Quote:
Originally Posted by Marasmusine
What would the actual delta-v be? An average of the two? (since they are sharing the same tanks?)
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Note: I saw your post pre-edit, and from that the first thing you have to understand is that delta-V means, literally, "change in velocity." The rated delta-V is basically a measure of how efficiently a particular engine type uses it's fuel - the higher the delta-V the more efficiently it uses it's fuel to convert it into a change in velocity. Since delta-V is basically a measure of efficiency it does not change no matter how many engines you add to a ship.
If they're sharing the same fuel tank the final delta-V is going to be some value between the higher and lower value. If you run the nuclear lightbulb engine to use 100% of the fuel the delta-V will be 0.9, if run the nuclear thermal rocket to use 100% of the fuel the delta-V will be 1.6. Or you can split the difference by running each engine for any percentage in between and wind up with a delta-V of any value between 0.9 and 1.6.
Realistically, and generally speaking, mixing two different engines is very inefficient, as the extra engine is going to cause you to have less fuel on board and cost more money as well. The main exception to that is when you have a high-thrust low delta-V engine to lift off from a planet then a low-thrust high delta-V engine to travel from orbit to wherever your destination is, for instance using a Chemical Rocket for take off and an ion drive for space propulsion to reach Mars. These two engines aren't really that far apart - replacing the Nuclear Light Bulb with another fuel tank will increase the delta-V of the NTR rocket to 1.35, which is close to the maximum delta-V having two drives installed.