JEC COMPOSITES MAGAZINE - Issue #78 - January/February 2013 - 54

aerospace
state of the art in thruster technology and
offer relatively low thruster efficiencies
(e.g. RAMPART’s design specific impulse
of 90 s), their simplicity and practicality
are well suited to pico- and nano-satellites. A key factor in the performance of
the propulsion system is the low weight
and cell structure of the propellant
tank made from Windform® XT material. Instead of utilizing state-of-the-art
technology, RAMPART uses a high
propellant mass fraction to provide the
required delta-V (320 m/s) for low-earthorbit in-plane orbital manoeuvres. Since
the dimensions and weight of a CubeSat
are its main constraints, Windform® XT
allows these parameters to be optimized
through the use of multiple and interconnected near-cubic cells. This offers the
advantage of maximizing the propellant
volume, integrated baffles provided by
the interconnecting walls, and possibility
of correlating material strength tests of
a unit cell to the entire propellant tank
design. One can easily compare this
design to traditional pressure vessels
(hemispherical caps) and achieve a twotimes difference in the propellant storage
volume.

Fig. 3: Top of propulsion module

To determine material performance, test
cubes similar in size to the baffled chambers (see Figures 3 and 11) were tested to
various pressures. After physically testing
the small test cubes that represented the
chambers up to 4 MPa, an FEA model
was developed by Whitney Reynolds
(U.S. Air Force Research Laboratory,
Space Vehicles Directorate (AFRL/RV))
to simulate the reactions of the small

chambers and then correlate the results to
a simulation of the final large propulsion
unit.

Material choice
The use of rapid prototyping plastics
in aerospace applications has been
somewhat limited to prototype work,
with the exception of Nylon 11 developed by Boeing (ODM) and Nylon 12
utilized by Northrop Grumman. (Programs include F-18 and Global Hawk).
These two materials are used in a production environment and are produced
using laser sintering (LS) machines. The
LS technology has been seen as the best
opportunity for durable plastic materials with a consistency in manufacture.
Selective laser sintering resolution for
plastics is commonly run at 0.1 mm per
layer, utilizing a CO2 laser that can be
adjusted to melt the plastics into a fully
dense material.
Fused deposition modelling (FDM) from
Stratasys, multi-jet modelling (MJM)
from Objet, and stereolithography (SLA)
from 3D Systems are some of the rapid
prototyping technologies examined. Each
technology and material has benefits for
prototyping. The key factors that helped
steer the material choice include heat deflection temperature, UV exposure, and
the requirement of plating the satellite in
order to make it radar reflective.
Windform® material was developed by
CRP Technology (Modena, Italy), whose
interest in the material was driven by a
customer request for an improvement
in stiffness specifically for wind tunnel
applications in Formula One racing
(Figure 4).

Fig. 4: F1 scale wind tunnel model created using
Windform® XT

54 jec composites magazine / No78 January - February 2013

Windform® XT uses a base polyamide
reinforced with carbon microfibres.
Racing teams were the first to begin using
the material, and determined it to be a
solution that could be utilized in “on-car”
applications. The material is currently
used by both F1 (Figure 5) and NASCAR
teams to replace components that would
typically require injection-moulded
materials. It has now been replaced by the
new Windform XT 2.0, which is characterized by improved properties. However,
because the new XT 2.0 material was still
under development, Windform XT was
used for this case study. The same CubeSat is now manufactured with Windform
XT 2.0 with excellent results.

Fig. 5: Windform XT used for a brake inlet duct on
an F1 car

Based on observations of other materials,
Windform® XT samples were subjected to
tensile tests and the cross-sections of the
brakes were examined under an electron
microscope. The micrographs showed the
carbon microfibres were encapsulated by
the Nylon base material. In addition, little
to no porosity was visible in the internal

Fig. 6: Micrograph of samples subjected to tensile
tests (courtesy Paramount Industries)
JEC COMPOSITES MAGAZINE - Issue #78 - January/February 2013

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