orion thermal protection system

The Effects of Friction on Orion’s Thermal

Protection System During Atmospheric Entry

K.J. Mattalo

St. Aloysius Gonzaga Secondary School

SPH-4U1

March 25, 2012

Abstract

We analyze the dynamics of the Orion Crew Module during the atmopsheric

entry phase and the properties of the Orion Crew Module’s Thermal Protection

System in protecting against frictional damage.

1 Introduction

Orion is a next-generation spacecraft being developed by NASA as part of its Con-

stellation program. The primary goals of the Constellation program and the Orion

spacecraft is to carry astronauts to the International Space Station by 2015 and to

the Moon by 2020 [1]. It will also serve as the primary successor of the Space Shuttle

Program as a means of exploration past low-Earth orbit and into the rest of the solar

system with the aid of the Ares I rocket. In order to achieve such feats, the design of

the Orion spacecraft must overcome various engineering issues. One of the primary

issues which must be overcome is the stabilization of the Orion Crew Module dur-

ing atmospheric entry onto Earth and other planets. Also, the Orion Crew Module

requires the construction of a durable ablative shield that can overcome excessive

temperature increases due to high-velocity atmospheric entry and the resultant air

resistance acting on the spacecraft. In this paper we will investigate the properties of

Orion through the aerodynamic design of the spacecraft, the properties of the mate-

rials used and in conclusion to understand how each of these aspects affect the overall

performance of Orion through analyzing the dynamics of ballistic objects.

2 K.J. Mattalo

2 Orion Crew Module Aerodynamics

The aerodynamics of the Orion Crew Module is a critical design component that is

important for the thermal properties of Orion, the dynamics resulting from the acting

forces and the stability of Orion through turbulent atmospheres at high-velocities.

The Orion Crew Module derived its design from the Apollo Command Module because

of its effective ability to increase stability and decrease thermal damage due to air

resistance [2].

Figure 1: Orion Crew Module [3] Figure 2: Apollo Command Module [4]

The design of these modules in Figure 1 & 2 are nearly identical in structure and

shape and by analyzing the aerodynamic properties and forces acting on this design,

its effectiveness becomes more obvious.

The primary aerodynamic component of the Orion Crew Module is the shape and

surface area of its heat shield. The circular shape of the heat shield allows for even

distribution of air resistance forces over the entire surface. This stabilizes the motion

of the module since according to Newton’s Second Law, an object accelerates in the

direction of the unbalanced force and it is the circular shape itself that minimizes the

deviation of forces acting over the surface area of the shield. The functions of the

Figure 3: Heat Shield [5] Figure 4: Shield Temperature Flux [6]

The Effects of Friction on Orion’s Thermal Protection System During Atmospheric Entry 3

heat shield’s large surface area is to maximize the air resistance forces contributing to

the module’s deceleration before parachute deployment and to separate the occupants

of the vehicle from the large temperature shifts occuring at the base [7]. Figure 4

provides an observational aid in visualizing how the heat shield effectively absorbs

and deflects the flow of the hot air [8]. As air resistance increases due to the rapid

density increase of the atmosphere the heat shield approaches temperatures of 2760◦C

as indicated by the red colouring in Figure 4 [9]. Further analysis of Figure 4 shows

that the heat flow is forced outwards, away from the module and it dissipates in the

air as it returns to a blue colouring.

During the atmospheric entry phase the Orion Crew Module enters the atmosphere

at an angle α relative to the direction of its velocity. This angle α represents the angle

of attack of the Orion Crew Module. The angle of attack is critical in producing lift

in airfoils and can also produce lift forces on the module.

Figure 5: Angle of attack during entry [10] Figure 6: Angle of attack [11]

As the value of α increases the lift forces acting on the module also increase; aiding

in the deceleration of the module [12]. Figure 5 shows the angle of attack during the

re-entry process of the Orion Crew Module. The angle between the intersection of

the flame trail (direction of velocity) and the line passing perpendicular to the heat

shield (direction of heat shield) is the angle of attack of the module. The resultant

lift force gradually decelerates the module as the density of air increases due to a

pressure differential formed over the top and bottom of the module.

3 Physical Properties of Heat Shield Materials

On returning from a deep space mission the Orion Crew Module will experience

extreme fluctuations in temperature as it enters the Earth’s atmosphere at 37, 000

4 K.J. Mattalo

km/h [13]. These extreme temperatures are capable of melting iron and various other

high strength metals and materials. To preserve the integrity of the heat shield, Orion

contains advanced ceramic and silicate materials that ablate (absorb heat and burn)

and redirect the flow of high temperature air around the capsule [14]. There are two

functioning thermal protection materials on the Orion Crew Module, the first being

AVCOAT which is composed of silcon dioxide (SiO2) embedded within a honeycomb

fiberglass matrix, mixed with a thermoset resin (cures irreversably) [15].

Figure 7: Honeycomb fiberglass [16] Figure 8: Thermoset polymer resin [17]

Figure 9: Silicon dixide: red is oxygen, silver is silicon [18]

This layer is the ablation component of the heat shield, it absorbs the heat and breaks

down chemically into carbon and silica. The heat absorbed by this material is directed

away from the module due to the flow of air being forced outwards by the circular

shield as shown in Figure 4. The secondary material is a ceramic composite material

known as AETB-8 tiles.

Figure 10: AETB-8 ceramic composite tile [19]


The thermal advantages of using a ceramic composiite is that it is extremely heat

resistant and only begins to decompose at temperatures beyond 2,000◦C [20]. Fur-

thermore, ceramic composites have very low thermal expansion and thermal conduc-

tivity aiding in the durability of the Orion Crew Module and in the safe keeping of

the occupants [21].

4 Ballistics of The Orion Crew Module

During the atmospheric entry phase of the Orion Crew Module, it undergoes a ballistic

entry which is when the force contributing to the deceleration (drag/air resistance)

is opposite the direction of the velocity v0.

Figure 11: Free Body Diagram Showing Forces and Velocity Components Acting On

Orion During Re-Entry.

In this diagram FD, the force of drag is acting opposite in direction to the direction

of the velocity v0 and has both vertical and horizontal components as does the velocity

v0. The Orion spacecraft is accelerating downwards at high altitudes because of the

lack of air density but as it increases FD also increases since it is a function of the

velocity of the object and the density of air. The full representation of the force of

drag is given by the equation:

FD =1

2ρv2CdA (1)

where ρ is the density of the medium, v is the velocity of the object, Cd is the drag

coefficient which is a dimensionless constant derived from the shape of the object and

A is the reference area (i.e the area of the heat shield) [22]. Since FD is angled the

forces can be divided into components giving the following two equations governing

the dynamics of Orion during re-entry:

6 K.J. Mattalo

∑Fx = ma (2)

FD cos(α) = ma

and

∑Fy = ma (3)

FD sin(α) − Fg = ma

By analyzing equation (2) you can see that there is only one force acting in the x-

direction therefore Orion will decelerate according to the values of the parameters

in equation (1). In the y-direction there are two forces acting on Orion, the force

of gravity Fg and the y-component of the force of drag FD sin(α). At high altitudes

Fg > FD sin(α) but since FD sin(α) is a function of air density (ρ) and velocity (v) as

the density of air rapdily increases FD sin(α) also increases thus counter-balancing Fg

and the result is Fg < FD sin(α). But, as the velocity slows down FD sin(α) decreases

and they reach a dynamic equilibrium where Fg = FD sin(α).

The dynamics of FD on Orion during the re-entry is the reason why that aerody-

namics of Orion and the thermal properties of its materials must be so precise and

finely engineered. It is this force which produces the immense heat wishtood by the

shield and the AVCOAT/AETB-8 materials. It is through the design of an efficiently

aerodynamic module and through the selection and testing of advanced materials

that the effects of friction on the Orion Crew Module and be reduced and safeguard

the future for human space exploration.

”Imagination will often carry us to worlds that never were. But without it we go

nowhere.” - Carl Sagan

Figure 11: Artistic Impression of Orion During Atmospheric Entry [23]


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8 K.J. Mattalo

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orion thermal protection system

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