Lightweight Airframe Components
|Andy Tomasch Entered this talk on vellum model
construction in R&D at NARAM 40 in Muncie Indiana.
The Development of Lightweight Airframe
From Drafting Vellum and Mylar Film
Andrew D Tomasch
Submitted to the Research and Development Competition at NARAM 41
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.
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:
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
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.
Table II compares the linear mass density for vellum and mylar based tubing
(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.
The prices for vellum and mylar film as of July 1, 1998 are:
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.
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.
(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
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
Mass Producing Parts With Power Tools