Rogue System looks promising as a semi-realistic combat space sim: <a href="http:&#x2F;&#x2F;imagespaceinc.com&#x2F;rogsys&#x2F;" rel="nofollow">http:&#x2F;&#x2F;imagespaceinc.com&#x2F;rogsys&#x2F;</a><p>Pioneer also exists as an open source Frontier remake, with the same sort of flight model and universe: <a href="http:&#x2F;&#x2F;pioneerspacesim.net&#x2F;" rel="nofollow">http:&#x2F;&#x2F;pioneerspacesim.net&#x2F;</a>