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Krushnik Effect, Parachute Tests

November 5 and 10, 2002 Meeting Notes

November 5 and 10, 2002 Meeting Notes


Krushnik Effect


We did tube hover tests on Tuesday.  It was cold, but no longer humid, so once the engines warmed up they were clear, which was a relief.  What was frustrating was that we did the first test with 250 psi tank pressure, and even though I wound up throttling all the way up, all the tube did was scoot sideways a little bit.  This did not make sense, because the test stand results indicated it should have over 500 pounds of lift with a straight shot to the engine.  We had modified our procedure to do the engine warmup pulses while the pressurization hose was still regulating the vehicle tank, so there was no drop in pressure, and it had plenty of time to get the initial pressurization cool-down pressure drop compensated for.


We loaded up again, and went to 300 psi, the highest I want to take this tank.  It did the same little scooting thing.  We were fairly mystified, so we pulled it back inside and very carefully balanced out two scales to weigh the vehicle (280 pounds without the parachute and draw tower), then used one of those scales with a hydraulic jack to calibrate the test stand load cell.  Everything checked out.  At 300 psi, it should have been pulling almost a full G of acceleration.


I checked the motor valve calibration to see if it might have only been opening halfway, but that was right on.


It finally hit me:


Krushnik Effect!


The base flare is only held about three inches off the ground by the wire rope isolator feet, so the rush of gas from the main engine has to accelerate a fair amount to escape radially.  The 4' diameter flare base has 1800 square inches of area, so even a small pressure drop would overcome the 250 or so pounds of net engine thrust, effectively sucking the vehicle to the ground.  This effect has been known to happen in model / high power rocketry when you have a smaller engine recessed in a body tube, right over the launch pad deflector.


We cut some more polyethylene blocks to stand the entire thing another 3.5" off the ground, and tried again.  It was a good thing I was ready for it, because during throttle up, the vehicle basically "popped" off the ground at a pretty good clip.  A caught it just before it came up on the tether, and got it back down.  When it hit the ground it knocked off most of our foot extensions, so it was sitting at a crooked angle, but everything came through nicely.  I just blew all the peroxide out the attitude engines, rather than try and lift off from a 20 degree tilt, but the test was a great success.  It flies nice and straight while accelerating under guidance.  It should be a pretty flight when we go to altitude.


We will be installing more permanent extensions with bolts before we fly it.





Parachute Tests


On Saturday, we transported the entire vehicle out to the test site as if we were going to fly it.  Got some funny looks on the highway.  The winds were gusting, so we decided to set it up off the concrete pad for the parachute tests, because a gust could easily grab the parachute and pull the vehicle over on its side.


We used spray-Teflon to lubricate the coupler that houses the parachute deployment bag, and that made a huge difference in how smoothly it pulled out.


Since we aren’t too worried about high speed aerodynamics on this vehicle, we have a terminal block on the outside of the cone for connecting the igniter wires.  The two Pro-38 rockets have conventional igniter installation, with the leads strain relieved at the top with a piece of tape.  On ignition, the wires are intended to be pulled out the bottom and left with the main cone.  On the second test, the igniter lead was wrapped around one of the tower legs, which broke the wire at the terminal block.  We are going to move to a head-end igniter assembly with well secured leads, and have a connector plug that separates with the chute tower for future tests.


The aluminum tower legs were protected with Kirkhill Fastblock-800 thermal insulation, which is rather fragile, but seemed to do a good job.


The tower base / rocket nose is a tough phenolic / fiberglass cone, but it was noticeably eroded after out two test fires.  We have coated half the cone with RTV and half the cone with Fastblock-200 to compare the effects on future flights.


The main parachute deployed absolutely perfectly in both tests, coming out all the way to line stretch, but the small parachute that we included in the bag to decelerate the rocket tower didn’t come out in either test.  In the first test, we had the small chute connected to the top of the big chute with rubber bands.  In the second test, we tried duct tape.  Neither one pulled the little chute out of the bottom of the bag, resulting in a ballistic impact of the tower structure.  The next test will use a thin Kevlar thread, which breaks at 10-20 pounds of force.  It is a bit touchy increasing the force on this, because if it doesn’t freely break after pulling the little chute out, the hot tower / rocket assembly will come back down on the main chute and burn a hole in it.





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