Are spark engine more efficient than nuclear engine ?

[Hdeedose]

Hi ,

Today , I want to build a ring to my station . I have the ring and it is stable , so that's right . I've put nuclear engines because I heard they are super efficient . Then , to see , I replaced them by spark engines . Here is the result :

With nuclear engines .

With spark engines .

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Big difference , isn't it ? So , what is better ?

[The Aziz]

Nuclear engines are terrible in the atmosphere. They're meant to be used only in vacuum, and that's where they shine. So what you see is probably your deltav at sea level. You can change that of course if you click the dv button.

Sparks could get you more dv technically, but you should check the twr - it's going to be ridiculously low I imagine. You need at least 1.0 to even move, and more to actually fly anywhere.

You need to find that sweet spot, as I don't think you could lift this ring into orbit with either of these engines.

Edited January 18, 2020 by The Aziz

[AHHans]

21 minutes ago, Hdeedose said:

So , what is better ?

Depends on what you want to do.

Nuclear engines are heavy, next to useless (well, not that bad, but sill bad) in an atmosphere, have on engine gimbal, but have very good vacuum efficiency and decent thrust. They also only burn liquid-fuel and will leave any oxidizer untouched.

Spark engines are lightweight, can gimbal, and have decent efficiency in atmosphere and vacuum.

So the unexpected readout is because you probably have the delta-Vee displayed for being at sea-level on Kerbin (this is the default) and because you are using the rocket-fuel tanks which contain both Lf and Ox - so the nuclear engines have less than half the fuel mass that they will use with the left-over mass adding to the dry weight of the craft. If you click on the "delta-V" tab on the bottom right-panel, select to display all parameters, set the environment to "vacuum", and then "show all" (stages that is), then you can see that: the ISP (roughly translated the "rocket efficiency") for the nuclear version is much higher, that it's TWR (thrust to weight ratio = how fast the rocket accelerates) is higher, but also that its start mass is higher (because the engines are heavier), and that the end mass is much higher than on the spark version.

So nuclear engines are great for transfer stages, that only need to work in vacuum and where high dV is important (when fed from the right kind of fuel tank). Spark engines are good for small, lightweight craft (probes, landers) where their small size and low weight is important and the low thrust and lower efficiency (than e.g. the terrier engine) doesn't matter.

You probably have an idea how to go on from here, if not then please ask.

[Fierce Wolf]

If the ship operates only on nuclear get rid of any oxidizer, it's dead weight.

[Starman4308]

There's also the question of "what is best?" to consider. Total cost? Mass? Redundancy? Burn time (and thus you time)? Compatibility with existing infrastructure?

 

While chemical engines have inferior vacuum Isp, they have the following advantages.

1) Almost always better sea-level performance

2) Much cheaper on a cost-to-thrust basis

3) Much better TWR

4) Scales up and down: there is no Ant-sized nuclear engine

It is important to note that nuclear engines are not inherently more efficient, they inherently have better vacuum specific impulse. What is actually most efficient depends on what you are attempting to do.