Bob, as a baseline how do propane (or LPG) and N2O perform as bipropellants? I've always thought they would be a good choice for amateur high-power rocketry since both are readily available, liquids at room temperature under reasonable pressure that can be used instead of pumps, and essentially stable and nontoxic.
I tested liquid nitrous oxide (N
2O) with liquid propane (C
3H
8). It seems to work moderately well if we can run the engine at a fairly high pressure. For example, I selected 34 atm (500 psi) and got a specific impulse of 235.8 s (sea level) with the nozzle expanded to 1 atm pressure. Of course that's a very high pressure to run at for a pressure-fed system. If we run at a pressure of 7 atm (which seems to be typical for many pressure-fed systems), we get an I
sp of only about 185 s. That's pretty poor; we'd be better off sticking with solid propellants.
On the other hand, N
2O and C
3H
8 seems viable for a low-pressure, high-expansion ratio engine. I selected a combustion chamber pressure of 7 atm and a expansion ratio of 50:1. Under these conditions I compute a specific impulse of 290.5 s (vacuum). However, I doubt an amateur would ever need an engine with a 50:1 expansion ratio.
Firestar Technologies is claiming an Isp of 300 s for their nitrous oxide fuel blend (NOFBX), a monopropellant consisting of an emulsified mixture of N2O and ethane, ethene or acetylene. It sure sounds explosive to me, but apparently it's pretty stable.
I tested all of these combinations, however I couldn't easily find the thermodynamics properties for liquid phase. Instead I ran the computations assuming gaseous reactants, which results in higher specific impulse. Again I used a pressure of 7 atm and a 50:1 expansion ratio. I first ran the computations for gaseous N
2O and C
3H
8 so we can see how it compares to the liquid phase computations.
N
2O + C
3H
8 ---> 297.6 s (vacuum)
We see that the liquid phase is 2.4% lower. We can probably assume about the same for the other propellants. Under identical conditions, here's what I get for the other combinations:
N
2O + C
2H
6 ---> 299.0 s (vacuum)
N
2O + C
2H
4 ---> 301.8 s (vacuum)
N
2O + C
2H
2 ---> 312.1 s (vacuum)
So we can see that 300 s is definitely feasible.