December 20, 2003 notes

 

Drum pump

 

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 didn’t 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 isn’t 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 don’t 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.

 

http://media.armadilloaerospace.com/2003_12_20/drumPump.jpg

 

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 doesn’t 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 won’t be possible to over pressurize with six bottles.  It doesn’t 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.

 

Powered Landing

 

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 don’t 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 wasn’t well received, but a naturally unstable vehicle that only lands under power has an extremely limited area that it can possibly hit.

 

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 wouldn’t 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 doesn’t work right you don’t 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 can’t 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 isn’t 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.

 

http://media.armadilloaerospace.com/2003_12_20/newTank.jpg

 

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 won’t 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.

 

http://media.armadilloaerospace.com/2003_12_20/bigDrogue.jpg

 

We are going to fly the 850 gallon vehicle until we crash it, then assemble the cabin-on-the-bottom vehicle.

 

Engine work

 

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 aren’t just going by sound, and plumbed the repaired coriolis flow meter in line with the big tank.  Unfortunately, the flow meter still isn’t 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 wasn’t 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 couldn’t 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 point.

 

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 didn’t 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 didn’t 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 didn’t give us more thrist:

 

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 bunched together.

 

http://media.armadilloaerospace.com/2003_12_20/weldedEngine.jpg

 

We will stick some more pressure taps in this engine next week, but it isn’t 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 weight targets.

 

Comparing the three different 5.5” diameter foil monolith masses:

 

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.

 

http://media.armadilloaerospace.com/2003_12_20/bigChamber.jpg

 

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.