May 5 and 10, 2001 Meeting Notes

 

Next meeting Saturday at LRS around 7:00 

 

In attendance:

 

John Carmack

Russ Blink

Phil Eaton

Neil Milburn

 

New supplies:

 

150 psi ASME composite tank

220 psi pvc tubing

100 psi plastic solenoid

Tefzel mil-spec hookup wire

Super big shot NOS solenoid (the 30 amp one)

Super pro shot NOS solenoid

1/2" 110 VAC solenoid for the test stand

More stainless fittings

Plastic hose barbs

Five sets of hearing protection.

Four XL disposable protective suits

Five rolls of mil-spec teflon pipe tape

2 40 amp DC solid state relays

2 40 amp AC solid state relays

5 4 amp DC solid state relays (for low cost, low pressure valve testing)

3 NOS blow off valves

10 -6 hose ends that turned out to be the wrong kind.  Grrr.

5 8" foam cubes for landing gear

Rubber anti-vibration mounting pads

 

To Get:

 

The correct –6 hose ends

Swivel ¼ inch to –4 fitting for fill cart

½ inch check valves

More plastic solenoids

Spill tray to put under our peroxide container

More distilled water

Contact cement for the foam pads

Extra hose clamps for the engine mounts

Some kind of shims for positioning the clamped engines

Some extra ¼ male to 1/8 male fittings

Various AN straight size adapters

 

 

On Saturday, we did some testing with a tiny (and cheap) little plastic solenoid (McMaster 7877K25).. We are seriously considering doing some work with low pressure rockets. The Isp will suffer, but being able to use plastic fittings and valves completely removes the peroxide corrosion problems, and you can flow more through a low pressure solenoid for a given amount of actuation force. It is also safer and cheaper.

 

I wanted to try McMaster 7876K28, which is listed as twice the flow, but Delrin is on the “severe reaction” list with peroxide.

 

Tip: the 220 psi tubing was incredibly difficult to get on the hose barb, but a little time in front of a heat gun softened it so it slid on a lot easier.

 

We did some water flow tests on the solenoid:

 

1000 msec at 100 psi: 59 ml

1000 msec at 150 psi: 75 ml

 

That is nearly as good as the small nitrous solenoid, which flowed 33 ml in 500 msec at 100 psi.

 

The valve would not open at 200 psi. The valve was rated for 100 psi, but a 24vdc version is rated for 150 psi.

 

We fired the 100lb thrust motor with the tiny little valve and made a steady 10lb of thrust with it at 150 psi supply pressure. A more reasonable nozzle to prevent huge overexpansion would have helped a bit. That valve had a listed Cv of 0.17, but the pressure drop was quite high with the big nozzle, so with a more reasonable chamber pressure it would flow less.

 

When we went to do a second run, the solenoid was dead. The coil had fused internally. We are going to try again with the coil that is actually rated for 150psi.

 

We also tested a big stainless solenoid that Bob had lying around at his shop. It appeared to be the same style as the NOS valves, but rated for a lower pressure (400psi vs 1500 psi), so we expected it to flow better. Surprisingly, it flowed worse than the tiny little plastic valve.

 

We were planning on firing the motor with the ½” manual ball valve, allowing it to reach it’s full 100lb thrust, but we ran out of Teflon pipe tape. We tried just tightening it up until the water tests didn’t leak, but we would up galling the threads. Lesson learned – never ever assemble stainless fittings without Teflon.

 

We probably won’t be able to use the ASME composite tank I ordered (I bought it from McMaster, but it is manufactured by Amalga Composites), because it has brass threads on the fittings. We may still do some water tests with it to evaluate the advantages of working with tanks with both top and bottom ports. Getting away from our current vacuum loading and pressurizing through a single port would eliminate some check valves and avoid dissolving nitrogen in the peroxide while pressurizing. Also, while I had seen one reference that epoxy was an acceptable short term material for peroxide, we heard from one of the Beal people that it should be avoided.

 

I talked with a rep from Structural Composites about some of their 150 psi pressure tanks. They offer polyethylene lined tanks, but their internal material compatibility chart only showed that to be good for 10% hydrogen peroxide. I think they are just mistaken, because we got our original 98% sample in a PE jug, and it hasn’t given us any problems at all. They wouldn’t give me any direct price quotes, but I got the number of a distributor to contact.

 

On Thursday, we had four firings of the big motor, and we began assembly of version 2.0 of the VTVL.

 

The new lander design is using foam blocks for landing gear:

 

media.armadilloaerospace.com/2001_05_10/lander2work.jpg

 

The much lower center of gravity should make it harder to top over, replacing the springy legs with foam will keep the liftoff jerk down, and the electronics box is soft mounted, which will help the gyros. We may fly it this Saturday.

 

 

We now have four different sized solenoids out of the NOS line that we test with. I wish they gave the Cv ratings for their valves, but the only thing the rate them with is the potential horsepower gain for auto applications.

 

175 hp 8.6 amps, 1/8 to 1/8 Super power shot

250 hp 10 amps, ¼ to 1/8 Cheater

350 hp 10 amps, ¼ to 1/8 Pro shot

400 hp 8.6 amps, ¼ to 1/8 Super Pro Shot

450 hp 8.6 amps, ¼ to 1/8 bottom discharge Pro race

600 hp 30 amps, ¼ to ¼ Super big shot

 

The question is if the top of the line is barely large enough to let us squeak by with a single solenoid for the manned vehicle.

 

All of the firings were at the max pressure of our regulator, about 435 psi, with 500 ml of 90% except for the last..

 

media.armadilloaerospace.com/2001_05_10/firings.xls

 

The first firing was with the Super big shot solenoid, no check valve, and a –4 feed line.

63 pounds peak thrust.

 

The second firing was with the 110vac piloted solenoid, no check valve, and a –4 feed line.

The first thing we noticed was that the piloted solenoid wouldn’t hold a vacuum, so we had to manually close the tank while loading, then open it before pressurizing.

When we fired it, the 200 msec warmup pulse emptied a third of the peroxide! The piloted solenoid apparently closes very slowly. Pulsing is obviously out of the question.

72 pounds peak thrust (from the warmup pulse!)

 

The third firing was with the 110vac piloted solenoid, no check valve, and a –6 feed line.

I cut the warmup pulse to 100 msec, and it didn’t fire anything during the warmup segment. From looking at the data, it looks like valve requires about 100 msec before anything at all flows through, and around a quarter second before it is near full flow.

88 pounds peak thrust.

 

The fourth firing was the same as the third, but with 800 ml of peroxide, and a tiny bit more peroxide concentration due to rounding off the mixing numbers. This is about as much as we can vacuum load into our little test stand tank (the lander tank is 2.5 times as large).

91 pounds peak thrust.

 

Analysis

 

The small tank on the test stand has a very noticeable pressure blow down effect with the large motors.

 

It looks like Juan’s analysis was just about spot-on in the motor design. If we had a regulated 450 psi of feed pressure, it would probably be right at 100 lb of thrust.

 

This is interesting, because we originally sized our 15 lb thrust motors by just scaling down Juan’s numbers, and we had to drill them out a bit to get what we wanted. We were only running 400 psi most of the time, but it looks like there are some additional inefficiencies with very small (under 0.25” throat) motors.

 

The last run is enough data to roughly calculate our Isp: 1233 newton seconds of thrust from 1.11 kg of propellant ( 800 ml of 90% peroxide of density 1.387 ) gives an Isp of 113 seconds. If it was kept steady state at 450 psi, Juan’s number of 125 seconds is probably right.

 

We need to test the Super Big Shot solenoid again with a –6 hose, because the hose made a much larger difference than just going to the big piloted valve.

 

We may have one more restriction in our test stand plumbing: the manual bottle valve doesn’t have all that large of an orifice. Russ is going to make us a custom manifold out of aluminum like the one on the lander, but with a ½” port straight out the bottom, and four ¼” ports around the side (the lander manifold has eight 1/8” ports).

 

We may be able to hit 80 pounds of thrust with the single NOS solenoid on the current engine with better plumbing, but that is probably about it. We could either increase the supply pressure or decrease the chamber pressure (by opening the throat more) to flow more peroxide through the valve, but to make it to 150 pounds of thrust we would need to nearly quadruple the pressure drop, which seems unlikely.

 

Our propulsion options for the manned vehicle are:

 

Live with less thrust. We could still lift off with four 100 pound engines if our pilot isn’t too heavy, but it wouldn’t leave a lot of margin for killing velocity when landing.

 

Gang two solenoids together on each engine. There would be twice as many hoses, but the solenoids could be run out of phase, giving much smoother pulse modulation of the thrust, and we could easily hit our 150 pound per engine target.

 

Keep the four engines with a single solenoid, but add a fifth engine in the center dedicated to lifting. Just adding one more identical single-solenoid engine would give a reasonable margin, but because it wouldn’t need to pulse, a piloted valve could be used, allowing much more flow. This would also give a growth path to adding very high thrust central engines for the vertical dragster.

 

Develop non PWM throttled valves that can flow a lot more. A ½” straight-through ball valve flows five times as much as the ½” 110 vac solenoid we have. I have been looking for an appropriate valve, but I haven’t found exactly what I want yet.