[1.8+] Real Fuels

[GiantTank]

Changed that in the RF stretchy cfg. I saw SRB and under it "max thrust". I changed that to 2k and the thrust still showed up at 1.3MN max, and 52 at part place. So, didnt work but thank you.

Edit: Scratch that, I need a way to make burn time less. Im going from 600 seconds to 500 (biiiiiiig rocket) which isnt enough for 1st stage. Any way to make the minimum burn time lower?

Anyone? Please.

[Raptor831]

Anyone? Please.

Normally I get 30-60 seconds as my minimum burn times. I'm usually in the range of 1.25-2.5m SRBs though, so if you have some massive 5+ meter SRBs, I'm not sure. Also, you generally don't adjust max thrust directly, you adjust it by burn times and solid fuel amounts. Or at least I do.

Logs might be helpful or maybe screenshots to see what's up.

[RocketBlam]

Is there some kind of chart or table that exists to tell me which fuels are best for what application?

[mecki]

Is there some kind of chart or table that exists to tell me which fuels are best for what application?

http://forum.kerbalspaceprogram.com/threads/64118-0-90-Real-Fuels-v8-3-Dec-20-14?p=876665&viewfull=1#post876665

[GiantTank]

Heres a screenie. The tank itself is invisible, its too big and the camera is inside it. I am going for a million tons, but I cant get burn time below 238 seconds. I want a burn time of 180 seconds, what cfg file and what line do i edit?

[Northstar1989]

NathanKell, Starwasher, etc.

Hi guys!

I was doing some research on atmospheric-compression of rocket exhaust-streams for inclusion of a thrust-loss effect on electric thrusters at sea-level in KSP-Interstellar, and it turns out the formula you guys use in RealFuels to simulate the relationship between Specific Impulse and rocket Thrust across different Background Pressures (ambient atmospheric pressures) could use some work as well! The formula that describes the relationship between rocket thrust an ambient atmospheric pressure is:

Thrust = (Vacuum ISP in seconds) * (9.80665 m/s^2) * (Mass Flow Rate) - (Exit Area in m^2) * (Background Pressure)

Vacuum ISP, Mass Flow Rate [equals: (Fuel Flow in Liters) * (Mass per Liter of Fuel)], and Background Pressure are all easily-accessible in KSP. The Exit Area is a result of the thruster-geometry for any given engine, and is simply the size of the exhaust opening for engines without nozzles like electric thrusters... (whereas chemical rockets normally have nozzles to increase the Exhaust Velocity, which has the side-effect of increasing the Exit Area...)

The Exit Area is unique to a given engine/nozzle combination, and generally remains constant throughout the flight (*unless*, like the Saturn V's F-1 Engine, you have a Nozzle Extension that deploys at higher altitudes to increase vacuum ISP; or you have some other variety of Extendable Nozzle).

Notice this formula leads to the following effects:

- Engines with high Vacuum ISP, but low Thrust, suffer comparatively-large losses of thrust at sea-level due to their very poor Mass Flow Rate. This matches the behavior of electric thrusters at low power-levels in the real world well...

- Engines with high Mass Flow, and comparatively small Exit Area (such as a launch-stage chemical rocket engine) suffer a comparatively small loss of thrust at sea-level. This matches the behavior of first-stage chemical engines in the real world well...

- Engines with medium Mass Flow, and comparatively large Exit Area (such as an upper-stage chemical rocket engine) suffer intermediate loss of thrust at sea-level as a fraction of their vacuum thrust. This matches the behavior of upper-stage engines in the real world well...

- Cutting the Mass Flow Rate, without reducing the Exit Area, results in a proportionally-larger loss of Thrust (and thus a lower sea-level ISP). This matches the behavior observed when throttling-down a rocket engine in-atmosphere in the real world well...

The biggest difference between the current formula in RealFuels and the one that applies in real life is that your atmospheric ISP declines when you throttle down a real rocket engine due to a reduction in the Exhaust Pressure (which is proportional to Mass Flow Rate, and factored out of the above equation entirely), whereas (as far as I know) it doesn't in RealFuels...

Regards,

Northstar

Edited February 5, 2015 by Northstar1989

[Raptor831]

http://i.imgur.com/fkTWxSc.png

Heres a screenie. The tank itself is invisible, its too big and the camera is inside it. I am going for a million tons, but I cant get burn time below 238 seconds. I want a burn time of 180 seconds, what cfg file and what line do i edit?

Holy crap! That's probably big enough that either the code is running into some rounding issues, or it may not be possible to have that much fuel burn that quickly without melting the KSC.

[GiantTank]

Holy crap! That's probably big enough that either the code is running into some rounding issues, or it may not be possible to have that much fuel burn that quickly without melting the KSC.

Hmmm... Ill change the diameter higher later tommorow, maybe that will help. Thanks! my computer is on life support right now lol

[lurkoholic]

....

The Exit Area is unique to a given engine/nozzle combination, and generally remains constant throughout the flight (*unless*, like the Saturn V's F-1 Engine, you have a Nozzle Extension that deploys at higher altitudes to increase vacuum ISP; or you have some other variety of Extendable Nozzle).

Notice this formula leads to the following effects:

- Engines with high Vacuum ISP, but low Thrust, suffer comparatively-large losses of thrust at sea-level due to their very poor Mass Flow Rate. This matches the behavior of electric thrusters at low power-levels in the real world well...

...

Northstar,

Do you have a good source showing the F-1 extendable nozzle? I thought the nozzle extension was just "bolted on" below the nozzle, and was the section that wasn't cooled by fuel lines.

Would engines like the LV-909 have worse Isp SL using this formula? I think it used to have something like Isp SL = 100 seconds.

Would it be difficult to vary Isp based on throttle position? This aerospike study suggest that it could have slightly increased Isp when throttled in vacuum: http://enu.kz/repository/2010/AIAA-2010-7060.pdf

Edited February 5, 2015 by lurkoholic
dug through archive, corrected Isp #

[NathanKell]

Northstar1989: you've described a formula for calculating Isp at sea level. However, in KSP, Isp at sea level is given via the atmosphereCurve, so we don't have to calculate it.

You're right, however, that throttling should lead to considerable losses in specific impulse in atmosphere (due to decreasing chamber pressure--one throttles by reducing chamber pressure, which doesn't matter that much in vacuum but matters a ton in atmosphere), and I plan at some point to address that issue in RF, decreasing specific impulse as needed due to throttling.

I highly recommend you try out Rocket Propulsion Analysis for a better feel for this stuff, the lite version is free.

Also, no, the F-1 did not have a deployable nozzle extension. You're confusing "nozzle extension" (which the F-1, along with pretty much all engines, had) with "deployable nozzle extension" (which is very recent and most famously used on the RL10B-2, though the idea of such a thing was current in the 1960s...).

Edited February 5, 2015 by NathanKell

[Northstar1989]

@NathanKell

The point was to not only suggest use of a formula that corrected for throttling in-atmosphere, but also was set up to easily be a adapted to any mods that introduced deployable extended nozzles in the future...

You're right about the F-1 (I think). I was getting a little confused from reading up no the Sea Dragon, which was proposed in the late 1960's (deployable extensible nozzles are *NOT* as new as you think) and unlike the Saturn V *DID* feature a deployable extensible nozzle on both the launch and upper stages... (the upper stage had a nozzle that would deploy to larger than the diameter of the launch stage, in fact!) Apparently, they decided a pressure-fed stage with a deployable nozzle was simpler and therefore cheaper (which was the whole point of the Sea Dragon, as a Big Dumb Booster) than a turbopump-fed rocket stage with comparable performance...

Anyways, the F-1 engine wasn't really the focus of the discussion at hand... The formula I suggested *WAS*...

The formula I suggested corrects for throttling. It corrects for deployable nozzles. *AND* if you come up with a good way to code it into KSP, we can re-use it for KSP-Interstellar for the electric and thermal rocket engines there (which utilize different engine modules than normal/stock rocket engines), which *BADLY* need a realism-pass to reflect the differences in sea-level performance of the different engines based on their Specific Impulse and Mass Flow Rates (it's possible in KSP-Interstellar to power a GW-scale plasma thruster with Microwave Beamed Power in-atmosphere, for instance- and at Mass Flow Rates that high, even an electric thruster gets good performance at sea-level due to the incredibly high Exhaust Pressure that results... Also, a Microwave Thermal Rocket utilizing Nitrogen or Carbon Dioxide should suffer from a *LOT* less atmospheric-compression that one utilizing Liquid Hydrogen, due to the much higher Mass Flow Rate involved- whereas right now *ALL* Thermal Rockets in KSP-Interstellar lose 40% of their ISP at sea-level regardless of propellant-choice, Mass Flow Rate, or Vacuum Specific Impulse...)

Regards,

Northstar

Edited February 5, 2015 by Northstar1989

[NathanKell]

though the idea of such a thing was current in the 1960s...

So yes, I am aware of the Sea Dragon plans (it wasn't even the only notional LV with a deployable extension dreamed up in the 60s...)

As I'm not involved in writing engine configs for RF itself (as the OP and second post say, one needs an engine pack) RF itself will not be ignoring specified Isp at sea level. If you find an engine with a crazy Isp at sea level, I do suggest you take it up with whomever is making the RF config for that engine. Realism Overhaul, since we model real engines (or prototyped engines, at least), for which there is existing data, again does not need to compute Isp at sea level, we have it from data. As I mentioned above, throttling *does* need to decrease Isp (when in vacuum but especially when in atmosphere), and that will get solved, but the solution is not going to flagrantly ignore already-known sea level Isp numbers.

I appreciate your work digging this stuff out, and it looks very helpful for people trying to write configs for engines for which there *isn't* real data, however.

[Northstar1989]

@NathanKell

If you configure the Exit Area for an engine right, it should perfectly reproduce the sea-level ISP of any real engine. It's just a matter of determining the right Exit Area (which the simplest solution is to geometrically compute, but for a 100% accurate solution you have to make some corrections for things like boundary-layer separation, so you can always tweak it for a given engine until the performance at full-throttle matches the real thing...)

The the equation I posted above is the one that describes real engine performance at different ambient pressures and throttle settings in real life, so as a rule it *HAS TO* reproduce the real performance, unless there are major deviations like, as I said, boundary-layer separation, or drastic over-expansion of the exhaust stream...

Regards,

Northstar

[NonWonderDog]

@NathanKell

If you configure the Exit Area for an engine right, it should perfectly reproduce the sea-level ISP of any real engine. It's just a matter of determining the right Exit Area (which the simplest solution is to geometrically compute, but for a 100% accurate solution you have to make some corrections for things like boundary-layer separation, so you can always tweak it for a given engine until the performance at full-throttle matches the real thing...)

The the equation I posted above is the one that describes real engine performance at different ambient pressures and throttle settings in real life, so as a rule it *HAS TO* reproduce the real performance, unless there are major deviations like, as I said, boundary-layer separation, or drastic over-expansion of the exhaust stream...

Regards,

Northstar

Your equation assumes 100% nozzle efficiency. Real nozzles don't have 100% efficiency, even without anything dramatic. And for any vacuum nozzle, you will have something dramatic (I think that's the equation that says most vacuum engines have negative thrust at sea level).

You can put an efficiency term in there (really you need to define things in terms of pressure recovery first, but whatever), but then you have to know the nozzle efficiency of each engine you want to simulate. You're not going to find that data. The best you can do is estimate it from the chamber pressure, mixture ratio, and expansion ratio, given the published Isp. Or I guess you can assume it's an 80% cone rao optimum (about 97.8% efficient), which is pretty close for most US-built engines. But not Russian engines -- they follow a different profile, and are maybe (maybe not) more efficient.

These details don't really matter for KSP, but you can't "perfectly reproduce" measured performance with that equation.

Edited February 6, 2015 by NonWonderDog

[Shurikeeen]

Is there a way to make deadly re-entry's ablative shielding a resource and have it shield my producial wings what can carry resources?

[Abdu]

Sorry if this is stupid but after installing RF in RSS TweakScale stopped working in engines, everything else seems to be ok.

is this normal if yes sorry.

[Raptor831]

Is there a way to make deadly re-entry's ablative shielding a resource and have it shield my producial wings what can carry resources?

Only procedural wing I know of that can carry fuels is B9's, and those already have the "passive" heatshield (and a darn good one at that). You might be able to get away with just adding the AblativeShielding to the tank definition(s) via ModuleManager. Or create a new one and add it to the typeAvailable in the wing part.

Sorry if this is stupid but after installing RF in RSS TweakScale stopped working in engines, everything else seems to be ok.

is this normal if yes sorry.

Yes, it's normal. The engines in RF are set up very carefully (both in configs and in the plugin code), and having them use TweakScale would throw all kinds of craziness into the mix.

Edited February 7, 2015 by Raptor831
Answering the next question...

[Abdu]

Yes, it's normal. The engines in RF are set up very carefully (both in configs and in the plugin code), and having them use TweakScale would throw all kinds of craziness into the mix.

Thank you !!

[Starwaster]

Is there a way to make deadly re-entry's ablative shielding a resource and have it shield my producial wings what can carry resources?

It IS a resource. Look at how DRE heat shields are set up.

As mentioned though by someone else, most wings have builtin shields that are non-ablative.

That's really the way to go too IMO because if you're putting ablative on the wing, it's going to be heavier which means greater ballistic coefficient. (unless you're playing stock drag where 95% of all parts are assigned the same drag value)

[Northstar1989]

@NathanKell, Starwahser, NonWonderDog

After considerable work to simplify the equation I posted before, I have come up with the following- which is mathematically equivalent, but should be much simpler to program/code and understand...

Atmospheric Thrust = Vacuum Thrust - (Exit Area Factor * Background Pressure)

The units of Exit Area Factor are in kN/atm, and the Exit Area Factor for a given nozzle is obtained by finding the physical surface area of an engine exposed to thrust in square-meters, and multiplying it by 101.325 (as "1 atm = 101.325 kPa"). The units are chosen as-such in order to reduce the complexity of code KSP has to run- as KSP stores atmospheric pressure as fractional Kerbin sea-level pressure (where 101.325 kPa = 1 atm). This way KPS won't have to perform a unit-conversion every time it wants to calculate an engine's thrust in-atmosphere!

As for your concerns about Nozzle Efficiency before, NonWonderDog- this can already *easily* be reflected in the numbers plugged into the formula. For a rocket with a Nozzle Efficiency less than 100%, you just plug an Exit Area Factor into the equation smaller than the physical size of the nozzle in square-meters * 101.325. This is a stealthy/clever way of reflecting that the Exhaust Pressure is actually higher than you would expect if you looked purely at the size of the exhaust nozzle and the amount of Thrust the engine produces in vacuum... Solution also works for intentionally highly over-expanded nozzles! (where the nozzle size/shape is actually used to SHIELD the exhaust-stream from some amount of atmospheric-compression)

Remember that the Exit Area Factor here is *ONLY* used to calculate atmospheric-compression. This is *COMPLETELY* separate from the effects of exhaust nozzles on increasing Exhaust Velocity at the expense of Exhaust Pressure- which should already be reflected in the Vacuum ISP values chosen for that engine...

At *extremely* low Thrust and Exhaust Pressure values (where predicted atmospheric thrust starts to approach zero) this equation breaks down and starts under-estimate thrust production. However the only time this should be a problem in KSP is if you throttle an engine down to *EXTREMELY* low throttle values- which isn't possible in real life anyways, and in which case your engine really ought not to be functioning in the first place (the only "deep-throttling" engines in real life are built for use on landers..)

Last, but not least, this equation has the beneficial result of solving not one but TWO of your problems with how RealFuels currently calculates atmospheric thrust!

(1) Atmospheric Thrust doesn't bottom-out at Kerbin sea-level pressure with this equation! Now your Eve landers will continue to lose thrust as they descend below the pressure = 101.325 kPa line!

(2) Throttle setting now affects sea-level ISP like in real life! Throttle down your engine and watch as your Thrust evaporates even faster than expected (if you actually were foolish enough to think 50% throttle would still produce 50% thrust in-atmosphere...)

Regards,

Northstar

Edited February 8, 2015 by Northstar1989

[junkie_business]

Sorry Nathan, you made RSS/RO so much more reliable and easier to use, it's tempted me to get involved again, I'd love to bring some of RaiderNick's pieces into the RSS.

I've been fiddling with engine configs and I either forgot or didn't ever know what to do with engine configs. Say I'm trying to RO'ize RN's Fregat, how do these numbers relate to what the game does by the time it's on the pad?

{

name = ModuleEngines

thrustVectorTransformName = thrustTransform

exhaustDamage = True

ignitionThreshold = 0.1

minThrust = 0

maxThrust = 45

heatProduction = 300

fxOffset = 0, 0, 0

PROPELLANT

{

name = LiquidFuel

ratio = 0.9

DrawGauge = True

}

PROPELLANT

{

name = Oxidizer

ratio = 1.1

}

atmosphereCurve

{

key = 0 359

key = 1 230

}

}

What units are the 'ratio' measuring in, mass isn't it? Does it work out fuel consumption on its own via the Isp?

I hate to ask but there doesn't seem to be a guide for this anywhere. Keep up the good work my man.

[Starwaster]

Sorry Nathan, you made RSS/RO so much more reliable and easier to use, it's tempted me to get involved again, I'd love to bring some of RaiderNick's pieces into the RSS.

I've been fiddling with engine configs and I either forgot or didn't ever know what to do with engine configs. Say I'm trying to RO'ize RN's Fregat, how do these numbers relate to what the game does by the time it's on the pad?

What units are the 'ratio' measuring in, mass isn't it? Does it work out fuel consumption on its own via the Isp?

I hate to ask but there doesn't seem to be a guide for this anywhere. Keep up the good work my man.

Ratio in this context is volume. So you need to convert mass ratio of F/O to volume. (Liters in RF /RO)

and be yes mass flow will work itself out mostly if you get the right Isp in there. Thrust in the config is vacuum thrust so when working out real world conversions make sure the thrust rating isn't SL. Which for first stage lifters it may well be. (Ie F1 at 33,000 kN being SL)

Edited February 8, 2015 by Starwaster

[Northstar1989]

Nathan, Starwasher, any thoughts on switching RealFuels over to the formula I suggested before?

Regards,

Northstar

[junkie_business]

Ratio in this context is volume. So you need to convert mass ratio of F/O to volume. (Liters in RF /RO)

and be yes mass flow will work itself out mostly if you get the right Isp in there. Thrust in the config is vacuum thrust so when working out real world conversions make sure the thrust rating isn't SL. Which for first stage lifters it may well be. (Ie F1 at 33,000 kN being SL)

Thanks for the reply. I've been playing around and things seem to be working out, but how do I find those ratios if I don't know the IRL flow rates? I could do it for F1 because it's well documented, but I'm having trouble finding information for other engines, or I can only find the total F+O flow, as with the Fregat engine.

[Agathorn]

Thanks for the reply. I've been playing around and things seem to be working out, but how do I find those ratios if I don't know the IRL flow rates? I could do it for F1 because it's well documented, but I'm having trouble finding information for other engines, or I can only find the total F+O flow, as with the Fregat engine.

The .xls file in post #2 has a calculator