1: Aluminum is fine to use, I believe the 3d printed alloy is 90%Al 10%Si
which is similar to the traditional casting alloys (A360) and should be
reasonably structurally sound till 500K (though you'll probably want to
verify for the specific alloy and temperature). It might be more worthwhile
to have some film cooling with aluminum as it will be more likely to
locally melt at hot spots than inconel.
2:You can do either fuel or LOX centered pintles, I wouldn't say either way
is better. The key is the momentum ratio. If you have a fuel centered
pintle you want a high momentum fuel jet that will penetrate the LOX
annulus, and with LOX centered you want a lower momentum LOX jet that does
not penetrate the fuel annulus. So if LOX centered is easier to plumb go
for it, just keep an eye out for hotspots inline with your LOX jets.
3: I think 3D print bosses and cutting threads is easier for test stand
work, other people disagree :) I also think brazing is the best option but
brazing aluminum is tricky at best and welding is hard to do on small
engines/ I don't know if the casting alloys support welding?
4: Machining the pintle separately is a good idea, especially if you can
change it out. You'll almost certainly want to change the injector at some
(many) points in the project so might as well make it easy. Other than the
obvious AL/SS thermal issue, if you don't anodize or otherwise treat the
aluminum make sure to keep it dry as galvanic corrosion can be an issue,
and everything gets wets from condensation with cryo unless you purge,
purge, purge. And it is super easy to strip aluminum threads with stainless
hardware so if you have any torque happy members in assembly just keep them
in check.
Overall it looks like a cool project! Good luck and thank you for sharing.
Lloyd
On Sun, May 1, 2016 at 3:34 PM, Kristin Travis <theeblueorchid@xxxxxxxxx>
wrote:
Hello All,
My senior mechanical engineering capstone team is designing a 250 lb
thrust liquid fuel engine for the Portland State Aerospace Society. This is
our first time posting on arocket. We are working with Kyle Meeuwsen who
has posted about designing the test stand.
Our plan is to 3D print the engine in metal so we can print regenerative
cooling channels in the nozzle and chamber walls. We are planning on using
a pintle injector and ethanol/liquid oxygen propellants. We have been told
we can print to a resolution of 0.03 for steels and 0.015 for aluminum.
Here is the link to our engine design document:
https://github.com/psas/liquid-engine-capstone-2015/blob/master/PSAS_Design_Review1.pdf
1. This design assumes we are printing in inconel, which results in very
thin chamber walls due to heat transfer properties of the metal. We are
wondering what people think about printing it in aluminum in order to have
thicker walls and larger cooling channels.
2. For the pintle injector we have been getting some mixed feedback on
whether we should have a fuel centered (fuel inside the pintle) or LOX
centered design. The plumbing gets much simpler with a LOX centered design,
but most of our research on small engines indicated fuel centered designs
are better because they spray the chamber walls with fuel instead of
oxidizer.
3. We are also looking for recommendations for connectors to the test
stand plumbing (not shown in our drawing). We can 3D print bosses and cut
threads or we can 3D print some type of male connectors.
4. We are thinking of machining the pintle injector out of stainless steel
and using EDM (electric discharge machining) to create the holes. We are
wondering about material compatibility issues (such as thermal expansion)
between the 3D print metal and a stainless steel pintle.
We are documenting our design calculations in Jupyter notebooks. They are
still a work in progress, but they can be viewed here:
https://github.com/psas/liquid-engine-capstone-2015/blob/master/Pintle%20Injector.ipynb
https://github.com/psas/liquid-engine-capstone-2015/blob/master/Nozzle_Construction/LFRE.ipynb
Any feedback would be greatly appreciated.
Thank you,
Kristin Travis