Aug 12

Hypersonic Propulsion Systems Review

Authors: Kyle Chiu, Anthony Dosev, Divij Gupta, Tiffany Hong, Nolan Kirk, and Rishi Madduluri

Mentor: Samuel Baker. Samuel is a doctoral candidate at the University of Oxford Centre of Excellence for Hybrid Thermal Propulsion Systems with research specialization in Machine Learning for Engine Airflows.

Abstract

The paper reviews the key concepts and recent work associated with the various propulsion systems used in hypersonic aircraft. Firstly, a brief historical background of the field is presented, followed by an overview of key concepts within hypersonic flight. Recent developments for the main classes of hypersonic propulsion technology are reviewed, namely airbreathing engines and rocket engines, as well as combined cycles and additional technologies. Finally, the key trends are summarized and presented alongside a future outlook for the field.

Introduction

Background

The realm of hypersonic flight and rocketry has witnessed a surge in activity and innovation in recent years. While the concept of hypersonic flight has been around for decades, recent advancements in materials science, computational fluid dynamics, and rocket propulsion systems have brought it closer to practical realization, especially in the field of rocket engineering, scramjet, and ramjet developments [1]. The development of jet engines in the 1940s paved the way for supersonic flight [1]. However, exceeding the speed of sound presented a new set of challenges, including aerodynamic heating (as objects approach the speed of sound, air resistance increases dramatically, leading to intense heat generation) and sonic booms (supersonic flight creates a loud sonic boom that can be disruptive and pose environmental concerns) [2]. By the mid 20th century, space launch systems were becoming widespread, with world powers fighting for space travel, and the utilization of hypersonics [2]. With the ability to travel faster than five times the speed of sound (Mach 5), hypersonic vehicles have the potential to play a key role in launching spacecraft as well as high-speed weapons systems.

Concepts

Lift-to-Drag Ratios (L/D Ratio)

The lift-to-drag ratio (L/D ratio) is a measure of an aerodynamic body's efficiency in flight. It's calculated by dividing the amount of lift an object generates by the amount of drag it experiences while moving through the air. The L/D ratio is also equal to the ratio of the lift and drag coefficients. For lift and drag coefficients of and respectively, the lift and drag equations are given as:

where L and D are the lift and drag in Newtons, ρ is the density of air in kg/m3 , v is the velocity in m/s , and A is the surface area in m2 .

Specific Impulse

Specific impulse, or Isp, is a measure of how efficiently a reaction mass engine, like a rocket or jet engine, generates thrust. It's the ratio of the thrust produced per unit of fuel consumed per unit of time, or the amount of thrust divided by the rate at which fuel is used. Specific impulse is usually expressed in Newtons of thrust per kilogram of propellant used per second, and its unit is seconds.

Mach Number and Flight Regimes

The Mach number is the ratio of flow velocity after a certain limit of the sound's speed. In other words, it is the ratio of the speed of a body to the speed of sound in the surrounding medium. This is the criterion that is used to classify categories of flights into subsonic, supersonic and hypersonic [3]. An object is subsonic if it's traveling slower than the speed of sound, or Mach 1, which is about 343.2 m/s (1,126 ft/s). This includes most general aviation aircraft, such as the Cessna 172, as well as ultralights and paragliders. An object is supersonic if it's traveling approximately between Mach 1 and Mach 5. These speeds are most commonly seen in jet aircrafts and supersonic cruise missiles as well as some ballistic missiles. An object is hypersonic if it's traveling at speeds greater than Mach 5. The most common applications today are in hypersonic cruise missiles and ballistic missiles, and rocket propulsion systems.

Literature Review

Airbreathing Engines

Overview

Airbreathing engines are a class of propulsion system that extract the oxidizer needed for the combustion reactions from the surrounding atmosphere [3]. There are a few main types of air breathing engines, including turbojet, scramjet, and ramjet engines. Turbojet engines are versatile, providing high thrust for both subsonic and supersonic aircraft up until around Mach 3 at which point the high temperatures at supersonic speeds begin to overwhelm the moving components [2], [4]. Ramjet engines are efficient at supersonic speeds, but the combustion system is limited to Mach 5 speeds, while supersonic combustion ramjets (scramjets) are designed to operate at speeds far exceeding Mach 5.

Turbojets

Turbojet engines are gas turbine engines that provide thrust by compressing intake air using a turbine, igniting the compressed air-fuel mixture, and discharging the accelerated exhaust gasses out of the rear of the engine [5]. Some engines also have afterburners, which are located at the back portion of the engine. These spray fuel directly into the exhaust from the engine, where it ignites and boosts the thrust even further [5]. In early aviation, rotary piston internal combustion engines were commonly used, adapted from the automotive industry. As technology developed new turbo engines which were rotary-reaction turbine engines became more commonly used. This is because turbine engines achieved better power-to-weight ratios by containing fewer moving parts, and they enabled aircraft to fly at higher speeds and altitudes due to being less dependent on the atmospheric air pressure to achieve efficient combustion [5].

However, drawbacks of turbojet engines include lower fuel efficiency when idle, a longer startup time, and lower responsiveness to changes in power demands. Additionally, if an afterburner is present, the fuel consumption rate is higher [5].

Ecological function (ECOL) and ecological coefficient of performance (ECOP) are both statistics that show the relationship between power input and loss of that power for turbojet engines, where ECOL is the ratio of power output to the loss rate of availability and ECOL as the power output minus the loss rate of availability [6]. Different fuels can be used in different turbojet engines. Balli et al. [7] investigated the advantages of using hydrogen instead of standard kerosene-based jet fuel on the performance of a turbojet engine, as shown in Table 1. The data shows that although standard fuel is more energy efficient than hydrogen fuel, hydrogen fuel is shown to be less damaging to the environment due to cleaner combustion and reduced emissions.

Table 1: Comparison of the energy efficiency, ECOL, and ECOP when using standard fuel and hydrogen fuel on a turbojet engine.

	Standard Fuel	Hydrogen Fuel
Energy Efficiency	15.40%	14.33%
Environmental Effect Factor	5.49%	5.98%
Ecological Effect Factor	6.49%	6.98%

One common issue that arises with the use of turbojet engines are thrust asymmetries between the engines on the aircraft. Unbalanced thrust along the plane results in unbalanced forces, causing decreased control of the aircraft. One of the key causes of asymmetrical thrust was identified by Burova et al. [8] to be the fatigue of the metal blades of their turbines, and erosion and corrosion of the blades. This demonstrates the importance of maintaining the state of turbine blades in turbojet engines. There have also been studies done on using precoolers in turbojet engines in order to reach hypersonic speeds. In the experiments done by Taguchi et al. [9], a pre-cooler for a turbojet engine was designed using cryogenic liquid hydrogen. The engine was operational over all speeds from stationary up until Mach 5, due to the improved heat management and air mass flow rates through the engine.

Ramjets and Scramjets

In the world of aviation and aerospace, scramjets have been one of the major types of hypersonic engines that fall within the categories of air-breathing jet engines, and are adapted from ramjet engines in order to facilitate combustion under supersonic conditions. Ramjets are categorized as engines for supersonic speed flights, operational over speeds between Mach 1 and Mach 5, with scramjets enabling hypersonic vehicles to exceed Mach 5 [10].

Ramjet Progress

According to Baidya et al. [11], ramjets are considered as the first engine to take “advantage of ram air compression … which eliminate the need to rely on axial compression.” As seen on Figure 1, this significantly reduces the complexity, space, and weight compared to turbojet engines, making the overall design simpler compared to previous ones.

Figure 1: Ramjet engine schematic, adapted from [11].

In order to compare the efficiencies of the ramjet combustion system and the optimum ramjet inlet configuration, simulations were run by the authors in references [10], [11], and comparative performances are shown in Table 2. As shown by the results, the authors successfully operated a ramjet over speeds of between Mach 2 and Mach 7 by running it in dual mode with a turbojet and a scramjet for a variety of altitudes, demonstrating the versatility of this type of engine.

Table 2: Concept aircraft target flight profile, adapted from [10], [11].

After numerous simulations and experiments, Veeran et al. [10] determined that the ramjet compression system's performance decreased with increasing Mach number, with pressure recovery dropping from about 80% at Mach 2 to 35% at Mach 5 leading to severe drops in efficiency. Meanwhile, Kariko and Egoryan [12] conducted an experiment to compare the thrust characteristics of air-breathing jet engines with supersonic detonation combustion engines to those with subsonic ("deflagration") combustion, such as ramjets. It was found that, contrary to popular belief, the thrust characteristics of pulse detonation engines (PDE) were not superior to those of the turbojet engines with deflagration combustion, with benefits of detonation combustion instead being related to facilitating design simplicity.

Scramjet Progress

Inspired by the simplistic and innovative design of the ramjet, the scramjet engine was later introduced in the field of aviation and aerospace. As well as facilitating combustion under supersonic conditions, scramjets also tend to be lighter compared to ramjets by using simpler geometries as there is less of a need to decelerate the airflow to such a degree for combustion [11]. Despite several differences between scramjets and ramjets, there is clear evidence of connection as well. As an example, the basic design structure for both ramjet and scramjet contains “an intake, an isolator, a combustor and a nozzle” [11]. The same elements, which are a mixture of air and fuel, are used for combustion reactions as well.

Combustion chambers play a significant role in the structure and properties of scramjets since they provide a space for the fuel and supersonic airflows, which are entering the engine, to be mixed and combusted in order to generate thrust for the vehicle. The generated thrust also helps to provide lift for hypersonic vehicles, especially during takeoff. The amount of fuel injected can be adjusted to control the amount of thrust generated. Gulothu and Nutakki [13] analyzed a scramjet combustor, particularly focusing on the flow dynamics and mode transition. Both experiments and simulations were conducted, for both cold-flow and combusting engine conditions. Figure 2 illustrates a scramjet combustor model that was used throughout simulations. The researchers concluded that the injection temperature had less impact on the flow characteristics under cold flow conditions. This was because of the “injection temperature [affecting] the parametric distribution in the diverging portion of the combustor, namely, static pressure and the Mach number distribution under the condition of engine ignition” [13]. The following major finding is supported by the fact that higher injection temperatures resulted in a significantly increased Mach number in the diverging section of the combustor.

Figure 2: Schematic of a typical dual-mode scramjet combustor adapted from [13].

One of the studies conducted by Scherrer et al. [14] specifically related to wall injection, which is discussed in terms of OH radical visualization. Wall injection is a method of fuel injection which is done at an angle relative to the main airflow, typically from the wall of the combustor. It affects the mixing and ignition processes within the combustor, and has connections with the shock wave structure and flow dynamics within the scramjet as well. OH (hydroxyl group) radical visualization is a technique used to detect the direction of the flame. This is done by measuring OH radicals. They have found that the injection temperature affects the diverging portion of the combustor, specifically the static pressure and Mach number distribution when the engine is ignited. By visualizing OH Planar Laser-Induced Fluorescence (PLIF) with acetone, Scherrer and colleagues figured that "the jet remains adhered to the wall ... high OH signals are visible only in the second zone, firstly at the jet periphery, then quickly inside the jet, which indicates the presence of large-scale oscillations of the jet.” Visualizations can be seen in Ref [14].

Rocket Engines

Overview

Rocket engines are energy conversion systems typically used for means of space transportation, with applications across ballistics and military purposes. Rocket engines may work with solid fuels, or liquid fuels, and have started to coexist together [15]. Liquid rocket engines are further divided into mono-propellent and bi-propellent engines, which are categorized on the basis of characteristics of the propellants used. A monopropellant fuel is a chemical rocket propellant that contains both fuel and oxidizer in a single substance, while a bi-propellant fuel is a rocket propellant consisting of separate fuel and oxidizer that come together only in a combustion chamber [16]. Monopropellants often are characterized with their relatively high performance together with simplicity, and better storability when compared to gaseous and bi-propellants, especially for miniaturized systems, while bi-propellants have improved thrust [17].

According to Haidn [15], low thrust rockets use mono-propellants such as hydrazine (N2H4) or nitrous oxide (N2O), while those requiring higher thrust utilize bi-propellants such as liquid oxygen, liquid hydrogen, or cryogenic fuels. The latter are used for prolonged space missions. Table 2 below shows bi-propellant fuels and oxidizers along with their values for their mixture ratios, specific impulse and density, where liquid oxygen provides higher specific impulse with most fuels on average.

Table 3: Analysis of fuels and oxidizers in rocket propulsion systems. Terms Mentioned: UDMH - unsymmetrical dimethylhydrazine, MMH - Monomethylhydrazine, UH25 - 75% unsymmetrical dimethylhydrazine (UDMH) and 25% hydrazine hydrate (N2H4) by weight.

Oxidizer	Fuel	Mixture Ratios [-]	Specific Impulse [s]	Density [kg/m3]
LO2 (Liquid Oxygen)	Kerosene	2,77	358	820
Liquid Hydrogen	4,83	455	700
Liquid Methane	3,45	369	430
N204 (Dinitrogen TetraOxide)	UDMH	1,95	342	791
MMH	2,37	341	880
UH25	2,15	340	850

Examining the table gives the conclusion that LH2/LO2 is an excellent choice for rocket fuel due to higher average rates of specific impulse. Also, the compound hydrogen peroxide acts as an oxidizer and fuel catalyst, which leads to higher engine efficiency. On the contrary, the degree of combustion tends to be violently exothermic, producing energies exceeding those beyond Nuclear Reactors (~3-4 GW), and thus is required to be stabilized by a coolant [15]. The various types of fuels used in hypersonic rockets are reviewed below, and the technique used to maximize the hypersonic speed, maintain stability, and optimize the efficiency.

Mono-Propellant Propulsion

According to Baek et al. [18], the use of a singular substance is the most efficient in economic resources. However, there was a slight issue regarding the toxic residue hydrazine left after the propulsion, which motivated the authors to work on green mono-propellants where a premixed liquid monopropellant based on hydrogen peroxide with ethanol blending was suggested to replace hydrazine in this research.

Bi-Propellant Propulsion

According to Connell [19], the utilization of bi-propellant fuels is heavily involved in rocket hypersonic engines. Impinging jet experiments were conducted to characterize ignition delay time for gelled n-dodecane and n-heptane hydrocarbon fuels containing varied sodium borohydride particle loadings, and aqueous solutions of hydrogen peroxide in bi-propellant propulsion. Experiments indicated that average ignition delay times ranged from approximately 78 down to 26 ms depending on sodium borohydride particle loading and fuel type. Dilution of hydrogen peroxide weight percent resulted in a significant increase in ignition delay.

Solid Fuel Propulsion

In the views of Adánez [20], the use of solid fuels in chemical looping combustion has been highly developed in the last decade and currently stands at a technical readiness level (TRL) of 6 which is attained when a technology's prototype system has been verified and demonstrated in an operational environment. Coal has been the most commonly used solid fuel in CLC, but biomass has recently emerged as a relatively cheap fuel for production and utilization.

Liquid Fuel Propulsion

As stated by Halchak [4], liquid propellant engines typically offer higher performance, that is, they deliver greater thrust per unit weight of propellant burned. They also have a considerably higher thrust to weight ratio. Since liquid rocket engines can be tested several times before flight, they have the capability to be more reliable, and their ability to shut down once started provides an extra margin of safety. However, they tend to materialize as a more expensive means of transport than solid fuels.

Cryogenic Fuel Propulsion

In the research proposed by Nøland [21], the combination of LH2 with all-electric cryogenic solutions is undoubtedly the best option to also reduce, or even eliminate, non-CO2 emissions and contrails and with the further avenues for development in Cryogenics. A theoretical solution of reducing the voltage level while increasing system-level power density and overall efficiency is explored for further research. The research paper shows how a next-generation H2-powered aircraft could take advantage of onboard cryogenic fuels to cool the electrical components, enabling a cryo-electric superconducting drivetrain that could lead to extraordinary performance.

Combined Cycle Engines

Overview

Typically, aircraft utilize engines that utilize a single mode of operation. However, combined cycle engines utilize a second component engine to broaden the operable speed range capabilities of an aircraft, particularly in the goal of achieving hypersonic flight. Combined cycle engines typically fall into two categories: turbine based combined cycle and rocket based combined cycle.

Ramjets and scramjets are industry leaders in hypersonic vehicles. However, they are significantly limited by their minimum functional speed due to a reliance on supersonic air intake [3]. Early designs avoided this issue by deploying hypersonic prototypes from carrier aircraft at high altitude [3]. However, for commercial, military, or general widespread use, this can be an impractical solution. Carrier aircraft require additional maintenance, fuel, and crew; the additional resources required for carrier aircraft will drive up operational costs significantly. Combined cycle engines solve this problem by using a secondary engine to propel the hypersonic vehicle to the minimum operational speed of a ramjet or scramjet.

Turborocket

Turborockets utilize a combination of turbojets, which are often seen on most modern aircraft, and rocket engines. The turbine component of this hybrid engine is a major limiter for this type of engine because it begins to lose its ability to accelerate around Mach 3 [22]. Consequently, the aircraft will have to utilize its rocket engine for any hypersonic flight. However, although rocket engines provide a large amount of thrust, they need to carry all of the oxidizer onboard the aircraft. This increases the weight of the craft and decreases the specific impulse provided by its engines. Therefore, attempting to sustain hypersonic flight in the atmosphere with rocket engines would be fuel inefficient when compared to airbreathing engines such as turboramjets [3]. For these reasons, turborocket designs are unlikely candidates for atmospheric hypersonic designs, but are a good basis for potential test beds or prototypes due to the use of proven and widely used technologies.

Turboramjet

Turboramjets use typical turbojets in combination with a ramjet, which is useful as the range that turbojets begin to cease functionality is the beginning of the range that ramjets begin to become functional [3]. These designs utilize turbojets which are proven to be effective at subsonic and low mach number flight. Secondly, the ramjets are effective at propelling the aircraft to hypersonic speeds of around Mach 8 [3]. A notable downside is that these engines have a speed ceiling set due to the need to decelerate in supersonic intake air down to subsonic speeds in order to facilitate stable combustion. This makes it so that the aircraft is not capable of as high mach number speeds as those enabled by a scramjet [3].

Baidya et al. [11] investigated the effect of multiple nozzles geometries, namely conical nozzle, bell nozzle, and dual bell nozzle, for use alongside turboramjet engines. This was done to create a successful model of sustained Mach 8 cruise flight for conceptual simulations. They attempted to achieve ramjets reaching hypersonic speeds by focusing on enhancing the thrust component. After numerous simulations and creations of mathematical modes, the authors concluded that dual bell nozzles ended up with best results. It demonstrated the best performance during altitude and pressure of 80,000 ft and Mach 4.5. The change of altitude was the one of the main challenges due to the dramatic changes of the atmospheric conditions; the models struggled to handle the pressure differences and higher degrees of over-expansion has led to “poor performance with inefficient utilization of the energy stored in the combusted exhaust” [11]. The study demonstrated the limitations of ramjet by itself. By adjusting necessary components such as the diffuser, combustion chamber, and nozzle, it was found that improvements to the ramjet’s efficiency didn’t produce the most stable results. As the nozzles tested were designed mostly for rockets, improved performances could be achieved by designing nozzle geometries specifically for airbreathing engines.

Detonation

Detonation engines tend to be less stable prototype engines that use a series of small explosions to aid in the compression of air. These engines provide greater fuel efficiency when compared to turbine engines by allowing increased mass flow rates due to the higher densities; however, detonation engines still suffer from universal limitations, mainly excessive temperatures, which limits the engine’s capabilities to speeds of about Mach 5 [23]. Furthermore, the significant use of detonations within the engine presents a potential for catastrophic failure, which would pose a serious concern if this design is to be used for commercial airlines.

Auxillary Methods

Pre-coolers

Pre-cooling is a promising technique for airbreathing vehicles, particularly turbojets, in hypersonic flight. It enhances engine performance and extended flight envelopes. In pre-cooled turbojet engines, incoming air is cooled using cryogenic liquid hydrogen or nitrogen fuel before entering the compressor, extending the speed range to Mach 5 by improving heat management as well as mass flow rates [9]. Recently, SABRE, a hypersonic vehicle designed by Reaction Engines, used pre-cooler technology to rapidly cool incoming air from over 1000°C to ambient temperatures in less than a hundredth of a second.

Magnetohydrodynamic

Magnetohydrodynamic (MHD) flow can interact with ionized plasma created from hypersonic movement to manipulate lift and drag [24] using electromagnets. This process is self-powering as electrons can be used in the flow around the vehicle to generate power. The working principle is to ionize air and use electromagnets to further accelerate the flow, resulting in increased thrust. According to Nguyen [24], while traditional control systems are limited due to “intense thermal and structural load during hypersonic flight,” MHD significantly reduces these requirements, lessening the need for control surfaces, center of mass control, and reaction control systems. Because MHD provides continuous aerodynamic control up to higher altitudes, typical control methods for flight at slower velocity are not needed.

Altitude Compensating Nozzles

Altitude compensating nozzles adjust to ambient pressure and aim to produce thrust efficiently at both high and low altitudes. In order to extract the maximum amount of work from the reacting flows through an engine, the exhaust pressure should be equal to the surrounding atmospheric pressure, which decreases with altitude. If the exhaust pressure is too high, less kinetic energy has been extracted from the fuel than was otherwise possible. Conversely, an exhaust pressure that is too low will force the air into higher-pressure air, also reducing the resultant thrust. Therefore, the exhaust pressure should be designed to change based on altitude to maintain optimal efficiency, which can be achieved through variable nozzle geometries [25]. Expansion–deflection nozzle (EDN) and dual-bell nozzle (DBN) are two commonly studied altitude compensation nozzles. Both have no moving parts and have two modes, one for lower altitude and one for higher altitude use [26]. An EDN produces higher thrust at a large nozzle pressure ratio, defined as the ratio of nozzle pressure to ambient pressure [26], while a DBN is more efficient when the NPR is low. EDNs operated at 75-82% efficiency for low NPR and 92% for high NPR. On the other hand, DBNs ran at 88-93% at low altitudes, a value which decreased to 64-73% when entering high altitudes.

Aerospikes

When designing a hypersonic vehicle, minimizing the effects of drag and aerodynamic heating is crucial for proper function. One such method is aerospikes, which have shown to be simple and effective methods for maximizing the tradeoff between these effects. Aerospikes consist of a large cylindrical rod placed at the central stagnation point of a blunt-body vehicle. Aerospikes manipulate the flow-field on the blunt nose in two main ways. First, they alter the strong detached bow shock wave into a system of smaller oblique shock waves. Second, they separate the boundary layer adjacent from the surface of the spike and creates a new shear layer that later reattaches to the the blunt-body [29]. This separated flow, as shown in Figure 4, creates a recirculation region of low temperature and pressure, which covers most of the front surface of the blunt vehicle, reducing the heat transfer rate and wave drag in both supersonic and hypersonic flight [29], [30]. However, aerospikes can create flow instabilities in the form of oscillations due to varying angles of attack and mach numbers, causing pressure and aerothermal fluctuations [31]. Since oscillations could arise, the spike's applicability is restricted, and lessens the spike's benefits and disrupt the flight's aerodynamics. Consequently, traditional spikes are only intended for hypersonic vehicles that perform only a few movements during flight.

Flow-field diagram of spiked and un-spiked blunt body

Figure 4: Simple flow-field diagram of both an un-spiked and spiked blunt-body.

To further improve upon traditional spikes, spikes with a large tip attached to the end, known as aerodisks, have been developed. Aerodisks operate similarly to aerospikes, but the disc-shaped tip pushes the reattachment point further down the surface. Aerodisks have shown to further increase the drag reduction and aerodynamic benefits of aerospikes, and are more applicable for greater ranges of mach numbers and angles of attack [32]. In particular, there are two main types of aerodisks: hemispherical disk spike and flat-faced disk spike. According to Ou et al. [31], hemispherical aerodisks are superior to flat-faced aerodisks in both drag and heat reduction. The effects of spikes and disks are heavily related to the geometry of the spike, primarily length [33]. Spikes are measured by the ratio between the length of the spike (L) and the diameter of the blunt-body (D), simplifying to an L/D ratio. As found by Deng et al. [34], the optimal L/D ratio for an aerospike is 2.0, producing a maximum aerodynamic drag reduction of 49.3%. Huang et al. [32] found that for a hemispherical aerodisk, the optimal L/D ratio is 1.0, producing a maximum drag reduction of 54.92%.

Conclusion - Hypersonic Propulsion Systems

To conclude, a major issue facing hypersonic designs in the modern day is that turbojet engines begin to lose their functionality as they go above mach three. New designs have been proposed to solve this issue by removing the turbine compressor, such as ramjets and scramjets, but those solutions have the same problem in its inverse: they have a minimum speed requirement. Combined cycle engines seek to solve this by utilizing multiple engines to reach hypersonic speeds. Examples of these designs being turborockets, a mix of turbojets and rockets; turboramjets, a mix of turbojets and ramjets; detonation engines, a unique version of a turbojet that utilizes explosives for air compression. Overall, combined cycle engines are leading candidates for experimental and practical implementations of propulsion mechanisms for hypersonic crafts. In addition, rocket propulsion systems are crucial for non-atmospheric hypersonic flight. They can use solid, liquid, or hybrid fuels. Liquid engines offer higher performance but are more complex and expensive than solid ones. Cryogenic fuels like liquid hydrogen and oxygen can provide high efficiency but require advanced technology. The choice of propellant depends on factors such as specific mission requirements, cost, and technological readiness. Overall, Rocket Propulsion Systems have proven effective, in maximizing efficiencies, along with their prevalent use alongside ramjets and scramjets. Finally, a variety of auxiliary systems that can enhance the performance of hypersonic propulsion technology are currently in development, with aerospikes and magnetohydrodynamic systems in particular showing significant promise.