Liquid air augmented rocket engine for a small reusable SSTO launcher (more rocket than any rocket-based combined cycle)

This paper discusses a new exceptionally synergetic combined cycle for vertical takeoff launchers capable of operating in two different propulsive modes. For the first airbreathing mode, part of the onboard oxygen is replaced by liquefied intake air prepared in an air/hydrogen heat exchanger/condenser and an air/oxygen mixer. At a high Mach number, the engine changes to the second mode, which is a conventional oxygen/hydrogen rocket engine. Both modes use common hardware, namely, the fuel and oxidizer turbopumps and the combustor/nozzle assembly. Engine can be demonstrated with an existing rocket engine using LOX/ LN2 mixture for oxidizer. Nomenclature DCTJ = deeply cooled turbojet DHR = double heat regeneration GTOW = gross takeoff weight KLINTM cycle = thermally integrated DCTJ and LRE LAARETM = liquid air augmented rocket engine LACE = liquid air cycle engine LAir = liquid air LH2 = liquid hydrogen LN2 = liquid nitrogen LOX = liquid oxygen LRE = liquid rocket engine NASA = National Aeronautics and Space Administration RLV = reusable launch vehicle SLS = sea-level static SSTO = single-stage-to-orbit USAF = United States Air Force VTO = vertical takeoff Introduction Liquid air cycle engine propulsion technology and its derivatives are listed among the four top priorities recommended for detailed study in the National Research Council 1998 Report (Ref. 1). The aerospace community seeks innovative approaches to develop technologies that can double existing rocket propulsion capabilities by the year 2010, particularly through specific impulse improvement and relatively low life cycle cost. In support of these efforts, MSE Technology Applications, Inc. (MSE) proposes to develop a combined propulsion technology for a small RLV based on existing expander rocket engines of the RL10 (RL50, RL60) family and other moderate combustor pressure LRE's. The combined cycle has been named LAARE*^ This paper describes the exploration research effort that has been undertaken at MSE. LAARE Concept LAARE is a combined cycle capable of operating in two different propulsive modes. For the first airbreathing mode, part of the onboard oxygen is replaced by liquefied intake air prepared in an air/hydrogen heat exchanger/condenser and air/oxygen mixer. At a high Mach number, the engine changes to the second mode which is a conventional LOX/LH2 LRE. Both modes use common hardware, namely, the fuel and oxidizer turbopumps and the combustor/nozzle assembly. The air inlet and air liquefaction system are the only attributes of the airbreather. This synergy is possible, because in the combined cycle mode, three fluids (LH2, LOX, and liquid air) are used in a combination that provides speed of sound in the nozzle throat which is designed for the rocket mode when two fluids (LH2 and LOX) are used. This results in high cycle performance and an extremely high thrust-toweight ratio for the combined cycle. The LAARE approach utilizes a philosophy developed in MSE's KLIN cycle technology development project (Ref. 2). The KLIN cycle is a thermally integrated DCTJ and LRE. As seen in Ref. 2, the two main findings from the KLIN cycle system study are: 1) Maximum KLIN cycle launcher efficiency corresponds to a rather high rocket engine contribution to total thrust (60%-70%); and 2) The most simple rocket engine control law (full thrust during the combined mode) provides the highest KLIN cycle launcher efficiency. Copyright © 2001 by V. Balepin. Published by American Institute of Aeronautics and Astronautics, Inc., with permission. * LAARE is a trademark of Dr. Balepin and MSE Technology Applications, Inc. U.S. Patent pending. 1 American Institute of Aeronautics and Astronautics (c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. LAARE inherits these features and has an even higher degree of cycle rocketization. In fact, LAARE is more rocket dominated than any other rocket-based combined cycle (RBCC). Figure 1 shows the LAARE cycle based on the RLlO-type expander rocket engine. Figure 1. RLlO-based LAARE cycle. Technology Benefits The LAARE cycle's unique benefits include the following. • The lightweight, very moderate specific impulse/high thrust LAARE cycle provides exceptional efficiency for small VTO SSTO launchers. • If based on an existing rocket engine, the LAARE cycle can provide low cost and very efficient earthto-orbit propulsion system that is fully within the current manufacturing capability of the aerospace industry. • The LAARE cycle can be based upon existing expander rocket engines of the RL10 class. Incorporation of these engines and their derivatives (RL50, RL60) into the LAARE cycle will allow them to be used as boosters for small launchers. The LAARE cycle can also be based on the projected Mitsubishi/Boeing MB60 engine of and its derivatives. • The air liquefaction system and triple mixture (air/oxygen/hydrogen) combustor/turbopump assembly can be separately developed and demonstrated. Initial demonstration can be performed with an old surplus engines without any hardware modifications using LOX/ LN2 oxidizer simulating the LOX/LAir mixture. • A small RLV utilizing a LAARE propulsion system will create a new market for on-demand launch services. Technical Approach Several technical papers and patents claim to use the same hardware (combustor/nozzle assembly) in both airbreathing and rocket modes, however, none have explained how they intend to do this. To operate a rocket engine having a single combustor and nozzle assembly in different modes, matching of the gas flows through the critical section or throat of the nozzle in each mode of operation is required. This critical nozzle section determines the total flow rate of propellant gases through the entire engine. Mach number at the throat is very close to unity and it is usual to include this approximation. In a selected propulsive mode, the mass flow rate of the fuel components may be determined in terms of the nozzle throat area, combustor pressure, characteristic exhaust velocity, and the mixture ratio of the total oxidizer mass flow rate and total fuel mass flow rate according to the equation: