Liquid methane as rocket fuel : why so late to the party?

[A1Ch1]

This actually came up on the AROCKET list recently, and it turns out that "bog standard" commercial methane (the stuff you can call and delivered tomorrow) contains a number of nasties that can cause problems in the cooling channels.

Which is why rocket-grade LNG would have to be standardized.

[IGNOBIL]

Liquid methane didn't have advantage over RP-1 or LH2 until two important facts came into play recently: for spacex reusibility is key and LNG has one major advantage over RP-1, no Coke buildup, extending engine life tenfold. Same applies over LH2, which corrodes metals over time, allowing the use of less exotic metals.

Another key feature both for NASA and spacex, isru. Methane is easily obtainable either combining hydrogen (taken onboard or separated from in situ water) with in situ CO2 (Mars) also available naturally on planets and moons (Titan)

 

[MatterBeam]

On 16/03/2015 at 9:10 AM, Streetwind said:

The price of RP-1 + liquid oxygen makes up less than 0.5% of the launch cost of a rocket (which, itself, is only a part of the total cost of the launch, and then there's also the cost of the payload...) Even if liquid methane was ten times as expensive as RP-1 is (and I assure you that it is not, RP-1 is super highly refined and certainly not common kerosene!), its price point would be irrelevant in the grand scale of things.

I can't tell you with certainty why it wasn't adopted earlier, but I can speculate that it was largely a question of practicality. RP-1/LOX not only worked just fine, but in fact did something very specific very well: it delivered high fuel density, allowing the designers to keep the rocket stage small and easy to build. Then, if you wanted a high Isp fuel and density was no longer important, you could go with LH2/LOX. It's the most effective (sane) chemical propellant mix that we have, even today.

Compared to that, liquid methane had no immediate key selling point. You had no reason to pick it over RP-1 if you needed a dense fuel, because it is much less dense. You had no reason to pick it over LH2 if you wanted performance, because its performance is much lower. And yes, it is much better storable long-term than LH2... but for deep space missions, you take hypergolics anyway, because they make the design of a restartable rocket engine a million times easier, and the resulting engine that much more reliable.

The problem with a propellant sitting somewhere in the middle is that it is a jack of all trades, master of none. It is good at many things but not the best at anything, so when rocket scientists looked to use only the best stuff to maximize their efficiency, it got passed over at every opportunity.

Fast forward a couple decades, and you get renewed interest in methane because nowadays, we have computer-aided engine design that allows us to understand the combustion process, and design specific behaviors for specific fuels. We have better materials to build rocket hulls and tanks. Thirty years ago the advantages of methane seemed not worth the extra effort you needed to go to to unlock it, but today we have minimized that effort. Additionally, the scientists today are looking at dfferent things than they did in the past, such as:

- Self-pressurizing tanks, which saves weight and complexity by removing the need to carry helium tanks and pumps and seals and valves and control electronics and software to maintain the pressure. Methane can do that, hypergolics or RP-1 cannot.

- The ability to refuel a rocket in situ, something that just doesn't work with most fuels due to the complexity of the refinement process. Methane on the other hand is super simple to produce, if you cannot just collect it from the environment outright.

- Cost reduction. Mixing a RP-1 first stage with hydrogen upper stages might be performing better, but it requires a more expensive rocket and a more expensive launchpad and a more expensive servicing and launch process. With just one fuel in all stages, and it being less cryogenic than liquid oxygen which is present anyway, you can radically simplify a lot of things, and that might be worth a small hit in performance.

- Special applications. SpaceX wants a higher Isp engine because that higher Isp is critical to avoid losing too much in the way of payload capabilities by saving fuel for a boostback and landing of the first stage. A conventional rocket gains a bit of mass to orbit from every extra second of Isp... but a rocket that needs to reserve some of its fuel for after decoupling gains many times more of an advantage - or more precisely, it sacrifices less in addition to gaining the same benefit that a conventional rocket would. So for SpaceX, the advantages of running on methane are larger than they are for conventional approaches.

To sum up, nowadays we have the know-how and the technology to get an actual advantage out of methane while mitigating its downsides, and we have gained a new appreciation of things only methane can do that designers decades ago weren't even in need of (or aware of). It's a different fuel for a different era.

Thank you for the insightful explanation. I'm out of likes for today, so I'm going to quote and thank you

[fredinno]

On 10/02/2016 at 8:10 PM, IGNOBIL said:

Liquid methane didn't have advantage over RP-1 or LH2 until two important facts came into play recently: for spacex reusibility is key and LNG has one major advantage over RP-1, no Coke buildup, extending engine life tenfold. Same applies over LH2, which corrodes metals over time, allowing the use of less exotic metals.

Another key feature both for NASA and spacex, isru. Methane is easily obtainable either combining hydrogen (taken onboard or separated from in situ water) with in situ CO2 (Mars) also available naturally on planets and moons (Titan)

 

Can't CH4 still cause coking as it still has some carbon?

Also, CH4 is planned for use in expendable LV, like Soyuz V. I think the lack of CH4 use is mainly since LH2 and RP-1 had already had lots of research- CH4 was really in the middle, and lacked a niche that neither could fill. Engine creation is expensive, so CH4 never took off.

 

On 16/03/2015 at 3:40 AM, EzinX said:

So, ok, there is this chart on wikipedia.

If you look, liquid natural gas (liquid methane) has better MJ/kilogram than kerosene but it is not as good as liquid hydrogen. It's in between.

Similarly, it has better MJ/liter than hydrogen, but worse than kerosene.

Similarly, it's harder to store than kerosene but easier than hydrogen. I'm unable to find specific numbers on what dry mass ratios look like, but I would expect intermediate numbers.

So, how come, way back when, they did this : http://www.reactionengines.co.uk/images/saturnv/Satirn-V-1024-cut.jpg

Instead of 2 fuels, why not use an intermediate fuel for the upper and lower stages, getting almost as good a performance, but being able to use the same kind of tanks and the same engines for both rocket stages.

I'm guessing I'm missing a subtlety here, and that it must in fact be worse than either fuel, and the only reason SpaceX wants to use it is because of Mars ISRU, and they want to reuse lower stages easily, and methane doesn't leave residue behind.

Intermediate fuels are a bad idea, since that raises the cost- you need to handle 3 (and possibly 4 if you use a hypergol upper stage) fuels instead of 2.

 

On 16/03/2015 at 9:10 AM, Streetwind said:

The price of RP-1 + liquid oxygen makes up less than 0.5% of the launch cost of a rocket (which, itself, is only a part of the total cost of the launch, and then there's also the cost of the payload...) Even if liquid methane was ten times as expensive as RP-1 is (and I assure you that it is not, RP-1 is super highly refined and certainly not common kerosene!), its price point would be irrelevant in the grand scale of things.

I can't tell you with certainty why it wasn't adopted earlier, but I can speculate that it was largely a question of practicality. RP-1/LOX not only worked just fine, but in fact did something very specific very well: it delivered high fuel density, allowing the designers to keep the rocket stage small and easy to build. Then, if you wanted a high Isp fuel and density was no longer important, you could go with LH2/LOX. It's the most effective (sane) chemical propellant mix that we have, even today.

Compared to that, liquid methane had no immediate key selling point. You had no reason to pick it over RP-1 if you needed a dense fuel, because it is much less dense. You had no reason to pick it over LH2 if you wanted performance, because its performance is much lower. And yes, it is much better storable long-term than LH2... but for deep space missions, you take hypergolics anyway, because they make the design of a restartable rocket engine a million times easier, and the resulting engine that much more reliable.

The problem with a propellant sitting somewhere in the middle is that it is a jack of all trades, master of none. It is good at many things but not the best at anything, so when rocket scientists looked to use only the best stuff to maximize their efficiency, it got passed over at every opportunity.

Fast forward a couple decades, and you get renewed interest in methane because nowadays, we have computer-aided engine design that allows us to understand the combustion process, and design specific behaviors for specific fuels. We have better materials to build rocket hulls and tanks. Thirty years ago the advantages of methane seemed not worth the extra effort you needed to go to to unlock it, but today we have minimized that effort. Additionally, the scientists today are looking at dfferent things than they did in the past, such as:

- Self-pressurizing tanks, which saves weight and complexity by removing the need to carry helium tanks and pumps and seals and valves and control electronics and software to maintain the pressure. Methane can do that, hypergolics or RP-1 cannot.

- The ability to refuel a rocket in situ, something that just doesn't work with most fuels due to the complexity of the refinement process. Methane on the other hand is super simple to produce, if you cannot just collect it from the environment outright.

- Cost reduction. Mixing a RP-1 first stage with hydrogen upper stages might be performing better, but it requires a more expensive rocket and a more expensive launchpad and a more expensive servicing and launch process. With just one fuel in all stages, and it being less cryogenic than liquid oxygen which is present anyway, you can radically simplify a lot of things, and that might be worth a small hit in performance.

- Special applications. SpaceX wants a higher Isp engine because that higher Isp is critical to avoid losing too much in the way of payload capabilities by saving fuel for a boostback and landing of the first stage. A conventional rocket gains a bit of mass to orbit from every extra second of Isp... but a rocket that needs to reserve some of its fuel for after decoupling gains many times more of an advantage - or more precisely, it sacrifices less in addition to gaining the same benefit that a conventional rocket would. So for SpaceX, the advantages of running on methane are larger than they are for conventional approaches.

To sum up, nowadays we have the know-how and the technology to get an actual advantage out of methane while mitigating its downsides, and we have gained a new appreciation of things only methane can do that designers decades ago weren't even in need of (or aware of). It's a different fuel for a different era.

"- The ability to refuel a rocket in situ, something that just doesn't work with most fuels due to the complexity of the refinement process. Methane on the other hand is super simple to produce, if you cannot just collect it from the environment outright."

Unless you're on the Moon. THEN LH2/O2 is better, since Lunar soil is depleted in Co2, and you need the stuff you breath out for crops and plastics.

On asteroids, carrying your own Xenon and refueling in LEO is probably less mass-costly due to low gravity.

The only place CH4 is good for ISRU is Mars (and Venus), TBH.

 

"- Self-pressurizing tanks, which saves weight and complexity by removing the need to carry helium tanks and pumps and seals and valves and control electronics and software to maintain the pressure. Methane can do that, hypergolics or RP-1 cannot."

You still need those pressurizing systems for O2. Also, you can use IVF to remove the need to carry helium, AND remove the need for large batteries (you make your own power via boil-off)

On 16/03/2015 at 0:38 PM, Pipcard said:

Also, SpaceX doesn't want to use hydrolox due to it being hard to handle.

No harder than Deepcryo O2 used for the Full Thrust F9. H2/O2 has been handled extensively since the 60s, you know.

On 17/03/2015 at 2:24 PM, prophet_01 said:

Back on topic: I would imagine that another reason why methane is getting more interesting, is the presence of space stations. Life support systems can use the sabatier reaction to recycle co2 into o2 by using h2. While that is pretty efficient, it also produces methane (ch4) that nobody uses atm ( http://en.m.wikipedia.org/wiki/Sabatier_reaction#/search ). I'm mostly guessing here, since I don't know if you can store methane and o2 efficently enough, but I think it could be a good idea to use that methane as fuel instead dumping it.

Curently almost all maned flights are going to a space station anyway, so it might be efficient to design maned vehicles upper stages to use the excess methane.

That amount is really negligible- unless you have a mars base, then it becomes essential. Moon bases would just use the Co2 for crops and/or store it to make plastics- Luna lacks carbon, unlike Mars, where it's plentiful, and you can produce lots of CH4, as long as you have (relatively light) H2.

 

On 27/03/2015 at 0:52 AM, RuBisCO said:

Both SpaceX and now ULA want to build a methane-lox fueled rocket system. Why? The advantages of methane above RP-1 (purified kerosene) is 4% higher ISP, cheaper, vaporizes, self-pressurizing and ability (theoretically) to work in a closed cycle. The disadvantage is that it is cryogenic and that it has nearly half the density of RP-1. The density disadvantage is frankly crippling for launching from the ground.

Propane would make a better alternative because it can be sub-cooled to LOX temperatures and have nearly RP-1 density, but with 2% greater ISP, it also vaporizes and could be used to self-pressurize and perhaps even operate in a closed cycle. Disadvantages is that it is heavier then air and without odor would be an extreme fire hazard if leaked.

Personally I just don't see the reason to leave RP-1, we have decades of experience and practice with it.

References:

http://settlement.arc.nasa.gov/Nowicki/SPBI1LF.HTM

I'm pretty sure we have even less experience with propane engines...

On 27/03/2015 at 0:54 AM, GreenWolf said:

Two words.

Mars. ISRU.

Technically, it's five words, but that's besides the point.

Like that'll happen in the next 30 years.

On 27/03/2015 at 3:51 AM, Bill Phil said:

I think that a better question would be: Why not?

Lower density, Marginally better performance than RP-1 doesn't really make it worth the investment- unless you are reusing engines.

On 27/03/2015 at 6:19 AM, Kibble said:

Kerosene is a fossil fuel right? If so, eventually we'll want to switch to a different fuel that fills the same category but that is renewable.

There will always be enough oil for rockets due to Unconventional Oil Production- at least for the foreseeable future.

[DerekL1963]

4 hours ago, fredinno said:

Can't CH4 still cause coking as it still has some carbon?

Yes.

4 hours ago, fredinno said:

There will always be enough oil for rockets due to Unconventional Oil Production- at least for the foreseeable future.

Yup.   And before you get to that there's also going to be a longish period during which 'normal' oil extraction will yield a product that's too expensive to use for cars or Happy Meal toys, but not too expensive for specialized uses (like rockets).   Even with reusables and price reductions, the hydrocarbons would have to increase in price considerably (more than tenfold, less than a hundred fold) before they become a cost driver.

[KSK]

17 hours ago, fredinno said:

Can't CH4 still cause coking as it still has some carbon?

Probably, but I wouldn't expect as much and I'd expect buildup to be slower than, for example, with kerosene.

Burning a hydrocarbon molecule is not an instant process. To completely oxidise it to carbon dioxide and water you also need to break all the carbon-carbon bonds in the molecule and this is essentially a random process in which your hydrocarbon chain gets fragmented into smaller and smaller pieces. Those pieces don't necessarily get broken down all the way to single carbon units (and oxidised to CO2), so you end up with hydrocarbon fragments building up on the walls of your burner.

Eventually this shows up as coking and it's a bigger problem for heavier hydrocarbon fuels (more carbon atoms per molecule), which is one reason why diesel, for example, tends to burn dirtier than gasoline. Conversely, it's less of a problem for lighter hydrocarbons - and methane is the lightest possible hydrocarbon.

Edited February 12, 2016 by KSK

[wumpus]

22 hours ago, fredinno said:

You still need those pressurizing systems for O2. Also, you can use IVF to remove the need to carry helium, AND remove the need for large batteries (you make your own power via boil-off)

No harder than Deepcryo O2 used for the Full Thrust F9. H2/O2 has been handled extensively since the 60s, you know.

 

Much like the original plan to use parachutes to return rockets to Earth (and then switched to the less obvious retro-rocket), Space-x originally planned on using LH2/LO2 for the MCT.  For whatever reason (I'd guess boiloff, but LH2 isn't friendly stuff) they gave up and moved on to methane.

To be honest, I doubt we will be able to draw any conclusions from this unless Space-x either switches again (and we learn that methane wasn't the right choice) or builds a third engine using methane (thus proving they really liked it).  Some issues:

* First stage is too big if you stick to all LH2/LO2.  Has anybody ever made a first stage *pure* LH2/O2?  The shuttle left the pad with H2, but was mostly carried at first by those SRBs.  It might just be too big to make.  Everybody that I can think of that used LH2 also used something else as well.  Space-x likes single fuel types: note that they might not think of the shuttle the same way.  SRBs are simply transported (full, heavy, and dangerous) to the launchpad.  Once they are there they don't have to bother with them any more.  I don't think space-x ever considered SRBs, but it would be interesting to know why.

* Boiloff.   A huge issue if you plan on leaving your craft in Earth orbit while its under construction.  Not that LO2 and LCH4 don't have boiloff issues, but LH2 has the most of any material.  Three parasols (light/heat shields) should be enough to block the solar heating.  Expect another shield for earthshine.  Don't ask what sort of insulation effect combination have (Webb will be at L2 and won't have significant earthshine).  Note that even LH2 leaks (through all known materials) and will require a few percent more fuel to cover losses, although I doubt this covers the Isp advantage (but Isp, plus bioff, plus big-honkin-extra-size...)

* cryogenics: I wonder if they started the deepcryo program before or after switching fuels.  They might not have been willing to bet the farm on LH2 cryogenics but might have made a different decision now (and find it too late to profitably change).

[DerekL1963]

3 minutes ago, wumpus said:

First stage is too big if you stick to all LH2/LO2.  Has anybody ever made a first stage *pure* LH2/O2?

Delta IV.

[sevenperforce]

On the subject of liquid methane, would any of you rocketheads like to weigh in here?

[fredinno]

1 hour ago, wumpus said:

Much like the original plan to use parachutes to return rockets to Earth (and then switched to the less obvious retro-rocket), Space-x originally planned on using LH2/LO2 for the MCT.  For whatever reason (I'd guess boiloff, but LH2 isn't friendly stuff) they gave up and moved on to methane.

To be honest, I doubt we will be able to draw any conclusions from this unless Space-x either switches again (and we learn that methane wasn't the right choice) or builds a third engine using methane (thus proving they really liked it).  Some issues:

* First stage is too big if you stick to all LH2/LO2.  Has anybody ever made a first stage *pure* LH2/O2?  The shuttle left the pad with H2, but was mostly carried at first by those SRBs.  It might just be too big to make.  Everybody that I can think of that used LH2 also used something else as well.  Space-x likes single fuel types: note that they might not think of the shuttle the same way.  SRBs are simply transported (full, heavy, and dangerous) to the launchpad.  Once they are there they don't have to bother with them any more.  I don't think space-x ever considered SRBs, but it would be interesting to know why.

* Boiloff.   A huge issue if you plan on leaving your craft in Earth orbit while its under construction.  Not that LO2 and LCH4 don't have boiloff issues, but LH2 has the most of any material.  Three parasols (light/heat shields) should be enough to block the solar heating.  Expect another shield for earthshine.  Don't ask what sort of insulation effect combination have (Webb will be at L2 and won't have significant earthshine).  Note that even LH2 leaks (through all known materials) and will require a few percent more fuel to cover losses, although I doubt this covers the Isp advantage (but Isp, plus bioff, plus big-honkin-extra-size...)

* cryogenics: I wonder if they started the deepcryo program before or after switching fuels.  They might not have been willing to bet the farm on LH2 cryogenics but might have made a different decision now (and find it too late to profitably change).

The Delta IV Medium (no srbs) is "Pure" H2/O2, and it needs a 5m diameter core for a 9T payload. I see what you mean.

[ChemMan]

Hey, why not store them together in the same tank. Roughly same M.P. (methane 90K; Oxy 54K), I'm still checking solubility. Sure, I know you're gonna say "hey, oxidiser and fuel in the same container?", but that's cold, man, and pretty low energy, might take a catalyst to wake them up. 

Silly me, just yer humble protein biochemist thinking out loud

[SomeGuy123]

They tried stuff like that in Ignition!.  Really, really, really, really, really bad idea.  TLDR, as I understand it, various metals can act like a catalyst - igniting a small part of the fuel/oxidizer mixture.  You've created a literal bomb - if the mixture is in the right molar ratio it will literally detonate as if the rocket were a fuel air bomb.  

Edited May 10, 2016 by SomeGuy123

[todofwar]

On Monday, March 16, 2015 at 6:29 AM, K^2 said:

Again, methane freezes solid in boiling oxygen.

As was mentioned, liquid nitrogen is used to insulate liquid helium, and the liquid nitrogen doesn't freeze. Clearly there's an easy way to do this. 

[p1t1o]

5 hours ago, ChemMan said:

Hey, why not store them together in the same tank. Roughly same M.P. (methane 90K; Oxy 54K), I'm still checking solubility. Sure, I know you're gonna say "hey, oxidiser and fuel in the same container?", but that's cold, man, and pretty low energy, might take a catalyst to wake them up. 

Silly me, just yer humble protein biochemist thinking out loud

To quote "Bit" from "Tron": "NoNoNoNoNoNoNoNoNoNoNo"

*edit*

Found this: 

http://pubs.acs.org/doi/pdf/10.1021/je60002a006

Explosive Systems Containing Liquid Oxygen
Liquid Oxyg en-Liquid Methane Mixtures
A. G. STRENG and A. D. KIRSHENBAUM
Research Institute of Temple University, Philadelphia 44, Pa. 

"SENSITIVITY TESTS
The sensitivity to impact, flame, shock wave, and spark was
determined only for the liquid methane-liquid oxygen mixture
which had the fastest rate of detonation (33 mole yo methane
and 67 mole yo oxygen). "

"Explosions were obtained in all these tests. Although they are
far from complete, they show that the liquid CH, + 20, mixture
is very sensitive and can be easily detonated."

 

Edited May 10, 2016 by p1t1o

[ChemMan]

I knew it weren't going going to work.  Those guys at NASA were smart. I'm going to try to build one with separate tanks and catalyst to set  them off

[SomeGuy123]

You know, as a side note, mixtures like this can work if the tank it is in is perfect, with no rusty spots or anything.  Atom by atom exact.  

That's not helpful or a wise plan for the engineering of 2016 but maybe in 2200 they'll be able to make things so exactly that inherently unstable stuff like this will be practical.  Just an observation - things that can't work today because we cannot lay down atom by atom an exact design, consistently and reliably every single time, would work then.  

[Kerbart]

19 minutes ago, SomeGuy123 said:

You know, as a side note, mixtures like this can work if the tank it is in is perfect, with no rusty spots or anything.  Atom by atom exact.  

That's not helpful or a wise plan for the engineering of 2016 but maybe in 2200 they'll be able to make things so exactly that inherently unstable stuff like this will be practical.  (...)

They might be able to produce fuel tanks like that in 2200, I don't think they'll find someone who is insane enough to sign off on it. Putting something in one tank that is going to make the MOAB look like a firecracker, but if everything works to plan it'll be just fine? Part of engineering is building in a safety margin to provide for mishaps. My suspicion is that the safety margin here is why don't we just not do that, ok?

 

[magnemoe]

6 minutes ago, Kerbart said:

They might be able to produce fuel tanks like that in 2200, I don't think they'll find someone who is insane enough to sign off on it. Putting something in one tank that is going to make the MOAB look like a firecracker, but if everything works to plan it'll be just fine? Part of engineering is building in a safety margin to provide for mishaps. My suspicion is that the safety margin here is why don't we just not do that, ok?

 

This, note that the fuel and fuel handling system also has to be atomic clean, you would not notice anything wrong until mixed. 

[kerbiloid]

Isn't LH2/LO2 mix such dangerous too?

[Kerbart]

1 hour ago, magnemoe said:

This, note that the fuel and fuel handling system also has to be atomic clean, you would not notice anything wrong until mixed. 

Although technically speaking, if you're riding that rocket, and anything would go wrong, you wouldn't notice either, as your brain is atomized before you even realize it.

33 minutes ago, kerbiloid said:

Isn't LH2/LO2 mix such dangerous too?

Well, this part of the discussion is about the concept of mixing liquid Methane and O2 in a single tank, as, theoretically, they wouldn't react until the temperature goes up.

This is the "theoretical" kind as in "theoretically, you can drink snake venom and it will exit your digestive system without harm, by nature of being a venom, and not a poison"—best left as a theory.

[magnemoe]

20 minutes ago, Kerbart said:

Although technically speaking, if you're riding that rocket, and anything would go wrong, you wouldn't notice either, as your brain is atomized before you even realize it.

Well, this part of the discussion is about the concept of mixing liquid Methane and O2 in a single tank, as, theoretically, they wouldn't react until the temperature goes up.

This is the "theoretical" kind as in "theoretically, you can drink snake venom and it will exit your digestive system without harm, by nature of being a venom, and not a poison"—best left as a theory.

True

And yes it would be an unstable monopropelant, you also has to make sure it don't ignite in the piping to the engine, have fun getting it trough an turbopump. 
More fun, the fire in the engine might even walk backwards up the piping. 

[Kerbart]

1 minute ago, magnemoe said:

True

And yes it would be an unstable monopropelant, you also has to make sure it don't ignite in the piping to the engine, have fun getting it trough an turbopump. 
More fun, the fire in the engine might even walk backwards up the piping. 

It would also be extremely Kerbal. So now I'm waiting for a mod that has combined LF/LO tank (with some attractive benefits of cours) that offer an increasing chance of Kaboom as time passes...

[kunok]

50 minutes ago, magnemoe said:

venom, and not a poison

Not really relevant, but what is the difference for non-english speakers? In spanish they translate to the same word (is curious the normal thing is that the same word in english is a lot different ones in spanish and not this way)

[kerbiloid]

5 hours ago, kunok said:

Not really relevant, but what is the difference for non-english speakers? In spanish they translate to the same word (is curious the normal thing is that the same word in english is a lot different ones in spanish and not this way)

Not a native speaker (obviously), but venom is a biological excrete mostly of protein nature. It may or may not react with the human biochemical processes, may or may not be neutralized by the immune system.
While poison is a more common word including simple inorganic compounds such as cyanides which stupidly disturb or corrode these processes or just kill the cells. The immune system is useless against inorganics, it's against proteins.

7 hours ago, Kerbart said:

Well, this part of the discussion is about the concept of mixing liquid Methane and O2 in a single tank

Missed this part, thanks.

Edited May 12, 2016 by kerbiloid

[Kryten]

14 hours ago, kunok said:

Not really relevant, but what is the difference for non-english speakers? In spanish they translate to the same word (is curious the normal thing is that the same word in english is a lot different ones in spanish and not this way)

The difference between poison and venom is the context they're used in. As an old saying goes: if it bites you and you die, it's venomous; if you bite it and you die, it's poisonous. 

Edited May 12, 2016 by Kryten