Terrestrial Environment Issues
Experience gained in developing terrestrial environment design criteria for previous aerospace vehicle programs has proven that to be most effective, the terrestrial environment design criteria for a new launch vehicle should be:
(a) Available at the inception of the program and based on the desired operational performance for the launch vehicle.
(b) Issued under the signature of the program manager and be part of the controlled program definition and requirements documentation.
(c) The design criteria document should specify the terrestrial environment for all phases of activity including pre-launch, launch, ascent, on-orbit, descent and landing.
Terrestrial environment phenomena play a significant role in the design and flight of all launch vehicles and in the integrity of the associated systems and structures. Terrestrial environment design criteria guidelines are based on statistics and models of atmospheric and climatic phenomena relative to various launch vehicle development, operational, launch and landing locations.
For terrestrial environment extremes, there is no known physical upper or lower bound except for certain environmental conditions; for example, wind speed does have a strict physical lower bound of zero. Essentially all observed extreme conditions have a finite probability of being exceeded. Consequently, terrestrial environment extremes for design must be accepted with the knowledge that there is some risk of the values being exceeded. The measurement of many environmental parameters is not as accurate as desired. In some cases, theoretical model estimates of design values are believed to be more representative for design use than those indicated by empirical distributions from short periods of record. Therefore, theoretical values are given considerable weight in selecting extreme values for some parameters, i.e., the peak surface winds. Criteria guidelines are presented for various percentiles based on available data samples. Caution should be exercised in the interpretation of these percentiles in launch vehicle studies to ensure consistency with physical reality and the specific design and operational problems of concern.
Launch vehicles are not normally designed for launch and flight in severe weather conditions such as hurricanes, thunderstorms, and squalls. Environmental parameters associated with severe weather which may be hazardous to launch vehicles include strong ground and inflight winds, strong wind shears and gusts, turbulence, icing conditions, and electrical activity. Terrestrial environment guidelines usually provide information relative to severe weather characteristics which should be included in design requirements/specifications.
Assessment of the terrestrial environment in the early stages of a new launch vehicle development program is advantageous in developing a vehicle with a minimal operational sensitivity to the environment. For areas of the terrestrial environment that need to be monitored prior to and during tests and operations, this early planning will permit development of the required measuring and communication systems.
Knowledge of the terrestrial environment is necessary for establishing test requirements for launch vehicles and designing associated support equipment. Such data are required to define the fabrication, storage, transportation, test, preflight design conditions and should be considered for both the whole system and the components which make up the system. This is one of the purposes of guideline data on terrestrial environment conditions for the various major geographic locations applicable to the design of a new launch vehicle and associated supporting equipment.
Good engineering judgment must be exercised in the application of terrestrial environment inputs to launch vehicle design analysis. Consideration must be given to the overall vehicle mission and system performance requirements. Knowledge is still lacking on the relationships between some of the terrestrial environment variates which are required as inputs to the design of launch vehicles. Also, interrelationships between vehicle parameters and terrestrial environment variables cannot always be clearly defined. Therefore, a close working relationship and team philosophy must exist between the design/operational engineer and the respective organization's terrestrial environment specialists. Although, a launch vehicle design should accommodate all expected operational environment conditions, it is neither economically nor technically feasible to design launch vehicles to withstand all terrestrial environment extremes. For this reason, consideration should be given to protection of launch vehicles from some extremes by use of support equipment and specialized forecast personnel to advise on the expected occurrence of critical terrestrial environment conditions. The services of specialized forecast personnel may be very economical in comparison with more expensive designing which would be necessary to cope with all terrestrial environment possibilities.
In general, terrestrial environment requirement documents do not specify how the designer should use the data in regard to a specific launch vehicle design. Such specifications may be established only through analysis and study of a particular design problem. Induced environments (vehicle caused) may be more critical than terrestrial environments for certain launch vehicle operations. In some cases the combination of natural and induced environments will be more severe than either environment alone. Induced environments are considered in the launch vehicle design criteria documents and should be consulted for such data.
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