July 30 and August 3, 2002 Meeting Notes
Vehicle Work
We got a tremendous amount of work done this week. We are nearing completion of two full
vehicles.
The seated lander should be flying again next week.
We straightened the bent frame by wedging it between the
trailer and Joseph’s tractor, and running a floor jack under the middle until
it straightened out. Russ gave it a
quick paint job.
The foam pad legs have been replaced by wire rope isolators
directly underneath the main frame triangulation points. The small vehicles worked fine with legs
sticking horizontally out from the vehicle, but the big vehicles were
constantly bending the legs on even fairly mild landings, so we are getting rid
of the concept. Initially, the vehicle
was designed so that it could be placed upside down without intruding on the
pilot, but given how readily the legs bend, that never would have provided
protection anyway. The wire rope
isolators (from www.enodine.com) that we
have on right now don’t have much travel (about 1.5”), but the basic technology
is almost ideal for a VTVL vehicle, because they absorb shock in all three
axis. We may wind up getting some
custom ones made, because the ones with larger compression strokes are way too
stiff for our needs.
All of the engine mounts are now positioned so that the
nozzles are above the frame. After
ruining two sets of nozzles all around in our big crashes, we are now trying to
protect them a lot better.
The tube vehicle is going to take a couple more weeks before
it is ready to fly.
Gem City metal spinning (www.gcmetalspinning.com) will have
our nosecones ready by the end of next week.
They aren’t going to fit perfectly, but we will fill in the gap with
foam, and it will work for this initial vehicle. I am hoping that custom filament winding is reasonably priced for
future vehicles.
We bought a new parachute and deployment bag from Butler
Parachutes (www.butlerparachutes.com). We will be cutting the nosecone and
fabricating the “escape tower” rocket system for pulling the parachute next
week.
We made some little 45 degree angle reflectors to mount on
the side of the rocket so we can have the laser altimeter and a video camera up
near the top of the vehicle. I bought
some first-surface laser mirrors for this, but our hand tests are somewhat
inconclusive on if this works with the altimeter or not. We will get it properly mounted and tested
soon.
Russ took apart the battery pack for the laser altimeter so
we can hook it up to be powered by the main vehicle power supply. Annoyingly, it needs 6v power, which means
we need to add another power converter.
We decided that the little 1” motors are not going to be
sufficient control authority for the tube vehicle. If it was perfectly balanced, they would probably be sufficient,
but they wouldn’t be able to cope with even a 2” offset in the center of
gravity with the main engine at full throttle.
We can statically balance it that well by balancing the vehicle on the
engine bell, but a gust of wind could probably cause the peroxide to slosh
enough to get it out of range. We are
rebuilding it for the 2” engines, which will give 4x the control authority.
We added a 1.5’ extension at the top that the nosecone will
be attached to. This allows us to have
a platform just for the parachute system, and attach the altimeter and video
camera underneath it, with clearance above the electronics box.
New Catalyst Packs
I traded some email recently with Arvil Porter ( www.arvilporter.com ), who is one of the
old time rocket dragster builders, as well as the guy building the rocket
motors for Brian “rocketguy” Walker. He
made some points about even pack compression and not needing anti-channel rings
later in the motor, so, since we needed to build a set of four 2” motors for
the tube, we experimented with a new packing sequence.
Our previous 2” motors had the following sequence:
2 ACR (anti-channel-ring) spacers at the top to allow the
peroxide to spread out
6 stainless screens as spreaders
ACR
10 sets of one stainless screen with two silver screens
ACR, press to 1500 psi gauge indicated (about 900 psi on the
pack)
10 sets of one stainless screen with two silver screens
ACR, press
10 sets of one stainless screen with one silver screen
ACR, press
14 sets of one stainless screen with one silver screen
press, retaining plate
A total of 50 stainless screens and 64 silver screens.
We weren’t using a solid spreading plate because it probably
introduces a bit more latency for pulsed engines, but not having a solid plate
to press against does keep the pack from being as evenly compressed.
We had resorted to the doubled silver screens before we had
a press gauge, so it was possible that they weren’t really needed with proper
compression, which would give the pack less opportunity to loosen with time.
I was pressing on the ACR because I was using a hollow tube
(a 1” engine, actually) to compress the screens, and it wouldn’t bear on the
center anyway.
The new pack experiment was:
2 ACR as spacers
Solid spreading plate with 29 0.060” holes, which should not
be a pressure drop compared to the main engine jet
6 stainless screens as spreaders
10 sets of one stainless screen with one silver screen
press to 2000 psi guage indicated (about 1200 psi on the
pack), using a full plate to get even pressure
ACR
10 sets of one stainless screen with one silver screen
press, ACR
10 sets of one stainless screen with one silver screen
press, ACR
10 sets of one stainless screen with one silver screen
press
10 sets of one stainless screen with one silver screen
press
10 sets of one stainless screen with one silver screen
press, retaining plate (it took quite a bit of force to
screw the nozzle on)
A total of 66
stainless screens and 60 silver screens.
There are a few other points of note for this test set: I exclusively used the 1 7/8” bore spiral
rings as ACR, while the earlier engines used 2”, which wrapped around to a
triple layer at the crossing point, taking up more depth. The 1 7/8” don’t exert as much force on the
wall, but sit flatter. We are also using
isopropal alcohol in our ultrasonic cleaner now, while previously we were using
methanol.
All tests were with the 1/4" ball valve instead of a
solenoid.
We did the initial break in run with two liters of peroxide
at 300 psi initial, with a 0.080” jet.
The first brief opening of the ball valve let a liquid slug of peroxide
come all the way out of the engine, as is common with fresh screens. After that cooked off, the second pulse was
perfectly clear, and running out the remainder was perfectly clear and smooth,
making 32 pounds of thrust.
The next run was at 600 psi. The warmup pulse was almost instantly clear, and the run was a
perfectly clean and smooth 49 pounds.
The next run increased the jet size to 0.120”, and we
skipped the warmup pulse altogether.
Thrust rose to 119 pounds, and was still perfectly smooth, but thrust
was now decaying proportional to the tank pressure, indicating that the
metering jet was no longer the primary restriction.
We then made a long duration run with about six liters of
peroxide, again skipping the warmup pulse.
It ran for 22 seconds, staying perfectly smooth, even as the pressure
and thrust decayed.
To see if the long run had made any permanent changes in the
pack, we did other things for a couple hours to allow it to cool back down to
room temperature.
We did another two liter run at the end of the day. We gave it a warmup pulse, but it probably
didn’t need it. The run was again
perfect.
I suspect that this packing arrangement would work fine, and
have a lower pressure drop, with 20 mesh silver screens, as opposed to the 32
mesh screens we use. With the current
arrangement, the maximum thrust is lower than with the looser packing, but it
is probably justified for the excellent catalyzation. I mistrust engines that aren’t jet restricted, because it is easy
to have them go rough after things settle in, so we will probably put a 0.100”
jet on these motors for the tube vehicle.
We’ll see how these behave after a few hundred seconds of run time.
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