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Fiberglass tanks, engine work, tube vehicle ground test, misc

June 11 and 15, 2002 Meeting Notes


In attendance:


John Carmack

Phil Eaton

Russ Blink

Joseph LaGrave


Fiberglass Tanks


I finally found a distributor willing to sell me single unit quantities of the fiberglass tanks from the Structural ( www.structural.com ) line:


Performance Water 714-736-0137


The 45 gallon tank we are looking at is very inexpensive, only $320.  It is rated for 150 psi use, with a burst pressure of 600 psi.  Doug Jones of XCOR reported that they had hydrotested one of these tanks, and the threaded closure blew out at 250 psi.  Kevin Bollinger of ERPS reported that they had pressurized a smaller Structural tank to over 450 psi, which caused some disturbing noises, but no ruptures.  I will probably buy two tanks and we will destructively test one, then proof test the other.


New Engine Work


We have a fuel injector ring and clamp ring for a 2” diameter biprop made, and we should be firing a brass mock up of the 2” radiatively cooled chamber next week.   We will be cooling it with multiple water jets on the outside, which will allow us to get the basic development and tuning done with brass.  After we have that done, we will be getting our 12” bar of TZM carved into three identical combustion chambers for us to experiment with coatings to prevent high temperature oxidation.  Our initial attempt will be platinum plating.  The goal is to be able to do multiple 60 second biprop burns on the engine.  We will be building a 2” regeneratively cooled engine later.


I have sent out some requests to metal spinning companies about fabricating our 12” engine.  The 5.5” engine is about as large as we want to consider cutting out of bar stock, so our options for bigger motors are either rolling and welding, or spinning.  Most spinning shops seem to be limited to 3/16” with stainless, which will be a little on the thin side, but should be ok.  A plus with spinning is that TZM is supposed to be spinable with some heat application, so if the stainless monoprop motor works out, and our TZM test motors work out, we may have an easy path to a several thousand pound thrust biprop.


In preparation for the biprop tests, we built another 2” catalyst pack with minor changes that may result in less pressure drop.  The packing order was 6 stainless at the top for spreading, then two sets of 10 x stainless / doubled silver, then two sets of 10 x stainless / single silver, and one final stainless screen before the retaining plate.  Compression on the pack was lowered from 1500 gauge to 500 gauge (about 300 psi) after each anti-channel ring, so it occupied the same pack volume as our previous packs, which had 4 more silver screens and 10 more stainless screens.


The run was smooth, but there was a hint of undecomposed peroxide early in the run, so it may be borderline.  Unfortunately, we didn’t match the parameters of our previous 2” engine test, so the results are inconclusive.  The May 14  test used a straight through –4 fitting, which is around a 0.130 jet equivalent, slightly different feed plumbing, and it was at higher tank pressure.  It may also be futile reducing the pack drop with the solenoid limiting the flow.  We should probably try testing with the larger ball valve to make direct comparisons, but it was worthwhile to see that the run was still smooth with less compression, which we needed to know before packing our 12” engine, because our hydraulic press gauge doesn’t go high enough to reach equivalent pack compressions for that much area.





Tube Vehicle Ground Test


We are trying a set of adjustable hydraulic shocks on the base.  There is a pretty good chance they will just break off if we come down with much side force, but they work nicely for direct impacts.  The wire rope isolators will handle multi axis loading a whole lot better.


We got the legs aligned and pinned, the thrust ring installed in the tube above the bulkhead, and the centering rings epoxy coated and installed.


We did several ground fire tests to make sure the structure is going to be strong enough to take the engine loads.  We flipped the vehicle upside down, so the engine was firing straight up, and ran a propellant line from the trailer to the vehicle.  We did three short incremental tests at 200 psi, 400 psi, and 600 psi.  On the high pressure runs we could see the fins flexing up a bit as the motor thrust against the bulkhead, but everything held together, so we did a three gallon run at 300 psi, which would be about the same loads as a typical hover test, and that also worked fine.  The insulation around the main engine did it’s job well, with nothing getting very hot after the long run.  We took a break after that, and we were a little surprised when we came back by how much heat soaked back into the propellant feed hose stainless overbraid from the engine.  It was quite warm three feet from the engine.  We will have to make sure to open the access hatch after flights if we don’t immediately do a water wash of the system, or we might cook some things down by the engines over the following half hour.




The filament wound tube has been really nice to work with.  It cuts easily with a hand held cutoff wheel, drills nicely (it usually chips a bit on the back side, but not too bad), and it quite strong.


We have been making our bulkheads and centering rings out of epoxy coated plywood, but we are probably going to move to aluminum in the future.  We have purchased a used CNC mill, but we haven’t cleared out the space to install it yet.  With that, we should be able to make our own optimized isogrid bulkheads, with exactly the mounting bosses that we need, and everything else cut way down.


We are probably going to get some basic sheet aluminum fabrication experience, but I am also investigating using metal spinning for nosecones and fairings.




We are moving to polyester webbing on our ratchet straps, because we noticed after the last crash that the nylon straps that had gotten some peroxide sprayed on them had turned into crusty goo.  We had seen this on some straps before, but we weren’t sure if it was due to heat from the engines or peroxide.  The polyester seems to hold up better to peroxide exposure.


On Wednesday before the OSIDA meeting, I met with several members of Jim Benson’s SpaceDev team ( http://www.spacedev.com/ ), including Marti Sarigul-Klijn, who had been the chief engineer and test pilot for Rotary Rocket.  After all the work at Rotary, he is pretty down on rotor systems, both for ascent and recovery.  Among other issues that he talked about, he mentioned that it may be very difficult to get out of the vortex ring state while transitioning from the windmill brake state towards autorotation or hovering.  Normal helicopters use forward airspeed to get through that, but if a vehicle was coming basically straight down, it could be a more severe problem.  This was a completely new concern for me, although I am inclined to believe that a rotor capable of generating 5x the lift needed to hover the landing weight might be able to power through the vortex ring state.  Marti is going to be getting copies made for us of some research that had been performed on high speed rotor recovery.  If we are thoroughly convinced that rotors are a Bad Idea, then we will probably wind up looking at deployable dive brakes as an initial decelerator.




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