The YES2 team built a 30+ km structure in space (YES2 champagne party/reunion coming up)

For 6 weeks we have been analyzing data to make a strong case for what really happened during the YES2 mission. All data sources (YES2 length and length rate data, Foton orbit measurement, Foton accelerations), point in the same direction and only one conclusion can be drawn: the YES2 tether was deployed to its full extent and on 25 September 2007 it became the largest structure ever created in space.

More than just having deployed completely, we can now explain why YES2 has been a major success not just in terms of education, but also technically - especially from a development and tether mission point of view:

The SpaceMail strategy's most challenging objectives were achieved

• controlled deployment of the tether to a stable position 3.4 km below Foton. This 'First Stage' is a key ingredient of SpaceMail, not tried before, and intended to obtain a sufficiently precise continuation of the deployment
• successful holding phase, release and initiation of the second stage, nominal to 2/3rd of deployment time
• 30+ km tether deployment in space (strong indications exist for the 31.7 km case)
• delivery of the capsule into an immediate re-entry trajectory 

YES2 has been a technical and scientific success

• The data from YES2 and YES2 SSAU (OLD's, GPS), DIMAC (accelerometers) and NORAD (TLE) allow to recontruct in high detail the full YES2 deployment (see pictures below for current status). 
• The tether deployer controller system has functioned fully according to plan, friction on the barberpole was close to that predicted. The system is mature for a follow-on mission.
• The tether spool performance can be analyzed thanks to the large amount of data. It has already been demonstrated that the tension required to remove the tether from the spool was several times higher than expected, but not too high, and the spool's friction did show a nominal dependency on deployment velocity (the second major performance parameter)
• Plentiful data exists to recreate the mission in detail and study differences between ground tests and space performance, such as stiffness effects, oscillations, shock waves etc.
• The data allows also to investigate what went wrong (length measurement signals intermittently failed to arrive -at least with proper time tag- in the on-board computer) and make the next effort a full success.
• The YES2 satellite, safety systems, staging systems and on-board software have all worked very well

Not bad for a bunch of students. Congratulations!

PARTY PARTY PARTY

A YES2 reunion & champagne party will be organized at Delta-Utec premises to celebrate this great outcome on Saturday December 1st, 1600-1900. Everybody who reads this sentence is automatically invited.

Also, in the party we will try to convertness our sadness about having not heard about Fotino into proudness that also this capsule's technology was properly qualified and from that we can derive hope that someone one day will find Fotino in Siberia or on Russia's Pacific beaches.

Till then!

Above picture shows the YES2 tether deployment trajectory (side view with Foton in 0,0 moving left, and Earth downward) as it was preliminarily reconstructed using the raw data. The left image is a zoom on the first stage (3.4 km), the right is the full view. The black line is a simple integration of tether dynamics using the raw length data from the OLD (and length rate after 6300 s). After that, Delta-Utec's advanced tether simulator YESsim was used to reconstruct the hardware's particular performance during the flight and to investigate oscillations and bouncing effects, as can be seen in the orange curve.

YES2 second stage reconstructed deployment speed versus nominal. A maximum of 16 m/s was reached, and deployment completed with about 13 m/s some 6 minutes early.

The deployment versus nominal deployment would have looked in space like this:

It can be seen that a larger swing was achieved, which would have resulted in a more effective re-entry trajectory than nominal. The nominal deployment did not aim for such an angle, as it was conservatively designed to include some margin in case friction in space would be very high. The result is a clear indication that the YES2 system could have easily performed the nominal deployment would the computer not be confused about the length and given up on decelerating the deployment near the end. Note that most likely the tether released at the deployment completion (red dot), so the swing to the Fotino release (pink dot) was probably not made. Future data analysis will clarify this point.

So how could the deployment be reconstructed, if the length data was corrupted after t=6300 s? A complex story, and many data sources were used. However, mostly, the loop rate (or number of tether loops that are unwound from the spool every second) could be determined as a function of time from the raw mission data. The curve is plotted below. From integration of this loop rate, the loops unwound at any point in time could be calculated and therefore also the speed. Integrating tether dynamics leads to the deployment plot side-view shown above.

The brown dots are considered higher quality data than the black. From 8000 seconds only sparse data is available with lesser reliability. A statistical analysis of the available data (not shown) has demonstrated that the deployment did continue, and that the loop rate was rising steeper. This seems also to be indicated by the few remaining loop rate data points. Finally, it could be determined that the deployment was completed around 8622 s (last OLD and after this time DIMAC measured no more disturbing accelerations of Foton, indicating tether was slack or released) and was most likely fully deployed (31705 m) at this time. Most likely, because Foton orbit change (measured by NORAD) indicates that the tether was released from the spool at this time. In order for deployment to complete, looprate between 8000 and 8622 s would have had to continue as indicated by the orange line. Note that this line (determined independently from the looprate data) matches quite well the sparse remaining data points. Quite neat, no? Soon we will have more data (DIMAC, GPS) to further refine this result.

With the deployment known, we can calculate the tension level required to follow this trajectory and compare it with the nominal profile, based on pre-flight measurements of the spool. It can be seen from below plot that the YES2 deployment (silver curve, Maximum Speed) follows well the nominal curve (dotted red line) as long as the velocity is close to nominal. As soon as velocity gets higher than nominal (due to lack of braking), friction in the canister increases as a result of unwinding physics, and which can be seen in the plot because the silver curve diverges from the red dotted line. It can be concluded that the friction behavior during YES2 deployment Stage 2 was fully nominal and could have well been controlled had the computer not been confused since t=6300 s.