Different Types of Rocket Nozzles 5322- Rocket Propulsion Project Report By Patel Harinkumar Rajendrabhai(1001150586) 1. Introduction 1.1 What is Nozzle and why they are used? A nozzle is a device designed to control the direction or characteristics of a fluid flow (especially to increase velocity) as it exits (or enters) an enclosed chamber or Pipe[9]. Nozzles are frequently used to control the rate of flow, speed, direction, mass, shape, and/or the pressure of the stream that emerges from them. In nozzle velocity of fluid increases on the expense of its pressure energy. A Water Nozzle[9] Rotator Style Pivot Sprinkler[9] 1.2 What is Rocket Nozzle? A rocket engine nozzle is a propelling nozzle (usually of the de Laval type) used in a rocket engine to expand and accelerate the hot gases from combustion so as to produce thrust according to Newton’s law of motion. Combustion gases are produced by burning the propellants in combustor, they exit the nozzle at very high Speed (hypersonic). 1.3 Properties of Rocket Nozzle Nozzle produces thrust. Exhaust gases from combustion are pushed into throat region of nozzle. Throat is smaller cross-sectional area than rest of engine, gases are compressed to high pressure. Nozzle gradually increases in cross-sectional area allowing gases to expand and push against walls creating thrust. Convert thermal energy of hot chamber gases into kinetic energy and direct that energy along nozzle axis.[1] Mathematically, ultimate purpose of nozzle is to expand gases as efficiently as possible so as to maximize exit velocity.[1] Rocket Engine[1] F m eVe Pe Pa Ae Neglecting Pressure losses F m eVe 2 Different types of Rocket Nozzle Configuration(shape) The rocket nozzles can have many shapes configurations. On the bases of there shapes they can be classified in three groups. 3 primary groups of nozzle types[2,3] 1. Cone (conical, linear) 2. Bell (contoured, shaped, classic converging-diverging) 3. Annular (spike, aerospike, plug, expansion, expansion-deflection)[1] 2.1 Conical nozzles[1,2,3] The conical nozzle is the oldest and the simplest configuration. It is ease to fabricate. The cone gets its name from the fact that the walls diverge at a constant angle. A small angle produces greater thrust, because it maximizes the axial component of exit velocity and produces a high specific impulse (a measure of rocket efficiency). Small nozzle divergence angle means long length and axial momentum and thus high specific impulse. It has penalty in rocket propulsion system mass, vehicle mass due to its long length. Large divergence angle reduces size and weight. But, results in performance loss at low altitude as the high ambient pressure causes overexpansion and flow separation. In practice the thrust, exist velocity, etc obtained from the ideal rocket equations are not the same. So, some correction factor has to be applied to this equations. The correction factor applied is called divergence factor and is denoted by Greek alphabet Lambda(λ). The expression for the divergence factor is given by 1/ 2*(1 cos) α is the half cone angle. λ = 1 for ideal rocket. For a nozzle with divergence angle of 30 deg. The value of α is 15 deg. Variation of correction factor with α is shown[2]. All 3 nozzles have same Ae/A* Red dashed lines indicate contours of normal flow[1] Flow is almost entirely axial (Best is uniform axial flow) Flow is mostly axial Flow has significant radial component Highly subject to separation 8 The design consist of an arc section which begins at throat. The arc section is followed by linear section with half cone angle α. The linear section has length L, which can be calculated mathematically as, 2 * Ae Where area * D 1 Ae D 2L tan * ratio is given by A * * L A D 2 tan Where, D* is the throat diameter. Ae/A* is the ratio of exit area to throat area. α is half cone angle. 2.2 Bell/Contoured Nozzle[3,4] Bell nozzle designs are similar to conical nozzle design but are more efficient and more compact than a conical nozzle. The bell nozzle is the most commonly used nozzle shape today. It is more advantages over the conical nozzle in terms of both, size and performance. It is Contoured to minimize turning and divergence losses. Reducing divergence requires more turning flow (more axial) which can result in compressions which in tern could lead to shock losses. This type of nozzles are designed such that all waves are isentropic and produce nearly axial flow at exit. The expansion in the supersonic bell nozzle is more efficient than in a simple straight cone of similar area ratio and length, because the wall contour is designed to minimize losses, as explained later in this section[2]. As shown bellow, the nozzle consists of two sections. Near the throat, the nozzle diverges at a relatively large angle(20 to 50 degree) (1) but the degree of divergence decreases downstream. Near the nozzle exit, the divergence angle is very small(less than 10) (2). The bell nozzle is a compromise between the two extremes of the conical nozzle since it minimizes weight while maximizing performance. The most important design issue is to contour the nozzle to avoid oblique shocks and maximize performance. This types of nozzle shapes are only optimum at one altitude conditions. 2 1 Bell Nozzle[1] Modern Day Bell Nozzle[11] The divergence loss at the exit of a bell nozzle is significantly less than that for a conical nozzle of the same design. The exit angle for a 15 degree conical nozzle is 15 degrees, while the exit angle of a Bell nozzle with the same exit diameter is only 8.5 degrees. This can be seen in Fig. below. Also the bell nozzle is shorter and has less mass than the conical nozzle because it is more compact. These characteristics make a Bell nozzle much more efficient than a straight conical nozzle[6]. Comparison of conical nozzle with bell nozzle[2] A change of flow direction of a supersonic gas in an expanding wall geometry can only be achieved through expansion waves. An expansion wave occurs at a thin surface, where the flow velocity increases and changes its flow direction slightly, and where the pressure and temperature drop. Wave surfaces are at an oblique angle to the flow. As the gas passes through the throat, it undergoes a series of these expansion waves with essentially no loss of energy. In the bell-shaped nozzle shown in Fig. these expansions occur internally in the flow between the throat and the inflection location I. the area is steadily increasing like a flare on a trumpet. The contour angle Ɵi is a maximum at the inflection location. Between the inflection point I and the nozzle exit E. The purpose of this last segment of the contoured nozzle is to have a low divergence loss as the gas leaves the nozzle exit plane. The difference between Ɵi and 0e is called the turn-back angle. When the gas flow is turned in the opposite direction (between points I and E) oblique compression waves will occur. These compression waves are thin surfaces where the flow undergoes a mild shock. The flow is turned, and the velocity is actually reduced slightly. Each of these multiple compression waves causes a small energy loss. It is possible to balance the oblique expansion waves with the oblique compression waves and minimize the energy loss with the help of Method of characteristics. The first set of curves[2] given below(left) gives the relation between length, area ratio, and the two angles of the bell contour. The second set of curves[2] given below(right) gives the correction factors, equivalent to the 2 factor for conical nozzles, which are to be applied to the thrust coefficient or the exhaust velocity, provided the nozzles are at optimum expansions, that is, P2 = P3. 2.3 Annular Nozzle[1,3] The annular nozzle, also sometimes known as the plug or "altitude-compensating" nozzle, is the least employed of those discussed due to its greater complexity. The term "annular" refers to the fact that combustion occurs along a ring, or annulus, around the base of the nozzle. "Plug" refers to the centerbody that blocks the flow from what would be the center portion of a traditional nozzle. "Altitude-compensating" is sometimes used to describe these nozzles since that is their primary advantage, a quality that will be further explored later. Expansion ratio: area of centerbody must be taken into account A A exit plug Athroat Another parameter annular diameter ratio, Dplug / Dthroat Ratio is used as a measure of nozzle geometry for comparison with other plug nozzle shapes There are two major types of annular nozzles developed to date. They are distinguished by the method in which they expand exhaust: (1) outward or (2) inward Radial Out-Flow Nozzles : Examples of this type are the expansion-deflection (E-D), reverse-flow (R-F), and horizontal-flow (H-F) nozzles Radial In-Flow Nozzles : Spike nozzles, linear-aerospike nozzle. Annular nozzles receiving significant research attention Several publications call these concepts ‘new’, but in actual, these ideas have been around for quite some time. These are the most complicated nozzles and hence have serious challenges with its implementation 16 2.3.1 RADIAL OUT-FLOW NOZZLES[1,3] Picture shows an example of an Expansion-Deflection (E-D) nozzle. Expansion-deflection nozzle works much like a bell nozzle. Exhaust gases forced into a converging throat before expanding in a bell-shaped nozzle Flow is deflected by a plug, or centerbody, that forces the gases away from center of nozzle and to stay attached to nozzle walls Centerbody position may move to optimize performance As altitude or back-pressure varies, flow is free to expand into ‘void’ This expansion into void allows the nozzle to compensate for altitude Pe adjusts to Pb within nozzle 17 Name of each of these nozzles indicates how it functions.