Optimal engine charts for 1.0.2

[Meithan]

Alright people, here are some atmospheric engine charts for TWR ranging from 1.2 to 4.0:

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The album was also added to the OP.

IMO, the highlights are:

1) The massive Isp drop of the KR-2L means the KS-25x4 engine cluster now outperforms it for high mass / high ÃŽâ€v.

2) The Poodle is completely replaced by the Mainsail at high masses and the LV-T30 at lower masses.

3) The LV-1R replaced the LV-1 (for some reason, the radial version has good atmospheric Isp).

You've done a lot of work, Meithan. Your charts/ graphs cover quite a range of needs. I didn't notice these on the Tutorials page; some people might miss this thread since its in General Discussion. I put up a graph of specific engine tests (60% dry mass, TWR 1.2 in VAB, so just over 1 on the pad). Its rather crude in comparison to your calculated graphs, but you might like to compare it with your work. Thanks for sharing with us!

Thanks. I'll look into your calculations to see how our results compare.

There's a set of calculations for fuel-cell powered ion engines in http://forum.kerbalspaceprogram.com/threads/116611-New-ion-craft-using-fuel-cell - the overall Isp is 1293, engine TWR of 5.4 with necessary fuel cells to run it, fuel unit ratio of 100 xenon == 4.495 LF+O to power the fuel cells. (I added some numbers starting on page 5 of the thread)

Is that enough to add as another engine in your (utterly fantastic) charts? I think it might be a viable alternative for missions past Dres.

Let me review that thread and get back to you on that.

[Brotoro]

I can also try to prepare colorblind-friendly charts. Since there are many engines it's a bit hard to choose a good color set, even for non-colorblind people. I had trouble finding one for these (and I'm still not happy about a couple of colors). Any suggestions on how I can help here? Are these good?

That set of colors you linked to look good...but, as you say, you have a lot more engines than those would cover. Yes, it would help if the key for the chart only contained the engines that actually appear on that chart.

[cybersol]

Alright people, here are some atmospheric engine charts for TWR ranging from 1.2 to 4.0

Wow, you are the man! Thanks again. I am actually quite surprised by how consistent the atmospheric charts are from TWR 1.2 all the way through 4.0. All throughout the thrust range it seems the story is very similar from Mammoth to Mainsail to Skipper to Reliant depending on how much you are lifting.

[nholzric]

Does anyone have an intuitive (or otherwise) explanation for the sawtooth nature of these charts?

For example, in chart two TWR=0.1, the LV-909 region at 100 tonnes is disconnected from the 909 region at 300 tonnes, separated by the Poodle. It looks wrong but given that I'm not up on the details and there are lots of smart people on this thread I'll bet I just don't understand. Thanks!

[GoSlash27]

Does anyone have an intuitive (or otherwise) explanation for the sawtooth nature of these charts?

For example, in chart two TWR=0.1, the LV-909 region at 100 tonnes is disconnected from the 909 region at 300 tonnes, separated by the Poodle. It looks wrong but given that I'm not up on the details and there are lots of smart people on this thread I'll bet I just don't understand. Thanks!

nholzric,

That's from the simulation adding engines to cope with increasing vehicle mass. When you add an engine, the total mass jumps. This often causes a stage using a different engine to become superior, at least temporarily.

The result of this is jaggedness in the plot.

Best,

-Slashy

[Meithan]

Does anyone have an intuitive (or otherwise) explanation for the sawtooth nature of these charts?

For example, in chart two TWR=0.1, the LV-909 region at 100 tonnes is disconnected from the 909 region at 300 tonnes, separated by the Poodle. It looks wrong but given that I'm not up on the details and there are lots of smart people on this thread I'll bet I just don't understand. Thanks!

nholzric,

That's from the simulation adding engines to cope with increasing vehicle mass. When you add an engine, the total mass jumps. This often causes a stage using a different engine to become superior, at least temporarily.

The result of this is jaggedness in the plot.

Slashy's quite right.

Here, I've thrown together a couple of charts to better explain where the jaggedness comes from. I hope it's not too long.

The first is an engine chart with only two engines, the LV-909 ("Terrier") and the LV-N ("Nerv").

The jaggedness is clearly visible here. Let's focus on the horizontal black line, which corresponds to a payload of around 3.4 t. I've marked as 1, 2 and 3 the points where one engine overtakes the other. As you can see:

• To the left of point 1 , the LV-909 is better.
  
• Passing point 1 , the LV-N becomes better.
  
• But at point 2 , the LV-909 gets back the lead. We'll see in a minute why.
  
• Finally, at point 3 the LV-N overtakes again, and will remain better for any higher ÃŽâ€v.
  

Now take a look at the next image.

This plot is a more detailed representation of the black line we saw in the previous image. Since it's computed for a fixed payload of 3.4 t, instead of showing just which engine is better it shows what the total mass of the ship would be, versus the ÃŽâ€v.

I've also marked three points 1, 2 and 3. These correspond to the same moments marked in the previous image. This is what's going on:

• Before point 1 , the LV-909 (red curve) yields lower total mass, which means it's better.
  
• As we move to higher ÃŽâ€v, notice how the LV-N (black curve) is increasing at a slower pace (this is because it has better Isp).
  
• At point 1 the curves cross over: the LV-N now yields a lower total mass.
  
• At around 2650 m/s (not marked) notice how the red curve jumps. Why? Because slightly above 10 t total mass, a single LV-909 with its 60 kN of thrust is no longer enough to provide a TWR of 0.6. So we need to add a second LV-909, which adds the mass of the engine (0.5 t) and about 2.5 t of fuel to compensate for the extra weight.
  
• As we continue to the right, we reach point 2, where what just happened to the LV-909 now happens to the LV-N: once it passes 10 t we need a second LV-N to make sure the TWR remains above 0.6. But see that this jump is much larger! This is because the LV-N is much heavier: not only do we add the 3 tonnes of the engine itself, but we also need to add about 7 tonnes of extra fuel (and tanks) to compensate. That's a big penalty!
  
• The result is that at point 2 the LV-N becomes worse again.
  
• Finally, as we move further right the better Isp of the LV-N lets it catch up, and it does so at point 3. By this point the curve for the LV-909 is starting to get more and more vertical (this is the exponential nature of the matter).

So as you see, the jaggedness is caused by these sudden changes in engine number that result in regions where one engine suddenly becomes worse than the others.

Why do they look like a sawtooth in the original chart? If we plotted the curves of the second image for a higher payload mass, you'd see the crossover points move to the left (and in different amounts for each engine). Hence the regions where one engine overtakes the other would start and end at smaller ÃŽâ€v.

Hope this helps understand the behavior.

[ToddEth]

Thanks for these, I will find these of much use when I get back off holiday.

[nholzric]

Hope this helps understand the behavior.

Brilliant, thanks Slashy and Meithan for the explanation. When I attempt design by calculation I often compute minimum TWR and sometime maximum out if curiosity but it's rarely a driving factor. If one was determined to smooth the boundaries, you could probably present the charts as TWR +/- some percent. Not a suggestion or request, just thinking out loud.

[John FX]

Can I make a request for charts?

With the ISp changes Eve is now a problem that needs more thought. Could you post a set of atmo chart for Eve sea level please?

Also I would be very interested in the single engine charts for a TWR of 0.1 to 3.

I often don't put multiple engines on craft so the multiple engine charts are of limited use for my style of gameplay.

Awesome to see this is being kept alive. +1 rep from me.

[cicatrix]

Meithan, I could kiss you if you were a pretty girl but god knows how I missed those charts

Great work!

[Meithan]

If one was determined to smooth the boundaries, you could probably present the charts as TWR +/- some percent. Not a suggestion or request, just thinking out loud.

As long as the number of engines needs to change to meet a TWR restriction, you'll get jaggedness. The only way to completely get rid of it is to do single-engine charts (or, equivalently, to set no TWR restriction; see the charts with no TWR minimum), like John FX says below.

Can I make a request for charts?

With the ISp changes Eve is now a problem that needs more thought. Could you post a set of atmo chart for Eve sea level please?

No problem. I can also change the TWR to be Eve-relative.

Edit: Actually, I'll need to figure out how to derive the Isp of each engine at 5 atm (Eve's sea-level pressure). I know KSP stores this information using atmosphereCurve, but I don't know how to evaluate these manually.

Taverius made a very detailed post on this but he uses an external Unity plugin to actually compute the curve. If anyone could give me Isp values at 5 atm I could make these charts quickly. Otherwise, I'll have to read up on cubic Hermite splines.

Also I would be very interested in the single engine charts for a TWR of 0.1 to 3.

I often don't put multiple engines on craft so the multiple engine charts are of limited use for my style of gameplay.

I can do that. Some engines won't be able to meet the TWR requirement though.

~~~

What I was thinking is that a webapp to generate these charts on demand would be very helpful. Sure, an in-game plugin, like some have suggested, would be cool too but I think a webapp is both easier to make and more generally useful.

I was thinking something along the lines of alexmoon's launch window planner, but I have little experience with web programming. Somebody told me Paper.js would be a good starting place. Any other ideas?

Edited May 18, 2015 by Meithan

[Xyphos]

Wow. VERY handy info, I'm surprised I didn't think of this.

[Meithan]

Also I would be very interested in the single engine charts for a TWR of 0.1 to 3.

I often don't put multiple engines on craft so the multiple engine charts are of limited use for my style of gameplay.

Here you go, dear sir:

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It's interesting that most engines are represented. No solution exists in the white zones.

If you need more TWR values let me know and I'll add them. But I really need to start programming that webapp so anybody can generate any chart on demand.

[FawkesFTK]

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Edited June 28, 2016 by FawkesFTK

[John FX]

I really need to start programming that webapp so anybody can generate any chart on demand.

I heartily agree. If the choice of engines can be up to the user that would be awesome.

"You must spread some Reputation around before giving it to Meithan again."

[Granite]

I'd be interested in seeing how the LFB KR-1x2 booster fits into the atmospheric chart. By subtracting out its included orange tank, it should become a 6.0 ton engine with more thrust but slightly less efficiency than the Mainsail. That should provide a good approximation for these charts since it would rarely be used with less than a full tank of fuel.

[Sobriquet]

I'm having a hard time getting my head around how to use these charts. It's like it's rocket science or something.

I want to launch a three man command module, with full science, from Kerbin's surface, land on Duna, and return to Kerbin. All with one vehicle.

Using the dV charts, I need a total of 4500 (escape Kerbin) + 1500 (transfer to Duna orbit) + 1400 (land Duna) + 1400 (escape Duna) + 1500 (transfer to Kerbin) + 3500 (land Kerbin). For a total dV of 13800 m/s.

Does this mean I should split my craft up into stages and build accordingly? i.e. Stage one is lifting 100T of lander so it should be comprised of 4x KS25x4 with enough fuel to make a total craft weight of 500T?

Are there charts split out into their respective engines so I can see the steps better?

Some questions I have about my space faring ambitions?

1. Is building a rocket of this type even advisable?

2. Are my dv calcs remotely correct?

3. Are there only two choices when playing this game. MechJeb, and all your engineering questions are answered for you, or stock and it's trial and error, and trial and error, and trial and error . . .

[Superfluous J]

I want to launch a three man command module, with full science, from Kerbin's surface, land on Duna, and return to Kerbin. All with one vehicle.

Using the dV charts, I need a total of 4500 (escape Kerbin) + 1500 (transfer to Duna orbit) + 1400 (land Duna) + 1400 (escape Duna) + 1500 (transfer to Kerbin) + 3500 (land Kerbin). For a total dV of 13800 m/s.

You don't need 4500 to launch from Kerbin. A mildly efficient launch with a fairly streamlined launcher should be able to do it with 3500 or less. Also, you don't need 3500 to land on Kerbin, because you'll get all that from aerobraking. Same with landing on Duna. Also, you should bring 10-25% extra dV for inefficiencies. As you get better and learn more, you'll need less but I try to always bring 10% and even then - I run out sometimes.

Does this mean I should split my craft up into stages and build accordingly? i.e. Stage one is lifting 100T of lander so it should be comprised of 4x KS25x4 with enough fuel to make a total craft weight of 500T?

That sounds reasonable, without actually building a craft, sure. That's the thinking you should have, yes.

1. Is building a rocket of this type even advisable?

2. Are my dv calcs remotely correct?

3. Are there only two choices when playing this game. MechJeb, and all your engineering questions are answered for you, or stock and it's trial and error, and trial and error, and trial and error . . .

[Meithan]

I'd be interested in seeing how the LFB KR-1x2 booster fits into the atmospheric chart. By subtracting out its included orange tank, it should become a 6.0 ton engine with more thrust but slightly less efficiency than the Mainsail. That should provide a good approximation for these charts since it would rarely be used with less than a full tank of fuel.

Ah yes, I see what you mean. I'll include it in the set of engines for the webapp I'm making.

I'm having a hard time getting my head around how to use these charts. It's like it's rocket science or something.

I want to launch a three man command module, with full science, from Kerbin's surface, land on Duna, and return to Kerbin. All with one vehicle.

Using the dV charts, I need a total of 4500 (escape Kerbin) + 1500 (transfer to Duna orbit) + 1400 (land Duna) + 1400 (escape Duna) + 1500 (transfer to Kerbin) + 3500 (land Kerbin). For a total dV of 13800 m/s.

Yes, what the charts say is that what you're asking is pretty much impossible. You can't build a single-stage ship that has a TWR of at least 1 (which is the absolute minimum you'll need to get off Kerbin's surface) and 13,800 m/s dv. Nope.

However, as 5thHorseman's pointed out, those dv figurs are a tad overestimated. Going to worlds with atmospheres means you can save a lot of fuel by aerobraking (and if you're landing there, the saving is mandatory, so to speak: the atmosphere will slow you down). Landing on Kerbin is close to free when coming from an interplanetary trajectory, so skip the capture and low orbit circularization dv.

Still, if you want the single-stage ship to launch from Kerbin's surface, it's a lot of delta-v. So let's assume you can get to orbit with 3500 m/s (I haven't been able to hit these values in 1.0 yet, but I'm far down the tech tree), then an aerobraking-assisted 1200 m/s Duna transfer. I have no idea how effective parachutes are in Duna in 1.0, but you won't need anywhere near 1300 m/s to land. Let's say you spend 500 m/s in a little propulsive assist. You will, however, need around 1300 m/s to takeoff. Ater that, you can do the transfer back to Kerbin's SOI probably on 800 m/s if you catch good windows, and then just aerocapture at Kerbin and land with almost zero fuel expenditure.

The total cost (ballpark) would be around 3500 (LKO) + 1200 (Duna transfer) + 500 (landing) + 1300 (takeoff) + 800 (Kerbin transfer) = 7300 m/s. That's also impossible, since you require TWR > 1 for Kerbin takeoff. So yes, no single-stage ship can pull this off (and there's a "good" challenge for anyone willing to try -- but they'll all fail).

If you relax the single-stage from Kerbin surface requirement, and instead assume you start in low Kerbin orbit (so you're allowed to use an expendable launcher that won't travel to Duna), then the total cost is about 3800 m/s. Duna liftoff requires Kerbin-relative TWR of 0.3, but let's multiply that by 2 so the Duna-relative TWR is at least 2. The 0.6 TWR chart says this is totally doable, and that the optimal engine is the LV-N.

Are there charts split out into their respective engines so I can see the steps better?

Not sure what you mean by this.

[Sobriquet]

Ah yes, I see what you mean. I'll include it in the set of engines for the webapp I'm making.

Yes, what the charts say is that what you're asking is pretty much impossible. You can't build a single-stage ship that has a TWR of at least 1 (which is the absolute minimum you'll need to get off Kerbin's surface) and 13,800 m/s dv. Nope.

However, as 5thHorseman's pointed out, those dv figures are a tad overestimated. Going to worlds with atmospheres means you can save a lot of fuel by aerobraking (and if you're landing there, the saving is mandatory, so to speak: the atmosphere will slow you down). Landing on Kerbin is close to free when coming from an interplanetary trajectory, so skip the capture and low orbit circularization dv.

Still, if you want the single-stage ship to launch from Kerbin's surface, it's a lot of delta-v. So let's assume you can get to orbit with 3500 m/s (I haven't been able to hit these values in 1.0 yet, but I'm far down the tech tree), then an aerobraking-assisted 1200 m/s Duna transfer. I have no idea how effective parachutes are in Duna in 1.0, but you won't need anywhere near 1300 m/s to land. Let's say you spend 500 m/s in a little propulsive assist. You will, however, need around 1300 m/s to takeoff. Ater that, you can do the transfer back to Kerbin's SOI probably on 800 m/s if you catch good windows, and then just aerocapture at Kerbin and land with almost zero fuel expenditure.

The total cost (ballpark) would be around 3500 (LKO) + 1200 (Duna transfer) + 500 (landing) + 1300 (takeoff) + 800 (Kerbin transfer) = 7300 m/s. That's also impossible, since you require TWR > 1 for Kerbin takeoff. So yes, no single-stage ship can pull this off (and there's a "good" challenge for anyone willing to try -- but they'll all fail).

If you relax the single-stage from Kerbin surface requirement, and instead assume you start in low Kerbin orbit (so you're allowed to use an expendable launcher that won't travel to Duna), then the total cost is about 3800 m/s. Duna liftoff requires Kerbin-relative TWR of 0.3, but let's multiply that by 2 so the Duna-relative TWR is at least 2. The 0.6 TWR chart says this is totally doable, and that the optimal engine is the LV-N.

Not sure what you mean by this.

The display on your charts "steps" where the graph tells you that another engine has been added. With all of the engines, their colors, the overlapping, etc. it is hard for me to see when another engine is being asked for. i.e. I cannot discern the steps with all of the disparate engine info being displayed. Is their a chart for just one engine? I may have missed it.

Also, I misspoke in my request for what I want my rocket to do. I expect it to be staged. A rocket of this nature would be doable, yes? Is it practical? (Heck does practicality even matter in sandbox game?)

Thanks so much for the reply and mucho kudos on your charts. Like i said "rocket science!"

[LordFerret]

Wow! Excellent! Bookmarked. Thank you!

Actually, I think a Mod should sticky this.

[Meithan]

The display on your charts "steps" where the graph tells you that another engine has been added. With all of the engines, their colors, the overlapping, etc. it is hard for me to see when another engine is being asked for. i.e. I cannot discern the steps with all of the disparate engine info being displayed. Is their a chart for just one engine? I may have missed it.

If you plotted the engine charts with a single engine, it would all be one color (or one color and white), which wouldn't be interesting. Perhaps you want to look at the total mass plots, where instead of just telling you which engine is better for any (payload,dv) pair, more detail is given about just how good or bad each engine is in providing dv for a given payload.

In those plots you can follow the line for a specifc engine and see how heavy the spacecraft would be to yield a specific dv, and then compare that to the other engines.

Don't know if this clarifies things a bit.

Also, I agree that there's probably too much information in these charts to be represented clearly. I'm working on a webapp (see below) that computes these charts on the fly with user-customizable parameters, and I'm thinking of several ways to interactively extract information from the analysis.

Also, I misspoke in my request for what I want my rocket to do. I expect it to be staged. A rocket of this nature would be doable, yes? Is it practical? (Heck does practicality even matter in sandbox game?)

Oh, then yes, it's totally doable. In fact, a single-stage ship that departs from low Kerbin orbit, lands on Duna, and returns to Kerbin is probably doable. The LV-N is probably the optimal engine for such a ship.

~~~~~

Alright guys, I wanted to give you a very quick sneak peek of the webapp I'm working on. A word of warning though: this is a very much work in progress at this point. The engine selection is not yet implemented (it's fixed to the LV-N, LV-909 and Mainsail, and there's no color key so you'll have to guess ), and many things can go wrong. But at least the core calculation and part of the UI is done:

http://meithan.x10.mx/KSP/engines/

Whad'ya all think? How fast/slow is it? Does it look good?

Edited May 21, 2015 by Meithan

[John FX]

How fast/slow is it? Does it look good?

It's very fast (on slow/accurate) for me. Looks good. Graph could be bigger on my screen and I'm sure it will make more sense with a key

One thing that would be handy although maybe not possible is to save the graph as an image on my PC.

[GoSlash27]

Oh, that webapp is gonna be very handy indeed! This is above and beyond the call of duty!

Best,

-Slashy

[Meithan]

It's very fast (on slow/accurate) for me. Looks good. Graph could be bigger on my screen and I'm sure it will make more sense with a key

One thing that would be handy although maybe not possible is to save the graph as an image on my PC.

Glad to hear about the speed . I can probably make the calculation faster by saving intermediate results, but if it's fast enough for most users then I'll give low priority to this.

Just added a link to save the generated image as a PNG.

Oh, that webapp is gonna be very handy indeed! This is above and beyond the call of duty!

I have some ideas for interactivity features, just you wait .