James A. Martin

Effects of tripropellant engines on earth-to-orbit vehicles

The effects of tripropellant engines on earth-to-orbit vehicles is examined in terms of their impact on engine configurations and launch capabilities. Hydrocarbon fuel with some oxygen is used in tripropellant fuels, with hydrogen as a back-up fluid for coolant and driving the pumps. Engine concepts which implement tripropellant fuels include a hydrogen-gas generator engine with staged combustion, a two-mode engine burning hydrocarbon fuel in one chamber and hydrogen in another, a dual expander engine with a central hydrocarbon nozzle and an annular hydrogen nozzle, and a dual throat engine. The use of hydrocarbons reduces the fuel weight by providing a higher specific impulse than with LOX-LH2 systems alone. Studies have shown that single-stage-to-orbit vehicles capable of lifting 13.6 Mg are possible with tripropellant engines. A dual-expander engine, is identified as offering the most fuel dry mass reduction for a given payload if cooling requirements can be satisfied. Further development of the tripropellant engines is concluded to be beneficial.

Hydrocarbon Rocket Engines for Earth-to-Orbit Vehicles

The Space Shuttle has initiated a new era in Earth-to-orbit transportation and studies are now under way to assess the technology requirements for more advanced vehicles. Potential derivatives of the Space Shuttle and completely new vehicles might both benefit from advanced hydrocarbon engines, several versions of which were recently studied. In the present paper, selected single-stage vehicles have been compared using these engines. The results indicate that propane staged-combustion engines are attractive for this application. The potential effects of advanced engine materials are also shown. HE Space Shuttle has initiated a new era in Earth-to-orbit transportation, and the large number of payload reservations indicates that the system will be used to capacity. Many advantages of operating in space with the Shuttle have been hypothesized, such as satellite checkout and repair in orbit. Assembly of large space structures from the Shuttle is also being considered. Beyond the first few years of the Shuttle era, Earth-to-orbit traffic will grow to the point that vehicles beyond the Space Shuttle should be developed for a more efficient transportation system. Advanced orbit-transfer vehicles are being considered in several future systems studies. Even though the Space Shuttle era has just begun, the time to develop the technology for future vehicles is now. Derivatives of the Space Shuttle have been studied in the past,1 and a current study2 will provide additional insight into such derivatives. For the more distant future, completely new, fully reusable vehicles have been proposed as in Refs, 3-6. One of the more promising concepts that have been con- sidered for new vehicles is the single-stage, dual-fuel vehicle. Vertical launch and horizontal landing, which is the operational mode of the Space Shuttle, may provide the lowest risk. One of the most significant design concerns for such a concept is the aft center of gravity. This concern is somewhat alleviated by dual-fuel propulsion, and recent work on control-confi gured design indicates that vehicles can be controlled during return from orbit to the runway with center- of-gravity locations considerably aft of the current Shuttle limits.7 Previous studies have shown that advanced hydrocarbon liquid rocket engines would be beneficial to both Shuttle derivatives and advanced vehicles. Replacing the Shuttle solid rocket boosters with liquid-fueled boosters could reduce environmental concerns and operational costs. Although derivatives of the Space Shuttle Main Engine (SSME) could be used for such liquid-fueled boosters, advanced hydrocarbon engines would reduce operational costs more. Several ad- vanced hydrocarbon engines have been studied in the past for use with hydrogen-fueled engines for new Earth-to-orbit vehicles, and the results have indicated a significant advantage over hydrogen engines alone.l One of the findings that surfaced in previous studies is that

Cost Comparisons of Dual-Fuel Propulsion in Advanced Shuttles

Econometric analyses of advanced Earth-to-orbit vehicles indicate that there are economic benefits from the development of new vehicles beyond the space shuttle as traffic increases. Vehicle studies indicate the advantage of the dual-fuel propulsion in single-stage vehicles. This paper shows the economic effect of incorporating dualfuel propulsion in advanced vehicles. Several dual-fuel propulsion systems are compared to a baseline hydrogen and oxygen system.

Ramjet Propulsion for Single-Stage-to-Orbit Vehicles

The concept of single stage earth-to-orbit transportation is studied with respect to existing and projected ramjet technology. Four types of ramjet are analyzed: fan ejector, fan ramjet, supersonic combustion ramjet, and fan ramjet with turbojet boosters. A fan ramjet with a removable fan, with separate rockets for the non-air-breathing flight phase, is considered superior to an ejector ramjet, for both ease of orbit insertion and payload boost capability. Vehicle design is also discussed in terms of trajectory integration and optimization, aerodynamic trim and stability, and complete mass estimation. Graphs are presented showing Mach number for air-breathing and non-air-breathing flight, specific impulse from various ramjet engines, and orbital-insertion parameters.

Propulsion evaluation for orbit-on-demand vehicles

Future earth-to-orbit vehicles may be required to reach orbit within hours or even minutes of a decision. A study has been conducted to consider vehicles with such a capability. In Phase I of the study, 11 vehicles were designed to deploy 5000 lb to a polar orbit. Changes in the designs were examined parametrically for increased on-orbit maneuvers, increased payload, and other mission variations. Based on the results, two concepts were selected for Phase II design work: a vertical-takeoff, two-stage system and a horizontal-takeoff, two-stage system with an airbreathing subsonic first stage. The results of several propulsion evaluations are presented, including liftoff thrust-to-weight effects, dual-fuel propulsion for a horizontal-takeoff concept, and the effect of using fluorine.

Parallel-burn options for dual-fuel single-stage orbital transports

A parallel-burn version of a single-stage vehicle for transport from the earth to low-earth orbit using two fuels and rocket propulsion is considered. New engine results were incorporated in vehicle performance and design studies. The results indicate that a hydrogen-cooled gas generator cycle engine provides attractive vehicle performance and that there is little incentive for increasing the chamber pressure beyond 27 MPa.

Two-Stage Earth-to-Orbit Vehicles with Dual-Fuel Orbiter Propulsion

Earth-to-orbit vehicle studies of future replacements for the Space Shuttle are needed to guide technology development. Previous studies that have examined single-stage vehicles have shown advantages for dual-fuel propulsion. Previous two-stage system studies have assumed all-hydrogen fuel for the orbiters. The present study examined dual-fuel orbiters and found that the system dry mass could be reduced with this concept. The possibility of staging the booster at a staging velocity low enough to allow coastback to the launch site ts shown to be beneficial, particularly in combination with a dual-fuel orbiter. An engine evaluation indicated the same ranking of engines as a previous single-stage study. Propane and RP-1 fuels result in lower vehicle dry mass than methane, and staged-combustion engines are preferred over gas-generator engines. The sensitivity to the engine selection is less for two-stage systems than for single-stage systems.

Economic and programmatic considerations for advanced transportation propulsion technology

Requirements for space transportation will increase after the Space Shuttle becomes operational. Shuttle derivatives and new earth-to-orbit vehicles are compared economically. The addition of both one and two new vehicle types are considered. Programmatic considerations are included in deriving a scenario which appears reasonable and includes the Space Shuttle, a heavy-lift cargo derivative of the Shuttle, a unique dual-fuel orbit transfer vehicle, and a small new earth-to-orbit vehicle in the foreseeable future. Large new orbital transfer and earth-to-orbit vehicles would be added when needed.

Comparison of methane and propane rockets

Martin (1983) compared methane and propane fuels for a single-stage-to-orbit vehicle, demonstrating a significant advantage for propane. Attention is presently given to the ways that this comparison changes when both methane and propane vehicles are optimized. It is found that while the difference is slightly reduced, propane remains the better hydrocarbon fuel for dry mass minimization.

Companion: An Economical Adjunct to the Space Shuttle

A Companion system which would operate with the Space Shuttle to provide lower cost transportation is proposed. The companion system operation would involve launching simultaneously with the Shuttle, booster fly back to the launch site for reuse, payload delivery, rendezvous with the Shuttle, and reentry in the payload bay of the Orbiter. The disadvantages of this system are that the Orbiter could return only the Companion in its payload bay and the Orbiter may need to remain in orbit extra days; however, the economic benefits of the system are significant. It is estimated that the system could deliver a payload and shroud mass of about 10,000 kg into low earth orbit.