Hypersonic Flight Vehicles
Hypersonic flight particularly has some special traits and some of which can be seen in every hypersonic flight. These traits are highly dependent on type of configuration, trajectory, etc. The nature of the hypersonic atmosphere encountered by the flight vehicle is decided as per the mission requirements.
For missions specifically for low fight duration with major deceleration closer to earth surface, vehicles with low angle of attack flight and sharp nose are used. Missions with longer flight duration prefer vehicles with blunt nose and high angle of attack flights.
The two parameters which govern the re-entry flight path of hypersonic vehicle are ballistic parameter and lifting parameter. Ballistic parameter is used for non-lifting re-entry of flights whereas Lifting parameter is used for lifting re-entry path. By applying conservation of momentum equation in the direction of the flight path and normal to it, we can obtain these equations. These governing flight parameters along with weight and surface area determine the velocity-altitude map of the flight.
Based on the design constraints of mission requirements, hypersonic flight vehicles are classified in four different types.
Types of Flight Vehicle
1. Re-entry Vehicles (RV)
Re-entry Vehicles use rocket propulsion system for launch and operations. These have blunt nose configuration and fly at high angle of attack. The control surfaces controls the re-entry of RV. Few examples are – BURAN and HERMES.
2. Ascent and Re-entry Vehicles (ARV)
Due to dual duty of ascent and re-entry, ARV have opposing specifications of their design. Duty of descent is dominated by fuel requirements whereas duty of re-entry is dominated by aero-braking. These flights are typically launched using rocket or air breathing propulsion system. Some examples of these vehicles are – Space Plane and HOTOL.
3. Cruise and Acceleration Vehicles (CAV)
The main features of Cruise and Acceleration Vehicles are the slender configuration and low angle of attack flight. CAV uses air breathing propulsion system of ScRAM / RAM jet types for launching and operation. With ablative cooling system, these vehicles can withstand high heating loads. First stage of Sanger is an example CAV as it operates using air breathing propulsion system.
4. Aero assisted Orbit Transfer Vehicle (AOTV)
The major concern of these vehicles is ionisation leading to the presence of plasma in the vicinity of the spacecraft.
Hypersonic Flow Regime
Hypersonic flow is when the flow exceeds the speed of sound by five times. In terms of Mach Number, one can define the flow regime. However, there are various characteristics of a flow which make it hypersonic flow.
Characteristics of Hypersonic Flow
1. Thin Shock Layers
Shock layer is the region between shock and the flight vehicles body. Shock angle, Mach angle and flow deflection angle share a relation such that for same flow deflection angle, there is a decrease in shock angle with increase Mack angle. Therefore, with increase in Mach number, shock layer thickness decreases. And therefore, hypersonic flow has thin shock layer. The interpretation of shock layer thinness is applicable for chemically reacting flow, calorically perfect gas and thermally perfect gas. However due to thin shock layer where boundary layer thickness and shock layer thickness are comparable, complexity of flow field increases.
2. Viscous Interaction
The no-slip property of the viscous fluid flow causes boundary layer formation near the wall. Such boundary layer formation leads to huge loss of kinetic energy at hypersonic speeds.
The conversion kinetic energy to thermal energy results to increased temperature of the flow in the vicinity of wall. This is known to be viscous dissipation. Due to increase in viscosity coefficient with temperature, viscous dissipation causes the boundary layer thickness to increase. Since this increased boundary layer thickness displaces outer inviscid flow.
Hence due to encounter between freestream hypersonic flow and an inflated object, there is a change in shape of the shock and parameters of the intern boundary layer along with surface pressure, skin friction, wall heat flux, etc. This communication loop between the outer inviscid flow and the viscous boundary layer is termed as Viscous Interaction.
Without the viscous interaction, the aerodynamic parameters deviate from the base value therefore, it’s compulsory to consider viscous interaction for hypersonic flow.
3. High Temperature Flow
Viscous dissipation causes higher temperature of the boundary layer fluid along with increase in thickness of the boundary layer. Therefore, in the vicinity of the flight vehicle high temperature fluid is experienced in hypersonic flow.
Also, blunt nose configuration is a reason behind high temperatures as normal shock is present at the stagnation point. The treatment of fluid such as calorically perfect gas leads to unrealistic solutions at elevated temperatures, therefore it’s necessary to consider the dependence of specific heats and their ratios as a temperature function.
4. Entropy Layer
Entropy layer is one of the main characteristics of the hypersonic flow. Each streamline passing through the shock faces different entropy rise. The weaker portion of shock leads to les entropy rise as compare to the weaker portion. A layer of entropy variation forming downstream of the shock defines the entropy layer.
5. Low Density Flow
At higher altitudes, hypersonic flights experience low density flows. The governing non-dimensional parameters of such flow regime I called Knudsen No. Knudsen number is the ration between the mean free path and the characteristic length of the object.