Design and Application of Synergetic Air Breathing Rocket Engines Joseph Pointer1 University of Colorado, Boulder, CO, 80309 May 11, 2017 The following report provides an overview of the design, operation, performance, feasibility, and application of a synergetic air breathing rocket engine (SABRE). The system is discussed in detail to provide understanding of the benefits the technology has to offer to the commercial transport and space launch industries alike. A view of the technological development is provided in terms of the efforts currently underway to develop and prove the feasibility of this system. The SABRE is shown to be a promising technology, but has several design obstacles to overcome before it can enter the market as a viable competitor to current propulsion systems. I. Background HE concept of a single stage to orbit (SSTO) vehicle is an attractive idea to competitors in the Tcommercial space industry of today. Such a vehicle would allow delivery of a scientific payload to a circularized low Earth orbit (LEO) at an altitude of roughly 300 km without requiring the use of expendable stages. These reusable launch vehicles (RLVs) would reduce operational cost in the long term and allow for rapid successive launches. In the 1980s, two British engineers (Alan Bond and Bob Parkinson) at Rolls Royce began working on the development of a propulsion system that could be utilized in a SSTO vehicle. The result was the RB545 that was envisioned to power the horizontal take-off and landing (HOTOL) vehicle. However, the project development never gained ground due to a lack of availability of other nations to assist in the development of such an extensive program, and the HOTOL project was canceled [1]. The company Reaction Engines Ltd. (REL) was formed by Bond and a team of engineers to further develop the technology and produce a design for a viable SSTO vehicle. The engine that has been in development is known as the SABRE, Figure 1, and is intended to power the envisioned aircraft-like SSTO vehicle known as SKYLON. The SKYLON spacecraft, at the current configuration C1, is designed to takeoff horizontally from a runway using two SABREs operated in air-breathing mode and continue on an ascent trajectory to a pre-determined altitude. The SABREs are then switched over to a more standard rocket mode of operation, followed by adjusting to a more aggressive ascent profile and continuing until the vehicle reaches an apogee of 300 km. The orbit can then be circularized and any scientific payloads aboard can be released into LEO [1]. As seen in Figure A1 of Appendix A, conventional rocket propulsion techniques currently dominate the commercial launch industry due to the high thrust to weight ratio of the propulsion system. However, as observed in Figure A2, the SABRE offers the highest thrust to weight ratio of the air-breathing propulsion methods and is the best candidate for a SSTO vehicle that incorporates air-breathing propulsion. From Figure A2, it is seen that the SABRE has a significantly higher specific impulse, compared to conventional rocket engines, which results in lower resource expenditure since the air captured during air-breathing mode acts as the oxidizer in the combustion process. 1 Aerospace Engineering Sciences, University of Colorado, Boulder, CO 80309. 1 American Institute of Aeronautics and Astronautics Figure 1: Synergetic Air-breathing Rocket Engine (SABRE). Source: Fig. 1 of Mehta et al. [6] II. SABRE Operational Overview The operation of the SABRE engine is split into two distinct modes: air-breathing mode and rocket mode. In air-breathing mode, the air intake at the foremost section of the engine ingests freestream air which is then used as the oxidizer that reacts with hydrogen in the combustion chambers. The combustion by- products are then expanded out of the rocket nozzles to generate thrust. At a predetermined altitude and velocity, the conical center body in the air intake translates forward and closes off the system from freestream air [2]. While this sealing process is occurring, the liquid oxygen (LOX) pump demand to the combustion chamber is gradually increased to prevent any discontinuity in thrust. The SABRE is then considered to be in rocket mode and utilizes gaseous oxygen and hydrogen for combustion (oxygen must be in the gaseous phase in order to utilize the same injector as is used in air-breathing mode). III. SABRE Key Components Combining the air-breathing and rocket modes of operation into one system increases complexity of the vehicle. However, the overall efficiency increases due to the absence of a separate inactive system that would otherwise increase the aerodynamic drag on the vehicle. The SABRE design is broken down to the subsystem level in order to discuss the aspects that are vital to the successful performance of the engine. A flow diagram of the system is shown in Figure 2 and is referenced frequently in the subsequent sections. Air Intake The air intake is comprised of a 20 degree axisymmetric conical center body, with the ability to translate axially, that actively controls the conditions at which freestream air enters the nacelle. During takeoff/subsonic conditions, the center body is fully retracted in order to maximize air intake at lower speeds. As the freestream velocity increases to supersonic conditions, the center body translates forward so that the oblique shock wave that develops at the front of the center body is not ingested by the air intake. The center body is continuously adjusted in order to provide inlet conditions that result in an appropriate pressure recovery value so that the turbo-compressor, shown in Figure 2, is supplied with a nearly constant pressure in order to maximize combustion efficiency. As an aside, the intake is pitched 7 degrees nose- 2 American Institute of Aeronautics and Astronautics down, seen in Figure 1, in order to compensate for the angle of the ascent trajectory of a SSTO vehicle such as SKYLON and maximize inlet air flow [2]. Figure 2: Simplified SABRE Flow Diagram Source: Fig. 5 of Longstaff et al. [1] Once the optimum conditions for transition from air-breathing to rocket mode have been achieved, predicted to be approximately Mach 5.1 at an altitude of 26 km, the system is switched over to the conventional rocket mode of operation. At this point, the center body translates forward and picks up three conical frustums that seal against the nacelle and isolate the system from freestream air. Pre-Cooler During air-breathing operation, the stagnation temperature of the air from the intake is in excess of 1223 K and must be cooled prior to contact with the compressor. This elevated temperature is due to the deceleration of the air from supersonic to subsonic velocities through a series of shock waves (one oblique and one normal). The pre-cooler is one of the most innovative components in the SABRE system and is a vital component in successful operation. As seen in Figure 1, in the current design, the pre-cooler system consists of four heat exchangers that operate in parallel to cool the high-temperature inlet air from the intake. Each heat exchanger is comprised of thousands of small diameter cooling tubes, as seen in Figure 3, that are approximately 1 mm in diameter with a wall thickness between 20 to 40 microns [3]. A portion of the air ingested by the intake is redirected by guide vanes (not shown) that force the air to travel radially inward through the heat exchanger matrix. Once through the heat exchanger, another set of guide vanes (not shown) redirects the flow back to an axial flow direction toward the turbo-compressor [9]. The cooling tubes that make up the heat exchanger matrix can vary in material in order to optimize the weight of the system. Cooling tubes near the exterior of the heat exchanger are exposed to higher temperatures and are made from a nickel alloy such as Inconel 718, while tubes at the cooler interior can be made from a lighter aluminum alloy [7]. Additionally, due to the high aerodynamic drag induced by the matrix, the cooling tubes in the heat exchanger are subject to high inward-radial loading and must be sufficiently supported. A lightweight interior structure is used to support the tubes axially and radially and shim-like structures are incorporated throughout the matrix to uniformly transfer the radial load while still allowing for relative motion due to thermal expansion and contraction of the tubes at various temperatures [9]. 3 American Institute of Aeronautics and Astronautics Figure 3: Simplified Pre-Cooler Heat Exchanger Diagram Source: Reaction Engines Ltd. [11] Seen in Figure 2, the closed Brayton cycle helium loop supplies the heat exchangers with a heat-sync as the helium, which can be as low as 30 K, circulates through the cooling tubes. The helium is cooled prior to entering the pre-cooler by passing through a heat exchanger with liquid hydrogen (LH). As an aside, helium is chosen as the coolant, as opposed to hydrogen, in order to avoid hydrogen embrittlement, remain in gaseous phase at temperatures as low as 30 K, and utilize the high ratio of specific heats to reduce the pressure ratio in the closed Brayton cycle loop [2]. Mock heat exchangers have been shown to be capable of cooling the stagnation temperature of the air from 1223 K at the inlet to below 133 K at the interior in 0.01 s [10]. During this rapid cooling process, the closely spaced heat exchanger matrix is vulnerable to clogging from the condensation and subsequent freezing of water that originated as moisture in the inlet air. A unique and innovative solution for the prevention of ice formation in the heat exchanger was made public in 2015 when REL published a patent on the technology.