Tuesday, September 22, 2009

Silver Status...

I finished the integration testing on the Silver ball last night.
the GPS cable turned out to be an intermittent antenna problem.
Swapped out the antenna and all is well.

We ended up about 30lbs over the target weight, at least 10 of these pounds are in the new stainless motor. In the present condition using carefully calculated ISPs adjusted for under over expansion we get a hover time of 182 seconds. we never ever get theoretical ISP, so we will need to do something. Most likely a careful weight diet and the addition of some external pressurization so we can fill past 50% point. If we fill the tanks till we have a takeoff Thrust to weight of 1.2 I get a hover time (using our historically achieved ISP fudge factor) on the order of 200 seconds. Some Parts like the Motor are really massive,....

Some parts like the roll control thrusters are as light as I can imagine...

We hope to test this coming weekend.
If I get home before its dark today, I'll drag it out in front of the garage and try to take an overall picture of the beast.

With the blue ball the "vector" point for the vanes is on the bottom of the motor so that there is always an offset from this point to the center of gravity ie CG. With the silver ball this point is the gimbals hinge and with the heavy motor we were worried abut the this getting too close to the CG, so we mounted the payload up high, since we mounted it up high. The (easily removable) leg weights we use on the blue ball won't work. The silver payload are steel blocks bolted to the top of the vehicle. This combined with the increased weight makes the silver ball a lot harder to move around. My son and I can still carry it, but we cant lift it 3 feet in the air to put it in the truck without assistance.

I've done some thinking and calculating. If I leave the system as blow down and do all the weight reduction things I can think of its still pretty marginal. If I put a pressurization system on the vehicle I can add 30 more pounds and get 75% of theoretical ISP (This is 0.75 * the number beyond already accounting for over/under expansion etc...) and still do the task.
If this weekends test goes well it looks like engineering a pressurization system gives me the most bang for the time invested. It has the side effect of almost doubling my propellant usage and that makes my propellant supply look tighter than I'd want for through testing, but it still looks like the best path.

A side note on calculating ISP for over or under expansion...

For under expansion I just use the ISP I calculate when the chamber pressure is exactly right for proper expansion, thus I get no ISP credit for the actual higher chamber pressure.

For over expansion I take the theoretical ISP for perfect expansion at this pressure, then calculate the area difference between the actual expansion cone and the perfect one. I then calculate this difference in area as a thrust equal to the (ambient pressure * this area).. in the wrong direction and use tht as a reduction in ISP.

I have some C code that will run propep as a sub process and can make these calculations automatically. So as an excercise I varied the actual expansion ratio up and down searching for the flight performance maximum. It turns out the simplifying assumptions I made in January 09 blog post of using the average flight chamber pressure was withing 1% of the more rigorious method, well withing the uncertanty in all these numbers.


Seb said...

What are candidates parts for the "liposuction"

Anonymous said...

Are you already using titanium fasteners? If not, based on the size of the nuts and bolts I saw in the photos you could certainly save at least a pound there.

Paul Breed said...

Some Sanitary Clamps.
Parts of the Motor.
Landing Gear.
Redundant Valves.
Nuts Bolts...
Electronics Cases.
Possibly the Kerosene/fuel tanks.
If we can make the presurization system work none of it may be necessary.