Controlled temperature rocket nozzle

A rocket nozzle having an axial double bell shape. The nozzle includes a first bell shape, a second bell shape and an inflection point where the first bell shape and the second bell shape meet. The inflection point is located between a location at the area ratio .epsilon.=10 and a location at 0.85.times..epsilon..sub.max of the nozzle, where .epsilon. is the narrowest cross-sectional area of the nozzle. The first bell shape and the second bell shape both have a contour line having an outwardly directed curvature of between 2 and 7 at the inflection point.

FIELD OF THE INVENTION
 The present invention provides a rocket nozzle having an axially double
 bell shape, that is, a so-called "Dual Bell" type, and having an outwardly
 directed change of curvature of the contour line or generatrix at the
 inflection point between the two bell shapes.
 BACKGROUND OF THE INVENTION
 The Dual Bell shape of rocket nozzles is known from the early 60's for
 providing an altitude compensation. In sea level operation mode of such a
 Dual Bell nozzle, the inflection point will force the flow to separate
 from the nozzle wall at the desired location, thus increasing sea level
 thrust. In altitude operation mode, the plume gradually expands until it
 finally attaches to the nozzle wall downstream of the inflection point. In
 reality, however, the Dual Bell nozzle concept has several inherent
 inefficiencies which reduce its performance from the theoretical optimum.
 On the other hand, the function of the rocket nozzle is to expand and
 accelerate the gas to high velocity, and thereby give thrust efficiency
 and payload capacity. The thrust efficiency is especially important to
 upper rocket stages. High thrust performance means high wall temperatures
 and as a consequence leads to exotic and expensive technologies. The
 temperature of the walls of a rocket nozzle is dependent on the pressure
 at the wall and the speed of the flow at the wall.
 For controlling the wall temperature of a rocket nozzle, particularly wall
 portions which are not actively cooled by convection cooling, several
 techniques have been suggested. First of all, the materials used are to
 have strength at very high temperatures, which of course is expensive. The
 nozzle walls also may be covered by coatings that insulate and allow high
 surface temperatures. This is also expensive. Finally, a cooling film
 might be used in combination with a continuous nozzle contour.
 In the case of using metallic materials, such materials have high cost and
 a nozzle structure must be built with many joints due to the material
 availability. The large number of joints, however, lowers the reliability.
 Alternatively, a ceramic matrix composite material may be used. In this
 case, the cost is very high and the reliability might be questioned due to
 little experience for application in rocket nozzles.
 Thus, coatings add cost and the potential to lower the steady state
 temperature is limited. A coating also means reduced reliability due to
 increased complexity. As to the case of film cooling, there is normally no
 gas to produce film available for closed cycle engines. Tapping of gas for
 film cooling purposes would mean serious performance lose.
 SUMMARY OF THE INVENTION
 It has now turned out that a simple and inexpensive way to obtain a control
 of the temperature of the nozzle walls might be obtained based on the Dual
 Bell shape but adapted as suggested according to the present invention.
 The invention thus is substantially distinguished in that for obtaining an
 improved cooling action on the nozzle wall the change of curvature amounts
 to between 2.degree. and 7.degree.. The said inflection point (I) is
 located between a location at the area ratio .epsilon.=10 and a location
 at 0.85.times..epsilon..sub.max of the nozzle.
 By introduction of a discontinuity in the meridional plane for a nozzle
 contour the wall temperature will be lowered faster than what would be the
 case for the normal continuous contour. The temperature of the nozzle wall
 from the point of the discontinuity is made close to constant. The
 temperature that decides the nozzle material, therefore, is lowered. As a
 side effect, by introduction of a discontinuity the behaviour of a cooling
 film could be controlled. At the inflection point the film close to the
 nozzle wall will be subjected to a sudden acceleration just downstream of
 the discontinuity which will stabilise the film and prevent mixing. The
 efficiency of the film is then maintained.

DETAILED DESCRIPTION OF THE INVENTION
 In FIG. 1 it is thus illustrated a rocket nozzle 1 of so-called "Dual Bell"
 type, i.e. having an axially double bell shape. Similar to well-known
 altitude compensating nozzle structures there is an inflection point I on
 the contour line or generatrix where there is a sudden change in the
 curvature of the contour line, in other words where the upper bell shape
 changes to the further bell shape next thereto. Unlike the known Dual Bell
 structures where the change of curvature amounts to at least 9.degree. in
 order to provide for a sudden direction change for obtaining the desired
 separation of the flow along the nozzle wall at the inflection point, the
 present invention suggests that the change of curvature amounts to only
 between 2 and 7.degree.. The inflection point I is located between a
 location at the area ratio .epsilon.=10 and the location at
 0.85.times..epsilon..sub.max of the nozzle. .epsilon. is the area ratio
 which amounts to .epsilon.=1 at the throat of the nozzle.
 According to the present invention, the inflection point might be located
 at any suitable location between the two stipulated limits stated above.
 The sudden acceleration of the film flow along the wall caused by the
 change of curvature of the wall contour line provides for an improved
 cooling effect starting immediately downstream of the inflection point and
 maintaining the effect to such an extent that the rest of the wall
 downstream of the inflection point will be kept almost constant. The
 reduced wall temperature thus allows the use of a wall material not at all
 as temperature resistant as requested in prior art and consequently a
 cheaper structure.