Why does the LV-N need oxidizer?

[zarakon]

*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)

The volume units in KSP seem to just be some arbitrary unit that is definitely not liters

I'm pretty sure it doesn't actually say liters anywhere

[SunJumper]

The volume units in KSP seem to just be some arbitrary unit that is definitely not liters

I'm pretty sure it doesn't actually say liters anywhere

I think it used to, back in 0.15

[Fel]

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.

Well, I'll concede my ignorance of rocket physics here.

But the biggest issue I keep seeing is that the hydrogen is ALREADY at an excited state while the oxygen is at a much lower energy state.

Despite the difficulties in getting the atomic collisions, wouldn't you be REMOVING energy from the hydrogen by interacting with the oxygen?

*and of course, the other way of injecting the oxygen into the core means you're wasting large amounts of energy getting the oxygen up to temperature... and then you're basically lighting the core on fire... which sounds so unbelievably dangerous to me.

Edited May 11, 2013 by Fel

[The Ideal Gas Lawyer]

I think it used to, back in 0.15

It definitely used to. They dropped the units around sometime before .17 (perhaps because they made no sense)

[nyrath]

But the biggest issue I keep seeing is that the hydrogen is ALREADY at an excited state while the oxygen is at a much lower energy state.

Despite the difficulties in getting the atomic collisions, wouldn't you be REMOVING energy from the hydrogen by interacting with the oxygen?

Yes. That's why initiating LOX injection in a LANTR lowers the specific impulse.

However, since the propellant stream is not just hydrogen, but hydrogen + oxygen you are increasing the propellant mass flow. This increases the thrust.

Which I why I originally said that in an LANTR, LOX injection trades specific impulse for thrust.

[Mattasmack]

It definitely used to. They dropped the units around sometime before .17 (perhaps because they made no sense)

I started playing KSP with version .18, and I've never seen a reference to liters in the game itself. I think the volume unit makes the most sense if you take it as being equal to five liters. Then, the liquid fuel and oxidizer densities work out to about what you'd expect of kerosene and liquid oxygen, and the capacities of the fuel tanks match up well with their sizes.

Closer to the original topic of the thread, it would be awesome to have an engine like the LANTR in the game, to be able to trade off ISP and thrust as needed.

[Teirusu]

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.

Would it be possible to have an option for LANTR type engines? Perhaps have a LOX mode that we can activate on the LV-N engine so that it performs near like it currently does? Something like that would allow us to keep our current designs roughly the same, but give more options. My thinking would be to have the LOX mode have around 600-ish ISP, but increase its thrust to compensate.. maybe make the engine a little heavier so its not quite the 'goto space engine' it currently is. Non-LOX mode offering higher ISP, but less thrust. (Thinking even less then it does now, but a ship would have much higher Delta-V because you could carry less LOX and more fuel)

[nyrath]

There is also a concept called a Bimodal NTR.

http://ares.jsc.nasa.gov/HumanExplore/Exploration/EXLibrary/docs/MarsRef/addendum/A5.htm#figA5-6

This is when your spacecraft uses a solid core nuclear thermal rocket for propulsion. After the burn is complete, you turn off the propellant, and turn on the heat exchanger. That is attached to a electrical power generator and a heat radiator. In other words, you turn your engine into a nuclear power plant. Not sure if this would be worthwhile in KSP.

Pratt and Whitney went one further. They took a Bimodal NTR and made the rocket into a LANTR. They call it the Trimodal NTR.

http://www.pwrengineering.com/dataresources/AIAA-2004-3863.pdf

(PDF file)

[The Ideal Gas Lawyer]

There is also a concept called a Bimodal NTR.

http://ares.jsc.nasa.gov/HumanExplore/Exploration/EXLibrary/docs/MarsRef/addendum/A5.htm#figA5-6

This is when your spacecraft uses a solid core nuclear thermal rocket for propulsion. After the burn is complete, you turn off the propellant, and turn on the heat exchanger. That is attached to a electrical power generator and a heat radiator. In other words, you turn your engine into a nuclear power plant. Not sure if this would be worthwhile in KSP.

Pratt and Whitney went one further. They took a Bimodal NTR and made the rocket into a LANTR. They call it the Trimodal NTR.

http://www.pwrengineering.com/dataresources/AIAA-2004-3863.pdf

(PDF file)

Oh man, I... I need this. Can someone with modding skills make one? It would be perfect for use with ion engines that require tons of electricity for another level of higher ISP lower thrust. Also for Kethane conversion in the outer solar system.

If it ever got included in the game it would have to come pretty far along in the career mode, maybe one step before an FTL drive

[Promii]

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.

The issue with Hydrogen is the storage requirements, not the thrust. It's density is low so the tanks must be HUGE (see space shuttle orange tank). Kerosene is much denser, is liquid at room temp, and tends to stay where you put it.

[nyrath]

re: Trimodal nuclear thermal rocket

Oh man, I... I need this. Can someone with modding skills make one? It would be perfect for use with ion engines that require tons of electricity for another level of higher ISP lower thrust. Also for Kethane conversion in the outer solar system.

Alas, the basic problem remains. Nuclear reactors, whether inside a NTR or as a dedicated power reactor, have too much mass. Currently the blasted things are 18 kilograms per kilowatt. This means a reactor for a 200 kilowatt VASIMR is going to be about 3.6 metric tons. Which will have a crushing effect on your thrust-to-weight ratio.

What is really needed is some advanced technology to drastically reduce the kilograms per kilowatts. Then the trimodal NTR will be incredibly useful.

[The Ideal Gas Lawyer]

re: Trimodal nuclear thermal rocket

Alas, the basic problem remains. Nuclear reactors, whether inside a NTR or as a dedicated power reactor, have too much mass. Currently the blasted things are 18 kilograms per kilowatt. This means a reactor for a 200 kilowatt VASIMR is going to be about 3.6 metric tons. Which will have a crushing effect on your thrust-to-weight ratio.

What is really needed is some advanced technology to drastically reduce the kilograms per kilowatts. Then the trimodal NTR will be incredibly useful.

Stupid radiation. Ruining all our fun

[Spatzimaus]

It would be perfect for use with ion engines that require tons of electricity for another level of higher ISP lower thrust

There's an ion engine mod that adds "hybrid" ion engines. 900 ISP regardless of atmosphere/vacuum, but it consumes Xenon, fuel, oxidizer, AND a lot of electricity. The 1.25m version is only 20kN thrust, while the 2.5m is 50kN (i.e., comparable to one LV-N). The thing is, the 900 ISP is slightly WORSE than the LV-N's vacuum 800, since the consumption ratios are additive (i.e., it consumes the same amount of fuel and oxidizer as a 900-ISP rocket engine, AND consumes xenon as a 900-ISP ion). But these engines are still my favorites for interplanetary vessels; my Grand Tour vessel (the Nimbus) uses twelve of them:

The triangular pods at the bottom of each of the four outer stacks have three 20kN hybrids apiece. 240kN is enough to get off any low-gravity moon, and I've got radial rocket engines to supplement for high-gravity environments. (Note that the amount of electricity these use is really critical; each of the 12 engines uses ~9 electricity per second, which means they need the huge solar panels, but you can't deploy those in an atmosphere. For Duna and Laythe I had to ascend purely on the radial rockets.)

So basically, if you want an engine that falls midway between ions and rockets, using a lot of electricity to give a usable amount of thrust, there's already a good mod for that.

Also for Kethane conversion in the outer solar system.

Once we get the new resource system, Kethane will probably go away. This is going to make a huge difference in how we deal with fuel; right now, one resource (Kethane) leads to four fuel materials. Because of the 5:1 conversion ratio, you're almost always better off building your refineries with big fuel tanks and small resource tanks, with your landers including a built-in refinery module for on-site conversion. But once we get the new resources, all that will change; there'll now be a lot of "raw" materials used for each refined fuel type, and it's unlikely that any one location will have all of the ingredients needed to refine fuel. This means that your landers will be mostly haulers for large amounts of raw materials, and the refining into actual fuel will take place at a central location (like a space station). You might even need to do this in multiple stages, i.e. a single fuel-refining central station around Jool that uses non-landing ferries to get deliveries from more specialized storage stations circling each moon.

Now, I think this'll lead to some interesting specialization. You won't use nuclear engines around planets poor in Blutonium, for instance, so the ships you use near Eve might be very different from the ones you use near Duna. But we'll have to wait and see exactly where the resources go.

Edited May 16, 2013 by Spatzimaus

[Fel]

The issue with Hydrogen is the storage requirements, not the thrust. It's density is low so the tanks must be HUGE (see space shuttle orange tank). Kerosene is much denser, is liquid at room temp, and tends to stay where you put it.

They use LIQUID Hydrogen....

LH2 / LOX has a significantly decreased Specific Impulse at sea level that translates into a significantly decreased relative thrust (what the engine can put out, what the engine will put out)

Kerosene has a much higher impulse which is likely due to it having a heavier propellant. (again, concede all knowledge of physics... except the above claim.); and it has a fairly constant relative thrust (err, the gain doesn't shift too much) which makes it good for that stage 1 lifter...

Once you get through the dense atmosphere, those LH2/LOX engines operate with enough efficiency to be used.

Edited May 17, 2013 by Fel

[nyrath]

They use LIQUID Hydrogen....

I'm not sure what you are saying. Promii said hydrogen has a very low density, and it does. Regardless of whether hydrogen is gaseous or liquid, it has an annoyingly lower density than other gases or liquids.

LH2 has a density of about 71 kg/m^3 while kerosene is more like 800 kg/m^3.

[AmpsterMan]

I see this thread about a week ago; Nova answers it in like two seconds.

1 week later 4 pages. AWAHHAH?!?!

On-Topic: I wonder what other uses there will be for tweakables.

[Mattasmack]

They use LIQUID Hydrogen....

LH2 / LOX has a significantly decreased Specific Impulse at sea level that translates into a significantly decreased relative thrust (what the engine can put out, what the engine will put out)

Kerosene has a much higher impulse which is likely due to it having a heavier propellant. (again, concede all knowledge of physics... except the above claim.); and it has a fairly constant relative thrust (err, the gain doesn't shift too much) which makes it good for that stage 1 lifter...

Once you get through the dense atmosphere, those LH2/LOX engines operate with enough efficiency to be used.

Err, I think you have that backwards. Under the same conditions, LH2/LOX gives a higher specific impulse than RP1/LOX, both in atmosphere and in vacuum. (RP1 = Rocket Propellant-1, essentially kerosene). Have a look at the big table about halfway down the Wikipedia page on liquid rocket propellants, here. V_e, effective exhaust velocity, is proportional to specific impulse (just higher by a factor of g). You can see that the specific impulse for LH2/LOX is approximately 30% higher than for RP1/LOX when exhausting to 1 atm pressure, and approximately 27% higher when exhausting to vacuum. In fact, RP1/LOX suffers proportionally more when exhausting to 1 atm pressure, its ISP being reduced by 16% while LH2/LOX loses only 14%.

I see this thread about a week ago; Nova answers it in like two seconds.

1 week later 4 pages. AWAHHAH?!?!

...

Topic drift! The conversation went in an interesting direction.

Edited May 17, 2013 by Mattasmack

[RoboRay]

They use LIQUID Hydrogen....

The issue of hydrogen isn't efficiency (it's the best, if pure Isp is the only concern), but about the problems of storing it. The much lower density (yes, even in liquid form) requires vastly more volume (and mass) to hold it. Not to mention that, over time, it boils off and escapes right through the walls of your huge, heavy storage tanks. In fact, the reason that RP1 or something else is typically used for fueling the first stage is simply because it's so much denser. You give up Isp in exchange for having a much smaller, cheaper, easier to design and fly vehicle.

LH2 is great for launch vehicles which you can fuel and send straight up, the upper stages in particular, but it's simply impractical for craft that will need to store it in space for long periods of time.

I think Methane/LOX is about the best option for storable propellents, especially when you consider that methane can be manufactured pretty easily and cheaply on any world with CO2 in the atmosphere. And the Isp isn't that much worse than LH2.

Edited May 17, 2013 by RoboRay

[Temstar]

In fact, the reason that RP1 or something else is typically used for fueling the first stage is simply because it's so much denser.

Fun fact: a litre of RP1 in fact has more hydrogen atoms than a litre of pure liquid hydrogen.

[Fel]

Eww Wikipedia! That is strange though, as I had written down ~200s for sea level ~430s vacuum. (Huh, and also remember kerosene wrong as ~350s for sea-level)

I assume that the storage issues are more so the time it takes to fill the tanks, as the ability of the atom to go through the valence shell is proportional to its energy state. (Hence, I would assume that there is a last minute cooling before pumping it into the rocket... thus making it slow to fill the tanks (inclusive of the volume flow).)

That being said, if it were possible to instantaneously fill the rocket with LH2, could you use paper as a storage tank? Or perhaps more like a sponge, Obviously other structural elements would be needed; or even "magical heat resistant paint." But if you only need to store the LH2 long enough to ignite it, how thick do your walls really need to be?

Would you instead say that the heat from the exhaust alone would be enough to jeopardize the fuel while in flight, and that tank elements would still be needed to prevent it from vaporizing?

[nyrath]

The trouble with blasted liquid hydrogen is that the atoms are so small they sort of wiggle out between the atoms of the fuel tank. To hydrogen, everything is porous.

[Dr. Muttonstache]

To hydrogen, everything is porous.

So you're saying hydrogen thinks everything is a

That's pretty cool, actually. I'm typing on, and looking at a sponge. I also am a sponge. But anyway, regarding the tweakables, would this mean that you could fill a xenon tank with normal rocket fuel instead of xenon gas? Would the fuel tanks just be shells before you set them to carry a certain fuel?

[Fel]

I understand that nyrath, but my question was more so about an F-18 (or w/e it is... I know the operation I want to describe)

As I understand it, the F-18 (or w/e) has a very loose fitting fuel tank, such that it actually leaks fuel while on the runway; this tank will expand into the body once the plane reaches supersonic speeds, but until then it will constantly lose fuel.

With liquid hydrogen, I know that it'll go through pretty much anything (and probably is actively leaking as the tanks are filled) but my question was "how secure do you really need it."

Let's use an impossible device for an example.

To greatly reduce launch weight, I'll bring 5Gg of LH2 into space and pour it down a hose, into a spout that flows directly to the engines.... so my "fuel tank" is now in space, while the "rocket" is on the planet.

So the question would be, just how thick does this hose need to be to get ENOUGH liquid hydrogen into the engine to propel it into space? (Ignoring all other variables).

[rosenkranz]

I'm having difficulty understanding why that is relevant? I'm not sure it matters how thick the sponge is, the leakage will be the same regardless. The only difference is how long it take the sponge to saturate.

Again, though, in any case, why is this relevant?

LH2 especially has an issue (called 'boil off'). It's just something it does. In KSP, you're rocket isn't on the launch pad long enough for it to make a difference. However, IRL, a rocket can end up sitting on the pad for days or weeks prior to launch waiting for it's launch window. They do this because as a fudge factor to account for issues that may delay the launch.

Anyways, without additional refrigeration, the tanks will leak out (or boil off) about 5% per month if i recall correctly. They budget extra fuel specifically for this. Generally enough that the rocket could sit there for 6 months and still meet mission requirements. With addition refrigeration, boil off can be minimized and/or eliminated. But that involves extra weight and complexity.

Just my 2c.

[DerekL1963]

I'm having difficulty understanding why that is relevant? I'm not sure it matters how thick the sponge is, the leakage will be the same regardless. The only difference is how long it take the sponge to saturate.

Again, though, in any case, why is this relevant?

You're right - it's not relevant. But neither is the rest your reply... because nobody (other Russian ICBM/SLBM operators) puts liquid fuels (other than maybe in a sealed RCS system, *maybe*) onboard a launch vehicle "days or weeks" ahead of time, refrigerated or not. It's unsafe as hell to have that much explosive sitting around essentially unshielded. (And depending on the liquid fuel/oxidizer involved, rough on the equipment too.)

Spacecraft are fueled as late in the launch sequence as possible, because once fueled, nobody sensible wants to be within miles of a fueled (read:explosive) booster.