Pratt Hobbies GOX Box recall

According to Jerry Irvine :

It's probably better to invest in proper solenoid drivers, irrespective of the "heavy dutishness" of the solenoids.

There's a huge difference between "pull in" and "hold" currents. No sense driving _any_ solenoid at "pull in" currents for more than you have to. Especially if you're worried about battery life.

They do this (mechanically) in car starters.

According to Joel Corwith :

One of the nice things about hybrids, is that it's almost impossible to overpressure one. Blow the ends off (defective closures) certainly, outright burst? Extremely rarely.

If the chamber overpressures, it simply stops the nitrous flow. This is part of the reason why some howl on boost - boost oscilation...

Not too long ago, as LCO, I watched someone's carbon fiber rocket on a solid J chuff - sort of. It threw the thing up about 100 feet. Started to arc over (at this point I was yelling heads-up over the PA system because it was still emitting smoke & flame), and it finally "caught" just as it was going below horizontal...

Skipped across the field under full thrust leaving burning bits in its wake...

Fill times have complex relationships that aren't immediately obvious. Size of vent hole, particularly near the end of the fill plays an important role, since it will tend to dominate the flow rate through the system near the end of the fill.

The solenoids that we use have a 3/64" flow orifice, and we've used them to fill 'M' motors in under three minutes. We use a simple capacitor/resistor solenoid saver circuit, but one could get more fancy by using a PWM approach.

According to Marcus Leech :

I'd be happier with a driver using semi-conductor switches (eg: power FETs) that had "engage" current transitioning to "hold" current using a timer. More efficient on the power.

Or, creative use of the poles on a small relay running in parallel with the solenoid.

GM car starters have a tapped winding on the solenoid, and some additional contacts. When you turn the key, current is applied to the "engage" portion of the winding. When the solenoid slams home, it opens the contacts for the engage portion, and the "hold" portion (a lot more turns) of the winding takes over. The "engage" current was on the order of dozens of amps, the hold just a few amps.

Whoooole lot of California jokes I am going to let go by here just cause I'm a nice guy :)))

I just got mine the other day and it has the white wires. Looks like I have to cancel some launches this weekend at the NEPRA launch. (...and I had it up on the website that we were launching hybrids!)

Oh well, we waited 18 weeks for it, what's another few?

-- Drake "Doc" Damerau

NEPRA President NAR Section 614 NAR 79986 L3 Remove "My Shorts" to reply

There are high pressure solenoid valves available with

12 volt continuous duty coils. Parker skinner make nice valves Why use cycle duty coils for a continuous duty application. The coils usually draw 20-26 watts. NOS valves are not designed for continuous duty. You most likely will not find high pressure 12 volt continuous duty valves on the shelf but, they can easily be special ordered. Be careful when ordering. there are dozens of options for valves. Working pressure, bust pressure, CV, body material, plunger material, seal material, seat material, ect. Make sure you check ever option for the valve. Gary Deaver

Hey, this IS RMR, you know.

;-)

Chris, I took a brute-force approach to the burned-solenoid problem. My solenoids have stronger coils. The guy who makes my valves for me (who made all the NOX valves used in the "Fast and Furious" movies) and I have gone through three iterations, and we've settled on one that I think is really bulletproof. I've also increased the size of the battery in the base RTLS unit to accomodate the amp draw of the stronger coils.

I guess I should have used the same philosophy on the high voltage transformers. These questionable transformers that I'm recalling worked great firing HyperTEK Js and Ks, but apparently aren't strong enough to put a hot spark across an igniter wire long enough to light an M. I don't know for sure that's the problem, but the solution is obvious: buy the more expensive transformers. Which I have done, and which I will supply at no charge to the five people who got the cheaper transformers. I've also changed the GOX Box faceplate to have an externally mounted fuse holder.

Live and learn, I guess. I'm pretty depressed over the possibility that I contributed to failures at the PB launch, and I'm damned determined to make sure it won't happen again. That's why I'm talking about it here in Flame City.

Doug Pratt

Doc, your unit will work fine for J and K Hypertek motors, and of course you don't need it for other kinds of hybrids. I'll have a new unit to you next week that will handle HyperTEK L and M motors.

On 9 Oct 2003 06:20:06 -0700, snipped-for-privacy@cswebmail.com (Doug Pratt) is alleged to have written:

Are your solenoids and fiitings available outside the context of the RTLS system? I'm interested in putting my own GSE together, but need to find a source of good, reliable nitrous and O2 solenoid valves. I'm in the process of designing my own hybrid motors, and need my own set of GSE to be able to test-fire them.

Also, are oxygen pressure guages compatible with nitrous? I have an extra oxygen regulator with gauges that I picked up on eBay for $6.00, but I want to make sure it will work with nitrous oxide before I spend any time or money looking at fittings, etc.

I assume that a gauge that is designed to work well with an agressive oxidizer should work just fine with a less-agressive oxidizer, but I want to confirm that before proceeding.

For the curious out there, I'm working on a design for hybrid motors using the standard Doctor Rocket/Aerotech cases as a base, and then adding a floating injector with dual silicone o-ring seals, and a "forward plug" that seats inside the forward closure, with silicone o-ring seals in the delay well and against the inside of the case, venting through the ejection charge "touch hole" to a tube leading outside the rocket. I'm still tweaking the design of the hardware, but, if I can keep the weight of the bulkhead and the plug down far enough, while keeping them strong, I might be able to make useable hybrid "G" motors and up, using all of the 38mm cases from the 38/360 on up. The 38/240 is looking *real* "iffy", but I'll probably burn a few in that size range just for giggles.

Cheers,

- Rick "hunka hunka burnin' plastic" Dickinson

I just received a great email from Doug. It went into the whole story of what happened, why it happened, and the "corrective action".

I'm happy. I don't like being in the dark.

Thanks Doug. I'm glad that we have a vendor that cares about the customer.

-- Drake "Doc" Damerau

NEPRA President NAR Section 614 NAR 79986 L3 Remove "My Shorts" to reply

Same here! To me, that's a major selling point. The downside was that the AT hybrids seemed to be awfully expensive, and extremely limited in variety.

Not since the fire, though I've heard that they may start production on them again at some point.

Yes, I sell the custom made NOX solenoid valves and the GOX valves I use on the RTLS-M4 separately. Drop me a note for pricing.

Doug Pratt

You never hear anything about the AT hybrids because Gary is a little busy doing other things at the moment. Gary was very excited about his hybrids, and supported the line even though I strongly suspect it never made any money.

I guess I can let slip a little news: I'm negotiating now to license the production of AT hybrid reload kits. "Negotiating" might be the wrong word; we agree, and I'm trying to pull together the cash to get started. In fact, I just delayed it another month by making a contribution to saverocketrynow.com...but I'll do my best to make sure it happens. Don't dump your AT hybrid hardware!

Doug Pratt

Thank YOU, for patience above and beyond the call of duty...or reason. Your GOX Box went in the mail this morning; I emailed you the Delivery Confirmation number.

Doug Pratt

Yup, that's what I use, but you could also reasonably use an inert-gas gauge.

I've been down this road. The problem with using casings intended for solids is that they aren't really long enough to make a "serious" hybrid motor. Not enough volume for N2O, once you make your fuel grain long enough to provide an adequate O/F ratio for the proposed thrust level. Robert Galejs has been down this road recently, you should get some of his thoughts.

Couldn't you make a device to use the reloadable case as the fuel grain and nozzle retainer and in stead of a bulkhead closure, screw a huge N2O tank on top?

Jerry