January 14 and 18, 2003 Meeting Notes

 

Trunk Latches

 

In the course of discussing robust electrically actuated release mechanisms for the main chute deployment, the idea of using auto trunk releases came up.  Russ and Phil went down to a local GM dealer and looked at a few different options, and brought one to our Tuesday work session.

 

The latch itself looked pretty sturdy, but the casing was just two pieces of stamped steel, so Russ welded the seams together for extra strength.  The actuator looked like a solenoid, but it is actually a motor drive, so polarity on the electric input matters.  We made a mounting bracket for the release, hung it from the chain hoist, and picked up one of the 250 lb boilerplate tank ends for testing.  We just lifted it a couple inches off the ground with 2x4s underneath it, and hit the actuator with one of our remote solenoid button boxes.  It released perfectly, so we decided to load it up and see how it held up.  With Russ standing on the tank end, bringing the weight up to 400 lb, it still operated fine.  Adding Phil, bringing it to 620 lb, the actuator would not open reliably with 12v, but putting two batteries in series for 24v still did it.  Adding Neil, bringing it to 850 lb, the actuator would only intermittently operate even with 24v.  We sprayed some Teflon lube in the actuator, and it seemed to work reliably with 24v after that.  At 850 lb, it didn’t show any signs of structural flexing, so it seems stout enough.  The release latch did have a plastic coating on it that had been abraded off by our overload tests, but it doesn’t seem to effect the operation.

 

The non-operating load on the 2’ vehicle drogue release will be fairly high, possibly in the 800+ lb range for a supersonic deployment, but the load at time of release will be under 200 lb.  We are going to be using two releases for redundancy, so the load on each will be halved.   This release seems to be perfect for the job, so we got another one, and welded the mounting brackets to our bottom bulkhead.

 

After some discussion on aRocket about completely pyro-free ejection systems, Rick Weber has graciously offered to build one of his CO2 ejection systems for us for initial drogue / stabilization chute deployment.  We could have lived with a black-powder solution for this vehicle, but we have been told that being completely pyro-free for our X-Prize vehicle will ease some of the regulatory hurdles, so we might as well test it out on this vehicle.

 

Crushable Nose Caps

 

We tested our crushable nose caps on Saturday.  We really had no idea how well they were going to work, so instead of testing with the full  vehicle, we bolted the nose cap bulkhead and a crush cone to the bottom of the boilerplate tank end, which weighs about as much as the final vehicle will weigh.  We had a +/-50g accelerometer mounted to the clamp plate inside the tank end, and I had paid for the WinDaq-Pro update (ripoff price, but I don’t have the time to write something better right now) so we could get high sampling rates from our data acquisition system.

 

The crush cones are 0.050” thick aluminum, rolled and welded to a 15 degree cone.  The top has a 1” disc welded to it to close out the rolled cone, and the bottom slides and bolts over a 6” diameter aluminum bulkhead with a matching slope on it at the top of the main vehicle nosecone (which is made of 0.125” thick aluminum).  There is about 6” of crushable space for deceleration.

 

We did the first drop from 3’ above the ground, giving a 0.43 second drop time, and a 13.9 ft/s velocity at impact.  The top of the cone crumpled up very nicely.

 

The second drop test was from 6’6” above the ground, giving a 0.64 second drop time and a 20.4 ft/s velocity at impact, which is about what we expect our impact speed under parachute to be.   One of the weld seams did split, and the bolt holes elongated a bit, but  the entire cone crumpled perfectly, bottoming one edge to the bulkhead.  Overall, these tests came out much better than we expected.

 

http://media.armadilloaerospace.com/2003_01_18/crush.mpg

http://media.armadilloaerospace.com/2003_01_18/cones.jpg

http://media.armadilloaerospace.com/2003_01_18/shortDrop.gif

http://media.armadilloaerospace.com/2003_01_18/tallDrop.gif

 

The accelerometer data didn’t come out very well.  The sensor is quoted as having 100hz bandwidth, so I expected it to show 30 msec or so of sharp positive deceleration on the main axis, but instead it had a vibration like pattern, bouncing above and below 0g.  I doubled the sampling rate for the tall drop, and you can clearly see the ringing at a much higher frequency than the sensor is supposed to output.  I should have mounted the sensor on a little slab of rubber to damp off high frequency oscillations.  The peak numbers look credible, at about 40 g maximum deceleration for the tall drop.  For equipment, that is completely reasonable, and we are going to fly without any modifications.  You can get a 40 g deceleration by dropping something hard only a few inches onto a solid surface.

 

The X-Prize crushable cone will be over 7’ tall, giving it 14x the deceleration stroke of the small cone, so it shouldn’t be a problem to get decelerations down to 10g-15g for the pilot.

 

We are probably going to try full-size drop tests for the X-Prize vehicle in the relatively near future.  We bolted one of the tank ends to our mockup cabin cone, and our plan is to add sandbags in the tank end until it is ballasted to the full landing mass of the final vehicle (around 2500 lb).  We will just weld some chain link fence over the end to keep the sandbags in place after the drop test.  We will need to rent a crane that can lift the weight 20’+ into the air for that test.  Before we can do this, we need to get the reinforced cabin hatch and the reinforced cabin-to-crush cone section fabricated, as well as a couple full size crush cones.

 

Electronics

 

I finally got the new Ampro PC104 board all working.  I had to upgrade to a newer linux kernel to use the on-board Ethernet, and I had some grief duplicating the entire filesystem from our previous computer, but it is all working now.

 

I bought a 512mb compact flash from Synchromesh, and I was able to finally use that for the filesystem.  The Kingston CF cards I had previously tried would always flake out with ide timeouts after they had been used with Linux for a while.  The synchromesh has given one non-fatal dropped interrupt message, but everything has been working well so far.  No filesystem access is performed during flight, so it can’t be a cause of an in-flight failure in any case.

 

One thing Ampro does which is still causing me grief is that they actually plug the sockets on their connectors that are supposed to be key pins.  None of my other hardware does this, and I can’t figure out a way to pull the plugs out, so they are basically forcing me to try ripping pins off of my other hardware.  Another gripe is that the CF socket on the bottom of the CPU board is too tall to allow another PC104 board underneath, but they also have top-mounted (as opposed to edge mounted) ribbon cable connectors for the floppy drive, so when you stack boards on top, you can’t get at it.  I would have preferred to have the CPU board on top, and just plug in rarely-used ribbon cables like floppy and external ide when needed, rather than leaving the cables on all the time.

 

My standard advice for people thinking about using PC104 for rocket applications:  Get one of the Parvus PC104 separating tools so you don’t bend a lot of pins changing things around, and make sure you use four metal standoffs between each board.  Most vendors only ship two, often plastic, standoffs with each board, but we have broken plastic standoffs and flexed the boards enough to pop the bus connectors off in crashes if all four corners aren’t screwed together.