puff engines

[mk1980]

i don't use asteroid mining a lot, but the few times i did, one purpose of the mining rig was to drag the asteroid into LKO as a cheap fuel source in orbit. (same logic would apply for dragging it to some other point of interest, though).

my first miner had only 1 or 2 nuke engines to get there on the cheap (high isp) but their thrust is annyoingly low when you suddenly have to push a few hundred tons of asteroid around.

i'll probably send out an asteroid miner soon in my current campaign. i think i'll use the new high Isp lf/o engine that was added by the expansion (wolfhound i think? the one that has over 400 vacuum Isp). will need some more fuel reserve to intercept the asteroid, but has much more thrust to push it into target orbit in a reasonably timeframe.

ions or puffs would be 1-2 orders of magnitude to weak for such a purpose ;)

 

oh also, don't know if anyone mentioned it but you shouldn't use the small IRSU for asteroids. it converts ore to fuel at a terrible ratio, so you waste most of the (limited) amount of ore on the asteroid in the inefficient conversion. i think it has something like 5 ore per unit of fuel (vs. 2 units of fuel per ore from the full scale ISRU - ie. 10 times as efficient). you can use the small drills on asteroids, though - they always work at full capacity there (only difference between small and large drills is that the small ones don't work on very low ore concentration, but that's not the case on asteroids)
 

[Zhetaan]

5 hours ago, OHara said:

In real life, monopropellant is good when you need to stop and start a thruster repeatedly.  KSP doesn't simulate the difficulties of restarting combustion engines, so monoprop seems to lose its main advantage.  I think the only 'Puff' engine is only useful if you want a monopropellant-only craft.

That raises an excellent point:  the Puff (or something Puff-like, anyway) makes a lot more sense in a Realism Overhaul game where you have to deal with propellant boiloff and limited engine ignitions, because a throttle-able engine that will start reliably an infinite number of times on a stable propellant is otherwise a rare thing:  the engine still has poor performance, but in that case it's a choice between the workable solution that's available and no solution at all.

5 hours ago, OHara said:

Are you using the monopropellant in the lander can for your puff test, and leaving it unused in the case of the spark ?

That is another excellent point; the lander can has 15 monopropellant in addition to the FL-R10's 20.

8 hours ago, farmerben said:

And a TWR of 1.03

True, but TWR doesn't matter so much once you're in space.  I concede that it does matter for purposes of burn time, so if you want to use a rocket to move ~100 tonne asteroids about, then you're likely going to want something that offers more than 20 kN of thrust.  For something of a KSP historical perspective, this is what the Rhino was made to do; it and the rest of the 3.75-metre parts were released along with asteroids in the KSP 0.23.5 update.

[Snark]

10 hours ago, farmerben said:

And a TWR of 1.03

Okay, sure, it's way overpowered, I get that.  The point being?

7 hours ago, OHara said:

[Edit: never mind all those numbers from version 1.3.1, you're using 1.4.x where the monopropellant tanks are much smaller-capacity, and the puff should have even more of a disadvantage.
Are you using the monopropellant in the lander can for your puff test, and leaving it unused in the case of the spark ? ]

1 hour ago, Zhetaan said:

That is another excellent point; the lander can has 15 monopropellant in addition to the FL-R10's 20.

The Spark wins even in that case.

• The Spark version masses 6.175t (assuming the lander can's not pointlessly lugging unusable monoprop).  Burning the Spark until the Oscar uses all of its 0.2t of reaction mass gives 103 m/s of dV.
• The Puff version masses 6.1t (assuming the lander's got its monoprop filled up).  Burning the entire 0.14t of monoprop (0.08t in the tank, 0.06t in the lander can) gives 57 m/s of dV.

So the Spark gives nearly double the dV of the Puff in that case.  Burning less fuel at lower Isp means less dV.  Quite a lot less, in this case.

Look, there's basically no way around the math.  The Puff has a much crappier Isp than pretty much any LFO engine, so in any apples-to-apples comparison, the LFO is going to come out the clear winner.  It's math.

And yes, the Puff has a slightly higher TWR, but in any practical situation I can imagine, that minor difference is going to be much, much less significant than its lousy fuel efficiency.  Bear in mind that though the Puff's TWR for the actual engine itself may be 20% higher than its small LFO counterparts... what matters when you're flying the ship is the TWR of the overall vessel.  In this example, for instance, the engine is only around1.6% of the overall mass of the ship.  When you've got a 6-ton ship, then "oh gee, to get 20 kN of thrust I need to carry an extra 0.02t of engine" just doesn't make any perceivable difference at all in the ship's performance.

The only reason I can think of where it would actually be of practical use is if there's some "extra" bit of context where the fact that it uses monopropellant is, per se, more useful in a particular situation-- e.g. if you've got a ship design with serious fuel-flow issues and really need monoprop's drain-the-whole-ship behavior, or you already happen to have lots of monoprop tankage for some other purpose, or something similar to that.

Because the engine math is swimming uphill.

(And of course, KSP isn't just about math.  It's also about fun.  There are lots of reasons why something can be appealing for reasons that have nothing to do with math.  For example, I happen to really really like SRBs simply because I enjoy watching ships lift of on a towering column of ravenous flame and boiling smoke.    Nothing wrong with that, just as there'd be nothing wrong with using the Puff simply because one likes its look, or it happens to fit one's play style or preferred way of building ships, or whatever.  Just wanted to make it clear I'm not criticizing anyone's design choices or making any "value judgments", in case it's not obvious.  I'm simply talking about the math.)

[OhioBob]

The only place I can see using puff engines is, (1) you need to carry monopropellant anyway for RCS, and (2) you need something that will produce high thrust but only a small amount of delta-v.  In that case, since you only need a small amount of delta-v, adding a bipropellant tank and engine just isn't worth it.  You have the monopropellant already, so use it.  And the lower specific impulse doesn't hurt you much when you don't need much delta-v.  However, I've never really found a need to do this.  I've found that when I need to perform a low delta-v burn using monopropellant, such as for a rendezvous or a deorbit, I can do so adequately simply using the RCS.

I also think the fact that the puff is radially mounted hurts its usefulness.  It typically needs to be used in pairs to keep mass and thrust balanced.  In the places where perhaps I could have used a monopropellant engine, I've rarely needed two of them.  I think I'd find more use for a small inline monopropellant engine.  Maybe something like a 1.25m attachment with four small spherical monopropellant tanks surrounding a center engine.  The monopropellant could feed both the small maneuvering engine and the RCS.
 

Edited July 26, 2018 by OhioBob

[Zhetaan]

2 hours ago, Snark said:

The Spark wins even in that case.

Exactly.  If the Spark wins when it has to push the dead weight of useless monopropellant and when the Puff is getting the benefit of a lower dry mass and extra fuel, then there isn't any point to a fair comparison:  the excellence of the point is that the Puff loses even when the test rocket is unfairly weighted in its favour.

2 hours ago, Snark said:

[...] I enjoy watching ships lift of on a towering column of ravenous flame and boiling smoke.   

You play KSP; that's essentially the same thing. 

1 hour ago, OhioBob said:

I also think the fact that the puff is radially mounted hurts its usefulness.  It typically needs to be used in pairs to keep mass and thrust balanced.

Agreed; I rather like @ExtremeSquared's jumping rover idea because it adds utility beyond a self-righting rover, which one can obtain from reaction wheels.  I would be less interested in jumping such a rover to orbit and more interested in jumping over ravines and the like, but there is a potentially decent use case there.  However, the need to have two Puffs to keep from spinning out of control inhibits my enthusiasm for the idea.

1 hour ago, OhioBob said:

I think I'd find more use for a small inline monopropellant engine.  Maybe something like a 1.25m attachment with four small spherical monopropellant tanks surrounding a center engine.  The monopropellant could feed both the small maneuvering engine and the RCS.

Agreed again; I would probably use something like that for a cheap re-entry pod from a science station or something similar, though I think I'd prefer an engine that is capable of re-entering from low Kerbin orbit using the on-board monopropellant in the pod.  

[farmerben]

My error I forgot to account for the lander fuel.  Spark tends to win on thrust now.  Two serious errors in one thread, my apologies.  

[OhioBob]

34 minutes ago, Zhetaan said:

... though I think I'd prefer an engine that is capable of re-entering from low Kerbin orbit using the on-board monopropellant in the pod.

I agree that would be useful for a deorbiting engine.  There's already enough monopropellant for that in the pod, so just slap on a small engine to make use of it in situations where only a small amount of delta-v is needed.  Of course what I've sometimes done in those cases is just add a couple Place-Anywhere RCS thrusters and angle them forward or aft to be used for deorbiting.

[Rocket In My Pocket]

No need to apologize, we're all just here to help you. 

The only way to learn is to be wrong.

We've all been there! (I've actually seen two or three of these monopropellent threads myself.)

[Snark]

2 hours ago, OhioBob said:

I also think the fact that the puff is radially mounted hurts its usefulness.  It typically needs to be used in pairs to keep mass and thrust balanced.  In the places where perhaps I could have used a monopropellant engine, I've rarely needed two of them.  I think I'd find more use for a small inline monopropellant engine.

That's a good point, never thought of that!

...I guess you could kinda-sorta fake it by using the new engine plates from Making History.  Start with an engine plate, attach the desired number of Place-Anywhere 7 RCS thrusters to it facing aft, twiddle their settings so they react to throttle instead of RCS toggle, save as subassembly.

But yeah, not the same thing as having an actual dedicated engine.  (Though given that I never use the Puff-- and that the reasons why I never use it have nothing to do with its being radially attached-- then even if there were such an inline engine available, I probably wouldn't use that one myself, either.  The need simply doesn't come up in my own gameplay.)

37 minutes ago, Zhetaan said:

You play KSP; that's essentially the same thing. 

Well, okaaaay.    But the SRBs do put on a lot more of a show than the LFO engines.  And I'm a sucker for that, even though the extra f/x doesn't actually affect the rocket's flight at all.

52 minutes ago, farmerben said:

Two serious errors in one thread, my apologies.

Well, if everyone understood everything perfectly from the get-go, then we wouldn't need to have a "Gameplay Questions" forum, now would we?

(Also KSP would probably be a lot less fun...)

These are all excellent questions and examples.  It's how we learn.  Also, I can guarantee that if something confuses you, it confuses somebody else, too.  Possibly a lot of somebodies.  So putting up questions like this does everyone a favor, because now all those folks who were too shy to ask about it themselves can refer to this here.  Everyone wins!