Making a true nuclear rocket engine high thrust low fuel demand

[Matrix501]

I was hoping someone might be able to help or explain to me how the game comes up with how much fuel anengine uses per second i tried to change the nuclear engine and now it uses 300 liquid fuel and 300 oxidizer a second!!!!!! with only a 200 thrust! i know thrust has a factor in the usage number and so does the numbers below the fuel and oxidizer i put both numbers at 0.0001 and it still doesnt help! I get insane fuel demands i'm trying to get my nuclear engine to put out 200-300 thrust and only use 0.01 fuel and oxidizer can it be done????? i know a real nuke uses no fuel but i'll sum it up to the amount of uranium fuel inside the engine! Please help or explain how this process works so i dont keep getting these fuel demands thanks!

~matrix- the kerbin with the broken engine

[NathanKell]

This question about developing an addon has moved to Addon Development!

[Taki117]

To get more fuel efficiency you are going to have to up the specific impulse. Take a look at the ion engine for comparison.

[wasmic]

I was hoping someone might be able to help or explain to me how the game comes up with how much fuel anengine uses per second i tried to change the nuclear engine and now it uses 300 liquid fuel and 300 oxidizer a second!!!!!! with only a 200 thrust! i know thrust has a factor in the usage number and so does the numbers below the fuel and oxidizer i put both numbers at 0.0001 and it still doesnt help! I get insane fuel demands i'm trying to get my nuclear engine to put out 200-300 thrust and only use 0.01 fuel and oxidizer can it be done????? i know a real nuke uses no fuel but i'll sum it up to the amount of uranium fuel inside the engine! Please help or explain how this process works so i dont keep getting these fuel demands thanks!

~matrix- the kerbin with the broken engine

In the nuclear engine .cfg file, you'll find something called ModuleEngines. It'll look like this:

MODULE
{
	name = ModuleEngines
	thrustVectorTransformName = thrustTransform
	exhaustDamage = True
	ignitionThreshold = 0.1
	minThrust = 0
	maxThrust = 60
	heatProduction = 600
	fxOffset = 0, 0, 1.6
	PROPELLANT
	{
		name = LiquidFuel
        	ratio = 0.9
		DrawGauge = True
	}
	PROPELLANT
	{
		name = Oxidizer
		ratio = 1.1
	}
	atmosphereCurve
 	{
   	 key = 0 800
  	 key = 1 220
 	}

}

You'll need to find AtmosphereCurve:

atmosphereCurve
{
key = 0 800
key = 1 220
}

This:

key = 1 220

is the Specific Impulse (fuel efficiency) of the engine at 1 atmosphere of pressure. This:

key = 0 800

is the efficiency in a vacuum. You'll need to crank them a bit up to increase fuel efficiency. Just remember: because the rocket equation is exponential, doubling the Isp will more than double the available delta-v.

To make the LV-N behave like the NERVA, you'll have to turn this:

key = 1 220

up to this:

key = 1 800

. You can then also reduce the mass of the rocket engine. If you increase the thrust of the engine, the fuel consumption will go up, unless you also increase the Isp. The real NERVA had an Isp of about 800, though, so you'll want to limit the efficiency to that level, and eventually use multiple LV-Ns with a lower mass.

[cantab]

The stock LV-N is pretty close to a "true nuclear rocket engine", it's based on the real-life NERVA that was ground tested but never used in any rocket. In the NERVA, LV-N, and any nuclear thermal rocket the nuclear reactor heats a propellant which is then expelled through the nozzle. So even though the reactor's nuclear fuel could last for years, the propellant gets used up quite quickly, though not as quickly for the same thrust as with a chemical rocket.

[Starwaster]

To make the LV-N behave like the NERVA, you'll have to turn this:

key = 1 220

up to this:

key = 1 800

. You can then also reduce the mass of the rocket engine. If you increase the thrust of the engine, the fuel consumption will go up, unless you also increase the Isp. The real NERVA had an Isp of about 800, though, so you'll want to limit the efficiency to that level, and eventually use multiple LV-Ns with a lower mass.

That would not make it behave like NERVA would have. That would just give it SL performance equivalent to its vacuum performance. Additionally, those numbers in NERVA were for hydrogen. And not a propellant with the density of stock propellants.

[Beowolf]

Might I suggest taking a look at this reference page:

http://www.projectrho.com/public_html/rocket/enginelist.php#ntrsolidcore

My impression from your OP is you're mentally mixing up the separate concepts of fuel and propellant in "nuclear thermal" engines. That's an easy thing to do, since they're the same thing in chemical rockets.

If you want a nuclear engine that doesn't need propellant, start with the "fission fragment" category on that page. There are several similar options scattered around the page using fusion or antimatter, too. And there are a couple of semi-magical approaches that consume electricity and produce thrust without reaction mass. See: http://en.wikipedia.org/wiki/Reactionless_drive

All these propellantless drives are highly efficient, but are also very low thrust. You won't be using them to blast off from Kerbin.

edit: Oh, and I guess an explosive Orion drive is also technically "propellantless", and that one has the thrust to put your whole VAB into orbit! Real estate values downwind will take a hit, though.

Edited December 8, 2014 by Beowolf

[HoneyFox]

because the rocket equation is exponential, doubling the Isp will more than double the available delta-v.

No no no... the part of equation that has relationship with dV and Isp is linear. So doubling Isp means doubling dV if other factors don't change.

[HoneyFox]

edit: Oh, and I guess an explosive Orion drive is also technically "propellantless", and that one has the thrust to put your whole VAB into orbit! Real estate values downwind will take a hit, though.

It should still be considered using propellant IMHO, which is the melt tungsten (according to the information given in that www.projectrho.com site) inside the nuclear charges. Though it IS quite different from conventional chemical rocket engines.

[wasmic]

That would not make it behave like NERVA would have. That would just give it SL performance equivalent to its vacuum performance. Additionally, those numbers in NERVA were for hydrogen. And not a propellant with the density of stock propellants.

I had read somewhere that the NERVA had almost as great performance at SL as in vacuum, but I can't find it right now. Anyway, you're correct on all the other points.

No no no... the part of equation that has relationship with dV and Isp is linear. So doubling Isp means doubling dV if other factors don't change.

Thanks for correcting me.

[cantab]

I had read somewhere that the NERVA had almost as great performance at SL as in vacuum, but I can't find it right now. Anyway, you're correct on all the other points.

The exhaust of the NERVA is actually cooler than of a chemical rocket, and thus I believe at lower pressure and more affected by ambient air. I think it was something like 400 s at 1 atm. (The greater exhaust velocity is because the exhaust is pure hydrogen, which being a light molecule moves faster at a given temperature.)

[Kerbart]

I know a real nuke uses no fuel but i'll sum it up to the amount of uranium fuel inside the engine!

While a nuclear thermal rocket engine does not require fuel (aside from the radioactive material in the reactor which would last longer than the lifetime of the engine), it does require propellant unless a major breakthrough in physics is made.

While you don't need a fuel/oxidizer combo KSP allows some "cheating" by substituting regular liquid fuel/oxidizer as propellant as you would have very limited choice of tanks for your nuclear engine (the jet fuel one only, I'd guess). But as long as Newton's 3rd law stands, you will need to expend some kind of propellent; your “fuel.â€Â

[Beowolf]

It should still be considered using propellant IMHO, which is the melt tungsten (according to the information given in that www.projectrho.com site) inside the nuclear charges. Though it IS quite different from conventional chemical rocket engines.

Thanks. I didn't remember that detail, but it did feel like there should be propellant in there somewhere!