Efficiency Question

[Kosmo-not]

UmbralRaptor answered the question quite sufficiently on page one. I guess my services are not needed here.

[GoSlash27]

This is sort of true in orbit, since you're not fighting gravity (though really low TWR can make for inefficient long burns.) But it's totally incorrect down in the gravity well: during ascents and (powered) landings. Even on airless worlds, higher thrust allows you to spend less time combating gravity during landings and ascents, which means it takes less fuel.

No, it is absolutely true. An inefficient engine can be replaced with an efficient engine or a cluster of efficient engines and generate adequate thrust for a stage. You can replace an inefficient engine with a more efficient setup and get more DV, but you *cannot* replace an efficient setup with an inefficient one and gain DV. The point is that "thrust efficiency" is a useless term. At least as far as our purposes go.

I understand what you're getting at, but your argument is based on a false choice fallacy.

Regards,

-Slashy

[Padishar]

Unless you're really oppossed to using any mods at all you are strongly advised to install VOID, MechJeb or Kerbal Engineer Redux as they will give you mass, TWR, deltaV and a mass of other figures for you in real-time, as you build and fly.

KER allows you to choose between vacuum and atmospheric stats when doing deltaV calculations in the VAB and also allows you to set the atmospheric pressure used for the atmospheric calculations with a tweakable on the engineer part (or the pod if you use an MM config file to add KER to pods). It sets the percentage of sea-level pressure on the selected reference body though will probably be changed to use altitude directly at some point.

[NecroBones]

This is sort of true in orbit, since you're not fighting gravity (though really low TWR can make for inefficient long burns.) But it's totally incorrect down in the gravity well: during ascents and (powered) landings. Even on airless worlds, higher thrust allows you to spend less time combating gravity during landings and ascents, which means it takes less fuel.

Yes, sort-of.

To clarify, a low TWR only reduces your efficiency when you're burning directly against gravity (during lift-off, or a powered landing, for instance), because a low TWR means you have to spend more time devoting a fraction of that thrust to offset acceleration due to gravity. A great visualization of this, is thrusting with a TWR of 1, right on the launch pad. You'll expend all of your fuel, and probably manage only to side-slip off the pad. (anyone trying this will note that you probably will lift upward, eventually, as the fuel gets expended and thus your TWR goes up due to reduced ship mass).

Otherwise, the actual strength of the thrust doesn't factor into the delta-V equation at all. It doesn't matter whether your burn is long or short. You can do your burns at half-throttle, and you'll get the same dV out of your engines (since KSP doesn't have variable ISP based on throttle settings). If your ship+engine mass is the same, and the ISP of the engines are the same, then it doesn't matter whether you use one big engine or lots of small engines, or even just one small engine. However the ship mass usually isn't the same if all you're doing is changing out the engine. If you replace a large engine, with a smaller one and more fuel to get the same mass, and that smaller engine is of equal or greater ISP, you'll get more dV out of it.

"Mass efficiency" is thus a term that gets used here. Generally speaking, any non-fuel mass in your ship, will serve to reduce the dV available to you. This includes the mass of the engines.

And likewise, ISP ends up mattering a lot more than the actual strength of the thrust of your engines, any time you're not landing or taking off. And even during ascent/descent, while TWR becomes extremely important, Specific Impulse (ISP) versus your ship mass, is still what determines the amount of dV available to you during those maneuvers.

This is where the LVN engines are a mixed bag. The mass is high compared to the thrust they can generate, but their ISP is wonderful in a vacuum. To get the most efficiency out of them, the idea is to never use them in atmospheres (except maybe Duna), and only use them for ascent/descent where the TWR is sufficient (low gravity moons). Use them almost exclusively for orbital maneuvers (instead of lower-efficiency engines), and use as few of them as you can get away with, to save on engine mass. Even a really huge ship will have better overall efficiency/dV with only one LVN instead of a lot of them. This can mean uncomfortably long burns, of course.

And that circles back to another point of clarification. In space, delta-V is delta-V, and the length of the burn doesn't matter.... right? Well, mostly. With low-thrust from your LVNs, it's possible to end up in situations where the burns take so long to complete, that you can move through a fairly large portion of your current orbit while executing your burn. If you just point at the blue marker on the nav-ball the whole time, a portion of the burn will be fighting gravity (or otherwise inducing some lateral acceleration, relative to the orbital trajectory), rather than burning prograde. For these long burns, it can actually be better to follow the orbital prograde marker. And in the case of escape burns (and transfers to higher orbits), also split the burn up over multiple orbits, so that you're taking advantage of the Oberth effect as much as possible.

Edited July 3, 2014 by NecroBones

[NecroBones]

When dealing with landers, the lighter the engine, the less total mass you need to bring down and return to orbit. Therefore, a lander using a LV-909 or Rocomax 48s will be far more efficient then using 2.25ton LV-N. For long interplanetary burns, the double efficiency of the LV-N will more then offset its extra mass by requiring less fuel for the burn.

Large 2 stage lander using 10 tiny engines that even have enough thrust to lift off from Kerbal. Note tons of fuel left after landing on Minmus.

http://i.imgur.com/768rNT5.jpg

Oh heck yea, the 48-7S is one of the best lander engines in the game currently. Those little radial engines in your picture have pretty terrible ISP, so that lander would be even better if you could replace them with 48-7Ss at the right angles, but of course you'd probably have more mass to get them all mounted.

I find that the LV-909 is really only worth it if you need the extra thrust, due to lander mass. The 48-7S is very light-weight for the thrust it generates, and so the slightly lower ISP isn't a huge loss here. But the 909 is still quite viable.

[Jouni]

This is where the LVN engines are a mixed bag. The mass is high compared to the thrust they can generate, but their ISP is wonderful in a vacuum. To get the most efficiency out of them, the idea is to never use them in atmospheres (except maybe Duna), and only use them for ascent/descent where the TWR is sufficient (low gravity moons)

Nuclear engines are a good engine choice for landers on all other planets and moons, except Kerbin, Laythe, and Eve. Obviously their efficiency depends on the circumstances:

• When you need a lot of delta-v from a single stage, Isp is more important than TWR. If you want to build a single-stage Tylo lander, nuclear engines are usually the most efficient choice.
  
• On low-gravity worlds, everything depends on the payload. For small payloads, conventional engines with high TWR and reasonable Isp, such as the 48-7S, are more efficient than nuclear engines. On the other hand, if your kethane miner weights 200 tonnes with its tanks full, nobody cares if it has 9 tonnes of engines.
  
• In many cases, it can be more efficient to use the transfer stage as a lander, instead of building a separate lander. Just leave unnecessary modules and fuel tanks in orbit, and land using the nuclear engines in the transfer stage.
  

[Mr Shifty]

Nuclear engines are a good engine choice for landers on all other planets and moons, except Kerbin, Laythe, and Eve. Obviously their efficiency depends on the circumstances:

• When you need a lot of delta-v from a single stage, Isp is more important than TWR. If you want to build a single-stage Tylo lander, nuclear engines are usually the most efficient choice.
  

You can see from [thread=45155]tavert's charts[/thread] that the LV-N is the most efficient engine only for larger payloads over a fairly narrow range of (Kerbin relative) TWRs: 0.2-0.9. Mostly this is due to the weight of the engine (at higher thrust, the 48-7S dominates for smaller payloads simply because it weighs so little: 25x less than the LV-N.)

[Jouni]

You can see from [thread=45155]tavert's charts[/thread] that the LV-N is the most efficient engine only for larger payloads over a fairly narrow range of (Kerbin relative) TWRs: 0.2-0.9. Mostly this is due to the weight of the engine (at higher thrust, the 48-7S dominates for smaller payloads simply because it weighs so little: 25x less than the LV-N.)

That narrow range happens to be precisely what's needed for Tylo landers: initial Kerbin TWR around 0.8, delta-v requirements around 6000 m/s, and payload in single-digit tonnes. Nuclear engines become even more efficient with my third point: using the transfer stage as a lander.

[SRV Ron]

Oh heck yea, the 48-7S is one of the best lander engines in the game currently. Those little radial engines in your picture have pretty terrible ISP, so that lander would be even better if you could replace them with 48-7Ss at the right angles, but of course you'd probably have more mass to get them all mounted.

I find that the LV-909 is really only worth it if you need the extra thrust, due to lander mass. The 48-7S is very light-weight for the thrust it generates, and so the slightly lower ISP isn't a huge loss here. But the 909 is still quite viable.

Here is the demo lander using the 48-S. Very efficient even landing on Mun.

I may experiment with a 48-s cluster on the unconventional lander if I can get a fuel line to work across the lander can.

[GoSlash27]

Yes, sort-of. ...

Well- stated.

Best,

-Slashy

[NecroBones]

That narrow range happens to be precisely what's needed for Tylo landers: initial Kerbin TWR around 0.8, delta-v requirements around 6000 m/s, and payload in single-digit tonnes. Nuclear engines become even more efficient with my third point: using the transfer stage as a lander.

http://www.cs.helsinki.fi/u/jltsiren/ksp/tylo_ship_3.jpeg

Yep, I neglected Tylo, as I was making general "tips" that are pretty easy to stick to. A lander with a Tylo-relative TWR of about 1, and multi-purpose engines is a more advanced concept for a lot of players.

Well- stated.

Thanks!

[SRV Ron]

The unconventional lander with 48-S replacing the less efficient side mounted rockets.

With Big Mother booster in orbit.

And in orbit around Mun ready for landing.

[Jouni]

Yep, I neglected Tylo, as I was making general "tips" that are pretty easy to stick to. A lander with a Tylo-relative TWR of about 1, and multi-purpose engines is a more advanced concept for a lot of players.

In that case, I'll state my points in the following way:

• Small landers use 48-7S engines or 24-77 engines. The choice between them is mostly a matter of convenience and aesthetics, because landers usually don't need that much delta-v.
  
• Bigger engines make lander design harder. The LV-909 is a borderline case that can sometimes be more convenient than the smaller engines.
  
• Still, big landers should use nuclear engines, no matter whether the size comes from fuel tanks or payload.
  
• Eve, Laythe, Kerbin, and possibly Tylo are special cases.
  

[UmbralRaptor]

The LV-909 is the Skipper of lander engines. Intermediate thrust, not the most efficient, but very conveniently sized. (See the endless "Toyota Corolla" variants for examples)

[SRV Ron]

The LV-909 is the Skipper of lander engines. Intermediate thrust, not the most efficient, but very conveniently sized. (See the endless "Toyota Corolla" variants for examples)

And, far lighter then the heavy LV-N when used only for landing and takeoff from moons or Duna.

[GoSlash27]

In that case, I'll state my points in the following way:

• Small landers use 48-7S engines or 24-77 engines. The choice between them is mostly a matter of convenience and aesthetics, because landers usually don't need that much delta-v.
  
• Bigger engines make lander design harder. The LV-909 is a borderline case that can sometimes be more convenient than the smaller engines.
  
• Still, big landers should use nuclear engines, no matter whether the size comes from fuel tanks or payload.
  
• Eve, Laythe, Kerbin, and possibly Tylo are special cases.
  

I wouldn't necessarily hang my hat on that. The Aerospike is an awfully good choice for a lot of bigger landers. A ton less mass and 3 times the thrust per engine makes up for a lot of Isp. I'm currently working with 2 standardized landers and neither one uses the LV-N. Between the two, I can land on and return to orbit from any body in the system.

I'd recommend modeling each choice in a spreadsheet to see what works out the best for your application.

Best,

-Slashy

[Random Tank]

I won't use a LV-N anywhere under 30km on Kerbin, and that's only really for the high alt. Spaceplanes anyway... I've also nerfed the LV-N's atm ISP down to 150, and weight up to 2.5t, just to make it a bit less useful.

EDIT:

I wouldn't necessarily hang my hat on that. The Aerospike is an awfully good choice for a lot of bigger landers. A ton less mass and 3 times the thrust per engine makes up for a lot of Isp.

Oh I totally agree, the Aerospike is very useful for lander, spaceplanes and even vacuum ships (it's flat out better than LV-909 and the Poodle for that sort of thing as far as I'm concerned). I just love the fact that it has the 390 ISP, and a TWR; and as a bonus its atm ISP basically doesn't change!

Edited July 4, 2014 by Random Tank

[Laie]

You can see from [thread=45155]tavert's charts[/thread] that the LV-N is the most efficient engine only for larger payloads over a fairly narrow range of (Kerbin relative) TWRs: 0.2-0.9.

One half order of magnitude doesn't qualify as narrow in my book: there's quite a difference wether your Jool transfer takes you four minutes or twenty. Further, that "narrow" range appears to be the one that matters for most practical applications.

I've designed few transfer stages so far, but aiming for a higher TWR seems to reduce the payload fraction, dramatically. My personal sweet spot, between how patient I am and how much fuel I'm willing to bring along, practically leaves me no choice but nukes.

[Pecan]

I pointed-out tavert's charts on page 1. From those and experience my engines of choice are:

• LV-1® for very light vehicles, eg; satellites. Because its tiny.
  
• 48-7S for light launch and transfer vehicles. It has great thrust for its weight and pretty good ISP.
  
• Aerospikes for launch vehicles too heavy for the 48-7S. Really good ISP.
  
• T30s when aerospikes just don't have the concentrated thrust you need. Probably the best all-round engine.
  
• LV-N for transfer vehicles when the 48-7S is too weak or the LV-N's fuel-efficiency is more important than its mass. Vacuum fuel efficiency.
  
• Turbojets, of course, for the atmospheric stage of (space)planes. Oxygen atmosphere fuel efficiency.
  

It's always surprising how many people overlook the T30, just because it's available right from the start in career mode. However; we shall have to see what rebalancing happens in 0.24, apart from the mainsail.

[UmbralRaptor]

And, far lighter then the heavy LV-N when used only for landing and takeoff from moons or Duna.

I'd argue that if you're taking more than a Mk1 pod or 800 L worth of LF+O, you're in the mass range where the LV-N comes out ahead. But, the LV-909 makes for easier lander design, so...

It's always surprising how many people overlook the T30, just because it's available right from the start in career mode. However; we shall have to see what rebalancing happens in 0.24, apart from the mainsail.

The LV-T30 is sneaky like that. No one expects it to do the Poodle's job better than the actual Poodle.

[Jouni]

I wouldn't necessarily hang my hat on that. The Aerospike is an awfully good choice for a lot of bigger landers. A ton less mass and 3 times the thrust per engine makes up for a lot of Isp. I'm currently working with 2 standardized landers and neither one uses the LV-N. Between the two, I can land on and return to orbit from any body in the system.

The Aerospike has a lot in common with the LV-T30. In principle it's a good engine, but in practice I almost never use it. It's just too big for landers and too small for launch vehicles. Because the Aerospike isn't stackable and the LV-T30 is tall and narrow, it's usually not a good idea to put them inline. With a practical minimum of 2 engines and ~400 kN of thrust, the lander has to be huge to make the engines worth their mass. My only landers (excluding Eve and Tylo) that have been big enough have been kethane miners, and nuclear engines work much better in them.

[LethalDose]

As of late, the KR-2L has been my go-to engine for transfers to other planets. It's not as efficient as the nukes or the "390" engines, but it has best TWR in the game and as more thrust than 40 nukes. I still use nukes for long-range probes (outside Eve-Kerbin-Duna) and medium sized vessels that get refueled frequently, but the advanced is quickly becoming my work-horse for large interplanetary payloads.

[NecroBones]

As of late, the KR-2L has been my go-to engine for transfers to other planets. It's not as efficient as the nukes or the "390" engines, but it has best TWR in the game and as more thrust than 40 nukes. I still use nukes for long-range probes (outside Eve-Kerbin-Duna) and medium sized vessels that get refueled frequently, but the advanced is quickly becoming my work-horse for large interplanetary payloads.

I've used the KR-2L for this as well on a few occasions, especially for the escape from Kerbin, It makes for nice short burn times.