I wouldn't necessarily say an ASI or torch igniter implies a restartable
engine. I think of it as more insurance against a hard start since you can
verify igniter chamber pressure prior to opening the main valves.
I have seen the U-shaped pressure curve on every one of my tests. I assumed it
was due to chill down issues but I hadn't tried attaching the valve to the
injector. I would think as long as the thrust in the "U" is less than the
vehicle weight, it wouldn't be an issue, other than wasting a half second or so
of fuel. Of course, maybe that super rich burn generates more soot which helps
out chamber cooling for a kerosene engine.
On my test stand I used individually linked valves for flexibility but I linked
them together for the flight vehicle.
-Bob
On Aug 2, 2016, at 04:17, Ben Brockert <wikkit@xxxxxxxxx> wrote:
If the LOX valve isn't directly attached to the top of the pintle, to
get a clean startup you're going to need a "prime" sequence where you
just trickle LOX to chill down the LOX plumbing. Otherwise you end up
with a big U-shaped chamber pressure excursion as the LOX massflow
changes wildly due to two phase flow.
You do not have to open the LOX first. The logic of not pooling
propellant in the engine applies to LOX the same as fuel. You can lead
one over the other for various reasons specific to the system, but you
can also open them simultaneously. Armadillo physically linked their
valves together and did thousands of successful starts that way.
What is the nitrogen valve? I still argue that you need purges if you
plan to fire this engine more than once, which is implied by having a
spark torch igniter.
Ben
On Tue, Aug 2, 2016 at 7:14 AM, Kristin Travis <theeblueorchid@xxxxxxxxx>
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
