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Structure complete, Drop G Forces

December 31 2002 and January 4, 2003 Meeting Notes

December 31 2002 and January 4, 2003 Meeting Notes


Structure Complete


We finished assembly of the nosecone (1/8” roiled aluminum) and engine bulkhead today.  The next time we do this, we are going to invest in some proper leveling tables and laser sights, because getting 2’ diameter tubes straight by eye is pretty hard to do repeatedly.  If this vehicle tests out exactly as it is supposed to, we will be moving to full X-Prize size vehicles next.  If, as is likely, we wind up planting this one during the test series, we have a spare cone, tank, and enough tube to build another one just like this in fairly short order.





The CNC mill cut the ½” thick aluminum bulkhead, the 4 degree engine canting shims, and a custom tooling block that aligns and locates the drill for creating all the side-bolt holes in the tube and bulkhead.  It is turning out to be a very good purchase for us.


We were prepared to make a large fin assembly out of honeycomb core composite boards, but we have decided to fly finless, relying on the control system for stability during the burn.  This saves a fairly notable amount of weight, forward drag area, and fabrication work.  The rocket will be immediately unstable at burnout, so we will rear-eject a drogue chute large enough to stabilize the vehicle either upon sensing an attitude change, or based on a timer.  A Kevlar drogue should be able to be ejected while the engines are still firing without harm, given the relatively cool temperature of the monoprop exhausts.  The drogue will be held by an electronic release, which will allow it to pull the main chute out after it has arced over.  We are going to investigate using electronic trunk releases as a cheap-but-sturdy possibility.  Popping a stabilization chute at burnout will severely limit the altitudes the vehicle will attain with this vehicle, but that doesn’t bother us at all, and should actually save us a lot of recovery work.  When we make a space-shot vehicle, the trajectory will be tailored to burn out at a very high altitude, mostly negating the issue.  This vehicle should still be capable of supersonic flight, given a full load of peroxide.


We are considering making the very first flight with the drogue pulled out by a static line shortly after liftoff, which would limit the altitude even more (it would be towing the drogue through most of the burn), but would let us factor out some uncertainties with the flight, and still get data on some other things we care about.


This assembly was 200 pounds almost exactly, but that doesn’t include the master cutoff valve/manifold, electronics, parachute, and nose cap.  Still, even with everything on, it will be lighter than the previous vehicle with the single engine and tail cone.


We have an Ashtech G12-HDMA speed/altitude unrestricted GPS with 10hz update rate, but it was quite expensive (> $7,000), so we aren’t going to put it in the vehicle until it has been successfully flight tested.  This unit is used in various space and missile systems, and is export controlled.


Still to do:


I need to get our new Ampro PC104 system configured, because the WinSystems SBC was killed in the last crash.  We could use the Mach-Z PC104 system that we still have, but the Ampro has soldered-on RAM, which removes one possible vibration failure point for us.  I was unable to find anything faster than a 486-133 with soldered memory.  The long SIMMS on modern systems seem like an invitation for failure.


New redundant power supply board, through-hole-plated driver board, and master cutoff watchdog board


Engine / release cabling.  It is tempting to run these actually through the propellant tank, but we will probably epoxy a little conduit on the side.


Fill connector and pressure gauge on bottom bulkhead


Crushable nose cap fabrication and testing.


Build and test three more 5.5” engine catalyst packs


Temporary landing pads for hover testing


Drogue cannon arrangement


Main chute release mechanism



Drop G Forces


We tested a 50G accelerometer for landing impact data acquisition, and learned a few things.  Somewhat surprisingly, you can easily give the sensor 40+ G’s by just making a fast pitching motion with your hand.  The sensor I got only has 100hz bandwidth, which isn’t enough to capture sharp decelerations.  If you just bang the sensor down on a flat table, it doesn’t register much of a signal at all, because it happens in too short of a time span.  I am probably going to have to get a vibration / shock accelerometer, which can give much higher bandwidth, at the expense of not measuring constant accelerations.


We upgraded to one of the professional data acquisition systems from Dataq a while ago, but I am still using WinDaq-Lite for data collection, which limits us to 240 hz sampling.  If I get a mult-thousand Hz bandwidth sensor, I am probably going to write my own acquisition software, because paying $2,000 to Dataq to basically just change a constant in their software seems like a ripoff after I paid >$2,000 for the hardware.  I’m not thrilled with the screw terminal breakout block on the system, either.


We strapped Russ and the accelerometer into the custom-foamed couch we made last week, and let him report how a few impact G tests felt.  The foam has almost no give to it, so you stop quite abruptly when it lands flat.  We dropped him a few times, and there is a big difference if it landed on one end and pivoted down, or landed completely flat.  The last completely-flat drop registered 15 G, but was probably higher due to the sensor bandwidth issue.  Russ said it was definitely a pretty hard impact, but non-damaging.  50G’s is a test point for a lot of race car safety equipment that is supposed to function in a crash.


We have been doing our decelerator estimates based on 10G deceleration for under a half second, which seems reasonable.  The current vehicle is only going to have about 6” of crushable nose cap, but the big vehicle is going to have over 8’ of cone to crumple up ahead of the cabin, so it should tolerate even fairly high parachute decent rates.




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