Film cooling, while mechanically simple, faces a much more complex
dynamic environment. It is not uncommon for flow instability,
combustion instability, or nucleate boiling to disrupt or destroy the
cooling film and leave part of the chamber fully exposed. And maddenly
difficult to diagnose when it does happen - I highly recommend that all
tests be monitored by a thermal imager, with strict abort criteria
including not just peak temperature but location of peak temperature.
But even that will only tell you so much. If you've got liquid flowing
through a tube or jacket, then a simple flow meter will tell you how
much coolant you've got flowing and the geometry of the system will
define where. With an internal film, there's always guessing involved,
hopefully educated.
And getting the injector absolutely right is critical, not just in the
design but in repeatable manufacturing processes. What constitutes
"absolutely right" in injector design, especially FC injector design, is
also guesswork.
The FC engines I work with, mostly 100 lbf liquid apogee engines with Pc
~250 psia, have an operating life on the order of ten hours when they
work well. As always, you can probably improve on that if you're
willing to dial down the performance a bit.
John Schilling
john.schilling@xxxxxxxxxxxxxx
(661) 718-0955
On 2/23/2018 3:43 PM, Brian Feeney wrote:
It has been discussed previously how XCOR (Doug Jones et al) achieved long engine life particularly with the Saddle Jacket (SJ) engine design -- hundreds of cycles on a given engine with lots of apparent life left after engine tear down.
How does a Film Cooled (FC) engine compare to the SJ design in terms of longevity. Clearly the SJ has a higher Isp all other factors the same. The FC engine is somewhat simpler in design and fabrication and does not appear to have the degree of inner surface to outer surface thermal stress that you get with a traditional regeneratively cooled non SJ engine design.
Is the FC engine chamber and nozzle essentially undergoing the same / similar lower thermal stress on the inner chamber as the SJ engine and therefore would have a longlife?
In this case I am choosing to rate longevity as a higher priority over Isp performance for suborbital flights as opposed to more recently discussed orbital efforts.
The SJ is somewhat more complex with added failure and maintenance modes compared to an FC design. Maybe I'm overstating it in the context of the overall complexity of the full rocket system?? That said, it's never a bad idea to reduce part count, complexity and failure modes, especially in a complex system if one can afford the performance drop off.
I appreciate all comments... Thank you.
Cheers
Brian Feeney