Purpose of Puff Engine

[Jestersage]

I am testing out a lander/mun flyer combo, and while it works (KER makes things easier!), I can't help but to wonder: Could I replace the descent stage with Spark Engine? This led to the more general question: What's the purpose of using Puff engine, aside from an all monoprop design?

[DD_bwest]

yes you can land on the mun using a spark. its my preferred engine for the task.

if you want a higher twr, since some people like that for landing. you can use 2 even.   the tinest cubic struts work well for getting around surface attach restrictions.

stacking the round 8 fuel tanks ontop of eachother has them clip inside eachother and take up less room than larger tanks with more fuel.  your weight ratios stay the same tho.

[Capt. Hunt]

I think the main reason it was added was that people asked for a monopropellant OMS like the space shuttle's.  I do find it useful for applications where I don't want to haul up an extra LF/O tank for the upper stage of a satellite launcher or a lander probe.  Also, it's handy for spaceplanes, because you don't need to carry Oxidizer to use in orbit, and you can save your LF for landing.

[TwinKerbal]

Another option is to use the Terrier. It is a bit over powered for the task, but is more efficient and has more thrust.

[Jestersage]

So short answer, especially for landers: because it's there.

 

EDIT: Does monoprop offer better TWR / fuel consumption for Mun and Minmus?

Edited November 2, 2016 by Jestersage

[Aegolius13]

I personally think the Puff is useless.  Yeah, you can get by on all monoprop, but it's not like it's generally difficult to add an LFO tank.  There are real life reasons for preferring monoprop in some cases, but I don't think they carry much to KSP.

I generally prefer the Terrier for all but the smallest Mun landers.  You can often get by on a smaller engine, but the extra TWR makes landing easier and saves some gravity drag.

[Kerbart]

21 minutes ago, Aegolius13 said:

I personally think the Puff is useless.  

But: one man's garbage is another man's treasure...

[bewing]

3 hours ago, Jestersage said:

EDIT: Does monoprop offer better TWR / fuel consumption for Mun and Minmus?

No, The monoprop thrusters have very low mass, so their TWRs are comparable with the other engines -- but their Isp values stink. So your fuel consumption is very bad.

[wibou7]

Personally, I like the Puff as a "last chance" engine.

Most (if not all) of my spacecraft have monoprop for docking, usually way too much of it (a 60 units tank is more than enough but a single one break symmetry, you see). Two Puff are light enough to barely show on DV but if worst come to worst, burning monoprop will give you a couple more Dv.

I once (barely) saved a Moho ascent stage using that trick. Puff is your friend

[Jestersage]

1 hour ago, bewing said:

No, The monoprop thrusters have very low mass, so their TWRs are comparable with the other engines -- but their Isp values stink. So your fuel consumption is very bad.

When i meant monoprop, I am also talking about the fuel tanks. the reason I am asking is that the wiki stated LF-O is 5kg/unit, While Monoprop is 4kg/unit. However, Puff is 250 ISP, While Spark is 300s...

Ugh, how can this be so difficult?!

[bewing]

Because it's Rocket Science!

[Aegolius13]

1 hour ago, Jestersage said:

When i meant monoprop, I am also talking about the fuel tanks. the reason I am asking is that the wiki stated LF-O is 5kg/unit, While Monoprop is 4kg/unit. However, Puff is 250 ISP, While Spark is 300s...

Ugh, how can this be so difficult?!

The game's ISP numbers work in the rocket equation with the mass of fuel, not the number of units.  In other words, ISP essentially measures thrust per kg of fuel, whatever that fuel happens to be.  So really, besides comparing the capacity of, say, one LFO tank with another, the number of fuel units is not a very important metric for comparing parts.  It gets even crazier with xenon, which comes in such little bitty units that even the smallest tank holds 400.  

 

2 hours ago, wibou7 said:

Personally, I like the Puff as a "last chance" engine.

Most (if not all) of my spacecraft have monoprop for docking, usually way too much of it (a 60 units tank is more than enough but a single one break symmetry, you see). Two Puff are light enough to barely show on DV but if worst come to worst, burning monoprop will give you a couple more Dv.

I once (barely) saved a Moho ascent stage using that trick. Puff is your friend

I carry so little monopropellant most of the time is not really an option for me, but even if you have a lot, I wonder how the Puff compares with the basic thrusters for this purpose.  Yeah, the thrusters' ISP is a bit worse, but you're saving a bit of mass by not having to carry the Puffs (I'm assuming you have thrusters anyway if you're carrying monopropellant at all).  And that mass is saved not just for the monopropellant stage, but all your previous stages as well.  The extra engines might add some atmospheric drag as well.  

On the other hand, in some rare cases (like getting to orbit, as you mention), the extra TWR from the Puff over thrusters might come in handy.  And I suppose the bigger the craft, the more helpful the ISP boost will be relative to the mass increase.  

 

[Plusck]

As others have said, Puffs are useful if you have to carry Monoprop but won't be using all of it, and you could do with a bit more thrust than the RCS thrusters but don't want to add a whole new fuel tank.

The downsides are that monoprop doesn't give great Isp, and monoprop tanks are slightly heavier than their LFO counterparts.

The upsides are that you don't need to worry about exact ratios of fuels between RCS and main engines (they'll both work until you are dead in space) and the smaller stacking monoprop tanks take up less space than their equivalent-sized LFO counterparts.

One situation I've used them is to send crew home after expanding a space station somewhere: I need monoprop for docking, but once docked the tanks are just useless parts. If they're attached to the command pod, I can simply decouple that and use Puffs to get back to Kerbin.

[SteveD80]

I toyed with using puff's for my last stage on crewed vessels.  And I will use them if I want extra thrust on a monoprop only vessel, but more often than not I just use RCS thrusters on those.

[EpicSpaceTroll139]

19 hours ago, Capt. Hunt said:

I think the main reason it was added was that people asked for a monopropellant OMS like the space shuttle's. 

Except that doesn't make sense because the shuttle used the hypergolic propellants MMH and N2O4, not a monopropellant.

[Raideur Ng]

Puff is incredibly useless. An RCS gives 240 to the Puff's 250. Just use the RCS you'd have on your vehicle anyways until it's empty.

Additionally, they buffed the Spark, it has a whopping 320 ISP now, better than even the Ant and WAY better than the Puff.

[septemberWaves]

The Puff engine is good for an aesthetic designer like me who likes to prioritize appearances over efficiency. There aren't many situations that I've found a use for the monopropellant engines, but they still get used (and in fact, the same goes for the tiny Spider engines).

[Jestersage]

Guess it's mainly just used to squeeze more thrust (since some craft have monoprop), or a really tiny design (read: mun/Minmus flyers) where one just want a tank for both RCS and Main thrusters. So as for landers: If one is not planning to hop around Mun/Minmus, stick with LFO.

[allegrocm]

I love the Puff.  I use it a lot on Minmus for landers because it doesn't need fuel lines and it doesn't seem to heat up other parts that are in its exhaust path.  In particular, when I assembled my Minmus output in orbit, I had Puffs all over it and used them to land it carefully (and then fly it up again when I wanted to add a new component).