Wind up tests for YES2 tether
The Second Young Engineers’ Satellite (YES2), which involves more than 400 students from across Europe and around the world, is making excellent progress towards a launch in September 2007.
One of the most important aspects of this innovative hands-on student-led mission is the deployment of a 30 km long tether to demonstrate an accurate change of orbit that does not demand attitude control systems or rocket engines. Not only will this be the longest artificial structure ever deployed in space, but it will also be the first time that a tether has been used to return a payload from space.
Preparations for testing and winding the thin, but extremely strong, tether are well under way at ESA’s Space Research and Technology Centre (ESTEC) in the Netherlands. The tether, measuring 32 km in length (30 km to be deployed in space plus a 2 km margin), is currently on the test rig in a clean room at ESTEC. In the coming weeks it will be wound onto the flight core – effectively the spindle for the tether - and then unwound a total of three times in order to prepare it for flight on the YES2 mission.
“We have to wind and rewind the tether three times because the tether characteristics change after each cycle but then stabilises,” explained Marco Stelzer, a member of ESA’s YES2 engineering team. “The tether is made from Dyneema, a lightweight synthetic fibre based on polyethylene, which is stronger than steel or Kevlar. It has a high resistance against abrasion, UV-radiation and impacts, but suffers from ‘creep’, so that it becomes more flexible as time goes by. Curiously, it also becomes a little shorter!
“We carry out the three wind-rewind cycles to stabilise its properties and to determine relevant mission parameters in order to gain valuable information for the active tether deployment control algorithms. Each cycle takes about four days – 12 days in all. Although we can rewind it quite quickly during the on-ground deployment tests, at up to 12 meters/sec, the actual winding process is slower and more time-consuming due to the necessity to ensure a high quality product which behaves in a predictable manner. We wind the tether onto the flight core at a rate of about 30 cm/sec.” Under current plans, the final flight core and its tether will be available for installation in the YES2 satellite at the end of March, in preparation for flight readiness tests.
YES2 will fly on ESA’s Foton-M3 microgravity mission, which is scheduled to be launched by a Soyuz rocket in September 2007. The experiment will be carried out on top of the battery pack of the Foton-M3 capsule. During the flight it will use the tether to deploy the Fotino mini-satellite and re-entry capsule.
“There is a spring-based ejection system on the FLOYD deployment mechanism,” said Stelzer. “The idea is to push the lower part of the satellite away from the FOTON to overcome the low gravity gradient force - the driving force behind the deployment - then, when the tether extends to a length of several hundred meters, the increased gravity gradient will become strong enough to accelerate the tethered re-entry capsule further, pulling out the tether to a length of 30 km.”