The YES2 project
YES2 will demonstrate a new concept called the ‘space mail’ service for the first time, thus removing the need to use retro-rockets for de-orbiting capsules from the Space Station. It will try to return a capsule with payload safely to Earth using a tether-assisted de-orbiting manoeuvre instead of a conventional retro-rocket propulsion system.
The purpose of the tether is to reduce the orbital energy of the capsule without the use of a retro-rocket as it is normally done. This is possible thanks to an ingenious trick where in a pendulum swing, the mother craft Foton-M3 is accelerated by a tiny amount, taking energy away from the small YES2 at the other end of the pendulum.
The reduced orbital energy results in a lower perigee of the orbit of the released Fotino capsule, so low that its orbit crosses the Earth’s surface. The braking effect of the atmosphere will slow down the capsule to allow a soft-landing. This ingenious trick only works if the orbit is low (280 km for Foton-M3) and the tether long enough (30 km for YES2) to generate a perigee of less than the radius of the Earth, in a single pendulum swing.
How does it work in practice?
Just before activation of YES2, the Foton-M3 mother craft makes a 90 degree rotation so that the experiment is pointing towards the Earth.
At release, three strong springs accelerate MASS and Fotino, still strapped together, towards Earth to a speed of 3 m/sec. Moving away, a thin tether is unwound like a fishing line. Closer to Earth, gravity becomes stronger and this force gradient gently pulls the capsule further down long after the initial energy has been lost in friction in the tether spool.
The motion of the capsule is controlled by the unwinding speed. When this speed is high, the orbital angular velocity increases, and MASS and Fotino swing forward, in the direction of flight. If the unwinding speed is low, the increase of gravity closer to Earth becomes the dominating factor and the capsule stays close to the local vertical. A friction brake on the tether can therefore take control of the pendulum movement.
Three and a half kilometres of tether unwind in 90 minutes, during which Fotino and MASS swing forward once by more than 50 degrees, and then back vertical again, as the friction brake slows down the tether release. When the 3500 metres of tether have been pulled out of FLOYD, the brake blocks the release completely for a few minutes: time to close in on the landing point.
As the brake is released again, Fotino and MASS rapidly pull down and with the lower orbit, their angular velocity increases, moving both ahead of Foton once more, by up to 40 degrees. After an hour, the release is suddenly blocked, at a release speed of some 30 km/hour.
Foton-M3 and YES2 has become a rigid pair now, a 30 km-long pendulum in space. At release speed zero, the gravity gradient forces the pendulum to swing back, against the direction of flight.
When it is nearly in the local vertical, the tether is cut at FLOYD and Fotino released from MASS by opening the straps … both slowed down enough for a direct path from space to Earth, plunging towards the atmosphere for re-entry.
Thirty kilometres of sun-lit tether is the longest object ever deployed in space and will be visible from Earth—but not for long. It rapidly burns up in the atmosphere together with MASS. Fotino takes course to the Kazakh steppe. Diving into the atmosphere, it loses energy as heat, protected by a shield tested to withstand 2000°C.
A small parachute opens at five kilometres when the air speed is low enough. The touchdown will be gentle—if everything works as it should.