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[1.11.x] Cryogenic Engines: Liquid Hydrogen and Methane Rockets! (Mar 9, 2021)


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I can't seem to get stock tanks to not boil off. I have CryoEngines, NearFuturePropoulsion and KerbalAtomics installed as well as ModularFuelTanks, and I switched my (stock) tanks to be LH2 only "Cryogenic" tanks, but then once I am on the launchpad, even with ample Ec reserves (and production) in place, the LH2 boils off, and no power is used. Any ideas? If I use a dedicated LH2 tank, there is no boiloff.

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45 minutes ago, entropy-- said:

I can't seem to get stock tanks to not boil off. I have CryoEngines, NearFuturePropoulsion and KerbalAtomics installed as well as ModularFuelTanks, and I switched my (stock) tanks to be LH2 only "Cryogenic" tanks, but then once I am on the launchpad, even with ample Ec reserves (and production) in place, the LH2 boils off, and no power is used. Any ideas? If I use a dedicated LH2 tank, there is no boiloff.

That's the idea. The dedicated tanks have slightly higher dry masses to simulate the equipment and insulation needed for zero-boil-off. The stock tanks lack that, so they're lighter.

edit: oops. I don't know how MFT is supposed to work with ZBO.

Edited by Bluebottle
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2 hours ago, Bluebottle said:

That's the idea. The dedicated tanks have slightly higher dry masses to simulate the equipment and insulation needed for zero-boil-off. The stock tanks lack that, so they're lighter.

edit: oops. I don't know how MFT is supposed to work with ZBO.

I assumed that's what the "Cryogenic" switch that showed up was for. I think it reduces the volume of the tank (presumably, since now space is taken by cryogenic equipment). Not sure if it does anything to the mass ratios.

I would be fine even slightly poorer mass ratios than the dedicated tanks, since the reason I am using stock tanks is the maximum temperature (1200 degrees for dedicated tanks vs 2000+ for the stock tanks) - yes I need to do aerobreaking and reentry with my hydrogen untouched :)

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10 hours ago, entropy-- said:

So the only way to carry around LH2 without boiloff is in the dedicated tanks?

I ported a patch to add ZBO to Ven's CryoX tanks (from Stock Revamp) so that I could use them for high-temp situations such as aerobraking. It's a few pages back, is now out of date since the latest rebalance, and it was only for Interstellar Fuel Switch anyway. But there's nothing stopping you from doing something similar with Nertea's Cryotanks patch for MFT.

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22 hours ago, entropy-- said:

I can't seem to get stock tanks to not boil off. I have CryoEngines, NearFuturePropoulsion and KerbalAtomics installed as well as ModularFuelTanks, and I switched my (stock) tanks to be LH2 only "Cryogenic" tanks, but then once I am on the launchpad, even with ample Ec reserves (and production) in place, the LH2 boils off, and no power is used. Any ideas? If I use a dedicated LH2 tank, there is no boiloff.

The MFT support probably needs to be adjusted for this. 

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1 hour ago, Koverpk said:

I would love to see the ability to collect H2 gas from boil-off. Saves me some tankage for H2 for my fuel cells. 

I think the point of boil off is that the fuel is escaping. Real life H2 is such a small molecule that it seeps out of any container, even when cryogenically cooled to a liquid. In the game, this provides a bit of extra challenge to compensate for the high efficiency of the engines in this pack.

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On 2/23/2016 at 8:35 PM, Nertea said:

The MFT support probably needs to be adjusted for this. 

I might give this a go later ... for now I have used a huge stack of dedicated LH2 tanks to get more or less the same performance as the previous stock tanks in cryogenic mode, save for the lower temperature resistance of the LH2 tanks (still have to test to see if the ship holds up in an aerobreaking scenario).

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On 28.2.2016 at 8:17 PM, Nightside said:

I think the point of boil off is that the fuel is escaping. Real life H2 is such a small molecule that it seeps out of any container, even when cryogenically cooled to a liquid. In the game, this provides a bit of extra challenge to compensate for the high efficiency of the engines in this pack.

I always assumed boil off has more to do with the fuel heating up, and therefor having to be 'vented' to avoid overpressure by the expanding cryogenic fuel.

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Btw, what's up with the cryo-engines performance on Eve? They only start working around a few kilometer height, and a volcano only reaches 300 ISP around 6.5km. Found those numbers in Kerbal Engineer, but eve on-site testing confirmed the results. Small bug or feature?

Really enjoying the mod. I've been a bit critical of the balancing at the beginning, but the inclusion of boil-off+increased performance really created a cool dynamic! Also meshes similarly well in the Kerbal Atomics pack. So thanks for that! :D

I find it's interesting how the current inclusion of cryogenic engines and boiloff adds a bit more realistic depth to KSP, while still fitting the games' theme, the complexity is very well balanced out by the additional efficience. To me, this is one of the very few feature-heavy mods that could be well implemented into vanilla.

edit: Seems like the nuclear engines have a very similar low eve altitude fall off. Guess the obvious reason would be the thrust curves not being adjusted for below kerbin sea level?

Edited by Temeter
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  • 3 weeks later...
On 3/3/2016 at 5:44 PM, Temeter said:

I always assumed boil off has more to do with the fuel heating up, and therefor having to be 'vented' to avoid overpressure by the expanding cryogenic fuel.

Yes.   ULA is proposing to burn the boiled-off gases in an internal combustion engine to provide power for vehicle systems - see their 2012 paper Development Status of an Integrated Propulsion and Power System for Long Duration Cryogenic Spaceflight

Edited by billkerbinsky
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On 5.3.2016 at 1:15 PM, Temeter said:

Btw, what's up with the cryo-engines performance on Eve? They only start working around a few kilometer height, and a volcano only reaches 300 ISP around 6.5km. Found those numbers in Kerbal Engineer, but eve on-site testing confirmed the results. Small bug or feature?

edit: Seems like the nuclear engines have a very similar low eve altitude fall off. Guess the obvious reason would be the thrust curves not being adjusted for below kerbin sea level?

If the engines lacked Isp curve definitions for below Kerbin sea level, they would work on EVE as if they were on Kerbin sea level. That would be the opposite of what you're observing. :P 

Much more likely: they have an intentionally poor performance in high atmospheric pressures. That is how it works IRL too. High Isp is achieved by having the most lightweight exhaust gas possible; for hydrolox engines, it's largely water vapor and related exotic species, and for NTRs, it's pure hydrogen. Much lighter than CO2 and the various incompletely burned carbon compounds exhausted by kerosene engines.

This lightweight gas can achieve a high exhaust velocity in a vacuum. But for precisely the same reason, it's poor at pushing against atmospheric pressure. Hydrolox engines lose huge amounts of Isp during atmospheric operation. Example: the RD-180, a staged combustion kerolox engine, posts a vacuum Isp of 338s that drops to 311s at sea level. That's 92% of the vacuum value maintained. The RS-25, a staged combustion hydrolox engine, posts a vacuum Isp of 452s which drops to 366s at sea level. That's only 81% of vacuum value maintained, losing more than twice as much as the kerolox engine.

At Eve, you have vastly more than just normal sea level pressure. Hydrolox engines, by their nature poor performers under pressure, are going to cave in dramatically, perhaps to the point of being unable to support proper combustion - due to the nozzle not being sized for this pressure, the exhaust will be strongly underexpanded, leading to flow separation that will kill your performance and can damage the engine. While none of these effects are technically modeled in KSP, reducing the Isp to zero at a value lower than Eve's pressure is a decent stand-in to represent it.

NTRs meanwhile, even though they don't need to worry about stable combustion, certainly won't fare any better. Their exhaust is even lighter, by an order of magnitude, than that of hydrolox engines. And the lack of combustion also leads to much lower internal chamber pressures.

 

On 21.3.2016 at 1:41 AM, Stone Blue said:

@Nertea any chance to get updated Dropbox & SpaceDock links? Pleeze... :D

Updated links will come with the mod update to 1.1, so give it a few weeks. :)

 

Edited by Streetwind
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4 hours ago, Streetwind said:

If the engines lacked Isp curve definitions for below Kerbin sea level, they would work on EVE as if they were on Kerbin sea level. That would be the opposite of what you're observing. :P 

Much more likely: they have an intentionally poor performance in high atmospheric pressures. That is how it works IRL too. High Isp is achieved by having the most lightweight exhaust gas possible; for hydrolox engines, it's largely water vapor and related exotic species, and for NTRs, it's pure hydrogen. Much lighter than CO2 and the various incompletely burned carbon compounds exhausted by kerosene engines.

This lightweight gas can achieve a high exhaust velocity in a vacuum. But for precisely the same reason, it's poor at pushing against atmospheric pressure. Hydrolox engines lose huge amounts of Isp during atmospheric operation. Example: the RD-180, a staged combustion kerolox engine, posts a vacuum Isp of 338s that drops to 311s at sea level. That's 92% of the vacuum value maintained. The RS-25, a staged combustion hydrolox engine, posts a vacuum Isp of 452s which drops to 366s at sea level. That's only 81% of vacuum value maintained, losing more than twice as much as the kerolox engine.

At Eve, you have vastly more than just normal sea level pressure. Hydrolox engines, by their nature poor performers under pressure, are going to cave in dramatically, perhaps to the point of being unable to support proper combustion - due to the nozzle not being sized for this pressure, the exhaust will be strongly underexpanded, leading to flow separation that will kill your performance and can damage the engine. While none of these effects are technically modeled in KSP, reducing the Isp to zero at a value lower than Eve's pressure is a decent stand-in to represent it.

NTRs meanwhile, even though they don't need to worry about stable combustion, certainly won't fare any better. Their exhaust is even lighter, by an order of magnitude, than that of hydrolox engines. And the lack of combustion also leads to much lower internal chamber pressures.

Thanks, so it is most probably a feature.

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So I suddenly started having Module Manager telling me that I was getting 4 mysterious errors related to the patch "CryoTanksFuelTankSwitcher.cfg." Not sure what changed exactly, since I haven't touched it, but looking over the patch, I noticed the first line is "/ Lifting tanks" when I'm guessing it should read "// Lifting tanks" as a comment with two slashes. Making the appropriate change made zero difference in my situation, but I figured I'd pass it along anyway, in the off chance it matters to somebody^_^. Yup.

(I was going to submit it on the github issue tracker for CryoTanks, but it didn't look like that gets used much...or there just aren't any issues:0.0:).

Cheerio.

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On 23.3.2016 at 8:06 AM, Deimos Rast said:

(I was going to submit it on the github issue tracker for CryoTanks, but it didn't look like that gets used much...or there just aren't any issues:0.0:)

The CryoTanks repository is very new. It only was created for 1.0.5 - so there hasn't been any reason for much activity there. You can still use the issue tracker, the 1.1 update will take care of it.

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