Experimental Study on Rocket Nozzle Side Load Reduction
Ralf H. Stark1 and Chloé Génin2
German Aerospace Center, Lampoldshausen, Germany, D-74239
The flow during transient start-up and shut down in all rocket engines operated on sea level under ambient conditions will separate inside the supersonic part of the nozzle. A separated nozzle flow is circumferential asymmetric distributed and induces therefore high side loads. DLR Lampoldshausen carried out a cold flow subscale test campaign to study devices that can reduce the side loads during transient engine operation. These devices can be installed either on ground test facilities or launch pads. A detailed parametrical study was performed and experimental data demonstrate the potential of the device to reduce side loads.
Nomenclature Ma = Mach number th = nozzle throat p = pressure w = nozzle wall X = axial coordinate FSS = free shock separation 0 = total GT = Guiding Tube a = ambient NPR = nozzle pressure ratio, pcc / pa or p0 / paD = design RSS = restricted shock separation cc = combustion chamber SLMD = side load measurement device e = nozzle exit SLRD = side load reduction device R = radius TIC = truncated ideal contour nozzle sep = separation
I. Introduction URING transient start-up of a main stage rocket engine, the highly overexpanded flow inside the supersonic part of the nozzle always separates until the engine reaches its design operation point. The separated flow
causes side loads, due to its unsteady nature and its unsymmetric circumferential distribution. The magnitude of these side loads are a function of the nozzle pressure ratio (NPR = pcc / pa) and the maximum has to be awaited when the Mach disc, turning the exhaust jet from a conical to a cylindrical shape, passes the nozzle exit cross section1, apart from flow phenomena like transition from free shock separation (FSS) to restricted shock separation (RSS) and vice versa in parabolic contoured nozzles. Unforeseen increased side loads can affect the nozzle’s structural integrity, the engine, the actuators, the rocket structure and even the payload, especially if next generation engines are foreseen to demonstrate a thrust reduced operation on the test bench, which leads to unavoidable separated nozzle flows.
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As start-up side loads are one of the deciding design factors of a rocket engine, the whole launcher would benefit from a reliable reduction. For this reason DLR Lampoldshausen carried out a cold flow subscale test study on a side load reduction device to be installed on test benches and launch pads.
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1 Head of Nozzle Group, Institute of Space Propulsion, Langer Grund. 2 Dr. Research Scientist, Institute of Space Propulsion, Langer Grund.
49th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition4 - 7 January 2011, Orlando, Florida
AIAA 2011-389
Copyright © 2011 by German Aerospace Center. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.
II. Experimental Setup
A. Test bench P6.2 The tests were performed in DLR’s high altitude test facility P6.2. Figure 1 shows a sketch of the assembly with
its 200 bar high pressure vessels on the left, providing dry nitrogen obtained from liquid phase. A line system with automatic valves, filters, pressure reducers, regulation valves and mass flow meters connect the vessels with the
settling chamber, feeding the nozzle. The settling chamber includes a combination of grids and honey combs to reduce the intensity of turbulence. The typical test configuration is shown in the middle where the test specimen is mounted inside the high altitude chamber. In combination with a diffuser the high altitude chamber acts in self evacuating mode. Test specimen can also be studied using an ejector stage (left aside) or mounted on a horizontal rig under ambient conditions (right aside).
Within the presented test campaign the high altitude chamber was used with open access windows (one window for mounting purpose and two windows on opposite sides for optical devices) and a subsonic diffuser to represent the properties of a main engine test cell and its attached guiding tube.
The test bench is able to supply the nozzle as well as the ejector settling chamber with feeding pressures up to 6 MPa and mass flows up to 4.2 kg/s, under total temperatures equal to the ambient temperature of the high pressure vessels. Dry nitrogen is used instead of air as a medium in order to prevent condensation effects (H2O, CO2, O2, etc.) and to assure a safe and cost-effective test bench operation.
B. Nozzle and pressure measurements
Figure 1. Test bench P6.2 and its possible test positions.
The tested nozzle was designed as a truncated ideal contour nozzle (TIC) with a design Mach number of MaD = 4.2 and an exit wall Mach number of Mae = 3.8, to achieve nearly a vacuum wall pressure profile for a NPR of 35. The nozzle was made of aluminum with a throat radius of Rth = 15 mm. Figure 2 shows the nozzle mounted inside the opened high altitude chamber. It was connected via a side load measurement device (SLMD) to the settling chamber and its jet was exhausted vertically into the guiding tube (GT). The nozzle was equipped with axial rows of pressure ports in 3 transversal planes. The wall pressure was measured via 0.5 mm orifices, perpendicular drilled into the nozzle wall. These pressure ports were connected with small metal and Teflon tubes to blocks of pressure transducers. In total 34 piezoresistive XT-154-190M type Kulite transducers were used. The transducers had a measurement range of 1 bar with an accuracy of 0.5%, relative to the upper range limit. The natural frequency of the transducers is higher than 50 kHz, but due to the low eigenfrequency of the Teflon tubes, the pressure signals were filtered with a cut-off frequency of 160 Hz and recorded with a LF rate of 1 kHz.
Figure 2. Initial test setup. TIC nozzle in high altitude chamber.
SLMD
TIC Nozzle
Guiding Tube
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C. Side load reduction devices Five ring shaped side load reduction devices (SLRD) were manufactured and tested. Like the nozzle, the SLRDs
were made of aluminium. They differ in the length as well as the profile of the inner contour. Figure 3 showes a sketch of the SLRDs, labeled from A to E. The axial contour length was 10 and 20 mm respectively. The inner contours were designed as a cone with an angle of 10°, 15° and 20°, related to the TIC nozzle exit angle (SLRD A to D). One contour was designed by applying a Prandtl-Meyer expansion at the TIC nozzle exit and calculating the resulting isobaric shear layer shape (SLRD E). The position with respect to the nozzle or the guiding tube could be adjusted using sleeves.
Three positions were tested for each device. Figure 4 gives an impression. Top down: directly mounted at the nozzle exit cross section, mounted on the nozzle with a displacement leading to a gap and fixed mounted on the guiding tube, without contact to the nozzle. In this way, a fundamental parametrical study could be performed. Table 1 summarizes the Profile parameters of the reduction devices.
Figure 3. Sketch of side load reduction devices.
Figure 4. Test configurations.
Table 1. Side load reduction devices. Label Contour Length Contour Profile A 10 mm 10° B 20 mm 10° C 20 mm 15° D 20 mm 20° E 20 mm Isobar
D. Side load measurements
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Test bench P6.2 features simultaneous side load measurements. A thin walled tube is connected upstream the convergent-divergent nozzle (Fig. 2 and Fig. 5 left). An asymmetric pressure distribution inside the nozzle causes a side load that bends the tube. The induced bending stresses on the surface of the tube are proportional to the load and are measured with strain gauges. Each of the two force components is picked up by opposite pairs of gauges, connected as a full wheatstone bridge. The pairs are wired on
Figure 5. Side load measurement device.
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opposite branches of the bridge (see Fig. 5 right). This faulty wiring compensates tensile, torsional and temperature stresses. The setup is calibrated with different weights (applied with a string) acting at the nozzle exit cross section. Releasing the weight by cutting its string gives the static as well as the dynamic response of the system. With this calibration all measured voltage signals can be interpreted as a force acting at the end of the nozzle.
E. Test Profile All the test were conducted with the same NPR (p0 / pa)
profile. Figure 6 illustrates the test profile with its up and down ramping gradients of dNPRup/dt = 4 s-1 and dNPRdown/dt = -4 s-1 respectively. In this way, the measured side loads can
be directly compared and related to NPR. Every setup (combination of SLRD specimen and assembly position) was tested at least three times.
III. Results and discussion
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Figure 6. Typical test profile.
Figure 7 shows the nozzle side loads of the initial TIC, means without any mounted SLRD. Given is the vectorial signal amount as a function of the NPR for up- (left) and down-ramping (right). The signals of the three test runs show a good repeatability. The side loads during up-ramping increase from NPR = 5 to NPR = 15 and decrease from NPR =22 to NPR = 28. Above NPR = 28, the side loads stay constant on a low level as the nozzle is flowing full. The side loads during down-ramping show a comparable distribution but with higher absolute values for the NPR interval from 12 to 22. A significant side load peak is given between NPR = 2 and NPR = 3. The just developed super sonic boundary layer passes the transition from relaminarized to turbulent flow separation. As this transition is not circumferentially uniform distributed the combination of oblique separation shock and Mach disc tilts, resulting in a partially reattached flow1 and therefore high side loads. As the influence of the SLRDs on the initial side loads are of interest, all discussed following side load data are normalized by the maximum voltage signal of 2 mV (found in fig. 7 right) and given in percentage.
Figure 8 left illustrates the influence on the side loads of the SLRDs being directly mounted on the exit of the nozzle. The initial side loads are given in red for comparison. SLRD A and SLRD E have no significant influence. SLRD B, C and D increase the side loads up to 150% and are counterproductive. As mentioned, the side load peak at low NPR is due to a partially reattached flow. The directly mounted SLRD elongate the nozzle and therefore increase the lever arm for this flow pattern, resulting in increased side loads.
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Figure 7. Side loads of initial configuration without SLRD. Up-ramping left and down-ramping right.
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Figure 8. SLRDs mounted directly (left) and with a gap (right) at nozzle exit.
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Figure 8 right shows the side load data of the SLRDs mounted with sleeves on the nozzle, resulting in a small gap between nozzle exit and SLRDs. In this configuration the SLRDs have no remarkable influence, except the still longer lever arm increasing the low NPR side loads. A positive effect is illustrated in Fig. 9 left, where the SLRDs are mounted on the guiding tube and are not in contact with the nozzle. The side loads are decreased to 75%.
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Figure 9. SLRDs mounted on guiding tube (left) and performance of SLRD E (right).
For all up to here discussed combinations, the distance between nozzle exit and guiding tube inlet was kept constant. An additional configuration was tested for the most promissing SLRD candidate: number E. The SLRD E was still mounted on the guiding tube but now with a much smaller distance with respect to the nozzle exit (4 instead of 11 mm). The result of this setup is shown in Fig. 9 right. The side loads for most of the NPR range could be reduced down to 50% and even the interval of high side load activity could be squeezed compared with the preceding setup combinations.
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Figure 10. Separation position as function of NPR. Up-ramping left and down-ramping right.
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A first explanation is given in Fig. 10. Illustrated are the separation positions Xsep/Rth for all configurations as a function of NPR (directly mounted on the nozzle, mounted with sleeves on the nozzle creating a gap, mounted on the guiding tube and SLRD E mounted on the guiding tube with a shortened distance), means the axial position of the incipient (lowest) wall pressure psep being present for a given NPR and introducing the separation zone. It can be seen clearly that the separation position of SLRD E, mounted on the guiding tube with a short distance to the nozzle exit, is shifted downstream for the same. The nozzle flows full for a much lower NPR. This explains the squeezed interval of high side load activity.
Figure 11 left shows separated wall pressure profiles for the initial TIC (red) as well as for SLRD E in short distance configuration (blue). Same separation positions, indicated by lowest wall pressure p
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w = psep, are reached for lower NPRs. In opposite to the initial TIC, where the separated region shows the awaited ~90% of the ambient pressure pa , the SLRD E reduces the pressure within the separated region with increasing NPR. This effect is well known for the back flow pressure inside a separated dual bell nozzle extension2. Figure 11 right shows the same wall pressures normalized by the total pressure p0. The normalized wall pressure profiles are identical. This indicates that both flows underlie the same separation conditions and the resulting separation zone is of same length. Therefore a deciding influence of the actual separation zone on side loads concerning its position and length can be excluded.
As over the whole nozzle wall, means for attached and separated flow, lower wall pressures are given with constant relative pressure fluctuations, the resulting side loads must decrease, as the pressure difference for opposite nozzle sides is lower.
IV. Conclusion The conducted test campaign, with its combinations of SLRD designs and assembly variations, demonstrated
that side load reduction devices can influence the flow within the back flow region of a separated nozzle. The NPR interval of high side load activity as well as the absolut side load values could be reduced down to 50%. It has been shown that mounting a side load reduction device directly onto the guiding tube of a test bench or a launch pad is the most promissing way.
References 1Stark, Ralf and Wagner, Bernd, “Experimental study of boundary layer separation in truncated ideal contour nozzles”, Shock
Waves, Vol. 19, No. 3, 2009, pp. 185-191. 2Génin, Chloé, “Experimental Study of Flow Behaviour and Thermal Loads in Dual Bell Nozzles”, Ph.D. thesis, Université
de Valenciennes, France, ISBN 978-3-8322-9230-0, 2010.
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Initial NPR 5.8 NPR 7.2 NPR 8.1 NPR 9.1 NPR 10.0 NPR 11.3 NPR 12.5 NPR 14.0 NPR 15.7 NPR 17.9 NPR 19.9 NPR 22.4 NPR 26.1 SLRD E NPR 4.8 NPR 5.8 NPR 6.4 NPR 7.0 NPR 7.5 NPR 8.2 NPR 8.8 NPR 9.6 NPR 11.0 NPR 12.6 NPR 14.0 NPR 16.2 NPR 20.0 Design
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Figure 11. Wall pressures of initial TIC and SLRD E in short distance configuration.