Liquid fuel mix ratio

[Paul Kingtiger]

In game fuel and oxidizer are mixed in a ratio of 1.2 (1.1 part oxidizer to .9 fuel).

In real world rockets the mix is about 2.5 for RP-1 (Kerosene) and 5 for Hydrogen.

So my question is, is there a reason the 1.2 value was used and are there any plans to revisit this later and change to a more real world mix ratio?

The question is, is there any value to the game to do so? I think there is. Firstly you don't have the invent anything new, the mixtures for engines and tanks can already be changed and secondly I see a benefit to playability when resource collections comes along. Finally it will increase the educational value of the game by giving players a closer approximation to real world rocketry without making the game any less fun.

Here is my suggestion.

First up, I'm sure at some point in the future you'll be running a balance pass on all the parts. At that point reconfigure the liquid engines to require the new ratio of 2, it's a nice clean number that is closer to the real world and has the advantage of needing double the oxidizer (easy numbers to work with). Then change the fuel tank parts to match the new ratio.

As a more advance change, link the fuel into the research system, so that LH/LOX equivalent engines and tanks can be researched (either are new parts or changes to existing parts) that would have the 5 ratio and an increase in ISP and cost to represent the more efficient fuel and the complexity of storing it. If and when teakables come in you can use the research to unlock the tweakable option to change to the more advanced engine and different ratio in the tanks.

[woppeldopple]

I support this.

This would be a good reason to introduce units, too!

90 "fuel" doesnt tell me anything! ^^

[Themohawkninja]

Higher fuel ratios mean less dV in rocket designs, and increasing ISP just makes the parts that much less realistic (the ISP's of the engines are the real life ISP's of the engines).

90 "fuel" doesnt tell me anything! ^^

You are given the amount of fuel burned per second, and knowing how much fuel you have gives you a rough idea on what you can do with the craft.

[woppeldopple]

You are given the amount of fuel burned per second, and knowing how much fuel you have gives you a rough idea on what you can do with the craft.

Oh really? Some peoples' profession tell them to be precise, you know?

[Themohawkninja]

Oh really? Some peoples' profession tell them to be precise, you know?

True, but if you know how much you burn per second, and how much fuel you have right now, and you know your current weight, and empty weight via the map, you can precisely solve for dV, so 90 "fuel" (which is Liters btw) can tell you quite a bit.

[woppeldopple]

Your answer is completely off-topic so i take the relevant part if you mind:

which is Liters btw

How do you know? Currently i dont have KSP access but last time i looked i did not find such an ingame information at least in part description.

(And this is what i am talking about btw. Feel free to join or go on talking about very obvious things )

[Themohawkninja]

Your answer is completely off-topic so i take the relevant part if you mind:

How do you know? Currently i dont have KSP access but last time i looked i did not find such an ingame information at least in part description.

(And this is what i am talking about btw. Feel free to join or go on talking about very obvious things )

Than what precise information did you want? I assumed that you wanted precise information where I stated you could get rough information. I'd also hesitate to call the rocket equation obvious, but then again as far as this community is concerned, it probably is. I'm very certain it's liters simply because it's the SI unit for volume, why would they use gallons if they use metric tonnes, and well... that's what everyone else says it is, and no dev has debunked that from what I have seen?

[Paul Kingtiger]

Higher fuel ratios mean less dV in rocket designs, and increasing ISP just makes the parts that much less realistic (the ISP's of the engines are the real life ISP's of the engines)..

Sort of. The ISP ranges are close to real life for RP-1 and requireing you to carry more oxidizer would reduce the dV for a given rocket. But LOX is considerably lighter than RP-1 so the dV loss isn't massive. The much larger ratio of LH/LOX is more than made up for by the lightness of LH and the higher ISP.

By using real world values you've got a huge amount of data available to help design and balance parts in game. It's a fair assumption to make that space agencies use the ratios and designs they do because they work best.

[Themohawkninja]

Sort of. The ISP ranges are close to real life for RP-1 and requireing you to carry more oxidizer would reduce the dV for a given rocket. But LOX is considerably lighter than RP-1 so the dV loss isn't massive. The much larger ratio of LH/LOX is more than made up for by the lightness of LH and the higher ISP.

Touche. I can't say I can think of how much less the dV would be given the fact that hydrogen is so light.

By using real world values you've got a huge amount of data available to help design and balance parts in game. It's a fair assumption to make that space agencies use the ratios and designs they do because they work best.

Part of the problem with your real life point is that the fuels in-game are based off of fictional materials, and plus, the entire Kerbol system is scaled down by a factor of 11, so real-world ratio does not necessary mean real-world performance (let alone the over-simplified aerodynamics engine).

[woppeldopple]

Oh dear. Units are mandatory. It belongs to the number like the verb in a sentense. Whats your problem at all?

[Themohawkninja]

Oh dear. Units are mandatory. It belongs to the number like the verb in a sentense. Whats your problem at all?

For doing certain equations, they are quite mandatory.

I don't have much of a problem at all, I just see reason to critique the OP's post, and as any suggestion on any forum should go, if a user finds reason to counter the points of the OP's suggestion to show that it doesn't need to, or shouldn't be put into the game, than said user should point out the problems with the post.

[woppeldopple]

I'm not the OP and me asking for units (an absolutely trivial wish) is not the topic and no reason to debate absolutely pointless. I end this now ...

@Topic:

For me one single reason is enough to introduce real world ratio: Education.

Edited July 31, 2013 by woppeldopple

[Themohawkninja]

I'm not the OP and me asking for units (an absolutely trivial wish) is not the topic and no reason to debate absolutely pointless. I end this now ...

@Topic:

For me one single reason is enough to introduce real world ratio: Education.

There was no debate on that in the first place.

Okay, let's try and figure out the math here. A higher ratio means less dV, but the OP is also suggestion real world density values which, since IRL densities may be lighter than in game, could negate that.

LH2 = .07 relative, or .07 kg/L, or .00007 T/L (we don't know what KSP uses for density values, so I am putting up three possibilities).

LiquidFuel = .005

LH2 might actually be denser by about a factor of 10, or lighter by a factor of about 100.

[woppeldopple]

What about adapting fuel consumption of the engines and tank sizes to dV = const?

[Themohawkninja]

What about adapting fuel consumption of the engines and tank sizes to dV = const?

What do you mean by dV=const?

[woppeldopple]

What do you mean by dV=const?

Your rocket provides the same dV before and after the ratio modification.

I named the variables you can adapt to reach this goal.

What else should it mean?

[Themohawkninja]

Your rocket provides the same dV before and after the ratio modification.

I named the variables you can adapt to reach this goal.

What else should it mean?

That would mean changing the ISP's from real-world values, to values that now suit real-world ratios. It doesn't seem like you can have both real-world aspects of the engines.

[Paul Kingtiger]

Part of the problem with your real life point is that the fuels in-game are based off of fictional materials, and plus, the entire Kerbol system is scaled down by a factor of 11, so real-world ratio does not necessary mean real-world performance (let alone the over-simplified aerodynamics engine).

I have to assume that even though Kerbal rocket fuel is fictional it is based on real world fuels. I've looking into Hypergolics a bit just in case that gave more kerbal numbers but the N2O2 / A50 mix used in the Titan booster still has a oxidizer ratio of 2. I've struggled to find anything that is 1.2 of KSP. Doesn't mean that you can't have that ratio, the game won't suffer from it. But I do think the game would benefit from something closer to real life (I have read a lot of books since starting KSP for ideas and inspiration as well as fun, would be cool if I could apply those ideas in game).

For the scale factor, the ratio shouldn't effect the performance, just the values. So we would expect craft in game to be proportionally smaller and less powerful to match the game scale, and I'm totally fine with that. But if we assume the engines are smaller and generate less thrust and other parts are proportionally smaller than the real world, then things like the fuel mix, fuel density and ISP can be the same as real world values.

[Themohawkninja]

I have to assume that even though Kerbal rocket fuel is fictional it is based on real world fuels. I've looking into Hypergolics a bit just in case that gave more kerbal numbers but the N2O2 / A50 mix used in the Titan booster still has a oxidizer ratio of 2. I've struggled to find anything that is 1.2 of KSP. Doesn't mean that you can't have that ratio, the game won't suffer from it. But I do think the game would benefit from something closer to real life (I have read a lot of books since starting KSP for ideas and inspiration as well as fun, would be cool if I could apply those ideas in game).

For the scale factor, the ratio shouldn't effect the performance, just the values. So we would expect craft in game to be proportionally smaller and less powerful to match the game scale, and I'm totally fine with that. But if we assume the engines are smaller and generate less thrust and other parts are proportionally smaller than the real world, then things like the fuel mix, fuel density and ISP can be the same as real world values.

While that is all true, it seems to me that to really get an idea on how much less the thrust provided would be required, a lot of math would have to be done in order to take into account, real fuel-to-oxidizer ratios, real ISP's, real densities, and so forth, and compare a the dV of a rocket design with these values, with those already in KSP.

[Scoundrel]

I personally would be happy if they simply tweaked the ratio to be 1.2:0.8, though I agree that a 2:1 ratio would be cleaner. And as for unit types, the fuel is in fact measured in litres, though I personally would have measured it in mass, but whatever.

[Paul Kingtiger]

You can make a lot of assumptions to take shortcuts and still get something good enough.

Lets start with an approximation of a RP-1/LOX engine from the 60s.

ISP: The maximum ISP for PR-1 on paper is ~353. In reality it's lower due to atmosphere and the engine not being 100% efficient.

So you assign each engine an efficiency rating in a vacuum, lets say that our engine is .9 that means the vacuum ISP of the engine running RP-1/LOX is 317

Next you work out how much less efficient the engine is at sea level, in this case lets say .75 which gives is an ISP SL of 264

Next we need a fuel and oxidizer ratio, to keep things simple and pleasing to the eye I'm going to approximate the real world value of 2.5 to simply 2. You need twice as much oxidizer as fuel.

So we have to change our fuel tanks from a 1.2 - 2 ratio of oxidizer to fuel, that's easy. Working out the weight is also easy just use real world values.

To balance the whole thing you adjust the power/flow rate of the engine and the size of the tanks until you have what you consider the minimum rocket to achieve orbit.

it's a fair bit of work, but once you've set one engine you should be able to adjust the others using the same calculation. (at this point I'm getting out of my depth as a game designer or a physicist, maybe someone else can comment?)

[Themohawkninja]

You can make a lot of assumptions to take shortcuts and still get something good enough.

Lets start with an approximation of a RP-1/LOX engine from the 60s.

ISP: The maximum ISP for PR-1 on paper is ~353. In reality it's lower due to atmosphere and the engine not being 100% efficient.

So you assign each engine an efficiency rating in a vacuum, lets say that our engine is .9 that means the vacuum ISP of the engine running RP-1/LOX is 317

Next you work out how much less efficient the engine is at sea level, in this case lets say .75 which gives is an ISP SL of 264

Next we need a fuel and oxidizer ratio, to keep things simple and pleasing to the eye I'm going to approximate the real world value of 2.5 to simply 2. You need twice as much oxidizer as fuel.

So we have to change our fuel tanks from a 1.2 - 2 ratio of oxidizer to fuel, that's easy. Working out the weight is also easy just use real world values.

To balance the whole thing you adjust the power/flow rate of the engine and the size of the tanks until you have what you consider the minimum rocket to achieve orbit.

it's a fair bit of work, but once you've set one engine you should be able to adjust the others using the same calculation. (at this point I'm getting out of my depth as a game designer or a physicist, maybe someone else can comment?)

But isn't the flow rate at least partially dependent on ISP, and therefore altering the flowrate, alters the ISP?

[Paul Kingtiger]

I don't think so, ISP is how many seconds 1 unit of fuel can generate 1 unit of thrust.

Flow rate would affect how many units of fuel you can push through the engine per second and so affect your total thrust.

So flow rate is your throttle and that doesn't change the ISP*

*In real life I guess each engine has a peak efficiency at a set throttle rating, which would affect the ISP. But we can ignore that for KSP and assume engines are equally efficient at all throttle settings (which is how they work right now).

[m4v]

heh, dunno. It isn't like Kerbin has the same size of Earth or that KSP is following real life ratios and values closely. The game is never too specific about the fuels used, it calls it "liquid fuel" and "oxidizer", you can say that liquid fuel is closer to RP-1 but the game never states it, and in the resources chart (albeit outdated now) showed that you could get liquid fuel from water, so it is RP-1 or hydrogen? The game might use RP-1 as a reference, but really it isn't RP-1.

In the end, what ratio the game uses should be decided with gameplay reasons first and real life rockets second. Not saying that the 1.1/0.9 ratio is fine though, I would rather have a more round number but either case I don't mind.