Nuclear engine question

[nikolay-spb]

Hello Kerbals or Humans xD,

Can someone explain to me why nuclear engine in the game use liquid oxygen and fuel? Liquid fuel/oxidizer does not require energy in order to produce thrust, so it is little absurd...

IMHO nuclear reaction (both fission and fusion) produce energy (heat) which can be transformed into some other kind of energy (reactive), and as far as i know it is done by boiling water or some other liquid in order to increase its pressure, after which that pressure is released therefore producing thrust.

[sal_vager]

Because it's a game nikolay, and the LV-N isn't a reactor, it's supposed to use the heat from sub-critical uranium to heat the propellant directly.

So Kerbals just pump their usual liquid fuel and oxidiser mix through the engine, why waste perfectly good reaction mass?

Edited February 7, 2015 by sal_vager

[regex]

Because when Squad first made them they literally had no idea how they worked. Now they still use two fuels because no one has bothered to change it and/or it is seen as a minor point in the game.

[geb]

Basically what regex said, but it's kept that way because if the LV-N suddenly only needed liquid fuel, or it's own special fuel, you'd suddenly need a load of extra fuel tank parts to support it. It would be silly to have to use half empty fuel tanks or aircraft fuselage for spaceships.

It's worth noting that some real world nuclear engine designs do use fuel/oxy mixes, although they were never built. A LANTR engine would heat the fuel with a reactor, then burn it in a conventional chemical reaction, like giving a NERVA engine an afterburner.

[helaeon]

and over in the mod section... Porkjet (the new mk2 spaceplane parts) made the engine geb talks about for us.

Atomic Age http://forum.kerbalspaceprogram.com/threads/104855-0-90-Atomic-Age-Nuclear-Rockets

[Red Iron Crown]

When the LV-N was added, there was no oxidizer. Liquid fuel and oxidizer were combined into a single resource, which all rocket and jet engines used. It wasn't until the next version that the resource was split into LF and O, and I believe the LV-N was made to consume both to keep craft from breaking.

[SorryDave]

It is not difficult to imagine an atomic engine that heats propellant. They can heat anything including water. The high ISP comes from an energy source that exceeds the output of chemistry alone.

Practically, I'd not like to see new tanks to support this engine. If it burned just fuel without oxidizer, it should burn enough to be balanced.

[kahlzun]

As it says in the .cfg for the nuke engine:

// Yes, I know this is wrong. NTRs don't actually burn fuel and oxidizer, but we don't want to jump into making separate tanks for the two yet.

[nikolay-spb]

As it says in the .cfg for the nuke engine:

// Yes, I know this is wrong. NTRs don't actually burn fuel and oxidizer, but we don't want to jump into making separate tanks for the two yet.

Yea i was guessing that it was made that way to keep game more simple, but still...

[SubzeroSpartan7]

If this is something that bothers you or something you just think would be cool to change to how it is in real life, you could download any number of mods or edit the config of the engine yourself so it only uses liquid fuel (that being said you would have to make you own fuel tanks or get modded tanks).

[PB666]

Hello Kerbals or Humans xD,

Can someone explain to me why nuclear engine in the game use liquid oxygen and fuel? Liquid fuel/oxidizer does not require energy in order to produce thrust, so it is little absurd...

IMHO nuclear reaction (both fission and fusion) produce energy (heat) which can be transformed into some other kind of energy (reactive), and as far as i know it is done by boiling water or some other liquid in order to increase its pressure, after which that pressure is released therefore producing thrust.

Because the game designers did not want to create another resource specifically for LV-N

That does not stop you however, you can create your own resource and mod tanks to put them into.

Since hydrogen is a gas it would likely be in a large round tank, or you could mod the jet fuel tank to take hydrogen.

The tricky part is creating the new resource.

You might also give yourself a bonus and tweak the ISP of the engine to the modern equivilent.

A bonus of using hydrogen, is that close to Kerbol you can harvest it, its something like 1 gram per year per meter^2 but . . . . . . . . . . . .

So production hydrogen gas in water modulated reactor is not a good thing, think explosive pressures.

The only serious nuclear accident in the US occurred in the 1961s, a worker was manually inserting fuel rods and he went to far, the reactor went prompt critical and all the water immediately vaporized resulting in the loss of reaction but, well ....., kind of KSPish. But anyway, the point is this, once you get to the point that your water is creating all kind of pressure and your uranium is oxidizing and making gas bubbles, you might be a few very short moments away from having reactor parts moving through your viscera at very high velocity.

http://en.wikipedia.org/wiki/SL-1

In a nuclear reactor the design is basically such that you want to heat water beyond its boiling point but not much more, so neutrons that are released are 'teased' so that they are delayed from reaching their targets long enough, given the plant operators time enough to adjust rod insertions and contain.

[BudgetHedgehog]

// Yes, I know this is wrong. NTRs don't actually burn fuel and oxidizer, but we don't want to jump into making separate tanks for the two yet.

And yet we have separate tanks for liquid fuel already (for planes and whatnot). So.. why haven't Squad made the LV-N use just those? They put the tanks in, regardless of what the cfg says. They don't need a whole other resource (and anyway, they've already done the 'different kind of engine needs a different resource' thing with the PB-Ion), just make them use Liquid Fuel. Simple.

[sal_vager]

So.. why haven't Squad made the LV-N use just those?

"Why don't you do X?"

"Um, we're only on M.....?"

I'm sure they're getting round to it, it's not exactly the end of Kerbin if LV-N's also heat and expel liquid oxygen

[Torquemadus]

You get a high ISP from a rocket by maximising exhaust velocity. There are practical limits to how much you can heat up the exhaust gas without melting the engine. Using an exhaust gas with a very low molecular weight, such as hydrogen, produces a higher exhaust velocity than heavier molecules.

Unfortunately, chemical rockets can't use hydrogen on it's own as an exhaust gas, since they need to burn it with an oxidiser to heat it up. This means that the absolute lowest molecular weight obtainable in a chemical rocket comes from burning hydrogen with oxygen to produce water.

Nuclear and solar thermal rockets don't need to use an oxidiser, they can heat up the hydrogen on it's own as an exhaust gas. This is why they obtain better ISP.

Thermal rockets can use water as a propellant, heating it up and expelling it as steam to produce thrust. This would produce the same ISP as a chemical engine using hydrogen and oxygen. This might be desirable if planning a mission to a place where water or ice was easily obtainable, as the water could be used to refuel. Other gases can be used instead if the ease of obtaining them matters more than ISP.

The performance of the KSP LV-N suggests that it uses hydrogen. It's never been made clear whether the liquid fuel used by engines in KSP is actually hydrogen. The ISP produced by some of the chemical rocket engines in the game suggests this, but the same liquid fuel that they use is also used in jet fuel fuselages, which are thought to contain kerosene. If the liquid fuel used in KSP is not hydrogen, then changing the LV-N to use liquid fuel only still wouldn't be realistic. It's worth noting that hydrogen is difficult to store long term in space because of the need to keep it extremely cold to avoid boil off.

To allow the LV-N to be used realistically, players should either be able to use hydrogen to get a high ISP, or use other propellants for easy deep space refuelling. Depending on how ISRU is implemented, there might be an advantage to having an engine that doesn't require oxidiser, especially if fuel and oxidiser can't be mined from the same places.

[Tuareg]

the main problem i think that this isn't intuitive at all. when a new player sit down next to ksp he has a general knowledge about the world. when he start playing he has to learn ksp's way instead of just using what he already knows...

[Red Iron Crown]

Thermal rockets can use water as a propellant, heating it up and expelling it as steam to produce thrust. This would produce the same ISP as a chemical engine using hydrogen and oxygen. This might be desirable if planning a mission to a place where water or ice was easily obtainable, as the water could be used to refuel. Other gases can be used instead if the ease of obtaining them matters more than ISP.

There's more to it than just the mass of the molecule expelled. Also important is the amount of energy added to it, in the case of LH2/LOX chemicals that is limited to the energy released by the chemical reaction. For NTRs and other rockets with a non-propellant energy source it is possible to add more energy than that, allowing higher Isps even though the molecule expelled is the same.

[Tuareg]

anyway. lv-n is op, thats the main problem with it. having a diff fuel for it could be the limiting factor...

[Commander Jebidiah]

why canyt you just edit the cfg yourself

[Tuareg]

why canyt you just edit the cfg yourself

because i dont play this game, my fun is modding it. we are talking about how squad should release the game to make it successful.

[Commander Jebidiah]

they wont listen a lot of people are telling them that releasing ksp will be a misake

[Red Iron Crown]

they wont listen a lot of people are telling them that releasing ksp will be a misake

Just from recent memory, Squad changed their minds about the way kerbal experience would work and releasing the "barn" style early buildings. They do listen, though the way they think the game should work is different from the way some players think.

[Commander Jebidiah]

They were origionally going to release it then when they realised it was bad they resceduled it, they didnt make it return because of us

[Red Iron Crown]

They were origionally going to release it then when they realised it was bad they resceduled it, they didnt make it return because of us

It's pretty clear in both of the examples I mentioned that community feedback is what made them reconsider. What else made them "realize it was bad"?

[Tuareg]

It's pretty clear in both of the examples I mentioned that community feedback is what made them reconsider. What else made them "realize it was bad"?

well, the barn wasnt that bad while the kerbal experience still is...

[Commander Jebidiah]

Most of the community wanted the barns and didn't care that they were bad it was bac9 that made them reconsider