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Space Access, teflon tubing, airplane parachute, new rotor testing

April 25 and May 4, 2002 Meeting Notes


In attendance:


John Carmack

Phil Eaton

Russ Blink

Neil Milburn

Joseph LaGrave


Space Access


Last weekend we were all at the Space Access ’02 conference. Watch the video linked off the main Armadillo Aerospace page if you have a decent net connection.


It was fun to see everyone else in the community, but the most important thing was the opening of a dialog with Joseph Hawkins, the deputy associate administrator of the FAA commercial space transportation division. We aren’t going to “need” their assistance for about a year, but we are going to do everything possible to make them comfortable with our plans as we go along.


Teflon Tubing


We pressure tested some thin, heavy wall Teflon tubing that I am considering using as a “sightglass” for our large lander tanks. It was only rated for 300 psi, but we pushed it up to 1200 psi without it bursting. The 1/16” tubing may be small enough that capillary action skews the level reading, so we are going to test larger tubing as well.


Airplane Parachute


We test fired the used 900 pound rocket-drawn airplane parachute that I had bought a while ago. We wanted to watch how the rocket deployment worked, because we are going to have to make our own system with electronic initiation instead of pull-cord initiation.


The rocket launched nice and straight, pulling the chute well clear of the packing bag, but after pulling halfway out of the internal deployment bag, it caught and ripped the bag, not fully deploying. If it had been airborne, it might or might not have pulled the rest of the way out when it partially inflated, but still, it wasn’t a particularly promising thing to see. We don’t know the history of the chute (it came from eBay, through an intermediary), so it could have been improperly repacked, but it might have just been the age of the materials. The actual parachute looks pretty nice, but the deployment bag is going to have to be replaced.


I tried to buy a new airplane parachute from BRSI last year, but they wouldn’t sell me one after I told them what it was for.



New Rotor Testing


We did high speed rotor testing at the 100 acres today, but we had some data collection issues.


Roughly summarized: 6.5’ blades, roughly 1’ hub, 8” chord, 20 degree pitch angle. From analyzing not-entirely consistent data, lift in pounds is approximately rpm*rpm / 800, so we will need to spin 700 rpm to make 600 pounds of lift. When the first rotor fell apart, it was making 320 pounds of lift at 675 rpm, so doubling the pitch and increasing the blade length by a few inches did not quite double the lift per rpm capacity. If you include the lift generated while the blade is coming back to rest, the measured Isp is a bit over 700. The Isp should increase some at higher RPM, where the tip rocket chamber pressure is higher.


There was noticeably more upward coning angle due to higher blade pitch, which makes perfect sense – we made the same amount of lift with about half the centrifugal force, so the ends of the blades bend up higher.




We had been having some problems with missed signals from the proximity sensor we were using as a tachometer in our previous tests, so we had converted over to a hal effect sensor securely mounted on a bracket connected to the hub bearing, with two large magnets mounted on the hub. We got a really wide, strong signal from this, and everything worked great during our basic spin test at the shop, but when we set up at the test range, we were getting phantom signal pulses. These made it look like the RPM shot up to a huge value, which immediately caused the system to inhibit the solenoid. Russ played with his sensor circuit a bit, but we were never able to get it working steady again. I wound up disabling the RPM limit and changing it to limit based on load cell values. The only RPM values that I trust in the data are when there is a clear change between two nearby numbers, indicating that it got at least three rationally timed pulses. We will have this fixed up one way or another before the next test.


We calibrated and used two new load cells from the recent aRocket group buy. Physically, I like them a lot better than the button cells I had previously gotten from Omega – they are big, beefy S shaped bars with tapped holes that can be used for compression or tension applications, and the cable is strain relieved and not the cruddy 28 gauge wire that some of our load cells use. However, the load readings we got on the rotor test were very noisy, while the last test we had with the omega button cell was smooth. I had to smooth over nearly a quarter second to get clean looking data. It probably isn’t the load cell’s fault, because we had a fair amount more preload in our test stand with the larger cell, and it didn’t have a good single contact point for measuring. We are going to use a big ball bearing on the load cell next time, and probably arrange to have a shorter spacer block. We will be using one on the horizontal test stand on Tuesday, so that will be another data point.


After going to the trouble of changing the auto-limter over to thrust, it turns out that rpm limiting wasn’t going to be a problem. After a trial run, the next run ran flat out through a liter of peroxide at 200 psi loaded tank pressure (it had decayed to 176 by the time the run started) and it stabilized at only 150 pounds of lift at 320 rpm.


We increased the pressure to 250 psi, and it worked its way up to 250 pounds of lift. It looked like it might have kept climbing, so we did the next run with two liters of peroxide. I missed the data logging on that run, but it showed it leveling off right at 250 pounds for the rest of the run.


We increased pressure to almost 400 psi, and it only made it a little higher, to 300 pounds of thrust. We then pushed it over 500 psi and saw no additional thrust whatsoever.


The limit is almost certainly due to the new tubing that we used inside the new blades. The tubing that we had in the first set of blades was 0.049 wall, 0.152 ID aluminum tubing rated at 2324 psi working pressure. The March 23 analysis showed that we would see over 5000 psi at our target rpm, and potentially 10000 psi at sonic tip speeds, so we built the new hub with much stouter tubing.


The tubing we used is 0.065 wall, 0.120 ID seamless stainless tubing rated at 8192 psi working pressure. This is a pretty small hole with a LOT of metal around it. We had to use swagelok fittings instead of flare fittings, because flaring that tubing was out of the question. Running the numbers directly, it appears to have a 4x safety margin built in.


We are going to change over to 0.035 wall, 0.180 ID tubing rated at 4375 psi working pressure, which should still have plenty of margin left over even if we overspeed the rotor a bit, and it will give us over twice the flow area, and allow us to put a flare jet right at the engines to make low rpm starting and running smoother. This should probably be adequate for our next gen vehicle, but if we build a rotor with full 13’ blades, we will need to move up to –5 or –6 tubing to get enough flow to the engines. The downside to larger tubing is that the rpm control will have more latency to deal with.


From the April 20 solenoid water flow tests, the solenoid should easily be able to flow enough peroxide to make 500+ pounds of lift at the higher pressures, but it may still not be adequate for the lift we will need in the vehicle (600+ pounds), so we have a few options:


Run the tanks at 600 psi, like we do for the pure rocket tests. That won’t hurt us on the current vehicle, but we are intending to use lighter tanks for the next generation vehicle.


Gang two Pro-Shot solenoids, which actually works out well with our new over-the-shaft rotary seal that has pairs of symmetric ports. This can also let the rotor RPM control be tri state instead of binary, which will smooth things out.


Use a Big-Shot solenoid, which flows about 50% more, but draws 30 amps of current.


Use a 1/4" throttled ball valve. This is probably the best solution, but our current test stand hardware can only open a ball valve fully, not proportionately. The vehicles have the right hardware, but that will make testing a bit trickier.


We will probably be back out at the test range next weekend to try for 600+ pounds of lift.



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