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Relief valves, Plastic issues, engine work, big vehicle work

Armadillo Aerospace probably wouldn’t exist if it weren’t for Walt Anderson

October 24, 2005 notes:

 

Armadillo Aerospace probably wouldn’t exist if it weren’t for Walt Anderson. The Space Frontier Foundation’s CATS Prize, which Walt funded, was the critical thing that took me from “this is interesting” to “this is something that I want to work on.”

www.justiceforwalt.com

 

There is another nice video of the X-Prize Cup flight here: 
http://search.aol.com/aolcom/video?invocationType=topsearchbox.video&aolplayable=&sort=&view=&query=x+prize+cup&category=&duration

 

Relief Valves.

 

An important lesson we learned a few days before we set out for the X-Prize Cup:

 

On that Saturday we did a test flight that aborted (automatic soft landing) after four seconds.  The abort was triggered when the pressurant tank pressure dropped below our abort level (600 psi).  The telemetry showed that one of the roll thrusters was on almost half the time, instead of the usual couple ticks per flight.  We took this to be the cause of the pressure drop, but we couldn't figure out what could possible be causing that much torque on the vehicle to make it fight that hard.  The vehicle didn't rotate at all, so the roll thruster was fighting something real, rather than incorrectly firing.

We scratched our heads for a while and decided to just try it again and see what happened.  Just before I was about to lift off, there was a big whoosh of escaping gas, and the pressure dropped below our no-go limit (1400 psi), which scrubbed the launch.  My first thought was that it was one of the computer controlled solenoids, either engine purge or a roll thruster, but it turned out to be one of the tank pressure relief valves.

Looking back at the telemetry from the first flight, I saw that the fuel tank pressure had dropped sharply right at liftoff, so it was pretty clear that the relief had popped open just as we lifted off.  The relief valve has a balanced flow diffuser, but it is mounted so that half the flow would immediately bang into a pressure gauge, giving an unbalanced flow and hence a roll torque, because it is mounted on the outside rim of the vehicle.  A lot more gas was flowing out the relief valve than out of the roll thruster, since the roll thruster had a nicely directed jet opposing the rough flow from the relief.

This was sort of a nice demonstration of our system robustness, because the roll thrusters did exactly what they should have, and we had a nice, safe automatic landing from the abort condition.

A few things contributed to this:

Our regulator seemed to slowly leak helium, and this seems to have gotten worse with time.  This causes a slow increase in tank pressure between the time we complete our pressurization and launch the vehicle.  About a month ago we had this hit the relief valve pressure, which dumps most of the pressure and scrubs the flight.  I made a change in the flight control software to have the computer act as an electronic relief valve, venting pressure through the purge solenoid when it exceeds a threshold.  This is a true "relief valve", because it only drops enough pressure to get it below the set level, rather than "popping" and dumping lots of helium.

Relief valves aren't the most precise things in the world.  The ones we are using tend to hiss a bit of pressure out a few tens of psi before they pop open.  Sometimes they don't seal real well when they re-seat.  The stock relief valve on our lox dewar occasionally needs a little tap with a wrench to get it to stop hissing.

I had set the computer relief to always relieve pressure before the relief valves started to hiss.  This had worked great all month, usually giving us a couple tiny pops of pressure relief while we are getting ready to fly, but that Saturday night was quite a bit cooler than it had been for a while, and some combination of temperature and possibly accumulated vibration had allowed the relief valve to pop below the computer relief pressure.  This was a movement of around 20 psi on the roughly 400 psi pop point.

The immediate work around was to increase the pressure on the relief valves a bit, which let us have a normal flight afterwards. We also swapped regulators to get rid of the slow helium leak.


We have had several issues with check valves over the years, and now relief valves.  There is a common element there -- "things with springs".  Regulators are probably the next thing to go on my evil parts list.  AST has mentioned regulators as a trouble source.

We are now relying on just burst disks and the computer relief for pressure safety, completely removing relief valves from our flight hardware.

Plastic Issues

 

We ran into two plastic issues in the space of a week that are worth mentioning:

 

While cleaning up the shop, I found a plastic pint sample jar with 50% hydrogen peroxide in it. It was labeled with a date that was 16 months old, but the plastic felt like it was twenty years old, making crackling sounds when you squeezed it. I disposed of the peroxide and tossed the container, but it was HDPE, which I thought had better chemical compatibility than that at 50% concentration. This gives me a little concern, because we still have a couple HDPE drums of 50% peroxide. I had been vaguely planning on doing controlled evaporation concentration experiments with them just to make the data publicly available, but I am leaning towards disposing of it now.

 

I ordered new Tygon peristaltic pump tubing to replace our pump hose that had split on us twice. When it arrived, I was very surprised at the difference between the new hose and the hose on the pump, even though they were exactly the same brand. The old hose was much harder, and slightly shrunk in diameter. It was so different that some people didn’t believe it was the same hose until the read the labeling along the tube. Lately we had to give the pump a bit of a hand start sometimes with the old hose because of the extra stiffness. We had been using methanol for a long time with that hose, which tends to be harder on plastics than Ethanol, so hopefully we won’t run into the issue as soon now.

 

Engine Work

 

The latest regen plus film cooled 2” tube engine has been running reliably with no signs of erosion, but Isp is only 155. This is a short-chambered (9” cooled section) engine with 1/3 of the fuel going in as film cooling roughly halfway down the chamber. I recently realized that I can easily mill spiral cooling passages, so the next test will be to try making an engine with spiral passages and no film cooling to bring the performance up some.

 

Chamber pressure has been a lot lower than we would like on these engines, generally at 150 psi chamber from 350 psi tank pressure. We upsized all of the plumbing on the test stand, going to ½” ball valves and 1” feed lines, and the performance didn’t change at all, so it must be injector limited. The current design has 16 x 3/32” holes for LOX injection and 20 x 3/32” holes for fuel injection, and it makes 500 lbf without any expansion section. We did have one test engine a while ago that used 1/16” holes and ran rough, but that was a counter-switrled injector with a transition to an ablative chamber, so it might not be an issue with a smooth regen cooled chamber.

 

We are preparing a 2.75” ID regen engine with a 12” cooled section. If it doesn’t melt, we will be replacing the engine on the little cone vehicle with it. This will give us enough thrust to do +1G liftoffs for boosted hops.

 

We have measured a 195 Isp on an engine with a bit more chamber volume and a proper expansion cone. Theoretical Isp is around 220 at this pressure, but we really don’t need to get that high for our purposes. We will probably be trying an unlike-impinging injector scheme sometime soon, which should give a higher Isp.

 

We fired the big, 6” ID throatless engine a few times. We tested the igniter several times on the stand, but we couldn’t get it to build much of any chamber pressure. Apparently the side injection works less well at larger sizes. We swapped the one foot long ablative chamber section for a two foot long section and moved it over to our horizontal test stand – Joseph’s truck hitch. For really big firings we are going to chain the engine mount to our “rocket anchor” that is planted deep in the ground at our test site, but this firing was still limited by the small valves on the test stand, so we weren’t worried about the truck jumping the curb and taking off.

 

We had a camera 100’ away zoomed in on the engine exit, which was interesting to see. After main throttle up (before the camera got blown over…) there is a clear ring of burning around the outside, and a black center. The lox gets vaporized at the top of the engine, but the fuel doesn’t penetrate very far into the chamber center before it starts burning with the gox, leaving a lot of gox unmixed with fuel. Higher chamber pressure should improve this (this run only made 36 psi chamber pressure, or about 1000 lbf), and it might work better putting the fuel in on top, but it looks like we probably need a different injector design for big engines. We will go ahead and fire this engine up at full thrust at the remote site when we get the big tanks plumbed up, but the efficiency probably won’t be good.

 

http://media.armadilloaerospace.com/2005_10_22/truckStand.jpg

http://media.armadilloaerospace.com/2005_10_22/onHitch.jpg

 

http://media.armadilloaerospace.com/2005_10_22/shopHitchTest.mpg

 

 

Big Vehicle Work

 

http://media.armadilloaerospace.com/2005_10_22/vehicle1.jpg

http://media.armadilloaerospace.com/2005_10_22/vehicle2.jpg

http://media.armadilloaerospace.com/2005_10_22/vehicle3.jpg

 

We are starting to fabricate parts for assembling the big vehicle. The original layout had the computer and pressure bottles in the nosecone, but we have since decided that it will be much better to put them in a slightly longer inter-tank section. We are using the current 4’ long section for mock-ups, but we should have the real 6’ long section in this coming week.

 

The current design calls for two pipes running down the side of the vehicle from the intertank to the base of the vehicle, one for fuel to the single gimbaled engine, and the other as a conduit for purge lines and wiring. I have been strongly considering another design change to try using four of our 2” tube motors in a differentially throttled configuration. This would drop the vehicle height by at least three feet, because the base of the bottom sphere could be almost touching the ground, and it would have the nice attribute of having all the plumbing and wiring in the intertank, with only a gps antenna going up to the top (there would be a dip tube to draw lox from the bottom tank through the top)..

 

The bigger ball screw linear actuators that we got for the 6” motor will have to be mounted differently, and currently need a 24V actuator. They are huge (and heavy), probably much larger than we would need for a 5000 lbf engine. If we do stick with the single gimbaled engine, it looks like we are going to start with a 4” engine and just use the smaller actuators.

 

Since we are going to be mounting lots of things on the 36” spherical tanks, I wrote a program to generate G-code that mills spherical sections out of whatever I want. This was an interesting exercise, but probably a bit of a waste of time – on tube sections of 2” diameter or so, the curvature is so close to a flat planar cut that it probably doesn’t make any difference. A 4” mount would have noticeable curvature, but I don’t think we will need anything that large.

 

I milled some nice mounts for the helium bottles, with 36” curves on the outside to weld on the intertank section, 7” curves on the inside to hold the helium bottles, and a wide lip to slip a hose clamp around. Making and mounting sixteen of these for eight bottles will be a bit of a chore, but it should work out well.

 





 






 
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