# Common usage of "dry-mass"

Usually, I would consider the dry-mass the mass of a vehicle without any consumables, passengers, or cargo.

This NASA article however describes the Apollo 11 Eagle as:

The lunar module was a two-stage vehicle designed for space operations near and on the Moon. The spacecraft mass of 15,065 kg was the mass of the LM including astronauts, propellants and expendables. The dry mass of the ascent stage was 2180 kg and it held 2639 kg of propellant. The descent stage dry mass was 2034 kg and 8212 kg of propellant were onboard initially.

Here, dry-mass seems to include crew and expendables (since the dry-mass + propellant = total mass). Which makes me wonder: When I read Spacecraft X has a dry-mass of Y, do I consider that including cargo, crew and expendables or not?

In rocketry, the two fundamental parameters of the Rocket Equation, $m_0$ and $m_f$ are referred to as wet mass and dry mass respectively. And, as you can guess, physics doesn't care what comprises the dry mass; everything has to be included.

While with other crafts, when writing specs, one can just drop the variable crew/payload and give a neat constant for the curious, in rocketry such liberties can't be taken; the exponential nature of the rocket equation can convert a kilogram of payload into a ton of fuel. This is such an essential parameter, that the common usage ("vehicle without cargo or fuel") is dropped in favor of "vehicle total mass at the end of burn".

Moreover, for purpose of stage calculation, dry mass refers to mass of the stage without fuel combined with mass of all upper stages with their fuel.

For other vehicles, their dry mass is a secondary spec of little importance: determining bridge accessibility for trucks, runway accessibility for airplanes, pretty much nothing important except broadly understood size for ships. However, with spacecraft, dry mass, combined with wet mass and ISp, comprise data necessary to determine $\Delta v$, which fundamentally defines what locations are accessible to the craft, which orbits, satellites, celestial bodies. So, in case of spacecraft you'll see the number that matters, not the number that serves to satisfy curiosity.

• Note that despite the fact that solid propellant is dry, it also makes the wet mass "wet". Commented Jun 27, 2016 at 13:23
• @MarkAdler As water is "dry" mass. Commented Jun 27, 2016 at 15:01

As a supplementary answer, "dry mass" even commonly goes as far as including propellant residuals and tank pressuriser (helium).

This can mildly affect performance calculations as the exact masses of these residuals can be hard to find, may actually be unknown, or vary between individual launches. This can furthermore be complicated by these residuals being vented after a burn, changing the mass slightly between individual parts of a stack being fired.

Back-of-the-envelope calculations typically assume all fuel to be burned, thereby assuming the dry mass to not include residuals. Common usage is therefore a little fuzzy.

Consumables for the astronauts like breathing oxygen and drinking water are also dry mass. Every mass that is not a propellant is dry mass. Propellants are used to gain speed, consumables dont produce any speed change.