Steves advice is good, Alum.AN 37 degree fittings are surprisingly cheap on
eBay compared to McMaster and yes, NPT should be avoided but are sometimes
useful in case like cryogent connections where the CTE mismatch of
dissimilar metals cause Teflon o-rings to leak in AN fittings and Boss
seals are too expensive long lead time and high min. order.
The 'torch' igniter is also good advice, I never had a failure after ~1000
ignitions one of which can be seen in action on - neverworl.net - under the
LR-11 test in El Mirage. You can make a torch ign. out of AN fittings, I
used an AN-938 which has 3 female ports,Ox and exhaust. I use a pressure
switch on the ignitor which enables my prop valve circuit once the ignitor
Pc is nominal, usually 50% of the main CC Pc. This way the ignitor shuts
off when the main CC is ignited and up to pressure. I wire the F&Ox prop
valves and high voltage spark plug circuits in parallel for simplicity. Oh,
and use the same propellants for the ignitor as the main engine, simplifies
things.One exception, vaporize your LOx before the ignitor, avoids
sporadic 2 phase flow plus everything ignites in a GOx atmosphere.
Grease in your Pc transducer line will protect it from the ignition
transient but be careful about grease in a LOx line as it could freeze,
just use a >6" line to your ducer.
Enbay also has used flaring tools, get the 37 degree not 45 though they
will still work especially with soft metals, even SS if seated on a softer
male flare. There's a nice Ridgid brand 37 degree flaring tool in a metal
carrying case on ebay right now for $65, 1/2" thru 1".
Use alum. tubing, easier to bend and flare, the LR-11 test video used 1"
0.035" wall 6065-T6 alum. tubing which I hand bent with a lever bending
tool. It was tough but the bends came out perfect with no wrinkling, flared
good too. Bot F&LOx were plumbed with 1" tubing.
And use cavitating venturis, they make life much simpler for a number of
reasons and plumb them as close to the tank outlets as possible because all
delta-P throughout your system is transparent to the venturi including the
injector of your rocket. Only caveat is you ;lose ~ b15 of your feed
pressure but it's worth it especially om ground test stands and even some
flight.
Much more on the topic but this was long enough, hope it helps you, it took
me 50 years to learn it all but a good mentor will shorten that by a
magnitude or two.
Good luck and please put safety first, accidents spoil your hobby or career
to be.
Ken
On Wed, Feb 3, 2021 at 8:21 AM Stephen Daniel <swd@xxxxxxxxx> wrote:
Pressure sensors on a budget: you can buy them on eBay for $35. No
branding, no specs. They have 3 wires, 5V power, ground, and sense out.
The sense out is 0.5V when there is no pressure, and 4.5V when at the max
pressure the sensor is rated for.
If you are going to use an Arduino to control everything, get one like
this: https://www.pjrc.com/store/teensy40.html. It's cheap, it is
_very_ fast, and it has a reasonable A/D converter for managing sensors.
You protect sensors by having 6 to 10" of 1/8" OD metal tubing between the
sensor and whatever you are sensing. That gives you enough distance so you
won't toast them. In oxygen service, pack the sensor with krytox grease.
That will keep the oxygen away from the active elements and any seals that
are in the sensor.
I know that fittings is a subject that many have opinions on. For small
projects, which means small orifices in your valves, I would avoid NPT
fittings when possible. I have spent more time opening up valves and
picking bits of teflon tape out of the valve than I care to admit.
Whenever possible I use 37-dgree flare fittings (AN type). No teflon tape
required, and they can be taken apart and put back together a reasonable
number of times. They are not cheap, but fittings that actually work never
are. In addition to the fittings you'll need a flaring tool. Of course,
some number of NPT fittings are inevitable.
On ignition sources: I would start by building a torch igniter. This is
essentially a small rocket motor that is ignited by a spark plug. You can
get spark plugs that will fit into a #10-40 threaded screw hole. You make
your igniter robust enough so that when the propellants light, even if the
igniter lights with a pop, nothing comes apart. Build the igniter with a
port for a pressure sensor. That will allow your arduino to know if the
igniter is lit, because the combustion chamber pressure will come up. Once
the igniter is lit you can sequence up the main propellant valves. I would
build the igniter and get it working reliably before I spent a dime on
parts for the rest of the motor.
-- Steve
On Wed, Feb 3, 2021 at 10:41 AM Robert Steinke <robert.steinke@xxxxxxxxx>
wrote:
Hi Charlie,
Welcome to arocket. It sounds like you are taking the right attitude of
experimenting safely and I hope you learn a lot and have fun with your
project. I'm going to give some meta-advice about how to proceed in
general.
1) Your first two questions in any situation should be "Do I have enough
knowledge to do this safely?" and "How could this go wrong and hurt me?".
When I started working with hydrogen peroxide as an oxidizer I knew nothing
about peroxide and little about rockets, but I had a mentor who did know
hydrogen peroxide that I could go to with questions. I also read a lot
about what other people had done with it before. So I felt confident, not
that I knew everything, but that I could experiment while being aware of
the hazards and how to manage them. If you don't have that confidence
don't give up, just read and ask questions more. Having someone more
experienced who can review what you plan to do is a great help. Asking on
arocket is a great resource. If you still have parts of the system that
worry you don't be afraid to switch to something safer. Just measuring the
thrust of a water rocket is quite an involved project.
2) Focus on subsystems. Don't think "I'm going to build a rocket", think
"First, I'm going to build (or get) a valve that I can open and close by
flipping a switch from the other side of that wall." You are in for a long
and involved process, and focusing on small goals will keep that feeling of
making progress. You may think that being able to demonstrate a remotely
operated valve is a small easy task. If you are doing it with stuff from
the local hardware store on a low budget it's not.
3) If you are building your system with inexpensive off the shelf parts
you will often be in the frustrating position of not being able to get as
much information as you would like about them. Rockets tend to have pretty
specific requirements, especially on the oxidizer side, and mass produced
consumer devices are where you can get complex parts cheap. You may find
yourself wondering, what material is this o-ring? and the answer is, this
part is for a Mazda MX-5. Why do you need to know that. Just install it
in your car. But you're not planning on using it in a car. There really
isn't any solution to this other than you are going to be spending a lot of
time hunting for spec sheets, and your decision of which part to buy may
depend on which has the most complete spec sheet. It's also depressing to
disassemble a brand new perfectly working part that you just bought just so
you know exactly what's in there and what materials the oxidizer touches
and then you replace all of the o-rings with ones you bought that you know
what the material is even though the existing o-rings might be that
material, but you can't be sure. Sometimes that's what you need to do to
be confident that you are operating safely.
On Tue, Feb 2, 2021 at 3:19 PM Charlie Jackson <
charliejackson8629@xxxxxxxxx> wrote:
Hi everyone,
I’m looking into various parts for the plumbing side of my engine
(valves, pipes, regulators etc) and I have a few questions.
1)Can generic, fairly inexpensive solenoid valves be used for
fuel(isopropyl alcohol) and Gox lines on the inlets? I’m not concerned
about high pressure as all lines will have a safety check valve downstream.
For fuel tanks I certainly will have high pressure valves.
2)I intend to implement a microcontroller for autonomous abort options,
any tips on ways to actively read combustion chamber properties like
temperature and pressure electrically with a sensor and how would I prevent
it from getting damaged?
3)Some parts of the plumbing will be quite awkward to bend into various
directions so I’ll have to use fittings, is this prone to leakage? I’m
using nitrogen, gaseous oxygen, and alcohol. And if so, how should I
mitigate the leakage?.
4)How do you suggest I implement ignition sources into my design? I’m
looking for engine re-start capability so things like electrical matches do
not suffice
Apologies for the non-stop questions I ask on here. I’m fairly young (14
years old) and I love learning about rocket propulsion so I can’t
articulate how thankful I am to all of you.
From Charlie.