Hi Everyone,
Thanks for the responses, it's nice to have such a knowledgeable community
on this list willing to dialog with interested engineering students. I'm
also helping out with PSAS's liquid propulsion project. This is planned to
be a restartable static test engine, hence the augmented spark igniter.
To expand a bit on Kristen's question, more generally speaking how do we
define design requirements for MOV/MFV actuators? The advice offered by
Huzel & Huang, and in NASA SP-8097 (Liquid Rocket Valve Assemblies) seems
broad and lacking in specifics. Sizing the actuator should be based on the
required response time and estimations of the effect of dynamic and
friction forces acting on the ball, stroke length, trapped fluid, the
inertia of moving parts (the ball and extension) inasmuch as they impact
the valve actuation timing, and how repeatable it is. However, I haven't
seen much information on determining the required response time for main
valves a priori. I assume much of the sequencing is detemined through
testing, however I'm still curious if there are any general guidelines or
rules of thumb. Based on anecdotes on this list, and elsewhere is seems
like a rule of thumb repeatable sequencing precision in the low 100's of ms
is required(thanks Lloyd). We are using a Worcester C44 cryoball valve with
a Worcester pneumatic Series 39 actuator (piston, double-rack and pinion
type). We expect the actuation time of the MOV to be about 2 seconds. I'm
not sure how precise or repeatable this timing is yet, though I assume we
will be able to have a ~200ms LOX lead (fully open post chilldown; the
pintle will connected to the main valve via braided flexline). We are not
sure if this is too slow or not. Can you guys offer any advice from
experience or point us towards references that might shed light on the
issue?
On Wed, Aug 3, 2016 at 12:54 PM Lloyd Droppers <ldroppers@xxxxxxxxx> wrote:
Looks like a reasonable first pass to me for timing.Ad astra
-I presume the nitrogen valve pressurizes both the LOX and the ethanol
tanks. If so make sure you wait long enough that your LOX tank is
pressurized to at least ~90% of full pressure before going to the next
step. This may take a surprisingly long time depending on your exact set up.
-Are you doing a prechill at all before or during this autosequence? It is
not necessary, but it is nice to have, and it will change the amount of
time it takes LOX rather than GOX to enter the chamber quite a lot.
I like to think about timing in terms of when the propellant enters the
chamber and back out the valve timings from that using the valve opening
time and hydraulic lag from cold flow data. For a small LOX ethanol engine
I think you want a LOX lead of between ~0.0-0.2 seconds into the chamber if
you have a torch igniter running. The initial Oxidizer will be GOX / Fluffy
lox so having OX come first by a small amount makes sense.
Also if you have long fuel lines make sure you purge them the same amount
each test, as the tube fill times, especially on test stands, can be quite
significant.
Lloyd
On Tue, Aug 2, 2016 at 1:15 PM, Thomas McNeill <thomas.mcneill@xxxxxxxxx>
wrote:
I agree with Ben. Solid igniters are very easy to get going. You can--
use model rocket motors, like I have done, or make your own grains. I used
Epoxy and KN for my home made ones.
On Tue, Aug 2, 2016 at 2:26 PM, Ben Brockert <wikkit@xxxxxxxxx> wrote:
On Tue, Aug 2, 2016 at 6:11 PM, Robert Watzlavick <rocket@xxxxxxxxxxxxxx>
wrote:
I wouldn't necessarily say an ASI or torch igniter implies arestartable 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.
You can put an interlock in a pyro igniter. On an end-burning igniter,
embed a thin wire near, but not at, the start of the igniter grain.
When it is disrupted you have a reasonable expectation that the
igniter is burning in a compter-sensable way.
That said I'm all for spark torch igniters, but if I was trying to get
a liquid engine project working as simply and quickly as possible it
probably wouldn't be included. Solenoid valves to run the igniter can
easily cost as much as main ball valves.
Ben
