6.12 Why is there no exhaust from the LM’s ascent rocket engine?

IN A NUTSHELL: Because the propellant of the Lunar Module didn’t generate a visible plume. The same occurs in other vehicles that use the same propellant type, such as the Titan launchers used for the Gemini program.

THE DETAILS: Bill Kaysing, in Fox TV's Did We Land on the Moon? (2001), objects that “In the footage of the ascent stage going up, what you don’t see is an exhaust plume coming out of the rocket engine nozzle... What do we see? We see the ascent stage suddenly pop up without any exhaust plume whatsoever as though it were jerked up by a cable”.

Actually, there’s no visible exhaust plume for a very simple reason: there shouldn’t be one. Not all rocket propellants produce a bright plume, especially in a near-vacuum such as the Moon’s surface. In particular, the Apollo LM used a mix of Aerozine 50 (50% hydrazine, 50% unsymmetrical dimethyl hydrazine) and dinitrogen tetroxide, which are hypergolic, i.e., they react spontaneously as soon as they come into mutual contact. This allowed a simple and highly reliable engine design. The product of the reaction of these substances is colorless and transparent: that's why there's no visible plume under the LM. A layperson might expect a plume and flames because that's what is usually seen during rocket liftoffs. But most large rockets use different, cheaper and less toxic propellants, which generate a bright, fiery exhaust.

The same type of propellant used in the LM was also used in the massive Titan launchers used in the Gemini manned spaceflight program (and also used as nuclear warhead delivery missiles), and their liftoffs produced a surprisingly small and colorless plume, with no flames (Figure 6-25). The Space Shuttle's maneuvering thrusters also used hypergolic propellants and likewise produced no significant plume.

It’s true, however, that NASA is partly to blame for this widespread misconception, because many artist’s illustrations published by the space agency actually show a plume both for the ascent stage and for the descent stage of the LM. However, there was no other self-explanatory way to illustrate the fact that the spacecraft was being propelled by its rocket motor.

Figure 6-25. Liftoff of a Titan rocket carrying the Gemini 12 spacecraft on November 11, 1966. Note the almost colorless engine exhaust and the lack of flames.