Right, firstly I appreciate I'm most likely being a moron and the answer to this is blindingly obvious. Humor me.
Commercial Jump Drives... Can I use them or are they for commercial (computer controlled) fleets only?
I have a jump ship with a 2250 to cap. military jump engine which transports 1400ton survey ships with no problems.
I have now created a jump ship with a 20k ton cap. commercial drive but it refuses to transport my 19k ton ships, all of which have commercial engines.
Sorry if I'm being a plank but if I'm reliant on military drives it's 45k rp points to research a drive that will allow my smallest freighter (19k tons) to leave the system!
[Edit - Think I've answered my own question. Does cargo add to ship tonnage? An unladen ship works fine. A fully laden one would take it over 20k cap which would explain it.]
1.) Cargo does not add to the ships tonnage. You might want to check that the jump engine ship is the same size or larger than the freighter you want to go through. The limiting factor is the smaller of the ship mounting the jump engine, or the jump engine size. You want the two to be identical if at all possible, otherwise things have a way of going wrong.
2.) At that rp cost it will actually be cheaper to upgrade your base tech first. The cost for a specific capacity jump engine will go down with the higher tech multiplier.
Brian