Protection against MMOD

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2. Protection against MMOD

2.1 Introduction

Space, the final frontier of mankind. Since the launching of the first artificial satellite,

Sputnik 1 by the Soviet Union in 1957, more and more artificial satellite and spacecraftlaunching out to space. Until now a days there are more than 560 operating satellite orbitingaround Earth, each serving different purposes. It is undeniable that artificial satellites are veryimportant for human civilization, satellites are used for a large number of purposes, rangedfrom military to civilian, which including communication, navigation, observation,forecasting weather, and list goes on. Space stations and human spacecraft in orbit are alsocategorized as satellites. Satellite orbits vary greatly, it can generally classify as low earthorbit, polar orbit and geostationary orbit.

MMOD, shorts for micrometeoroids and orbital debris are objects that orbiting around

earth. Micrometeoroids are small particle of rock, typically metallic in space, usuallyweighing less than a gram. Mircometeoroids are very common in space, yet they have lessstable orbits compare with meteoroids due to their small size. Orbital debris, also known asspace debris, space junk and space waste. It is the collection of objects created by humansthat orbiting around the earth. Unlike satellites, orbital debris no longer serve any usefulpurpose, they consist of broken satellite, spent rocket stages, surface degradation productsand so on. Without the force required to send them back to earth or move to other place, thesespace junk will remain in space, orbiting earth almost indefinitely until external forces thatchanges its course.

Fig 2.1.i: The orbital debris orbiting around earth



Spacecraft that are launched to operate at orbit are susceptible to impacts by MMOD,which are shorts for micrometeoroids and orbital debris. These impacts possess a threat to theoperating satellite. Depending on where the impact occurs, size, composition and speed of theimpacting object, it could cause damages to orbiting satellite. The impact can occurs atextremely high speeds, damaging crucial system which caused the satellite to lost function or

even causing casualty in space station. Thus, it is important for spacecraft designer to beaware of these threats and providing protection in order to ensure the operational of satellites.Yet, this is very challenging as there¶s a lot of limitation such as weight and the conditionunder space make it hard for repairing and maintenance.

So, how many MMOD are there in our orbits? The United State Space SurveillanceNetwork (SSN) has been tracking space object since 1957. SSN able to track space objectsfrom 10 cm in diameter. Currently, SSN tracks more than 8,000 man-made orbiting objects.The objects range from satellites that weight about few tons to spent rocket bodies thatweight few kg. Out of the 8,000 tracked objects, around 560 are operating satellite, the restare space junk, which is about 7% only. This number still doesn¶t include small size orbitaldebris and micrometeoroids. This shows that providing protection for the satellite is veryimportant to minimize the damage caused by impact by the MMOD.

In order to protect satellite against impact by the MMOD, the approach is by placingextra bumper as shield to the satellite. However, most spacecraft can¶t include thick protective shield due to limitation of cost, design, weight and/or mission constrain. Thereforethe idea is how to modify the original wall of the satellite to wall system with outer and inner bumper that will be able to protect the satellite, with little or none changes to the design andweight.



2.2 Consideration

To successfully modifying the wall of the satellite, there are certain consideration andlimitation to the choice of materials and design. These considerations are important for engineer and designer to look through before choosing the suitable material and design that

will able to provide the protection required.

Weight: Weight is definitely one of the major considerations for space applications.As each extra kilogram will cost a fortune to send it to outer space. Currently an extrakilogram will cost an extra of about 50k to launch it. This is due to several factor, first of all,thousands of professional workers are required to build and launch a rocket. Secondly, designand development state of a rocket could take years and billions. During this time, thecompany doesn¶t obtain any income. Finally and most important is the fuel, the escapevelocity for earth is 11.2km/s. However depending on the altitude of orbit, the velocityrequired is lower, for a 9000km altitude, the velocity required is around 7.1km/s. As we cansee, huge amount of energy are required to achieve such high velocity. For most rockets, thefuel can makes up over 90% of the total weight. Thus it is essential to control the weight of asatellite.

Atomic oxygen (AO) Degradation: in the low earth orbital (LEO) atmosphere,atomic oxygen is the main components. Under long exposure, it can eventually degradecertain materials especially polymers. It will gradually reduce the thickness and reduce theproperties of the materials. These will greatly affect the service life of the wall-system andeventually the service life of satellites. Generally, this can be controlled by an extra thin layer of aluminium wrapping around the exposed materials.

Mechanical Properties: For sure the mechanical properties of the material areanother main consideration. Mechanical properties like ductility, strength, toughness,stiffness are the main factors in deciding the effectiveness of the wall system of the satellite.The mechanical properties will directly affect on how much impact the wall can hold andhow well it can protect the satellite from any damages.

Temperature: Temperature is another important consideration to make, as in theorbit, the temperature varies greatly depending by the position of the satellite relative to thesun. The temperature variation may range from -100 to 100 . These temperatures cancause thermal shock which produce residual stress due to the change of temperature. It isestimated that a satellite with an orbital period of 90 min will experience 175 000 thermal

cycling during a 30 year service life. In the same time, it is important to ensure that theselected material able to retain their properties under the critical temperature. Thermalstability is essential criteria in choosing the suitable material for the spacecraft.



H ypervelocity impact ( HV I): Earth orbiting spacecraft are exposed to ultra highspeed impact by the micrometeoroids. This is why a HVI test is a required consideration for our design and choices of materials. HVI refers to impact speed on the order or 2km/s or higher. The study of composite materials under such condition is relatively new in theindustry. Oppose to ordinary low velocity impact, the impacting materials under HVI will

behave like fluids. Extreme velocity will cause vaporization of both the impacting materialand the target. As soon as the impact occurs, the impacting material can undergo temporaryliquefaction, it can generate plasma discharges which can interfere with the electronics onboard of the satellite. Under earth orbit, a HVI by micrometeoroids can caused anything fromminor degradation to the complete annihilation of the satellite without the proper protection.

Fig 2.2.i: A laboratory simulation of a MMOD Hypervelocity impact on spacecraft

S erving Life: Serving life is another consideration that designer need to take inaccount. As most of the time, the satellites are expected to be working for a long period of time due to the high cost of launching it to space. Thus it is important to choose a materialwith long durability and stability over time.



2.3 Choosing Composite as Materials

As we can see choosing the right material for space application is very importantbecause it involves a lot of different consideration. Depending on the choices of materials, itcan directly affect the performance and serving life of the satellite. Under such challenging

requirements and limitations, it makes composite our number one choice of materials. This isbecause composite materials can provide wonderful properties such as thermal stability andgood mechanical properties at a low weight ratio. As technology getting more advance by thedays, it allow engineers to precisely design and create specified composites that can fit in thespecific application that can fulfil the requirements.

In fact, more and more space applications are designed to be made of compositesmaterials such as space station trusses, robotic arms and booms, fuel tanks and pressurevessels. As one of the example design, the truss structure in low earth orbiting (LEO) spacestations which is used to support the crew modules, lab modules and solar arrays. This trussstructure can be made of composite with hybrid construction with ply orientation. Thiscertain hybrid composite are made by the combination of T-50 carbon fibre-epoxy laminates,P-75 carbon fibre-epoxy laminates, aluminium foil as coating and adhesive layers.

As another example, a metal-matrix composite - Graphite/Al composites are used inhigh-gain antenna boom for the Hubble Space Telescope ( Fig2.3.1 ). This composite is madewith diffusion-bonded sheet of P100 graphite fibres in 6061 Al. This boom offers the desiredstiffness and low Coefficient of thermal expansion to maintain the position of the antennaduring space maneuvres.

Fig 2.3.i P100/6061 Al high-gain antenna boom for the Hubble Space Telescope (HST)



From the examples, we can see that the composites being used are carefully designedwith the specific materials and specific orientation to suit each of the specific application.Using this advantage, the composites can optimize their performance for each of theapplication which using the minimal amount of materials which is important for spaceapplications. More composite materials are being invented and created each years, from the

choice of materials, composition, orientation, structure and so on, there¶s are limitlesscombinations which means there are limitless possibilities and potential for compositematerials. In future, it is believed that composite materials will be used for most of the major components in space applications.

Concerning our main application for this assignment, which the wall system for satellite. This certain wall system is expected to provide protection for satellite under MMODenvironment. There are several designs and material choices available for the satellite, but tohave a better comparison, we¶ll focus on wall that consists of 2 bumpers which are the inner bumper and the outer bumper. Each bumper will be a multi-wall system, consistingcomposites laminates of different composites.

Our choice of composite materials included Kevlar, graphite/epoxy and spectra fibres.Kevlar fibres are chosen mainly due the good strength-to-weight ratio of Kevlar fibres, in thesame time, Kevlar fibres can maintain its strength down to -196 . Similarly, graphite epoxyis chosen for their light weight, good strength with the extra stiffness and corrosion resistance.Spectra fibre, another name for ultra-high-molecular-weight polyethylene (UHMW), is one of the manmade fibres with highest strength-to-weight ratio. The high strength provided byspectra making it another excellent selection for building the bumper.

As we can see, the choices of materials are mainly emphasize on the strength-to-weight ratio as weight is an important issue for space applications. It is very important tominimize the weight to cut down cost. To fabricate the laminates bumpers, the prepreg lay-upprocess is chosen due to the high requirement of high fibre volume fraction to further improve the mechanical properties of the bumpers. In the same time, using composite fibresable to eliminate severe cracking and petaling.



2.4 Composites Manufacturing Process - Prepreg Lay-up Process

2.4.1 Introduction

Prepreg lay-up is one of the two major methods of hand lay-up process. It is a very common

process in the aerospace industry as well as making prototype parts Wing structures, radomesand yachts parts are made using this process. Sometimes, it also called as autoclaveprocessing or vacuum bagging. This is a labour intensive process, the labour cost are 50 to100 times greater than other high-volume process such as filament winding and pultrusion.This process is chosen as our composite manufacturing process because it can manufacture

complicated shapes with very high fibre volume fractions. In the same time, Kevlar andgraphite/epoxy are some of the usual material used for prepreg lay-up process.

Fig 2.4.i Series of pictures showing products and process of prepreg lay-up process

2.4.2 Prepreg preparation

Before the lay-up process, one must prepare the raw material or prepreg. Prepreg arerolls of uncured composite materials in which the fibres have been preimpregnaterd or combined with the resin, thus the name, prepreg. In order to make prepregs, the fibres aredrawn, guided and then flattened onto a belt, forming a web or sheet of fibres. The fibershave to be controlled at a precise thinkness, oriented nicely without overlap or gap in between.

After that, the fibres are mated with two backing sheets, on the top and bottom. Thesebacking sheets have been coated with resin to the proper thickness; the resin used would havebeen mixed with catalyst, accelerator, and other required material. Using sets of rollers, thesandwich of backing materials, resin and fibres is compacted. Finally, the sandwich is beingheat to a specific temperature for a specific time. This heating will cause the resin to be



slightly cured. This process is to ensure the sandwich will intact nicely but not overcured.This is because the material still needs to go through a series of prepreg lay-up process.

Finally, the sheet of material is then trimmed and wound up as prepreg material.Because the resin has already been initiated, the prepreg must be kept in refrigerator to

prevent premature curing.

Fig 2.4.ii Prepreg Process

2.4.3 The lay-up Process

For aerospace applications such as our main application, the prepreg lay-up processhas to take place in a very clean and neat atmosphere with controlled humidity andtemperature. Workers are required to cover their heads, body and shoes with clean clothingaccessories and duct is prohibited in the room. Such precautions are required to ensure thequality and results of the product are carefully controlled and consistent.

Fig 2.4.iii Pictures of clean room for prepreg lay-up process



First of all, the raw material or prepreg is kept refrigerated before the process starts.

To make the parts, the prepreg is removed from the refrigerator and brought slowly to roomtemperature. Once the pregreg reaches room temperature, the prepreg is laid on a cuttingtable, using the reciprocating action of knife, laser, or ultrasonic cutter, the prepreg is cut into

desired shape and length. To ensure the efficiency, consistency, repeatability and quality of the cutting, the cutting machines are computer controlled by through software.

After the cutting is done, prepregs are laid on top of an open mould. Release agent isapplied to the mould for easy removal of the part. The backing film, both top and bottomfilms are removed from the prepreg. The prepregs are laid in the sequence and orientationaccording to the design. For parts for greater safety issue or aerospace application such as thisone will required more care. Quality control personnel will check the ply sequence after eachlayers of prepreg are laid down. In the same time, it is very essential to ensure that there¶s noentrapped air as it will cause porosity. By using squeezing rollers, the entrapped air isremoved and an intimate contact is created.

2.4.4 Vacuum bagging

Once all the prepreg layers are arranged in the desired orientation and sequence,vacuum bagging preparations are made for curing and consolidation of the parts. Theprocedures for vacuum bagging are:

I. A release film is applied on top of the prepreg. This is to allow entrapped air,excess resins, and volatiles to escape.

II. Apply bleeder, a porous fabric, on top of the release film. This is to absorbmoisture and excess resin from the prepregs.

III. Apply barrier film on the top of bleeder. The function is similar with releasefilm but it is not porous.

IV. Apply breather layer on top of barrier film, a porous fabric similar to bleeder.This is to create even pressure and allowing air, volatiles to escape.

V. The final layer is the vacuum bag. It is a expendable polyamide (PA) film or reusable elastomer.

VI. The film is sealed on all sides of the prepregs using seal tape, a wide rubberymaterial that sticks to both the mould and bagging material.

VII. A nozzle connected to vacuum hose is inserted into the vacuum bag to createvacuum inside the bag.



Fig 2.4.iv Vacuum bagging for prepreg lay-up process

2.4.4 Autoclave

After the bagging, the mould is placed inside an autoclave for curing andconsolidation. An autoclave is a chamber that can maintain the required pressure andtemperature for the processing of the composite. After placing the parts, the autoclave door isclosed and connections to thermocouples and vacuum hoses are made. Depending on theresin material and geometry of the parts, a cure cycle is run to process the parts. The curecycle is computer-controlled, one only required to key in the cure cycle data.

Fig 2.4.v Vacuum bagged parts are going into the autoclave

After the curing process is done, the parts is taken out and allowed to be cooled down.Finally, the vacuum bag is removed and the parts can be taken out.



to its low melting temperature of 120 . The temperature on the space varies greatly and theimpact might produce high heat. This might melt the spectra fibre if used as the material for the outer bumper. As for the inner bumper, a triple-wall system with Kevlar and Spectrafibres are used. This is to provide the maximum protection to the satellite using the stronger spectra and Kevlar instead of graphite/epoxy. These 2 bumpers will the main protection for

the satellite under MMOD environment.

The impact under MMOD can be occurs under a very high speed, which producegreat impact and energy. However, the impact can be very small as well. This huge range andrandomness make designer hard to estimate the component¶s probability of survival andconfidence level. Therefore, it is important to create the multi-wall system bumper as strongand stiff as possible which will be able to provide the protection necessary for the electronicparts in the satellite. Yet, there¶s a limitation to the weight and cost of the bumper as well asother considerations. This makes the design a very challenging works for the engineers andscientists.

2.6 Conventional Design

Fig 2.6.i Conventional Metal Design, Manufacturing and Testing

Before the usage of composite materials, the walls of satellite are made of lightweight alloys, mainly aluminium alloys due to its low density. Fig 2.6.i shows the sequencesof a conventional metal design, manufacturing and testing. As an example, the 1 st satellite

Sputnik 1 by Soviet Union, it is shield with aluminium-magnesium-titanium AMG6T.However due to the limitation in weight, the protection given are usually not enough. Someof the satellite don¶t even include bumper in the design to minimize the weight of the satellite.



2.7 Comparison

This part discussed on the assessment done to compare the effectiveness of compositematerials as the bumpers. The comparison is done by comparing the composite with an equal-weight aluminium alloy for both of the inner and outer bumpers. The test carried out was

mainly HVI tests.

2.7.1 Outer bumper

The dual-wall systems with Kevlar and Graphite/Epoxy (Gr/Ep) outer bumper was comparedagainst an equal weight all aluminium dual wall systems. This test was carried out bySchonberg in1990. It is found that aluminium bumper is more effective spreading out thedebris created by the initial impact. This is because Kevlar bumpers prevented completebreak-up of the projectiles, which decrease the dispersion of debris cloud fragments, thusincreasing the likelihood of pressure wall perforation. The pressure wall damage in the Gr/Eplayer is more wide-spread compare with Kevlar layer. Perforations in Gr/Ep consist of severalsmall holes while Kevlar shows a large hole. From these result, brought to conclusion thatusing a laminated composite as the outer bumper does not offer any protection advantage ascompare to the protection provided by aluminium.

Similar studies were done by Christiansen (1987), Colombo et al. (2003) and Li et al. (2004).All the studies showed that composite material bumpers only give slight improvement in theouter bumper.

2.7.2 Inner Bumper

Similarly, the response of triple-wall system with Kevlar and Spectra inner bumpers was

compared against all metallic triple-wall system. This study was done by Schonberg andWalker at 1991. In most of the test, it is showed that Kevlar panel were not perforated, whileboth aluminium and spectra panel were perforated. However, the pressure walls in Spectrapanel show little or no damages. These results clearly show that composite materials doincrease the protection of spacecraft against MMOD impacts.

Similar studies were carried out using different composite materials and design under HVI.Some of it included a double-bumper shield with a glass reinforced metal laminate (Lambertand Schneider, 1995) and hybrid Nextel/aluminium multi -shock shield (Christiansen, 1993).The studies show that composite shows superb results as the inner bumper.

The studies also show that combination of composite bumper with aluminium bumper produced less damaging. This is due to the reasons that it will increase the efficiency of converting the kinetic energy into internal thermal energy. The combined walls were lesssensitive to impacting projectile shape and obliquity. It will result in less cumulative damageto the pressure wall.



2.8 Conclusion

Protection of satellite against MMOD impact is a very important issue in aerospaceapplications. Under MMOD environment, orbiting satellites are exposed to threat by impactof space debris and micrometeoroid with HVI. However, designing the protection system is

very challenging because of many limitations and considerations such as AO degradation,and weight. Using composites as the main materials are very promising due to the highstrength-to-weight ratio, and the thermal stability. Several good selections included Kevlar fibres, Graphite/epoxy and Spectra fibre due to their high strength-to-weight ratio.

The fabrication techniques that widely used for such application are prepreg lay-upprocess. This is because prepreg lay-up allows fabrication of high fibre-volume-fraction andcomplex parts. The prepreg lay-up included few main processes which are preparation of prepreg, cutting and arranging the prepregs on a mould, vacuum bagging and autoclave. It isnoted that aerospace applications required extra care and precautious by the workers.

The application consisted of wall system with inner bumper and outer bumper. After comparison done with conventional aluminium multilayer wall, shows that using laminatedcomposites as the outer bumper didn¶t offer any protection advantages. In other hand, usingcomposite materials as the inner bumper able to improve the protection against MMODimpacts. It is also found that combination of aluminium layers in the wall system able toimprove the overall performances.

As a conclusion, designing a wall system protecting the satellite from MMOD usingcomposite materials are very challenging. Many research and development and still requiredto create better and lighter composite materials for aerospace applications. However, it isvery potential materials as there are limitless combinations and design are available, beingcreated from time to time.

Protection against MMOD

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