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.

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Two cold flow subscale nozzles were tested for side load characteristics during simulated nozzle start transients. The two test article contours were a truncated ideal and a parabolic.

During over-expanded operation of rocket nozzles, which is defined to be when the exit pressure is greater than internal pressure over some part of the nozzle, the nozzle will experience a transverse forcing function due to the pressure differential across the nozzle wall. Overexpansion occurs during the nozzle start-up and shutdown transient, even in high-altitude engines, because most test facilities cannot completely reproduce the nearvacuum pressures at those altitudes. During this transient, the pressure differential moves axially down the nozzle as it becomes pressurized, but this differential is never perfectly symmetric circumferentially. The character of the forcing function is highly complex and defined by a series of restricted and free shock separations. The subject of this paper is the determination of the magnitude of this loading during sub-scale testing via measurement of the structural dynamic response of the nozzle and its support structure. An initial attempt at back-calculating this load using the inverse of the transfer function was performed, but this attempt was shown to be highly susceptible to numerical error. The final method chosen was to use statically calibrated strain data and to filter out the system fundamental frequency such that the measured response yields close to the correct dynamic loading function. This method was shown to capture 93% of the pressure spectral energy using controlled load shaker testing. This method is one of the only practical ways for the inverse determination of the forcing function for non-stationary excitations, and, to the authors' knowledge, has not been described in the literature to date.

A variety of altitude compensating nozzle (ACN) concepts were evaluated for application on a reusable single stage-to-orbit vehicle.The analysis examined ACNs applied to both verticaltake-off/vertical landing (VTVL) and vertical takeoff/horizontal landing (VTHL) vehicle configurations. Twelve basic ACN concepts were identified, with multiple design variants existing for some concepts. The performance of each ACN concept was analyzed parametrically for a 3000 psia H 2 /O 2 engine, with the engine quantity and nozzle expansion area ratio(s) varied. The result of the parametric analysis was an optimal design for each ACN concept, i.e. the design that produced the maximum weight-corrected trajectory averaged specific impulse.A mechanically translating nozzle appears to offer the best ACN capabilities for both vehicle trajectories, but the mechanical reliability of the translation mechanism was identified as the primary area of risk. Bell clusters on an annular plug and E-D surface appeared to have the best capabilities for continuous altitude compensation, however both nozzle concepts have significant costs and risks associated with their development. In light of the inaccuracies in the analysis, all ACN concepts that satisfied the performance requirements for both vehicle concepts are being evaluated during the coldflow testing program. Since the performance requirements are most stringent for the VTVL trajectory, coldflow models were developed from optimal full-scale nozzle designs for a VTVL vehicle. The testing will provide the data necessary to further evaluate the performance capabilities of these ACN concepts. Since each "optimal" ACN concept nozzle is designed to meet the same overall performance goals, this testing will provide a straightforward comparison of performance under anticipated RLV operating conditions.