First step to a YES2 sequel mission: understanding the mishaps

We have been working on a small ESA contract to evaluate critically the YES2 mission. Goal is to understand the problems that occurred during YES2 in order to prepare better for a future mission. Next step is to provide a new conceptual mission design. So far we have analyzed the mission in detail (see earlier reports or request the relevant papers); developed a hypothesis for the OLD problem matching flight and test data and demonstrated it in a test set-up (see previous report); and now we did the same for the quite high initial deployment tension. Cause of the high tension seems to be not so much the suspected increase in stickiness of the tether, but stiffness and memory and a settling of the tether into the grooves of the spool.

Tension test set-up.

Typical tension development as the tether unwinds from the criss-cross spool. The level at the Top of the spool is an indication for stickiness. The Up level provides an idea of the stiffness and settling effect. The peaks of the layer crossing passes indicate the "hooking" effect of the tether bent around the layer transitions. The Down level is the highest level, because the tether has to jump a layer as it is pulled off the spool and travels towards the top. The increase with respect to the Up is another indication of shape memory and hooking effects.

The tension signature as a function of test condition and depth into the spool (from left to right about 3 mm).

The fresh wound tether has low unwinding tension (1,2). Thermal changes cause a settling, memory and hooking effect (4,5). Different subsequent test conditions have little further influence (7,8,9,10,12,13), suggesting that chemical/mechanical changes in the tether finish and maybe also exposure to humidity contribute not so much. After a break, the tether relaxes over a lenght of about 15 m resulting in lower unwinding tension (3,6,11). Vacuum seems to reduce tension a bit (7 vs. 5), but deeper in the spool the tension grows again to the prevacuum condition (14), possibly due to reduced outgassing at increased depth. Another, less trivial, explanation of the results could be that humidity needs many days to penetrate the spool and has a lubrication effect at room temperature, but increases tension at low temperatures.