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Manned drop test, failed hover test

March 11 and 15, 2003

March 11 and 15, 2003


Manned Drop Test


We had two crush cones prepped for testing today.  They are essentially the same as the first one tested, with the addition of two 2” diameter pressure vents.  The cabin bulkhead has also been reinforced with welded retainers and a much better bond, in case the pressure rise is still significant.


Both tests were with the nose 10’ off the ground, and a total drop weight of 2400 pounds.


The first drop landed straight and square, but our secondary restraining line came off, so it still tipped over.  We were a little concerned that it might crush all the way, because the last test had a significant air-spring effect with the un-vented cone eventually popping the main bulkhead off.  However, this time it only crushed a little bit farther, and without the overpressure induced buckling, it actually ended up taller than the last one.  The accelerometer traces showed a longer deceleration, without the increase and spike from the air spring effect.  The accelerometer data is still vibrating back and forth hugely, but if I filter it way down, it shows a steady rise to 4G of deceleration, which is about the behavior we expect – crushing the wider part of the cone requires more energy.




For the next test, Russ got in the cabin and strapped down.  The pilot accommodations include a five point racing harness with strap pads, a 2” thick hard foam plate that covers all the seat belt and reinforcement brackets on the bulkhead, a 4” thick memory foam pad under his back, 4” thick memory foam pads on his sides, and a couple other foam pads positioned wherever he wanted.  He wore a helmet and neck brace.


The drop went perfect – 0.8 seconds of zero G, then about 0.25 seconds of deceleration.  We finally got the catch rope right, so it didn’t even tip over.  Russ reports that it was absolutely no problem at all, it felt like you were just standing on a big aluminum can that someone poked in the side.


As an experiment, we had left one of the vent holes completely open, and one covered.  The covered one did still blow out, which confirmed our calculations that a single 2” hole would still generate several PSI of pressure.  We intend to go to permanent louvers in the cabin flange section in the future, so additional fabrication work doesn’t need to be done on the disposable sections.


In practice, under parachute the vehicle will be coming down at some angle, and with some horizontal velocity, so it is highly unlikely to work out this nicely for real recoveries, but we have tons of margin in both pilot comfort level, and maximum energy absorption, so it will probably be ok.  We will learn from the parachute flights of the 2’ diameter vehicle, and then a helicopter drop test of a full size vehicle.



(warning – 16 megs)


Failed Hover Test


We got the 2’ vehicle out under the lift for its first attempt at hovering.  We don’t have landing gear on this vehicle, so it needs to be supported from above if it isn’t going to come all the way over and land nose-first under parachute.


The engines are all brand new, only having been fired once each.  We spent a long time trying to carefully warm them up, but because we were at 73% humidity, they never did clear up.  I slowly brought the engines up to warm them all together while the vehicle was still on blocks, and it unexpectedly hopped off the ground a little bit.  I killed the throttle, so it came back down and banged one of the legs into a cinder block, but nothing seemed to be hurt.  However, the Crossbow IMU had stopped updating.  The computer and telemetry were still fine, but the Crossbow wouldn’t reset until the entire system was power cycled.  This was worrisome.


This showed up an issue with the flight control software that I then fixed – if the crossbow was dead, my flight control software would automatically exit, my original thinking being that you can’t possibly fly it without the IMU, so you shouldn’t be allowed to try.  However, that overlooks the valid use of the manual flight control mode for venting the tank and engines.  We have a manual switchbox that also serves that purpose, but it is much more convenient to use the computer when it is functioning.


We loaded more peroxide, and, after verifying that the engines were still hot and producing thrust, we pulled the blocks out from underneath the legs to let the vehicle just dangle, then throttled up for a liftoff.  Very shortly after liftoff, the crossbow died again, leaving the vehicle to careen off course, then the telemetry ceased another second later.  The vehicle swung like a big pendulum under the lift, but didn’t suffer any damage except for a dent in the cone.  This was close to the behavior at the Oklahoma crash, but is probably unrelated, because the main voltage was not collapsing.


This did show an error in the new abort logic that hadn’t been tested yet – the engines should have gone into immediate throttle down on losing the crossbow, but they didn’t.  One of the changes I made when we moved to the four servo engines is to drive the fundamental tic rate directly from IMU update packets, rather than the system timer.  This removes an average of ½ the update period in sensor latency, but it means you have to fall back to a different way of cycling when the IMU is dead.  The code was in there for that, it just didn’t work right.  All it took was moving one line, and it seems to do the right thing now.


The crossbow didn’t come back after power cycling, and now seems to be really dead.  It was probably fatally damaged in the Oklahoma crash, and only happened to be barely working in static situations since then.  I have had a new model ordered for three months, but Crossbow is having delivery problems.  I hope they take that into account and put a priority on fixing this one when I ship it back to them…


We have a few other anomalous things that we don’t understand:  The feedback calibrations on the valve pots has shifted.  The valves got fairly hot while we were slowly pushing all the peroxide through the engines at ground level, but they still work fine, so it seems odd that the resistors would have been damaged.  The main parachute chain releases were fired at some time during the flight.  The vehicle did tip past horizontal, but that should only be a trigger in “parachute mode”, not “hover mode”, which it was definitely in.  A crashing computer can often trigger random actuators, which is probably what happened, but I wish we knew for sure.


On looking inside, we found that the wireless transmitter’s power cable wasn’t very secure, so it is conceivable that the computer didn’t actually die, and we just lost telemetry, but that doesn’t fit with the fired parachute releases.  I want to find a small VGA screen we can put on the vehicle to let us sort those cases out, but the smallest analog VGA LCD screen I have found is 10” so far.


There are at least two intermediate connections on the electronics board that can be removed – the Esteem power connector can be replaced with a crimped connection, and the Crossbow cable goes to a DB9 / ribbon connector on the PC104, which can be replaced by making a custom cable that goes directly from the DB15 on the Crossbow to the power terminals and the ribbon connector.


We have been planning to replace the current power supply board, because it doesn’t have proper ring terminal connectors on it, and we suspect it of some possible failures.  We are going to do this before flying the vehicle again.  We will also be using a regulated 12v power supply for the Esteem and pressure transducers, instead of letting them run directly from the battery.  They should be fine down to 10v, but we want to completely remove the uncertainty.  The Crossbow is rated for 15v to 30v operation, and we currently have a 15v power supply for it, but we have been cautioned by a reader that running at the minimum input may not be the best thing for reliability, so we are looking into possibly increasing this.  The next-gen Crossbow FOG IMU takes 8v-30v, which will be nice, but they have slipped several months in delivering them so far.


This is discouraging, because we had made several reliability improvements on the current electronics board over our previous ones.  If the Crossbow can be repaired quickly, we are going to change a few more things and try again.






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