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Cabin work, Small vehicle engine tests, Aborted hover test

February 18 and 22, 2003 notes

February 18 and 22, 2003 notes

 

Cabin Work

 

We refitted the main bulkhead to the cabin, using an angle grinder to make much deeper gouges in the aluminum before bonding than the sanding we had done previously.  We also welded four custom brackets above the bulkhead location, so we could put backing plates on the pilot side as an additional precaution against popping loose.  Ideally, we would have had the custom brackets on the bottom side, but we can’t weld that close to the bulkhead without cooking the epoxy bond.

 

The hatch sealing lip is almost finished being welded on, and we are working on the pressure seal and hatch dogs.

 

Small Vehicle Engine Tests

 

We broke in all four new engines on the small vehicle, and we saw something rather interesting.  Because we didn’t want to disassemble everything, we just flipped the bulkhead upside down and clamped it to the trailer, so the engines would fire straight up.  This is a Really Bad Idea for biprop engines due to the possibility of propellant pooling, but for a monoprop break in, we decided to do it anyway.

 

When fresh cat packs are first fired, the exhaust is very cloudy (after the first liquid gush of peroxide has cooked off), and clearly visible.  On several of the break-in runs, the exhaust jet shot off at a pretty significant angle instead of straight up.  This was a result of flow separation on the highly overexpanded nozzle (the throttle was only cracked, so chamber pressure was very low) causing the jet to attach to only one side of the nozzle.  If this nozzle was designed to be minimum weight, it would have buckled, but all of our engines are way, way overdesigned, so it doesn’t hurt anything.

 

We had never seen this effect so dramatically before, because we never had the right combination of firing direction and non-broken-in engines.  It was interesting enough that it got me thinking again about alternate nozzle designs.  I think we are going to fabricate and test an expansion-deflection nozzle for our bench test 5.5” engine.  We have two sets of nozzles for these engines, 1.25” D throat, and 2” D throat.  An E-D insert can be easily made to give the 2” D throat the same annular throat area as the smaller nozzles, which would make for a perfect back-to-back set of test to contrast the purported pressure compensating effects of an E-D nozzle with a normal nozzle.

 

In any case, the engines are ready for liftoff now.

 

Aborted Hover Test

 

We were hoping we were going to have everything ready for a hover test of the new control software (this will be our first use of four servo valved engines for attitude control, all previous vehicles used solenoids), but we didn’t quite make it.

 

We insulated all the engines with about a 1/8” thick layer of Fastblock-800, which should dramatically reduce the temperatures that the parachutes and vehicle see.  We already had some Nomex gores RTV’d to the bottom of the tank for some insulation there, but insulating the engines is a superior solution, because it keeps the heat away from everything at once.

 

On the advice of a reader, I tried moving everything to a SANDisk brand Compact Flash for the flight computer.  That seems to have fixed all the stability issues.  To recap:  Kingston CF cards I was never able to get Linux installed without running into fatal errors.  Syncrotech CF cards were just reliable enough to be dangerous – I could get things installed, but there would be an occasional IO error during disk intensive operations like compiling.  The new card hasn’t shown any problems whatsoever.  The AMPro BIOS still has a quirk of some kind that causes it to not boot the CF on initial power on, but it finds it after a quick power cycle.  I am probably going to ask AMPro about this.

 

Another improvement to the electronics that I finally got around to doing was to add a proper connector for battery charging / external power.  I had always been just clipping battery chargers to each battery, but now we have a multiple charger setup that can be connected with a single CPC connector.

 

We took measurements on the vehicle in basically the condition that it will fly:

 

Dry weight:  280 pounds

Diameter: 24”

Total height, ground to nose cap tip: 120”

Leg length: 9”

Tube length: 68”

Nose length: 43” (15 degree half angle)

 

We measured the CG to be 29” from the bottom of the tube, which is nearly one caliber of negative stability.  Hopefully the control system is up to it…

 

What actually killed the hover test this week was the quad motor drive board.  We had tested it at 50hz cycling rate, but the flight computer cycles at 200hz, which started killing off the transistors.  Russ built this one slightly differently than the previous one, so he is going to change back to the old design, and use higher spec transistors.  The next board will have the full complement of actuators necessary for the X-Prize vehicle.

 

I’m out of town next Saturday, and we are moving to a bigger, dedicated facility (yeah!) the first week of March, but we will probably be able to combine the hover test and the repeat of the crush cone drop test the week after that.

 

 





 






 
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