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Pure Silver Foam

October 30 and November 3, 2001 Meeting Notes


In attendance:


John Carmack

Phil Eaton

Russ Blink

Bob Norwood



Pure Silver Foam


Our pure silver foam blocks finally made it in, and we did several test stand runs with it.


I tried to use a new laptop for the testing, but WinME seems to leave one of the serial control lines in a different state than Win98, so it was leaving the test stand solenoid on all the time. I switched back to the old laptop for today, and I will investigate later.


We now have a lexan shield for the test stand that covers the engines. I’m not very worried about our monoprop engines, but I think it is a good measure for when we do our hybrid testing.


The silver foam is in half inch thick blocks, with 60 pores per inch. It is a fairly rigid, slightly crumbly sintered metal deal. We asked for 80 PPI, but they informed us that they tried a few times and couldn’t get that working, so we settled for 60 PPI.


The first test we ran, with three lightly compressed blocks separated by control rings in the engine did not provide enough back pressure, and the run was wet. Compressing a fourth block into the engine made it mostly clear, but it still clouded up some, and when we opened the engine back up, the bottom block had severely deformed, and the nozzle had a lot of silver plating on it.


We had been cautioned by Mark Henry, the lead propulsion engineer for Beal, that pure silver foam was going to be too soft, and it looks like he was right.


The control ring plates do seem to be doing their job, so we assembled new packs for all four engines using mostly our existing silver plated nickel foam. We left one pre-compressed pure silver foam brick at the top, so we can see how that holds up long term.


The current engine sets are made up with a double layer of coarse perforated metal for the base plate (we need to get thicker stainless base plates for the small motors), eight plated discs, a control ring, eight more plated discs, another control ring, then a pure silver block that gets compressed down as the cover is tightened down.


We are probably going to put twenty gallons of peroxide through our engine sets in the next couple weeks, so we will be able to see if the control rings are the key to longevity.


Flight Test


We did another short-form lander hop with the new engines. I used new laptop for this, which avoids any chance of the S3 graphics-driver crash we had last week during testing. We had it on a single tether, but the two gallon hop flew fine. However, the flight computer hung after it bounced down on the ground. Telemetry was lost in the midst of the settling bounce, and when we opened the box up, the LEDs were not lit on the 802.11 card.


Power cycling brought the computer back up, but I didn’t want to fly it again until we had done something to try to address the problem. We are assuming that shock is twisting the PC104 stack, which can cause all sorts of bus failures. I have a very rugged versa-tainer to move all the PC104 boards to, but it will mean moving away from the 802.11 card to a separate wireless Ethernet device, and a lot of other changes. I am going to see about doing that over the next week.


We still wanted to run the engines some more to see how the control rings hold up, so it was suggested that we have a couple of the guys stand on the lander so it can’t possibly take off, and run the engines through a load of peroxide. We would find out some of the human factors involved with our piloted flights, like what the noise level and fumes are like right next to all the engines.


Joseph and Russ were in the chem. Suits, so they stood on opposite sides of the lander. They had goggles, ear protection, and filter masks on. Phil stood by with the hose. After firing each of the attitude engines separately, I throttled the central engine up, eventually going to full throttle. With only 300 psi in the tank, and a total weight of 550 pounds, it definitely wasn’t going to take off (although we did leave the tether on, just in case someone jumped off). It did manage to slide the vehicle along the ground a little bit, mostly pivoting on Joseph’s side, because he weighs over 100 pounds more than Russ.


They reported that the initial pulses of the attitude engines when I was testing them individually felt completely smooth. The noise level was not a problem at all. There wasn’t any problem with peroxide fumes, but these are all almost brand new cat packs, so that will likely get worse with wear. They could feel a lot of hot air coming up from the ground, but nothing dangerous. The ride did get rougher when the attitude engines were working to level the platform dynamically and the engine got throttled up, but it wasn’t at a jarring level.


After the flight, we noticed two interesting things: at 75% throttle, there were pulsations in the tank pressure telemetry, showing that the feed system wasn’t choking the engine well enough at that point. That is reasonable, because a 1/2" ball valve would be more appropriate for an engine a couple time larger than our current one.


The other thing we saw was that the central engine exhaust had eroded some of the landing pads on the inside, depositing some melted bits on the chem. suits and vehicle. We had never seen this before, because it probably required a couple seconds at high throttle right on the ground, while all of our previous flights lifted off before it could happen.





New Vehicle


The new twin-tank, seated-pilot-under-a-roll-bar vehicle frame is taking shape. Unfortunately, the only tanks that we can get a matching pair of at the moment are heavy hoop wrapped aluminum NGV tanks. We have access to five different pure-carbon tanks, but all of them are different sizes.


We will fly it originally with these tanks, then if we want to save 100 pounds, I can get some tanks made to spec.


Other Stuff


The FAA denied our request for a standing 5000’ waiver over my 100 acre property out of town. I’m pretty sure we will be able to work it out after making a few more trips up and down the bureaucracy (the waiver got passed between four different people before someone finally said “no”), but if we are reduced to applying for one-shot waivers with a 30 day turnaround for every test flight, it is going to have a significant impact on our development speed.


BRSI (www.airplaneparachutes.com) was unwilling to sell us recovery parachutes for our unusual application due to liability concerns. They had the ideal chutes for our next couple vehicles, but we will have to find something equivalent elsewhere. They did provide some information about their systems: the drawing rockets are 294 total NS with a 1.1 second burn time, are attached to the chute with two angled cables, and don’t have any fins or guide rod.


We got a new book on parachute recovery systems, which makes for some interesting reading. For high mach, high altitude deployment, towed ram air ballutes get the highest marks. A complete parachute recovery system for an X-Prize class vehicle is looking more complex than I had originally thought, probably involving three chutes (not counting backups): a small supersonic chute to get it below mach one, then a drogue to get it bellow 300 mph, then a reefed main chute. We spent a while discussing various dive break options that might be able to get the vehicle to a 300 mph terminal speed, as well as having the ability to reduce the peak reentry G loads by varying the extension, and possibly providing some steering during descent by non-uniform extension. For the past couple weeks we have also been semi-seriously considering building a roton, which neatly solves all the recovery aspects.








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