atomic_fungus (atomic_fungus) wrote,
atomic_fungus
atomic_fungus

#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.

* * *

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....
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