The total mass of the lander will be important in determining how much energy will need to be absorbed and the static load to be supported after touchdown. According to the subsystems mass budget (8/18/95), the mass at launch is scheduled to be 520 kg. If the 245 kg of fuel is depleted prior to landing, the remaining mass would be 275 kg. Because additional payload is expected, the landing gear will be designed for a 400 kg total lander mass at touchdown. This value was derived from the idea that additional payload will be added before completion of the project. Of equal importance is the mass budget of the landing gear structure, which will be taken as 10 kg.

The landing regime was initially assumed to be a free fall to the surface from 10 meters. The calculated velocity at impact would be 5.7 m/s. The conceptual designs in this document will reflect this. As of the GNC progress report of 9/22/95 the landing regime has been changed to be a controlled soft touch landing. The expected maximum vertical and horizontal velocities listed in this document are 6.0 m/s. The conceptual designs can accommodate this change.

The maximum acceleration to protect the internal equipment is assumed to be 10g’s. To account for the variation in acceleration during the impact, 5 g’s was assumed to be the maximum average acceleration. Based on this, 33 cm of deflection was calculated for the landing gear system, including possible soil deformation. These assumptions were based on data from the Surveyor V landing data.

Another important requirement is that the landing gear structure must integrate with the existing bus structure. The forces on the bus structure must be distributed adequately to prevent failure. Also the integration with other components on the lander may be important such as propulsion equipment and other subsystem equipment. The other key importance is the integration of the lander structure with the launch vehicle. The launch vehicle is currently assumed to be a Delta II Rocket.

A third specification to be considered in the design is possible lunar conditions. Some of these conditions that effect the landing gear are the possibly temperature variations and lunar surface conditions.

RATING CRITERION

1. Feasibility of each design is considered to be the most important. The designs must meet the design specifications to function properly. This will receive priority among other criteria due to its critical importance.

2. Compatibility of the landing gear determines if the design will fit into the launch vehicle. If the landing gear is found feasible, but will not fit the geometry of the launch vehicle, then it can not be used.

3. Simplicity will assure that the design has a low cost to actually manufacture. Also the weight of the landing gear may be kept to a minimum with a simple design. The reliability is increased with a design that contains simple members and actuation.

4. Vertical travel exhibited by the geometry of the members during impact gives some indication of the ability to absorb impact energy to protect equipment contained in the bus structure.

5. Surface contact of the foot with the lunar surface is unpredictable due to possible obstruction and surface discontinuities. The foot of the landing gear in contact with the lunar surface needs to have a minimum horizontal movement.

The conceptual designs will be measured against these five criterion. This will produce several landing gear designs that are suitable to landing the oxygen plant on the surface reliably and safely.

CONCEPTUAL DESIGN ALTERNATIVES These design specifications have led the team members to a number of perceptions of the physical structure which will make up the landing gear system for the oxygen plant lunar lander. The feasibility of each conceptual design has obtained the greatest amount of interest up to this point. A feasible design for the lunar lander is expected to meet the criteria set by the design specifications. The major feasibility specifications being designed for include the preliminary travel zone to be experienced by the landing gear structure, the dimensions for which the structure must fit within during the launch phase, structural integrity and simplicity, and preliminary placement and selection of the energy absorption devices to control the landing accelerations.

Several side view drawings of the designs being considered have been included in this report. The nature of the energy absorption devices illustrated have not been determined. Each design illustrated on the following pages has the potential to be developed into a feasible design for the lunar lander landing gear. The motion of each of the conceptual designs considered will be observed through the use of Fourbar Software.

Design Alternative 1:

The simplicity of this design gives it a high reliability factor. Due to its small number of components, the weight of this structure should also be feasible. It has not been determined if this design will be able to carry the loads to be experienced upon impact.

Design Alternative 2:

The horizontal movement of the footpad for this design calls for little horizontal translation while the landing gear structure is being compressed. The fold-up capability of this design could prove to be feasible with the design specifications set. The single energy absorption device aids in maintaining a low weight for the landing gear structure.

Design Alternative 3:

Any vertical or horizontal velocities to be absorbed upon impact could be accounted for in this design. The long compression member directly above the footpad could pose a weight problem upon final sizing of the members in order to reduce the possibility of buckling. The necessity for two energy absorption devices also introduces excess weight to the design.

Design Alternative 4:

 This design could be sized to allow for optimum sizing of a spring and damper shock absorption system. The loading function of each device could be optimized in order to accommodate a minimum weight while maintaining performance characteristics.

Design Alternative 5:

A minimal horizontal translation is maintained in this design. The use of a single energy absorption device helps to reduce the total weight of the design. The fold-up capability of this design may also prove feasible with mechanically stored energy for deployment of the landing gear structure.

The energy absorption devices being considered include coil springs, dampers, gas shocks, crush zones, air bags, and torsional springs. The landing gear design will include one or a combination of several of these devices. The selection of the optimum energy absorption device will be determined according to the loading profile needed for a smooth landing. The characteristics of these typical suspension system are noted here for future reference.

A coil spring and damper system could be designed to apply a fairly even load through the landing. This system is typical for automobile suspensions. The technology to design such a system is available. The weight of a spring and damper system may exceed the desired maximum allowed in the design.

A gas shock system may be designed to take the loads during landing without the excess weight inherent of a spring and damper system.

Crush zones may be designed into the landing gear system at the feet and between the bus structure and the landing gear members. The crush zones allow for a controlled failure, protecting the remainder of the landing gear structure from high stress. Crush zones are fairly light in weight and will work with other energy absorption devices.

Torsional springs could be used at the pivoting joints on the landing gear structure. They have also been considered for a mechanical energy storing mechanism used to deploy the landing gear structure.

A completely different approach to the landing technique has been considered through the use of an air bag deployment. The technology for such a landing system is not readily available, but the system does seem feasible and the light weight of materials gives it an advantages. This type of system is to be used next year in the Mars Pathfinder.

CONCLUSION:

After the examination of previous designs, the group members began a basic brainstorming of ideas for a possible landing gear structure configuration. At this point the group is continuing brainstorming and will rate the conceptual designs and chose the ones most feasible for the objective of the Texas Space Grant Consortium project. The group plans to develop each of these designs in more detail, modify them to meet our requirements, and create several workable design for the project. Some of the things that must be examined in each of these conceptual designs is the feasibility, ability to fit in a launch vehicle, the ability to provide the safe landing of the oxygen production plant as well as the other objectives stated earlier in this report.

After the completion of preliminary development of conceptual designs chosen to date, the group will narrow the options to one design. The process used to make the final selection of a conceptual design will be a rating system. The rating system will assign a numerical value to each specification in order of importance. The design which garners the highest rating will be selected as the primary candidate for development of a final preliminary design. This is what the Lamar Landing Gear Group expects to have completed by the end of the semester. To this point the group is on schedule to complete our semester objective of obtaining a detailed conceptual design.