December 5th, 2009

#1838: The Hot Potato

Okay, I'll tell the tale, damn it.

...in an issue of High Power Rocketry magazine there was an article about some big launch out west, and one guy tried making a rocket out of Pringles cans. Pringles cans being what they are, they turned out not to be well-suited to deal with the thrust of a big (for a model) engine.

As I recall, the failure mode for this vehicle was compression: the fuselage (made of Pringles cans stacked end-to-end) buckled, resulting in a catastrophic loss of stability.

There's an easy way to prevent that, one I'm surprised the builder didn't use because it seems fairly obvious to me. You take a fuselage tube made of some reasonably strong material and use it as a structural core, epoxying the Pringles cans to it (and installing internal bulkheads as needed). The internal core tube then needs only to be the diameter of the desired engine (29mm would seem reasonable) and can run the length of the rocket. There are several techniques to build fins which will stand up to the aerodynamic load of a flight using a high power model rocket motor; needless to say just attaching fins to the outside of the bottom Pringles can would be a sub-optimal solution.

And in fact, if all you want is a rocket that looks like it's made from Pringles cans, just get a commercially-available kit of the right size and cut-and-paste the cans around the outside of the thing.

That's for a high power flight; and you'd have to do that if you use several Pringles cans. But what if you use only a single can?

I thought, WTF, I can make that work!

I actually built two of them. The first was a proof-of-concept and worked very well, but it was a bit ugly. The second was more refined, and I took pains to make it look like an unopened can of Pringles with fins.

In model rocketry, using metal parts is verboten, generally speaking. (At least when you're working on the Estes end of the scale.) But my Pringles can rocket--the Hot Potato--used the can's metal bottom as a thrust bulkhead.

First I cut about 2.5" off the top of the can and emptied out the potato chips. I cut a 24mm hole through the can's metal bottom; then I installed a bulkhead about 1.5" below the cut. I installed a 24mm body tube through the bottom and bulkhead, using epoxy to secure it. A thrust ring went inside the tube to provide something to hold the motor in position.

I used the cap from a can of hair spray to make the slip coupling that held the nose "cone" (it was actually a cylinder, of course) in place. I had to add about half an ounce of BBs to function as ballast, held in place with epoxy, and attached a shock cord between the nose cone and the forward bulkhead.

The fins were balsa. On the prototype they were simple square fins; on the "production" model they had elliptical ends. I carefully removed three narrow strips of the can label, exposing the cardboard underneath, 120° apart, and epoxied the three fins in those places.

Add an 18" parachute and the rocket was complete.

The first test flight was successful, though the method I used for aligning the fins ended up canting them about a degree off vertical, so the rocket spun as it ascended. I figure an apogee of around 300 feet on a D12 rocket motor, which was acceptable.

The fins: if you place a rocket fuselage against a door jamb, you can draw perfect straight lines parallel to the tube's centerline. But the Pringles can has a lip at the bottom, which cants the tube slightly. I did my best to correct this on the production model, but it was not a serious flaw; it wasn't even really a flaw in the sense that it caused undesirable flight characteristics. There's nothing wrong with a rocket spinning as it flies, as long as stability is maintained.

At Danville Dare that year, then--I think it was 1994--the Hot Potato made its public debut; and when the issue of High Power Rocketry magazine came out that reported the launch, the rocket and I were mentioned by name; so I guess it made a decent impression. Can't complain about that.

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My problem is that--once I get to know something--I get bored with it.

Rocketry was no different; I'd pretty well done everything I could reasonably do as a rocketry hobbyist. I had given thought to building bigger rockets--in fact, I bought a rocket with a 29mm mount that would have been a good start--but I realized that bigger is just bigger; the flight profile is otherwise exactly the same: launch, coast, apogee, ejection, recovery. Sometimes something goes wrong and you get back mangled wood and cardboard, or you can't recover the rocket because it's stuck in a tree some 35 feet off the ground.

Faced with choosing between more of the same and bigger and MUCH more expensive that's still pretty much the same thing, I decided to hang up my rocket scientist cap.

There were still things I could do, but none of them sounded interesting to me. Adding electronics to monitor the rocket? Meh. Video? Meh. Guidance system? Meh. (Besides, adding a guidance system makes the feds antsy.) And all that stuff means weight, and weight means more propellant, which means bigger motors; and pretty soon you're talking about building a rocket you can fly maybe two or three times per year and only at major launches. And which costs $50-$100+ per flight even after a successful recovery, because rocket motors don't grow on trees.

I'm not kidding; in the 1990s reloads for K motors ran more than $50 each, and that didn't include the motor casing which ran around $80. And thanks to the way the law is, you can't ship those motors through the mail; you had to order your motors in advance and pick them up on launch day.

I doubt the laws or prices are any better now.

I suppose its coincidence that I got into anime about the time I was getting out of rocketry. But all else being equal I think rocketry was cheaper....

#1839: I don't believe it.

Headline on the Chicago Sun-Times this morning: "JOB LOSSES SLOW TO A TRICKLE".

The economy lost 11,000 jobs in October, claims the article, and the unemployment rate declined to 10%.

...what?

AP changed the story I linked to in this post. The original headline opened with "Job creation near"; the link now leads to an article headlined with "Unexpected drop in jobless rate sparks optimism".

They got rid of the story which said unemployment was bad and replaced it with an optimistic story saying it's not. They didn't post a new story with a new URL; they replaced the story at that URL with a new story.

By doing a search I found the article which was replaced.

So: 130,000 jobs lost in September, 111,000 lost in October...suddenly only 11,000 are lost in November? The rate shrank by an order of magnitude in one month with no change in market conditions?

I don't believe it.

To make matters worse, the old article no longer says that U3 was 10.5%; now it says that U3 was 10.2%.

The former article reports 190,000 jobs were lost in October. The new article says 111,000 were lost in October; were the other 79,000 jobs "saved or created"? 130,000 job losses were estimated for November; we're told we only lost 11,000. Were those 119,000 other jobs "saved or created" too?

The people who are responsible for tracking this stuff generally don't make such egregious mistakes as to produce estimates which are an order of magnitude too large. Can anyone point me to an example in the past 30 years? (Republican or Democrat, I don't care. Be prepared to defend your answer with facts and figures.) This stinks, it stinks bad, and it has the odor of "let's help the boss look good".

Check me on this: if there were a mere 11,000 jobs lost in November, that's unalloyed good news about the economy, isn't it? I mean, earlier this year we were hemhorraging 600,000 jobs per month, and now it's just 11,000 in the past 30 days, so isn't that wonderful news? It's good economic news, a sign that the economy is getting better!

...why release that report on a Friday?

It's going to be old news by Monday. The way the news cycle works, politicians have a habit of releasing bad or questionable news on Friday afternoon specifically so that it doesn't get the full-on press whammy. Obama has proved to be no different in this respect. Heck, the Democrats have started having important votes on ObamaCare on the weekend to avoid the press as much as possible.

If I were Obama, I'd want that news released on Monday morning, not Friday afternoon, because I'd want to give the press all week to praise me for it. Objectively speaking, it's good news, so why wait until Friday afternoon to release it?

My theory? It's bullshit. It's the result of tinkering with definitions and numbers to give a falsely positive result. You release that kind of thing on a Friday because you want to give your people time to talk it up on the weekend news and commentary shows, before the commentators and reporters who are hostile to your administration can dig down into the details of the thing and explain what's wrong with the numbers.

And it comes right on the heels of Obama's "jobs summit". Coincidence?

My predictions are usually wrong, so God knows WTF is really going on. But from here I can't see how it can be anything else.

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By the way: isn't October-November about the time stores do a lot of seasonal hiring? How many of those jobs will still exist come January or February?