[Enthalpy]

Hello dear friends!

Here I'd like to discuss somewhat speculative rocket fuels: efficient, but whose mass-production route is unclear, or whose stability isn't established.

We may call Candy a first candidate as on the appended sketch: C1CC12C3C2C4C3C45CC5, C10H12, 132.21g/mol, estimated bp=+144°C hence storable but regrettably flammable. It's a collision between spiropentane and tricyclohexane, which nearly retains the bond angles of both, hopefully as stable as both - or does it offer easier decomposition routes?

I combined the heats of formation measured at the 298K liquid cyclopropane, cyclobutane, housane and spiropentane to obtain +502kJ/mol for the anti shape. That would make it 9s better than Rg-1 "kerosene", wow.

The synthesis could pass by the dispiro diene (from cyclohexadiene or pentaerythritol?), at least in my imagination. Then:

• Ideally, a light-induced 2+2 cycloaddition would make Candy. Same as from norbornadiene to quadricyclane, with bond candidate atoms waiting at the same distance, but now the spiro carbons stay in the way and more atoms must move.
• Or add Br₂ then Cl₂ on the double bonds: if the conditions permit it, the hydrocarbon is a gas and the dibromo rains down to avoid further bromination. Syntheses of bicyclo[1.1.0]butane and [1.1.1]propellane suggest that liquid metal couples preferentially the bromine-bearing carbon with the chlorine-bearing one, leading then to the tricyclohexane rather than to cyclohexadiene.
• Or add BrI and use concentrated laser pulses to abstract two iodine atoms, then two bromine atoms, to create the new bonds - if and when the laser process works...
  http://www.chemicalforums.com/index.php?topic=77307.0
  (if the bromines and iodines are crossed, the improbable compound resulting from halogen abstraction would be even more efficient. Later.)

Comments and suggestions welcome, of course!
Marc Schaefer, aka Enthalpy

[Enthalpy]

Candy is a bit volatile and too easily flammable, but the sketched bigger variants shall remedy it.

• Spectacles C1C2C1C3C2C43C5C4C6C5C6 (C₁₁H₁₂ 144.22g/mol Hf=+601kJ/mol hand-estimated for the liquid at 298K) is to boil at already decent +164°C according to Stein and Brown. It keeps Candy's +10s (my mistake) advantage over Rg-1.
• Lollipops C1CC12C3C2C4C3C5C4C56CC6 (C₁₂H₁₄ 158.25g/mol Hf=+585kJ/mol) is safe with Bp=+182°C but brings +9s.
• Fishkiss C1CC12C3C2C4C3C45C6C5C7C6C78CC8 (C₁₅H₁₆ 196.29g/mol Hf=+847kJ/mol) offers Bp=+226°C and +10s.

A central spiro lowers the freezing point, but C₁₅ is already much.
Marc Schaefer, aka Enthalpy

[Yggdrasil]

I don't have any substantive science to add to the discussion, but when I think about speculative rocket fuels, I think of a very entertaining science fiction short story, called "A Tall Tail".  Here's a short excerpt:

During the 1960s some bright spark at ARPA got a bright idea and handed it to the CIA: why not pretend we were using some extremely high reactivity oxidizers and fuels in our latest missiles, and leak plans and blueprints to the bad guys’ spies? Obviously this wouldn’t play with the Soviets, but small fry like East Germany or North Korea or Iraq might fall for it. Worst case, it would send them on a wild goose chase; best case, they might really damage themselves trying to build and fly this stuff.
So we brainstormed the most suicidal rocket motor we could come up with. And you wouldn’t believe just how mad it was.

You can read the full story here: http://www.tor.com/2012/07/20/a-tall-tail/

[Enthalpy]

Fun! How guys at Cia (and the other agencies, they don't differ much...) spend their time.

During early rocketry times, developers did try many thousand oxidisers and fuels that showed very concrete drawbacks; it's possible that spooks took advantage of the situation by claiming responsibility.

One nasty combination that has survived is N₂O₄ with N₂H₄ and its monomethyl and dimethyl variants. Yuk for real when using 200t of Udmh in a rocket. Many people seek a 1-to-1 replacement for hydrazine and Mmh (it must catch fire in milliseconds with the tetroxide, for tiny thrusters that push 1N*1s at satellites to control the orientation), but the replacement is presently under way by ionic propulsion, and if needed, I proposed a sort of Diesel glowplug equivalent to use kerosene.

The other horrors of that time have disappeared. For instance N≡C-C≡C-C≡N was tried, OF₂ as well. 98% H₂O₂ is still in use occasionally and 70% regularly at Soyuz. Recently, after I suggested a (harmless) amine recomposition pre-chamber to power the turbopump
http://www.scienceforums.net/topic/83156-exotic-pumping-cycles-for-rocket-engines/?p=805383 (drawing with green tank)
a group began to develop "azetam", a liquefied acetylene plus ammonia mix - apparently they stopped on time.
(And a new forum member here asked how to handle ethylene diamine, if you remember).

When handling such quantities, even nitromethane is dangerous. Since Saturn V was 100m tall, the rocket guy answered the fuel guy "try a drop on concrete from 100m height" and the result was "better not with 200t". Recently, even N₂O created an accident.

A funny text on the Internet: "Ignition!" by John D. Clark
http://web.gccaz.edu/~wkehowsk/ignition.pdf
incredible, what they all tried.

[Enthalpy]

Diasterane and triasterane are other prospective rocket fuels, advocated by Laurent Catoire and others. See the sketches: few unusual bonds between cyclobutanes increase their strain energy but keep much hydrogen, making these fuels efficient.

• Diasterane is tricyclo[3.1.1.1(2,4)]octane (homonyms exist) or C1C2CC1C3CC2C3 or C₈H₁₂ and weighs 108.18g/mol. Estimated Bp=+123°C makes it flammable.
• Triasterane is C1C2CC1C3C4C2C3C5CC4C5 or C₁₂H₁₆ and weighs 160.26g/mol. Estimated Bp=+198°C makes it hard to light. A mixture with some diasterane retains this advantage and could stay liquid at colder temperature.

Diasterane was finally synthesized in 1987 by Otterbach and Musso. Finding no heat of formation, I depend on software estimates, porca miseria!

• The isomer anti-3-ladderane, liquid at 298K, has measured Hf=+169kJ/mol. Am1 imagines +186 and +325kJ/mol for the ladderane and diasterane, both gaseous at zero K, so I extrapolate Hf=+308kJ/mol for diasterane, liquid at 298K.
• The isomer anti-5-ladderane, liquid at 298K, has Hf near +334kJ/mol. From Am1's +370 and +540kJ/mol, I extrapolate Hf=+504kJ/mol for triasterane, liquid at 298K.
• Then, according to Propep, triasterane outperforms RG-1 by 9s and diasterane by 10s, wow again.

Since bicyclo[1.1.0]butane looks rather simple to synthesize, it would be nice to dimerize it to diasterane as sketched... This mere electron reshuffle releases almost 300kJ, or 130kJ more than if producing two cyclobutenes. It's just that bicyclo[1.1.0]butane uses to polymerize instead, at least under present concentrations and catalysts.

Marc Schaefer, aka Enthalpy

[Enthalpy]

To convert two bicyclobutanes in a diasterane, would it be conceivable to use something like CO or SO₂, helping to make a first cycle, bigger and less strained, and in a second step under different conditions, contract the ring by expelling the auxiliary group, say with light?

[Enthalpy]

Tetraspiroundecane, a standard frenzy, cannot be absent here. With a heat of formation of +489kJ/mol
http://webbook.nist.gov/cgi/cbook.cgi?ID=C129872306&Mask=2#Thermo-Condensed
it would outperform RG-1 "kerosene" by 9s - as good as methane and cyclopropane - and its software-estimated Bp=+157°C would make it uneasy to light.

No idea about a synthesis nor a melting point - I thought you would.

[Enthalpy]

Dailey among others synthesized cyclopropenes. While the parent needs cold, spiro variants reach room temperature. Simpler synthesis paths were found meanwhile
  Dailey - en.wiki - de.wiki
They make Candy and Lollipops less speculative. Maybe a poison can hamper the polymerization, then light and cold favour the cycloaddition?

Marc Schaefer, aka Enthalpy

[Enthalpy]

Bicyclopropylidene cPr=cPr has syntheses in the kg amount. I ignore if my scheme for tons that cyclizes tetramethylethylene
  chemicalforums
is as "robust" as commented, so the present bicyclopropylidene and its heirs are speculative fuels. Illustration.

De Meijere et al studied the decomposition of cPr=cPr, so the temperatures and 55% yield to methylenespiropentane aren't optimized. Better: the other product is an excellent fuel too. Mix and use both products, and this step gives >86% before optimizing.

Methylenespiropentane offers 12s Isp more than RG-1. If produced more easily than spiropentane (+10s), this fuel is magic. It's less badly volatile and flammable than spiropentane. It demands an oxygen-cooled engine.

Bicyclopropylidene could be as efficient as Methylenespiropentane. Wow too!

Tetraspiroundecane might possibly be made from methylenespiropentane. It was already synthesized, and its +9s are fantastic for a fuel reluctant to flame.

Tetraspirododecane must be as efficient, fantastic too.

Marc Schaefer, aka Enthalpy