Hi dear friends!

I'd like to show some sensible rocket fuels that improve over the RG-1 "kerosene", which is approximately a Diesel fuel freed of all constituents volatile or unstable at heat, since it flows in the jacket to cool the engine. The US RP-1 is a tad lighter and more energetic.

Rocket fuels must be (1) stable (2) even at heat (3) healthy (4) not easily flammable if possible (5) cheap and abundent (6) efficient. RG-1 has all that, so improving isn't easy. For instance nitromethane and propyne aren't stable enough, alkenes polymerize at heat, boranes are toxic and expensive. If a fuel is easily flammable, we prefer methane or hydrogen. Also, burnt hydrogen brings more heat per mass unit than carbon, so only strained cycles are useful.

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How much an improvement? A 2% better Isp (ejection speed / g) makes a difference every time the rocket has accelerated by one ejection speed, or more due to inert masses. To a low-Earth orbit (9500m/s cost) it acts >3 times to carry ~10% more payload, or 4M$ over a 40M$ launch. 200t of fuel must then cost <<20$/kg - but 40t that gain 5% at the second stage must cost <<50$/kg. To the geosynchronous orbit it carries ~14% more, and landed on the Moon (Leo+5800m/s) it's nearly 20% more, where the last stages are tiny.

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To begin, here are banal commercial compounds that outperform RG-1. Amazing for rocket designers; future posts could me more exciting to chemists. The appended images compare (expansion from 245bar to 0.8bar) and depict the molecules; log in to see them.

Cis-pinane is strained by the trees, saturated by paper factories and sold as a byproduct. Much more available than RG-1, a bit better. Flammable under +55°C alas, but alkylation, cyclopropanation or cyclobutanation instead of hydrogenation would improve it.

Branched alkanes differ by the liquid range. The heavier products of an alkylation unit are good; farnesane, phytane would stay liquid on Mars but must be mass-produced first.

Branched amines are easier to make and better than branched alkanes because N₂ produces heat; tertiary amines improve the liquid range. Pmdeta and Pmdpta are mass products, and I've put there how I imagine to synthesize Detmdpta, hopefully liquid on Mars:
http://www.chemicalforums.com/index.php?topic=56069.msg272080#msg272080

JP-10 (saturated dimer of cyclopentadiene, good to burn with air) illustrates the cycles being less efficient if unstrained.

Boctane (di-cyclobutyl) is considered as an RG-1 upgrade
http://www.chemicalforums.com/index.php?topic=50579.0
but banal amines are safer, cheaper and almost as efficient - yes, I'll describe strained amines also.

Marc Schaefer, aka Enthalpy

Here are some hydrocarbons that have been considered as RG-1 improvements - list and molecules appended. Many have drawbacks, but they belong to rocketry fantasy, help comparisons and show general trends.

Only Syntin has flown, on Soyouz three decades ago, is abandoned allegedly because of cost (synthesis on Wiki), but permitted one cosmonaut more per flight. In the existing tanks and pumps, it replaced RG-1 at identical volume. Notice that four hydrogen atoms less lose 5s over plain cyclopropane, the big cost of a safer liquid.

Soctane seems abandoned, but the easier Boctane is fashionable.
http://www.chemicalforums.com/index.php?topic=50579.0
Some mixed "trimer" would improve the flash point. One could also dissolve cyclobutane and cubane if meaningful. Worry: I've suggested here cyclic amines that outperform it, and some must be easier to synthesize.

With 4-Ladderane we approach the performance of simple cyclic amines - nitrogen brings so much. It just remains to mass-produce... If lucky, the laser pulses or the metal vapour I described there will help:
http://www.chemicalforums.com/index.php?topic=77307.msg283262#msg283262
http://www.chemicalforums.com/index.php?topic=72951.0

The hexyl-5-ladderane, is made with a carboxylic end by a bacterium, so Wiki's article about ladderanes raised big hopes
http://en.wikipedia.org/wiki/Ladderane
but the unstrained tail botches the advantage. Consider it only if its cost or liquid range are magic, or if a cheap method isolates the strained part.

The volatile Housane (Bicyclo[2,1,0]pentane) and its civilized "dimer" are difficult to synthesize, and easier molecules outperform them.

Quadricyclane has a decent synthesis path
http://www.orgsyn.org/demo.aspx?prep=cv6p0962
so sizeable amounts were characterized for rocketry:
"Chemical, Physical and Hazards Properties of Quadricyclane"
www.dtic.mil/dtic/tr/fulltext/u2/a345589.pdf
but a cyclopropyl-aza variant would be less flammable and keep the performance
http://www.chemicalforums.com/index.php?topic=79658.0

Methane is often considered the successor of RG-1 because it's cheap and its hydrogen gains 9s. Several adapted engines tested it with success. It leaves the engines cleaner, important if they're reused in the future. Though, it's very flammable and needs bigger tanks and pumps, so no designer has chosen it up to now. I claim that cyclopropane and spiropentane are better: more efficient, denser, easy to produce. The density would even allow a better chamber pressure not included in the comparison. More importantly, di-spiropentyl (and di-bicyclopentyl) bring methane's performance without the flammability.

Bicyclo[1,1,1]pentane is nicely stable and would have some synthesis paths, often through propellane
http://www.orgsyn.org/demo.aspx?prep=v75p0098
so it has been considered as a propellant
http://www.dtic.mil/dtic/tr/fulltext/u2/a267508.pdf
as well as its seducing "dimer", to blend with the "trimer" as needed. With a good process, it's useable.

Cyclopropane is a mass product, with only advantages over methane. Its dimer is volatile, ts trimer is Syntin.

Tricyclo[3.1.0.0(2,4)]hexane is more difficult to synthesize; maybe my laser pulses or metal vapour help.

Spiropentane is made since the 19th century just from pentaerythritol, bromine and zinc or sodium
http://de.wikipedia.org/wiki/Spiropentan (German or Polish as you prefer)
where bromine and the metal could be recycled on the site. Spiropentane is liquid at room temperature, rather dense, nicely stable, and it outperforms methane. Its "dimer", helped by some trimer, is not easily flammable, and equals methane through the gained pressure. Could a pentahalo-neopentane reacted with metal make the dimer directly?

Cheap ethylene gains 2s more if a designer accepts to cool the engine's walls with oxygen, which isn't trivial. But heavier alkenes are bad, even tetravinylmethane, alkynes too unstable, allenes probably too.

cubane is still a fantasy, stable and so efficient. For solid engines, or dissolved, or tuned to be liquid.

Nitrogen would ease some syntheses: azacubane, ladderanes, housane, diasteranes and more exotic ones
http://saposjoint.net/Forum/viewtopic.php?f=66&t=2372&start=20 (take with mistrust)
but the toxicity of aziridine versus azetidine suggests that nitrogen shouldn't be too strained, and the harmlessness of new candidates be tested early. Cyclopropanone might help
http://www.chemicalforums.com/index.php?topic=78226.0

Marc Schaefer, aka Enthalpy

Thanks phth!

As they flow in a jacket to cool the chamber, heated fuels shall leave no solid decomposition products, or a runaway hot spot would destroy the wall. Some  present designs even wish a clean jacket after flight to reuse the stage.

The appended two-sheet xls (cut the txt extension) and result image estimate a temperature where 1000 moles of a fuel leave 10g decomposition products over 300s. The sources are in the xls; some fuels are deduced from much faster rates, which is inaccurate and pessimistic.

The fuels in green lines fly successfully. Most others refute their reputation.

Bicyclo[1.1.0]butane seems too fragile, but oxygen instead might cool a future engine. Hvap=427kJ/kg for cyclobutane and Hf=+217kJ/mol for gaseous bicyclobutane at 298K according to
http://webbook.nist.gov/cgi/cbook.cgi?ID=C157335&Mask=1
so Hf=+194kJ/mol for the liquid, and then bicyclobutane is 14s better than Rg-1, 3s better than ethylene, just 1s behind cubane, fabulous. Do you know if it may detonate or polymerize as a liquid without air?

Marc Schaefer, aka Enthalpy

Finally, I've found a measured decomposition speed for the Rocket Propellant N°1 or Rp-1, called "kerosene" but nearer to Diesel oil, at Nist of course
http://ws680.nist.gov/publication/get_pdf.cfm?pub_id=901027
but still not for hydrazine, Mmh nor Udmh despite they were investigated.

For the comparison, I take the temperature that decomposes 10g from 136kg of Rp1 in 300s, or +316°C. It tells that the small cyclo- and spiro- are stable enough, azetidine probably too. Data about the commonly used hydrazines would validate more candidates.

Until we cool the engine walls with the oxygen to leave the engines clean for reuse and enable better fuels like ethylene, bicyclobutane, spirohex-1-ene...

Marc Schaefer, aka Enthalpy

Do you have any connections to try to get such calculations into the hands of the engineers at these companies? SpaceX especially seems like it has a culture where it will take a good idea no matter where it comes from.

Cheap allene (propadiene) oligomerizes easily to precursors of spiroalkanes.

Methenylcyclobutanes can be cyclopropanated. Maybe the final alkane is cheap enough. I didn't find if a 2+2 cycloaddition provides spiroheptanes; light with decent efficiency would be clean and affordable.

As heats of formation, I've seen only spirohexane, dispirooctanes and [4]rotane in
 The Chemistry of Cyclobutanes, by Rappoport and Liebman
which suggests that spirohexanes bring no more strain energy than the cyclopropane plus cyclobutane, surprisingly. I wish more measurements were available. So to estimate spiroheptanes, I added the rings' strain energy, not forgetting the heat lost in branchings. This is pessimistic.

5s over RG-1 equals hydrocarbons more difficult to produce, or more flammable, or unstable at heat. The listed C₈ is flammable and the C₁₀ marginal, but limited proportions can stay in a mix to lower the melting point. One could also cyclopropanate one methylidene and cyclobutanate the other, or fully cyclopropanate the trimer and cyclobutanate the dimer. The density is unknown but probably excellent.

I've found a decomposition speed for hot spirohexane, no idea if it's the main path
https://kinetics.nist.gov/kinetics/rpSearch?cas=157459
Spirohexane would then bear excellent +317°C under the criterion here, 1000 moles of a fuel leaving 10g decomposition products over 300s.

Marc Schaefer, aka Enthalpy

(Pseudo-) Rotanes are energetic, some are flame-safe, or heat-resistant, or freeze uneasily, or look easy to mass-produce. Maybe a mix of them makes a good rocket fuel?

They all lack badly data. The few heats of formation look very wrong. As I have nothing about spiroheptane and spirohexane isn't credible, I can't even extrapolate additively. I've no Mp neither, just the very symmetrical C₁₂H₁₆ is solid.

Software isn't too wrong at estimating Bp. I subtracted 120K for rough Fp, please doubt.

Thoughts about synthesis and production may follow.

Marc Schaefer, aka Enthalpy