1051. Parametric Weight Prediction Method for Spacecraft Vehicle

Paper

Abstract

This paper deals with the development of a unique digital computer program to predict the weight
of various types of manned spacecraft. The Spacecraft Parametric Weight Computer Program provides
the capability of predicting, synthesizing and trading-off weights for mannned spacecraft. Such a
program enables rapid disseminator of weight data to support Space Shuttle Vehicle studies.The
program is oriented to operate on a selected input data to produce both parametric and design
synthesis information. The two-fold objectives of this program is to provide a high quality weight
prediction method and parametric weight scaling of manned spacecraft with a reasonably short, e
conomical operation.
The main program is formulated with a series of subroutines assembled to simulate the actual sizing
and synthesis routines involved in any parametric analysis. Respective iterations are required to
accomplish the interrelationships of subsystems with mission and design requirements. This approach
is particularly applicable to the math model of the stage for sizing and weight scaling. For example,
unit weight equations for structure are replaced by parametric routines for loads and stress computations.
The program logic for successive sizing and weighing of the selected spacecraft stage is accomplished by
two unique iterative loops. Essentially, the first loop provides for matching of the estimated and
computed stage volumes by the application of subsystem packaging densities, expendables and internal
payload to dimensional scaling relationships. Another iteration loop involves the applicable performance
relationships which utilize the matching of estimated and computed stage mass fraction. Because of the
sensitivity to divergence of the mass fraction, three comparison checks are executed. The first check is
to determine if the estimated and computed propulsion mass fraction is within a tolerance of matching.
The second check to determine if the value of the computed value exceeds the estimated for the stage mass
fraction. The third check is accomplished by ensuring that the stage mass fraction is greater than the
performance mass fraction. This procedure resets the assumed value in larger increments steps of estimated
stage mass fraction until either the computed values matches the estimated or is less than the performance
mass fraction. In the event the performance mass fraction becomes greater than the stage mass fraction,
the previous estimated value is assumed and systematic reduction in size of the increment is provided until
a match of the computed and estimated values, within tolerance, are obtained or a preset indicator
terminates the iterations.