3.1 APPROACH
It is a major goal of this book to familiarize the student with the detailed techniques used by industry in liquid propellant rocket engine systems and component design. The authors feel that to convey a feeling for this subject effectively, nothing serves better than a set of realistic sample calculations. To promote a good feeling for the interrelationship between major subsystems, the principal calculations were made for the engines of a hypothetical multistage space vehicle. These calculations and their associated designs were especially prepared for this book and are not related to existing or planned engines. As the various subsystems of liquid rocket engines are discussed in subsequent chapters, most of the supporting sample calculations will be for the engines of that assumed vehicle, which thus will appear throughout the entire book. For simplicity of reference, the space vehicle will be called the "Alpha" vehicle; it is assumed to be composed of four stages: A-1, A-2, A-3, and A-4. Table 3-1 lists the major parameters of the Alpha vehicle.
The Alpha vehicle combination is realistic, though not necessarily optimized. For instance, a different propellant combination has been chosen for each stage to permit sample calculations and designs for a number of typical propel-
Table 3-1.-4-Stage Space Alpha Vehicle
| Stage | Stage thrust. lb | Number of engines | Engine thrust, lb | Propellant | |
|---|---|---|---|---|---|
| A-1 | 3000000 | 4 | 750000 | ||
| A-2 | 600000 | 4 | 150000 | ||
| A-3 | 48000 | 3 | 16000 | ||
| A-4 | 15000 | 2 | 7500 |
[^0]lant combinations, feed systems, and thrust levels. In practice for logistics reasons, or to permit multiple use of parts, fewer combinations would be chosen. In fact, the student and the teacher using this book may find it interesting and instructive to modify the designs chosen by the authors. For instance, the student may wish to determine what engine-design parameters would result if stages A-2 and A-3 were to use the same propellant combination; or, what design parameters would be obtained if stages A-3 and A-4 were combined into one, capable of restart and throttling to 30 percent nominal thrust.
It is not intended to suggest a specific mission for the Alpha vehicle. However, a "primary mission" for it could be the landing of an unmanned scientific payload on the Moon to gather samples and return them to Earth. The staging sequence may then be as follows:
Stage A-1: Boost to 250000 -foot altitude. Stage A-2: Boost to 300 -nautical-mile altitude and inject into Earth parking orbit. Stage A-3: Accelerate to escape velocity and inject into a translunar trajectory. Stage A-4: First start: Deceleration for lunar orbit and soft Moon landing of scientific payload Second start: Moon takeoff for return to Earth In addition to its main powerplant, stage A-4 will require very-low-thrust attitude-control jets. Even if designed for a given "primary mission," a vehicle combination retains a certain degree of
[^1]flexibility. Within the limits of existing propellant tank configurations, the following principal possibilities of modification exist:
Omission of the upper two stages, for Earth-orbital tankers, shuttle vehicles, spacestation assembly, and supply ships.
Omission of stage A-4, for unmanned deep space probe assignments, with no return intended.
Off-nominal tanking of one or more stages. This may yield some overall performance gains for special missions. It is emphatically not intended to say that the stated modifications can be made a few days before launch. Rather, the stages and certain of their subsystems, in particular the engines, should be regarded as building blocks. Their availability can permit meeting a new requirement within, for example, a year, as compared to several years when "starting from scratch." In such ways, substantial gains have been obtained in practice. The earlier Thor, and the Redstone and Atlas Mercury boosters are well-known such cases.
Brief mention should be made here of a special type of system: experimental engine systems, sometimes referred to as breadboard engines. Because of time and fund limitations, the design and development of liquid rocket engines for a given mission rarely permit the investigation of novel ideas and principles. New ideas must then be tried out independently, detached from rigid schedules. Here the test effort can be conducted with full awareness that many of the principles under investigation will not "make the grade." However, while those that succeed can be applied to advanced operational systems, the eliminated marginal ones are just as valuable, as they were early prevented from finding their way into operational engines. If experimentally verified advances are selected for operational use with strong emphasis on vehicle application, true progress will have been made. The major U.S. liquid propellant engine manufacturers have been conducting experimental engine programs with excellent results for a number of years.
The reader will now be acquainted with some of the characteristics of the engines which have been selected for the different stages of the Alpha vehicle. While discussing and implementing these in greater detail in subsequent chap- ters, and through calculations and layouts, this summarizing description can serve as a guide and reference, throughout the book.