The present invention relates to thrust chambers for bipropellant liquid rocket engines

Conventional bipropellant liquid fuel rocket engines have thrust chambers in which fuel and

oxidiscr arc reacted together. The reaction between the two constituents generates thrust

and exceed the melting temperature of the material from

which the chamber is constructed or cause attack by corrosion of the chamber metal o\cr a period of use.

In order to reduce the peak temperature reached by the chamber wall, the technique of film

cooling is frequently employed. In this technique, the two bipropellant constituents arc fed

into the thrust chamber in a predetermined manner. To generate a combustion reaction, fuel

and oxidiscr arc both fed through a plurality of pairs of icts in an injector into the thrust

chamber where they react together to generate thrust. Simultaneously, one constituent,

usually the fuel, is fed through a ring of icts adjacent the thrust chamber wall so as to form a

thermally insulating film along the chamber wall and separate the burning bipropellant therefrom by a cooler liquid and gaseous layer.

The problem caused by film cooling is that as the distance from the injector jets increases

downstream therefrom, the fuel component decomposes into uncombusted but very corrosive species which cause attack by pitting of the combustion chamber wall up to about

shortly about before the throat of the nozzle at which point such pitting tends to stop

The reason for this cessation of attack is that the corrosive species produced by

decomposition of the fuel may become fully burnt at this point probably due to the rising

temperature which occurs along the length of the chamber.

However, the pitting corrosion problem is very serious in that rocket thrustcrs used for

positioning and spatial adjustment of space vehicles such as satellites, for example, need a

relatively long life and pitting corrosion can result in premature failure of the thrust

chamber.

A further disadvantage of the technique of film cooling is that it is an inefficient use of the

fuel constituent. The ability to reduce or eliminate the quantity of the fuel constituent used

in this way would enable an increase in the duration of a satellite, for example, on-station

through more of the fuel being available for manoeuvring, or an increase in the launch

vehicle payload through the need to carry less fuel or the ability to build smaller, less costly

but improved performance thrustcr motors.

US-A-4 882 904 describes a rocket motor employing a baffle in the thrust chamber to

deflect the uncombusted fuel film cooling constituent into the high temperature burning core

of fuel and oxidiscr before it can decompose to form corrosive species. However, disadvantages of this approach arc that cooling of the chamber downstream of the baffle

may be inadequate, the presence of the baffle itself impairs motor performance and a

consequence of disrupting the film cooling is that more heat is conducted into the rocket

engine body causing problems with vapour locks in the propcllant supply to the injector and

degradation of mechanical components such as plastics material value scats for example.

US-A-4 917 968 seeks to provide a chamber liner by chemical vapour deposition of a

corrosion resistant layer such as iridium onto a highly temperature resistant material such as

rhenium. Other materials such as refractory oxides of hafnium or zirconium may also be

employed as surface layers. However, although the rhenium layer is mechanically very

strong at elevated temperatures, it is very susceptible to oxidation. Furthermore, the iridium

surface layer exposed to the film cooling fuel layer has been found to be susceptible to grain

boundary corrosion by the fuel decomposition products, as described above, causing the

loss of surface grains of iridium producing pitting and allowing attack of the underlying

rhenium layer.

It is an object of the present invention to provide at least a chamber liner able to withstand

the corrosive effects of the decomposition species of the fuel film cooling layer and to

withstand the high temperatures of bipropellant combustion.

It is a further objective to improve the performance of thrust chambers.

According to the present invention there is provided a thrust chamber for a liquid propcllant

rocket engine characterised in that at least the surface of the inner wall of the chamber is

lined in a first zone with a material including rhodium and in a second zone, downstream of

the first zone, with a material including iridium.

Preferably, the first zone extends from a position where in use the production of corrosive

species formed by decomposition of a cooling layer of fuel injected along the surface of the

thrust chamber begins to a position where the formation of such corrosive species ends and

said second zone extends from the end of the said first zone. The second zone preferably

extends beyond the nozzle throat at the downstream end of the thrust chamber.

Preferably, the first zone comprises substantially pure rhodium. It has been found that

rhodium is resistant to the corrosive effects of the decomposed fuel species generated

during film cooling.

Preferably, the rhodium is in the form of a lining to a backing layer or a liner provided within a backing layer or main body; the backing layer or the main body of the thrust

chamber may comprise a refractory metal or metal alloy. Although the corrosion resistance

of rhodium is exceptionally high, the high temperature strength of rhodium may be

inadequate .

The first zone is also preferably prov ided in the form of a lining or a liner in view of the

high cost of rhodium.

Preferably, the second zone downstream of said first zone comprises substantially pure

iridium.

Again, the second portion is preferably provided in the form of a lining or a liner to a

backing layer or within a main body as in the case of the first zone.

A further reason for preferring the first and second zone to be provided in the form of

linings or liners to a main body or backing layer is that refractory alloys used for such

applications arc well characterised in terms of their mechanical and material properties.

Thus, rocket engine thrust chambers according to the present invention may be produced

with a greater degree of certainty with regard to materials and may require less resources

for testing and certification.

According to a second aspect of the present invention the first and second zones of the thrust chamber may comprise an alloy including rhodium and iridium.

The alloy of rhodium and iridium preferably has a composition of from about 25wt

rhodium/ 75wt% iridium to about 75wt% rhodium/ 25wt% iridium, and more preferably, the

composition of the alloy may lie in the range from about 4()wt% rhodium/ 6()wt% iridium to

about 60wt% rhodium/ 4()wt% iridium. One example of an alloy liner for a thrust chamber

according to the present invention includes about 55wt%/ rhodium and about 45wt%

iridium.

In this second aspect of the present invention it is again preferred that the first and second

zones are provided in the form of a liner composed of the rhodium and iridium alloy on a

backing layer or within the main body of the thrust chamber, the backing layer or main body preferably being of a refractory metal or metal alloy.

A lining or liner may be produced by various physical and/or chemical vapour deposition

methods. Alternatively, a liner may be produced by fabrication from sheet materials and

applied to the inner wall surface of a main body or backing member by employing metal

working and joining techniques known in the metallurgical art.

In order the present invention may be more fully understood, examples will now be

described by way of illustration only with reference to the accompanying drawings, of

which:

Figure 1 shows a schematic representation of a thrust chamber of a first embodiment of a

rocket engine according to the present invention; and

Figure 2 which shows a schematic graph of temperature against position within the thrust

chamber of Figure 1.

Referring now to the Figures and where the same features arc denoted by common

reference numerals.

A schematic cross section of a rocket engine thrust chamber is shown generally at 10. The

chamber has an injector 12 at one end of the chamber, the injector having jets 14 in a

central region through which bipropellant comprising fuel and oxidiscr is introduced for

combustion and icts 1 around the outer pcπphcrv of the lnicctor for the introduction of a

peripheral curtain of fuel alone adiaccnt the inner wall for the purpose of film cooling of the

inside surface 18 of the chamber The chamber comprises a mam body member 20 formed

from a refractor) metal alloy and a liner 22 bonded to the inner surface 24 of the mam body

The liner 22 comprises two principal elements, a first portion 28 of rhodium extending from

adjacent the injector 12 through a parallel portion of the chamber to a region 30 at which the

liner material changes to a second portion 32 comprising indium and which extends through

the throat 34 of the chamber to the end of the divergent nozzle 36

In operation the temperature of the inner wall 18 rises from the injector end towards the

nozzle 36 as represented schcmaticallv in Figure 2 The temperature of the inner wall is

prev ented from becoming as hot as would otherwise have been the case by the action of the

film cooling of the curtain of raw fuel prov ldcd through the injector jets 16 The fuel

coolant decomposes under the action of the heat of the combusting bipropellant in the core

of the chamber and at a region, indicated by the arrow at 40, a distance downstream of the

injector 12. the decomposition species from the fuel begin to attack the iridium liners used

in prior art thrust chambers In the present invention, the use of rhodium from at least the

region 40 has been found to prev ent such attack of the surface The corrosiv e species

generated bv the coolant fuel decomposition exist in the thrust chamber during operation until about shortlv before the region indicated at 30 where the joint between the first portion

of rhodium and the second portion of indium occurs At this region the temperature has

nsen significantly, as indicated in Figure 2, but the corrosive species from the decomposed

fuel have themselves been combusted due to the higher temperature in this region In the

absence of the corrosive species but under the influence of the higher temperature, the

material of the liner from the region 30 downstream thereof changes to indium which has a

greater temperature capability than rhodium

In the embodiment described abov e, the first portion of the liner 22 comprises rhodium from

adjacent the injector 12 to the region 30

In a second embodiment of the present inv ention the liner 22 is composed entirely of an

alloy of rhodium and iridium from adiacent the injector 12 to the end of the nozzle 36 One

alloy used with particular ad antage for the liner comprises 55wt% rhodium/ 45wt%

iridium. The manufacturing procedure is substantially the same as that described with reference to Figure 1, except that the joint at the region 30 docs not exist.

Alloys comprising rhodium and indium of the type described have sufficient resistance to

attack by the decomposition species of the fuel coolant and adequate thermal resistance to the temperatures reached m use.