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Oct 17 meeting notes


Location: Norwood Autocraft


In attendance:


John Carmack

Phil Eaton

Russ Blink

Darin Smith

Bob Norwood


I brought my GPS telemetry system, mounted in a 4" diameter payload bay. It is basically a Garmin GPS unit connected to a tigertronics 1200 bps radio modem and 300mw transceiver.  A laptop connected to another radio modem logs the data and displays a 3D view of the incoming traces that you can fly around in.


I asked for advice about sharing a single power source among different devices.  Currently there are separate batteries in the GPS and transceiver, plus a 9V battery for the radio modem (normally it is run off a spare signal line from the computer, but the GPS doesn't have an extra power out).  That bothers me, and I would much rather have a single power source for everything.


The problem is that the transceiver takes 4.5V, while everything else can run off of 9-12V.  I could make a voltage divider, but that would waste a fair amount of power.  Russ also mentioned that even at 300mw, the transmitter would drain down a 9V battery pretty quick.  I may try setting up a six cell power supply that can be tapped off at either 4.5V or 9V.


The simple solution is to just use components that will all work at a common voltage, like 12V, which is what the VTVL platform will probably run at.


I also bought a kenwood handheld radio that has a built in packet radio modem, as well as going up to 6W of power.  I need to buy another one to start using it, but the complete package would save some clutter in the flight system, and it has the very nice feature of being able to transmit and receive at the same time if they are split between the 2m and 70cm bands.  I think a full duplex connection is going to be important when we are flying a controllable vehicle.


I was pretty disappointed with how quickly the data link went to shit with the little radios.  As soon as it goes around the block, the 1200 baud link starts getting errors and quickly drops to useless, even though the voice is still pretty good.  It may not be a problem launching straight up into the empty sky, and we can certainly just throw more transmitter power at it, but I am somewhat tempted to analyze the signal directly on the laptop CPU and see if I can do a better job than whatever discrete components are in the radio modem.


We discussed flight computers a bit.  I am going to do my uplink control testing with a basic stamp, but the lack of multiple background serial channels prevent them from being used for real, when we will need at least a few serial inputs (uplink, GPS, possibly a commercial attitude sensor, and the downlink) running simultaneously.


Darin and I don't have a preference, so Terry and Russ should discuss what they prefer and what capabilities it will have.  I would like us to settle on something, then get identical dev systems to the four of us that are likely to be doing coding.


Russ has started working on turning some motors, and has gotten directions to FMC for the 90% peroxide site inspection.  We don't have a schedule for when we can expect it yet.


Darin has the evaluation board for the accelerometers.


We fired a 28mm G motor on the test stand, which produced the expected data.


We then tapped the forward closure and connected it to our fill system so we could try using it as a cold gas thruster.


With 450 psi, we only got about one pound of thrust out of it, which is way down at the limits of the 100lb load cell and the slight stick in the test stand slides.  I expected more from it, because the standard reload got over ten pounds of thrust.  That was much hotter and probably somewhat higher pressure, but that shouldn't account for the order of magnitude difference.  The nozzle wasn't sealed well, and the fill lines may have been somewhat restrictive, so we may want to try this again sometime. Tapping the casing for a pressure gauge would be useful to see what our losses are.  We also need to get a higher pressure regulator, although we should pressure test the casing and check valve before we consider running any higher pressures.


It doesn't look promising for building a cold gas flight testing platform, but we should still test the pulse width modulation limits with it.


We left that open casing motor on instead of putting on Juan's motor with the baffle plate, so we would get a nice water jet from our testing this time.


After making a bit of a mess when the launch valve wasn't set right, we sat down and properly wired up our launch valve switch box. It has two momentary switches for open or close, so we can easily make a squirt to warm up the catalyst pack. It works fine.


New fittings for the fill manifold were installed.  We are now leak-free.


Added the check valve after the launch valve to prevent vacuum leaking through the engine.


We did two cycles of filling and firing with water at our full 450 psi pressure with no problems at all (and it made the expected impressive spray this time).  Once we get a bottom draining tank for the peroxide (so someone doesn't have to maneuver the hose end to the tank corner as it drains), it will just be a matter of pressing buttons in sequence to load everything up.


The water rocket thrust was noticeably stronger than the cold gas thrust. We didn't have the load cell in, but it did shove the stand slide back by itself, which the gas-only thruster didn't do. A nice demonstration of mass flow vs exhaust velocity.


We cut and plumbed a longer hose for the connector from the fill cart to the tank. We are going to need to have the line elevated above the tank fill point to prevent the line from filling with peroxide after pressurization. We need to keep in mind the tank height when installed in a rocket on the launch rail as well.


Stuff still in transit:


Larger load cells.

Silver screens.


To do for hot fire on the test stand:


The fill apparatus needs to have everything peroxide compatibility checked.


While it isn't mandatory, I hope we get the fill cart into its final form, with everything secured on a cart instead of spread out on a table.  We still need a 12V vacuum pump, a vacuum catch tank with gauge, and a bottom-draining tank for the peroxide.


Safety gear.  Buy whatever FMC recommends.


Wait for delivery of the drum of 90%.  We can do a short firing with our remaining peroxide, but we won't be able to collect much data with it.


To do for launch:


We need a 7" diameter rocket for the tank and engine.  If Phil or Neil don't want to cut up one of their existing big rockets, we need to build a new one.  Planning for this should begin soon.


Launch the rocket with a certified plugged motor, but with the tank installed and loaded with water.  We won't make this month's HPR launch, but we should seriously aim for next month's.


Arrange for our private experimental launch.  We will need the site permit, the FAA waiver, and the cooperation of the fire department.  Anything else?  Assuming we don't blow anything up, we can try a few flights with different amounts of peroxide loaded.  We will launch with my GPS telemetry, but I assume we will use a conventional altimeter for recovery for now.


We will need our own launch rail.  We may be a little light on liftoff thrust, so I want to be conservative and get as long of a rail as we can deal with logistically.  Depending on how we build it and how full we load it, the rocket should be between 20 and 40 pounds at launch.


To do for VTVL craft:


The next weekend I get to work on stuff (not this weekend), I am going to try to get a rough cut of joystick control of four PWM LED's through an uplink.


Transistors and control system for pulse width modulation testing of the solenoids.


Flight computer selection.


Full duplex communication.


Accelerometer testing.


Order a gyro and a magnetic field sensor for testing.


Cut our own small motors and catalysts.  They will only need to be about 10lbf each, because four of them will be used, and we don't want much more thrust than the platform will weigh.


Get Bob's metallic foam silver plated to test as an alternative catalyst to the silver screens.


I'll order a pre-machined distribution block so we don't need a bunch of T's in the plumbing, and I will get four of the smallest solenoids I can find.


We could probably start building the vehicle any time we want.  We can plumb everything up and water test before we have functional engines.  I still like the idea of building it on top of an inner tube.  We should discuss rough drawings next meeting.



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