Building & Testing a Rocket Injector 

We are building a new impinging Injector this year for our next rocket. I think a collaboration could be in interesting idea. There is definitely lots to be shared on the topic of injectors!

Man, second method sounds pretty fancy. We might need to upgrade our tools to pull off a strategy like this. I think the plate method might be a bit too inaccurate.

@@ASTRABremen you could still drill and polish the openings with the plate method. Any way you could try with electroplating, get conducting ink (silver ink) and use wax for modelling. Electroplating is slow but simple. I saw a video on youtube where one guy used it to make some part of a rocket engine.

They are essentially the same, just not designed for 55 bar and 3000+ K

Wow, lots of questions. I will try to answer them all: 1. The placement of the elements was a bit accidental. It was supposed to be just 4 bolts with six holes. But after the first try at manufacturing we realized that 6 holes was too much for one bolt. Thus we have to drill 2 more holes and we couldn't make them arranged circularly anymore. At least we have two more slots for bolts if we end up needing more flow after the hot fire test so at least this way we are a bit flexible for an increase in power. 2. Great to hear that you want to start a team. We need more in the North of Germany for sure. I started the project by just putting out a notice to the students in my study program that I wanted start a group that builds a rocket that goes to space. There were about 15 people that were in to it after the first meeting and that essentially became ASTRA. In the end we decided to become an e.V. as that seems to be the easiest way to operate in Germany as a group like this. If you want more info on this, feel free to reach out (you can probably find me on linkedIn or you can write to astra.bremen@gmail.com) 3. We have not been able to convince the uni to support us... A bit sad, but we still have hope. Maybe when we get to space they will realize what a disasterous mistake they have made :). Most of the money has been raised privately so far, but we have managed to raise around 6k€ in our crowd funding campaign - which is still ongoing... (hint, hint to those who might be able to support). 4. We are doing our first hot fire test at DLR Lampoldshausen (not exactly close to Bremen unfortunately). One of our long term missions would be to set up a hot fire type of facility somewhere in the North that could accommodate student rockets. Maybe with more teams in this area we will be able to pool resources and political capital to make it happen. So, definitely encourage more of these groups to form!

@@ASTRABremen Thanks a lot for your explaination. I was just a little worried about your engine going boom because of uneven burning due to the injector. But i understand this one was just for development and you dont plan to fly it that way. Interesting to hear about your experience starting and financing this project.(i guess you`ve already asked the local space industry like arianegroup for funding). I will probably have some more questions when we really get started, but thanks for offering more info. Good to hear that there is a test site. But would be nice indeed to have some place here in the north. I mean in theory one could ask some farmer to use their field but it would require a movable test stand including all the Gse and stuff. I heard that the DLR also has a program for student rockets, so when you say you can use their facility, you are probably involved in that as well? Again thank you for sharing so much of this project and good luck!

We were looking for 30 bar in the combustion chamber or more. In the end we got exactly 30 bar so it actually worked pretty well. We had to up the injection pressure a bit to get that though. Overall, we had 25 bar drop across the injector. We are still analyzing the data to determine the injector Cd but it looks like it was around 0,6.

That was based on FEM simulation. We have definitely over engineered it though. The safety factor for the fasteners is somewhere around 10. Theoretically we could hold the plate on with just 4 bolts.

It is a hybrid rocket so only the Nitrous Oxide goes through the funnel. The fuel is solid paraffin wax which is already in the combustion chamber.

Unfortunately our drill press doesn't have that feature. I wish it did... Also, if we had that feature, it would still be very difficult to make sure the flows converged without very accurate placement. The contraption at least helps with that.

this was an older design. We have learned since that we need to make the angles steeper. It is just tricky to manufacture them ourselves at those steep angles...

We attempted to get the holes drilled professionally and sent many quotes all throughout the Bremen region with none under €500. The waterjet company just does the discs (no holes). Holes are really no issue for us, it is the threading that sucks. Also, the plastic template is 3D printed so has a reasonable degree of precision - at least enough for our purposes. If you check out some of our later videos, you will see that everything lined up perfectly and to top it all off, we had many successful hot fire tests with the injector plate even surviving the explosion that ended the campaign. Overall, I would say we did a pretty good job on the injector at least...

@ASTRABremen I never said you didn't do a good job. On the contrary, what you did is remarkable. I know professional machining services can get quite pricey, I was thinking more like "a friend who can let you use a mill in his shop on a weekend" kind of arrangement. And you can power tap holes on a mill (with some practice).

@@andreipetrenko2422 I guess we just need to find the right friends... As the group matures, I think we may find more opportunities like that.

It did angle them. It holds them at a 4° angle. It is not much, but enough to have them converge a few centimeters away from the plate.

@@ASTRABremen did you thought to make it like 45ish degree ? Speed of collisions would be much greater

@@bartekpudo9624 We did, but we would have needed much thicker bolts... This may be an improvement in the future. Probably would need M16 or M20 to pull of angles greater than 20°.

It is a hybrid rocket, so the fuel is in solid form. The oxidizer is what goes through the injector.

Would have preferred that but we don't have easy access to one. It would have just cost a lot of money.

@@ASTRABremen Yes I know. :( I use to run one and that is what you really need. And $

@@thisoldminer We totally agree.