NASA developed only conceptual designs for Altair. No Altair spacecraft were built—plans called for a first landing on the Moon in 2018. Like the
Apollo Lunar Module (LM), Altair was envisioned as having two
stages. The descent module comprising propellant tanks, a main engine, landing gear and supporting structure and an Ascent Module with a pressurized crew cabin, life support systems, docking systems, avionics, propellant tanks and engine for lunar ascent. Like the Apollo LM, the Altair's crew cabin was based on that of a cylinder. Initially a horizontal cylinder, like that of the LM (despite the "boxy" appearance on the outside), contemporary blueprints and computer simulations showed the use of a vertical cylinder. Unlike its two-man Apollo ancestor, Altair was designed to carry the entire four-person crew to the surface, while the temporarily unoccupied
Orion crew module would have remained in lunar orbit. Altair was intended to be capable of operating away from Earth (in space and on the lunar surface) for up to 210 Earth days.{{cite web • Crewed sortie mode • Crewed outpost mode (with no airlock) • Uncrewed cargo mode, capable of transporting up to 15 metric tons to the lunar surface Altair, like the LM, was planned to have two hatches; one on top for docking and internal transfer between Altair and Orion, and a main hatch for accessing the lunar surface. Unlike the Apollo LM, Altair would have an airlock similar to those on the
Space Shuttle and the
International Space Station between the cabin and main hatch. The airlock allowed the astronauts to don and doff their
spacesuits without tracking
potentially hazardous Moon dust into the main cabin and allowed the vehicle to retain its internal pressure. Unlike the Apollo LM, in which the entire cabin was depressurized during
extra-vehicular activity, the airlock would allow a crew member with a malfunctioning spacesuit to quickly return to the Altair spacecraft without having to terminate the entire EVA, and allowed the landing party to complete most of their tasks during their 7-day lunar stay. Also, the airlock would remain as part of the Altair's descent stage, allowing NASA to utilize the airlock as a component of the Lunar Outpost. Because the
Ares V payload shroud was planned to have a diameter of and height of (including landing gear), the landers were designed to retract so as to fit within the Ares V's payload shroud. The spacecraft would also have included an improved miniature camping-style toilet, similar to the unit now used on the ISS and the Russian
Soyuz spacecraft, a food warmer to eliminate the "cold soup" menu used during Apollo missions, a laser-guided distance measurement system (with radar backup), using data acquired by advanced uncrewed lunar orbiting spacecraft, and new "
glass cockpit" and
Boeing 787-based computer system identical to that on the Orion spacecraft.
Engines Altair intended to utilize current
cryogenic technologies for the descent stages and
hypergolic technologies for the ascent stage. The Apollo LM, as advanced in both computer and engineering technology in its day, used hypergolic fuels in both of its stages, chemicals that combust on contact with each other, requiring no ignition mechanism and allowing an indefinite storage period. Both the cryogenic and hypergolic systems, like that of the Apollo LM, would be force-fed using high-pressure
helium, eliminating the need for the
turbopumps utilized in most rocket engines. Mission requirements obliged the vehicle to be able to descend from an equatorial or high-inclination lunar orbit to a polar landing site, along with bringing it and the Orion spacecraft into lunar orbit, as the Orion spacecraft's onboard
Aerojet AJ-10 rocket engine and the amount of fuel it carried would have been insufficient to brake the Orion/Altair stack into lunar orbit (also necessary if flown without Orion for cargo-only missions). The new lander would have been powered by a modified
RL-10 engine (currently in use on the upper stage of the
Delta IV rocket and
Centaur upper stage of the
Atlas V rocket), burning
liquid hydrogen (LH2) and
liquid oxygen (LOX) for the descent phase. A single
AJ-10 rocket engine, like that on the Orion, was intended to power the ascent stage. Originally, NASA wanted to power the ascent stage using LOX and liquid
methane (LCH4) engines,
RS-18, as future missions to
Mars would require the astronauts to live on the planet. The
Sabatier Reactor could be used to convert the carbon dioxide (CO2) found on Mars into methane, using either found or transported hydrogen, a
catalyst, and a source of heat. Cost overruns and immature LOX/LCH4 rocket technology forced NASA to stick with cryogenic and hypergolic systems, although later variants of Altair were meant to serve as testbeds for methane rockets and Sabatier reactors after a permanent lunar base was established. == On-orbit assembly ==