Would adding Boron Trifluoride to rocket fuel increase ISP?

[Agelastos]

Since Boron Trifluoride is soluble in many Hydrocarbons, I have been wondering what the Isp of an engine burning a mixture of 20% Boron Trifluoride and 80% Ethane, as fuel, and Oxygen as oxidizer would be.  Furthermore, is Boron Trifluoride the secret ingredient of the fuel Naphthyl used by Roskosmos?

[Proponent]

According to replies in this Russian-language thread, the napthyl fuel burned in the Soyuz 2 launch vehicle is kerosene with fewer aromatic compounds and more napthenes.  There's no mention of boron compounds. 

[lrk]

If the fuel contained significant amounts of boron it would be easy to tell- the exhaust flames would be bright green. 

[Agelastos]

According to replies in this Russian-language thread, the napthyl fuel burned in the Soyuz 2 launch vehicle is kerosene with fewer aromatic compounds and more napthenes.  There's no mention of boron compounds.

Use of Oxygen and Naphthyl gives a vacuum Isp of about 370 seconds.  Could the complete elimination of aromatic compounds and the increase of napthenes explain such a high specific impulse?

[Agelastos]

If the fuel contained significant amounts of boron it would be easy to tell- the exhaust flames would be bright green.

Can we find out what the Isp of a combination of Boron Trifluoride, Ethane (20% Boron Trifluoride, 80% Ethane), and Oxygen would be?

[Gliderflyer]

If the fuel contained significant amounts of boron it would be easy to tell- the exhaust flames would be bright green.

Can we find out what the Isp of a combination of Boron Trifluoride, Ethane (20% Boron Trifluoride, 80% Ethane), and Oxygen would be?

You could run it through NASA's CEA code to find the theoretical performance. There is an online version that is free to use.

[Pete]

Use of Oxygen and Naphthyl gives a vacuum Isp of about 370 seconds.  Could the complete elimination of aromatic compounds and the increase of napthenes explain such a high specific impulse?

I'd very much like a link for the source of that number.

370 is a *lot* for a kerosene-based compound

[Genial Precis]

I ran the free online CEA on liquid ethane with and without 20 mol % BF3. Chamber pressure 300 bar, shifting equilibrium, probably a few other parameters I'm not sophisticated enough to include. Output files attached. Inclusion of BF3 increases specific impulse by about 10 s at optimum O/F. Optimum mixture ratio is slightly more "fuel" rich presumably due to inclusion of oxidizing material in the fuel.

Since I know I'm not sophisticated enough to assure accurate real-world numbers with CEA I quote the difference of 10 s, which should be much more robust to setup choices on my part. So whatever is possible with liquid ethane as fuel you can do approximately that much better with BF3 included, if it really is soluble.

[Agelastos]

I ran the free online CEA on liquid ethane with and without 20 mol % BF3. Chamber pressure 300 bar, shifting equilibrium, probably a few other parameters I'm not sophisticated enough to include. Output files attached. Inclusion of BF3 increases specific impulse by about 10 s at optimum O/F. Optimum mixture ratio is slightly more "fuel" rich presumably due to inclusion of oxidizing material in the fuel.

Since I know I'm not sophisticated enough to assure accurate real-world numbers with CEA I quote the difference of 10 s, which should be much more robust to setup choices on my part. So whatever is possible with liquid ethane as fuel you can do approximately that much better with BF3 included, if it really is soluble.

I  checked your .txt files from the CEA program.  I am no expert either - I still do not know how to use the CEA program - but I see an Isp increase of more than 20 seconds in the data you gave.

[Genial Precis]

With 20 mol% BF3 the best vacuum specific impulse at expansion ratio 300 is 3650 m/s at mixture ratio R=1.2 (O/F =1.96, second entry). With pure liquid ethane the corresponding best Ivac is 3570 m/s at a mixture ratio of R=1.1 (O/F=3.38). So it's actually only 80 m/s or 8 s difference in specific impulse.

I learned to use the CEA browser tool in order to respond to this question; I found it pretty easy to pick up as I went.

[edzieba]

Borated fuels ('Zip fuels') are regularly proposed for jet and rocket engines, and have been for many decades. Their chief drawbacks are:
- Forming abrasive particulates during combustion that eat your turbomachinery
- Hideously toxic exhaust products that are bad news for anyone vaguely nearby

John D. Clark's fabulous Ignition! covers many of the experiments with these fuel additives, and how well the purported benefits work out in practice.

[libra]

They were the hype of the late 50's for both B-70 Valkyrie and early rocketry, to the point so ill-advised people started speculating on stocks of that stuff.

Then it was found it was not only toxic, it also let ultra-hard deposits on rocket and turbojet combustion chamber; that were nearly impossible to clean up.

[WBailey]

They were the hype of the late 50's for both B-70 Valkyrie and early rocketry, to the point so ill-advised people started speculating on stocks of that stuff.

Then it was found it was not only toxic, it also let ultra-hard deposits on rocket and turbojet combustion chamber; that were nearly impossible to clean up.

Unless you burn it with everyone's favorite liquid fluorine as the oxidizer, then the combustion products are all gasses.
I recommend The Green Flame by Andrew Dequasie to anyone interested in the effort to produce borane fuels in quantity for the USAF and the struggle to keep the pilot plants from exploding.