112 range of the gimbal, and can be used for routing the flex ible propellant tubing across the gimbal. Early in  the gimbal development, it was realiz ed that the propellant tubing desi gn would dictate the size of the  clearance volume and therefore the size of the gimbal ring. For light weight a nd high stiffness, it was  desirable to minimize the size of the gimbal ring. However, design of the metal propellant tubing coils for  adequate flexibility required a relatively large coil diameter and therefore a larger gimbal ring.   These opposing requirements were resolved throu gh an unorthodox approach to design and material  selection. Material selection was the more powerf ul tool, since the geometry  of the gimbal ring is  constrained by gimbal and propellant coil dimensions . Available materials were surveyed for the relevant  properties, primarily density and bending modulus. Tradi tionally, titanium alloy has been the material of  choice for high stiffness and light weight. In terms of  specific stiffness (stiffness per unit mass), however,  titanium is similar to other material choices. A more  creative approach to material selection was needed.    To best meet the design requirements for the Gimbal  Ring, Moog selected an aluminum grade containing  reinforced composite material. To produce the gimbal ring, a near-net-shape ring of  the material was first  forged. Using the material supplier’s recommended cutting and fabricating techniques, that billet was then machined to produce the one-piece Gimbal Ring structure.  

Propellant Line Assembly   From the fluid system design standpoint, cleanliness and smoothness of the interior surfaces were the driving requirements for the Xenon propellant tubing. A stainless steel alloy was selected as the tubing material. A small tube diameter supports the small r equired flow rate, and a nominal wall thickness was  adequate to contain system pressure. From the gim bal design standpoint, flexibility of the tubing was  most important.  Full articulation of the thruster gimbal is ±35 degrees , on both axes, from the symmetrical or zero position.  The metal propellant line assembly must accommodate that range of motion. To incorporate the necessary flexibility into t he tubing assembly, t he geometry of a large helical coil was selected. The tubing  coils can accommodate both axial (stretching/compre ssion) and lateral (bending) deflections. The height  of the gimbal and the inside diameter of the gimbal ring allowed a tubing coil design which can accommodate the maximum deflections with acceptable material stresses in the tubing and low torque loads induced on the gimbal actuators.  The geometry of the tubing coil is that of a soft hel ical spring constrained only at its ends- a necessary  feature in the gimbal application. Vibration input to such a structure can induce a large response, and this  was a major concern with the tubing coil.  The fluid system design for the thrusters requires  two Xenon supply lines. This feature was used to  design a vibration-resistant tubing coil assembly. The two lines were designed as concentric coils having  the same number of turns end-to-end. The differenc e in coil diameters resulted to separate natural  frequencies for the two coils, and it was thought that tying the two together at intervals would force a  response at a different frequency and much  lower amplitude (Figure 6, Figure 6a).    An experiment was conducted on an engineering prot otype with the inner and outer propellant lines  coupled together at every 90 degrees around the coils.  Exposure to random vibration showed response at  a single frequency, as intended. However, amplitu de of the response was still unacceptably high,  achieving only a partial solution to the problem.  The vibration concern with the propellant tubing coils arises only at launch, and the stowed configuration  of the gimbal at launch is not at the symmetrical or  zero position, but rather at the fully articulated  position; i.e., +36 degrees rotation on both the X and Y axes. In that gimbal conf iguration, it was found  that the coils could be tied to gimbal structure in or der to limit and control the vibration response. Properly  spaced flexible tension members performed this functi on in the launch configuration, and then went slack  NASA/CP-2010-216272