In the 1960s they made trials with rocket engines that used high-energy fuels such as hydrogen and fluorine.

-Fluorine is a highly reactive liquid, physiologically dangerous and difficult to handle.

-And for its condition it was preferable to use it in higher stages of launchers.

-These tests were grouped in the HORA program (HOchenergetischer RAunsondenantrieb), which is high energy propulsion for deep space.

-The aspect of this test engine, prepared to be used in a NASA Atlas-Centaurus, is as follows.

"Dornier HORA"