Ben's got the answer there, but if your running a test stand two
additions are
-you need to think though the flow rate after main chamber ignition, i.e
you will be probably be running the igniter for a second or two after main
chamber ignition, so make sure you don't shift into some really hot O/F. If
you are on a test stand it is easier to make sure your igniter feed
pressures are 50% over PC or greater and you should be fine, on a vehicle
you don't always have that luxury.
-Also, if you can, running a purge through the igniter after main chamber
ignition at about the pressure as PC will save the igniter from torching in
the case of leaks and some instability. Probably not worth it on a vehicle,
but for injector characterization tests it is very worth it. Unless you
really enjoy rebuilding igniters i guess.
Lloyd
On Sat, Oct 26, 2013 at 5:22 PM, Ben Brockert <wikkit@xxxxxxxxx> wrote:
1. Doesn't matter. Igniters are often run heat sink mode, and heating
goes up with higher pressures, so running at low pressures is
advantageous for minimizing heating. At the same time you do want to
run choked so that you can detect go/no go with a pressure transducer.
That sets the reasonable minimum around 30psi, so for off-nominal
propellant conditions a nominal operating pressure of 60-100 psi is
good. If tank pressure is lower than that then you take what you can
get.
2. If the igniter is plumed to main feed pressure, by nature you can't
get chamber pressure to flow up the igniter lines because there has to
be pressure drop across the rocket injector for the rocket to rocket.
You can add check valves if you're really worried about it, but I've
never bothered. If you hard start the engine or have severe
instability the Pc can be higher than propellant pressure, but then
you have other problems worse than igniter trouble.
A much more realistic concern is making sure that all of the
connections on the igniter are absolutely leak-free. Leaks will turn
into (likely rapidly enlarging) hot gas leaks when the igniter is off
and the engine is on.
It's common to have 1/8" tubing feeding small igniters. Use stainless
tubing, the weight difference with aluminum is inconsequential. Never
use aluminum tubing smaller than 1/4", it can't survive even small
fires when pressurized.
Ben
On Sat, Oct 26, 2013 at 5:57 PM, Graham Sortino <gnsortino@xxxxxxxxx>
wrote:
Apologies, in advance as I'm almost positive this question was already
answered but I can't seem to find the responses.
Re-phrased:
1. When designing a (GOX/Ethanol) igniter should the chamber pressure
be
higher or lower then the main engine pressure? Also, are there any
rules for
choosing an injector pressure relative to the main engine pressure?
2. If the igniter pressure should be lower than the chamber pressure
then
how does one prevent propellants flowing back through the injector
orifices?
Thank you very much.
Graham
On Thursday, May 19, 2011 10:16 AM, Joel Scotkin
<arocket@xxxxxxxxxxxxxxxxxxxx> wrote:
On 05/18/2011 10:45 PM, Robert Watzlavick wrote:
I'm going to be working on an augmented spark igniter (ASI) to try
and
improve the starting of my 250 lb LOX/kerosene motor. I was
wondering
about the chamber pressure for the igniter - I would think it needs
to
be >= than the main chamber pressure to keep it from flowing back
into
the igniter. Am I missing something? I've noticed that some designs
shut the igniter off after a few seconds and in that case, how do you
keep from the gases from the main chamber from backing up through the
igniter orifices and into the feed lines? The few igniter pictures
I've seen show small solenoid valves that typically aren't rated for
enough pressure to handle the main propellant tank pressures. My
kerosene tank pressure is ~490 psi (large drop through cooling
jacket)
and the LOX tank pressure is ~340 psi.
Thanks in advance,
-Bob
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