Simulation and Analysis of Fairing Jettison from Sounding Rocket *

Transcription

1 Journal of Aeronautics, Astronautics and Aviation, Series A, Vol.40, No.4, pp (2008) 237 Simulation and Analysis of Fairing Jettison from Sounding Rocket * Huiwen Hu **, Jieming Wang, and Wei-Jun Lu Composite Materials and Lightweight Structures Laboratory Department of Vehicle Engineering, National Pingtung University of Science and Technology 1, Hseuh Fu Road, Neipu Hsiang, Pingtung Taiwan, R.O.C. ABSTRACT The fairing and payload are designed to depart from rocket vehicle at an altitude over 100 km. At this moment, the velocity of rocket vehicle is about 1900 m/s with a spin of 3 rps. If the departing velocity of fairing is too small, the spin of fairing may cause collision between the fairing sections and the payload. The collision may induce instability and unredeemable damage in payload. It is therefore essential to evaluate the separation mechanisms which are designed to provide sufficient departing velocities for fairing jettison. In this study, a commercial code RecurDyn is used to establish a dynamic model to simulate the fairing and payload jettison. Simulation is performed under the conditions with and without gravity. Analysis results show that the original design of spring constants is insufficient to safely eject the fairing and collision between fairing sections and payload is inevitable. The increase of spring constants is required to avoid the collision, attenuate the harmonic nutation rate and stabilize the trajectory of payload after jettison. Keywords: Fairing, Payload, Jettison, Collision, Nutation rate I. INTRODUCTION The development of sounding rockets is used to support the researches of atmospheric science, for instance, the measurement of temperature, pressure, ionosphere from 80 to 400 km above the earth's surface is very important to meteorology. Recently, the researches of bioastronautics, exobiology, and some experiments under the circumstance of low gravity have been done using sounding rocket. Fairing is designed to protect the payload and to reveal the payload at a proper altitude to perform experiments and measurements. Traditionally, rocket fairing requires a lightweight structure with high stiffness and strength to withstand the aerodynamic buffeting, explosive reaction of the rocket motors, shocks from the stage separations, and the thermal loads induced by aerodynamics [1]. A reliable jettison at a proper altitude to release the payload without any collision between the fairing and payload is its ultimate goal. For example, the fairing of Japanese H-II rocket comprises of a aluminum alloy nose cap, a cone and a cylindrical of sandwich structure, coating of thermal insular on nose cap and cone, pyrotechnic separation system between two fairing halves, and between each fairing half and the rocket, and jettison system with eight spring thrusters and four pivoting hinges furnished on the fairing bottom [2]. Fig. 1 shows the conventional design of the jettison methods for nose fairing. Inglis et al. [3] designed a method of onward jettison as shown in Fig. 1-(b). Low pressure gas is required to pressurize the inner space, and high pressure gas is produced for exerting force to shear the retaining pins and accelerate the fairing forward as shown in Fig. 2. This design seems to take the risk of collision between fairing and payload, and thereby to suit the protective cover for guided missile. The missile angle of attack creates lateral aerodynamic forces on the nose fairing to clear it from the missile. Another method designed by Hatakeyama et al. [4] is shown in Fig. 1-(c). The releasable clamshell fairing * Manuscript received, Apr. 8, 2008, final revision, Jul. 9, 2008 ** To whom correspondence should be addressed, huiwen@mail.npust.edu.tw

2 238 Huiwen Hu Jieming Wang Wei-Jun Lu sections is ejected from a spinning rocket vehicle body, in which each fairing section is retained at its lower trailing edge portion. The separation mechanism shown in Fig. 3 (a) Fairing (b) Onward jettison (c) Lateral jettison Figure 1 The early designs of fairing jettison (a) before separation cords (ESMDC), a frangible bolt, aluminum alloy consists of two expandable shielded mild detonating housing and holder, shock absorbers, and detonator blocks, etc. When ESMDC is ignited, the resultant dynamic pressure expands the elliptic tube and fractures the bolts. After separation, the fairing sections are rotated outwardly from the rocket through a predetermined angle. It is therefore substantially eliminate the possibility of the fairing section striking the vehicle payload. The objective of this research is to support the design evaluation of fairing jettison for the sounding rocket developed by the National Space Organization (NSPO) of Taiwan. The sounding rocket is designed to eject the fairing sections at an altitude about 100 (km), and then subsequently eject the recyclable payload from rocket vehicle. Fig. 4 shows the schematic configuration of fairing and payload. Each fairing sections is initially retained at its nose cap and lower trailing edge portion, i.e. 0 o and 180 o. Fig. 5 shows the separation mechanism, which consists of latch system and ejecting system, respectively. A retaining pin is initially latched by a piston, which is designed to unlock the retaining pin by using high pressure gas. Three spring thrusters, i.e. one at the nose cap and two at the lower trailing edge portion 90 o and 270 o, are designed to eject the fairing sections outwardly from the rocket vehicle. It is very important to design a proper spring constants which provide sufficient departing velocity to safely eject the fairing. If the initial velocity of fairing is insufficient, the spin of fairing may cause the collision between fairing and payload. Therefore, the main task of this study is to simulate the fairing jettison from SR and to analyze the trajectory of fairing sections and payload. Any collision between fairing and payload is not allowed. (b) after separation Figure 2 Separation mechanism of onward jettison (a) side view (a) before separation (b) after separation (b) bottom view Figure 4 Schematic configuration of fairing and payload Figure 3 Separation mechanism of later jettison

3 Simulation and Analysis of Fairing Jettison from Sounding Rocket 239 (a) before separation Figure 6 Departing sequence and gesture of fairing sections before and after jettison (b) after separation Figure 5 Separation mechanism of SR II. SIMULATION MODEL The fairing sections are designed to simultaneously depart from rocket. The departing velocity for fairing jettison is originally given about 1.55(m/s). The spring constants are thereby estimated based on the concept of energy conservation. However, the initial spin of rocket makes this estimation relatively complicated. Fig. 6 shows the separating sequence and gesture of fairing sections. The initial spin and departing velocity of fairing sections produce centrifugal force and Coriolis force simultaneously, and therefore make the fairing sections move outwardly with translation and rotation. Since the radius of lower trailing portion of the fairing sections is larger than the nose cap, more inertial forces in the lower tailing portion may generate pitch motion. Besides, masses of components attached to fairing sections may significantly affect the inertial forces as well. It seems that the only one given condition of departing velocity is insufficient for examining this complex motion. An underestimated departing velocity and spring constants may cause the collision between fairing and payload. Moreover, trajectory prediction for the fairing sections and payload are also important to avoid any unwanted collision. It is therefore essential to establish a detailed three dimensional simulation model for this study. For instance, the physical properties of aluminum alloy cone structures covered with a heat insulation material and all components of separation mechanism. Those parts and their properties are required to calculate the center of gravity and mass moment of inertia. Figure 7 Simulation models A commercial code, RecurDyn, is used to establish the dynamic models and simulate the ejecting motions of the fairing and payload. Fig. 7 shows the simulation model of the sounding rocket. The complete model includes seven parts, i.e. rocket vehicle, two parts of fairing, two parts of nose cap, base disc and payload. All parts are connected and fasten by using bushing which is defined to restrain the certain degrees of freedom of each part. Two parts of fairing are initially constrained together at three locations, i.e. one at the nose cap and two at the lower trailing edge portion 0 o and 180 o. Two thrusters with spring are located at the lower trailing edge portion 90 o and 270 o. Before jettison, the spring and thrusters are compressed into the position where all parts are fastened and latched together. Once the fairing sections are separated, bushings are removed and compressive springs with thrusters are released to push the fairing sections apart from the rocket vehicle. Payload is initially fastened at base disc and then subsequently ejected from rocket vehicle. An initial condition of line speed 1900 (m/s) with a spin of 3 (rps) in axial (z-) direction is simulated. Two loading conditions, i.e. with and without gravity, are applied to the simulation model. As a matter of fact, gravity is indispensable at the altitude of 100 (km), and results in the values of (-9.516, , ) (m/s 2 ) in this simulation. With a total simulation of twenty seconds, the fairing and payload are assumed to depart from the rocket vehicle at the fifth second and the tenth second, respectively.

4 240 Huiwen Hu Jieming Wang Wei-Jun Lu III. RESULTS AND DISCUSSION In the first place, the simulation of no contact among the parts is assumed for the demonstration purpose. Fig. 8 shows the position and gesture of the first five steps of fairing sections after jettison, respectively. Apparently, the result shown in the third step demonstrates that fairing section has intruded into the payload at 5.07 (sec). (a) t = 5.01 sec It means that collisions occur between fairing section and payload. Fig. 9 shows the position and gesture of the first five steps of payload after jettison, respectively. The nutation of payload occurs. If the condition of contact is applied to the model, the collision can be found from the impulse of impact force as shown in Fig. 10. The simulations show that collisions occur simultaneously at both sides between the fairing sections and payload no matter whether the gravity is applied or not. As a matter of fact, the departing velocity 1.55 (m/s) seems insufficient to safely eject the fairing sections. Fig. 11 shows the ground test of fairing jettison performed by CSIST (Chung-Shan Institute of Science and Technology) to validate the reliability of separation mechanism design. Although the payload is not installed in this test, the collision has been found through a high-speed camera. The high initial spin of fairing sections causes the collision between the lower tailing portion of fairing and rocket. Since the effect of gravity is significant in ground test, fairing sections are rapidly fall after jettison. Gravity effect may be less significant over the altitude of 100 (km), but the high initial spin of fairing sections should remain the same. If spring constants and spin rate are remained the same, the collision between the fairing sections and payload is inevitable. (b) t = 5.04 sec (without contact) (with contact) (c) t = 5.07 sec (a) t = 10 sec (b) t = sec (d) t = sec (c) t = sec (d) t = 17 sec (e) t = 5.22 sec Figure 8 Position and gesture of fairing jettison (e) t = 25 sec Figure 9 Position and gesture of payload jettison

5 Simulation and Analysis of Fairing Jettison from Sounding Rocket 241 The higher spring constant is considered to reach higher departing velocity. The simulation without gravity shows that spring constant should be increased up to at least five times higher than the original design to avoid the collision. However, if gravity is considered, it required at least six times higher than the original design to avoid any collision. The stability of payload after jettison is of concern since a predictable trajectory of payload is very important to recycle it. Dynamic analyses of the simulation model with and without gravity are therefore performed to analyze the stability of payload in terms of the nutation rates of rocket vehicle and payload. Fig. 12 and Fig. 13 show the nutation rates of rocket vehicle and payload before and after jettison without gravity. A shock of nutation rate of the payload occurs while the rocket vehicle ejects the fairing sections and then turns into harmonic nutation after payload jettison. Figure 11 Ground test of fairing jettison (a) simulation without gravity (b) simulation with gravity Figure 10 The impact forces of fairing (original spring constant) Figure 12 Nutation rate in x-axis (simulation without

6 242 Huiwen Hu Jieming Wang Wei-Jun Lu Figure 13 Nutation rate in y-axis (simulation without Figure 14 Nutation rate in x-axis (simulation with Apparently, when the original design of spring constants is used, the collision between fairing and payload happens first and then payload departs from the rocket vehicle moving with a harmonic nutation. If the spring constant is increased to five times higher than the original design, the avoidance of collision significantly attenuates the harmonic nutation rate of payload after jettison. Fig. 14 and Fig. 15 show the nutation rates of rocket vehicle and payload before and after jettison with gravity. The behaviors are very much alike to those of the simulation without gravity. When spring constant is increased to six times higher then original design, the avoidance of collision significantly attenuates the harmonic nutation rate of payload as well. In other words, the collision between fairing and payload will contribute the harmonic nutation rate of payload. This small disturbance in nutation rate may complicate the prediction of payload trajectory after jettison. The preliminary trajectories of rocket and payload after jettison are predicted through the first twenty second simulations with and without gravity. The analysis data during such short period of time is good enough for the further trajectory analysis including the effects of gravity and aerodynamics. The effects of spring constant before and after changed on the trajectories are also investigated. Figs. 16, 17 and 18 show the simulation without gravity. The trajectory of payload deviates from the rocket in x-direction after jettison. When spring constant is increased, the trajectories of rocket and payload turn to Figure 15 Nutation rate in y-axis (simulation with

7 Simulation and Analysis of Fairing Jettison from Sounding Rocket 243 Figure 16 Trajectory in x-axis (simulation without Figure 18 Trajectory in z-axis (simulation without Figure 17 Trajectory in y-axis (simulation without Figure 19 Trajectory in x-axis (simulation with

8 244 Huiwen Hu Jieming Wang Wei-Jun Lu Figure 20 Trajectory in y-axis (simulation with the opposite x-direction. However, the trajectories of rocket and payload in y- and z- directions are almost identical in both simulations before and after the design change of spring constant. Figs. 19, 20 and 21 show the simulation with gravity. The trajectory of payload slightly deviates from the rocket in x-direction after jettison, but design change of spring constant does not affect the trajectories of payload in all three directions. IV. CONCLUSIONS This paper presents the simulation of fairing and payload jettison from a rocket vehicle at an altitude of 100 (km). The design of separation mechanism is evaluated through the dynamic analysis especially for the evaluation of spring constant. Simulation and analysis results show that original design of spring constant is not sufficient to safely eject the fairing sections, and consequently the collision between fairing and payload is inevitable. To avoid the collision, the spring constant should be increased to at least six times higher than the original design. The avoidance of collision may also significantly attenuate the harmonic nutation rate of payload and stabilizes its trajectory after jettison. Nevertheless, the increase of spring constants may also increase the allowable pressure to remove the retaining pin in latching mechanism. Another risk is raised in the failure of delatching fairing. Generally speaking, the current design of fairing jettison seems to increase the Figure 21 Trajectory in z-axis (simulation with complexity in comparison with the conventional design and to raise the potential risks of failure in ejecting fairing. A more reliable and simpler design is required in the future work. ACKNOWLEDGEMENT The authors would like to acknowledge the support of National Space Organization (NSPO) of Taiwan to this research through the grand number 96-NSPO (B)-SE-FA REFERENCES [1] Timmins, A. and Heuser, R., A Study of First-Day Space Malfunctions, NASA TN D-6474, Sept [2] Yasunaga, Y., Fukushima, Y., Nakamura, T., and Fujita, T., Separation Jettison Test of Japanese H-II Rocket Satellite Fairing, AIAA , 28 th Aerospace Sciences Meeting, Reno, Nevada, Jan [3] Inglis, R. T., Bastian, T. W., and Schertz, C. W., Jettisonable Protective Cover Device, US Patent No , Sep. 19, [4] Hatakeyama, L. F., Method and System for Ejecting Fairing Sections from a Rocket Vehicle, US Patent No , Dec. 19, 1972.