Why does the LV-N need oxidizer?

[The Ideal Gas Lawyer]

I was doing some Wikipedia surfing and I stumbled into the article on nuclear thermal rockets and was surprised to learn that no actual combustion takes place when the rocket is powered up. It's merely a nuclear reactor that heats a working fluid (usually LH2) to great temperature and pressure before expelling it through a rocket nozzle to produce thrust very efficiently.

Given that's the case, shouldn't the LV-N only require liquid fuel? One possible explanation is that the liquid fuel used at KSC is not hydrogen but kerosene, which would explain why it can also be used as jet fuel.* in that case the oxidizer could be the working fluid instead since some NTR engines run off of LO2 or other chemicals but in that case, why require liquid fuel? I'm sure the reason why is game balance and simplicity since this isn't Orbiter, but still. It strikes me as a weird oversight.

*This doesn't agree with the numbers however. The Mk.2 fuselage has a total mas of 1.0 tonnes, a dry mass of 0.25t and holds 150 liters of LF. That gives a density of 5kg/L which is hugely more than kerosene's 0.8 kg/l or LH2's 0.07 kg/l. A similar calculation for oxidizer yields the same density (5kg/l) which is at odds with LO2 (1.1 kg/l) and liquid nitrous oxide's (1.2 kg/l)

[NovaSilisko]

We don't have rocket-specific fuel-only tanks yet. Once we have tweakables (which will let you right click a tank and choose whether you want fuel, oxidizer, or both), the oxidizer requirement will go away.

[The Ideal Gas Lawyer]

We don't have rocket-specific fuel-only tanks yet. Once we have tweakables (which will let you right click a tank and choose whether you want fuel, oxidizer, or both), the oxidizer requirement will go away.

Thanks for the fast answer. You guys rock

[rakutenshi]

this makes me happy.

Larger stock LV-Ns would be nice too (or perhaps a cluster of smaller ones?).. just saying.

Between clickables in the VAB and 'fixed' ISP/throttle handling.. is definitely going int he right direction

[ROFLCopter64bit]

We don't have rocket-specific fuel-only tanks yet. Once we have tweakables (which will let you right click a tank and choose whether you want fuel, oxidizer, or both), the oxidizer requirement will go away.

That was on the resource system. That reminds me, has there been any changes to the resource map? Or is it the same? I know I shouldn't ask (0.21/0.22 seems like mid-july-ish), but would you be so kind?

[Fel]

\KSP_win\Parts\nuclearEngine\part.cfg

// 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 the resource weight values are temporary.)

(And nothing really makes sense, does it XD)

[Frostiken]

We don't have rocket-specific fuel-only tanks yet. Once we have tweakables (which will let you right click a tank and choose whether you want fuel, oxidizer, or both), the oxidizer requirement will go away.

Does this also include 'empty'? Because it should if it doesn't

Edited May 10, 2013 by Frostiken

[simonh]

I'm wondering why they bother accounting for fuel and oxidizer separately. I know it's realistic, but in practice what difference does it make? Why not just list how much 'propellant' the tank contains, then just have different propellants for chemical, nuclear and ion engines.

Simon Hibbs

[ComradeGoat]

It would be nice if, when we get sources, LV-Ns could still use the oxidiser itself as a working fluid, but perhaps with a reduced ISP.

[SunJumper]

I'm wondering why they bother accounting for fuel and oxidizer separately. I know it's realistic, but in practice what difference does it make? Why not just list how much 'propellant' the tank contains, then just have different propellants for chemical, nuclear and ion engines.

Simon Hibbs

This is to differentiate jet fuel tanks and rocket fuel tanks. When resource mining comes along, you'll most likely have to mine the hydrogen and oxidiser differently.

Any word on Kerosene or other heavy liquid fuels for the future?

[RoboRay]

I'm wondering why they bother accounting for fuel and oxidizer separately. I know it's realistic, but in practice what difference does it make? Why not just list how much 'propellant' the tank contains, then just have different propellants for chemical, nuclear and ion engines.

Simon Hibbs

Why would there be separate chemical and nuclear propellants? Hydrogen offers the best performance in both cases.

And if methane or another fluid is chosen instead, due to the difficulties in storing hydrogen, it still makes more sense to use the same propellant for both.

Originally, there was no oxidizer, just fuel. Oxidizer was added as a rocket requirement when working intakes were added for jet engines.

Edited May 10, 2013 by RoboRay

[nyrath]

It seems to me that the sticky bit is the old problem that fuel is not propellant. Except in odd cases like chemical rockets.

Chemical Rockets: fuel is fuel+oxidizer, propellant is combustion products

Ion rockets: fuel is electricity (solar cells or RTG), propellant is xenon

Fission nuclear thermal rocket: fuel is uranium/plutonium, propellant is hydrogen

Magneto Inertial Fusion: fuel is deuterium, propellant is lithium foil liners

VASIMR: fuel is lots of electricity (nuclear reactor), propellant is argon or xenon

[Tw1]

Chemical Rockets: fuel is fuel+oxidizer, propellant is combustion products

Ion rockets: fuel is electricity (solar cells or RTG), propellant is xenon

Fission nuclear thermal rocket: fuel is uranium/plutonium, propellant is hydrogen

Magneto Inertial Fusion: fuel is deuterium, propellant is lithium foil liners

VASIMR: fuel is lots of electricity (nuclear reactor), propellant is argon or xenon

To clarify, propellant is the substance expelled by the engine to make it go, fuel provides the energy.

I'm a fan of the main fuel for liquid engines being hydrogen.

It's versatile, it contains lots of energy, and it burns to produce water vapor. I'd rather not muck around with more pollutive fuels like kerosene if hydrogen an option. Assuming the Kerbals have mastered storing it properly.

I have wondered why there isn't a design that combines nuclear and chemical power.

If you're using a fuel that burns, why waste the chemical potential energy? There must be design challenges that make it impractical.

[OrbitusII]

We don't have rocket-specific fuel-only tanks yet. Once we have tweakables (which will let you right click a tank and choose whether you want fuel, oxidizer, or both), the oxidizer requirement will go away.

That might be something for me to look at for a plugin... Heat sinks are basically done so I could mess around with some code to see if I can figure it out...

[nyrath]

I have wondered why there isn't a design that combines nuclear and chemical power.

If you're using a fuel that burns, why waste the chemical potential energy? There must be design challenges that make it impractical.

There is one. It is called LOX-augmented Nuclear Thermal Rocket. The chemical and nuclear parts interfere with each other.

What it boils down to is that when it is in Nuclear mode, it has an Isp of 940 seconds, but a thrust of only 67 kiloNewtons. In LOX augmented mode, it has an Isp of only 647 seconds but a thrust of 184 kiloNewtons.

In other words, in LOX augmented mode it trades reduceds Isp for increased thrust.

http://www.projectrho.com/public_html/rocket/enginelist.php#id--Nuclear_Thermal--Solid_Core--LANTR

Once you start dealing with liquid and gas core nuclear thermal rockets, the chemical part is superfluous. It is sort of like attaching a firecracker to the side of a nuclear warhead: yes, the explosion will be bigger, but not enough to measure.

[magnemoe]

There is one. It is called LOX-augmented Nuclear Thermal Rocket. The chemical and nuclear parts interfere with each other.

What it boils down to is that when it is in Nuclear mode, it has an Isp of 940 seconds, but a thrust of only 67 kiloNewtons. In LOX augmented mode, it has an Isp of only 647 seconds but a thrust of 184 kiloNewtons.

In other words, in LOX augmented mode it trades reduceds Isp for increased thrust.

http://www.projectrho.com/public_html/rocket/enginelist.php#id--Nuclear_Thermal--Solid_Core--LANTR

Once you start dealing with liquid and gas core nuclear thermal rockets, the chemical part is superfluous. It is sort of like attaching a firecracker to the side of a nuclear warhead: yes, the explosion will be bigger, but not enough to measure.

One issue in KSP is that many ships are hybrid, they use an combination of nuclear and chemical rockets, my kerthane miners is an good example, they typically land on take off on all engines but uses the nuclear elsewhere. It will have an totally different profile if mining on Gilly and Moho.

One feature of LANTR is that you has to have the oxygen, for the sort of hybrid use it makes sense. On the other hand I think we will start to having far more fuel than oxygen on ships later.

[Fel]

I'm a fan of the main fuel for liquid engines being hydrogen.

It's versatile, it contains lots of energy, and it burns to produce water vapor. I'd rather not muck around with more pollutive fuels like kerosene if hydrogen an option. Assuming the Kerbals have mastered storing it properly.

But isn't Hydrogen extremely poor against the heavier elements in the atmosphere, hence meaning your first stages would produce very little thrust? (Well, far less than KSP currently models)

While Kerosene is much heavier and works extremely well in the lower atmosphere.

I'm in favour of having the "first stage engines" use fuel that first stage engines should use. If you want to fight global warming though, I'm sure the kerbals will appreciate it ;p

I have wondered why there isn't a design that combines nuclear and chemical power.

If you're using a fuel that burns, why waste the chemical potential energy? There must be design challenges that make it impractical.

Your solution is to set fire to a "barely controlled fission reaction"?

The nuclear core heats the hydrogen up, producing large amounts of pressure and hence thrust. As hydrogen has a MUCH higher specific heat than oxygen (Let's say 14:1), you'd have to saturate the core with oxygen to get a 2:1 ignition ratio... but since the gas is already pressurized I guess you'd be igniting within the core to further increase the pressure (and the excited gas form isn't as effective as the liquid form, if I recall correctly).... I think H2O would be the Lox/LH2 propellant... which probably would reduce pressure (hydrogen bond)... but with increased excitation...

I'm inclined to say that the additional energy supplied by oxidizer doesn't outweigh the difficulties / weight of said engine.

I'd also say that part of the benefit of the NTR Engine is that you reduce the fuel load you need; this only increase it.

[Scialytic]

There are larger issues than that. The limiting constraint on NTR efficiency is how hot you can safely run the reactor. Assuming constant temperature, the exhaust velocity is proportional to the inverse square-root of the propellent's molar mass. Why does that matter? The specific impulse is equal to the exhaust velocity divided by standard gravity. Thus, using lower molar mass propellent gives higher Isp. The molar mass of water is about 9 times that of molecular hydrogen.

[Giggleplex777]

One issue in KSP is that many ships are hybrid, they use an combination of nuclear and chemical rockets, my kerthane miners is an good example, they typically land on take off on all engines but uses the nuclear elsewhere. It will have an totally different profile if mining on Gilly and Moho.

One feature of LANTR is that you has to have the oxygen, for the sort of hybrid use it makes sense. On the other hand I think we will start to having far more fuel than oxygen on ships later.

You can toggle and control the LOX injection.

Edited May 11, 2013 by Giggleplex777

[nyrath]

Yes, Scialytic has it correct. For an NTR you want the lowest molar mass possible. And Giggleplex777 is correct as well, you can toggle the LOX injection. That is the point of LANTR, it has a low-gear and a high-gear. In those cases where you suddenly need more thrust, you can shift into high gear by toggling the LOX. You get more thrust with the expense of a lower specific impulse. Kind of like an afterburner.

[Fel]

Yes, Scialytic has it correct. For an NTR you want the lowest molar mass possible. And Giggleplex777 is correct as well, you can toggle the LOX injection. That is the point of LANTR, it has a low-gear and a high-gear. In those cases where you suddenly need more thrust, you can shift into high gear by toggling the LOX. You get more thrust with the expense of a lower specific impulse. Kind of like an afterburner.

That I can see being made much more practically. You effectively only need to shield the nuclear core when disengaging the NTR / engaging the LOX.

[nyrath]

That I can see being made much more practically. You effectively only need to shield the nuclear core when disengaging the NTR / engaging the LOX.

Ummmm, I fear you do not understand. In LOX mode the nuclear core is still running. The only difference is that the hot exhaust has LOX injected into it as an afterburner.

[Atanar]

How do I delete my own message here?

Edited May 11, 2013 by Atanar

[Tw1]

Basically, the need for both oxidizer and hydrogen reduces the advantages of the nuclear heating? I think I get it.

[Giggleplex777]

Basically, the need for both oxidizer and hydrogen reduces the advantages of the nuclear heating? I think I get it.

Since H2O molecules are more massive than H2, they have a lower exhaust velocity and therefore lower Isp.