1. INTRODUCTION

In today’s realm of aeronautics and astronautics, the reality of costs to access space has been

astoundingly high, making it quite difficult to make space access affordable. Some of the

biggest contributing factors to the high cost in accessing space are in the design of the

propulsion system, fuel cost, and regulations. It is given that improved rocket nozzles can

lead to heavier payloads, less weight, longer range, and much lower costs. The multi-purpose

low-cost reusable Single-Stage-To-Orbit (SSTO) transportation system concept is a strong

trend of aerospace technology development. For this type of system, the aerospike nozzle is

critical. Over the years, the aerospike nozzle has already attracted much attention with its

outstanding advantages of automatic altitude compensation ability of altitude performance,

which would complement of SSTO applications.

More recently, during the 1990’s, NASA invested in the development of aerospike

technology for Single-Stage-To-Orbit (SSTO) Reusable Launch Vehicles (RLV) as part of

the now-defunct X-33 program. This program led to the development of several linear

aerospike engines, RS 2200, which were tested repeatedly. To date, however, no aerospike

engine is known to have powered a rocket in flight. Unlike conventional bell-shaped nozzles,

which operate optimally at one particular altitude, plug nozzles allow the flow expansion to

self-adjust, thus improving thrust coefficients. This improvement over conventional bell-

shaped nozzles occurs at altitudes lower than the design altitude. This is particularly critical

for SSTO vehicles, which operate both in the atmosphere and in vacuum . At altitudes higher

than the design altitude, plug nozzles essentially operate similarly to bell nozzles.

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1. HISTORY

NASA and its industry partner in the X-33 Advanced Technology Demonstrator program,

Lockheed Martin Aeronautics Co., of Palmdale, Calif., have taken a 30-year-old idea the

linear aerospike engine and updated it for the 21st century by incorporating new technologies

and materials. The effort is managed by NASAs Marshall Space Flight Center in Huntsville,

Ala., NASAs Lead Center for Space Transportation Systems Development and Center of

Excellence in Propulsion.

The aerospike engine is being developed from groundwork laid in the 1960s and 1970s by

the Rocketdyne Propulsion & Power unit of The Boeing Company in Canoga Park, Calif.

Unlike conventional rocket engines, which feature a bell nozzle that constricts expanding

gasses, the basic aerospike shape is that of a bell turned inside out and upside down. When

the reconfigured bell is "unwrapped" and laid flat, it is called a linear aerospike. The

aerospike engine is being developed from groundwork laid by power unit of Boeing

Company in California. The effort is managed by NASAs Marshall Space Flight Center.

NASA engineers at the Marshall Center have conducted a number of tests for the linear

aerospike engine. Rocketdyne conducted a lengthy series of tests in the 1960s on various

designs. Later models of these engines were based on their highly reliable J-2 engine

machinery and provided the same sort of thrust levels as the conventional engines they were

based on; 200,000 lbf (890 kN) in the J-2T-200k, and 250,000 lbf (1.1 MN) in the J-2T-

250k (the T refers to the toroidal combustion chamber). Thirty years later their work was

dusted off again for use in NASA's X-33 project. In this case the slightly upgraded J-2S

engine machinery was used with a linear spike, creating theXRS-2200. After more

development and considerable testing, this project was cancelled when the X-33's composite

fuel tanks repeatedly failed.

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3. NEED FOR NEW DESIGN

The revolution in aerospace propulsion was increased greatly during World War 2. Faster,

bigger and more efficient aerospace vehicles were required which led to the birth of Space

research organizations like NASA. Speaking about the future, advanced rocket propulsion

systems will require exhaust nozzles that perform efficiently over a wide range of ambient

operating conditions. Most nozzles either lack this altitude compensating effect or they are

extremely difficult to manufacture. Bell nozzles are currently used for all aerospace

applications. As stated earlier, the main drawback of this design is the decrease in efficiency

with the increase in altitude as there is a loss of thrust in the nozzle. This occurs due to a

phenomenon called “separation” of the combustion gases. For conventional bell nozzles, loss

mechanisms fall into three categories:

Geometric or divergence loss,

Viscous drag loss,

Chemical kinetics loss

Geometric loss results when a portion of the nozzle exit flow is directed away from the

nozzle axis, resulting in a radial component of momentum. In an ideal nozzle, the exit flow is

completely parallel to the nozzle axis and possesses uniform pressure and Mach number. By

calculating the momentum of the actual nozzle exit flow and comparing it to the parallel,

uniform flow condition, the geometric efficiency is determined. By careful shaping of the

nozzle wall, relatively high geometric efficiency can be realized. A drag force, produced at

the nozzle wall by the effects of a viscous high-speed flow, acts opposite to the direction of

thrust, and therefore results in a decrease in nozzle efficiency. The drag force is obtained by

calculation of the momentum deficit in the wall boundary layer. The third nozzle loss

mechanism is due to finite-rate chemical kinetics. Ideally, the engine exhaust gas reaches

chemical equilibrium at any point in the nozzle flow field, instantaneously adjusting to each

new temperature and pressure condition. In real terms, however, the rapidly accelerating

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nozzle flow does not permit time for the gas to reach full chemical equilibrium. A long

nozzle is needed to maximize the geometric efficiency; but simultaneously, nozzle drag is

reduced if the nozzle is shortened. If chemical kinetics is an issue, then the acceleration of

exhaust gases at the nozzle throat should be slowed by increasing the radius of curvature

applied to the design of the throat region. The optimum nozzle contour is a design

compromise that results in maximum overall nozzle efficiency. Nozzle contours can also be

designed for reasons other than for maximum thrust. Contours can be tailored to yield certain

desired pressures or pressure gradients to minimize flow separation at sea level. A nozzle

contour designed to produce parallel, uniform exit flow, thereby yielding 100 % geometric

nozzle efficiency, is called an ideal nozzle.

This ideal nozzle is extremely long and the high viscous drag and nozzle weight that results

are unacceptable. Some design approaches consider truncating ideal nozzles keeping in mind

the weight considerations. Most companies have a parabolic curve-fit program, generally

used to approximate optimum contours, which can also be used to generate desired nozzle

wall pressures. For nozzles at higher altitudes, vacuum performance is the overriding factor

relating to mission performance and high nozzle area ratio is therefore desirable. However,

nozzle over-expansion at sea level does result in a thrust loss because the wall pressure near

the nozzle exit is below ambient pressure. If the nozzles exit area could be reduced for launch

and then gradually increased during ascent, overall mission performance would be improved.

The ideal rocket engine would make use of a variable-geometry nozzle that adjusted contour,

area ratio and length to match the varying altitude conditions encountered during ascent. This

feature is referred to as Altitude Compensation.

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4. BASIC PRINCIPLES

Aerospike nozzles can be described as inverted bell nozzles where the flow expands on the

outside of the nozzle instead of being completely constrained by the nozzle walls. compares

the flow at low (liftoff) and high (space) altitudes for both types. Unlike conventional bell-

shaped nozzles, which operate optimally at one particular altitude, plug nozzles allow the

flow expansion to self-adjust, thus improving thrust coefficients. This is particularly critical

for Single Stage-to-Orbit (SSTO) vehicles, which operate both in the atmosphere and in

vacuum. This improvement over conventional bell-shaped nozzles occurs at altitudes lower

than the design pressure ratio. At altitudes higher than the design altitude (or pressure ratio),

plug nozzles essentially operate similarly to bell nozzles. Many references discuss these

advantages8,9 as well as typical flow characteristics on plug nozzles.10,11 The main

drawbacks associated with the aerospike nozzle are the often higher cooling requirements

because the throat regions typically cover larger areas than for conventional bell

shapenozzles12 and the strong engine vehicle interactions. While the terms plug and spike

nozzles are interchangeable, some authors associate aerospike nozzles with truncated spike

nozzles with base bleed. In this paper, all three terms are used interchangeably. In addition to

these performance advantages over bell nozzles in atmospheric flight, plug nozzles may also

offer improved packaging, reduced cost and increased reliability for space engines.13 For

launch vehicles, both of conventional (cylindrical) type like typical expendable systems and

of other shapes such as the formerly proposed Venture Star, aerospike engines typically

cover the entire base of the vehicle, leading to a lighter structure for transferring the

propulsion loads to the rest of the vehicle along with reduced base drag.

The basic concept of any engine bell is to efficiently expand the flow of exhaust gases from

the rocket engine into one direction. The exhaust, a high-temperature mix of gases, has an

effectively random momentum distribution, and if it is allowed to escape in that form, only a

small part of the flow will be moving in the correct direction to contribute to forward thrust.

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Figure: 1 Comparison between the design of a bell-nozzle rocket (left) and an aerospike

rocket (right)

Instead of firing the exhaust out of a small hole in the middle of a bell, an aerospike engine

avoids this random distribution by firing along the outside edge of a wedge-shaped

protrusion, the "spike". The spike forms one side of a virtual bell, with the other side being

formed by the outside air—thus the "aerospike".

The idea behind the aerospike design is that at low altitude the ambient pressure compresses

the wake against the nozzle. The recirculation in the base zone of the wedge can then raise

the pressure there to near ambient. Since the pressure on top of the engine is ambient, this

means that the base gives no overall thrust (but it also means that this part of the nozzle

doesn't lose thrust by forming a partial vacuum, thus the base part of the nozzle can be

ignored at low altitude).

As the spacecraft climbs to higher altitudes, the air pressure holding the exhaust against the

spike decreases, but the pressure on top of the engine decreases at the same time, so this is

not detrimental. Further, although the base pressure drops, the recirculation zone keeps the

pressure on the base up to a fraction of 1 bar, a pressure that is not balanced by the near

vacuum on top of the engine; this difference in pressure gives extra thrust at altitude,

contributing to the altitude compensating effect. This produces an effect like that of a bell

that grows larger as air pressure falls, providing altitude compensation.

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The disadvantages of aerospikes seem to be extra weight for the spike, and increased cooling

requirements due to the extra heated area. Furthermore, the larger cooled area can reduce

performance below theoretical levels by reducing the pressure against the nozzle. Also,

aerospikes work relatively poorly between Mach 1-3, where the airflow around the vehicle

has reduced pressure, and this reduces the thrust.

5. ROCKET NOZZLE THEORY

Rocket motor nozzles have typically represented the shape of a cone/conical (De Laval

Nozzle), or a bell shaped nozzle designed to the specific altitude. These nozzles suffer from

reduced efficiency at low and high altitude because of the difference between the pressure of

the exhaust gases and the ambient pressure of the atmosphere . At low altitudes, the exhaust

gases are lower in pressure than the atmospheric pressure. This causes the jet exhaust to

constrict and become separated from the nozzle walls reducing the amount of thrust

generated. This condition is known as over expansion as shown in Fig. 2a. When the exhaust

pressure is the same as the ambient pressure, a column shaped exhaust plume is formed. This

condition corresponds to thrust at maximum efficiency as shown in Fig. 2b. At high altitude,

the exhaust gases are at a higher pressure than ambient and continue to expand past the

nozzle exit as shown in Fig. 2c. Since additional expansion occurs outside of the nozzle, the

under-expanded exhaust plume results in thrust loss and lower nozzle efficiency. In a bell

nozzle combustion gases flow through a constriction (throat) and then the expansion away

from the centerline is contained by the diverging walls of the nozzle up to the exit plane.

Bells nozzles are a point design with optimum performance at one specific ambient pressure

(i.e., altitude). Careful design is needed to achieve desired high altitude performance while

avoiding flow separation at the walls of the nozzle near the exit when operating at low

altitudes (launch), which can lead to loss of performance and possible structural failure of the

nozzle due to dynamic loads [flow separation is responsible for the large nozzle motion on

the SSMEs during startup transient - watch closely during next launch]. Therefore a

compromise altitude must be used for the design point of a bell nozzle.

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Figure 2. Bell nozzle behavior during flight

6. AEROSPIKE NOZZLE THEORY

An aerospike nozzle utilizes a similar principal for propulsion, except that the exhaust

remains attached to the center-body, also known as a plug or aerospike . Since the aerospike

has no outer walls, the pressure of the atmosphere (outer free jet boundary), causes the

exhaust gases to hug its outer surface. At low altitudes, high ambient pressure forces the

exhaust gases inward increasing the pressure on the spike as shown in Fig. 3a. At optimal

pressure (exhaust pressure equals ambient pressure), the flow becomes column shaped, much

like a bell nozzle, for maximum thrust efficiency as shown in Fig. 3b. When operating at low

ambient pressure, the flow is constrained by expansion and compression waves that direct the

exhaust axially to maintain the thrust force on the spike shown in Fig. 3c. Many believe that

this characteristic of aerospike nozzles is their ultimate strength, a concept called, “altitude

compensation.” Altitude compensation results from the fact that the outer plume boundary is

acted on by the ambient pressure of the atmosphere. A full aerospike nozzle has a full-length

conical or isentropic spike, where as a truncated aerospike nozzle has a truncated conical or

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Figure 3 Annular Aerospike nozzle.

isentropic spike in which an "aerodynamic spike" is created from the re-circulating flow aft

of the flat nozzle base. Losses from truncating the aerospike can be offset by adding a

secondary gas flow(base bleed). This modification can help add pressure to the base portion

of the spike, therefore aiding in the overall thrust performance of the truncated aerospike.

The base bleed is found to be efficient when using only 0.3-3% of the total exhaust flow of

the nozzle according to past studies. In addition, the base region is open to high ambient

pressure resulting in a greater "base" thrust component. Experimental data on altitude

compensation using full and truncated aerospike nozzles collected by Rocketdyne.

According to theory, the full aerospike should meet or exceed the performance of the bell

nozzle at all altitudes due to the inherent characteristics of altitude compensation. In some

though, experimental data have shown that these predictions are not necessarily true for all

cases. There needs to be actual flight tests to evaluate both sets of data. On that note, there

are many potential sources of performance losses in the design of a practical

engine. In the design process of an aerospike nozzle, many shapes are considered. The most

common shapes of aerospike nozzles are annular/conical, annular/isentropic curve (ideal

shape), and linear versions of the plug, sometimes with truncation. Performance losses in a

conical nozzle instead of an isentropic aerospike can be than 1% for cone half-angles up to

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30°. The aerospike shape allows the exhaust gases to expand through an isentropic (constant

entropy) process. This process allows the nozzle efficiency to be maximized. Even though

the isentropic spike is the most efficient shape, it is realistically longer and weighs much

more than a conical nozzle. In summary, the advantages of aerospike nozzle altitude

compensation result in higher nozzle efficiency at all altitudes and a truncated aerospike can

be far smaller than a typical full aerospike and bell nozzle for nearly the same performance.

In contrast, the disadvantage of an aerospike nozzle is much greater heat flux than a typical

conical or bell nozzle. One of the solutions to this problem can be fixed by truncating the

spike to reduce the exposed area and to cool the aerospike by regenerative cooling. A base

bleed or that is, secondary flow, can also help reduce the temperatures of the spike with

increasing the performance of a truncated nozzle. The aerospike nozzle is more complex and

difficult to manufacture than the conical or bell nozzle. As a result, this can be more

costly. There have been very few aerospike flight tests in rocket or space applications.

Figure 4. Aerospike nozzle behavior during flight

5.1 WORKING PRINCIPLE OF AEROSPIKE NOZZLE In an aerospike nozzle, ambient pressure places a limit on the expansion process and the

thrust loss associated with nozzle over-expansion does not materialize. Since ambient

pressure controls the nozzle expansion, the flow area at the end of the aerospike changes with

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altitude. A key advantage of an aerospike is that a very high area ratio nozzle, which provides

high vacuum performance, can also be efficiently operated at sea level. The primary exhaust

can be seen expanding against the center body and then around the corner of the base region.

The interaction of this flow with the re-circulating base bleed creates an inner shear layer.

The outer boundary of the exhaust plume is free to expand to ambient pressure. Expansion

waves can be seen emanating from the thruster exit lip, and these waves reflect from the

centerbody contour to the free jet boundary. At low altitude, the free boundary remains close

to the nozzle causing the compression waves to reflect onto the center body and shear layer

themselves. The waves impacting the center body increase pressure on the surface, thereby

increasing the center body component of thrust. The waves impacting the shear layer, on the

other hand, increase the circulation of the base flow thereby increasing the base component

of thrust. Ideal behavior results from the fact that the outer plume boundary of the primary

flow is acted upon only by the ambient pressure of the atmosphere. From Aerospike thrust

characteristics the high ambient pressure at low altitudes forces the exhaust inward increasing

the pressure on the "centerbody" and the centerbody component of thrust. In addition, the

base region is open to high ambient pressure resulting in a greater "base" thrust component.

At design pressure, the flow becomes column shaped, much like a bell nozzle, for maximum

efficiency.

5.2 DESIGN OF AEROSPIKE NOZZLE

The design of spike contour of the aerospike nozzle is the most important step in the overall

design of the nozzle, which varies according to the operating conditions and application.

However the design procedure, including the basic physics behind the spike design remains

the same. The design of spike can be done using a simple approximate method or Rao’s

method based on calculus of variation. The simple approximate method assumes a series of

centered isentropic expansion waves occurring at cowl lip of the spike nozzle. Using this

method the annular spike contour for a given pressure ratio, area at throat, and ratio of

specific heats can be determined.

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5.3 AEROSPIKE NOZZLE ANALYSIS

The aerospike nozzle uses an annular/ toroidal and full conical spike configuration. The

geometry and design of the nozzle is based off of the identical throat areas for the conical

nozzle that came with the hybrid rocket motor. This was done in order for the experimental

data to be comparable in both nozzle systems. One of the most important parameters in

rocket nozzle design is the expansion area ratio.

In previous designs, other academic and industry groups used aerospike nozzles with a center

shaft from the main injector bulkhead down to the aft end of the combustion chamber that

supported the spike. Previous results have had design issues with the central graphite shaft

fracturing and leading to catastrophic rocket motor failures. Different methods of

modifications were used to compensate for this problem. UCLSB incorporated a solid

metallic alloy rod down the center of the graphite spike for their bipropellant aerospike

rocket motor. Boston University used a steel rod with a solid fuel grain molded around the

support shaft for their solid propellant aerospike rocket motor. Both methods proved too

heavy in material and added unnecessary weight to the rocket motor. Two different aerospike

nozzle designs are being considered in this investigation. Both nozzles incorporate a short

compact version of the aerospike, eliminating the need for a central support shaft for the

spike. Coincidentally, both aerospike nozzle designs had ported heads into an aft mixing

chamber. Both aerospike nozzles had 8 ports (flow passages) with a total area greater than

the choked exit area. The post mixing chamber not only eliminates the need for the support

shaft, but it also aid in the mixing process of the oxidizer and fuel grain before expulsion of

the exhaust gases for improved thrust efficiency. The full aerospike design was fabricated

from mild steel (1018). The aerospike nozzle also had a greater mixing chamber total

volume. The steel aerospike nozzle had countersunk flow passages to improve oxidizer

turbulence and flow. The graphite aerospike nozzle also had a similar feature where the entire

head end of the graphite aerospike nozzle was ramped towards the center axis.

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7. ALTITUDE COMPENSATION

To begin our discussion of altitude compensation, we must first understand how a traditional

bell nozzle functions as it operates from sea level to high altitude, as illustrated below. As

discussed in the above figure, bell nozzles suffer from reduced efficiency at low and high

altitude because of the difference between the pressure of the exhaust gases and the ambient

pressure of the atmosphere. At low altitudes, the exhaust pressure is too low, and the higher

atmospheric pressure pushes the exhaust inward. This inequality causes the exhaust to

become separated from the nozzle walls reducing the amount of thrust generated. This

condition is known as overexpansion. At high altitude, the bell nozzle suffers from the

reverse problem.

Figure 5. Bell nozzle behavior during flight

Here, the exhaust pressure is much higher than the ambient pressure causing the exhaust to

continue to expand past the nozzle exit. Since the additional expansion occurs outside of the

nozzle, that expansion does not exert thrust on the nozzle, so that thrust is lost. This condition

is known as under expansion. At the optimum altitude, for which the nozzle is tailored during

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the design process, the exit pressure equals the ambient pressure. Note in the above figure

that the exhaust plume becomes column shaped indicating that the exhaust is perfectly

expanded thereby maximizing efficiency and thrust. Thus, both overexpansion and

underexpansion reduce overall engine efficiency and thrust. If we could somehow minimize

the exit area at launch and increase it as the rocket ascends, we could optimize the nozzle for

each altitude and maximize the thrust. Such a nozzle, would markedly improve overall

performance. An ideal nozzle would be able to continually adjust its contour, area ratio, and

length to maximize thrust at each altitude (see the figure below), a concept known as altitude

compensation. However, a bell nozzle that continually changes its geometry is clearly not

practical, and the designer must instead make a series of tradeoffs in selecting the area ratio

of a nozzle. The following graph gives some idea of the issues the designer must consider.

 

Figure 6. Ideal nozzle that continually adjusts its geometry as altitude increases

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Figure 7. Fixed area ratio and continuously variable area ratio nozzles.

The ideal nozzle, indicated by the thick solid line, represents the maximum performance

possible. At low altitudes, we want a small area ratio, meaning the exit area is small, to

minimize overexpansion. However, the graph shows that we will pay for this excellent

performance at low altitude with underexpansion at high altitude resulting in large thrust

losses. The converse is true of a high area ratio--high altitude performance is maximized at

the expense of overexpansion and poor efficiency at low altitude. While altitude

compensation is certainly desirable in any rocket application, this adaptive quality is a virtual

necessity in any Single Stage to Orbit (SSTO) vehicle, like the X-33. Some believe that a

reasonably sized payload simply cannot be carried from sea-level to orbit in a one-staged

vehicle with current engine technology. Numerous methods of incorporating altitude

compensation have been tried. Examples include extendible nozzles that "grow" through

flight, "aspirated" nozzles in which ram air is injected into the nozzle to fill the exit and keep

flow attached, and a step along the nozzle wall that prevents the flow from overexpanding at

low altitude.

However, many claim that the ultimate strength of the aerospike nozzle is its inherent altitude

compensation capability, as shown below.

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Figure 8. Aerospike nozzle behavior during flight.

This ideal behavior results from the fact that the outer plume boundary of the primary flow is

acted upon only by the ambient pressure of the atmosphere. Recall from our discussion of

aerospike thrust characteristics, high ambient pressure at low altitudes forces the exhaust

inward increasing the pressure on the "centerbody" and the centerbody component of thrust.

In addition, the base region is open to high ambient pressure resulting in a greater "base"

thrust component. At design pressure, the flow becomes column shaped, much like a bell

nozzle, for maximum efficiency. When operating at low ambient pressure (at high altitude or

in a vacuum), the flow is constrained by expansion/compression waves that direct the exhaust

axially to maintain the thrust force on the centerbody. At low pressures, however, the nozzle

operates in a “closed wake” state. Since the base is not subject to a high ambient pressure,

there is no altitude compensation benefit, and the aerospike behaves like a high area ratio bell

nozzle. Thus, in theory at least, the aerospike nozzle meets or exceeds the performance of the

bell nozzle at all operating pressures.

8. BACKGROUND AND RELATED SEARCH

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The X-33 is being developed under a cooperative agreement between NASA and Lockheed

Martin, which began July 2, 1996. It is a subscale technology demonstrator prototype of a

RLV which Lockheed Martin has named "Venture Star," and which the company hopes to

develop. Through demonstration flights and ground research, the X-33 will provide

information needed to decide whether to proceed to the development of a full-scale,

commercial RLV.

Lockheed Martin plans to conduct up to 15 flight tests of the X-33. These tests will originate

from Edwards Air Force Base, Edwards, Calif., and fly to Michael Army Air Field at Dug

way Proving Ground, Utah, and later to Malmstrom Air Force Base, Great Falls, Mont.

NASA engineers at the Marshall Center have conducted a number of tests for the linear

aerospike engine. In the spring of 1997, Marshall Center conducted tests of three hydrogen-

cooled thrusters, or thrust cells, mounted side by side and attached to a 4-foot-long copper

alloy ramp. The test series collected data on cell-to-cell plume interaction, cell-to-cell feed

system interaction and heating. Marshall Engineers followed the thrust cell tests with tests

the same year on aerospike ignition and gas generator systems.

In 1997, NASA and Lockheed Martin tested a 5-percent scale model of the lifting

body/aerospike configuration in the supersonic wind tunnels at the Air Force's Arnold

Engineering Development Center near Tullahoma, Tenn. The wind-tunnel tests were aimed

at characterizing the interaction between the aerospike engine exhaust and the lifting body's

aerodynamic behavior. Engine flight qualification testing began in 1998 at NASA's Stennis

Space Center in Mississippi. By the spring of 2000, the first aerospike engine had been

successfully operated at full power and exceeded the expected operating time that it will

experience in test flights from California to Utah.

The first aerospike engine for the X-33 program successfully completed 14 planned hot fire

tests in the spring of 2000, accumulating more than 1,460 seconds of total operating time. In

addition, engineers have successfully demonstrated the aerospike engines ability to vary its

thrust from side to side and top to bottom. This capability to vary its thrust -- called

differential throttling -- will be used to control the X-33s direction of flight.

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The test stand at Stennis was upgraded in the fall of 2000 to accommodate testing of two

engines side-by-side, as they will be installed in the X-33. The dual-engine configuration is

scheduled to begin testing in 2001.

9. AEROSPIKE ENGINE EFFICIENCY

The key to a conventional bell nozzle's level of performance is its width. At high pressure --

i.e. sea level -- the gasses are more tightly focused, so a bell nozzle with a narrow interior

surface works best. At low pressure -- i.e. higher altitudes -- a wider interior works best as

the gasses will expand farther. For example, the initial stage of the Saturn rocket which

carried the Apollo astronauts to the Moon featured a narrow nozzle to produce an ideal

straight-edged column of exhaust at sea level. However, the command module which orbited

the Moon featured a much wider bell nozzle better suited for controlling the combustion

gasses in the vacuum of space. Since the width of the bell nozzles can't change to match the

atmospheric pressure as the rocket climbs, bell nozzles are normally designed to provide

optimum performance at one certain altitude or pressure. This is called a "point design," and

engineers accept the performance loss the nozzle will encounter at any altitude other than the

one it was designed for. The aerospike eliminates this loss of performance. Since the

combustion gasses only are constrained on one side by a fixed surface -- the ramp -- and

constrained on the other side by atmospheric pressure, the aerospike's plume can widen with

the decreasing atmospheric pressure as the vehicle climbs, thus maintaining more efficient

thrust throughout the vehicle's flight.

However, the aerospike engine's ramp is cut short to save weight, since the amount of thrust

contributed by the end of the ramp is small compared to its weight. Designers compensate for

this by pumping secondary exhaust from the aerospike's gas generator into the base region to

add to the atmospheric pressure and elongate the wake.

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10. ADVANTAGES

SMALLER NOZZLE:

Shortened nozzle length for the same performance, or increased performance (higher

expansion area rations) for a given length.

SUPERIOR PERFORMANCE:

Improved performance at sea level or low altitudes. (Annular nozzles with high expansion

area ratios can be used for a single-stage sea level to vacuum vehicle mission.)

STAGNANT REGION IN NOZZLE CENTER:

The relatively stagnant region in the center of the nozzle can possibly be used for installation

of gas generators, turbopumps, tanks, auxiliary equipment, and turbine gas discharges.

MODULAR COMBUSTION CHAMBER:

A segmented combustion chamber design approach can be used, easing development effort

(individual segments can be built and tested during the early phases) and improving

combustion stability.

LESS RISK OF FAILURE:

The lower chamber pressure reduces risk of explosion.

LOWER DRAG:

The vehicles with aerospike engine have lower vehicle drag.

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THRUST VECTORING:

Due to combustion chamber control, the vehicle is maneuvered using differential thrust

vectoring. This eliminates the need of heavy gimbals and actuators used to vary the direction

of traditional nozzles.

LOWER WEIGHT:

Vehicle with aerospike nozzles having less weight than a vehicle with plug area nozzles.

11. DISADVANTAGES

Relatively high cooling requirements, because of higher heat fluxes and greater

surface areas to be cooled.

Heavier structural construction in some applications.

Manufacturing difficulties. Increase in Production cost.

12. FUTURE SCOPE

The simplicity of the aerospike design coupled with its altitude compensating properties

makes it the best option in aerospace applications. With some more research done into these

nozzles it can be seen as a means of propulsion for reusable vehicles in near decade.

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13. CONCLUSION

This independent study was successful in carrying out hybrid rocket motor tests with

an aerospike nozzle. We were able to test the design of a compact aerospike nozzle

without a center support shaft through the combustion chamber and supported at the

main injector as done by other university and private studies. The aerospike nozzle

was proven to be compatible with a hybrid propulsion system and ignition was

successfully achieved. The plume displayed typical Prandtl-Meyer expansion waves

and Mach/shock diamonds with over expansion at near sea level conditions. No

motors catastrophically failed and data were collected for the next phase of analysis.

The CD nozzle works best only for the particular designed altitude whereas

Aerospike nozzle maintains its efficiency for the changes in the altitude.The results

obtained from CFD analysis will be compared with theoretical and experimental

values thus accomplishing the validation of the code. The aerospike rocket engines

are also a brand new technology. NASA has spent over $500 million for developing

these engines. The risk of failure as well is less, hence aerospike engine is an effective

solution loping program. It can be concluded that aerospike engine is a solution to the

loses in efficiency.

 

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REFERENCE

[1] Erich Besnard, Hsun Hu Chen, Ton Mueller, Design Manufacturing and test of a plug

rocket nozzle, Mechanical and Aerospace engineering department, California State

University, Long Beach.

[2] J.J, Korte, Parametric Model of an Aerospike Rocket Engine, NASA Langley Research

Centre, Hampton, Virginia.

[3] Klaus W.Gross, Performance Analysis Of Aerospike Rocket Engines.

[4] Brandon Lee Denton, Design And Analysis of Rocket Nozzle Contours For Launching

Pico-Satellites, Department of Mechanical Engineering, Kate Gleason College Of

Engineering, Rochester Institute Of Technology, NY.

[5] Maciej Matyka, Prandtl-Meyer Expansion Wave, Computational Physics Section of

Theoretical Physics, University of Wroclaw.

[6] John Cipolla, AeroRocket and Warpmetrics http://aerorocket.com/MOC/MOC.html

[7] Chang Hui Wang, Yu Liu, Li-Zi Qin, Aerospike Nozzle Contour Design and Its

Performance Validation, Beijing University Of Aeronautics and Astronautics, Beijing.

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