Gaseous Nitrous Oxide
In previous tests, we saw that the flame changes from a nice mach-diamond plume to a fat something-is-wrong plume. We then started thinking about the density of nos with the change in temperature. Nos changes to vapour at around 38 degrees (at vapour pressure 72 bar). This changes the mass flow rate through the injectors since the mass flow rate through a specific orifice is much less for vapour than liquid. I created a spreadsheet to try and understand this change:
If I did this right, then it means that there is a big drop in mass flow rate for the nos, which affects the O/F ratio. The graph below shows that if you start with an O/F ration of 11 running on liquid nos, the O/F ratio will drop to below 4 when the nos heats up before flowing into the injector holes. Running on vapour from the start, doesn't affect the O/F ratio so much. But it does mean much bigger injector holes.
With this shifting of O/F ratio in mind, I wanted to see if I can run the motor on nos vapour. So I increased the nos injector holes to 2.2mm (from 1mm) and mounted the motor upside down.
And this is what the burn looked like (in sequence over about 4 seconds)
As you can see, the plume shape and colour stayed pretty much constant until the burn through on the last screen shot.
The vapourising of the nos (with the upside down tank) cooled down the tank so much, that there was a little bit of liquid nos left that boiled of very, very slowly. A few minutes after the burn when we walked over to the motor, liquid nos was still boiling off.
- It is possible to run the nos motor on nos vapour.
- The combustion chamber of stainless steel just can't handle the heat.
- It is not feasible to run the motor on vapour from the top of the nos tank. Evaporation cools down the nos so much that the pressure drops to nothing.
What I want to do now, is to feed the motor liquid nos (to prevent the the nos tank from cooling down so much) and to vaporise the nos in a jacket around the combustion chamber and then feed nos vapour through the injector holes. In other words, build a regeneratively cooled motor that is cooled by the evaporation energy of nos (376.14 kJ/kg). (Water requires 4.18kJ/kg per change in temperature in degrees). If I understand this correctly, one kilogram of nos evaporating, will drop the temperature of one kilogram of water (liquid) by about 90 degrees.