University of Wisconsin-Madison Physical Sciences Lab

Independent Firn Drill

[Firn drill picture]

The melting of the solid ice is done with a straight, high-pressure stream of hot water but this method doesn’t work in the snow (or “firn”) layer at the surface. The water just seeps away through the snow without melting the desired large hole.

Firn drill ready to go

This meant that two drills were required for drilling the 2400m deep holes in the ice. A “firn drill” was used for the first 50m of the hole. Hot water was circulated through copper tubing on the outside of the drill to melt snow by contact. Some water was squirted in a solid stream from the tip of the drill to melt a leading hole and some was leaked from circuits on the side of the drill to help with enlarging the hole. When the firn drill completed its portion of the hole, the hot water drill melted the solid ice with a stream of hot water, which finished the hole.

This firn drill head used the same support tower and water supply that was used by the main drill. This saved on equipment but meant that the firn drill had to be set up, used and then dismantled before main drilling could start.

The Design

It soon became apparent that quite a bit of time could be saved if the firn holes could be drilled independently at the same time as other main holes were being drilled. The two processes could be run in parallel instead of in series.

This required:

  • A drill with its own water supply and heater
  • A drill head that could work without releasing any water since it takes too much heat to melt that much snow on a self-contained sled. (It takes as much heat to melt ice as it does to heat the resulting water from ice temperature to boiling!)

The key to designing a drill head that melts through snow at a reasonable rate is to pay attention to the rate at which heat can be conducted from the water inside the drill into the copper that makes up the drill surface. Conducting heat through the copper, and from the copper surface into the snow, are faster processes so they don’t limit drilling speed.

Firn drill tip

Increasing heat transfer from the water into the copper is achieved by keeping the water moving over the copper at a high speed. In the coils this is done by using small diameter tubing so that the water moves through it quickly. In the tip, shown at left, a fast stream of water impinges against the inside of a hollow copper cap. This gives good heat transfer to the cap and keeps it melting through the snow.

The independent firn drill also required a reel to hold the supply and return hose, a shelter to house electronics and people during drill and a catch pan to catch any spills of the drilling fluid. (A mix of non-toxic propylene glycol and water was used instead of plain water so that the liquid would not freeze when the drill was not being used.)

Firn drill rig