Comprehensive Space Engine Efficiency By-Usage Analysis

[cantab]

Well I'm something of an ion man myself. If you're OK with the TWR and you have high delta-V needs they trounce the nerva utterly. You try getting 20 km/s with nervs.

The original question of a "shuttle" that's making multiple trips does skew things somewhat. In that usage fuel mass per trip matters more, and that's where the nuke can shine even if it means a slightly heavier craft overall.

But people are talking about 10, 20 tons. To me that's heavy for a lander. You'll run that with a big ISRU setup, a lot of Kerbals, or just being showy, but you can get much lighter. My Moho lander for example was under 4 tons, putting a nuke engine on that would just be plain silly.

[Streetwind]

Under 4 tons? o.O That must have been super tiny!

My tiniest lander so far with an actual pod (not an external command seat) has been a Mun lander weighing 3.2 tons, and Moho has two thirds more gravity than the Mun. Although two radial parachutes and the ~275 m/s dV for a direct Kerbin return were included in that weight, I still wonder if less than 1 ton difference could make up the dV and TWR gap... especially since that design of mine comes from alpha KSP where 390s vacuum Isp was typical, not 345s as it is now.

[Freshmeat]

If you have a small lander with rover, the situation changes somewhat. Suddenly, you need a minimum of two engines for symmetry in a skycrane, and 6t vs 1t of engine can be the deciding factor. Unfortunately, I have not researched nukes in my current save, so I have no hard numbers to back my intuition up with. I remember deciding on nukes in 0.90, but the result was an unwieldy beast, and the fuel savings was rather small compared to an equivalent chemical design.

[juanml82]

32 minutes ago, Freshmeat said:

If you have a small lander with rover, the situation changes somewhat. Suddenly, you need a minimum of two engines for symmetry in a skycrane, and 6t vs 1t of engine can be the deciding factor. Unfortunately, I have not researched nukes in my current save, so I have no hard numbers to back my intuition up with. I remember deciding on nukes in 0.90, but the result was an unwieldy beast, and the fuel savings was rather small compared to an equivalent chemical design.

Or if your lander is a rover. In that case, you need either Thuds or low profile engines.

The Nervs are the best engine for tugs and while they have the best combination of isp and thrust, sometimes it's best to sacrifice their isp.

If you have a 100ton+ interplanetary ship, you'll loose a lot of dV to the inacuraccies caused by the long burns. Basically, if your TWR falls below 0.2, it might be better to sacrifice efficiency and use Rhinos instead.

If it's a ship for use within the Kerbin system, the last ascend stage can double as the transfer stage, so you save weight by not adding another stage with nervs. If you're going to Duna or Eve and you have a refueling station in LKO, it can also be a good idea to refuel that ascend stage and do the interplanetary leg of the journey with a refueled chemical engine than throwing in another nerv stage. Or, conversely, your space program might have ended with returned interplanetary transfer stages with nervs in LKO, so you can refuel them, dock them to a new payload launched from Kerbin and reuse them.

 

[sdj64]

"Nukes are best" is a good rule of thumb, with some considerations:

1. are you in vacuum (or in Duna atmosphere)?

2. is the required TWR less than 0.5?

3. will a tall / long craft be acceptable for the purpose?

4. is your payload greater than 10 tons?

5. is the delta V required more than 1500?

In career mode, add:

6. does the craft need to be "infinitely" reusable?  OR  7. does a nuclear transfer stage minimize the cost of the entire mission?

 

If you answered no to any of these, there might be good reasons to use something other than nukes.  In my experience, Rhino is best for heavy disposable transfer stages (mostly for part count reasons), Poodle is good in career transfer stages before you unlock nukes, Spark is best for small landers, Terrier is best for medium landers, Aerospike is best for large landers.  Of course, nukes all the way for reusable tugs and interplanetary craft.  Single use craft should use whatever minimizes total mass, while reusable craft should use whatever minimizes fuel mass consumed.

[riocrokite]

For me the stock game strayed too far from reality (and difficulty) with the introduction of the god-like all-fuel producing isru. Thus I modded my game (rebalanced isru and fuel mix) so the isru can produce only oxidizer.

This creates nice scenarios when some engines have better effective dV than NTR since 75% of their fuel (oxidizer part with 3:1 fuel mix) can be refuelled:

http://forum.kerbalspaceprogram.com/index.php?/topic/126813-wip-fuel-and-isru-rebalance-mod-fir/#comment-2303379

Also this opens up a nice mod space for less efficient propellants that are easy to make by isru unlike liquid fuel (RP1 / liquid hydrogen).

Edited December 2, 2015 by riocrokite

[panzer1b]

On 12/2/2015, 5:15:31, Venusgate said:

without those honkin' NERVAs hangin out

One thing i hate about nukes, yeah they are massive and dont look very pretty on my builds.  The solution: clip the entire nuke minus the nozzle inside a fuel tank or whatever, looks prettier, more compact ship, and the only downside is increased suceptibility to kinetic weapons (as clipped parts tend to explode when hit hard enough by a ibeam at 500m/s).  Military aside (i know im one of the few that finds KSP as a military/sci-fi game instead of whatever the devs intended), there is nothing wrong with clipping (especially inside that new mk1 fuselage with teh hollow internals.

[UmbralRaptor]

So, it seemed like a good idea to try to work out the relative performance in detail. The exact point where the LV-N offers better performance is variable, but unless you have a weirdly high desired TWR or payload low enough that the LV-1 should be considered...

[GoSlash27]

It is possible to mathematically prove which engine is best for any application simply by reversing the rocket equation and plugging in the thrust to weight/payload/ DV requirement. Spoiler: Meithan's chart already does this and shows the correct answers.

 This is how I design all of my stages before I enter the VAB.

1) Determine the lifting capacity of the engine at your desired t/w ratio.

 This is done by adjusting the thrust to your prevalent atmospheric condition and converting your engine's thrust into mass, and dividing by your desired t/w ratio.

 ex. an LV-909 generates 60kN of thrust in vacuum. It can therefore handle up to 60/(.5*9.81) = 12.2t total at .5 t/w.

2) reverse the rocket equation to determine wet to dry ratio.

e^(DV/9.81Isp)=Rwd

An LV-909 has a wet to dry ratio of 1.81 when accelerating a payload to 2,000 m/sec DV

3) Convert the wet to dry ratio to fuel fraction.

(Rwd-1)/Rwd= Ff

44.6% of this rocket would be fuel.

4) Factor in the tank mass.

1/8 of a loaded LF&O tank is tank by mass,

An LF&O tank weighs 1/8 as much as the fuel it contains, so

Ff/8=Ft

5.58% of this rocket would be tankage.

5) Multiply these by the lifting capacity to find the mass of tank and fuel.

.446* 12.2= 5.44t of fuel

.0558*12.2= .67t of tankage

6) Add in the mass of the engine

.5t+5.44t+.67t=6.61t of our rocket that is not payload.

7) Subtract this from the lifting capacity.

12.2-6.61= 5.59t of payload.

8) Divide the payload by the lifting capacity.

5.59/12.2= 45.8% payload fraction.

For most purposes, the engine with the highest payload fraction in a given situation is best. In some cases, you may want to design for least expensive stage or limit your build to an arbitrary number of engines. In this case...

9) Divide your desired payload by the payload capacity of a single engine and round up to the nearest integer to give the required number of engines. Afterwards, tally up the cost to build the entire stage.

I have set up a spreadsheet to do all of this for me on the fly with all engines simultaneously, but Meithan's charts work just fine for planning purposes.

Best,

-Slashy

 

 

 

Edited December 4, 2015 by GoSlash27
correction

[Streetwind]

7 hours ago, GoSlash27 said:

4) Factor in the tank mass.

1/8 of a loaded LF&O tank is tank by mass, so

Minor nitpick: 1/9th of a loaded tank is dry mass, not 1/8th.

The fuel:dry mass ratio is 8:1, meaning 8 tons of fuel accompany 1 ton of dry mass. The total mass is 9 tons, not 8.

Other than that, great writeup!

Edited December 4, 2015 by Streetwind

[GoSlash27]

1 hour ago, Streetwind said:

Minor nitpick: 1/9th of a loaded tank is dry mass, not 1/8th.

The fuel:dry mass ratio is 8:1, meaning 8 tons of fuel accompany 1 ton of dry mass. The total mass is 9 tons, not 8.

Other than that, great writeup!

Streetwind,

 Good catch! I meant to say that a tank weighs 1/8 the fuel it contains (what you said). It's not a minor nitpick, either. If you hadn't corrected it, somebody might've wandered off into the weeds and started overestimating their DV.

Thanks,

-Slashy