Optimum rocket engine performance is obtained when the outlet pressure from the diverging portion is equal to ambient pressure. This leads to the ratio between the cross-sectional area of the nozzle throat and the outlet cross-sectional area of the diverging portion to be fixed in such a manner as to match the outlet pressure from the diverging portion to ambient pressure.
For conventional diverging portions having a fixed area ratio and for use with rocket engines operating from the ground, the area ratio is determined for proper operation at ground level. As a result performance falls off at altitude, since the external pressure falls off and is no longer matched to the outlet pressure from the diverging portion which remains constant.
In order to improve the average performance over the duration of a flight, and in particular performance at high altitude, proposals have already been made to provide diverging portions of variable area ratio, such as extensible diverging portions in the form of truncated cones or having single curvature, as described, for example, in patent specification No. FR-A-2 457 390. Proposals have also been made to use fixed diverging portions with discontinuous curvature.
The use of a single curvature extensible diverging portion on a rocket engine suffers from constraints related to engine architecture in addition to any possible problems relating to the unfolding system per se. With a single curvature extensible diverging portion, the moving portion which is retracted around the engine in the folded starting position is very long and has a small inside diameter. This imposes very strict constraints on architecture right up to the top of the engine.
The principle of a fixed diverging portion with discontinuous curvature is to have a jet that breaks away at the discontinuity in curvature while on the ground, i.e. the jet ceases to follow the diverging wall and a dead zone is thus set up between the jet and the wall. Thereafter, as altitude increases and as external pressure falls off, the jet becomes reattached to the bottom portion of the diverging portion, thereby increasing the apparent area ratio of the jet and thus improving its performance at altitude. Nevertheless, this system suffers from drawbacks relating both to stability and to performance. It is very difficult to ensure that the jet detaches cleanly on the ground or at low altitude and then that it becomes reattached both symmetrically and stably at higher altitude. Further, the discontinuity in the curvature gives rise, after the jet has become reattached, to an increase in the boundary layer and thus to a loss in performance.
The present invention seeks to remedy the above drawbacks and to enable the outlet section area of a fixed or extensible type of diverging portion to be properly matched without giving rise to severe architectural constraints nor to a substantial limit on performance or stability in the operation of the rocket engine.
The invention also seeks to improve the operation of unfolding a diverging portion of the extensible type.