© 2020 Lockheed Martin Corporation   380 Moment Test at Cold Temperatures  

Long after all qualification tests had been completed and flight MM Solar Arrays were getting ready to be  delivered to the vehicle, a question was asked regarding moment testing at cold temperatures. The concern  was raised by an analyst who asked the team “I know you tested your composite boom tubes at ambient  temperatures but how did they perform when they were tested at cold temperatures?” The team did not  have an answer for this question. The team followed standard protocol for a composite boom tube, whi ch  consisted of fabricating the tube, bonding in the titanium end fitting, thermal cycling the tube  to the hottest  and coldest temperature predicts plus margin, and then proof -loading each tube. However, the proof load  test was always done at ambient temperature. No cold temperature moment testing was ever conducted,  even on the qualification unit. But the design of the MM Solar Array relies on the strength of a single boom  tube that connects the entire array to the vehicle. And this single tube sees a very  high moment load at the  time the vehicle’s liquid apogee engine (LAE) fires to raise the orbit of the vehicle (recall from an earlier  section of this paper that the MM Solar Arrays are fully deployed prior to firing the LAE). And this tube could  be cold a t the time the LAE fires. Knowing that there is some reduction in the strength of bond lines and  composite structures at cold temperatures, there was suddenly an urgent need to fabricate a flight coupon  and moment test it at cold temperatures. A coupon was fabricated and subjected to the appropriate moment  load at temperatures as low as - 64˚C. Several different load cycles were conducted to simulate the different  loads that the boom would experience as the LAE is fired at different times during  orbit raising. The results  were that the boom tube passed every test and the late- breaking concern was laid to rest . The lesson  learned here is that critical bonded structural joints should be tested at temperature extremes to ensure  they will survive this environment. However, the costs of such a test can be prohibitive so care needs to be  taken when identifying critical joint s. A side lesson learned from this experience is that it is a good idea to  include bolts in bonded joints where possible, a.k.a. chicken fasteners . Most composite tubes rely on the  strength of the bond line that bonds in the end fi tting. But if bolts are also present in this joint, then the  strength of the joint is increased. The MM Solar Array was originally designed with bolts in these bonded  joints as an added measure of margin. One question that was never answered by the cold moment test is  “what is the reduction in strength of a boom tube that is tested at ambient temperature versus a boom tube  that is tested at cold temperature?” Is there a 10% reduction in strength? 25% reduction? 50% reduction?  The team had hoped to answer this question by breaking a boom tube at ambient and then breaking one  at cold. But due to budget constraints, there was only enough funding to build one tube and verify that it  would survive the worst -case moment at the worst -case temperature, i.e. , a quali fication load of 618 Nm  (5471 in- lb) at a qualification temperature of - 64˚C. Further research in this area would be prudent to  determine the actual percent reduction in strength at cold temperatures.  

   Figure 17 - Critical MM Solar Array Joint  Figure 18 – Boom Tube Test Article