December 20, 2003 notes
We have been experimenting with different ways to load the
big vehicle with propellant. Without a
launch license, we are limited to somewhat less than three drums of propellant,
so our current handling procedures are based around drums. When we fly larger loads, we will probably
be loading directly from a large capacity storage tank
We have previously loaded the big tank by filling the 80
gallon tank on the trailer, then pressure feeding that into the vehicle tank. This leaves the problem of getting it into
the transfer tank, and also gives us another good sized piece of equipment to
cart around. Another problem we had was
that pressurizing the transfer tank to push the propellant into the vehicle
tank caused the big hose to kick around quite violently.
A while ago we had tested a McMaster 4326k28 air filling
drum pump that basically lightly pressurizes the drum to push fluid out. It didnt flow close to the listed 18 gpm,
but it was still a pretty nice way of draining a drum. We put together our own system to do the
same thing directly from out trailer plumbing and directly into the vehicle
tank. We learned several things doing
this. 7 psi will cause a plastic drum
to bow up noticeably, and is about as much as you want to pressurize one of
them, but that isnt much pressure at all for transferring fluids. Our first test with a ½ diameter line
produced a pathetically low amount of flow.
Conveniently, the 2 diameter polyethylene tube we have will just barely
fit down inside drum bungs with 2 NPT threads, so we were able to build a pump
outlet that was 2 diameter all the way to the vehicle. This flowed quite well, emptying a drum in a
little over two minutes.
This would work, but would require a manual operation to
disconnect the 2 PE tube from the vehicle and connect a smaller reinforced
tube for pressurizing, and we have found that if you dont blow the big lines
clear with fairly high pressure gas, they tend to hold a LOT of liquid. We tried reducing the line from the drum
pump to a 16 hose, and found the time of about six and a half minutes to drain
acceptable. We then tested it actually
filling into the vehicle, which means flowing through a (big, low resistance) check
valve, and up into the tank, fighting some pressure and head forces. This was less of a problem than expected,
and it only took an extra 15 seconds or so.
A couple extra parts will be in next week to let us pressurize directly
through the same line without disconnecting anything.
Pressurizing the mostly empty vehicle tank to operating
pressure is going to take longer than loading the liquids. It takes two minutes to take one nitrogen
bottle from 2500 psi down to 400 psi, and that doesnt even get 50 psi into the
big tank. We still have to do
experiments to find out what the relative times for gang filling all at once
versus cascade filling will be. We might
also bypass the (high flow Tescom) regulator for the tank filling and just let
bottles flow directly into the tank with no intervening restrictions, since we
know it wont be possible to over pressurize with six bottles. It doesnt look like a single six pack of
nitrogen bottles will get us to 300 psi unless we vary patiently cascade
load. This is a highly convenient
operational point, so it may dictate our operating pressure.
We have been leaning towards powered landing instead of
parachutes for a couple months now. The
primary incentive has been that range safety and potential casualty
calculations really dont like big parachutes, because under the right set of
failures that causes them to mis-deploy, the drift can be many tens of
miles. Arguing for redundant interlocks
on parachute deployment wasnt well received, but a naturally unstable vehicle
that only lands under power has an extremely limited area that it can possibly
The huge advantage of powered landing is operability. Even without any effort to correct for wind
on the ascent, the landing point will only be a mile or two from the launch
point, and there wouldnt be any parachutes to repack or crush cones to
replace. Forget reflying in two weeks,
it could fly again in an hour.
There are two primary drawbacks: While it is safer for third
parties, it will be a much nervier ride for a pilot, because if the powered
landing doesnt work right you dont really have enough time to do anything
about it, unlike a parachute failure at 10,000. It also adds a significant amount of weight the landing gear is
100 pounds, and the reserve propellant is about 400 pounds. Deleting drogues and parachutes, including
the redundant ones, only gets you about 150 pounds back. The 850 gallon tank we are using cant
possibly make X-Prize flights with powered landing.
We purchased the largest tank of this class we can get
1600 gallons. The workmanship on the
tank isnt as nice as the smaller ones, with visible waviness on the sides,
likely due to the very long unsupported liner distorting more under winding
tension. Of course, this tank weighs
almost twice as much, so the vehicle will also need heavier landing gear, yet
more reserve propellant for the weight and worse ballistic coefficient, and
more engines, but there is still quite a bit of margin for us.
For a powered lander, the cabin is going to be a cylindrical
section at the base of the tank, both to keep the CG far back for stable
descent before throttling the engines up, and to allow people to step in and
out of the vehicle without requiring ladders.
With the cabin at the bottom, the entire nose is going to be
a thin gauge fairing. We may stick our
tank pressurization system up there if we need to fill the tank really full,
but it will still be mostly empty volume. We will probably have an emergency drogue parachute even though we
wont have a main canopy. If the flight
control has completely failed, a 14 drogue and the very long crush cone may
bring the vehicle down at a rate (probably 80 100 mph) that would at least
allow us to recover some of the equipment.
We are going to fly the 850 gallon vehicle until we crash
it, then assemble the cabin-on-the-bottom vehicle.
We have done a few more things to improve our test trailer,
adding a three-way valve to let us change between the big and small tanks
without disconnecting the feed lines, finally added a vacuum gauge to the
vacuum pump so we arent just going by sound, and plumbed the repaired coriolis
flow meter in line with the big tank.
Unfortunately, the flow meter still isnt working. Running through the 2 diameter S curve
inside the flow meter also gives a very messy cutoff, with propellant
sputtering out over several seconds, rather than cleanly depleting.
We added two more 100 gram catalyst layers to the test
engine, giving a total of five layers, all initially loosely packed, but after
running, most of the layers were packed down to half their size.
This gave a steady and repeatable 510 lbf at 282 psi tank
pressure with the bored out nozzles, and no hint of undecomposed peroxide.
This still wasnt what we wanted, so we drilled a 0.21 hole
in the spreading plate to allow more peroxide flow. No change in thrust.
We started adding pressure taps to the engine to find out
where we were losing our pressure. A
swage-lok fitting welded to the chamber flowed by a short length of 1/8
stainless tube going to a porous metal snubber in front of a transducer gives
us reliable pressure data.
For a 271 psi tank, we had 221 psi in the middle of the
engine, underneath the cold catalysts and opposite the glow plug. That sounded very good 12 feet of 16 hose,
a ball valve, a 90 degree fitting, a spreading plate, ten 20 mesh screens, and
two 400 cpsi catalyst monoliths later only dropped 50 psi.
We couldnt conveniently add a pressure tap underneath all
of the catalyst, because the bottom layer was directly over the nozzle, so we
put the top at the level of the final catalyst layer. On the next test run, at the same 270 psi we had 195 psi at this
This was extremely odd, because at that chamber pressure we
should be making 900 lbf, not 510.
Rechecking the load cell and pressure transducer calibrations didnt
show anything odd.
Finally, we welded yet another extension section underneath
the chamber with a pressure tap in completely clear space before the nozzle. For 292 psi tank, we only had 105 psi
chamber pressure there, which means we dropped 100 psi over the very last block
of catalyst. The last section is the
hottest, and has the most gas flowing past it, but it still seems odd that the
one layer would drop several times as much as all the previous layers.
We decided to build up a new all-welded engine, again in
hopes that it might be suitable for flying the big vehicle. We received a few new 900
cells-per-square-inch foil monoliths, so we used one of those at the top,
followed by a 600 cpsi monolith, then three 100 gram layers of catalyst bale,
then two 50 gram layers, which should offer less back pressure than the final
layers in the test engine.
On the first run, this engine chugged very violently, but
after about ten seconds it settled down, and it didnt repeat on the second
run. This was likely the catalyst bale
finding its final compressed set. The
runs were good and clear, but reducing the catalyst at the end didnt give us
450 lbf, 276 psi tank, 94 psi chamber pressure
500 lbf, 300 psi tank, 108 psi chamber pressure
This was slightly less than the test engine, almost
certainly due to the fact that the welded engines are 5.5 ID throughout, instead
of 6.125 ID in the test chamber sections, so the catalyst bale was a bit more
We will stick some more pressure taps in this engine next
week, but it isnt looking good for avoiding the flow losses with this catalyst
at the bottom of the motor. We will
probably try putting a monolith at the bottom after three sections of bale, and
there are a couple other things to try.
We could just build up three more engines just like this, but we would
have to increase the tank pressure more than we care to if we want to get any
significant liftoff acceleration. We
will also need a better thrust to weight ratio for the big engines to hit our
Comparing the three different 5.5 diameter foil monolith
400 cpsi 267 grams
600 cpsi 327 grams
900 cpsi 404 grams
Even the 900 cpsi ones have very little flow restriction, so
there is probably no reason to not always use the 900s.
On the big engine front, we drilled new sets of holes in the
12 machined top and nozzle to match the two extension pieces we had
fabricated, and we have started some assembly work.
The weld-on 12 nozzles for the X-Prize flights are starting
to come in from EnTek. The flange is
smaller than the bolt-together one, because it just needs to serve as a ledge
for positioning the chamber, and the nozzle extension is tapered from 0.25 to
0.15, for a total savings of 6.5 pounds per nozzle. Each nozzle starts out as a 429 pound blank of 316 stainless
steel, and winds up at 17 pounds.
Surprisingly, even in an order for nine pieces, this is still
significantly cheaper than fabricating by metal spinning or rolling operations.