The nominal part of YES2 deployment

With the data from OLD, OBC and MASS combined, we can now understand what happened during the first 5.4 km of deployment that went rather nominal and very promising for the remainder of deployment (had not some OBC or OLD related problem decreased the OLD interrupt performance heavily at about 6300 s). Deployment speed during this nominal part of deployment is shown above, as registered by OLDs (until 6300 using OBC data, from there using integrated OLD rate data). After initial high tension phase due to excessive braking, the deployment speed is a bit too low. In order to correct, the brake turns to zero at t=70 s. The deployment friction is however a bit higher than expected. At zero turns it is identical to that of the nominal deployment friction at the nominal brake position (0.8 turn), so the deployment speed cannot catch up and remain slightly slower than nominal. In order to make up for the lost length, the deployment is allowed to accelerate until 2200 s by the controller. Then the controller continues to make corrections abruptly to keep the deployment on its path (2200-4000 s). We can be lucky that the deployment did not get stuck in this period, and it shows the winding proceeded to deploy smoothly. First stage completes with perfect length and an oscillation of 10 degrees, which is still acceptable for an accurate re-entry of Fotino.

Below plot shows the reconstructed tension, which is derived by integrating orbital dynamics of a tether system using the length and velocity data obtained from the YES2 mission. Grey curve is nominal. It can be seen that during the first stage the nominal deployment tension is quite accurately followed, albeit at zero turns (so at edge of controllability) and apart from the initial heavy braking (see tension plot earlier in this blog). As said, the control effort after 2200 s seems a bit too rough for this level of disturbance. The carefully selected control gains should therefore be reviewed in the light of the mission data. Note the stabilizing value at the end of the first stage is a bit off from the nominal due to the centrifugal effect of the 10 degree oscillation. The second stage shows a close-to-nominal start and friction level is nearly perfectly nominal. Unfortunately the OBC turned the brake down for too long (due to above described yet poorly understood problem), causing the friction to be lower than nominal and causing deployment to diverge after about 6500 s.

Information on what happened in the YES2 mission hour after this magical 6500 s will follow soon!