Task: Question Generation
Input (English): Passage: The two main requirements of an anti-torque system are that it should prevent rotation of the sonde, and it should allow easy movement of the drill up and down the borehole.  Attempts have been made to design drills with counter-rotating components so that overall torque is minimized, but these have had limited success.  Five kinds of anti-torque systems have been devised for use with cable-suspended EM drills, though not all are in current use, and some drills have used a combination of more than one design.  The first drill to require an anti-torque system was used at Camp Century by CRREL in 1966; the drill incorporated a set of hinged friction blades that swung out from the sonde when the drill motor was started.  These were found to have very weak friction against the borehole wall, and were ineffective; the drill had to be controlled carefully to prevent twisting the cable.  No other drills have attempted to use this approach.For the next deployment of the drill leaf springs were installed, and this has proved to be a more durable design.  These are mounted vertically, with a curve outwards so that they are easily compressed by the borehole wall, and can slide up and down with the movement of the drill.  They pass easily through any areas of irregularity in the borehole, but the edges of the springs cut into the borehole wall and prevent rotation.  Leaf springs are very simple mechanically, with the additional benefit of being easy to adjust by changing the spacing between the end points.  They can be placed anywhere on the drill that does not rotate, so they do not add length to the sonde.  The shape is usually a fourth-order parabola, since this has been determined to provide the most even loading against the borehole wall.  Leaf springs have been found to be so effective that they can prevent rotation even in heavy drills running at full power.Skate antitorque systems have blades attached to vertical bars which are pushed against the borehole wall; the blades dig into the wall and provide the anti-torque.  Skates can be built with springs which allow them to keep the blades pressed against the wall in an irregular borehole, and to prevent problems in narrower parts of the borehole.  Although skates are a popular design for anti-torque and have been used with success, they have difficulty preventing rotation in firn and at boundaries between layers of different densities, and can cause problems when drilling with high torque.  When they fail, they act as reamers, removing chips from the wall which can fall to the drillbit and interfere with drilling.
Output (English): What has difficulty preventing rotation in firn and at boundaries between layers of different densities?