I
t stands for a reason that early stage of development of spaceflight focuses on reducing the high launch costs of satel-lites and spacecrafts. Reuse of the launch-er was identified as the most promising way to obtain a really drastic reduction in launch costs but this appeared to be an enormous technological challenge, particularly the atmospheric re-entry at hypersonic speed. A major and impressive step forward was the design and develop-ment of the Space Shuttle that also be-came fully operational during more than thirty years. NASA aimed at a spectacular reduction in launch costs by launching each shuttle every fourteen days. It caused a lot of concern in Europe where we just started the operational phase of the Ari-ane launcher. Unfortunately, for NASA the technically successful Space Shuttle ap-peared to be an economical failure and this was mainly caused by the extremely high maintenance and refurbishment costs. Figure 1 shows the damage of the thermal protection tiles that occurredaf-ter every flight. Repair was difficult and time consuming because the tiles were bonded to a layer of felt underneath and could not be easily removed. Rather than launching every two weeks each shuttle was launched every ten months and sometimes even more.
It became clear that the vulnerable ther-mal protection tiles should be protected by a robust but low mass containment and should also be easily removable. Re-search was done in the US, Germany and the Netherlands to develop better tiles based on the application of metallic heat resisting materials that develop a protec-tive self-healing oxide scale against hot oxygen. An example of such a design is shown in Figure 2.
HYPERSONIC FLOW
A re-entry vehicle has to pass several dif-ferent flow regimes: free molecular flow, transitional flow and continuum flow. In the continuum flow we have hypersonic,
transonic and subsonic flow. The highest heat and mechanical loads for a Reus-able Launch Vehicle (RLV) occurs in the hypersonic regime, usually a Mach num-ber greater than 6. In the hypersonic re-gime, there is a very strong bow shock with a discontinuous jump in pressure, density and temperature. Behind the bow shock the temperature is raised to about 10,000K, bi-atomic gasses like oxygen and nitrogen rapidly dissociate and the temperature decreases to about 6,000K. The dissociated gasses may chemically re-act with other atmospheric components and as a result, the chemical composition along a streamline is not constant. Also, the chemical reaction and the diffusion rate are important parameters that are a function of temperature and density. All these effects shall be included in a hyper-sonic computer code and as a result these codes become much more complicated than the subsonic and supersonic codes. The verification of the hypersonic codes is another problem. Duplication of a
hy-Insights into new advances in re-entry vehicles and materials
We are already used to a widespread commercial utilization of spaceflight, particularly
with satellites for communication, earth observation, navigation, weather forecast
and many other applications. Launching satellites is a rather costly affair, particularly
because twenty minutes after lift-off the greater part of the expensive part of the
launcher is transferred into scrap on the bottom of the ocean.
TEXT K.J. Sudmeijer
HYPERSONIC RE-ENTRY TECHNOLOGY
personic flow in a ground-based facility is presently not possible. The available wind tunnels can reproduce some parameters but not all of them at the same time. So, we have to work with a number of test facilities that can each reproduce some aspects of hypersonic flow and try to syn-thesize all the results.
It stands for reason that there is a high demand for flight tests because it assures a real hypersonic environment. However, flight tests are expensive and the associat-ed hypersonic database is limitassociat-ed. In addi-tion to this it is not possible to correct for scale difference of the test model by simi-larity parameters like the dimensionless Reynolds and Mach numbers because the additional hypersonic similarity param-eters are not dimensionless. Hypersonic boundary layer theory and particular tran-sition phenomena are poorly understood and so hypersonic flight tests are needed to support ground based facilities and validate the hypersonic computer codes. Already in the early nineties, the first stud-ies were done at the Aerospace faculty and a conceptual design was made of a small experimental re-entry vehicle that could be used for aerodynamic research (see Figure 3 left). Such a vehicle was ac-tually developed by ESA (IXV depicted in Figure 4). IXV is expected to be launched by the new VEGA launcher at the end of
this year.
Hyperion-1 and IXV have both have a rath-er large blunted nose in ordrath-er to reduce the stagnation point heat flux and keep the nose temperature down to acceptable values. The stagnation point heat flux is inversely proportional to the square root of the nose radius and as a result of which, the IXV has got a large nose radius, a rela-tively high drag, and a low L/D.
The maneuverability and the experimen-tal utility of the vehicle could be increased considerably if we reduce the nose radius to 2.5cm. There are no materials available that can withstand the extremely high temperature and as a consequence of this, active cooling is required. Such a design is depicted in Figure 3 right. Recent stud-ies at the Aerospace faculty demonstrated that new experiments are possible that cannot be done with blunt nosed vehicles like Hyperion-1. An example of this is the capability of Hyperion-2 to fly controlled suborbital flights with a constant Mach number during about thirty seconds with a large variation of Reynolds number [4]. That allows for transition measurements at a fixed Mach number that is possible with blunt lifting bodies like Hyperion-1 and IXV.
Next step in our research will be to add wings with sharp leading edges in the Hyperion-2 design with the aim to
in-crease the L/D ratio even further. Such a re-entry vehicle will be able to fly longer at high altitudes and enter the denser atmosphere at lower altitudes at a lower speed to reduce heat fluxes. This is a com-pletely new re-entry strategy. The atmo-spheric part of the re-entry will last much longer (one and a half to two hours) rather than twenty to thirty minutes as is pres-ently normal for winged re-entry vehicles. Other interesting features are the low an-gel of attack (α < ten degrees) rather than thirty to forty degrees as was usual for the Space Shuttle as well as α-control instead of bank angle control. The accelerations during a high L/D re-entry are low and so the return flight back to earth is quite comfortable. The specific properties of this new re-entry strategy must be inves-tigated by simulations and optimization of the re-entry trajectories to find the ad-vantages and disadad-vantages of such high L/D designs.
THE EXPERT RE-ENTRY VEHICLE
In 1989, the Faculty of Aerospace En-gineering started a daring project that aimed to design, development and flight of a small and low cost re-entry vehicle that would be launched from a subma-rine with the Russian Volna rocket. The re-entry module had a full metallic ther-mal protection system with active cooling of the nose by nucleate boiling and
en-N
ASA
Figure 1. Damage of the thermal protection tiles found after
re-entry Figure 2. Removable thermal protection tile, designed by NASA Langley.
Figures 3L and 3R. Conceptual design of the experimental re-entry vehicles Hyperion-1 (left) and Hyperion-2 (right)
N
hanced radiation cooling of the flare. The shape of the vehicle depicted in Figure 5, was a blunted bi-cone obtained after a numerical optimization process taking into account all relevant constraints and requirements.
Within two years after the inception of the project, ESA took over the project and started a much bigger project under the name EXPERT with important European space industries like Alenia (main contrac-tor), Astrium, Dutch Space and institutes like DLR, ONERA and IRT.
The new re-entry vehicle became bigger (450kg) and longer (1.5m) and much more expensive (40 million Euro). The outer skin is made of a Nickel based alloy (PM1000), an Oxide Dispersion Strengthened mate-rial produced by powder metallurgy, ex-cept for the nose cap and dummy flaps that are made of a ceramic composite (C-SiC) consisting of carbon fibers with a matrix of silicon carbide. The metallic skin is pre-oxidized during one hour at 1100ºC in laboratory air that naturally develops a 5μm thick self-healing chromium oxide scale.
An exploded view of EXPERT is depicted in Figure 6, showing the hot Thermal Protection System (left), the internal cold structure (right), and the interface struc-ture with the Russian Volna launcher. Dutch Space builds the metallic outer skin in The Netherlands and they were also
re-sponsible for the thermal protection sys-tem. The complete TPS including internal insulation is shown in Figure 7 as integrat-ed in the Dutch Space facility in Leiden. The EXPERT re-entry vehicle is now fully integrated in the Alenia facilities in Turin, Italy and waiting for launch.
HEAT RESISTING MATERIALS
The main obstacle in the development of reusable re-entry vehicles is the availabil-ity of heat resisting materials. The environ-ment during re-entry is very harsh and so are the material requirements: high yield and rupture strength, good ductility over the whole temperature range, resistant to hot oxygen particularly to atomic oxygen, self healing oxide scale with high emissivi-ty and low cataliciemissivi-ty to reduce recombina-tion of atomic gasses at the outer wall of the re-entry vehicle. These requirements are is particularly difficult to meet if we want to apply sharp noses and leading edges in order to obtain a high L/D ratio. A material that meets all these require-ments is “unobtainium” but this wonder-ful material has not yet been found or developed. As a consequence, we have to work with the available heat resisting materials and apply active cooling in the sharp noses and leading edges.
Only a limited number of heat resistant materials are presently available for tem-peratures over 1,000ºC. Ceramic com-posites can withstand the highest
tem-peratures; Carbon-carbon (C-C) materials consisting of carbon fibers in a matrix of graphite can be used to nearly 1,800 ºC. Such C-C composites are applied in the nose and wing leading edges of the Space Shuttle. It stands for reason that carbon cannot be exposed to hot oxygen and thus a protective coating is required. C-C components that are exposed to the ex-tremely hot hypersonic boundary layer shall be treated very carefully because the slightest damage of the coating may cause serious damage during re-entry. That is why much work has been done to develop self-healing coatings that can automatically repair damage. The results of this work that was also done in our faculty had only limited success. Small crack in the coating were actually closed but larger cracks and scratched could not automatically be repaired. These coatings must be carefully inspected after every flight and every crack that is found must be repaired. Such properties cause high maintenance and refurbishment costs that are not desirable for application in reusable launchers. Other ceramic com-posites like C-SiC that can be used up to 1,400ºC also need self-healing coatings and suffer from the same disadvantage. There are a few metallic materials that may be useful, particularly two ODS mate-rials (PM1000 and PM200) where strength and stiffness at temperatures up to 1200 ºC are improved by dispersion strength-ening. PM1000 is a Nickel based material
ESA
ESA
Figures 4L & 4R. ESA’s experimental re-entry vehicle IXV
Figure 5. The Delft Aerodynamic R-entry Test bed, DART
Figure 6. Exploded view of the EXPERT re-entry vehicle. Left the Thermal Protection System TPS
that naturally develops a chromium ox-ide scale at elevated temperatures and PM2000 is an iron based alloy that devel-ops an aluminum oxide scale. Both ma-terials appeared to have a stable and self healing oxide scale when tested at tem-peratures up to 1250 ºC in laboratory air. These materials where also tested in the plasmatron (Figure 8) of the Von Karman Institute in Brussels, Belgium where in the hot plasma flow nearly all the oxygen is dissociated.
Several heat resisting alloys were tested in the Plasmatron, among them also PM1000 and PM2000. For PM1000 the results were disappointing as can be seen in Figure 9. The left picture shows the surface of the chromia scale after the pre-oxidation and before the exposure to the hot plasma. The right picture shows the results after 15 minutes exposure to the plasma flow at a temperature of 1300 ºC. The chromia scale is completely restructured and even partly destroyed by reactive evaporation. Large conglomerations of Nickel oxide were observed and it became clear that
the protective function of the oxide scale was strongly reduced. PM1000 is not a good material for the outer skin of a reus-able launcher; a single use is the only op-tion. The alumina scale of PM2000 on the other hand appeared to be very success-ful in the Plasmatron tests. Even after ex-posures of 30 minutes no degradation of the oxide scale was found and also the self healing property of the oxide scale with respect to scratches was outstanding. All scratches applied to the pre-oxidized test samples were closed by oxidation dur-ing the Plasmatron test and hardly visible anymore. So a self healing alumina scale is the preferred coating for the outer skin of a reusable re-entry vehicle. A remaining problem is the rather low emissivity and high catalicity of the alumina scale. A useful alternative may be the applica-tion of a new brand of ceramic materials: the MAX phases that are made up of three elements in the general form:
n+1 n
M
AX
n=1,2 or 2where M is an early transition metal, A is
a group A element and X is carbon or ni-trogen in the periodic table of elements (see Figure 10). There are three groups of MAX phases with n=1,2,3 respectively in-dicated by 211, 312 and 413. Particularly interesting are Ti AlC and 2 Ti AlC be-3 2
cause they naturally develop a protecting alumina scale at elevated temperatures. These materials can be used at high tem-peratures (up to 1,200ºC), not brittle and can easily be machined. They posses a relatively high conductivity both for heat and electricity quite similar to metals. However, the MAX phases are much stron-ger in compression than in tension and that is a serious constraint for structural applications of these materials but when used in the extremely hot nose of re-entry vehicles the occurring stress will mainly consist of compression if well designed. Nevertheless, useful work has been done to improve the mechanical properties of a MAX phase by a ZrC particle reinforce-ment of Ti AlC [9].3 2
There are two interesting MAX phases that develop an alumina scale at elevated
Figure 7. The thermal Protection system of EXPERT, integrated in the Dutch Space facilities in Leiden
Figure 8. The Plasmatron facility of the Von Karman Institute in Brussels
Figures 9A & 9B. Microphotographs of the Nickel based ODS material PM1000. Up (9A) the surface after pre-oxidation at 1150 deg. Celsius dur-ing one hour. Down (9B) after 15 minutes exposure at 1100 deg. Celsius in the Plasmatron
temperatures: Ti AlC2 and Ti AlC . But 3 2
the oxide scale does not only contain the stable Al O2 3 but also the unstable TiO2
(Rutile). It has been observed by Song et al [11] that during the oxidation process at 1100 ºC in laboratory air a porous layer was formed in the oxide scale with an outer layer of TiO2. In our Plasmatron tests with PM2000, we found small rutile crystals on the surface of the samples that were not stable and did fall apart after an exposure time of 15 minutes. Thus, we preferred the 211 phase Ti AlC2 because
of the lower Ti content rather than the 312 phase Ti AlC .3 2
A good quality of Ti AlC material is pro-2 duced by the Swedish company Kanthal under the name Maxthal 211. A small amount of Maxthal 211 was made avail-able by Kanthal for the first investigation of the oxide scale. Some results are shown in Figure 12, left the alumina scale with a columnar structure and a constant thick-ness of 3 micron and right the character-istic kinkbands of the MAX phases. Even though the material seems promising, it is necessary to test Maxthal 211 in a plas-ma wind tunnel in a chemically reactive environment with hot atomic oxygen. A proposal for such tests has already been presented to ESA.
Figure 12L & 12R. Microphotograph of Maxthal 211. Left a fractograph showing the alumina scale after 48 hours of pre-oxidation at 1200ºC in laboratory air and right a micrograph showing the characteristic kink bands of this material.
Figure 11. Summary of presently known MAX phase materials [8]. Figure 10. Periodic table of elements with the three groups of elements
that constitute the MAX phase materials, ref[8].
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[4] M.Dijkstra, Trajectory optimization of Hyperion-2 for the study of hypersonic aerothermodynamic phenomena, Final Thesis 2013.
[5] R.Monti, D.M.Paterna, A low risk reentry: looking backward to step forward, Aerosp. Science and techn. 2006.
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[7] J.Buursink, K.J. Sudmeijer, “Experimental studies of an Enhanced Radiation Cool-ing System”, AIAA
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