The Injection system

To have a good combustion it is essential that the paraffin steam and Nitrous oxide are mixed homogenous. This is achieved by the interaction of swirling and spraying of the Nitorus oxide in the thrust chamber. The characteristics of the injector are identified during cold flow tests with water. Here the different spray forms are qualitatively observed and also the characteristics like the pressure lose Δp over the injector head and the mximum oxidizer mass flow m'_Ox are detected.

Specifications

It is assumed that the Nitrous oxide arrives still in its liquid phase at the injector. From our previous design calculations a mass flow of 0.79 kg/s is needed to get the desired performance. That is the propertiy that defines the area of the injector and therefore limits the mass flow. Further you need a specific pressure drop over the injector to decouple the oxidizer feed lines from pressure oscillations in the thrust chamber. A value of 10 to 12 bar will be desired.

Realization

To reach a good atomization of the oxidizer, which is desired for a good mixing andcombustion, there are thermal and mechanical ways to do so. The mechanical treatment can be reached by shear forces. Classical injector geometries are the swirl injectors and the impingement injectors. In a swirl injector the fluid flow is being rotated and swirled so the flow gets riped open. But the manufacturing of a swirl injector is ambitious so we decided to develope and impingement injector.

We tried to design an injector that could be constructed easily and rapidely. A double impingement injector was excluded because of the formation of an elliptical flow pattern that would not be good for our cylindrical wax grain. A triplet impingement injector would not have that drawback but still would be even more difficult to manufacture due to the high accuracy needed for manufacture.

The simplest impingement injector is the showerhead which has only some boreholes perpendicular to the injector area. To get higher shear forces, we designed a modified showerhead, which also results in a better mixing and swirling of the oxidizer. In a normal showerhead the individual streams do not interact very much with each other which is not good. Our new design enables rapid mixing and atomization of the flow which you can see on the picture below. The oxidizer stream is widened up on its way through the thrust chamber and by varying the distance between the injector and the wax grain you can also regulate the fuel mixture generation.

Construction

The injection system is composed by the injector plate with welded pipe connection and the injector heads so you can realize and test different injector heads. The influence of the injector head on the flow coefficient will be analysed during our first tests.

Determnination of specific parameters

To characterize the parameters like the pressure loss Δp you have to do flow measurements with water, so called Cold flow tests. To get exact values extensive experiment set-up is needed. We have had the oppurtunity to test our Injectors over a wide pressure range and with different mass flows at the test bench M11 of the DLR in Lampoldshausen ( http://www.la.dlr.de/ra/te/Airbreathing/ thanks once again ). So we could determine the optimum operating point.