Ode to Marcel

The above plot show the deployment tension in the first critical minute after ejection, as measured by the MASS tensiometer. The scale is TBD, but approximately in mN.

The braking of the cotton knots in the Kevlar that held the tether together during launch (tie downs) can be observed in the off-scale peaks at the expected times (0.3 and 2.5 seconds). The low average during this period suggest that the tie downs broke cleanly (also indicated by the nominal deployment speed after ejection). A clear rise in tension to about 0.2 N is then observed for several seconds (t=4.5 to t=6.5), as can be thought to be associated with the deployment of the 6 m long triple-thickness ripstitching section, expected to deploy at about this time interval. Then the tension level sinks down to a level of about 5 cN at t=8 s, which is very high compared to the expected level of about ~2 cN. The brake just started to increase at this point (in order to stabilize any oscillation of the MASS/Fotino). 

The high level in the period afterwards is very neatly associated with the barberpole being at quite a high level of turns (see below, time origins as yet +/- 2 s). It will allow us to determine the friction coefficient in space, and we can derive an important property of the spool and deployer: the zero-brake tension level. A preliminary check indicates that in case of a nominal pole friction level (TBD) the graph corresponds to a zero-brake tension level of about 3 cN, compared to value of 1 cN measured before launch. This is a considerable difference. Note also a rather mysterious delay in rise of tension, that could be some dynamic effect of the tether on the barberpole or possibly indicates that the pole did not start turning until about t=14 s. All these hypotheses require extensive check against all other data sources to build up one consistent image, and this process will take some months.

Unfortunately we have no clean data available (yet) for the few seconds after t=60 s, when the brake returns to zero position and could give a clear and direct image of the zero-brake deployment tension level and could confirm whether the brake was really at zero turns at this time. Hopefully we manage to decode some of the corrupted packets received since. An alternative route to obtain this value is by calculation from the OBC's length and velocity profile measured during the zero-braking phase of the first stage. The result obtained here is consistent with the 3 cN level obtained from the MASS data, but a direct confirmation in addition would be very valuable.

With these measurements we could begin to explain the controller difficulty during the first stage, and if it is really true that the zero-brake deployment tension was so high, it is an important result. Never before a mechanical tether was kept 11 days in space before deployment, although various people have been hypothesizing that the tension may increase due to thermal cycling effects. No such effect was found during ground tests, but these tests never included all effects together. Note that the control algorithms were designed to deal with such an off-nominal tension and this high level is not related to the problems with deployment control in the second stage. 

In short, we can learn from this graph (after careful investigation):

• proves the usefulness of this tensiometer for this purpose in space
• confirmation on tie down breaking
• confirmation of ripstitch deployment
• confirmation of barberpole and stepper driver functionality
• barberpole friction coefficient
• zero-brake deployment tension and effect of storage of the tether in space
• why could the first stage be controlled just barely within acceptable limits?

Not bad, eh? Remember that during the shaker test there was (just) a little problem a bolt from the center of the MASS structure had come loose from the tensiometer, and due to the tight schedule it was tempting to give up the tensiometer completely. The problem was however fixed, by a heroic team effort with help of Marcel in Samara last Summer, using MacGyver style bent wires and an endoscope. See http://www.yes2.info/node/78 :) Thanks Marcel!