Featherweight and Tumble Recovery

Featherweight recovery: the model is very light compared to its size, so drag vs weight is high and terminal velocity is low, even if the model comes in stably (i.e. flies straight down).

Tumble recovery: the model creates high drag by rotating its "broad side" into the airstream often; if drag vs weight is high, terminal velocity is low.

Clearly all recovery techniques are related--get drag high enough so that the drag/weight ratio gives a safe terminal velocity.

In featherweight recovery the rocket typically remains stable, and the model usually ejects the engine. The old Astron Streak had a super-lightweight mylar body tube just long enough for the engine and the nosecone sholder, 3 fins, and ejected the engine. If it hit you on the way down it was so light that it wouldn't hurt.

Tumble recovery usually involves making the model unstable, so that it tumbles (and slows is descent) as it falls. The old Astron Scout used this method. The ejection charge just moved the engine back, where is was held in place by a hooked piece of piano wire. This shifted the CG (center of gravity) behind the CP (center of pressure), making the rocket unstable so that it would tumble down rather than streamline in.

Hope this helps!

My first rocket was a Scout--my favorite Estes rocket to date, because it was my first. Remember *your* first? (rocket that is...) Anyway, even though there was a vent hole in the BT, and gauze glued down the side to retain the hook, every time I launched my bright orange (Chevy engine block paint) rocket, no matter what engine I used, it was kicked out at apogee past the engine hook. The Scout then came in ballistic. Usually it stuck in the dirt with no damage, but when it did hit the asphalt of the school yard parking lot, the nc suffered badly. After many flights the nc resembled the shape of a Bertha. I flew my first one many times until the fins were actually burned (literally!) by the exhaust of the engine. Anyway, to stay of topic: Tumble recovery sounds good on paper, but my experience was less than perfect.

"David J. Braunegg" wrote in news:e4r7vg$euj$ snipped-for-privacy@newslocal.mitre.org:

Classic examples of the featherlight recovery include the Mosquito and the Birdie. They both kick the engine,then lazily floated down in a stable manner. They are just so darned light that they couldn't hurt themselves despite how far they fell.

The tumble recovery method is often seen on the booster sections of multi- stage rockets. They are mostly fin, and end up pinwheeling through the air on their way to a gentle bounce with the engine still onboard.

Moving the CG back behind the CP to make it unstable is how "tumble recovery" is described, but I think that's wrong. Think about it: an object is stable if it's CG is ahead of it's CP. Well, moving the CG behind the CP just makes the rocket stable in the rearward direction. Assuming there's no longer any thrust out the back end then the rocket will "fly" stably, fins first.

I think tumble recovery is more complex. The CG shift is real but the goal is to make the rocket is neutrally stable, or nearly so, by moving the CG coincident with the CP. The ejection gases expelled out the single hole in the rocket (Scout, Sprite) or the act of stage separation of a booster pitches the rocket or lower stage over, to produce the onset of tumbling. And as the rocket tumbles the CP moves back and forth as the angle of attack changes. This "pumps" the tumbling to keep it going.

The original Scout had three holes equally spaced around the airframe. This changed early on to just one hole. I suspect this was done to improve tumbling, by giving a single directed "jet" that will be sure to pitch the rocket over.

That would match my experience. The Sprite seemed to fare somewhat better.

The Scout was also my first rocket. Mine as far as I can remember never ejected the motor. Eventually the tube got so dirty that the motor would not move far enough back to cause it to tumble. Then it came in ballistic and bent the body tube at the vent hole. I still have it and it flew in the payload bay during my L1 flight.

True, as long as the rocket maintains a 0 deg angle of attack in the reverse direction. Any divergence from that 0 deg AOA will cause the CP to move back toward the tail (and because of the geometry - fins 'forward' - will do so rapidly) past the new CG and the rocket will want to again decend nose first. Then it gets in that attitude, it will again be unstable and the cycle repeats.

My first was the Astron Streak. I'm not exactly sure how the thing landed, though, as it performed its usual trick of completely disappearing into an alternate dimension at the tip of the launch rod. If it did come down (I can't truthfully say that it did, as I never managed to get any actual proof of this) it would have been stable and extremely light on the way down.

My second rocket was a WAC Corporal (Estes kit #K11). I didn't get a Scout until my 3rd or 4th. It always worked as designed, locking the motor in the rearward position and tumbling relatively slowly down, usually landing on one fin which stuck into the lithosphere (ground). Since the fins were so thick and reinforced with the gauze there was never any damage. I probably got 15-20 flights on it before I lost it to a motor that for some reason burned with equal energy output from both the normally fiery-and-smoky bottom end AND the normally non-fiery-and-smoky top end. The remains of the rocket burned on the pad.