Preliminary Mission Report [updated 29 Sept. 2007]

At the time of writing it seems that with YES2 we deployed 8.5 km, rather than 30 km of tether. [NOTE D.D. 10 NOV 2007: It is now clear that the tether DID get fully deployed, but length measurement error lead to failure to stop the tether smoothly]. 

What does it mean, how much of a success was YES2? We shouldn't jump to conclusions, but there is some story-telling to do...

THE SUCCESS OF YES2

Even if the YES2 SpaceMail as an end-to-end demonstration failed, as experiment the success is determined by what can be learned from it, and this is a lot: plentiful data has been gathered to learn about all stages of tether deployment and use this to achieve full deployment success next time. Also technologically the project has been a success. The satellite electronics and software have functioned perfectly, as well as the ejection mechanism and the brake control (as already confirmed for stage 1), not a trivial feat indeed for a student-built satellite: the quality-oriented approach (aiming at ESA standards) and the extensive testing has paid off.
Although we didn't break the record for longest tether in space (SEDS, 20 km) we can be satisfied to see that many of YES2's mission objectives have still been achieved. The tether deployment was an innovative and challenging plan with several stages: pre-ejection, ejection, first stage (3.5 km), hold phase, continuation of deployment, second stage, end brake, release Fotino, release tether, opening parachute. 80% of total deployment time has been (close to) nominal. Of the previous list of phases, as far as we can judge only the final part of second stage and end brake were not performed correctly. Then there are the successes of technology development and qualification of tether and capsule, the satellite design, its construction and its problemless operation by 400 students. Possibly the Fotino landing objective will still be confirmed later this week. On top of that we managed to organize that a tether mission was flown again after a decade period of silence on this front. This was only possible through the development and application of a number of safety measures, which all seem to have performed well, therewith demonstrating that a tether deployment can be safely performed autonomously from a highly valuable platform. Conclusion YES2: from a perspective of technology development: full success. Tether science: near full success. Education: more than full success. Tether deployment: better than half. Tether safety: full success. Re-entry: still TBD :) From perspective of team work and spirit: just outright amazing. We are not done yet, as we have a lot of data to plough through now.

THE FIRST PASSES
It was the thriller of a lifetime, one orbit after another, awaiting ground contacts to receive the latest data of YES2. The first ground contact (TSU data over Canada) we were very worried about YES2's temperature after being switched off for 11 days in Foton's shadow and cold of space - afraid to kill YES2 by cold. But data did come in, so at least we were alive and...we were room temperature in the end. Good job MLI team! This was the first reward we received to day for our hard work. 1 Hour later. After the second contact (Foton TM over Russia) we received a signal that we had armed the pyro's nominally and the computer (OBC) was still ok. So congratulations to the OBC and PDU (Power Distribution Unit) designers. Important, as without this news YES2 would have been switched off for safety reasons. Next worry was the ejection: (finally!) the start of the tether deployment through three springs pushing subsatellite MASS/Fotino away powerfully from the rest of YES2.

A complex moment involving many mechanisms, dynamics and safety systems. That's why there could be many worries: would the ejection system work properly, would not too much rotation be induced due to minor asymmetries (submillimeter!), would the tether have survived launch and were the tiedowns (little knots) that held the tether during launch properly released? Would one of the 3 independent safety systems (2 autonomous cutters and a passive slip system) activate and cut the tether before any real deployment? Luckily, amazingly (or just because of excellent design work!) none of this happened, and the ejection was nominal. Congratulations to the Lulea and Patras teams!
Then, next orbit, the news on deployed length of the first stage was expected. It was wonderfully nominal, even better than in most tests: cause for great cheers and warm moments. The deployment data we received was of excellent quality and very complete. A bit later, closer inspection of the data showed significant braking action, friction or control anomalies, that caused an unsteady deployment, but well controlled by the software around the nominal profile, so still a vertical was achieved albeit with a 10 degrees estimated oscillation. This 3.4 km first stage is a unique element of YES2, an extra challenge over just deploying the tether all at once, and was intended to increase landing precision. It was thought the hardest part of the YES2 mission, because friction levels are extremely low (centi-newton, or grams) and thus hard to control - if not properly controlled you would end up with an accidental early stop. Its completion is therefore also a big success for YES2. The challenging transition into second phase also seems to have occurred properly. Herewith the principle of the two-stage deployment and feasibility of tether control methodology has been demonstrated.

Also MASS data was received through the UHF link to FLOYD, providing valuable scientific data on the deployment to be analyzed in the near future, and success for this freeflying autonomous satellite.

UNDERDEPLOYMENT
Later, during the second stage, the tether slowed down for yet unknown reason, somehow the friction level was significantly too high and could no longer be controlled. We still have to find out the details. It reached a length of 8.5 km as it came to a stretched-out slow stop, and must then have been swinging below Foton for about 15-20 degrees oscillation, before the preprogrammed commands released Fotino from MASS and cut the tether. The picture shows the estimated trajectory that MASS/Fotino has followed on the tether during deployment. This short deployment was unexpected, at least for me, because such a thing never showed up in the 560 km (!) of deployment tests we did and may indicate some mechanism failure rather than off-nominal tether control or friction. Nevertheless, as part of our off-nominal case studies, we have looked over the years at various scenario's that could lead to such a problem and we will try to find out which has been the most likely cause.

Not right now, as we are currently assessing the orbit that Fotino is in to understand when and where the capsule will return to Earth on its parachute.

[PRELIMINARY reconstruction of tether deployment from length and velocity data]

WHAT NEXT
It seems that the tether was cut OK as Foton's DIMAC payload measured a shock of the pyros. This means that MASS is freeflying with the tether and will burn in a few days. [Note that two ground observation reports of the tether have come in and will be analyzed]. Further analysis of their accelerometer data could show also the release of Fotino from MASS/Fotino.
All right, you may say, and what can be done now? Well, we will track the Fotino capsule, thanks to NORAD, the American tracking system, and as well looking for its UHF transmissions. Fotino's batteries are sufficient to survive until re-entry so we can expect tranmission for several days to come. We are looking also for MASS transmission, and as it is tethered, that tether is also orbiting and can be observed from the ground for two or so more days - which then would be useful for scientific and mission analysis. The Fotino electronics for parachute and beacon are designed to deal with this off-nominal case, to guarantee a safe landing, and for us to be able to get detailed information on the whereabouts of the eventual landing, which is to be expected in 4-11 days.

Congratulations with the success of your hardware and software, thanks so much once more for your efforts and dearest regards, till soon, Michiel