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Rocket Construction: Backgliding Rockets

Like most of us, I have observed that a certain kind of rockets do not nose dive immediately after reaching apogee. At first they fall backward for a while, then they turn sideways and, after gaining more and more speed, turn into the dreaded nose dive. They go straight into earth's surface, this is called a lawn dart.

Discussions in the WR mailing list brought up the idea to design rockets in such a way that they go up like usual, but do NOT turn into lawn dart mode, but falling sideways or slightly backwards - EVERY TIME! Hereby the rockets do not gain a high speed and land therefore on the ground safely without destroying itself.

The theory of this so called Super Roc Gliders (others call them Backsliders) can be found here. I made a practical approximation to these ideas without much CLA / CG calculations. Some WR list members asked for more construction details - here they are.

If you want to see the BackMaxes fly, visit me or click at this here launch report with a video.

The Red Slim FTC Backglider with a video and CLA/CG/BCP discussion is here.

Rockets BackMax1 and 2 These are the 2 rockets I was testing. Both consist of two Robinson coupled 0.5 liter Fanta bottles of 65 mm diameter; these are made of 0.3 mm PET. The top bottle got a guppied nose. The fins are taped on modular skirts.

Left: BackMax 1 (26th August 2001)
4 PET fins, taped on a modular slightly tapered skirt.
Mass 122 g, Length=47.5 cm, CG=28 cm from nose tip.
Fin size is 42 mm x 52 mm.

Right: BackMax 2  (20th October 2001)
3 laminated paper fins, taped on a modular cylindrical skirt.
Mass 116g, Length 48 cm, CLA= approx. 26 cm from nose tip, CG=27.8 cm from nose tip, BCP > CG.
Fins have 61 mm x 38 mm max. dimensions.

Click here for a larger image (50 KB).

CG=Center of Gravity
CLA= Center of Lateral Area
BCP=Barrowman Center of Pressure
explanations here.

Detail BackMax 1 BackMax 1: Optimization method: adding weight

Optimization: the rocket was repeatedly shot with air at only 2 bar pressure. Small weights were added (first 2, then another 2, ...), making sure that after each weight increase the weight distribution stayed symmetrical. I used flat nuts and lamp rods - that's what I had with me. When the rocket descended in the desired backgliding way, I increased the pressure to 4 and 6, later 8 bars. With increasing pressure, small weight increases were necessary. Too much weight at the tail must be avoided: instable ascent would be the sure consequence.

BackMax 1 Crew The crew of neighbours that helped with optimization of BackMax 1.

Rocket science takes patience,  doesn't it?

Detail: BackMax 2 BackMax 2: Laminated fins with cut lines

To ease the optimization process, I made fins with printed-on square lines. This helps maintaining identical fin size while cutting the fins to a smaller size.

Click here to see how to make these Laminated Cardboard Fins.

Optimization Process
BackMax 2:
reducing fin size

The rocket was repeatedly shot with air at only 2 bar pressure. The fins were cut off with strong scissors strip by strip, until the rocket stopped to nose dive and descended sideways instead. The squares on the fins help to cut each fin to the same size. Then we increased the pressure to 4 and 6, later 8 bars. With increasing pressure, small further trims were necessary, because with increasing speed from the greater altitude the rocket went into a nose dive before reaching the ground.
After we made a fin-cut at the last 6 bar step, we felt that the stability during ascent was short before becoming unstable. Therefore we did not trim the fins further down. However, the back gliding was not satisfying yet, so the last step of optimization was made by taping three short pieces of solder above the fins (under the red tape, above).

I know, it would have been academically more correct to have the two optimization methods clearly separated by doing exclusively fin cuts here. But, following the concept of the mentioned Alway brothers' paper, reducing the fin size further probably leads to the situation that the BCP comes too close to the CLA, that is why adding tail weight (=moving the CG back) was probably the right cure for BackMax 2. At least it ended up in a working backgliding rocket.

My next backglider will be longer than BackMax 1 and 2, because a longer rocket will be much easier to trim  for backgliding than these relatively short ones. On longer rockets, BCP and CLA are further away from each other, eliminating the discussed problem.

Countdown for the next  step of optimization Here you see one of my two YRSAs (Young Rocket Science Apprentice), David,  from the neighbourhood, doing the countdown for the next optimization step, with security glasses and ear protection (under the hood - we are in the higher bar region ...)
Filling BackMax 2 After successful trimming, we added water. The rocket had the same flight characteristics as with the air-only, but yielded greater altitude.

Left: both YRSAs, David and Johannes, in teamwork. We had a really nice afternoon!

Pumping is a strenuous business

Johannes pumps her up, supervised by my friend Claus. The higher bar regions (8 to 10 bar) are difficult for the kids, but they didn't give up!

We achieved altitudes above 100 m with 8 bar pressure and 250 ml water. Typical up time: 4 secs, down time 14 secs.

It was JUST GREAT TO SEE THE ROCKETS COME DOWN SLOWLY AND SIDEWAYS from up there WITHOUT A SINGLE SCRATCH on touchdown. The angle of attack was never constant, though. If 0° is the direction "straight up", the rockets wavered between about 30° and 120°. Spinning was non-existent to moderate.

Of all deployment methods, this is the most KISS one, once you have it worked out.

Passing the water ...

We were working all afternoon until it got dark. Look a the pants - the grass was wet from autumnal October dew.

After the "hard" scientific work, we had sent up a good part of my rocket arsenal several times, but still plenty of water was left. The kids found a funny way of passing the surplus water via our lucky mascot, the Pink Panther. We made him to be a parachutist before, but his equipment needs some more polishing. Maybe next time!

Detail: parabola of water The Pink Panther gave us another good lesson in practical physics: a mass, here water, thrown off horizontally, falls down following a true parabola!
Soldering Lead for BackMax1 After these optimization processes I decided a few days later to make BackMax1 nicer - I also needed the lamp rod and nuts, which were used as ballast, to build more Robinson couplings. So, these and the yellow tape were removed and weighted.
The new position for the ballast I selected to be under the skirt. I calculated how much weight needs to be put there - of course the flight characteristics should not change, therefore the CG should  stay at the same position as before.

This photo shows the new ballast, symmetrically arranged, ready to be inserted.

New ballast taped in place. The new ballast for BackMax1, taped inside the skirt.

Did I mention that the new CG position of the assembled rocket was EXACTLY the same as the old one? Surprising, what a little math can do.

Are you interested, how the new rocket (and further developments) flies? Click here for videos of uHornstein's backgliders / backsliders.

Send me an email, if you have got any ideas to evolve this topic. 



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Stand / Last Revision:  03.04.2017 new link to Alway paper.  01.11.01

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