Specific heat for Nitromethane ?

[HattoriHanzo]

I am going to calculate Isp for Nitromethane CH3NO2 and need specific heat(gamma). I am only familiar with specific heat regarding monatomic gases γ=Cv/Cp<5/3.
I also wonder about the molecular weight of Nitromethane. I have seen numbers from 61-70g/mol. What is correct?

[MJD]

The higher numbers are for nitromethane containing carbon or hydrogen isotopes versus regular C and H. The mol weight of regular nitromethane is easily calculated from the periodic table:

12.011 + 3x1.0079 + 14.0067 + 2x15.9994 = 61.04

Cp/Cv at 1000psi is 1.2427, at 100psi is 1.2434. 1.243 is a good round number. Specific gravity is 1.127.

MJD

[HattoriHanzo]

The higher numbers are for nitromethane containing carbon or hydrogen isotopes versus regular C and H. The mol weight of regular nitromethane is easily calculated from the periodic table:

12.011 + 3x1.0079 + 14.0067 + 2x15.9994 = 61.04

Cp/Cv at 1000psi is 1.2427, at 100psi is 1.2434. 1.243 is a good round number. Specific gravity is 1.127.

MJD

Thank you very much!

[HattoriHanzo]

I cant edit my first post, I see that I have made a bad mistake above. γ=Cv/Cp<5/3 not correct, it should have been Cp/Cv=5/3!


However I have done some calculations for nitromethane:

R=136

With combustion chamber pressure=2MPa, temperature=3400K and expanding to atomspheric pressure at sea level I get Isp=150s

Cp=700J therefore exhaust temperature=1900k
I get exhaust velocity=1450m/s

If I am all correct with my calculation it looks like nitromethane can be used as a lowperfomance monopropellant.

Because nitromethane contains oxygen I suppose it is possible to dentonate it. But under what circumstance will it detonate?

Marksteamnz stated in the thread "Rocket How to" that Turbonique used nitromethane as rocket engine fuel. They have to "abondon the market", I suppose it was due to some detonations :lol:

[marksteamnz]

Marksteamnz stated in the thread "Rocket How to" that Turbonique used nitromethane as rocket engine fuel. They have to "abondon the market", I suppose it was due to some detonations :lol:[/quote]

Sorry I wasn't clear enough in my post that Turbonique system was just an example of a monopropllent unit. The Turbonique normally used n propyl nitrate, the documentation states other mono propellents could be used but there is no disscussion of how well they worked. The Turbonique system on restart apparently, if not carefully purged of any pooled liquid, would detonate.

Nitro methane is mentioned in various references but the only link I have found for some one who has mastered it is the swissrocketman web site http://www.swissrocketman.com/

[MJD]

http://www.dow.com/angus/pmo/basic.htm

Here you will find a data sheet broadly describing the conditions that promote detonation, but not from the detailed viewpoint of a rocket experimenter.

MJD

[HattoriHanzo]

Very nice detailed information regarding nitromethane! Thanks

[HattoriHanzo]

In "Rocket Propulsion Elements" by Sutton there is a chapter regarding propellant analysis. One example is derived for nitromethane. A very nice example of the summation of products and reactances and a more exact calculation of molecular weight, temperature and exhaust velocity.

A highly recommended book, a bit expensive however. Knowledge is expensive these days :wink: