Development of a Cold Gas Propulsion System for the ... - SSL - MIT
Development of a Cold Gas Propulsion System for the ... - SSL - MIT Development of a Cold Gas Propulsion System for the ... - SSL - MIT
Therefore, the vertical test stand did not provide as constrained a testing environment as had been desired. If time had permitted, adjustments probably could have been worked out to mitigate these problems to at least some extent. However, because of schedule pressure, the vertical test stand was abandoned slightly earlier than had been planned. Instead, as work on the EDFs was completed and all of the subsystems were integrated together on the TALARIS vehicle, validation testing moved on to 6- DOF free flight. 7.1.3 6-DOF Flight Testing Full 6-DOF flight testing of the TALARIS hopper is currently ongoing at the time of writing this thesis. For these tests, the hopper is hung beneath a pyramid-shaped frame that connects to four hard points on the structure. The hopper is then belayed by a single rope attached to the tip of the pyramid, as shown in Figure 7-6. Figure 7-6. Full 6-DOF flight testing of TALARIS hopper. As shown in Figure 7-6, the hopper sits elevated on four pylons before the start of a flight test, which then fall away as the propulsion systems engage and the hopper begins to rise. The belay line is usually kept slack, so the hopper is free to rotate and translate in all dimensions, but if it begins to go out of 112
control the belay line can be put under tension to arrest the hopper’s motion. Various tethers attach the TALARIS hopper to weights on the ground; these lines are long enough to allow the hopper to move freely within a restricted area, but if it gets too far out of range, the tethers catch and prevent the hopper from moving further. These precautions both help to prevent crashes and to protect hopper operators and test observers. Initial plans are for the TALARIS vehicle to first demonstrate an ascent, hover, and controlled descent. Once control is demonstrated for this primarily vertical maneuver, flights can expand to include significant attitude changes and horizontal motion. Eventually, the goal is to perform a demonstration hop at 2 m altitude with a 30 m horizontal traverse, as illustrated at left in Figure 3-1. 7.1.4 Summary of Validation Testing Efforts Although the flight tests had not yet led to full validation of the CGSE at the time of writing this thesis, they had revealed some important aspects of CGSE operation. The CGSE in general was found to be quite reliable; in nearly all tests where the CGSE did not perform as expected, the problem was traced back to another subsystem such as avionics or software, although in one case CGSE performance was reduced because the cable carrying the signals from the RIO to the CGSE control circuit was not plugged in tightly and the connection was intermittent. Furthermore, the majority of the CGSE components were found to hold up well over the intensive testing schedule. The one hardware element found to be subject to wear and tear was the seat of the valve in the main orifice of the Tescom flight regulator; however, this seat is simply a small plastic gasket, and it is inexpensive and easily replaced when it becomes too worn down to function properly. The flight tests, in combination with the static characterization tests, also provided some points of comparison to the flight times predicted by the MATLAB model. As mentioned in section 4.1.7, the MATLAB model predicted maximum flight times of approximately 15 s. During the static characterization tests, the CGSE was found to hold about 36 to 40 thruster-seconds of gas, as indicated by Figure 6-8. If all four vertical thrusters had an average duty cycle of 50% (an approximate percentage based on the total thrust available from all four vertical thrusters, the anticipated total vehicle mass, and the changing thrust-to-weight ratios required during a hop), this would allow them to fire for 18 to 20 s. However, a hop also includes horizontal maneuvers and attitude control; once these are taken into account, estimated flight times decrease toward 15 s and below. The flight tests that were run generally supported these estimates, although a full hop had not yet been flown at the time of writing this thesis, 113
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There<strong>for</strong>e, <strong>the</strong> vertical test stand did not provide as constrained a testing environment as had been<br />
desired. If time had permitted, adjustments probably could have been worked out to mitigate <strong>the</strong>se<br />
problems to at least some extent. However, because <strong>of</strong> schedule pressure, <strong>the</strong> vertical test stand was<br />
abandoned slightly earlier than had been planned. Instead, as work on <strong>the</strong> EDFs was completed and all<br />
<strong>of</strong> <strong>the</strong> subsystems were integrated toge<strong>the</strong>r on <strong>the</strong> TALARIS vehicle, validation testing moved on to 6-<br />
DOF free flight.<br />
7.1.3 6-DOF Flight Testing<br />
Full 6-DOF flight testing <strong>of</strong> <strong>the</strong> TALARIS hopper is currently ongoing at <strong>the</strong> time <strong>of</strong> writing this <strong>the</strong>sis. For<br />
<strong>the</strong>se tests, <strong>the</strong> hopper is hung beneath a pyramid-shaped frame that connects to four hard points on<br />
<strong>the</strong> structure. The hopper is <strong>the</strong>n belayed by a single rope attached to <strong>the</strong> tip <strong>of</strong> <strong>the</strong> pyramid, as shown<br />
in Figure 7-6.<br />
Figure 7-6. Full 6-DOF flight testing <strong>of</strong> TALARIS hopper.<br />
As shown in Figure 7-6, <strong>the</strong> hopper sits elevated on four pylons be<strong>for</strong>e <strong>the</strong> start <strong>of</strong> a flight test, which<br />
<strong>the</strong>n fall away as <strong>the</strong> propulsion systems engage and <strong>the</strong> hopper begins to rise. The belay line is usually<br />
kept slack, so <strong>the</strong> hopper is free to rotate and translate in all dimensions, but if it begins to go out <strong>of</strong><br />
112
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