aerodynamics of reentry

8/10/2019 Aerodynamics of Reentry

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A Re-entry Vehicle is a spacecraft that travels through space and re-enters theatmosphere of a planet (Earth), and most of the time, does not have an engine.

When returning to Earth or when landing on another planet, a safe reentry through

the atmosphere is needed.

Safe re-entry can be difficult, because the very high speed of the spacecraft creates

very high temperatures, when entering through the atmosphere.

Hence we need to study the aerodynamics, which involves the prediction of forces

produced on the vehicle by the atmosphere.

This seminar topic mainly concentrate on the aerodynamics of such reentry vehicles

like shape of the vehicle to be used, early designs and effect of different parameters

on heating and shock waves.


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Newtons sine squared law, states that the pressure coefficient is proportional to the

sine square of the angle between a tangent to the surface and the direction of free

stream.

Under these assumptions, the nondimensional pressure coefficient, Cp , at any point

on the surface of a body can be obtained from the Newtonian sine-squared

relation.

= 2

fig(1): momentum transfer of particles on inclined surface


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Newton originally applied his theory to model the pressure on the walls of a water

channel. Experimental tests performed by dAlembert later concluded that this

model is inaccurate for subsonic flow conditions.

However, as the Mach number increases to hypersonic speeds, the shock wave

approaches the surface of the body. Thus as the flow velocity changes directionafter crossing the shock, the flow appears to be deflected by the body similar to

Newtonian flow theory as shown in fig(1)

As the Mach number continues to increase, the shock continues to approach the

body surface. Thus, as Mach number increases, Newtonian flow theory improvesin accuracy and the aerodynamic coefficients are computed independent of Mach

number


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Step 1: Surface Parameterization

r = [f(u, v) g(u, v) h(u, v)] T

Step 2: Compute pressure coefficient

sin = Step 3: Compute Shadow Boundary

Step 4: Compute Reference Values

=

Step 5: Evaluate Surface integral

Step 6: Output Aerodynamics Code


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1.Sharp Cone FamilyThe sharp cone family is parametrized by the cone half-angle, andlength along the axis of revolution, L. The surface of the sharp cone is

parametrized using the local radius from the axis of revolution, = andrevolution angle, =

Comparisons between the analytic relations and CBAERO in both force andmoment coefficients for a 20 sideslip and various cone half angles and angles ofattack excellent agreement was observed between the analytic moment equationsand CBAERO.


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Sharp cone force coefficient validation, = 20


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2.Spherical Segment FamilyThe spherical segment family is parametrized by the nose radius,

and cone half-angle,

3.Cylindrical Segment FamilyThe cylindrical segment family is parametrized by the nose radius, ,

and blended wedge half-angle, .


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The shape of common entry vehicles can be determined throughsuperposition of basic shapes.

Sphere-cones can be constructed using a spherical segment and a singleconical frustum, and

Biconics can be constructed using a spherical segment and two conicalfrustums.


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Ballistic coefficient

During reentry phase the drag is force is resisting the vehicles motion. So it is

decelerating the vehicle. From Newtons second law of motion this deceleration

can be written as

=

This quantity has had a special significance in describing how an object

moves through the atmosphere. By convention, engineers invert this term and

call it the Ballistic coefficient, =


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From this basic relationships, we can show that the amount of deceleration anobject experiences while traveling through the atmosphere is inversely related tothe object's ballistic coefficient.

In everyday terms, we would say a light, blunt vehicle (low BC) slows downmuch more rapidly than a heavy, streamlined (high BC) one, as shown in Figure


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Generally shock wave will be developed during reentry phase, Depending onthe vehicles shape, the shock wave can either be attached or detached .

If the vehicle is streamlined (high BC, like a cone), the shock wave mayattach to the tip and transfer a lot of heat, causing localized heating at theattachment point.

If the vehicle is blunt (low BC, like a rock), the shock wave will detach andcurve in front of the vehicle, leaving a boundary of air between the shockwave and the vehicles surface.


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We can quantify the heating rate, a re-entry vehicle experiences as

= 1.83 10

we can plot heating rate, versus altitude for various re-entry velocities as sown

in graph. We see that the maximum heating rate increases as the reentry velocity

goes up.


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We can find the altitude and velocity where the maximum heating rate

occurs using

=

and = 0.846

We also can vary the re-entry flight-path angle, to see how it affects the

maximum heating rate as shown in graph

Steep re-entry angles cause high maximum heating rates but for a shorttime

Shallow re-entry causes low maximum heating rates but for a long time


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The higher the BC (the more streamlined the vehicle), the deeper it

plunges into the atmosphere. This means a streamlined vehicle spends less time

in the atmosphere and reaches the ground long before a blunt vehicle.


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To protect the vehicle from severe hot condition we use specially formulated

materials and design techniques called thermal protection systems (TPS). Well

look at three approaches to TPS

Heat sinks

Ablation

Radiative cooling


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Heat sink using extra material to absorb the heat, keeping the peak temperaturelower.

In ablation technique the vehicles surface is coated with a material having a

very high latent heat of fusion, such as carbon or ceramics. As this materialmelts or vaporizes, it soaks up large amounts of heat energy and protects thevehicle.

The process of reducing equilibrium temperatures by emitting most of the heatenergy before a vehicles structure can absorb it is known as radiative cooling


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1. Re-entry at high total angle of attack provides a reduction in both

maximum heating rate and total heating encountered.

2. It is possible by suitable choice of configurations to obtain a vehicle

with desirable aerodynamic characteristics during entry and good

subsonic flying qualities for landing.

3. We can meet requirement on the design front by changing vehicle

shape & size, vehicle thermal protection systems.


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Michael J. Grant and Robert D. Brauny, Analytic Hypersonic Aerodynamics for

Conceptual Design of Entry Vehicles, 48th AIAA Aerospace Sciences Meeting

Including the New Horizons Forum and Aerospace Exposition 4 - 7 January 2010.

John D Anderson Jr, Fundamentals of aerodynamics, fifth edition Tata Mcgraw

hill publication.

Sheikh Arslan Ali, Dr. Mukkarum Husain, Dr. M. Nauman Qureshi, Effects of

Nose-Bluntness Ratio on Aerodynamic Performance for Re-entry Vehicle, Journal

of Space Technology, Vol 1, No. 1, July 2012.

Steven P. Schneider, Hypersonic laminar turbulent transition on circular cones and

scramjet forebodies , Progress in Aerospace Sciences, Volume 40, Issues 1 2,

February 2004, Pages 1-50


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aerodynamics of reentry

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