Mylar Magic-2

Steve Griffin's Balloon Building

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This is quite a different system in a number of respects. For one it is double skinned that is it is a balloon within a balloon and secondly it is made out of a thin polyester film rather than a woven fabric. It has been designed to fit within the FAI AX-2 category and was used to break the AX-2 world duration record in February 2004.

This record was not completed without some re-engineering work to both balloon envelopes and highlights some of the dangers of building and operating 'experimental' aircraft. During the inflation for the balloons first flight the acrylic transfer adhesive that was used to stick the envelope panels together failed in the cold temperatures (-27^oC). The failures were confined to a couple of small areas so a flight was conducted at low level for about an hour to prove the systems performance. This flight was ultimately terminated well short of the record mark.

Subsequent research revealed that acrylic based adhesives are generally only acceptable for use down to about -10^oC. The specification sheet for the transfer adhesive used on this project states that it is good down to -30^oC, this figure appears to be overly optimistic. Reinforcing of all of the envelope panel seams was required before further flight and was effected using a silicon adhesive based cloth tape. Following this reinforcement work the balloon performed superbly and broke the world AX-2 duration record.

Mylar balloons are not new to Australia or to other parts of the world for that matter. Much of the early development of balloons was conducted using film balloons, in Australia this work dates back to the days of the Aerostat Society in the early 60's when a balloon called Archimedes was made, this was followed shortly after by Phil Kavanagh's Little Bear which flew some 170 hours before being retired. Grahame Wilson's website has a good rundown on the early history of balloons in Australia including some of the early film balloons.

Film balloons are more fragile than fabric balloons hence they are not generally used for recreational or commercial ballooning operations. Mylar however has much better thermal performance than a similar weighted woven fabric which makes it particularly good for high performance / short operational life balloons.

From a construction point of view a film balloon is very easy to build but you need to have at least 2 people on hand for almost all of the taping operations as the film tends to be difficult to maneuver and impossible to join on your own. About the only tool you need is a pair of scissors or a sharp knife. In contrast a fabric balloon can easily be built as a solo operation but you need to have at least one and probably 2 specialized sewing machines, a swaging tool and the skills to use them.

As part of this project I also built a handheld burner to be used for inflating the balloon. See the FireStik picture below.

Technical Overview

Skin Material Aluminized 23 µM Mylar® Film - outer skin
Clear 23 µM Mylar® Film - inner skin

Envelope Size / Design Outer Skin - 400 m³ 16 gore x 8.5 panels, custom designed ground resetable, concertina rip panel
Inner Skin - 360 m³ 16 gore x 8 panels, triangular rip panel

Deramecourt Shape for both skins

Construction Method Panel joints using double sided adhesive tape with polyester substrate and monomer acrylic adhesive
- short term temperature rating* 230^oC
- long term temperature rating** 150^oC
NOT SUITABLE FOR OPERATIONS BELOW -20^oC
Structural reinforcing using Glass cloth tape with a thermosetting silicon based adhesive
- short term temperature rating* 250^oC.
*Short term rating expressed in minutes / hours
** Long term rating expressed in weeks / months

Weight Certified double envelope weight 28.5kg

Construction
Work commenced on this project in October 03 with completion and certification planned to be completed before Christmas. You can view the view the construction log here.

Gallery

Vertical seam section prepared for structural testing Vertical seam under long term static load Vertical tape tensile strength testing Vertical seam tensile strength testing

Mike Lloyd assisting with panel cutting Yours truly rubbing down the final seam on gore 1 Closed envelope of worktable. The blue bag contains my Eye-Strain fabric envelope Commencing the final seam

Outer skin in its VERY loose bag (red). The blue bag on the left is ESN fabric envelope On the scales for the first time. Outer skin in the bag came in at 14.3 kg Highly conspicuous experimental sign Fire Proof ID Plate

Registration Marks Bill Scheeles (left) and yours truly (right) adding the last panel to complete the inner skin gores Mike Lloyd, Andrew Fewtrell & Kenn Williams (left to right)helping out joining the gores for the inner skin First Inflation 9/12/3
Inner Skin in clear mylar

First Inflation 9/12/3
Inner Skin - deflation panel Hot Inflation 9/12/3 First Inflation 9/12/3
Outer Skin First Inflation 9/12/3
Destructive rip panel

Hot Inflation 9/12/3 Destructive Rip #1
prior to operation Destructive Rip #2
partial operation Destructive Rip #3
completed operation

FireStik Burner First Flight 2nd Feb 2004 - partial delamination is evident in a lower panel Outer envelope inflated during repairs Grahame Bateman destroying his knees during envelope repairs

Takeoff towards the World Duration Record 11th Feb 2004 Progressing Smoothly Landed after 4 hours 39 minutes and 50 secs