Wall Pressure Unsteadiness and Side Loads in Overexpanded Rocket Nozzles

Abstract

Surveys of both the static and dynamic wall pressure signatures on the interior surface of a sub-scale, cold-flow and thrust optimized parabolic nozzle are conducted during fixed nozzle pressure ratios corresponding to FSS and RSS states. The motive is to develop a better understanding for the sources of off-axis loads during the transient start-up of overexpanded rocket nozzles. During FSS state, pressure spectra reveal frequency content resembling SWTBLI. Presumably, when the internal flow is in RSS state, separation bubbles are trapped by shocks and expansion waves; interactions between the separated flow regions and the waves produce asymmetric pressuredistributions. An analysis of the azimuthal modes reveals how the breathing mode encompasses most of the resolved energy and that the side load inducing mode is coherent with the response moment measured by strain gauges mounted upstream of the nozzle on a flexible tube. Finally, the unsteady pressure is locally more energetic during RSS, albeit direct measurements of the response moments indicate higher side load activity when in FSS state. It is postulated that these discrepancies are attributed to cancellation effects between annular separation bubbles.

Effect of Stagger on the Vibroacoustic Loads from Clustered Rockets

“…1 During startup however, the internal flow within the nozzle progresses through a series of flow and shock wave patterns that produce both lateral and vibroacoustic loads on the vehicle and surrounding structure. 2 A quantitative assessment of these loads is a necessary step to ensuring that the vehicle and its occupants are safe. Here we seek to add new insight to this problem by characterizing the effect engine stagger has on these vibroacoustic loads during the startup of a clustered rocket configuration comprising TOP nozzles.…”

Numerical investigation of flow separation behavior in an over-expanded annular conical aerospike nozzle

“…1. 20 A single incipient separation of the flow along the interior surface of the nozzle is triggered by an adverse pressure gradient between the regions of isentropic expansion and subsonic entrainment. The shock that originates from the incipient separation line interacts with a reflected shock; this shock emanates from the triple-point which is the location where the Mach disk, internal and reflected shocks coincide.…”

“…1 Illustration of the internal shock structure in a thrust optimized parabolic nozzle during an FSS state. 20 the flow reattaches and continues down the length of the nozzle as an attached boundary layer. Depending on the nozzle contour and expansion ratio, more than one bubble may be present, as shown in Fig.…”

“…2. 20 Upon further increases in the NPR, the RSS flow regime will translate downstream and the enclosed separation bubble opens up to the ambient environment when passing over the nozzle lip. When the last annular separation bubble downstream from the incipient separation shock opens up, the flow structure then switches to an FSS state due to the presence of a single separation shock with an associated separated flow region downstream.…”

“…Illustration of the internal shock structure in a thrust optimized parabolic nozzle during an RSS state 20. …”