The DIMAC data shows accelerations of Foton resulting from disturbing forces. During the YES2 mission, one of the major disturbances is the YES2 tether tension. So in a way, the DIMAC was also a tether tension sensor. The data is high quality and very rich in information for YES2. It will keep us busy for months. A first glance tells us:
- The tether deployment stopped abruptly at 8625 s, as expected. The DIMAC derived tension profile is generally consistent with the 31.7 km best fit deployment case reported in the YES2 data analysis report. Although it must be admitted that presently sensor drift makes it difficult to distinguish between deviations and drift effects (this will be improved in the near future) the swing behavior and bouncing, deployment angle and Foton orbit raising all match the best-fit 31.7 km scenario and together provide a very strong case.
- A large shock (~30 N) followed, higher than expected.
- The tether remained attached to Foton, so the piece of tape at the end did NOT come lose.
- The tether was cut from Foton according to the timeline (9364 s).
- It seems that the release of Fotino can also be confirmed from the data, there is a sharp drop in tension visible some seconds after Fotino release time (9344 s).
This could mean that the entire SpaceMail mission was completed, and Fotino was sent into a trajectory towards Kazakhstan, and must have landed upstream (roughly between Baikonur and the nominal landing point near Astana), case G in the report, possibly towards case J.
We still need to study when exactly Fotino came lose from the belts to determine where in between these cases Fotino ended up.
- The delay between Fotino release time at one end of the tether and tension drop at the Foton end would be valuable information on the speed of sound in the tether, and seems to be consistent with expectations (several kilometers per second).
- The proper performance of the stepper driver and barberpole brake can be confirmed, as the tension signature due to braking is clearly recognizable. This data allows for separating several types of oscillation observed in the tether during the first stage deployment so we can learn about details of deployment dynamics that do not show up in ground tests.
- The tension during hold phase is as expected, and shows oscillations which will tell us about stiffness (EA = 120 N at 0.2 N tension, matching ground measurements) and damping of the tether, allowing us to improve simulations.
- The bouncing of the tether after deployment (picture on left) matches well the preliminary deployment reconstruction simulation (see insert and image on right), the difference in timing tells us more about the tether stiffness and thus in a next iteration we can better simulate the bounce and learn about the dynamics of Fotino at time of release (was it stable or spinning?).
- The DIMAC measurements shortly after ejection can be overlayed well with the YES2 tensiometer data, and demonstrate the DIMAC data is a good representation of tether tension. (See separate article).
- The tether angle with respect to Foton could be derived from the direction of the acceleration in Foton body frame. Combined with the YES2 deployment reconstruction performed earlier, a detailed coherent picture appears on the Foton attitude during the deployment and swing. See picture below. For the second stage this conclusion was already confirmed to several degrees precision from DIMAC magnetometer data.
- Foton attitude control thrust could be filtered from the data and it was shown that Foton Attitude Control System (ACS) maintained an angle with the vertical of about 30 degrees while the tether climbed to its swing angle. The Foton torque followed well the tether-tension induced torque during this time. See picture below. While the tether was at lower angles, it appears Foton attempted to follow the tether direction, minimizing the control torque requirement.
Potentially critical open issues are still:
- uncertainties due to sensor drift over time, which will be estimated by RedShift at a later stage.
- There is possibly a deviation of about 10 degrees in the hold phase swing angle between RedShift magnetometer data and YES2 deployment reconstruction which may indicate a slightly different first stage deployment from so far assumed. Further work into the first stage deployment reconstruction is necessary.
OTHER NEWS
Further analysis of earlier ejection data shows that the pitch-off rate measured by the gyro was about 4 degrees per second, and maximum amplitude just after ejection would then be about 30 degrees. This information should allow for improved subsatellite simulations.
