A problem that frequently shows up is estimating the mass of the propellant tanks of some hypothetical rocket configuration. This need arises from the important role of the dry mass in a rather famous equation.
Two assumptions that are often made:
- Tankage mass is proportional to propellant mass. ("the mass fraction is scale independent")
- Tankage mass and propellant mass have a square-cube relationship.
The actual values obviously differ both by propellant type, and by rocket configuration (launch vs. deep space). But the problem is still:
How well do the two assumptions above compare to real-world data?
And if those don't fit well at all, what better heuristic can be made?