Advantages of monopropellant as primary fuel?

[Pwnstarr]

What, if any, are the advantages of using monopropellant as the main orbital maneuvering fuel rather than fuel / oxidizer? It seems to me that no monopropellant system in KSP provides anything close to the power or efficiency of a fuel/ox system. 

[Mikki]

As far as i can see it drains the fuel evenly from all attached tanks, but the ISP is not so overwhealming. You may save parts at smaller crafts, not much but sometimes handy.

Edited December 18, 2015 by Mikki
drains...

[nosirrbro]

There is no advantage whatsoever, other than being able to use your RCS you have for maneuvering as a fuel source, however as the main thing its a bad idea especially due to the low ISP

[cantab]

The O-10 engine has a high TWR, which may in some cases offset the low Isp, though I think it's unlikely. The poor mass ratio of the monoprop tanks is what really kills it. That leaves just convenience, of not needing separate tanks for RCS and main engines, and personal taste and fashion.

[wumpus]

The only real advantages a monopropellant has are the obvious ones: only one turbo pump (big) only one fuel tank (small).  The dissadvantages are pretty huge: ISP.  I wouldn't be surprised if a pressure fed NO2/"rubber" hybrid motor works better overall than most possible monopropellants.

Note that there are a few RP1/LOX rockets that share the turbopumps: the density of kerosene is close enough to LOX that they can effectively be 1.5 turbopumps (or less.  I know pretty much nothing about turbine design).  Once this is possible, it doesn't make much sense to use turbopumps with weaker fuels.

[wibou7]

 

The only use case is if you want to have both RCS and main engine while saving a bit on weight (since you won't need to add liquid fuel then). But the ISP efficiency of O-10 engine (or RCS for what that matter) is worst than entry line engine (i.e. Terrier) so it is probably a bad option anyways.

That said, I always add a couple of O-10 engine on ship that has large amount of monoprop. This is my "last chance fuel reserve" in case everything goes wrong. It saved a least 1 mission from disaster.

[fourfa]

I sometimes use 100% vernor engines and empty all the pod monoprop supplies just for simplicity, aesthetics, heat resistance and low drag, whatever.  With the stock RCS parts the all-vernor option is pretty attractive, especially on large craft that need their much bigger thrust.  With very small craft, using just one ring of 4 RV-105s and only the pod monoprop supply is cool for low part count. But the 1500K heat resistance can be a problem.

Aren't there some cool mods with more/better monoprop RCS parts?

[Caelib]

The only reason I have found to use mono-propellant over LFO is for ship design in terms of how I want the parts stacked (or radially-attached) -- sometimes the LFO engines don't work as well as Mono.  And if you're using Kerbal Engineer, sometimes will see a lower ISP still gives more delta-V because of the part weights of LFO vs Mono.

Edited December 18, 2015 by Caelib

[Corw]

I prefer to have my satellites (final stage) monoprop only for circularization and fine positioning, which can't be really done without RCS. I like to keep it simple.

[Fearless Son]

What Corw said.  I like to use mono-prop engines when I want something cheap and light, like a satellite.  Particularly when I want to build it really small, like a tiny sat that gets deployed from a large vessel making an interplanetary journey.  I find a few RCS control points and a pair of mono-propellant engines on a small tank gets me plenty of maneuvering options without driving up cost or complexity.  

[OhioBob]

If you need RCS and have only a small Δv requirement, then using monopropellant makes great sense.  You can use one monopropellant tank to fuel both RCS and main propulsion.  If you went with bipropellant main propulsion, then you'd have to have two tanks and likely more propellant than you need.  Of course you could use the slider to reduce the propellant load, but then you have more inert mass than you need by carrying oversized tanks.  On the other hand, if you have a large Δv requirement, then the greater I_sp of bipropellant makes having the two separate fuel sources worth it.  There is probably a breaking point in there somewhere that swings the advantage from monopropellant to bipropellant.

[juanml82]

Suppose you have a light lander. The lander has RCS for maneuvering. Suppose the monoprop tank remains attached to the lander can. You can squeeze extra dV by separating everything beneath the landing can (except for monoprop and a docking port) and continuing with two monoprop engines. They won't kill your dV in previous stages because of their light mass but, having to handle just a landing can, batteries, solar panels, a monoprop tank and a docking port, you suddenly have several hundred more dV.

 

It's not useful for Mun/Minmus, but it can be useful for places like Moho, Dres or Eeloo, where every extra dV can count

[GoSlash27]

It's rare, but it does come into play occasionally. If you have a very low DV and thrust requirement and wish to keep your lander cheap and simple, monoprop/ RCS can be a good choice.

Best,

-Slashy

 

[Overland]

It's quite useful as a transmission fluid too if your into modding and driving, my 40 class locomotives made the change from diesel electric where lights would go out if the traction motors were pulling too many units of power 

now the Detroit engines generate monoprop as the transmission medium..wheels being modded to use it the same as electricity

It's got like the above posters has said the advantages of having RCS thrust on tap..something useful to both aircraft, trains and spacecraft alike

 

same goes for spacefaring or vtol aircraft with the underside jets

 

having more monoprop is always a good thing.. It's a valuable resource

 

depends how "pure" you want your missions to be though I guess too

 

 

i

[numerobis]

IRL hydrazine is principally useful because it's much easier to reliably ignite and than are most bipropellant combinations. That lets you do lots of small burns over many years of space flight.

It's also easy to store in long-term space conditions.

[cantab]

Yeah, the IRL advantage of simplicity doesn't really apply in KSP where *all* the engines offer deep throttling, unlimited ignitions, and 100% reliability.

[Cupcake...]

4 hours ago, Pwnstarr said:

What, if any, are the advantages of using monopropellant as the main orbital maneuvering fuel rather than fuel / oxidizer? It seems to me that no monopropellant system in KSP provides anything close to the power or efficiency of a fuel/ox system. 

The one major advantage is that O-10 engines don't burn stuff which is great for cargo lifters, light tugs etc... as you can position the exhaust anywhere without any danger of destroying anything.

 

Cupcake...

[WuphonsReach]

3 hours ago, fourfa said:

Aren't there some cool mods with more/better monoprop RCS parts?

FASA, Tantares, SpaceY has a scaled up vern thruster

[Corw]

9 hours ago, GoSlash27 said:

It's rare, but it does come into play occasionally. If you have a very low DV and thrust requirement and wish to keep your lander cheap and simple, monoprop/ RCS can be a good choice.

Best,

-Slashy

 

In my space program, it is actually very common. Bulk of my crafts are smallest/simplest/reliable probe designs and that mostly ends up as monoprop final stage. Few examples, satellite and Minmus biome research probe:

 

http://imgur.com/a/cSQC2

 

EDIT: Who in his right mind thought this reply sistem was a good idea? This is pure unworkable bullkraken.

Edited December 19, 2015 by Corw

[Wanderfound]

16 hours ago, Pwnstarr said:

What, if any, are the advantages of using monopropellant as the main orbital maneuvering fuel rather than fuel / oxidizer? It seems to me that no monopropellant system in KSP provides anything close to the power or efficiency of a fuel/ox system. 

Monoprop fuel tanks are much more physically compact than LF/O tanks, especially if you go heavy on the radials. They're good for making very small landers and probes to fit within spaceplane cargo bays.

[FlyingPete]

It's useful for small craft, as the pod's internal monopropellant supply is added to the total which gives a slight improvement in mass ratio. A lone capsule with a couple of the O-10 engines added is capable of deorbiting itself from LKO. The Mk1 lander can with some radial monoprop tanks makes for a good minimalist lander.

[Geschosskopf]

I find mono-only to be good for landers at places with approximately zero gravity, such as Gilly.  The dV required to land and take off is so low that Isp doesn't have enough time to make a difference, so the whole efficiency thing goes away and you can just look at sheer functionality.  At such ;places, even RCS has a significant TWR and the mono engines are super powerful.  So that part's covered.  But more importantly, flight operations at such places are PAINFULLY slow because just being within the miniscule SOI of the moonlet puts you too close to the ground to warp at all, and the gravity is so insignificant that it takes forever merely to fall to the ground.  Thus, you have a real need to thrust TOWARDS the ground just to minimise the boredom of landing there.  For this, having some up-pointing "Place Anywhere" RCS thrusters are IMHO the must convenient option, which means you need mono on the ship anyway (not to mention if it needs to dock at some point).  So you simplify the whole design if you just use mono for the main engines, too.

[FancyMouse]

Squad really, really should make EVA prop non-infinite and share with the monoprop in pod. That's really what I think monoprop should be used most of the time.

[monophonic]

28 minutes ago, FancyMouse said:

Squad really, really should make EVA prop non-infinite and share with the monoprop in pod. That's really what I think monoprop should be used most of the time.

Oh, they tested it already in-house. That's why command pods and cockpits got their small monoprop tanks added. But they found that it just was not fun at all to be limited in your EVA ability. So they didn't release it.

Remember KSP follows the game first philosophy. Where fun and realism cannot be reconciled, realism must take the second place. You may not agree, but it does make good business sense: The majority of paying gamers are not into realistic simulations. And profitable KSP means continued development of KSP.

[Geschosskopf]

5 hours ago, monophonic said:

Oh, they tested it already in-house. That's why command pods and cockpits got their small monoprop tanks added. But they found that it just was not fun at all to be limited in your EVA ability. So they didn't release it.

Remember KSP follows the game first philosophy. Where fun and realism cannot be reconciled, realism must take the second place. You may not agree, but it does make good business sense: The majority of paying gamers are not into realistic simulations. And profitable KSP means continued development of KSP.

I heard it a bit differently.  The real problem with jetpacks buring regular ship mono was that at the time Squad tested this in-house (and added the small mono tanks to the capsules for this purpose), mono used a different fuel flow mode than it does now.  Back in those days, mono tanks drained from the top down just like LFO tanks, except considering the whole ship so decouplers didn't stop it.  Thus, with the capsule usually at the top of the rocket, if the ship as a whole used any RCS en route to the destination, the 1st tanks to drain would be those in the capsule.  This created a large trap for the unwary and a large potential for ragequits, from folks forgetting to pump mono back into the capsule prior to separating the last stage below it

Squad decided this was bad but didn't want to rewrite the mono fuel flow code at that time.  So as a temporary fix, Squad created "EVA Propellant" which has the same physical properties as mono but is infinitely available in capsules.  But the intent was to rewrite the mono code at some point, which is why the capsules still have their little mono tanks even though they're never used.  Besides, by leaving the mono in the capsules, once the change happens, ships from before the update will already have the mono aboard ready for EVA use (unless the user has removed it to save weight and/or launch costs --- I've gotten into the habit of doing that myself).

That was long ago.  In the time since then, in fact several updates ago, Squad changed the mono fuel flow code so that now mono is stopped by decouplers.  This keeps capsule mono safe from the rest of the ship.  I'm thus rather surprised that EVA packs don't use mono now, seeing as all the pieces for it are now in place.  So either Squad has forgotten about this, or has thought better of it for reasons of fun, as you suggest.