Fuel Depot for Cryogenic Propellants

Cryogenic Fuels for Moonships

We would like to use liquid hydrogen and liquid oxygen as the propellants for our moonships. Hydrogen and oxygen give us the most energy per pound of fuel of any chemical propellants, so we have far more efficient rockets if we use them. That's why these are the propellants used by most large liquid-fuel launchers, including the main engines of the Space Shuttle.

These elements have an additional advantage of being extremely clean fuels -- when you combine hydrogen and oxygen, you get water and nothing else. The water comes out the rocket nozzle as high-velocity, extremely hot vapor. So you can think of a Space Shuttle as clean-burning steam engine.

The only practical way for a rocket to use hydrogen and oxygen is to fill the rocket's tanks with these elements in their liquid form. To be a liquid, hydrogen and oxygen must be very, very cold; that is, in a "cryogenic" state. This usually works fine for rockets which launch from Earth and accelerate until all their fuel is used up, but it becomes a special problem when we get into zero-gravity conditions.

Storing cryogenic fuels in zero gravity can be tricky. To keep the fuel liquid, we cool it continously by allowing excess heat to be carried away as the gasses boil off. That's the vapor you see venting away just before a Space Shuttle launch. In zero gravity, however, the liquid may flash into vapor away from the nozzle that dumps the gas overboard, pushing liquid propellant ahead of it. If this happens, we waste tremendous amounts of propellant when the liquid gets dumped into space.

We would need some elegant engineering solutions to be able to store cryogenic fuels at a small facility such as the initial LEO staging base shown at the left. This facility will serve us well for the initial phases of the Artemis Project, where we launch ships directly from Earth and can maintain a constant, small acceleration to keep the cryogenic fuel settled. However, to store fuel for weeks at a time in low Earth orbit, to refuel commercial spacecraft arrive, we will need an expanded space station.

Upper-stage liquid-fuel boosters, which begin their mission in low Earth orbit, get around this problem by using propellants which are liquid at room temperature. One of the most common propellant combinations is hydrazine and nitrogen tetroxide. These chemicals also have an advantage of being hypergolic. That is, they burn spontaneously when they come into contact.

However, hypergolic fuels have the disadvantage of being much less energetic than hydrogen and oxygen. If we launch a moon-bound rocket from low Earth orbit using hydrogen and oxygen, about 75% of the mass of our moonship is fuel, and the rest is useful payload landed on the moon. However, for hypergolic propellants, we will typically have more than 90% of the mass of the rocket as fuel, so we get less than 10% of the ship's initial mass landed on the moon.

All these fuels are relatively cheap -- liquid oxygen costs less than gasoline; however the cost of getting them to low Earth orbit is not. Currently, we estimate that about 75% of our Reference Mission cost (see section 4 of the Artemis Data Book) will be spent on just getting our moonships and their fuel to low Earth orbit. This will change as new, less expensive launchers are developed; however we expect that getting to Earth orbit will always be a major portion of the mission cost.

Gravity Gradient Provides a Solution

If you're 300 feet (about 100 meters) away from the center of gravity in an orbiting spacecraft in LEO, up or down, you'll experience about 25 microgravities of acceleration due to Earth's gravitational field. That would be enough to keep the cryogenics settled.

We can build our fuel depot as a tall 600-foot (about 200 meters) tower built with a lightweight truss in Earth orbit. This tower will orient itself along the gravity gradient, so that the long axis will always point along a radius to Earth's center of gravity. That means our fuel depot can maintain this desirable orientation without the need to expend fuel.

Boil-off gasses can be used to reboost the station against atmospheric drag, so we can use fuel that otherwise would be expended just to keep the cryogenics cold to maintain the fuel depot's orbit. We would do this with small H₂-O₂ thrusters mounted at the fuel depot's center of gravity.