Lunar Transfer Vehicle Requirements

In the current Artemis Project reference mission, the Lunar Transfer Vehicle is based on the Spacehab module. (See section 4.3.6.1 for more information about Spacehab. Originally, this space tug was based on using a Node from the International Space Station; we would have removed all the equipment from the Node that we didn't need for the mission, thus getting rid of the great majority of the node's dry weight. (Dry weight is the mass of the vehicle when you leave out all the fuel.) However, after discussions with Spacehab, Inc., we discovered that we could reduce the mass of the module even more, and acquire a module more suited to our mission at a fraction of the cost.

We added some rockets for the main propulsion system and maneuvering, and additional systems for propulsion, electrical power, thermal control, and life support. The Spacehab already has the internal equipment needed for controlling all these systems and distributing their functions, but external sensors and antennas have to be added for guidance, navigation, and communication.

Lunar Transfer Vehicle Artwork by Vik Olliver

The body-mounted radiators, antennas, and maneuvering rockets are mounted on ring-shaped service modules attached forward and aft of the pressurized crew compartment. Solar cells are on wings attached to the forward service module. For more information about the size of the LTV, see the document listing LTV design references in this section of the Artemis Data Book.

If you add up all the rocket burns needed to get from Earth orbit to an orbit 60 nautical miles above the moon, you get about 13,500 feet per second. That defines the LTV's propulsion requirements: it must be able to impart a delta-V of 13,500 ft/sec to the entire Artemis stack. (About 80,000 pounds of spacecraft and crew start out from Earth orbit; roughly the weight of two railroad engines.) Then, after the crew has done their work on the moon, it must again handle a delta-V of 13,500 ft/sec to get itself, the crew, and the Ascent Stage vehicle (about 7,000 pounds total dry weight, a couple of mid-sized automobiles) back from lunar orbit to an orbit around the Earth. We maintain more a more detailed table of engine burns in the Reference Mission Timeline in section 4.1.1.

The LTV must be able to keep the crew alive and comfortable for at least six days (three days out, three days back) of active use, and remain unattended in lunar orbit for several weeks while the crew works on the surface of the moon. Because of the nature of the Artemis Project, the LTV has to support very high data rates with its communications systems' two channels of compressed high-resolution video. So it has the radio power of a small communications satellite.

The basic concept for refueling the LTV is to replace its empty fuel tanks with full ones in low Earth orbit. For the first few missions at least, the spacecraft will be refueled with full tanks brought up from Earth's surface. The empty tanks will be stored in Earth orbit so they can be reused later, refilled with hydrogen from Earth and oxygen shipped in from the moon.