As others point out, depends on details. I've gone LOX first for two
reasons - LOX valve sluggishness and combustibility limits. For LOX
valve sluggishness, enough lead (and we are talking small fractions of a
second) gives the engine controller time to figure out if the LOX valve
opened and react appropriately if it hasn't. With combustibility limits,
if the torch igniter is fuel rich to keep the igniter cool enough, then
the LOX lead provides a better mix ratio in the chamber than a fuel lead.
Physically linking the two propellant valves gives all the same benefit
if you are regenerative cooling with the fuel, and the LOX valve is
sufficiently close to the injector. If the LOX valve is sluggish or
frozen stuck then the fuel valve isn't opening any sooner or faster, and
the longer fuel flow path gives a nice oxygen lead.
Dave
On 8/1/2016 10:14 PM, Kristin Travis wrote:
Hello,
I'm working on the Portland State Aerospace Society liquid fuel engine and I'm wondering about the ignition sequence for main valve timing.
It's a LOX/ethanol pressure fed system. We have a LOX centered pintle injector and we plan to use a GOX/ethanol spark torch igniter similar to Robert Watzlavick's design. http://watzlavick.com/robert/rocket/
At the injector, I've heard we need to lead with LOX so that fuel can't pool in the engine and create a hard start. I've been digging in the archive and online and I can't find specifics.
Does anyone have thoughts about how much time should elapse between stages of the ignition sequence? Specifically how much time should pass between injecting the LOX and the ethanol into the combustion chamber?
Would this be the correct sequence?
close fuel vent
close lox vent
open nitrogen valve
start igniter spark
open GOX igniter valve
open fuel igniter valve
check igniter pressure
open main LOX valve
open main Ethanol valve
Thank you,
Kristin Travis
