While　polynitrogens　are　proposed　to　be　the　hyper　rocket　fuels,　many　of　their　properties　in　the　solid　states　are　unclear.　Herein,　we　comprehensively　investigated　the　thermodynamic　and　kinetic　stabilities,　polymorphs,　and　rocket　performances　of　platonic　and　cubic　gauche　nitrogens,　i.e.　tetrahedral　N4,　cubic　N8,　platonic　dodecahedral　N20　and　cubic　gauche　nitrogen　(cg-N),　using　the　first-principles　calculations.　The　results　showed　that　the　dissociation　barrier　of　tetrahedral　N4　in　the　gas　phase　(＞200 kJ mol−1)　is　high　enough　to　keep　stable.　It　would　be　a　liquid　monopropellant　under　slight　external　pressure　or　low　temperature.　The　specific　impulse　(Isp)　of　tetrahedral　N4　is　comparable　to　that　of　liquid　H2/O2,　but　it　provides　a　density　of　1.459 g cm−3　with　the　body　centered　tetragonal　packing.　The　conversion　barrier　of　cubic　N8　in　the　gas　phase　(∼85 kJ mol−1)　is　marginal　for　stability.　It　may　be　a　solid　at　ambient　temperature,　and　its　Isp　exceeds　that　of　liquid　H2/O2.　The　crystal　density　of　cubic　N8　is　1.964 g cm−3　with　the　rhombohedral　centered　hexagonal　packing.　The　dissociation　barrier　for　stability　of　platonic　N20　in　the　gas　phase　(∼25 kJ mol−1)　is　insufficient,　however　it　possesses　a　lattice　energy　above　100 kJ mol−1.　Although　the　Isp　of　platonic　N20　is　as　powerful　as　that　of　the　liquid　H2/O2,　it　has　a　crystal　density　of　2.132 g cm−3　with　the　face　centered　cubic　packing.　For　surface　relaxation,　cg-N　is　unstable　at　the　ambient　condition　with　a　dissociation　barrier　of　∼17 kJ mol−1.　While　the　Isp　of　cg-N　is　less　than　those　of　platonic　nitrogens,　its　effective　impulse　(Ief)　is　the　best　due　to　an　extremely　high　density　of　3.401 g cm−3.　In　respect　of　the　thermodynamics,　the　enthalpies　per　atom　of　platonic　nitrogens　are　always　higher　than　those　of　cg-N　and　Ε-N2　from　0　to　100 GPa.　©　2016　Elsevier　B.V.