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Streamlined hovers and landings

June 13, 2004 notes

June 13, 2004 notes


Streamlined hovers and landings


We completed the streamlined subscale vehicle and conducted several hover tests this week.  Total vehicle weight is 310 pounds dry.






We did some drop tests from the hoist to evaluate the shock absorber landing gear, and they did a very good job, even when the vehicle was swung before release.  Without any bounce, the vehicle just settles down, even at slight angles.


It has been raining a lot, and it wouldn’t let up for us at all on Tuesday, but we really wanted to test things out, so we put a big plastic bag over the rocket nose to give some extra cover to the electronics, and went ahead and gave it a try.  The flight control software had been modified to predict acceleration some time in the future to compensate for the time lag between moving the valve and engine chamber pressure changing, which should smooth out the up/down oscillations in hover and landing modes.


We stood the vehicle up on the big foam blocks so we didn’t need to trust it to come down straight, but this put them too close to the engine on the narrow vehicle, and we burned off a lot of the aluminum covering we added.  We are probably going to get some stainless foil next time.





This engine didn’t warm up as nicely as the engine on the lander, which we attribute to the lack of a compressed hot pack.  After it finally got warm, the hover looked very good, but it ran out of propellant before it landed, which made us very happy we had done a block launch instead of a ground launch.  We did another run with extra propellant, and everything went normally.  The acceleration prediction seemed to be working very well.


At hover, the chamber pressure was only 100 psi, so on a boosted hop with even a 0.5G stabilize level, we are looking at a 50 psi chamber pressure, which is certainly going to separate inside the nozzle.  We seem to have pretty good luck with even separations, but we have certainly seen some single-wall attachment separations, which I have been very concerned about disrupting flights.  We decided to cut off some of the nozzle, taking it down to only about a 1.8x expansion ratio.  This will be underexpanded on boost, and still rather overexpanded during coast, but it will hopefully be a reasonable compromise.


We also cut the engine open to make a compressed hot pack for better warmup.  There was one burned spot in the catalyst retainer, but overall it didn’t look too bad.  We replaced the catalyst anyway, and added one of the thicker water jet cut retaining plates so we could put 3000 psi gauge pressure on the 7” diameter pack.  With the extra height of the new retaining plate, we weren’t quite able to get all 1000 grams of catalyst in.


When we were setting up for a ground liftoff test, we bent one of the shock absorber rods while lifting it off the cradle.  It was just carelessness in this case, but it would be unavoidable on a big vehicle to lean on the shocks, so we are going to have to make removable covers or some other kind of removable support when we move to shocks there.  We bent the rod back into shape, but it definitely has an uneven landing to it now.


The compressed engine hot pack did make the engine warm up steadily without needing any throttle adjustment, but on throttle up it did show a stream of clouds in the exhaust, meaning that the cold pack isn’t in great shape.  We don’t have any unused 5.5” catalyst monoliths, but we have 7” chambers coming, so we will eventually make a completely fresh engine just for this vehicle.


On liftoff, the valve insulating cover ripped off right at throttle up.  We aren’t exactly sure if this is due to incremental wear on the insulation allowing the epoxy to come apart, the cut down expansion cone putting more pressure on the side, possibly in conjunction with the pressure spike this flight had at throttle up, or just the proximity to the ground in this aerodynamic configuration.  We are making replacement covers in metal, and screwing them to the bulkhead plate.






Contrast the altitude hold and landing with previous hover tests.  I should be able to get the horizontal position hold tighter, but we are very happy with this behavior.  I am concerned that the visible exhaust cloud seems to be coming off at a fairly large angle, but it seems to have plenty of control authority.  If it doesn’t rain any more, we are going to try a boosted hop with this vehicle on Tuesday.


I have the software set up now to take differential GPS corrections over our normal telemetry stream, but we were getting such good GPS signals today that I wasn’t able to tell if it got rid of the discontinuities in the GPS stream.  Having the antenna really at the top of the vehicle, with a nice metal ground plane below it, may well be giving us a lot better GPS reception than in the previous vehicle, which had some metal bars above the antenna, and a composite base.


We also set up the remote battery boxes in the big vehicle, and rebuilt the 2” valves that were allowing propellant to leak into the engine during pressurization.  We are still afraid that we might have killed this 12” engine due to the leaking during our last test.  Trickle flows get the engine started without any ignition source at all, because the methanol decomposes on the catalyst into hydrogen and formaldehyde, and the hydrogen will catalytically burn with oxygen even at room temperature, eventually raising the temperature enough to start free-space combustion.  Unfortunately, this allows burning to take place in the cold catalyst pack, because the flow isn’t high enough to keep it down at the flameholder where we want it.  This starts locally killing the catalyst.  The lesson is to not live with leaky valves.




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