Aerothermodynamics of Hypervelocity Toroidal Aerobrakes

Ivy Lourel (2008). Aerothermodynamics of Hypervelocity Toroidal Aerobrakes PhD Thesis, School of Engineering, The University of Queensland.

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Author Ivy Lourel
Thesis Title Aerothermodynamics of Hypervelocity Toroidal Aerobrakes
School, Centre or Institute School of Engineering
Institution The University of Queensland
Publication date 2008-06
Thesis type PhD Thesis
Supervisor Richard Morgan
Timothy McIntyre
Total pages 251
Total colour pages 36
Total black and white pages 215
Subjects 290000 Engineering and Technology
Formatted abstract The use of aerodynamic drag for various orbit insertion or planetary entry missions has been
shown to offer considerable mass savings over the conventional chemical propulsion
technique. Compared to rigid aeroshells, inflatable aerobrakes carry the potential for reduced
system mass, simplified navigation and control, and applicability across all payload shapes
and sizes, enabling new possibilities for several high value missions. Of particular interest is
a new trajectory concept involving the use of a large toroidal ballute (a hybrid of a balloon
and a parachute) in aerocapture missions. An aerocapture ballute is an inflatable structure that
is stowed in the parent spacecraft and inflated just prior to entry into the upper atmosphere of
the planet. Various fields of study are producing promising evidence in support of the ballute
technology, even though considerable work remains to be done to mature the technology to a
mission readiness status.
The inception of ballutes dates back to the 1960s. Originally developed to serve as a
supersonic decelerator/stabiliser for sounding rockets and air weapons, the evolving ballute
took many shapes and forms. Investigations into the use of a ballute as a decelerator for
astronauts, aeroassisted orbital transfer vehicles and various planetary descent probes
continued but did not gain widespread interest. Based on advanced thin-film materials, recent
ballute studies have yielded design and trajectory concepts that circumvent the major
technical obstacles that limited the usability of the ballute in the past, spawning renewed
interest in further technology development.
Timely deployment and detachment of the ballute has been shown to provide sufficient
trajectory modulation capabilities to enable aerocapture at various planets despite atmospheric
uncertainties and navigation errors. Of the areas requiring further research, ballute shape,
heating and flow stability have been highlighted as having priority, followed by experimental
verification of the preferred solutions. Out of the various ballute shapes and configurations
being investigated, a large, towed, toroidal ballute has been identified as one of the most
promising designs offering attributes pertaining to flow stability and reduced system mass.Preliminary computational fluid dynamics (CFD) studies and experiments on the towed
toroidal ballute have yielded positive results. This study aimed to further explore and validate
the dominant aerothermodynamic characteristics associated with a rigid toroidal ballute, as
well as a spacecraft and ballute flown in tandem. Experiments were conducted on a number
of model configurations in the T3 Free Piston Shock Tunnel at the Australian National
University in Canberra and the X2 Superorbital Expansion Tube at The University of
Queensland in Brisbane. A large metal ring was used to represent a rigid, toroidal ballute and
a sphere mounted on a cylindrical sting acted as the leading spacecraft. The overall diameters
of the T3 and X2 ballute models were 112 and 42 millimetres respectively, which were 7
times the cross-sectional diameter of the rings, and 8.8 times the payload diameter.
The T3 experiments aimed at unveiling the complex flowfield generated by a towed ballute
and spacecraft system at a tow length of between 3.9 and 11.8 times the payload diameter.
Two low enthalpy (4.3 MJ/kg) N2 conditions at M∞ = 8.35 and 11.01 were used to allow
good details of the wake flow to be captured by planar laser induced fluorescence (PLIF)
imaging. A total of fourteen shots were documented. Steady flow interaction was achieved
up to a tow length of 8.8 times the payload diameter, with the toroidal ballute successfully
swallowing the spacecraft shock. As the tow length increased, shock-shock interaction and
shock-impingement-induced boundary layer separation were observed. Experiments
conducted on the toroidal ballute alone showed the formation of a steady Mach disk
downstream of the ballute orifice. The dynamics of the interacting flowfield were uncovered
by transient heat transfer and pitot pressure measurements at the ballute geometrical
stagnation point.
In X2, near-resonant holographic interferometry was used to visualise density variations at the
higher enthalpy conditions for the study of shock interaction. Two moderate enthalpy (around
17 MJ/kg) and two high enthalpy (around 54 MJ/kg) conditions relevant to aerocapture entry
at Mars and Titan were produced at M∞ = 7.24 to 10.30 using CO2 and N2 as the test gas.
Twenty shots were documented in this test series. Throughout the experiments, the ballute
trailed behind at a nominal distance of 3.9 times the payload diameter and steady flow was
established across all four test conditions. The experimental heat flux measurements were
generally in good accordance with the theoretical stagnation point heat flux predictions
obtained by treating the toroidal ballute as a two-dimensional, wrapped-around cylinder. One
blocked-ballute experiment was carried out at each condition to study the extreme effects ofan adverse flow interaction, such as in the case of a choked or a disk ballute. Highly unstable
flowfields ensued: evidenced through visualisations of the flowfield and also through
transient heat flux measurements.
One-dimensional methods for modelling the flow across the ballute intake were developed,
based on equilibrium and finite rate chemical kinetics. These were used to predict the critical
area ratios required for choking through the ballute core for two low density (around 2×10-3
kg/m3) and two high density (around 3×10-2 kg/m3) N2 and O2 conditions at a total enthalpy
of between 17 and 18 MJ/kg. The range of conditions provided opportunity for the effects of
a large difference in post shock Reynolds number and gas chemistry to be studied. Flow
visualisation and heat transfer measurements were produced from seventeen experiments in
X2 to validate the one-dimensional numerical predictions. Axisymmetric Direct Simulation
Monte Carlo (DSMC) modelling was carried out on ideal (non-dissociating) and real
(dissociating) oxygen flows to validate the effect of chemistry on the choking limit. These
studies yielded a unanimous conclusion that dissociation or gas chemistry decreases the
chance of choking by allowing the flow to undergo more geometric contraction. This is
manifested in the decreased level of ballute shock self-interaction in the core flow seen in the
DSMC results and also in an earlier CFD study. The overall agreement between
experimentally measured critical throat size, and that predicted by one-dimensional and
axisymmetric modelling suggests that the ballute core flow is relatively insensitive to intake
geometry, as the results are predominately dictated by mass continuity.
Dynamics of choking was found to depend on Reynolds number, in accordance with
published results on flow rate stability of critical toroidal nozzles. A bulge in the ballute
shock witnessed in one of the interferograms was related to a previous CFD study on a shock
tube nozzle, where it was postulated that shock reflected off the symmetry axis created a
vortex ring which sustained a bulge in the reflected shock ahead of the nozzle.
From the various aerothermodynamic aspects of the toroidal aerobrake explored in the present
work, it appears as a workable solution for future planetary missions. Only part of the
groundwork has been covered; however, as there is still a multitude of technical issues to be
addressed, requiring a continual research effort across many specialist areas.

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