Efficiency Question

[DeskLamp]

I noticed while comparing the LV-909 and the LV-N Atomic Rocket Motor (Nuclear Engine) that the LV-909 is the most "efficient" in terms of thrust compared to overall fuel use, although I commonly hear that the Nuclear Engine is the most efficient. What pieces am I not looking at when considering efficiency? Thanks!

Edited July 8, 2014 by DeskLamp

[Alshain]

What pieces am I not looking at when considering efficiency? Thanks!

Weight and atmosphere. Nuclear engine is more efficient in a vacuum it's terrible in the atmosphere due largely to how heavy it is. If you look at the wiki for the LV-909 and LV-N you will notice Fuel Consumtion has 2 entries (the larger the number the less efficient). In atmosphere it's ~3 vs ~5, but that gets reversed in a vacuum to ~2 vs ~1.

EDIT: Read Mr. Shifty's comment, he's probably more correct than me.

Edited July 2, 2014 by Alshain

[Mr Shifty]

I noticed while comparing the LV-909 and the LV-N Atomic Rocket Motor (Nuclear Engine) that the LV-909 is the most "efficient" in terms of thrust compared to overall fuel use, although I commonly hear that the Nuclear Engine is the most efficient. What pieces am I not looking at when considering efficiency? Thanks!

Specific Impulse (Isp) is a measure of the rocket's efficiency. Specific impulse is inversely proportional to the weight of fuel it takes to generate a specific amount of thrust. The LV-N has a specific impulse of 800 seconds, while the LV-909's Isp is 390 seconds. Both engines generate about the same thrust (the LV-N is slightly higher), so with the same amount of fuel (and total rocket weight), the LV-N will burn more than twice as long as the LV-909.

The LV-909's advantage is that the engine itself is 5 times lighter than the LV-N, which could be important for small probes, and in career mode, the LV-909 is earned very early on in Tier 3, while you have to get to Tier 7 to get the LV-N.

[Rhomphaia]

I'm not sure what you mean by "overall fuel use", sure the 909 has better ISP in atmosphere, but that isn't really a consideration for the intended use of either rocket, they are both designed for vacuum.

The key advantage the LV-N has, is its vacuum ISP, it has a major disadvantage in weight though. for small rockets using the 909 means you can take a couple of extra tons of fuel for (roughly) the same TWR.

[michaelhester07]

ninja'd but...

The Lv 909 while not as efficient as the LV-N can be used as an early transfer stage for interplanetary vessels. You can do a mission to Ike with landing and coming back with LV 909s in the transfer stage (my may carrer I had done just that with all of T4 tech unlocked and the long boosters unlocked)

[UmbralRaptor]

I noticed while comparing the LV-909 and the LV-N Atomic Rocket Motor (Nuclear Engine) that the LV-909 is the most "efficient" in terms of thrust compared to overall fuel use, although I commonly hear that the Nuclear Engine is the most efficient. What pieces am I not looking at when considering efficiency? Thanks!

The fuel consumption figures in the VAB make bad assumptions (it gives the results at 1 atm), and might as well be wrong everywhere but Eve.

It's best to pay attention to the I_sp figures, with a focus on the vacuum side. Notably, the LV-N will become more fuel efficient than the LV-909 at 0.837 atm (891 m on Kerbin). And the LV-N will have a higher Isp than even the aerospike at a mere 1717 m (0.71 atm)

As for the why of I_sp, it's a measure of fuel efficiency. For KSP purposes, ÃŽâ€V == I_sp*9.82*ln(wet_mass/dry_mass)

[Pecan]

Tavert wrote the ultimate guide to mass-optimal engine choices. Specifically, if you can get where you're going in a short burn (low deltaV requirement) the low mass of the 909 will beat the fuel efficiency of the LV-N. Note, however, that the 909 is almost never the 'right' choice - a better light engine is the 48-7S.

[DeskLamp]

Specific Impulse (Isp) is a measure of the rocket's efficiency. Specific impulse is inversely proportional to the weight of fuel it takes to generate a specific amount of thrust.

Can you explain Specific Impulse more explicitly, perhaps in laymans terms? I understand that it denotes efficiency, but you noted that it's a unit of seconds, which threw me off.

[UmbralRaptor]

Can you explain Specific Impulse more explicitly, perhaps in laymans terms? I understand that it denotes efficiency, but you noted that it's a unit of seconds, which threw me off.

Specific impulse can be though as pound*s of impulse (change in momentum) per pound (yes, weight) of propellant. The pounds cancel, leaving seconds. Arguably, effective exhaust velocity is clearer.

[Pecan]

Wikipedia explains Specific Impulse pretty nicely.

[DeskLamp]

In the wikipedia definition, it's stated that there's a conversion factor between the mass definition and the weight definition. Could I work out Delta-V from there, or is there a more simple way to do it?

[Laie]

The fuel consumption figures in the VAB make bad assumptions (it gives the results at 1 atm), and might as well be wrong everywhere but Eve.

Let me expand on this a little: as you climb, an atmosphere *quickly* becomes thinner. Very quickly. Yes, your rocket is still experienceing considerable drag -- that is misleading. If you right-click an engine at launch and check it's ISP during early ascent, you'll see that by 5000m you're already in near-vacuum. Did this by accident, it was an eye-opener.

[Pecan]

In the wikipedia definition, it's stated that there's a conversion factor between the mass definition and the weight definition. Could I work out Delta-V from there, or is there a more simple way to do it?

In post #6 UmbralRaptor gave you the deltaV formula. The '9.82' in that is the 'g' to use on Kerbin.

...If you right-click an engine at launch and check it's ISP during early ascent, you'll see that by 5000m you're already in near-vacuum. Did this by accident, it was an eye-opener.

IIRC the LV-N becomes the most efficient engine as low as 2,000m on Kerbin. Lol, still not a good choice for a launch engine, of course, because of its low thrust.

Edited July 2, 2014 by Pecan

[NecroBones]

In post #6 UmbralRaptor gave you the deltaV formula. The '9.82' in that is the 'g' to use on Kerbin.

Yep, the delta-V equation is also called The Tsiolkovsky rocket equation, which wikipedia explains pretty nicely.

The equation is built around "effective exhaust velocity", which is a number that can be replaced with ISP*G, where ISP is the Specific Impulse, and G is simply the acceleration due to gravity at the surface of Earth/Kerbin. ISP is a value that's represented as seconds, by normalizing it around 1 G of acceleration, so to get velocity out of it, those need to be multiplied back together.

So in that equation, Isp is specific impulse, G0 is 9.82 m/s, and m0 is the starting (fueled) mass of the entire spaceship, and m1 is the empty mass (the mass after the fuel is spent). "ln" is simply the Natural Logarithm.

Inserting some of these values, as was pointed out earlier in the thread, it'll look something like:

dV = Isp * 9.82 * ln(fueled_mass/empty_mass)

Edited July 2, 2014 by NecroBones

[Laie]

IIRC the LV-N becomes the most efficient engine as low as 2,000m on Kerbin. Lol, still not a good choice for a launch engine, of course, because of its low thrust.

Until quite recently, I assumed that LVNs should not be used in any kind of atmosphere, lest they waste fuel as if there was no tomorrow. Am I the only one who has fallen to this fallacy?

Actually, for any engine that will keep running longer than one minute after launch, the vacuum figure becomes the vastly more important one. Might even be 40 seconds.

[Pecan]

No, you're definitely not the only one to avoid the LV-N at launch for that reason. It takes most of us a while to catch-on to how quickly it becomes efficient, I think. My standard workhorse tractor is designed never to re-enter atmosphere once launched and so uses LV-Ns for their vacuum efficiency. As you can see in that link I fire its engines directly at launch, with the comment "Although they don't provide much thrust and this is using the payload's fuel the LV-Ns are very efficient so, overall, whatever contribution they make saves a little fuel that you can transfer back once in orbit."

Yes, they'll be inefficient right on the ground but it's not worth the effort to ignite them a few seconds later ^^.

[Mr Shifty]

Until quite recently, I assumed that LVNs should not be used in any kind of atmosphere, lest they waste fuel as if there was no tomorrow. Am I the only one who has fallen to this fallacy?

Actually, for any engine that will keep running longer than one minute after launch, the vacuum figure becomes the vastly more important one. Might even be 40 seconds.

It's the low thrust/high weight of the LVN that makes them bad for atmospheric ops, not the low atmospheric Isp. You need a TWR of around 2 to get the most efficient balance between fighting gravity and fighting air resistance. A single LVN, with its thrust of 60kN, can only lift 3 tons at that TWR, i.e. only 0.5 tons more than the weight of the engine itself. It doesn't matter how efficient your rocket engine is if it can't lift the rocket off the pad.

[DeskLamp]

So in that equation, Isp is specific impulse, G0 is 9.82 m/s, and m0 is the starting (fueled) mass of the entire spaceship, and m1 is the empty mass (the mass after the fuel is spent). "ln" is simply the Natural Logarithm.

Inserting some of these values, as was pointed out earlier in the thread, it'll look something like:

dV = Isp * 9.82 * ln(fueled_mass/empty_mass)

Mass in the game is given by (I assume) adding all of the "mass" numbers of each component on a craft together. What unit is used by the game, and is there a specific unit called for in the equation?

[Mr Shifty]

Mass in the game is given by (I assume) adding all of the "mass" numbers of each component on a craft together. What unit is used by the game, and is there a specific unit called for in the equation?

Mass in the game is typically in metric tons (1,000 kg). In the Tsiolkovsky equation, you're dealing with mass ratios, so the units don't matter as long as they're the same.

[Pecan]

Mass is in metric tonnes. Watch out though, several small parts report a mass in VAB/SPH but are actually massless in-flight - to limit the work the physics engine has to do it just ignores them. The easiest way to check your total flight mass, without mods, is to click 'Launch' and then 'M' for map mode. The 'i'nformation button on the right of the map screen will tell you the value KSP will really use. Then just 'revert to VAB' to get back to building.

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.

ETA: Oh, and as Mr. Shifty Ninja'd - you can put the total tonnage straight into the rocket equation.

[Laie]

It's the low thrust/high weight of the LVN that makes them bad for atmospheric ops, ...

yesyesyes, no question about that. Never mind the TWR, I assumed the ISP to be abysmal "in atmosphere", so I re-equipped my lander with 909s when I sent it to Duna. Because, you know, Duna has an atmosphere and LVNs suck in atmosphere and well, by now you should really get my drift.

(may well be that 909s were the better engine anyway, as I made only a single landing, but that's beside the point)

[Red Iron Crown]

LV-Ns are actually quite good on Duna, the atmosphere is so thin that it's highly efficient and Duna's lower gravity means the bad TWR isn't such a big deal.

[GoSlash27]

Basically you've got 2 ways of looking at efficiency: Mass efficiency and Isp. An engine that can generate a DV with less total mass for a stage (which includes the engine itself) is highly mass efficient. This is critical for later stages in a stack, as they make for a smaller payload for earlier stages to contend with.

An engine that delivers a high Isp is more fuel efficient. All else being equal, an engine with high Isp will generate more DV with a given amount of fuel. This can be important for longer burns, where the fuel efficiency overcomes the mass penalty.

The important thing is that thrust doesn't figure into efficiency considerations. You need to generate the required thrust for a stage regardless of other concerns, but there are many ways to do that and nobody says you have to use just one engine to do the job.

Best,

-Slashy

[Mr Shifty]

The important thing is that thrust doesn't figure into efficiency considerations. You need to generate the required thrust for a stage regardless of other concerns, but there are many ways to do that and nobody says you have to use just one engine to do the job.

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.

[SRV Ron]

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.

Edited July 3, 2014 by SRV Ron