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Furfural alcohol, engine tests, lander modifications

September 3 and 7, 2002 Meeting notes

September 3 and 7, 2002 Meeting notes

 

Furfuryl Alcohol

 

We finally got our furfuryl alcohol from Penn Specialty Chemicals, and we ran some tests.

 

It is not at all hypergolic with peroxide, so that avenue of investigation is shut down.  It is supposed to be rapidly and cleanly hypergolic with pure nitric acid, but with 50%, the only grade we had on hand, it just rapidly polymerized and changed colors.

 

The density is very high, about 40% more so than kerosene, which is readily apparent when carrying a five gallon container of it.  However, the O/F ratio with peroxide is about half that of kerosene, so it doesn’t really improve total propellant density, since it is still less dense than peroxide.  We might try some dissolved catalyst hypergolic experiments, but I am still leery of the safety issues with that, and it is more expensive than kerosene, so this is probably a dead end.

 

As it seems most have determined in the past, autoignited kerosene looks to be the best all around choice for fuel with peroxide.

 

Engine Tests

 

To test our theory that the bent-pipe injector was performing worse than the drilled-hole injector due to the shortened distance inside the venturi, we shortened the exit section up another 1/8”, and got improved performance.  We then went back to the drilled-hole injector, which gave still better performance.  We believe that the bent pipe will provide better performance if we can give it an equal length of venturi underneath it, but that will require increasing the thickness of our injector plate.  The desired venturi height is probably proportional to throat diameter, so this does mean that sticking with a single venturi will result in a several inch thick injector plate for the big engine.

 

We have had a couple problems with accidentally twisting the tubing inside the injector so that it no longer points straight down. When tightening an AN fitting into the flare on the end of the tube, it seems we sometimes rotate the drilled-through swagelok that holds the tube in place.  We may start brazing the tube either directly in, or brazing the swagelok into an immovable piece.

 

We tried running a higher performance mixture in the regeneratively cooled engine.  With 250 psi, 0.80 peroxide / 0.50 kerosene jetting and the drilled-tube injector, the plume “got funny” after a few seconds of hot firing, so I got off of the kerosene and let it finish out with monoprop.  The better injector and resulting better combustion was clearly increasing the heat load into the coolant (we ran a 0.60 fuel jet with the older injector for 30 seconds without problems previously).  We tried a run at 500 psi, which somewhat decreases the heat load per unit of coolant (at the expense of higher rate of transfer), but the engine failed abruptly after a few seconds, bursting the tube that runs from the top of the cooling jacket to the catalyst pack inlet.  No gentle warning of imminent failure at the higher pressure.

 

We weren’t sure if it was because we had a vapor detonation in the line (it wasn’t all that violent), or if it was because the aluminum line had lost too much strength (it was rated for 1400 psi, but at 100F, and it might have been 300F at the time of rupture.  We replaced the aluminum line with a stainless line, but the engine seemed to be completely not working when we tried it again, with peroxide gushing out undecomposed.  It turns out that we had burned a hole through the combustion chamber near the top, which then fried the line exiting the cooling chamber.  It probably wouldn’t have just started gushing peroxide out of the engine instead of bursting the line if we had been using stainless line at that point.

 

We had seen on the radiative engine that this injector caused a hot spot high up in the combustion chamber, and that is likely exactly where the burn through occurred.  We are hoping that the bent-pipe injector will provide more uniform combustion, which should help with this.  The biggest help will just be making larger engines, where the square-cube law helps cooling significantly.  The 0.5” throat regen engine has 25 square inches of cooled surface area to 0.2 square inches of throat, while the 2” throat regen engine will have 124 square inches of cooled area to 3.2 square inches of throat, so we should only have 1/6th the heat transfer to the coolant (we cut the volume slightly, otherwise it would be exactly 1/4th)   The question at that point will be if the thicker section at the throat can transfer the heat fast enough to keep the throat from melting.

 

One thing we are still unsure about is the value of trying to film cool with decomposed peroxide.  The current injector has a ring of 16 1/16” holes around the periphery to allow some gas to enter the chamber without mixing with fuel in the main chamber.  You can see clean streaks in the chamber under the holes, but I’m not sure if that is good or bad.  It seems likely that if stray droplets of fuel or getting to the wall, then there will be lean burning there, but I can see how it might reduce total heat transfer in a well-distributed engine.  Since total heat load shouldn’t be a problem, we are probably going to skip the holes in the big injector

 

We have the valves, jets, and lines set up for the 1000 lbf biprop.  I have two separate high flow tescom regulators on order, so we will perform mixture ratio tweaking by changing pressures with fixed jets.  I also got enough of the Kzco servo ball valves for us to build a large four-engine, differentially throttled vehicle, which is looking like our chosen platform for the high performance vehicles, instead of separate attitude jets.  I broke a 1/16” drill bit inside one of the balls while venting it for peroxide service, but I think we have enough remnants from previous valves we have broken parts off to replace it.

 

Russ is slowly boring out the big chamber.  Boring a 14” long chamber from a solid bar of aluminum is not much fun, so we will probably farm the work out in the future.

 

 

Lander Modifications

 

I ran the electronics box through a full duty cycle to try to reproduce the gyro misbehavior that we saw last Saturday.  After 80 minutes of operation, my “battery warning” light started flashing, but power did not completely fail until 129 minutes, so that is a very conservative indicator.

 

The inertial unit’s temperature sensor did rise steadily through the entire time, but it did not reach the levels logged on Saturday because I was testing inside, and there weren’t any rockets firing.  It did not misbehave in any way, but the plastic box was noticeably warm underneath the inertial unit, so a thermal problem is still our best bet.  On opening the box after power finally failed I found the internal temperature to be somewhat warm, but not too bad.  The inertial unit was fairly warm to the touch, and the switching power supplies were hot, as expected.

 

Because the total heat load does not seem excessive, we decided to try to just improve the cooling of the inertial unit inside the box.  We stood it off about 3/8” from the box to allow air to circulate underneath it, and we added a fan inside the box to force air around.

 

I was also happy this week to come across a store online that still had some old stock of the Logitech Wingman Warrior joysticks that we had originally planned to use for the pilot on the lander.  It was the last of the RS-232 serial joysticks with throttle and hat buttons, before everything moved to USB.  The PC104 computer we were using for a while had USB, but we moved back to the larger SBC with more serial ports, and lost the USB port.  I will be getting things set back up for local serial joystick this weekend.

 

Since tipping the vehicle over doesn’t seem all that difficult, Russ has started welding in support bars in the front to protect a pilot in case it rolls over forward.

 

 





 






 
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