Structural Specification Sheet

Tim Crain
August 1995

The structural subsystem of a spacecraft is analogous to the skeleton of the human body in function and form. Both serve to carry loads for motion through inertial space and to maintain the shape of a covering membrane that protects internal components against adverse environmental factors. Specifically, the spacecraft structure will serve to: (1) carry loads applied during launch, lunar delivery, and landing; (2) support the spacecraft thermal material, the skin of the craft; and (3) provide the internal components with infrastructure attachment points and micrometeorite impact protection for the duration of the mission, in space and on the lunar surface.

Structural Constraints:

  1. Total internal equipment volume and mass must be calculated and must be accommodated by the launch vehicle chosen by the propulsion group.

  2. Component acceleration tolerances must be taken into account when designing the landing gear and number of attachment points per component. The landing regime will be defined by the Navigation, Guidance, & Control (NGC) group in coordination with the Structures group. The landing accelerations must be moderated by the landing gear to prevent damage to internal equipment.

  3. Launch and landing loads must be within the dynamic load parameters and ultimate strength limits of the system. Launch system loads will be provided by the propulsion group.

  4. Component mapping must be conducted in coordination with the NGC group to determine the spacecraft mass properties, and thus, spacecraft stability.

Structural Concerns by Mission Phase:

Pre-Launch Phase

The pre-launch environment must be considered for the possible Earthside effects on structural and internal hardware. The coastal location of nearly any NASA launch site carries associated airborne moisture and salt concerns. Also, temperature variations Earthside will be less than those on the moon but may be aggravated by the presence of moisture. The possibility of unexpected launch delays should be investigated for the effects that extended storage could have on the performance of the structure subsystem.

Launch Phase

The transition from Earth to space will introduce acoustic and dynamic loads to the spacecraft. The magnitude of the launch loads will probably not exceed 7 g's and operate at fundamental frequencies of 15 Hz radially and 35 Hz axially. The internal hardware attachments to the spacecraft bus structure should be analyzed for this phase. Also, it is suggested that uniform attachments should be used throughout the spacecraft.

Orbital Phases

This phase of the mission addresses the environmental concerns presented to a spacecraft by the conditions of interplanetary space. Radiation protection for all sensitive internal hardware, especially computer equipment, should be provided through the use of non-load bearing skin panels. The level of protection provided by these panels will be a function of the susceptibility of hardware to radiation and the contingency needed to withstand possible increased solar activity during lunar transit. Thermal effects must also be provided during this phase, although this is the responsibility of the Thermal group, it is possible that the radiation protection system could double as thermal insulation.

Micrometeorite protection should also be provided by the skin panels but further study needs to be conducted to resolve what level of kinetic energy protection from incoming particles should be provided. Outgassing from the spacecraft materials could also be a problem during this portion of the mission. Material selection should consider outgassing as a criterion.

The spacecraft bus structure should be able to withstand the NGC actuation and propulsion forces needed for maneuvering and orbit changes. Careful planning with these groups should be made for thruster placement, thruster strength, and momentum wheel placement. The spacecraft should be designed so that any antennae, solar panels, or other externally deployed equipment will not be physically hindered in there operation by the spacecraft structure.

Landing Phase

Attitude actuation and main engine translation loads will play a major role in this phase of the mission. Likewise the external hardware must not be hindered by the physical proximity of the spacecraft as it approaches the lunar surface. There is a possibility, to be determined by the communications and science groups, that a microsatellite and/orseveral water detection penetrator devices will be deployed in this phase of the mission. Loads and support structures for these activities must be accounted for in the general bus design.

As mentioned previously, the actual landing regime is yet to be determined. The structures group should work in close coordination with the oxygen production and propulsion teams to ascertain the jeopardy posed to the oxygen production mission by lunar soil sample contamination by landing exhausts. The possibility of the use of a lunar rover for sample collection may make a soft touch landing possible even if the immediate landing site is contaminated. The likelihood of a controlled freefall landing has been discussed and would include a ten meter drop to the lunar surface. The landing gear and possibly a crushable material system, similar to that used in military equipment airdrops, must limit the acceleration of the general bus structure to the g-force tolerances imposed by the WEAKEST internal hardware member. The prime consideration of the landing accelerations is the SURVIVAL OF THE OXYGEN PRODUCTION PLANT AND THE SOIL COLLECTION SYSTEM. Currently 10 g's has been set as an arbitrary maximum for the landing accelerations of the bus structure and its internal payload. Also, the final maximum displacement of the spacecraft from the lunar surface horizontal must be considered of reasons of spacecraft stability and operation of possible gravity intensive devices such as shakers on the oxygen production plant.

Post-Landing Phase

After landing, the structural subsystem will operate as a platform for the lunar Oxygen production experiment. Radiation, thermal, and debris impact protection must be provided as well as secure platforms for communications and power arrays. The spacecraft will hopefully survive the 14 Earth day lunar night. This goal should be included in design considerations. The very fine and very sharp edges of the lunar soil must be protected against. Although there is no weather per se on the moon considerable dust may be kicked up at landing. Any movable parts on the spacecraft must be properly sealed to insure that they do not become contaminated by lunar debris and cease to function. This consideration is very similar to the problem that the desert sands present to tank commanders in the military.

Structural Suggestions and Questions:

Structural Interfaces

Input Group Interface/Information Output Group

Initial Spacecraft Mass EstimatePropulsion
PropulsionLaunch Vehicle Payload Size Constraints
PropulsionRefined Spacecraft Mass Estimate
All SystemsComponent Mass/Volume/Mounting Constraints
All SystemsComponent Acceleration Limits
All SystemsDynamic Loading Constraints
All SystemsMicrometeorite Protection Requirements
All SystemsRadiation Protection
All SystemsThermal ProtectionThermal
ThermalThermal Analysis/Suggestions
Sensors/O2Sample Collection System
SensorsRover Integration
Sensors/O2Penetrator??? Integration
SensorsMicro Satellite Integration
PropulsionMain Propulsion Vibrations
NGCManeuvering Loads (DeltaV's, Torques)
NGC/PropulsionRCS/MPS Propellant Consumption
PropulsionRCS Thruster Placement
PropulsionRCS/MPS Propellant Tank Volume/Pressure
PropulsionRCS/MPS Pressurization Tank Volume/Pressure
NGC/Com.Antenna Pointing Requirements
CommunicationsComm/Antenna Placement
PowerElectrical Bus Grounding
PowerBattery/Solar Panel Attachment

Landing Phase Accelerations (max)NGC

Landing Hazard IdentificationNGC

Landing Design RequirementsNGC

Mass PropertiesNGC

Structure/Component LayoutThermal

Structural References

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