Lightweight components have been developed for high-performance spacemodels flown in duration competition.  These components (tubing, transitions, conical nose cones, and conical shrouds) are constructed from drafting film (vellum or mylar) and mylar tape. They exhibit excellent strength at substantially reduced weight when compared with traditional cardboard, balsa, or plastic components. The initial development was undertaken to produce lightweight FAI class S3 A (2.5 N-s parachute duration) and S6 A (2.5 N-s streamer duration) models without the use of fiberglass and at competitive weights. This effort has been successful, producing models comparable in weight to those with fiberglass airframes, and with excellent strength. Subsequently, these components have been incorporated into NAR duration models (streamer, parachute, and egg loft) in impulse classes ranging from 1/4 A to D, all with complete success. Numerous places won in NAR competition over the last five years have proven the value of these construction techniques. To date, no component has failed structurally during flight. This method of construction has the additional advantages of moderate cost, readily available materials, and simple, rapid assembly. The original goal of competing in the US spacemodeling team trials using vellum models for S3 A and S6 A was achieved in 1997, with a best placing of sixth in S3 A. These construction techniques are well proven and provide an excellent competitive advantage for low-impulse duration events. The most important overall finding is that traditional model rocket construction materials produce models which are grossly over built with respect to the loads encountered in flight, and that a factor of two weight saving is possible for small models. The weight saved provides increased duration due to both the reduced airframe weight suspended from the recovery device, and the increased  recovery device area possible for a given total weight at liftoff.

Introduction

Lightweight models for the FAI classes S3 A (2.5 N-s parachute duration) and S6 A (2.5 N-s streamer duration) were first developed in 1991, to meet the need for  simple competitive models which could be built quickly for use in the upcoming  US team trials. A detailed account of this early development work has been  published (1) and is submitted for judging in Appendix A, along with the following sections which are included to supplement and update the contents of this original publication.

Materials and Methods

Three basic materials have been employed in the construction of lightweight1airframe components:

1. Drafting Vellum.
2. Mylar Drafting Film. This can be clear, fogged on one side or fogged on both sides. I prefer at least one fogged side, since ink better adheres to the fogged surface.
3. Scotch Magic Transparent mylar tape.

Table I reports thickness and mass densities (per unit area) for these  materials. All masses were measured on a digital scale with a 200 gram full scale and 0.01 gram least count. Thickness were obtained with a precision  dial caliper accurate to 0.0003''. The last reported decimal place has been
rounded.

TABLE I: MATERIAL PROPERTIES

These basic materials can be used to fashion a variety of lightweight airframe components: tubing, conical transitions, conical nose cones, and long conical shrouds for egg lofters. The egg-lofter shrouds can be fully structural, taking all flight loads without additional internal structure for classes up to C (10 N-s), and can also be used for supplemental streamlining in larger impulse classes such as D egg loft duration. The construction methods are simple and straightforward. A brief summary of the construction techniques is given below.

For most applications, vellum has proven to be structurally adequate. For some high stress applications (e.g. C egg loft shrouds) mylar is substituted wherefor extra strength, provided the factor of two additional mass per unit area is not a significant penalty within the context of the overall weight budget. To date, no component, vellum or mylar, has failed under flight loads.

Weight Savings

Table II compares the linear mass density for vellum and mylar based tubing
with that for various types of commercial tubing. These data are useful for
making detailed ``weight budget'' estimates prior to construction.

TABLE II: LINEAR MASS DENSITY FOR TUBING

Taking the 13 mm diameter as an example, mylar tubing is only 54% as massive as phenolic, and vellum is only 31% as massive for a given length of tubing.When compared to white paper tubing mylar is 56% as massive, and vellum 32%.These represent excellent weight savings, which are a particular advantage for low-impulse duration classes. As an example, consider the weight savings achieved by substituting an 8'' long 10.5 mm diameter vellum tube for the equivalent length of white paper tubing in a 1/4 A PD model. The difference in mass is

(8'')(2.54 cm/in)(0.114-0.033(g/cm))= 1.65 grams.

This can be compared with an overall mass of 1.61 grams for the 1/4 A PD model shown in Photo 7. Substitution of white paper tubing for vellum tubing would approximately double the overall mass of the model.

Finish

A major advantage of this construction is that graphics can be applied to the component blank prior to rolling the part to final shape and joining the seam.  To date, a digital plotter driving a rapidograph pen charged with India ink, and an ink-jet printer have both been used to print the blanks, with the required NAR number also drawn on the blank. The blanks were laid out using AUTOCAD mechanical design software, although any computer drawing program could be used. India ink is particularly well suited for this application, since it is highly water resistant, is difficult to smear, and can have clear aircraft dope applied over it, without bleeding, for additional water resistance. If an ink-jet printer is employed, a fixative must be applied immediately after printing the blank, since the ink-jet ink smears easily. Excellent success has been obtained using Krylon brand spray matte finish. A light application over the fresh ink sets quickly and eliminates smearing. It also provides sufficient finish to vellum parts, which then require no further doping for water resistance. The mylar drawing film is essentially waterproof and requires no additional finish other than that required to fix the graphics. The use of a "paint" type computer program should allowcolor graphic decorations to be ink-jet printed on the blanks with minimal additional weight. Laser printed blanks should also be possible.

Cost

The prices for vellum and mylar film as of July 1, 1998 are:

Vellum:  $9.66 + Tax for 100 \ 8 1/2 x 11'' sheets of 16 lb drafting vellum.

Mylar Film:  $33.68 + Tax for 100 \ 8 1/2 x 11'' sheets of  mylar graphics film (fogged both sides).

These prices were quoted by Dunn Blueprint (3) of Ann Arbor MI. The 100 sheet quantity represents a lower price per sheet than that obtained for single-sheet purchases at local graphics stores, and is a sufficient supply for many years. Scotch Magic Transparent Tape ranges in price from $1.50 to $4.00 a roll depending on quantity and width. Single sheets of mylar film can be obtained from graphics supply stores for $0.50 a sheet, and smaller pads of vellum can be purchased for under $10.00, so experimentation is  possible for a nominal initial outlay. These construction techniques have proven to be very cost effective, particularly with careful layout work to maximize the number of parts obtained from a single sheet of material.

Examples

To Illustrate the many possible applications for these components, the following examples of models for NAR and FAI competition are included. With the exception of the 10.5 mm diameter 1/4 A PD model, all have been extensively flight tested. Please consult the attached photo booklet for the corresponding photographs. Because the nose cone or capsule weight often dominates the overall weight of the model, the weight without nose cone is reported, with the overall weight including nose cone included in parentheses. Weights do not include recovery system or rigging (e.g. shock cords). Photographs were taken with a Pentax K-1000 SLR camera using a stock Pentax 50 mm f/2.0 lens, on Fuji ASA 100 speed color print film, in natural daylight.

Photo 1: FAI S6 A (Streamer Duration).  Vellum paper tubing, aft conical transition and nose cone. This is Paper Tiger III, shown in Figure 1, and is nearly identical to Paper Tiger I, as described in
Appendix A. These models were used to compete in the 1997 US team trials. Paper Tiger II, presented in Appendix A, was ultimately judged too extreme, and was not flown. Note that the photographed version differs slightly from the drawing, with a shorter tail cone and the fins bonded to the blackshaft tail tube. Paper Tiger III represents the current state of development for this design. Recently placed second in A SD (SpringThing '98). Weight: 3.79 (4.53) grams.

Photo 2: FAI S3 A (Parachute Duration). Modified Paper Tiger III. Body tube is extended 5 cm for additional 'chute capacity. Un-hollowed balsa nose cone is too heavy, and was not hollowed due to lack of time. A photograph of this model also appears in Appendix B, showing the piston/tower combination used for launch. This design was used to place 6th at the 1997 US team trials. Recent NAR placings include: second place A PD (Falling Leaf '97), and first place 1/2 A PD (SpringThing '98). Weight: 4.42 (8.84) grams.

Photo 3: NAR 13 mm Parachute/Streamer Duration. As described in detail in Appendix A. Excellent performance on 1/4 A - A 13 mm engines. Numerous contest wins, including first place in A PD (MSC '97). Weight: 2.00 (3.03) grams. (plan--not to scale indicated in original)

Photo 4: NAR B Streamer Duration. Similar to 13 mm model, but for 18 mm engines. Mylar airframe tube, with blackshaft tail tube and 0.015'' G-10 fiberglass fins bonded with CA. Two recent first place finishes in B SD (MSC '98) and C SD (MSC '97). Weight 5.96 (7.52) grams.

Photo 5: NAR C Egg Loft Duration.  For use with a C6-5 and a piston/tower launcher. Blackshaft 18 mm tail tube with 0.015'' thick G-10 fiberglass fins, bonded with CA. Shroud is 0.004'' thick mylar film, NAR number is printed with an ink-jet printer. Has placed 3rd in C ELD (MSC '98). Weight: 8.00 (19.20) grams.

Photo 6: D Dual Egg Loft Duration:  For use with a D12 engine, piston launched from a tower. Core tube from 24 mm Blackshaft tubing, shroud from 0.004'' mylar film, 0.015 G-10 fins, bonded with CA. Has flown successfully with a vellum shroud. No placings to date, model was lost on a power line for non-recovery of the egg when last flown in D ELD. This model will fly at NARAM 40. Weight: 15.88 (32.80) grams.

Photo 7: NAR 1/4 A PD.  New for NARAM 40. Basically similar to the 13 mm model, but with Totally Tubular 10.5 mm white paper tubing for the tail cone assembly. Weight: 1.39 (1.61) grams.

Conclusions

Light weight vellum and mylar components are now well proven in competition and are a standard part of my contest construction. They offer substantial weight reduction, which is a particular advantage for the low-impulse duration classes which dominate the contest schedule. The techniques for producing these parts are simple, and the cost moderate. When combined with mass production techniques (4), these components can be used to construct large numbers of lightweight models in a short time, with minimal effort. This is a further hidden advantage to this approach, allowing highly specialized models to be produced for each event flown at each meet. Finally, other flyers (5) have employed these construction methods in competition, successfully winning the A Streamer Duration event at NARAM 33, and further demonstrating the potential for gaining a competitive advantage with these construction methods.

References

(1) "Rocket Boosted Origami, or How to Build Really Light Contest Rockets From Drafting Vellum Without (Hardly) Even Trying,'' A. D. Tomasch, T Minus Five, Vol. 7 Number 4 (1992). Included in Appendix A

(2) The Art of Scale Model Rocketry, Peter Alway, ISBN 0-9627876-3-9, Saturn Press, P.O. Box 3709, Ann Arbor, MI 48106-3709 (1994).

(3) Address: 2825 Boardwalk, Ann Arbor MI 48104. Phone: (734)-663-2471.

(4) "Mass Producing Parts With Power Tools," A. D. Tomasch, Sport Rocketry, Vol. 41 Number 2 (1998). Included as Appendix B

(5) "The VDM-3 Streamer Duration Winner at NARAM 33," Al de la Iglesia, T Minus Five,  Vol. 7 Number 4 (1992). Included in Appendix A

Appendix A:

Rocket Boosted Origami -or- How to Make Really Light Contest Rockets From Drafting Vellum Without (Hardly) Even Trying,  By Andrew D. Tomasch

The VDM-3 Streamer Duration Winner at NARAM 33

Appendix B:

Mass Producing Parts With Power Tools