Spacecraft Active Protective Shielding
Development of Spacecraft Active Protective Shielding against Meteoroids and Orbital Debris and New Methods of Shielding Testing at Hypervelocity Impact
Tech Area / Field
- SAT-SAF/Space Safety/Space, Aircraft and Surface Transportation
3 Approved without Funding
Institute of Physical Chemistry and Electrochemistry, Russia, Moscow
- GOSNIIMASH, Russia, N. Novgorod reg., Dzerzhinsk\nInstitute of Applied Mechanics, Russia, Moscow
- NASA / Lyndon B. Johnson Space Center, National Aeronautics and Space Administration, USA, TX, Houston\nFraunhofer Institut Kurzzeitdynamik, Germany, Freiburg\nEuropean Space Agency / European Space and Technology Center, The Netherlands, Noordwijk
Project summaryThe aim of the project is the development of spacecraft active protective shielding against meteoroids and orbital debris and, also, new methods of shielding testing at hypervelocity impact.
Modern protective structures of spacecraft and orbital modules are based on the effect of impact fragmentation of orbital particles colliding with a shielding. Protection performance is mainly determined by the efficiency of projectile fragmentation, i.e. by the characteristic size of the fragments and the expansion of the fragments cloud. Physically, the shielding is called upon to strongly decrease the momentum flux density to the rear wall, and therefore to reduce the probability of perforation or dangerous damage of a spacecraft hull.
Shielding design stems from the compromise between two contradictory requirements: reliable protection and light-weight of the structure. Rational shielding design problem is of great importance because of high cost of orbital cargo delivery and the increase in safety requirements for long-term space station operation.
Spacecraft protection is presently based on the so-called “whipple concept” proposed in 1947. whipple protection represents the bumper (plate) located at some distance from a spacecraft hull. To reduce the weight of whipple shielding, a great number of its modified versions were suggested. The advanced versions incorporate spaced supplementary bumpers, new composite materials and rational geometric parameters of the structure (the experience in that field is summarized in the Proceedings of Inter-Agency Space Debris Coordination Committee). The weight of the modern protective structures is nevertheless comparable to the weight of a spacecraft hull. For instance, the weight of protective structure of each ISS modulus exceeds 1000 kg.
It was recently found out that the use of mesh bumpers permits the substantial weight reduction of protective shielding. The advantage of mesh bumpers over solid ones is caused by a more efficient transfer of momentum and energy to the projectile. This is governed by the penetration of mesh elements (strings) into the projectile. the penetration of thin string occurs by the forming of groove-shaped crater whereupon the string is subjected to strong transverse deformation. As a result, the transverse momentum of projectile substance turns out to be much greater than for the case of solid bumper with the same mass per unit area. Impact fragmentation of spherical projectile on mesh bumpers was investigated in ISTC project 1917.
the penetration of the mesh elements into a projectile provides the potentiality of further improvement of fragmentation. Namely, it is possible to insert into a projectile an active substance bearing chemical and/or phase transformation with intensive gas phase production. gas phase production at high shear rate has to increase the pressure inside the forming crater and thereby to increase the transverse momentum of the most dangerous heavy fragments. The complete ISTC project 1917 demonstrated that active polymer coatings on metal mesh bumper make the fragmentation better.
To produce the efficient active protection one should however solve a number of key problems. The orbital condition imposes rigorous restrictions on the materials and structure of protective bumpers. The requirements to safety, radiation stability and durability exclude the use of easily evaporated materials or homogenous substances which are able to high-rate exothermic transformations: high-energy and ultraviolet irradiation causes fast degradation of such materials. Of course, the possibility of spontaneous combustion and propagation of self-maintaining exothermic transformation are bound to be completely precluded. On the other hand, mechanochemical and/or phase transformation at high-velocity shear deformation have to be adequately fast. The transformation-induced contribution to the kinetic energy of the flying apart fragments can be substantial if only the transformation time is of the order of the time of acoustic wave propagation along the projectile.
A detectable contribution to the kinetic energy of fragment cloud expansion can result from mechanochemical reaction between active components of the mesh bumper and orbital debris substance. In spite of heterogeneity, the rate and completeness of the reaction in penetration of mesh element into a projectile can be rather high. That fact was experimentally established in high-velocity slipping impact of aluminum plates and teflon films.
The use of mechanochemical reactions to improve impact fragmentation seems to be quite practicable. The point is what exactly compound and structure of the bumper are the best for that for purpose.
The promising active substances seem to be composite materials involving fluoropolymer matrix and metal (for instance, magnesium or aluminum) filler. Such composites are highly reactive in high-rate shear deformation and yet sufficiently stable in orbital conditions. As for the structure of the bumper, one can specifically use metal meshes with composite active coating. Wire mesh bumpers outperform the solid ones with the same mass per unit area. The better protection should be however expected with the structures containing isolated heavy particles fixed in a light-weight substrate (fabric or polymer mesh). According to theoretical notion developed in ISTC project 1917, the advantage of such structure results from the useful redistribution of impact energy.
A possibility to use fabric-based or another soft bumper is also very attractive. This allows to open protective structure upon the orbital injection of a spacecraft and to install the first bumper at a great distance from the hull. An increase of the spacing between the first bumper and the rear wall reduces the momentum flux density and the probability of dangerous damage of the hull in the event, of course, that the bumper provides the required finesse of the fragments.
The advisability of pursuing research into active protection is supported by the first experiments performed within the limits of ISTC project 1917. The preliminary results were presented in the project proceedings, in the presentations of 4th European Conference on Space Debris (2005) and in the meeting in Lyndon B.Johnson Space Center (2005).
The main objective of the proposed project is set as:
Task 1: the detailed theoretical and experimental investigation of active protection concept.
The second task is related to the testing of protective shielding. projectiles acceleration for that purpose is presently based on the light-gas guns technique. The other methods do not provide the required velocity or the retention of shape and mass of the projectile. light-gas guns however give the conventional projectile velocities up to 7-8 km/s, whereas the impact velocity of 80% of orbital debris is higher than 8 km/s. Besides, the projecting in the limiting velocity sharply intensifies the wear of the barrel and therefore increases the cost and labour-consumption of the testing.
To attain the most probable velocity of orbital debris (about 10 km/s) with a projectile of prescribed mass and form, we propose the new method of acceleration. The method is based on the traveling reduction of the barrel by a metal shell. The shell is projected by an explosive charge with a constant detonation rate, but the reduction velocity smoothly varying along the barrel. This provides the “soft” acceleration and prevents deformation or damage of the projectile. The size of the device is not too large: for the projectile diameter of about 1 cm and finite velocity 7 km/s the length and diameter of the unit is about 40 cm and 10 cm respectively.
The suggested scheme of acceleration was first realized in ISTC project 1917 for the velocities up to 7.3 km/s. To obtain the higher velocities it is necessary to perform experimental and theoretical investigation to select geometric parameters, loading regimes and structural materials of the device.
To improve the quality and economic feasibility of the testing we propose to carry out
Task 2: the development of the blast accelerator of calibrated projectiles for the velocities up to 10 km/s.
The experts of Lyndon B.Johnson Space Center (NASA) E. Christiansen and K.Nagy proposed to study within the project the highly important problem of auto-opening of the shielding in an orbit. The presently using rigid structures give no way of this. In contrast, the above “soft” structures can be delivered into orbit in a portable form followed by automatic opening by means of more or less simple provision using power elements from shape memory alloy or gas supply unit.
Increasing the spacing between the first bumper and the hull clearly improves protection providing the fragments sizes are adequately small. We therefore intend to perform
Task 3: the development of basic design of auto-opening protective shielding.
The scientific team of the project has a wide experience in investigation of super-velocity impact and projecting systems, physical/chemical mechanics and mechanochemistry. The key persons showed the good skills and creative potential in the preliminary study of the problem
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