December 27, 2003 Notes

 

Hydrotest

 

We received a new 100 gallon tank from Structural last week to do another hydrotest on.  The previous tank we tested failed at 600 psi, but it had a clear and obvious weak point in the lightweight and incomplete cast aluminum winding flange on the bottom of the tank.  The new tank has proper flanges on both sides, and we bolted our own milled aluminum flanges to the ends.

 

Neil borrowed a real hydrotest machine from his work for this test.  We had to change some things on it, because it was set up with a 20,000 psi gauge.  Neil said this was their low pressure one!  The machine runs on shop air, and it took a painfully long time to bring the pressure up.  Neil commented on how different hydrotesting a fiberglass tank was from the steel vessels they normally work with – the steel vessels have so little stretch that they come up to pressure very rapidly.

 

The tank made the normal snap-crackle-pop sounds all through the pressurization, but it passed 600 psi with no problems at all.  Our air compressor was having a hard time supplying the test pump at the higher pressures, running flat out all the time.  We let it sit at 800 psi for 20 minutes while we ate dinner, then went back and continued pressurizing it.  Zooming way in with the video camera we could see a few drops of water leaking past the O-ring, and the ½” thick flange bowing slightly upwards, but it was still holding fine.  We let it continue pressurizing until it reached 900 psi, but it was going up so slowly that it didn’t look like we were going to burst the tank that night, so we decided to release the pressure and keep the tank.

 

Big Vehicle work

 

I had to order a glow plug harness, because none of the local auto parts stores stocked them.  We chopped off the connectors and wired everything up so the computer can control the glow plugs by itself, but it turns out that I accidentally bought an AC coil power relay, so we were only able to test it by pushing the manual button.

 

We fabricated custom aluminum mount points for the engine pressure transducers.  We were considering just hose-clamping them to the support struts, but then we would have had to pull them off and let them dangle whenever we pulled an engine plate off the vehicle.

 

We improved the propellant drum loading system so it can load and pressurize without disconnecting anything.  Going through an additional check valve slowed it down a little bit, taking nine minutes to load a full 55 gallon drum into the vehicle tank.  We also timed gang-pressurizing an entire six-pack of nitrogen bottles into the vehicle tank.  By five minutes, the tank was at 200 psi, and by nine minutes it was at 265 psi and almost equalized with the tanks.  These were transducer readings, which look to be high compared to the gauge, which read about 218 psi at the end.  We were considering bypassing the big regulator (which was set at 300 psi) completely and just letting the nitrogen bottles flow out at full pressure, but the times weren’t so bad getting most of it through the regulator.  The low pressure times stretch out a lot, so if we decide to cascade load the individual bottles, we may consider it again.  Unfortunately, even with cascade loading, it doesn’t look like we are going to get to 300 psi tank pressure with a single six-pack, so we are probably going to have to take two of them with us, which is a pain.

 

We hung the entire vehicle with the scale to get a current ready-to-fly weight for the vehicle, which comes out to 1700lb, assuming the final engines weigh the same as the last couple ones we welded together.  This has grown a bit more than we expected (which should be expected…).  We want to do the initial flight tests with about 500 pounds of propellant, so we really do need to see 700 pounds thrust minimum from each of four engines to get even a slow liftoff.  We are contemplating adding a fifth engine in the center to crutch things up if we don’t get the engine efficiency up soon.  Unfortunately, adding the fifth engine now isn’t very straightforward due to the way we have things set up.

 

We got the 1600 gallon tank cradle fabricated, and we have started some layout work on the cabin-at-bottom design.  It looks like this will actually be roomy and comfortable, compared to our strap-in-upside-down arrangement on the crush cone vehicle.  The big tank weighed somewhat less than we expected, it was only 1400 lb, plus about 50 pounds for the flange closures.

 

Engine Work

 

Optimization of the engines has continued, still without any spectacular results.  We reworked our test stand glow plug / valve control electronics to reduce clutter, which always seems to be a good idea.  We keep hoping we are almost done with this test series, but it looks like it may stretch on for a while longer.

 

We removed the last two layers of catalyst bale from one of the test engines and replaced it with 15 very heavy 3-mesh screens that were used to contain the catalyst bale in commercial use.  They were coated with the same catalyst as the bale, but the total surface area was quite low compared to the other catalysts.  On testing, it didn’t ever clear up, giving a densely cloudy (low Isp) exhaust, but it did produce very high thrust:

 

650 lbf at 278 psi tank and 139 psi chamber (this run started off high thrust for 1.5 seconds, but then dropped to a lower level)

580 lbf at 272 psi tank and 126 psi chamber

 

Three x 100 gram layers of bale plus the small amount of catalyst on the heavy screens was obviously not enough for that amount of flow, but the bale didn’t bunch up and restrict the flow under pressure.

 

On Saturday we had a new type of catalyst to test that we have good hopes for.  Instead of being a solid mass of catalyst, it is made up of hundreds of ¼” diameter by ¼” long metal rings with some extra perforations.  This prevents any straight through flow, and each little ring should be a great flameholder for combustion in the hot section.  We got 3 pounds of the rings, which is about 140 cubic inches of volume.  The rings are made of quite thin material, only 0.008” thick, but they are reasonably strong at room temperature. 

 

We cannibalized the first welded engine parts to make a 5.5” ID test engine for this.  We are concerned that they will still self-compress under pressure loads at high temperature, so we built the first test engine with two layers of 200 grams of rings, each supported by a separate perf-plate.  Each layer was about 1” deep.  This engine had two 600 cpsi monoliths for the cold catalyst.  This engine was very difficult to get preheated.

 

584 lbf at 271 psi tank and 120 psi chamber

 

When we tried a re-test, we couldn’t get it to fully preheat at all.  It seems like as we have reduced the flow restrictions in the hot section, the oxygen/methanol vapor combustion under the cold pack just flows out without transferring much heat to the hot section catalyst.  It is getting bad enough that the old propane / air forced preheats are starting to look good again.

 

We decided to try a different test, switching the 2.2” throat nozzle for our smallest one, a 1.25” throat.  This warmed easier, and produced dramatically different pressure numbers:

 

401 lbf at 283 psi tank and 229 psi chamber

 

That is an extremely efficient feed to chamber pressure for a catalyst motor, but a 5.5” catalyst diameter to 1.25” throat diameter is a pretty bulky contraction.

 

We next switched to a 1.83” nozzle, which is the same 3 : 1 diameter ratio we have on the big motor ( 12” : 4” ).  This was more difficult to preheat again.

 

545 lbf at 287 psi tank and 176 psi chamber

 

The increased flow is starting to lose a lot more by the time it gets to the chamber.  We added another pressure tap to the engine under the cold catalyst for comparison.

 

520 lbf at 275 psi tank and 186 psi mid pressure

 

This shows a very tiny pressure drop across the two ring layers, but it also shows that the higher flow is starting to choke somewhat higher, in either the cold catalyst or the spreading plate.  The engine with five by 100 gram bale layers last week produced a higher mid pressure (271 psi tank gave a 221 psi mid pressure) at a little less thrust.  What we really need to do is have three pressure taps, one under the spreading plate, one under the cold catalyst, and a third under the hot catalyst.  I have ordered more data acquisition channels so we can start reading all of these simultaneously next week.

 

When we took the engine apart, we noticed a drawback of the ring catalysts – the supporting perf plates had bowed down enough that the rings pooled in the dome, exposing the outside edges to straight-through flow.  We hammered it back into place, but it looks like it will be a good idea to use deeper ring catalyst sections in the future.  We added a third section of 200 grams of rings to the engine to see if engine performance improved with additional catalyst.

 

487 lbf at 270 psi tank and 153 psi chamber

 

So even the rings are a producing some increased flow restriction at the full hot flow rates

 

We tried opening up the top spreading plate with a 0.25” drilled hole down the center, but that let too much rush through, giving us a central cloudy stream in the engine.

 

We went back to the all-welded 2.2” throat engine we made last week with the 3 x 100 and 2 x 50 bale layers to test it at higher pressure.

 

612 lbf at 410 psi tank, 282 psi mid pressure, 124 psi chamber pressure

 

The catalyst bale in the hot section gives huge pressure losses, but the pressure drop to the mid section is still very high.  I have a hard time seeing the foil monoliths being a dominant pressure loss, so I am probably going to have to make some more spreading plates with different hole sizes to experiment with.  It is possible that we just have a critical catalyst diameter to throat diameter ratio that we are wasting our time fighting, but I’m not quite ready to quit yet.  If necessary, we can reasonably easily use a 7” ID chamber with our existing nozzles, but it would require another custom order of catalyst, at which point I would probably just consider building the 12” engines.