Low Cost GN&C System for Launch Vehicles Thomas P. Bauer

A low cost Guidance, Navigation, and Control (GN&C) system for launch vehicles has been developed that exploits recent developments in Global Positioning System (GPS) and computer technology. To obtain low life cycle cost, simplicity and adequacy have become the design metrics. Use of commercial or "off-the-shelf" hardware and algorithms is the norm. Modular designs that avoid expensive and complicated integration and optimization are adopted to restrain cost. This system is now available as a low cost, commercial, off-the-shelf (COTS) product for sounding rockets, launch vehicles, and satellites.

Introduction

The cost of launching payloads to space is too high. A new approach is needed to lower the cost with an emphasis on minimizing the overall system's life cycle cost. In the past, launch vehicles were designed to maximize performance or for specific military requirements such as launch on demand from a silo. The Microcosm clean sheet approach to a low cost system results in a design that, in general, minimizes complexity, manpower, exotic materials and processes, and number of parts. Use of commercial or industrial grade, "off-the-shelf" components is the rule rather than the exception.

The availability of new technologies such as GPS, composites, and small but powerful computers together with the valuable experience gained in the launch industry over the past 40 years has enabled the development of a new breed of launch vehicles that has the potential to lower launch costs by nearly an order of magnitude compared with the current stable of U. S. vehicles. In many ways this revolution in launch vehicles parallels that of computers where the clean sheet development of "personal" computers, ones that minimized the cost to the user, took the industry by storm. The idea of developing a low cost launch vehicle is not new. For example, the "Big Dumb Booster" concept of the early 1970's was overtaken by that other low cost approach: the reusable Shuttle. In retrospect, the Shuttle system was not designed for low life cycle cost.

Microcosm has designed a suite of launch vehicles (Scorpius) from single stage sounding rockets to a medium lift launch vehicle using the approach of minimizing the overall cost of launching a payload. This paper describes the avionics system of this family of vehicles and its applicability to other new, low cost vehicles.

Method

The Scorpius GN&C system was designed by choosing components that together would provide the lowest life cycle cost with adequate performance. The analysis and design process described in (Larson and Wertz 1993) is used in the Scorpius program. The availability of GPS and low cost computer chip sets was a driving force behind the selection of GN&C techniques, hardware, and software.

The ability of GPS to provide absolute position and velocity states has made the launch navigation problem nearly as easy as plugging a GPS receiver into a computer. Moreover, the availability of commercial GPS receivers with adequate velocity and acceleration tolerance has rendered the cost of navigating a launch vehicle into low earth orbit nearly insignificant. The wide availability of modestly powerful, small computers raised the possibility of dramatically reducing the cost of the computer hardware as well.

Southwest Research Institute, under contract to Microcosm, has realized this important capability with the development of a low cost space flight computer, the SC-2DX. The control requirements of the Scorpius family of vehicles are very amenable to digital computer control. In addition, the common use of bus structures and distributed processing has lent convenience to the multipod, Scorpius architecture. It should be emphasized, however, that most of the components of the modular GN&C system (hardware, software, techniques, and the simulation) are applicable to a wide variety of vehicles.

Results

The approach of designing a GN&C system for low cost has resulted in a design that combines modern and widely available technology (GPS and single board computers) with conventional, established algorithms.

Navigation

A search of the various means for navigating to low earth orbit led to a dismissal of the old and exotic (ground based radio navigation; laser tracking) leaving inertial navigation and GPS. The technique, adopted by essentially all operational vehicles according to (Isakowitz 1991), of using inertia measurement units (IMUs) is feasible, but expensive. The use of GPS receivers alone is not practical because of dropouts and data rates. The combination of a GPS receiver with inertial sensors offers accurate, inexpensive navigation. However, the problem of attitude determination is not solved with a conventional (translation only) GPS receiver. Inexpensive inertial attitude or rate sensors do not provide sufficient accuracy by themselves. A GPS receiver capable of providing attitude determination through interferometry could periodically update the rotational state of an inexpensive inertial system in the same way that a GPS receiver can periodically update the translational state of an inexpensive IMU. However, a less expensive and less complicated method is to use an integrated GPS/INS system. Such an integrated system combines an inexpensive GPS receiver with inexpensive inertial sensors together with a Kalman filter in a single package. The complementary features of the two technologies provide good navigation and attitude determination for an accelerating vehicle at low cost, with high data rates, and with resistance to dropouts. In addition, alignment of the inertial platform is a problem that virtually disappears with the integrated GPS/INS system since determination of the absolute position is not dependent upon the initial orientation.

Guidance

Conventional guidance schemes are employed because of their familiarity and applicability to a range of vehicles and because they provide adequate performance with relative simplicity. There is a conventional vertical rise, roll to azimuth, pitch kick, pitch pause, and gravity turn sequence employed via an attitude lookup table (as a function of velocity). For the upper stages of the launch vehicles, a linear sine (closed loop) steering law is employed. Delivery accuracies using the integrated GPS/INS navigation system are expected to be less than a kilometer in position and a fraction of a meter per second in velocity.

Control

The control law is conventional in that the commanded torques are primarily linear functions of the attitude and rate errors, and, therefore, would apply to most vehicles. However, the implementation of the control torques for the Scorpius vehicles is unique as it would be for many vehicles. The current control system has been designed for the relatively easy problem of controlling a vehicle that is statically stable at subsonic and low supersonic Mach numbers. Extension to the more general vehicle that is always unstable is primarily a matter of data rates and careful actuator and system modeling.

The Scorpius vehicles employ thrust magnitude steering in addition to thrust vector control. This presented some unique challenges in the design of the control torque implementation. The control torque selection makes maximum use of throttling before thrust vector control is invoked. The resulting control authority with thrust magnitude control for the smaller Scorpius vehicles is very large because of the high moment arm to inertia ratio. If anything, these vehicles are overcontrolled by differential throttling. The larger Scorpius vehicles have progressively lower levels of control authority using thrust magnitude steering because control acceleration scales with about the fourth power of linear dimension (engine thrust mg r3 and moment arm r0.8) whereas inertia scales with the fifth power ( m r2 r3 r2). The availability of the thrust vector control maintains high tolerances in the larger vehicles for center of mass offset, thrust misalignment, and attitude transients, although these problems diminish somewhat for the larger vehicles. The low cost GN&C system is general enough to accommodate both thrust magnitude control and thrust vector control.

Vehicle Status

The calculation of mass properties such as inertia and center of mass is generic with vehicle specific parameter values admitted through input. These quantities are used by the control torque implementation which converts angular acceleration commands to engine (valve) selections. For some applications, these calculations may be unnecessary due to their implicit incorporation in the control gains.

Event Sequencer

The event sequence and its hardware implementation is vehicle specific, although the avionics design is adaptable to particular configurations. Given the nature of the bus architecture, great flexibility is available in both space and time for controlling events.

Avionics Hardware

Microcosm and Southwest Research Institute have developed a low cost flight computer, bus, and distributed processing architecture to control and monitor launch vehicles. The entire hardware suite is designed for robust operation including memory error detection and correction, single event upset protection, and system watch dogs. The 80C186-based SC-2DX computer is shown in Figure 1. The microcontroller-based Pod Electronics Box, which receives and executes the computer's commands and obtains sensor data from various vehicular systems, is shown in Figure 2. The integrated GPS/INS system has its own dedicated central processing unit. Accommodation can be made using the bus architecture for other, specialized components such as telemetry encoders or for communication with the payload. The bus system provides an expedient means for accessing the avionics system during checkout and prelaunch.

Simulation

A launch vehicle simulation has been developed using modular software components to exercise a vehicle's configuration in 3 or 6 degrees of freedom. All of the Scorpius vehicles with several variations have been simulated for various trajectory types. A sample plot of 3-D performance results for the Scorpius Liberty light lift vehicle is given in Figure 3. A myriad of output parameters can be obtained in different formats using several output files. A search capability is available to obtain specified endpoint conditions such as impact point, insertion altitude, and apogee/perigee. The simulation was written in standard FORTRAN and is run on a 486 personal computer.

Simulation in 6-D has shown that SR-1, the one stage Scorpius sounding rocket, is controllable using the approach described in this paper. An example attitude history for the first 20 seconds of a vertical flight is given as Figure 4. For the case illustrated in Figure 4, winds are modeled, the center of gravity (c.g.) offset has components in both transverse axes, the thrust vector control (TVC) fluid is water, the span of an equivalent cruciform fin pair is 3.5 m, and the fineness ratio of the nose of each pod is 3. The 5 transverse control gains are listed in the lower right. The solid, wave-like curve is the yaw angle error whereas the solid, squared-off curve is the commanded torque plotted against the right vertical axis. Note that the commanded angle designated by the dashed line (Ang:Cmd) differs from the yaw angle error by the amount of the bias compensation. Also note that the control law torque designated by the long dashed/short dashed line (C Law T), which on this Chart indicates the yaw component of the 2-dimensional transverse torque that is obtained from the linear control law before thresholding is applied, more or less follows the commanded torque.

Conclusion

An inexpensive GN&C system that exploits the recent developments of GPS and small but powerful computers has been designed for sounding rockets, launch vehicles, and short-lived satellite applications. The system is expected to provide similar or better delivery performance than current systems at a fraction of the cost.

FIGURE 1. SC-2DX Flight Computer.

FIGURE 2. Pod Electronics Box.

FIGURE 3. Performance of Scorpius Light Lift Launch Vehicle, Liberty, to 185 km Circular Orbit

FIGURE 4. Typical Yaw Attitude History for Scorpius Single Stage Sounding Rocket, SR-1

Acknowledgments

The work in this paper, in part, was sponsored by the Launch Vehicle Technology Office (VT-X) of the United States Air Force's Phillips Laboratory and directed by Ken Hampsten under a Small Business Innovative Research contract. The author acknowledges the contributions to the GN&C algorithms made by Dr. Peter Joseph and Leo Early. Several subroutines from the Aerospace Corporation's Generalized Trajectory Simulation were used in the rotational portion of the simulation.

References

Isakowitz, S. I. (1991) International Reference Guide to Space Launch Systems, ISBN 1-56347-002-0, AIAA, Washington, DC.

Larson, W. J. and J. R. Wertz, editors, (1993) Space Mission Analysis and Design, Second Edition, ISBN 1-881883-01-9, Microcosm, Inc., Torrance, CA and Kluwer Academic Publishers, Boston, MA, pp.1-13.

Nomenclature

English g: acceleration due to gravity m: mass of vehicle r: linear dimension of vehicle Computer mnemonic CK1: control gain for angular rate error term CK2: control gain for angle error term CK5: control gain for acceleration to angle conversion and threshold CK8: pitch/yaw torque threshold factor CK10: pitch/yaw angle error special threshold

This paper is copyright 1996 American Institute of Physics

Email us at microcosm@smad.com HOME