Pre- and Post-Combustion Chamber

[josborne]

Hey all,

I have a general question about pre- and post-combustion chambers. I was reading a paper (Karabeyoglu et al., 2003, AIAA, SCALE-UP TESTS OF HIGH REGRESSION RATE LIQUEFYING HYBRID ROCKET FUELS) which discussed in particular about how for hybrid engines the use of a post-combustion chamber was necessary due to the higher residency time of the propellant to ensure more complete combustion. My worry with post-combustion chambers is that at that location the temperatures are going to be extremely high and I'm not certain how easy it would be to prevent against chamber liner failure at those temperatures. 

Moreover, I wanted to know what opinions were about pre-combustion chambers. I imagine it would have benefits to ensure that the nitrous has had time to evaporate and to ensure a more even spread of nitrous once it reaches the fuel. Last year we had neither of these but I feel like they may both be important to consider.  

[Yiqing Wang]

As I have not yet designed a complete engine I don't know about the technical details much, so I cannot comment on if they offer benefit or how much.

I do however, have experience with insolating them. Pre-combustion chamber does not require much isolation as the nitrous cools it very effectively. All you need is a 1cm layer of high temperature silicon or HTPB. For post combustion chamber however, you need a thick layer of HTPB (no AL), about 3cm thick.

Our current engine has pre-combustion chamber but not post-combustion chamber. Our previous design had both. And with the insolation described above, we had more than 3 test fire each. So should be pretty reliable.

[josborne]

Regarding the post-combustion chamber, my concern would be that with such a thick layer of HTPB you wouldn't get the complete combustion happening. What I mean is, increasing the residency time is accomplished by tripping the boundary layer of your flow at the far downstream side of your fuel grain, causing a recirculation region in your post combustion chamber. You want your post-combustion chamber to be long enough to allow for one full rotation of the eddy that forms because of this tripping. My concern with the thick layer of HTPB is that you will have a considerably smaller recirculation region, and so the effectiveness of your post-combustion chamber would be smaller.

Ah so many things to think about. I need to do some proper research and figure out a way around this.

[Yiqing Wang]

Right, haven't thought about HTPB in post combustion chamber vaporizing. I suppose making it our of graphite might work, like making it an extension of the nozzle.