[AR] Re: Fw: Igniter questions
- From: Robert Watzlavick <rocket@xxxxxxxxxxxxxx>
- To: "arocket@xxxxxxxxxxxxx" <arocket@xxxxxxxxxxxxx>
- Date: Mon, 28 Oct 2013 12:27:53 -0500
I run my main chamber at 250 psia and the igniter at 130 psia. I don't use a
purge but shut the igniter off 0.5 sec after the main chamber starts. The
igniter pressure tap eventually matches the main chamber pressure. I do have
check valves installed in the igniter lines and I clean the igniter gox line
after each run. An igniter purge would help that. Haven't noticed any damage to
the igniter after the runs but it is made of SS-304 vs. aluminum for the
chamber.
-Bob
On Oct 28, 2013, at 12:00, Graham Sortino <gnsortino@xxxxxxxxx> wrote:
>
> Hmm... Thanks. I wasn't planning on using a purge because the oxidizer is gox
> (not lox) and the engine is pretty small but definitely worth considering.
>
> Actually I was planning on doing exactly as Thomas suggested ie. Turn off the
> oxidizer once the main engine has been started and then just run fuel as a
> coolant. Otherwise how do you keep the igniter from getting too hot?
>
> This is purely a test stand engine BTW.
>
>
> From: Jonathan Goff <jongoff@xxxxxxxxx>;
> To: <arocket@xxxxxxxxxxxxx>;
> Subject: [AR] Re: Fw: Igniter questions
> Sent: Mon, Oct 28, 2013 3:45:19 PM
>
> Thomas,
>
> That's a dangerous one in case you get an igniter oxidizer leak, say due to
> debris getting into the igniter ox valve and preventing it from closing
> properly. Not saying that we learned that one the hard way at Masten, or
> melted any engines due to an igniter not actually shutting off because of
> such a failure...
>
> ~Jon
>
>
> On Mon, Oct 28, 2013 at 9:22 AM, Thomas McNeill <thomas.mcneill@xxxxxxxxx>
> wrote:
>> Instead of running a purge you can keep the fuel flowing through the igniter
>> to keep it cool.
>>
>> Thomas
>>
>>
>>
>> On Mon, Oct 28, 2013 at 10:59 AM, Lloyd Droppers
>> <lloyd.droppers@xxxxxxxxxxxx> wrote:
>>> 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
>>>> >> _______________________________________________
>>>> >> aRocket@xxxxxxxxxxxxxx
>>>> >> http://exrocketry.net/mailman/listinfo/arocket
>>>> >
>>>> > _______________________________________________
>>>> > aRocket@xxxxxxxxxxxxxx
>>>> > http://exrocketry.net/mailman/listinfo/arocket
>>>> >
>>>> >
>
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