Star Trader 2 https://steamcommunity.com/sharedfiles/filedetails/?id=311638058 unfortunately it not out yet, think the first one was only on mobile.

As you mentioned Traveller, you might want to keep an eye on Stellar Tactics http://www.stellartactics.com/ should be on Steams EA within a month or

two.

The smugglers games http://store.steampowered.com/app/291450/ never played them so no idea what they are like.

Blue Solar http://store.steampowered.com/app/501430/?snr=1_7_7_151_150_7 might be another but again I not played it.

thanks will check them out.

Star Traders 2 has two predecessors from the Trese Brothers, I think there are free versions on Android you can check out.  Those games are based on the ORIGINAL "Star Traders" aka "Star Lanes" from 1984, you can probably find it and play in Dosbox.

Speaking of Dosbox, there are some old games that are exactly what you are asking for.  Like, literally:

Megatraveller and Megatraveller 2, you can find on Abandonia.com

You will see Whales Voyage linked at the bottom of those pages as a "Similar Game".  It is also good.

If you get an Atari ST emulator, take a look at a brilliant game called Sundog The Frozen Legacy by FTL studios, the same guys that made Dungeon Master.

There was an old BBS Door game called Trade Wars 2002 that might be up your alley.  It has a relatively recent remake that sort of sucks, but you can probably still find the original running online somewhere.

There is something of a "roguelike" remake of privateer called ASCII Sector, has much more going on than Privateer did, but the general concept is similar -- fly, trade, fight, upgrade your ship.

You might also like Escape Velocity and its various successors such as Naev and Endless Sky, both of which are very much worth playing.

This is exactly the kind of game I am trying to find, please let me know if you find something close to it!

This is the closest game I have found so far:

http://www.roguebasin.com/index.php?title=Prospector

download link

http://www.prospector.at/forum/dm_eds/dmedsdl.php

Yeah, Prospector RL is awesome.  Unfortunately the latest version has a bug with ranged combat that trivializes the game, and development has stopped.  But it is still fun to play and explore.

You might enjoy Starflight, there is a fan made spiritual successor you can get here, which is pretty good:   https://sites.google.com/site/starflighttlc/downloads

SGJ does a let's play series you can check out here:   https://www.spacegamejunkie.com/series/starflight-the-lost-colony-review/

and since this forum does some swanky automatic yootoob embedding, I'll direct link the first one in his series here:  

The original Starflight games you can probably find and get working in dosbox.  They all feature free form exploration, traveling and fighting in space, diplomacy with aliens, getting off your ship and driving around, harvesting resources and encountering local flora and fauna... Only Starflight 2 really has a robust system of interaction with alien cities, the others just use the starting station as a central hub.

But a couple of the ones in my earlier post fit this bill to one degree or another, so I'll offer more details:

In the sense that you can fly your ship between star systems, get off, walk around cities, trade cargo, and shoot at pirates both in space and on the streets, Sundog (on an ST emulator) and the two Megatraveller (on Dosbox) games come close to that.  Neither of them have truely freeform sandbox play, though Sundog does let you take your rover out and drive cross-country between settlements.

See some Sundog:  

See some Megatraveller:  

 (the sequel had some significant technical advances, but the gameplay formula was similar)

Another one that might be interesting is "Ascent" which is in Early Access.  SGJ talks to the developer here:   https://www.spacegamejunkie.com/interviews/sgj-podcast-157-ascent-space-game/

I guess it is in early access, I haven't been following it too closely.  It looks like it has some X3 / Unending Galaxy style colony and industry management, but at its core seems to be an open universe ship-piloting exploration and trading game.  It is probably the furthest from what you want, in comparison to the games listed above -- but it is probably thematically similar enough that it is the sort of thing you guys would be interested in.

Here is a let's play of ASCII Sector.  Not the best video, but the only one I found that wasn't instantly offensive:  

I love that you dock at a station, walk to your airlock, and then walk around the station to visit the various offices.  It is very similar to Prospector RL in the way that it handles ship and station interiors.  Has less going on in terms of wide open universe, but much more in terms of quests and being a finished game.