Past and Future Design Challenges of Hypersonic Flight Vehicles

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Past and Future Design Challenges of Hypersonic
Flight Vehicles

• E. H. Hirschel
• Institut für Aerodynamik und Gasdynamik
• University Stuttgart
• and
• formerly EADS Military Aircraft
• München
• Germany

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Contents 1. Introduction 2. Thermal
Surface Effects in Hypersonic Flight 3.
Aerodynamic Performance (L/D) of Non-Winged and
Winged Re-Entry Vehicles 4. A Particular
Challenge with Large Airbreathing Hypersonic
Flight Vehicles 5. Concluding Remarks
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Introduction Three vehicle classes are in the
background - non-winged re-entry vehicles
(RV-NW), - winged re-entry vehicles (RV-W),
- airbreathing cruise and acceleration/
ascent and re-entry vehicles (CAV/ARV).
Generally flight in the Earth atmosphere and
in the continuum regime. Attached viscous flow
is considered to be of boundary-layer type.
Below 60 to 40 km altitude laminar-
turbulent transition. Now systematic review,
completeness is not attempted. Propulsion
systems are not considered.
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2. Thermal Surface Effect in Hypersonic Flight
The classical heat flux concept is
extended into the concept of the thermal
state of the surface. The thermal state of the
surface (qgw, Tw) includes both the thermal
loads and the thermal surface effects.
Thermal surface effects are the effects of
qgw, Tw on wall and wall-near viscous flow and
thermo-chemical phenomena. Especially
airbreathing hypersonic flight vehicles are
viscous-effects dominated. Necessary and
permissible surface properties are of utmost
importance for any hypersonic flight vehicle.
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The thermal state of a surface
(one-dimensional).
? Surface radiation cooling is the major passive
cooling means of external flow path surfaces.
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Thermal surface effects the influenced
phenomena and issues (qgw ? ? T/?n?gw, qw ?
? T/?n?w, Tgw Tw)
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Example SÄNGER lower stage, forebody RANS
computation, M? 6.8, H 33 km, ? 6.
Wall temperature and skin-friction coefficient
distributions in the lower and the upper
symmetry line. (Schmatz et al. 1991)
- Radiation-adiabatic (radiation-equilibrium)
temperature (qw 0) - flat surface
laminar
turbulent
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The influence of the state of the boundary layer
(laminar/turbulent), the gas model, radiation
cooling, and the location on the wall
temperature. ? Wall temperature
influencesalso inlet-onset flow andcontrol
surface flow.? Ground-facility simulationnot
possible, transition predictionproblem in
computational simulation,NASP/X-30 factor 2 in
GTOW!
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Excursion RV-NWs and RV-Ws have large
effective surface radii positive for wave drag
and for radiation cooling. Design dilemma for
CAVs and ARVs small nose and leading radii
keep wave drag small, but reduce radiation
cooling capabilities. The
radiation-adiabatic temperature usually is a
good approximation of the real wall
temperature. At sharp noses, edges etc.
(M. Hornung, 2003) tangential heat
transfer in the structure ? potential of
efficient thermal load relief.

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Excursion continued Example nose of the
SÄNGER lower stage, coupled RANS/structure
compu- tation, M? 6.8, H 33 km, ? 0.
Material assumption titanium. (Haupt and
Kossira, 1998) ? Other material or
thermal load relief by active cooling, better
by tangential heat transfer with e.g. pitch
fiber ceramics. At ? 300 K kpf? ? 1150 W/(m
K), kpf?theor. 2400 W/(m K) vs. kTi ? 33 W/(m
K), kCu ? 400 W/(m K), Orbiter TPS at ? 1000 K
k? ? 0.15 W/(m K), k? ? 0.04 W/(m K)).
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The influence of the state of the boundary layer
(laminar/turbulent), the gas model, radiation
cooling, and the location on the wall temperature
at the lower symmetry line. (H2K solution
Radespiel, 1994)
? Fly an external surface as hot as possible! ?
tailoring of surface emissivity.
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2. Aerodynamic Performance (L/D) of Non-Winged
and Winged Re-Entry Vehicles Several
non-winged re-entry vehicles (capsules, RV-NW)
and one winged re-entry vehicle (RV-W) are
operational. Mission demands, atmospheric
density uncertainties and wind make down range
and cross range modulation capabilities of
re-entry vehicles important. These
modulation capabilities are very important in
view of possible contingency situations.
Analytical considerations (equilibrium glide
trajectory) yield down range ?x ? L/D, cross
range ?y ? L/D. Modulation usually is made
with roll of the vehicle around the flight-path
vector (bank angle) ? thrusters, aerodynamic
control surfaces. In the following
consideration of RVs in view of hypersonic L/D
properties, no discussion of missions and of
thermal loads.
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Particularities of non-winged re-entry
vehicles- Flight with z-offset of cog and
negative angle of attack.- Problems parasitic
trim points, dynamic stability at lower flight
Mach numbers. - L/D small.- Example L/D and
trim points Viking 2.- Moment reference point x
0.34 D1, Z 0.0218 D1. (Data Blanchet et
al. 1997)
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Larger L/D of RV-NWs with e.g. bicones-
Bluff, slender, bent bicones were studied, but
became not operational.- Bicones fly with
positive angle of attack. - L/D ? 0.6 to 1.4.-
Example L/D and trim of the slender bicone
shape of Tsniimash (data only up to M 5.95).-
Different moment reference points. (Data
Ivanov, 1994)
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Large L/D with winged re-entry vehicles- The
US Space Shuttle Orbiter is only operational
RV-W.- First re-entry flight April 14, 1981.-
Together with Apollo it is one of the great
milestones of hypersonic flight.- L/D ? 1 at ? ?
40?, statically unstable flight at high M.-
Design methodology verified (Hoey, 1983) -
overall lifting re-entry design, - reusable
thermal protection system (TPS), - application
of aircraft test techniques, -
aerothermodynamic flight test methods, -
unpowered, low L/D landing technique. - Design
prediction discrepancies (Hoey, 1983) -
Hypersonic pitch trim and normal force
coefficients, - jet interaction effects, -
lower surface heating (overpredicted),
IRIS image, M 15.6, H 56.5 km - upper
surface heating (locally underpredicted), -
subsonic L/D (underpredicted).
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Aerodynamic data of the Orbiter- Coefficients
of longitudinal motion, moment reference point x
0.65 Lref. - Flight L/D (?) (Hoey,
1983) (Data Aerodynamic Design Data
Book, 1980)
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A radically new shape approach SHEFEX (sharp
edge flight experiment, (DLR)) - facetted
thermal protection system concept,- plane
ceramic composite panels bolted to the primary
structure, - cost saving potential seen in
manufacturing and maintenance,- thermal loads
increments at edges and chamfers thermal load
relief needed!- much improved aerodynamic
performance possible RV-NW configuration
possible RV-W configuration L/D ?
1.4 to 1.5 (to 2)
L/D ? 2.5


(Longo, DLR)
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4. A Particular Challenge with Large Airbreathing
Hypersonic Flight Vehicles Most critical
challenges in design and development of a large
CAV-type flight vehicle (SÄNGER Technology
Development and Verification Concept, 1995) -
Determination and verification of
laminar-turbulent transition location,
viscous drag, viscous inlet-onset flow,
control-surface effectiveness, thermal
loads. - Airframe propulsion integration. -
Ground-facility verification of a ram/scram inlet
in a realistic environment. - Ground-facility
verification (free-jet testing) of a ram/scram
propulsion system (inlet, core engine,
nozzle) under real flight conditions (SÄNGER
2.0 m diameter, 30.0 m length). -
Ground-facility tests and verification of static
and especially dynamic aero-
servo-thermoelastic properties of a hot primary
structure (SÄNGER 80.0 m length, Tstructure
? 1,000 K).
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Excursion I Take-off mass sensitivity of a M
5 CAV (Breguet equation). The specific impulse
of the propulsion system has been omitted here in
order to make the figure better readable. 
- Case A weak sensitivity. ? Moderate
design margin regarding L/D can be taken.
  - Case B vehicle is mass critical. ?
Design with small uncertainties in massempty
and L/D is necessary.
(Lifka, 1987)
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Excursion II Product phases and aircraft
ground-facility simulation and
verification. ? In the background is
Cayleys design pradigm, first and second aspect.
 
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Excursion II (continued) Product phases and
aircraft ground-facility simulation and
verification. 
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A particular sensitivity of airbreathing
hypersonic flight vehicles, illustrated with
SÄNGER lower-stage data. Sensitivity increases
with increased forebody pre-compression.  
- Schematic of mass-point force polygon in
steady level flight. -
Influence of ?? net installed thrust.
(Schaber, 1994)
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Classical problems in transport aircraft
development - general mass increase, center
of gravity location, - aeroelastic (static and
dynamic) properties, - mass increase due to
repair solutions.??? Commercial risk mitigated
by high payload fractions (30-45 per
cent).Problems in (large) airbreathing
hypersonic flight vehicle development - the
same as above.??? Risk dramatically enlarged by
the small payload fraction (only a few
per cent, SÄNGER-TSTO ? 3 to 4 per cent). The
higher the flight Mach number, the more important
becomes aerodynamic shape fidelity!
X-43A, M 10, L 3.66 m
lower stage SÄNGER, M 6.8, L 82.5 m

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Resumé in view of large airbreathing hypersonic
flight vehicles - large sensitivities
regarding aerodynamics, propulsion, propulsion
integration in the presence of highly
coupled functions (Cayleys design paradigm is
invalid), - static and dynamic aeroelastic
airframe properties are of very large
importance but very problematical -- late
structural ground tests, if possible at all,
-- numerical evaluation only for perfect-elastic
structures, not for real-elastic ones
(joints modelling problem), - very small
payload fraction.Conclusion - the classical
aircraft definition and development approach is
no more applicable.Necessary new approach
(Prerequisite 1/3) - revamping of definition
and development processes (post-Cayley
paradigm), - truly concurrent approaches
instead of discipline-oriented iterative ones
(Cayleys paradigm, 2nd aspect), - use of
numerical multidisciplinary simulation and
optimization tools, - employment of dedicated
experimental vehicles (EVs).
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Transfer model approach sketched in the German
Hypersonics (SÄNGER) Technology Programme
(AIAA 93-0752, 1993) - must allow to
transfer knowledge and data found with ground
simulation, (experimental vehicles,) and
computational simulation to the full-size design
problem without the classical ground-facility
verification. Goal step-wise
opening of technology and flight envelope in
order to reduce risks and cost of future
hypersonic flight vehicle/system development.

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Prerequisite 2/3 - adequate flow-physics
(laminar-turbulent transition, turbulent
separation etc.), thermodynamics (combustion
etc.) and structure-physics (joints etc.)
models, - numerical multidisciplinary
simulation and optimization methods for the
external and the internal flow path including the
real-elastic structure together with the
whole (!) flight control chain (ADS, IMUs
actuators, movable control surfaces) ?
Virtual Product/Virtual Flight Vehicle, - a
succession of dedicated experimental vehicles
HYTEX vehicles studied in the SÄNGER technology
programmePrerequisite 3/3 - systematic
identification of sensitivities and their
causes, - search for and study of -
effective ? disturbances, - control-induced
disturbances, - reduced pre-compression up
to podded engines (e.g. LAPCAT A2), -
alternative structure and materials concepts,
- alternative vehicle shapes, - dedicated
technology development and verification with
experimental vehicles.
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5. Concluding Remarks Technology development
for hypersonic flight proceeds since several
decades. Much and important system and
technology knowledge has been gained, but only
capsules and the Space Shuttle are operational,
not airbreathing hyper- sonic flight
vehicles. Important lessons have been learned
from past technology programmes. No future
single extraordinary technology break-through can
be expected, but major steps are made, e.g. in
materials, vehicle shapes, propulsion.
Airbreathing hypersonic flight still poses major
challenges. Numerical disciplinary and
multidisciplinary simulation and optimisation
has very large potential. Needed are
adequate flow-physics, thermodynamics, and
structure-physics models. Changes are needed
in definition and design phases and in systems
engineering. The overall challenges are very
large. Long and continuous technology
development effort and experimental vehicles
are still needed to arrive at highly efficient
flight vehicle and transportation systems.
Hypersonics has a bright future!