Roger A. Lepsch

Response Surface Model Building and Multidisciplinary Optimization Using D-Optimal Designs

This paper discusses response surface methods for approximation model building and multidisciplinary design optimization. The response surface methods discussed are central composite designs, Bayesian methods and Doptimal designs. An over-determined D-optimal design is applied to a configuration design and optimization study of a wing-body, launch vehicle. Results suggest that over determined Doptimal designs may provide an efficient approach for approximation model building and for multidisciplinary design optimization.

Design options for advanced manned launch systems

Various concepts for advanced manned launch systems (AMLS) are examined for delivery missions to Space Station and polar orbit. Included are single- and two-stage winged systems with rocket and/or airbreathing propulsion systems. For near-term technologies, two-stage, reusable rocket systems are favored over single-stage rocket or two-stage airbreathing/rocket systems. Advanced technologies enable viable single-stage-to-orbit (SSTO) concepts. Although two-stage rocket systems continue to be lighter in dry weight than SSTOs, advantages in simpler operations may make SSTOs more cost effective over the life cycle. Generally, rocket systems maintain a dry weight advantage over airbreathing systems at the advanced technology levels, but to a lesser degree than when near-term technologies are used. More detailed understanding of vehicle systems and associated ground and flight operations requirements and procedures is essential in determining quantitative discrimination between these latter concepts.

Dual-Fuel Propulsion in Single-Stage Advanced Manned Launch System Vehicle

As part of the United States Advanced Manned Launch System study to determine a follow-on, or complement, to the Space Shuttle, a reusable single-stage-to-orbit concept utilizing dual-fuel rocket propulsion has been examined. Several dual-fuel propulsion concepts were investigated. These include: a separate-engine concept combining Russian RD-170 kerosene-fueled engines with space shuttle main engine-derivative engines: the kerosene- and hydrogen-fueled Russian RD-701 engine; and a dual-fuel, dual-expander engine. Analysis to determine vehicle weight and size characteristics was performed using conceptual-level design techniques. A response-surface methodology for multidisciplinary design was utilized to optimize the dual-fuel vehicles with respect to several important propulsion-system and vehicle design parameters, in order to achieve minimum empty weight. The tools and methods employed in the analysis process are also summarized. In comparison with a reference hydrogen- fueled single-stage vehicle, results showed that the dual-fuel vehicles were from 10 to 30% lower in empty weight for the same payload capability, with the dual-expander engine types showing the greatest potential.

Using dual response surfaces to reduce variability in launch vehicle design: A case study

Space transportation system conceptual design is a multidisciplinary process containing considerable element of risk. Uncertainties from one engineering discipline may propagate to another through linking parameters and the final system output may have an accumulation of risk. This may lead to significant deviations from expected performance. An estimate of variability or design risk therefore becomes essential for a robust design. This study utilizes the dual response surface approach to quantify variability in critical performance characteristics during conceptual design phase of a launch vehicle. Using design of experiments methods and disciplinary design analysis codes, dual response surfaces are constructed for the mean and standard deviation to quantify variability in vehicle weight and sizing analysis. Next, an optimum solution is sought to minimize variability subject to a constraint on mean weight. In this application, the dual response surface approach lead to quantifying and minimizing variability without much increase in design effort.

Multidisciplinary Analysis of a Lifting Body Launch Vehicle

As part of phase 2 of the X-33 Program, NASA selected an integrated lifting body/aerospike engine configuration as the study vehicle for the conceptual analysis of a single-stage-to-orbit reusable launch vehicle. A team at NASA Langley Research Center participated in the screening and evaluation of a number of proposed vehicle configurations in the early phases of the conceptual design process. The performance analyses that supported these studies were conducted to assess the effect of the vehicles lifting capability, linear aerospike engine, and metallic thermal protection system on the weight and performance of the vehicle. These performance studies were conducted in a multidisciplinary fashion that indirectly linked the trajectory optimization with weight estimation and aerothermal analysis tools. This approach was necessary to develop optimized ascent and entry trajectories that met all vehicle design constraints. Significant improvements in ascent performance were achieved when the vehicle flew a lifting trajectory and varied the engine mixture ratio during flight. Also, a considerable reduction in empty weight was possible by adjusting the total oxidizer-to-fuel and liftoff thrust-to-weight ratios. However, the optimal ascent flight profile had to be altered to ensure that the vehicle could be trimmed in pitch using only the flow diverting capability of the aerospike engine. Likewise, the optimal entry trajectory had to be tailored to meet thermal protection system heating rate and transition constraints while satisfying a crossrange requirement.

Utilizing air-turborocket and rocket propulsion for a single-stage-to-orbit vehicle

With appropriate technology advances, a horizontal-takeoff single-stage-to-orbit (SSTO) launch vehicle could be designed which utilizes a combination of air-turborocket (ATR) and rocket propulsion systems. Such a vehicle is currently under study at Langley, and this paper presents the results of that study. Estimated vehicle weight characteristics, engine characteristics, geometry, aerodynamics, performance, structures, and subsystems are summarized. Trade studies performed on the reference vehicle to optimize the initial thrust-to-weight ratio (T/W) and the T/W after transition to the rocket phase with respect to vehicle weights are also presented. Throughout the vehicle presentation, special attention is given to the design issues, sensitivities, and performance parameters involved.

Conceptual design of a fully reusable manned launch system

The conceptual design of a rocket-powered, two-stage fully reusable launch vehicle has been performed as a part of the advanced manned launch system (AMLS) study by NASA. The main goals of the AMLS study are to provide routine, low-cost manned access to space. Technologies and system approaches have been studied that would contribute to significant reductions in operating time and manpower relative to current systems. System and operational characteristics of the two-stage fully reusable vehicle are presented, and the various tools and methods used in the design process are summarized. The results of a series of trade studies performed to examine the effect of varying major vehicle parameters on the reference two-stage fully reusable vehicle are also summarized.

A Multidisciplinary Performance Analysis of a Lifting-Body Single-Stage-to-Orbit Vehicle

Lockheed Martin Skunk Works (LMSW) is currently developing a single-stage-to-orbit reusable launch vehi- cle called VentureStar™. A team at NASA Langley Re- search Center participated with LMSW in the screening and evaluation of a number of early VentureStar™ con- figurations. The performance analyses that supported these initial studies were conducted to assess the effect of a lifting body shape, linear aerospike engine and me- tallic thermal protection system (TPS) on the weight and performance of the vehicle. These performance studies were performed in a multidisciplinary fashion that indi- rectly linked the trajectory optimization with weight es- timation and aerothermal analysis tools. This approach was necessary to develop optimized ascent and entry tra- jectories that met all vehicle design constraints. Significant improvements in ascent performance were achieved when the vehicle flew a lifting trajectory and varied the engine mixture ratio during flight. Also, a considerable reduction in empty weight was possible by adjusting the total oxidizer-to-fuel and liftoff thrust-to- weight ratios. However, the optimal ascent flight profile had to be altered to ensure that the vehicle could be trimmed in pitch using only the flow diverting capability of the aerospike engine. Likewise, the optimal entry tra- jectory had to be tailored to meet TPS heating rate and transition constraints while satisfying a crossrange re- quirement.

Single-stage-to-orbit — A step closer

Abstract Over the past several years there has been a significant effort within the United States to assess options to replace the Space Shuttle some time after the turn of the century. In order to provide a range of technology options, a wide variety of vehicle types and propulsion systems have been examined. These vehicle concepts which are representative of the classes of concepts mat could be proposed for any future vehicle development is being used in the initial phase of the access to space activity to identify requirements for the technology maturation effort and to assess approaches to achieve the required low operations cost. This paper provides the results of recent systems analyses and describes the ongoing technology maturation and demonstration program supporting the Reusable Launch Vehicle Program.

Design for Quality Using Response Surface Methods: An Alternative to Taguchi's Parameter Design Approach

ABSTRACTThis article presents the response surface methodology as an alternative approach to Taguchis parameter design methods for optimizing designs for quality. The method is briefly explained, and its application is illustrated by an example of a preliminary design study of an advanced space transportation vehicle. The results indicate that the response surface methodology is a systematic and efficient approach that can help engineering managers design for quality, performance, and cost.