Brett,
That sounds interesting. I suspect a large cryogenic sliding seal will be
practically impossible to get leak tight, so plan for some leakage. I also
do not think you should attempt to make it perfectly leak free. A tight
seal will mean that the plunger will have added friction and be more
difficult to push down the bore. This is trade off that has to be made in
hydraulic cylinders as well, where the highest efficiency cylinder will try
to balance the mechanical friction against the power loss from fluid
leaking through.
I'm surprised no one else has pointed this out, but any valve can be a cryo
valve if you have enough torque. Since you will be pumping your oxidizer
with direct electromechanical actuation, you should be able to easily
control pressure. Because of this, you may not even need a main oxidizer
valve, but you could instead use a check valve with a high spring force.
When driving the piston the check valve will open, and when you stop
driving the piston the pressure in the tank will rapidly drop, allowing it
to close. There is the caveat that you should expect such a cryogenic check
valve to leak. That is going to add operational complexity but I can't
judge whether or not it is a good trade-off.
Your plan for instrumentation seems to be the correct approach, but it
never hurts to have a few more thermocouples.
This sort of pressurization scheme is very rare, because rather than using
stored energy in the form of pressurized gas to drive out the propellant,
you have to convert electrochemical energy in the batteries through a motor
and gear boxes. This is going to weigh significantly more than simply using
a gas pressurization system.
Thanks,
Lars Osborne
On Mon, Jul 16, 2018 at 3:16 PM Carl Tedesco <ctedesco@xxxxxxxxxxxxxx>
wrote:
Of course you could add your own actuator, but the servo may need to be
quite large to handle the torque.
--- Carl Tedesco
*From:* arocket-bounce@xxxxxxxxxxxxx <arocket-bounce@xxxxxxxxxxxxx> *On
Behalf Of *Brett Coles
*Sent:* Monday, July 16, 2018 2:05 PM
*To:* arocket@xxxxxxxxxxxxx
*Subject:* [AR] Re: Cryo-Valves (and other related questions)
Thanks Carl!
Do to think we could just use a regular cryo valve for the others and
actuate them with a servo? They won't return to a position if there's power
loss, but the only problem I can see with that is that the run valve may
get stuck open.
On Mon, Jul 16, 2018, 11:33 Carl Tedesco <ctedesco@xxxxxxxxxxxxxx> wrote:
I would make the fill valve normally closed and the vent valve normally
open. That way if you lose power you vent off the pressure, but not lose
your propellant load right away.
FlowServe/Worcester makes actuated cryo ball valves. Price is about $600
each for 3/8” pipe size. They are only rated to 1000 psi, but we have used
them higher. We do have to replace the stem seal occasionally. Cryo
solenoid valves are a little harder to find in high pressure. Try Valcor,
Asco and Jefferson. Those that are high pressure have very small orifices
(0.05” and 1/16” are common). High pressure, high-flow valves are not yet
common.
--- Carl
*From:* arocket-bounce@xxxxxxxxxxxxx <arocket-bounce@xxxxxxxxxxxxx> *On
Behalf Of *Brett Coles
*Sent:* Monday, July 16, 2018 8:07 AM
*To:* arocket@xxxxxxxxxxxxx
*Subject:* [AR] Cryo-Valves (and other related questions)
Hi all,
I'm doing a little research on hybrids that will involve building a small
hybrid motor. I've built solids before, but don't have any experience with
hybrids or liquids. I want to make sure I'm not overlooking anything.
Chamber Pressure is 1,000 psi
Pressure Drop across the injector is 200 psi
We designed the LOX lines to have a maximum oxidizer velocity of 10 m/s.
Nominal thrust is 200 lb.
I have a couple of questions:
1. We are planning on using a mechanically-driven piston instead of
regulated high pressure gas. In my head, it seems like it should work as
long as we can get it sealed correctly, but that doesn't mean it will
actually work in practice.Thoughts?
2. I am having a difficult time finding actuated cryo-valves that are
normally open (in case we lose power, I want to automatically vent.) The
valves I'm finding are either high pressure or cryo, but not both; or they
are not normally open. Anyone know any good suppliers? Could I modify a
high pressure valve by swapping out the seals and drilling a vent in the
ball?
3. I am attaching an image of the proposed flow diagram. Is it missing
anything/should I take anything off? (We are curious if the main LOX valve
needs to be there since the check valve could fill that purpose, but I am
nervous about that since the crack pressure is 5 psi.)
4. How can we measure the level of the LOX in the tank/is it even
necessary? Also, what instrumentation is necessary/good to have? I'm
thinking Chamber and Tank Pressures, Chamber wall temp, LOX tank temp,
Injector Temp. A flowmeter would probably be good, but we think we can
measure the flow rate based on the position of the piston.
Any advice you guys could offer would be great! Thanks in advance!
--
Brett J. Coles