Fixed the box fins and added the access hatch. Also gave it a touch of colour.
The design is a variant of Hermann Oberth's Modell E
Rocket design depicted in one of his books from the 1920s. There might be an explanation for the box style fins in the book but nothing I read in the Spaceship Handbook
mentions the thinking behind the them. They do serve as supports for the ship's take-off, but what isn't apparent here is how this all works in the film. In the film the ship is standing in water
for take-off, which is supposed to protect the ship from acoustic shock from its own take-off. Apparently it is somewhat similar to what is done with modern rockets prior to take-off (they're hosed down I understand) and that's where all that billowing smoke (actually steam) comes from.
The ship is a multistage rocket just like modern rockets, and Oberth had all this worked out years before the film. Indeed it's why Fritz Lang contracted Oberth to a be the film's technical advisor. In the film, while the ship's crew and passengers lack any specialized flight suits, the film correctly depicts the effects of acceleration and zero gravity. I think a lot of the film silly except for the depiction of the rocket and its flight---those parts are fascinating not only in how it's shown, but in how essentially right so much of it is. And this was at a time when space travel was considered pure fantasy.
Apparently a number of people have built working scale models of the Friede
and flown them successfully. I understand it's also available as a resin kit from Fantastic Plastic.