A small study of technology performance: ion/nuclear/chemical

[bitzoid]

I was thinking about what scaling factors where necessary to make drop-in replacements between ion and nuclear drives. For completeness I also compared this to chemical engines (represented by the Terrier).

So, the question is at what requirements, what tech should be used. As a baseline, I picked 60kN of thrust and added the correct fuel until all contestants reach the same mass. This means that they also will have the same acceleration (when completely full). The result is therefore only dv, with everything else being equal. For the purpose of this comparison we neglect the dry mass of the tank.

The following graph shows a comparison of five configurations:

• ion: 30 Self-sufficient Dawn engines, with enough RTGs (330) to run at full thrust independently from the sun.
• ion no EC: The same Dawn engines, but relying on the payload providing the EC.
• ion fc: The same Dawn engines, but being supplied by 15 Fuel-Cell Arrays. This configuration needs a combination of Xenon, Liquid Fuel and Oxidiser.
• nuclear: A single Nerv engine with fuel to match the mass of the ion baseline.
• chemical: A single Terrier engine with fuel to match the mass of the ion baseline. Disclaimer: I do not use the Wolfhound because I need all configurations to have the same thrust. Raising the others to the Wolfhound's thrust would make the comparison less useful.

Then we add more fuel proportionally, such that the mass of all four configurations remains the same. To read this graph, the scale for dv is on the right side. In the top part of the graph, the exact formula for all five configurations is shown.

There are some interesting things do note here, but first, only the RTG/Ion engine intersects noteworthy with others (we can pretty much ignore the intersection at x=36).

Mass and Performance

The mass at these intersections are:

• x = 311: 49.5t (1.2 m/s²)
• x = 416: 54.7t (1.1 m/s²)
• x = 523: 60.1t (1.0 m/s²)

That means, whenever you have a configuration that needs more fuel than 56.5t per Nerv, you should always use Dawns+RTGs (in vacuum). This is only limited by Career-funds.

Dawn configuration

The turquoise configuration with magic EC blows every other engine out of the water errr... space. Obviously. Using Fuel Cell-Arrays with the Dawn engine isn't necessarily a bad idea, but the Nerv is consistently better. For higher dv requirements, the magic power coming from the RTG gains over the weight of the fuel you would need to power the FCA.

Net Acceleration

This is difficult to see, but the background indicates the acceleration you get from the bottom, which is the wet acceleration and the same for all engines, up to the top, which is the dry acceleration. As is to be expected the chemical engine wins flat out wrt to dry acceleration (with a whopping 120m/s²), as it burns the most fuel-mass. That's why the background is primarily purple. On the lower bit we see a strip of orange (nuclear), with a dry acceleration of 20m/s² and further down a dark patch where the ion drives live (ranging from 1.78m/s² to 8m/s²). This is to be expected.

 

Conclusion

What I take from this is that only past 20km/s every non-atmospheric ship should use Ion engines (if money is not an issue). With enough RTGs there is no need to awkwardly wait for the gigantor to fill a huge battery.

 

                                     

The graph and computations were made with KRP. See also the forum thread.

 

Edited January 18, 2019 by bitzoid