Purpose of Different Fuels?

[xdreaper15]

I have been toying around with different long range rockets in the Interstellar mod, and i just cannot for the life of me find a comprehensive guide to the different types of fuels and how they interact with the engines.

I have seen notated here and there in the forums that things like Hydrazine provide extra thrust?

But i also see some places that Hydrogen is used often for first stages, and most engines have "higher base isp" for Hydrogen; yet, when i compare hydrogen to LiquidFuel for example, i come out with FAR less dv according to KER.

Unfortunately, i cannot find a definitive explanation and comparison between the types of fuels, and the Interstellar wiki seems to come up short. I have schooling experience with liquid water reactors, and i enjoy learning about high tech systems and the physics behind how they function, but like i said earlier. Just having trouble finding some kind of write-up with the niche uses of each type of fuel. OBVIOUSLY they would have a use if they exist in the game...right?

[Streetwind]

Welcome to the forums!  

While I have no experience with KSP Interstellar, "liquid fuel gives me much more dV" sounds like a very common fallacy that a lot of players run into when comparing different fuel types, especially when hydrogen is involved. Which is: mistaking the amount of fuel or the tank size as being the significant factor.

The rocket equation does not care how large your tanks are, or how many unnamed units of a resource are stored within them. The rocket equation cares about one thing only: your mass fraction. That is the mass of the stage with fuel divided by the mass of the stage without fuel. In other words, the only thing that mattters is how much propellant mass you're packing.

Now, hydrogen naturally has a very low density. That means that any given mass of it will fill out a much larger volume than other materials. Or, in reverse: any given tank can store less hydrogen than other fuels. While KSP itself doesn't simulate volume at all, mods sometimes do; and even those who don't will usually fudge things by simply putting less of a resource into a tank to give the appearance as if there was volume and density involved.

Which leads to the following situation: a player puts up a capsule, a LFO tank and an engine in the editor. He notes the dV provided. He then switches the tank to LH2/O2, and replaces the engine with a hydrogen-burning engine that has like 30% more Isp. He notes that the dV is now a lot less. This confuses the heck out of the player, and he goes onto the forums to inquire what's going on.

What the player didn't notice is that he's comparing something like two tons of LFO propellant with one ton of LH2/O2 propellant. Of course that will result in less dV!  Try giving the rocket the same propellant mass in LH2/O2, and compare figures again.

[xdreaper15]

So if i'm understanding this correctly, it is more effective to use the denser fuels, because you are reducing the amount of "non propellant" mass(i.e. tank mass) added to the craft?

I took a tank and emptied the amount of Methane/Ox to match the amount of Hydrogen/Ox and the Methane/Ox tank got similiar dV(a small difference due to not having perfect control over fuel amount); however, i notice that it seems a "waste" to use the hydrogen since it takes up the same amount of "craft" space(the same size tank) when the Methane tank is damn near empty.

 

If this is the case, i still am confused about usage case for fuel types. Since endgame, we are not limited by the size of our craft, you should always use the most dense fuel, since this decreases your Tank:Fuel mass ratio, therefore utilizing the most Propellant Mass you can in that stage.

[danielboro]

i did compere 1 time a ship of 50 tons fueled by H2 and a ship of 50 tons fueled by LF (KSPI/nerta nuke)
the H2 ship was considerably larger, had more parts and ~10% more dv
start TWR was the same for both, but end TWR was ~*2 more for the LF ship
for me, the less parts and hi TWR at end of fuel was better then having an extra 10% dV
i rarely use H2/LOX motor or H2 for nukes

 

edit
after reading the new posts i think i should add on the "rarely use H2/LOX "
i used H2/LOX as my transfer stage to duna, moho and jool bifor i started using nukes (bigger pay load for same first stage)~ 6 probes all together
but for nukes i use LF. i like the low part count and hi stop TWR at target over the extra DV/payload

Edited July 27, 2017 by danielboro

[Streetwind]

7 hours ago, xdreaper15 said:

If this is the case, i still am confused about usage case for fuel types.

A large part of this is up to the individual mod, too. For example, a hydrogen tank could have less dry mass than a LFO tank of the same physical dimensions because it needs to be less sturdy because it needs to contain less propellant mass. All in all, you have a tradeoff where the higher engine Isp stands against the worse mass fraction of the tanks. Which side wins, and by how much, is usually decided by the mod author who stats up the parts. Sometimes mod authors choose numbers you may disagree with. (As I mentioned, I have no experience with KSPI, so I can't speak about its balance choices.)

Also, keep in mind that it is entirely expected that one specific fuel is particularly good in onespecific use case. Having niches is the ideal way to differentiate fuel types. If you examine only one use case, you may come to the false conclusion that the fuel which is best in this use case is always best.

For example, hydrogen has hidden advantages when used in upper stages. If you can build two different stages with the same dV, it might not matter to you which one you pick for a first stage; but if one of the two has less total mass, then it makes a better upper stage because you can then downsize and save money and bulk on your first stage (or on all stages below, in case there's more than one). Or, make the upper stage have the same total mass but increased capabilities. This compounds with the fact that hydrogen engine's lighter exhaust, which gives them their superior Isp, also doesn't work as well in an atmosphere.

The famous Saturn V moon rocket had a RP-1/O2 first stage, LH2/O2 second and third stages, and hypergolic service modules and landers. It chose to employ three different fuels on the same rocket instead of standardizing on one, because each fuel was the best choice for its particular application then. Dense kerolox for sheer brute force to push the thing off the pad and out of the atmosphere; lightweight hydrolox supplying massive dV for orbit insertion and trans-lunar injection; and hypergolics for ease of storage and ultra-reliable engine ignition across a long mission duration.

[OhioBob]

The following doesn't have anything to do with KSP, but it does provide some explanation of real life rocket propellants.  If describes some of the pros and cons, gives physical properties, and compares performance.  I haven't used any mods that provide different propellant types, but I presume all in some way attempt to reproduce at least some aspect of real life performance.

http://www.braeunig.us/space/propel.htm

[regex]

47 minutes ago, OhioBob said:

The following doesn't have anything to do with KSP, but it does provide some explanation of real life rocket propellants.  If describes some of the pros and cons, gives physical properties, and compares performance.  I haven't used any mods that provide different propellant types, but I presume all in some way attempt to reproduce at least some aspect of real life performance.

http://www.braeunig.us/space/propel.htm

Real Fuels will. Anything else is suspect in terms of realism towards the actual fuels because Real Fuels is designed to be used with replica engines, although from personal experience I can suggest that KSPIE, Cryo Engines, and SSTU at the very least pay tribute to differences in fuels that mimic real life.

E: Regarding the OP, in general, what I have observed:

Cryogenic fuels and oxidizers using lighter molecules (liquid hydrogen, liquid oxygen, liquid methane) tend to have higher isps because it is easier to accelerate those molecules (isp is a measure of exhaust velocity) but also generally produce less thrust in comparable engines due to using lighter molecules. They also tend to have much broader isp ranges between sea level and space. Cryogenics generally aren't storable, at least liquid hydrogen, due to boil-off.

Hypergolic fuels and oxidizers (dinitrogen tetroxide, unsymmetrical dimethylhydrazine, monomethylhydrazine, hydrazine, inhibited red fuming nitric acid, high test peroxide) tend to have lower isps but higher thrust because these complex molecules are harder to accelerate but heavier. Compare the Протон (UDMH, NTO) in size to something using cryogenics with similar LEO capability, maybe like Delta IV Heavy (the difference isn't that big to illustrate the point). Hypergolics are very storable and are generally used where the fuels may be sitting in a tank for years.

Lastly, hydrocarbon fuels (kerosene, liquid methane might fit here as well) provide a decent mix between the two in terms of isp and thrust. Kerosene and liquid oxygen (kerolox) is a very popular combination for launch vehicle lower stages because of this.

Edited July 25, 2017 by regex

[Nertea]

It's important to realize that the true performance of a fuel in KSP is related only to a few key parameters:

• The mass ratio of the tanks that contain it (amount of fuel you get for an amount of dry mass)
• The mass of the engines that use it (heavier engines mean you may lose advantages)
• The specific impulse of the engines that use it
• Other, optional parameters of the fuel itself, like whether it boils off, its cost, its ISRU abilities, etc, are less important to the DV-TWR tradeoff

If these parameters are not balanced well by the mod that is working with the fuels, you may not see any benefits from different fuels. I go to great pains to try to do this with my mods, which isn't easy. 

[OhioBob]

2 hours ago, regex said:

They (cryogenic fuels) also tend to have much broader isp ranges between sea level and space.

That's mainly because almost all cryogenic engines are designed for use on upper stages.  Engines that have been adapted to work in a vacuum will always have a much broader range of Isp than engines adapted to work near sea level.  It's not really a function of the fuel per se.  The Rocketdyne RS-68 (Delta IV) is the only LH2/LOX engine that I can think of that is designed exclusively as a first stage engine.  Its sea level/vaccum Isp is 365/410 s, which is about the same range* as other first stage engines that use different propellants. 

* The range might be a smidgeon larger than some other propellants, but the ratio is about the same.
 

Edited July 25, 2017 by OhioBob

[regex]

59 minutes ago, OhioBob said:

That's mainly because almost all cryogenic engines are designed for use on upper stages.  Engines that have been adapted to work in a vacuum will always have a much broader range of Isp than engines adapted to work near sea level.  It's not really a function of the fuel per se.  The Rocketdyne RS-68 (Delta IV) is the only LH2/LOX engine that I can think of that is designed exclusively as a first stage engine.  Its sea level/vaccum Isp is 365/410 s, which is about the same range* as other first stage engines that use different propellants. 

* The range might be a smidgeon larger than some other propellants, but the ratio is about the same.

Good point, I should have mentioned that as a function of the engines as I took care to note thrust in comparable engines. Thanks.