I may sound silly. I think that the Rocket systems and missiles might be having high power amplifiers and circuits inside them. Then how the Electric ground is accommodated in those kind of circuits. I have heard about floating ground but I am not sure whether that concept applies here.
Ground (separate from earth) is an arbitrary point in a circuit which is chosen to be 0V. All other potentials in the circuit are then referenced to this arbitrarily chosen 0V. Ground does not have to be the lowest voltage in a system.
Earth on the other hand is typically a low impedance, high current conductor stuck literally into the earth.
To answer your question, the ground in a missile could be any point in the circuit that the designers chose to define as 0V. A "floating ground" is usually meant to indicate that the chosen point is not "tied" to earth (the physical conductor buried in the ground).
Always remember voltage is the difference in electric potential between two different points and not an absolute value like current (the flow of a discrete number of electrons).
There's a long, long wire that connect the missile to the earth. :-)
No, serious, all the voltage are floating, but referred to the "chassis" or any equipotential point used as reference.
I think that the principle is the same as in automotive (and in all battery-powered systems).
Remember, voltage is always a difference (in Italian is common to see voltage indicated as the acronym "ddp", that means "differenza di potenziale", potential difference in english, and this really helps the understanding).