A resettable and shockless hold-down and release mechanism , comprising: a) an active housing assembly having an active housing having a distal end which includes an attachment feature to attach it to a first object , said active housing having a first interface, a reversible locking mechanism mounted in said active housing which includes engagement members extending beyond said first interface; b) a passive housing having a distal end which includes an attachment feature to attach it to a second object , the passive housing having a second interface complementary to said first interface to form a coupling between it and said first interface to give a structural connection between said active and passive housing capable of reacting rotational and in-plane loads, said passive housing being configured to receive therein said engagement members when a structural connection is formed between said active and passive housing , said passive housing having an internal geometric structure complementary to the shape of said engagement members, said reversible locking mechanism including a reversible drive mechanism which, upon establishment of said structural connection, is configured to drive said engagement members radially outwards into physical contact with said internal structure thereby reversible locking said active and passive housing together in a shockless manner, and reversing said reversible drive mechanism causes said engagement members to physically disengage from said complementary internal housing structure to release said active and passive housing from each other in a shockless manner; characterised in that said first and second interfaces are Hirth interfaces . a) an active housing assembly having an active housing having a distal end which includes an attachment feature to attach it to a first object , said active housing having a first interface, a reversible locking mechanism mounted in said active housing which includes engagement members extending beyond said first interface; b) a passive housing having a distal end which includes an attachment feature to attach it to a second object , the passive housing having a second interface complementary to said first interface to form a coupling between it and said first interface to give a structural connection between said active and passive housing capable of reacting rotational and in-plane loads, said passive housing being configured to receive therein said engagement members when a structural connection is formed between said active and passive housing , said passive housing having an internal geometric structure complementary to the shape of said engagement members, said reversible locking mechanism including a reversible drive mechanism which, upon establishment of said structural connection, is configured to drive said engagement members radially outwards into physical contact with said internal structure thereby reversible locking said active and passive housing together in a shockless manner, and reversing said reversible drive mechanism causes said engagement members to physically disengage from said complementary internal housing structure to release said active and passive housing from each other in a shockless manner; characterised in that said first and second interfaces are Hirth interfaces . <EOS>