Unrelenting or unyielding in severity.
Unremitting, steady and persistent.
Rigorous and unbending.
Stiff, rather than flexible.
Fixed, rather than moving.
Uncompromising.
An airship whose shape is maintained solely by an internal and/or external rigid structural framework, without using internal gas pressure to stiffen the vehicle (the lifting gas is at atmospheric pressure); typically also equipped with multiple redundant gasbags, unlike other types of airship.
A bicycle with no suspension system.