Tuesday, November 27, 2007

Tank and Helicopter Testing.

Today at lunch I flew the helicopter again and I seem to have solved all the electronics glitches. I changed from a 72Mhz RC Rx to one of the new 2.4Ghz units and it was much smoother. I'll analyze the recorded telemetry in the next day or so and post the results.

This afternoon my son started hydro testing all of the vehicle tanks. We had planed to test the entire vehicle with plumbing and everything, but with recent tank failures on the test stand and our sample heat treated tank we decided to individually test the vehicle tanks. The tanks were designed to be hydroed to 500PSI. However for the 90 second vehicle we did not ned to go so high. We backed off to 450PSI and still did not fair well. We tested all 8 tanks and got the following results: Failures at 350, 385,415 and 435. Four tanks passed all the way to 455. Since thanks are welded together in pairs we have one good pair, that will go on the test stand. The rest of the vehicle tanks are probably scrap. All four tanks failed in exactly the same way, at the interface between the endcap and the tubular tank. The weld stayed attached to the end cap and the weld pulled away from or failed at the tubular part of the tank. The tubular part is 6063 and the end cap 6061 so this failure mode sort of makes sense.This result really complicates the decision process. What do we have from last years effort? We have 1/4 of a vehicle tank. We have some structure. We don't have valves, plumbing electronics we can use. We think (confidence 80%) that we have motors that will work. Do we scrap the design for a simpler solution or do we fix what we know is broken? There is a strong desire to start over from scratch. If I were to start again I think I'd build a 3/4 scale pixel. However this would mean scrapping the one part we think we have working the motor. Arghhhh decisions and only 11 months to be ready.

9 comments:

noel.wade said...

I'm noticing a similar tone of frustration from both you and JC over at Armadillo. I can understand it, since I don't deal well with frustration myself...

But try to remember this: Its not just about the physical parts, its about the knowledge and the first-hand experience you have gained. Knowledge and experience, while not physically material objects, DO have value!

If they don't, why do we pay through the nose for things like college degrees or technical certifications? The pieces of paper we get for completing these does not, in and of itself, justify the cost of the entire experience. There must then be some value we assign to the information and experience gained by grinding through it all; or no one would do it!

Try to keep that in mind. Your knowledge and experience is what allows you to make improvements and move towards success. Even if it does feel like that progress is only inches at a time, its more than you would make if you weren't learning through your work on these projects...

Take care,
--Noel

Jim said...

3 ideas...

If you go for a Pixel style vehicle, instead of designing a new engine could you just cluster your engines close together using a manifold and single set of valves?

Or...

Keep your tank setup, but again, cluster the engines and gimbal-mount them below the top of the vehicle. Keeping approximately equal propellant usage between the 8 tanks may be difficult though with a manifolded engine setup.

Or...

What about taking your four good tanks, leaving them in approximately the same configuration as your current vehicle and use them for just for the fuel, and use a single spherical tank centered below them for the LOX. Although this would still keep the 4 independent engine/valve system which I've gotten the impression you want to do away with. On the plus side though you wouldn't have the added mass/complexity of a gimbal system, plus a single LOX tank may have a better mass fraction than 4 tanks. Perhaps moving the engines inboard a bit would help your ball valve speed issues (closer to c/g would mean slower throttle speed required for control, right?)

Good Luck.

-Jim

Jim said...

Re-reading your post I see now that your 4 good tanks wouldn't be usable in my last suggestion and new ones would need to be made, but the concept may still be worth some thought.

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-Jim

Jesse said...

Have you pretty much given up on the PET tank idea?

Paul Breed said...

I realize that I only have 11 months.
If I had two or three years I'd work on some composite tank ideas.

I just don't have the time at this instant.

Paul

Carl Tedesco said...

I agree your motor development seems complete, however, I think the last few static tests you had some ignition problems. Ignition may have been the culprit with Armadillo. I wouldn't under estimate the amount of time it may take to get your igniter to work reliably. If you're lighting four motors and they all must light at once ignition may even be more critical.

--- Carl T.

John Carmack said...

Our single module design is a lot easier to build, work on, and operate than the Quad designs. It doesn't have quite the performance to get to 180 seconds (about 150 tops) with the extra weight of the legs and half the tankage to amortize the engine and computer over, but if you just want to make something work well...

John Carmack

Jon Goff said...

Paul,
I'm with John on this one. A single-engine vehicle with vertically stacked in-line tanks like MOD seems to be a much easier project than a Quad or a multi-engine vehicle.

~Jon

Bob Steinke said...

>A single-engine vehicle with
>vertically stacked in-line tanks
>like MOD

...or Laramie Rose...

Sorry, I just had to take the cheap shot.

Paul, If you are going to scrap the two tanks that did not fail, but are welded to ones that failed I think it would be interesting to test them to failure too.

Bob