[AR] Re: Contraction Ratio and Characteristic Length Estimates for a 500 lbf Engine
- From: Ben Brockert <wikkit@xxxxxxxxx>
- To: "arocket@xxxxxxxxxxxxx" <arocket@xxxxxxxxxxxxx>
- Date: Fri, 11 Nov 2016 19:49:30 -0800
On Fri, Nov 11, 2016 at 7:00 PM, Graham Sortino
<dmarc-noreply@xxxxxxxxxxxxx> wrote:
Thanks all for the responses. I realized this is a bit of a subjective
exercise but I wanted to just make sure my assumptions weren't way out of
bounds and it sounds like they weren't. I also did find a few other
instances of projects posted online in addition to Bob's comments below and
it seems like most are using a higher contraction ratio to get a short/fat
chamber. At first I was a bit worried that this would not provide enough
particle stay-time but it sounds like this is not the case.
L* is meant as a way of comparing the dwell time of chambers without
having to get too deep into the math. All else equal, two chambers
with the same L* but different geometry will end up with roughly the
same dwell because the gas flowing through a fatter chamber will be
traveling slower than the gas in a thin chamber, but travel a shorter
distance.
Chambers tend to be short and fat because it cuts down the total
surface area that needs to be cooled; a sphere is theoretically
optimum. People rarely build spherical liquid rocket chambers any
more, though the advent of single piece additive manufacture will
hopefully lead to reevaluating some of the assumptions for
manufacturability baked in to most engines now. Rectangular channels
in the walls are another such assumption based on what was practical
to machine.
Ben
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