Location: Norwood Autocraft
Next meeting at Long Range, unless notified.
We tested a small motor tonight, and learned a few things.
I took some pictures, which I will get on a web site
I brought a checklist for the procedure this time:
Fill and Fire Checklist
Everyone put on goggles and gloves.
Prepare buckets of water for emergencies.
Set the test stand up on the other side of a concrete wall
from the controls.
Position the observation mirror for a view of the test
Load the peroxide into the fill container.
Set the nitrogen regulator to 450 psi.
Open and close the launch valve. (it should have been left
Connect the fill cart to the flight tank.
Open the vacuum valve. (it should have been left open)
Start the vacuum pump.
Wait until the vacuum is drawn.
Stop the vacuum pump.
Close the vacuum valve.
Ensure the peroxide hose is properly submerged.
Hold the peroxide switch until all the peroxide is loaded.
Watch the tank pressure gauge for any unexpected increase
Hold the nitrogen switch until pressure is equalized.
Disconnect the fill cart and move it away from the test
TEMP UNTIL WE HAVE A CATCH CAN: disconnect the vacuum line
from the pump
Open the vacuum valve to release pressure in the fill system
and leave it
in a safe state.
Start logging the load cell readings on the computer.
Optionally pulse the launch valve open and closed to warm
Open the launch valve and let the engine fire until the tank
After the engine has cooled, make sure it is drained of all
Should we always wash the engines with water?
Put a plastic bag over the engine before putting it away.
There was temptation to overlap different parts, like
drawing the vacuum while someone is getting water. I think we need to be
more disciplined about following exact procedures, because we certainly aren't
doing everything flawlessly while winging it.
The other checklist lesson learned is that we must
acknowledge each checklist item before proceeding on. "Start
logging" was called out, but not acknowledged, and we didn't get a data
I wrote a little utility last week that simplifies the
logging of load cell data for our testing. We used to capture the meter
readings in hyperterminal, then clip out the relevant sections in an editor and
paste it into excel. The utility lets us start logging from the command
line, and closes the file when you hit escape, which lets you avoid having lots
of extra junk in the file. It automatically increments file names, so you
don't need to worry about stomping on anything (which I did once when saving
from hyperterminal), and it also automatically puts the file on the windows
This was the fourth or fifth time we had set everything up
for filling and testing, but it is still a huge hassle arranging power,
nitrogen, tanks, and the vacuum pump. We need to bump finishing the fill
cart up on the priority list. We should not (try to) fire an engine again
until all the fill apparatus is completely nailed down.
Related to that is the load cell meter, which currently
takes 120V. We should try and work something out so we can run that
without an extension cord.
We were testing a small 30mm motor that Russ made to the
same basic design as the large motor we fired successfully last week.
This is the basic size we are planning for each of the four lifting engines on
the VTVL platform. We had another pint or so of hand distilled peroxide.
Positioning the mirror for viewing the test stand was more
of a hassle this time because of the limited space to move around behind the
stand. The wall and door at long range was more convenient.
There was a miscommunication about the catalyst warming
pulse. I had expected we were going to look at how the pulse went before
deciding how to proceed, but Phil fired the motor quickly after the pulse
(which is what we did last time).
The motor didn't work. It rapidly jetted out all the
peroxide as a foam, with almost no catalyzed gas.
The peroxide seemed good, as it was clearly decomposing on
The catalyst pack was cut from the same foam as the last motor,
and there was obviously some amount of decomposition going on inside the motor,
so it wasn't completely bad catalyst.
We finally decided that we were just flooding the engine
with too much peroxide for it to catalyze.
We had catalyst surface area proportional to the throat size
of the bigger (successful) motor, but the feed system was still the same size.
We had been assuming that the feed system could be oversize
without causing any problems, thinking that a greater peroxide flow would
result in a higher chamber pressure, which would lower the peroxide flow, and
everything would balance out with the smaller motors having a smaller pressure
differential between the chamber and feed system.
That may or may not be true in steady state, but the lesson
learned was that when you are starting at zero chamber pressure, a high feed
pressure can push the peroxide through fast enough that you never get a chance
to build up chamber pressure to slow the feed down.
We ran water through the little engine, and we were
surprised at how strong a jet blasted out, compared to the gurgling flow we got
out of the big engines.
With computer control of valve pulses it would probably be
possible to build up the chamber pressure in a controlled manner, but the
general lesson is that the feed system should be restricted so that it doesn't
flow more than you need.
We changed the plumbing around so that we could use NOS
nitrous jets in the feed line before the engine. A 45 thousandths jet
gave a water flow that looked similar to what we got with the big motors.
If we had more peroxide, I bet it would have worked.
Once we have engines firing reliably, we need to tap a port
for the pressure transducer so we can see what chamber pressure to feed
pressure deltas we are actually getting.
Something that we didn't consider is that we might want to
try compressing the foam catalyst in the engines. We were feeling good
about the very low pressure drop across the catalyst bed, but it might turn out
that a significant pressure drop actually makes it easier to start the engines.
It certainly isn't crucial, because last weeks engine started well, but it is
something to think about.
We have the basics down well enough now that we need to
begin doing scientific development work. To start efficiently
accomplishing things, we need permanent facilities and a good supply of
I am hopefully going to be looking at some places this week
for us to set up permanent shop at. I talked with the mayor about the
various regulations, and it looks like the most serious issue for us will be
the noise. There are regulations regarding the maximum db at the edge of
the property line. We may wind up in an area outside city incorporation
We need to have a large enough supply of peroxide that we
can run several experiments in an evening. If we had had more tonight, we
probably could have had more successful runs after installing the restrictor
The home-brew seems to work well enough, but I don't like
the extra uncertainty that it adds to our results, and I am still nervous about
us doing it in any quantity.
The 30 gallon drum of 90% from FMC is $1200 plus about $1500
in shipping, but they won't sell it to us until we have a concrete containment
for the drum. They sell gallon samples for $300, which Russ is pretty
indignant about, but I think we should go ahead and get one. That would
only be a single full length firing of the large motor, but we can do a lot of
work with the small motors with a gallon.
Dumb rocket update:
Neil has started picking parts for the dumb peroxide
rocket. It will be a simple 7" diameter rocket with a single stage
recovery system. Mounting the flight tank will be the primary
challenge. We should probably try to include my GPS telemetry system,
which is currently contained in a 2' x 4" tube section.
We will make an adapter for either Russ's big motor or
Juan's motor so that it fits in a standard 98mm motor mount, allowing us to
test fly the rocket with water loaded into the tank on a certified motor to
make sure all the recovery systems work and that the rocket is stable before
running it with a peroxide motor.
It's going to be heavy. At a minimum, we are going to
have 12lb of dry hardware in the tank, manifold, launch valve, and engine.
More likely, it is going to be closer to 20lb unless Russ replaces the manifold
and launch valve with smaller custom cut parts.
With 10lb of peroxide loaded, plus the normal rocket mass
for airframe, fins, and recovery, it is clear that a 100lb thrust motor won't
get it going fast enough to be stable off the rail.
Juan's motor was speced so that we could open the throat up
and get 170lb thrust when we want to, and Russ's motor is even larger, but our
current launch valve is not even flowing 100lb/s worth of peroxide based on the
length of last week's engine test.
We have quite a bit of room for increasing tank pressure,
which will help some. We have been testing with 450-500psi, but the tank
is designed for use at 1100+ psi (it is DOT approved, so the burst pressure is
probably 3x that). Bob has some 2000psi N2 regulators, but we need to
pick one up for the fill cart.
We need to do more exact tests, but I am nearly certain that
the primary restriction is our launch valve. We are using an NOS remote
bottle valve, which is functionally perfect for our needs (two position
latching, current only required to change positions), but it doesn't flow all
that well. It is clearly more than sufficient to flood the 10lb engines,
but the large engines seem to become feed restricted around 50lb thrust or so.
We know that the large nitrous solenoid flows significantly
better than the bottle valve, but there would be two drawbacks to using
it. We would need to carry a battery on the rocket to keep the valve open,
and when the battery wore out it would leave the tank closed instead of vented.
We could go to a large normally open solenoid for the launch
valve, which would solve both the issues with the normal solenoid, but that has
some significant safety issues during fill. If the battery on the ground
wore down during filling, the rocket would launch by itself. That would
Russ and Phil are investigating machining our own
pyro-operated slide valve. I would like to avoid pyro if we can, in the
name of simpler turnaround.
I want to bring back up my earlier idea: use a decent
sized, pneumatically actuated valve, but put a check valve on the air
inlet. Connect an air hose to the check valve with a push-on barb.
To launch, pressurize the air hose, which will open the valve, which will
launch the rocket, which will pull away from the air hose, but the check valve
keeps the valve control pressurized anyway. Russ: consider using an air
hose instead of pyro in your design.
Both the pyro and pneumatic valves would prevent us from
pulsing the engine to warm the catalyst, which may be an issue.
We are going to have to get our own launch insurance, since
Tripoli's won't cover liquid propellant rockets. I am pinging John Powell
of JPA about their flight insurance provider. Theirs is pretty expensive,
so I hope it is pro-rated based on our far lower altitude.
We have the four small solenoids for the engines now.
I took two and Russ took two. I ordered a couple solid-state relays that should
be able to drive the solenoids, but Russ is also building a custom circuit for
The fittings we need should be here any day now. The
engines, solenoids, fittings, and jets connect together into a nice rigid
assembly for each corner of the platform.
I wrote code that runs multiple channels of PWM off of my
laptop parallel port. I tested it with LED's, but it should be ready to
hook to the engines as soon as we get driver circuits.
How well PWM modulation will work is our big uncertainty right
now. We should be able to find out in the next couple weeks. If we
have to change to something based on proportioning valves (probably driven by
stepper motors), I am concerned about finding something that will be small and
accurate enough for the 10lb motors. The full-flow jet orifice is already
The square aluminum tubing I brought looks like it is going
to work out fine. A 3' square platform should be good for us.
Phil had a very good idea for the initial testing of the
VTVL platform. Set up four posts around the testing area and hang the platform
between all the posts with shock cord or cable. Post and cord
lengths can be selected so that the platform is hanging off the ground, and has
several feet of free movement, but positively cannot hit anything or tip over.
If we start with this setup, we won't need landing gear
immediately, we just need the frame for four engines and support for the tank
I have a basic flight console program working. It is
designed to work with a modern joystick with twist for yaw and a separate
throttle. I wrote and tested it on a desktop system, but I have a USB
joystick on the way that will work on my laptop.
The current "flight logic" is dead simple, direct
control. The four primary lifting engines are set to the throttle value
modulated by the X and Y joystick positions. The twist yaw action
controls two additional engine levels (we don't plan on adding the yaw engines
until we are forced to).
The engine levels are used to control PWM outputs on the
parallel port. I am defaulting to 10hz for the PWM, which is pretty
marginal, but we will be experimenting to find out how fast we can go with our
Theoretically, we could hook this up and fly with it.
In reality, it is a fair bet that a person will not be able to control it at
all, let alone do anything intelligent with it. One of the first things I
will do to make it more controllable is limit the joystick modulation values to
the minimum needed to tip the platform, instead of the full 0% to 100% range it
If it turns out that it is manually controllable, we will
build landing gear and give it a longer leash.
If it isn't controllable, we will need to install the
attitude sensor and go to a closed loop flight control system. I think
that taking joystick position as a small range (say, +-5 degrees) of desired
pitch and roll values and letting the flight computer try to maintain that will
be sufficient control to let a human control the craft. Manual throttle
control and indirect positional control by tipping the platform should be
within the realm of what a person can manage.
Later, we may want to investigate fully automated landing
and flight paths, but that isn't an overriding goal for me if remote piloting
is working out ok.
Once we get it out from the posts, we still won't be flying
it high enough for a parachute to do much good in the early stages, so there is
a lot to be said for giving it a foam or inner tube base and flying it over
When it is hung between the posts, the direct control system
could just be wired directly to the laptop parallel port with a long cable, or
we can do a very simple one-way telemetry system. While we decide what
"real" flight computer we want to use, I can do crude telemetry PWM
control by having a basic stamp just copy the serial byte stream from a radio
modem onto the solenoid driver bits. That would only allow 120 switching
points a second, but the times could be dithered, and it would probably work out
This would be raw FSK data, not packet. To be legal,
we would need to stay under one watt of power in the amateur bands.
Once we put a sensor on board, we are going to want a full
duplex telemetry link with two radios. The flight control logic will
eventually migrate to the on-board computer, but development will be a heck of
a lot easier if we can treat the platform like a device, reading its sensors
and writing its actuators from the flight console.
I am getting the information for the serial output
magnetometer board, which sounds perfect for our attitude sensing needs.
Once we let the platform off the leash completely, we will
want a GPS on the platform.
Landing will be a lot easier if we have a laser
rangefinder. I have found a couple with serial outputs, but they are
$2000 surveying units.
So, our real flight system will probably need four serial
inputs (radio in, magnetometer, gps, range finder), one serial output (radio
out), and at least six digital outputs (PWM engine drivers).
We are currently debating a very wide range of possible
At one end, Russ is looking at tiny microcontrollers.
He is going to give me one of the dev kits to look at next week.
Terry Parks is suggesting something like the 68332 board
or the MPC555 board available from:
And I am looking at using a WinCE palmtop as the flight
computer with all the peripherals hung off the USB bus.