FFSC engines using fuels more optimal than methane

[RFspace]

I assume one of the major influences to SpaceX's decision to use methane was manufacturability on Mars, in addition to it being a generally high performing choice for FFSC. But if a company were to design a FFSC rocket engine for just earth orbits, would methane still be the best choice? Or not even FFSC in particular, it seems like a lot of companies are wanting to use methane for more common cycles.

 I'm essentially talking about other few carbon fuels. Various alcohols like methanol, ethanol, isopropanol, (Alcohols are interesting. Water soluble. They have a room temp advantage. I also heard that they deal with sooting relatively well, since they have an oxygen group, cant confirm that though.) Short chain hydrocarbons, thinking a lot about propane due to its extremely high thermal contraction, it can gain 12.3% density as a liquid just by cooling. Compare that to methane <1%.
I guess the big question is coking. Would these just produce lots of soot in the fuel rich burner? Also performance factors.

[redneck]

The fuels you listed would have a lower Isp for the most part. The question would be if the handling properties and development cycles would make it worthwhile. the A4 (V2) flew 80 years ago on alcohol/LOX. The first methane to orbit flew last year.

I doubt long term advantage over methane but am willing to be proved wrong. There would seem to be possibilities in getting a short development cycle and cost for entry level companies.

P.S. Welcome to the forum.

[sdsds]

The question was about the choice of fuel, but don't forget the choice of oxidizer.

Liquid fluorine.... Safety becomes an issue. "Under proper conditions, fluorine reacts with practically every element or compound except the inert gases."
http://www.astronautix.com/l/lf2.html

[JayWee]

I assume one of the major influences to SpaceX's decision to use methane was manufacturability on Mars, in addition to it being a generally high performing choice for FFSC. But if a company were to design a FFSC rocket engine for just earth orbits, would methane still be the best choice? Or not even FFSC in particular, it seems like a lot of companies are wanting to use methane for more common cycles.

There's quite bit of a discussion in this thread:
Should have created a Hydrolox engine for the second stage, SpaceX?
https://forum.nasaspaceflight.com/index.php?topic=57489.0

[rpapo]

Should have created a Hydrolox engine for the second stage, SpaceX?

That would help except for one thing: refilling the fuel tanks after having landed on Mars.  Starship is not an optimal second stage for any other purpose.  But then again, SpaceX has never been a subscriber to the Formula One approach to designing and operating rockets where everything has to be absolutely perfect, no matter the cost.

Perfect is the enemy of good enough.

[JayWee]

Should have created a Hydrolox engine for the second stage, SpaceX?

That would help except for one thing: refilling the fuel tanks after having landed on Mars.  Starship is not an optimal second stage for any other purpose.  But then again, SpaceX has never been a subscriber to the Formula One approach to designing and operating rockets where everything has to be absolutely perfect, no matter the cost.

Perfect is the enemy of good enough.

That was the name of the thread. I edited the post to avoid confusion as I strongly disagree with the idea too

[ZachF]

FFSC and methane fuel synergize pretty well…. Honestly methane is kinda crap unless you use staged combustion and fuel densification, then it becomes great.

[Robotbeat]

There’s also the fact that, besides the slight Isp advantage, lack of coking, etc, it’s also the cheapest source of energy in the world right now.

US natural gas is like $3/MMBTU or 1¢/kWh-thermal right now. Even seaborne coal is around $100/tonne, or 1.4¢/kWh-thermal. Refined oil products and natural gas liquids like propane and ethane are more expensive. In my state, propane is like $36/MMBTU or higher, similar to highway gasoline or diesel or jet fuel.
That works out to around $1700/tonne for propane at bulk rural prices. (Granted, prices are like half that for spot prices).
Natural gas is like $150-200/tonne plus has slightly higher energy content and Isp. Granted, that’s not the liquefied shipborne price, which can be double (but not if you do it yourself).

So for a booster fuel, methane has a price advantage that becomes significant if you want to get launch prices below $10/kg. Starship requires about 500MJ/kg of fuel energy to get stuff to orbit. Propane fuel cost alone would make it cost more than that ($17/(500MJ) for residential propane, just $1.5/(500MJ) for natural gas or methane). Plus methane is easier to synthesize with clean electricity (solar, wind, or nuclear), whether on Earth or Mars.

If you want $10/kgLEO or lower, you almost HAVE to use methane.

(It’s possible to improve the energy efficiency of orbital launch to maybe 200-300MJ/kg, even better if you use mechanical launch assist.)

And if you’re using methane for first stage, makes sense to use it for the upper stage as well, at least at first.

Rule of thumb: 1GJ=1000MJ= ~1MMBTU. Also, 2kg to LEO per GJ or MMBTU. So to find the fuel costs to LEO, just divide price per MMBTU in half.

methane is 56MJ/kg, propane is 49MJ/kg, hydrogen is 142MJ/kg.

[deltaV]

There’s also the fact that, besides the slight Isp advantage, lack of coking, etc, it’s also the cheapest source of energy in the world right now.

1. Climate change legislation may eventually force rockets to be carbon neutral. This is likely to make methane much more expensive than it is today and may make hydrogen, which is probably easier to make from (carbon neutral) electricity, or ethanol, which is easy to make biologically, the cheapest propellant.

2. There are lots of ways to make first stages more energy efficient. Some will presumably be implemented once launch costs drop enough for propellant costs to be a big deal. For example something air breathing, a tri-propellant rocket using liquid oxygen, whatever fuel is cheapest per Joule, and water, or something using a mega structure (https://en.wikipedia.org/wiki/Non-rocket_spacelaunch).

[Greg Hullender]

There’s also the fact that, besides the slight Isp advantage, lack of coking, etc, it’s also the cheapest source of energy in the world right now.

1. Climate change legislation may eventually force rockets to be carbon neutral. This is likely to make methane much more expensive than it is today and may make hydrogen, which is probably easier to make from (carbon neutral) electricity, or ethanol, which is easy to make biologically, the cheapest propellant.

If you figure a CO2 cost of $185/mt ($0.185/kg), as suggested in this Nature article, that comes to a carbon cost of $0.60/kg. If we use Robotbeat's numbers, we need 8.93 kg of CH4 for every kg of payload to orbit. That comes to 7.14 kg of carbon per kg of payload, for a total extra cost of $4.29 added to the "untaxed" rate of just $1.50/kg. (Someone should check my math, of course.)

That looks really bad, but now try to figure the cost of the other fuels you had in mind. Green Hydrogen costs $5/kg at a minimum, so we're looking at $17 for each kg of payload to orbit--three times worse than natural gas even with the carbon tax added.

[deltaV]

If you figure a CO2 cost of $185/mt ($0.185/kg), as suggested in this Nature article, that comes to a carbon cost of $0.60/kg. If we use Robotbeat's numbers, we need 8.93 kg of CH4 for every kg of payload to orbit. That comes to 7.14 kg of carbon per kg of payload, for a total extra cost of $4.29 added to the "untaxed" rate of just $1.50/kg. (Someone should check my math, of course.)

That looks really bad, but now try to figure the cost of the other fuels you had in mind. Green Hydrogen costs $5/kg at a minimum, so we're looking at $17 for each kg of payload to orbit--three times worse than natural gas even with the carbon tax added.

The molar mass of CO2, C, and CH4 are 44.01, 12.01 and 16.04 respectively. That paper's carbon tax is then 0.185 * (44.01/12.01) = $0.68 per kg of C. I dunno where you got $0.60. Also 8.93 kg CH4 has 8.93 * (12.01/16.04) = 6.69 kg of C. I dunno where you got 7.14 kg.

The overall carbon tax is then 0.185 * (44.01/12.01) * 8.93 * (12.01/16.04) = $4.53 per kg of payload. This differs from your $4.29 but doesn't change your overall conclusion much. So at present prices it does seem that you're right that fossil methane is the cheapest rocket propellant even with a carbon tax. However the price of green hydrogen is likely to fall in the future relative to the social cost of carbon so this may change in the future.

Another big caveat: the above calculations implicitly assume that Congress makes economically sound policies. That's possible but not a given. Congress could easily decide that it's unethical for billionaire hobbies like Mars colonization, suborbital tourism and lunar tourism to damage the planet and require carbon neutrality with no option for paying a tax instead.

[Robotbeat]

There’s also the fact that, besides the slight Isp advantage, lack of coking, etc, it’s also the cheapest source of energy in the world right now.

1. Climate change legislation may eventually force rockets to be carbon neutral. This is likely to make methane much more expensive than it is today and may make hydrogen, which is probably easier to make from (carbon neutral) electricity, or ethanol, which is easy to make biologically, the cheapest propellant.

2. There are lots of ways to make first stages more energy efficient. Some will presumably be implemented once launch costs drop enough for propellant costs to be a big deal. For example something air breathing, a tri-propellant rocket using liquid oxygen, whatever fuel is cheapest per Joule, and water, or something using a mega structure (https://en.wikipedia.org/wiki/Non-rocket_spacelaunch).

Already addressed these points:

...Plus methane is easier to synthesize with clean electricity (solar, wind, or nuclear), whether on Earth or Mars.
...

(It’s possible to improve the energy efficiency of orbital launch to maybe 200-300MJ/kg, even better if you use mechanical launch assist.)
...

So I took all that into account with my statements. To expound:

Hydrogen isn't just hard to handle, it's also *less efficient* for a first stage from a chemical energy perspective if operated with a mixture ratio similar to any hydrogen rocket so far.

You're right about hydrogen potentially being an acceptable propellant for first stage if you do some weird things like operate extremely oxygen rich or blend in water.

You might point out the energy needed to capture CO2 for methane, but that's small compared to the total. The Sabatier Reaction itself loses energy, but there are ways around that. But hydrogen also takes a huge amount of energy to liquefy, which often isn't taken into account. The lower typical bulk density contribute to making it typically less efficient for a first stage.

Another issue is that stratospheric water vapor, which hydrogen maximizes, is actually *worse* than CO2. But that can potentially be engineered around through trajectory modification.

Ethanol is lower performance, and corn ethanol is barely, if any, better than just straight fossil fuels, so in a more environmentally conscious future, I doubt it'd continue.

Also, the overall discussion was looking at hydrocarbon fuels, which methane is the easiest to synthesize. Terraform Industries intends to do it for as cheap as fracking.

[Robotbeat]

...
Another big caveat: the above calculations implicitly assume that Congress makes economically sound policies. That's possible but not a given. Congress could easily decide that it's unethical for billionaire hobbies like Mars colonization, suborbital tourism and lunar tourism to damage the planet and require carbon neutrality with no option for paying a tax instead.

Not only can you possibly synthesize methane for as cheap as fracking, you can do carbon neutrality by enhanced weathering as well.

[ZachF]

...
Another big caveat: the above calculations implicitly assume that Congress makes economically sound policies. That's possible but not a given. Congress could easily decide that it's unethical for billionaire hobbies like Mars colonization, suborbital tourism and lunar tourism to damage the planet and require carbon neutrality with no option for paying a tax instead.

Not only can you possibly synthesize methane for as cheap as fracking, you can do carbon neutrality by enhanced weathering as well.

Theoretically, if you use biogas with an Allam-cycle turbine and carbon capture, you’d have a carbon-negative source of power.

[john smith 19]

I assume one of the major influences to SpaceX's decision to use methane was manufacturability on Mars, in addition to it being a generally high performing choice for FFSC. But if a company were to design a FFSC rocket engine for just earth orbits, would methane still be the best choice? Or not even FFSC in particular, it seems like a lot of companies are wanting to use methane for more common cycles.

There's a post showing a long list of fuel/oxidizer combinations at standard chamber conditions from (IIRC) Bruce Dunn decades ago.https://yarchive.net/space/rocket/fuels/fuel_table.html.

As you can see Methane benchmarks pretty well against the other short chain HC's. Methylacetylene (major ingredient of "Mapp gas" industrial fuel) is a dark horse here. AIUI this is (was?) a by product of the process Quaker Oats used to make porridge.[EDIT Ooops. The Quaker Oats compound is actually propargyl alcohol, but it's performance is also impressive under certain circumstances] It's mentioned in Clarke's "Ignition" there was a concern it's cancerogenic (not sure what happened to that). But near 0 c BP it would densify pretty well and it's bulk density is very high.

For carbon neutral/mfg using electrochemistry ammonia is the only other choice apart from H2 AFAIK. Multiple Australian groups seem to be pursuing this option. Ironic, given Australians large coal exports and small(ish) population make it the second highest CO2 producer per head of population on the planet (Saudi Arabia is #1, due to about 10% of the oil they pump out running their petrochem plants).

[deltaV]

For carbon neutral/mfg using electrochemistry ammonia is the only other choice apart from H2 AFAIK.

Carbon neutrality means no _net_ release of carbon. It's fine to include carbon atoms in carbon neutral propellants as long as you source the carbon from CO2 in the air rather than fossil fuels.

[john smith 19]

For carbon neutral/mfg using electrochemistry ammonia is the only other choice apart from H2 AFAIK.

Carbon neutrality means no _net_ release of carbon. It's fine to include carbon atoms in carbon neutral propellants as long as you source the carbon from CO2 in the air rather than fossil fuels.

Good point.

However that may not be enough given the level of CO2 already in the atmosphere. 

In which only H2 and NH3 eliminate carbon entirely.   

I will note that (AFAIK) no one has ever actually tried to design a fuel IE create a compound solely optimised to store and release energy by combustion. The nearest I know of is JP10, which is a pure compound (with strained bonds, which seems to help store energy) that is cruise missile fuel, although I don't know it's development history well enough to know if it was designed to do this. IIRC the last price I saw for it was $10/Kg, so not cheap. 

[Robotbeat]

For carbon neutral/mfg using electrochemistry ammonia is the only other choice apart from H2 AFAIK.

Carbon neutrality means no _net_ release of carbon. It's fine to include carbon atoms in carbon neutral propellants as long as you source the carbon from CO2 in the air rather than fossil fuels.

Good point.

However that may not be enough given the level of CO2 already in the atmosphere. 

In which only H2 and NH3 eliminate carbon entirely.   

This is totally illogical. If you want to be pedantic, synthesized hydrocarbons are carbon NEGATIVE as some extra atmospheric carbon is locked up while you’re storing the fuel before it’s burned. In that sense, H2 and NH3 are *worse*.

NH3 is also pretty toxic FWIW.