The helium tank in the LEM which landed on the Moon? What for?

Answer 1

The LM was a system that required very high reliability. In order to achieve that reliability the system of the engines was made as simple as possible. The engines of the first stage of the Saturn V, for example, used liquid propellants that required pumps, valves and recirculation systems, and an ignition spark. That's too complicated a system to use 400,000km from home with no hope of rescue. The LM engines were superbly simple devices. They used hypergolic propellants (that is propellants that ignite on contact with each other, no ignition spark required), and they had just two moving parts: the valves that controlled the flow of fuel and oxidiser to the combustion chamber of the engine. To ensure the fuel flows into the engine at the right rate other engine designs use pumps, but the LM simply pressurised the fuel tanks with an inert gas to force the fuel out when the valve opened. That inert gas was the helium, and that is what the helium tank on the LM was for.

The problem with helium is that it has the lowest boiling point ever, and even a fractional increase in temperature could lead to boiling and a potentially dangerous pressure build-up. To relieve this problem the helium system was equipped with burst-discs, which would rupture if the tank pressure rose above a certain point (well below the point at which the tank itself might explode) and safely vent the helium into space. The problem with that, of course, is once it goes there is no longer any pressure for the fuel. The system was designed so that under good conditions the discs would not burst until well after the mission was complete and the LM had been abandoned. On Apollo 13, which was not performed under ideal circumstances, a helium disc did burst on the homeward journey. Fortunately by that time all major course correction manoeuvres that needed the LM engine had been completed.

Answer 2

According to this page, the helium was used to pressurize the propellant system.

Answer 3

it also made the astronaughts talk funny....