alt Hacker NewsWhy use fuel, though? Is there something about its specific density and weight distribution that rules out using other types of ballast?
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If you are writing an integration test for some new and potentially bug-ridden code then you might opt to mock, say, the database connection.
Doing so risks having to write so much database logic — with all the potential for getting that code buggy as well — that it’s often better to avoid the mock and test the entire system, end-to-end.
This was an end-to-end rocket test.
The vehicle is designed to hold all that fuel, plus whatever payload it carries on top, but it's not designed to have heavy loads attached to it in any other way. Rockets are so intensely optimized for weight that sometimes they're barely strong enough to stand upright if you fuel them the wrong way: https://www.youtube.com/watch?v=imkdz63agHY.
Where would you put the other ballast?
You've got two large tanks making up the bulk of the stage's structure - one for oxidizer, one for fuel. They have large diameter pipes that feed propellant to the engines. You can't mix the ballast with either the oxidizer or fuel, and you can't feed the engines from anywhere but the propellant tanks...