AnEvaluationofAlternativePropulsionSystemsforFutureSpace

Exploration

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Table of Contents

Introduction 2Criteria 2MagnetoplasmadynamicThrusters(MPDTs) 4Overview 4HallEffectMPDTs(HETs) 4PerformanceandApplicationofMPDTs 5LifetimeLimitingFactorsofMPDTs 6

ElectrostaticIonThrusters(EITs) 6Overview 6PerformanceandApplicationofEITs 6LifetimeLimitingFactorsofEITs 7

PulsedPlasmaThruster 7Overview 7PerformanceandApplicationofPPTs 7LifetimeLimitingFactorsofPPTs 8

SolarSails 8Overview 8PerformanceandApplicationofSolarSails 9LifetimeLimitingFactorsofSolarSails 9

Evaluation 9Conclusion 10Bibliography 12Glossary 16

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Introduction In the early years of space exploration many people dreamt of travelling to distant planets and other solar systems. They dreamt of boarding a spaceship and arriving at some interstellar destination in just a matter of days, of visiting neighbouring solar systems, exploring the outer edges of our galaxy and even beyond. This has remained very much the subject of science fiction, and although there have been many advancements and improvements in space technology, we seem no closer to our nearest neighbouring star Proxima Centauri than we were 53 years ago when Yuri Gagarin became the first human ever to leave the Earth’s atmosphere. In fact it has been 42 years since man last stepped foot on the moon. The biggest challenge faced by modern space exploration is the longevity of the spacecrafts. Most rockets use chemical propulsion, combusting a flammable propellant to generate thrust. Although this process is very good at producing a large amount of thrust, typically in the range of mega-Newtons, chemical propulsion is extremely inefficient. This is illustrated by looking at the ratio of the payload mass to the total mass. For example the Saturn V rocket used in the Apollo lunar project had a payload which was just 1.6% of its total mass. Because chemical rockets burn their fuel so quickly, they are unable to continue thrusting and will typically ‘coast’ through space, firing only occasionally to adjust the course of the craft. Building a chemical rocket with continuous thrust capabilities would be incredibly expensive and almost completely unfeasible. This means many space missions are one way. Return trips, such as those carried out to the moon by the Apollo space program usually utilise gravity to slingshot around a celestial body, rather than changing course by firing its own thrusters and reversing its direction. Making missions as economical as possible is another major consideration in space travel. Usually spacecraft are not reused and so each mission requires new hardware. Reducing the cost of missions would allow us to launch probes more frequently. Electric propulsion seems to present a viable alternative. First suggested in a paper by Konstantin Tsiolkovsky dated 1911[1], electric propulsion theoretically produces specific impulses a whole order, or sometimes two

orders, of magnitude greater than that of traditional chemical propulsion. In this essay I will examine what criteria must be fulfilled in order to make long distance space travel more viable. I will first look at various forms of alternative propulsion before comparing them and drawing a conclusion as to which method I believe is best suited to long-duration spaceflight. Throughout the EPQ I refer often to the raw thrust and specific impulse produced by the various forms of propulsion. In reality this is not necessarily the best form of comparison as the various thrusters will operate at different powers, and will have different dimensions. It would be almost impossible, however, to compare thrusters of the same size, with like for like operating powers. Simply comparing the raw thrust and specific impulse produced by the thrusters is a good enough measure to know whether these values are of the same order of magnitude. Throughout my research I have tried to use a few of the best performing examples to demonstrate the capabilities of that type of thruster. It is understandable that a 50 year old design is not as good as a 5 year old design, as technology has moved on. By making examples of the best individual thrusters documented, I am displaying the optimum performance we are able to produce from the theory of that particular design in the modern age.

Criteria For the application of long-duration spaceflight there are a number of criteria that the chosen thruster must meet:

• Firstly, it must have a sufficiently long lifetime. The structure of the thruster should be durable and reusable so that the spaceship may return to Earth and be reused for successive missions. This is important because one of the biggest factors making the exploration of space uneconomical is the fact that most rockets are not reusable. The thruster must also have a long thrusting lifetime, meaning that the thruster must be able to produce thrust for more or less the entire duration of the space flight. This is inherently important in electric propulsion because the thrust produced is so much smaller than that produced by chemical propulsion. Consequently the thrust must be applied for longer

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to reach a high speed, currently chemical rockets fire for just 5-15 minutes[2,56].

• It must at least have been tested in the laboratory, and had some of its performance values measured experimentally. I have chosen this criterion to help separate out forms of propulsion that are only in their theoretical stage. There is very little useful or technical information available on these thrusters and so it would be difficult to include them in my EPQ.

• It must have a specific impulse that is not matched by any form of chemical propulsion. Specific impulse can be thought of as a measure of efficiency for spacecraft, much like miles per gallon in a car. Having a high specific impulse will help to improve the economic feasibility of long-distance space travel.

Specific Impulse Specific impulse can be thought of as a measure of efficiency in spacecraft. One of the most attractive features of electromagnetic propulsion is its incredibly high specific impulse. Specific impulse describes the change in momentum delivered per unit mass of fuel. For a given mass of fuel, a large change in the momentum of the spacecraft would mean it had a high specific impulse. I will now show you how we derive impulse mathematically for an object of constant mass. Electromagnetic thrusters can be though to have constant mass as the mass of propellant on board the spacecraft is usually very small in comparison to the total mass of the spacecraft. Symbol Corresponding Value

𝑝 Momentum 𝑚 Mass (ambiguous) 𝑚# Mass of Fuel 𝑣 Velocity 𝑢 Initial Velocity 𝐼 Impulse 𝐼'( Specific Impulse 𝐹 Force (ambiguous)

The momentum of an object is given by its mass multiplied by its velocity:

𝑝 = 𝑚𝑣

So the change in momentum (impulse) is the momentum after the event subtract the momentum before the event (in this case the event is the expulsion of propellant):

𝐼 = 𝑚𝑣 − 𝑚𝑢 Which can be factorised to give:

𝐼 = 𝑚(𝑣 − 𝑢) As it is the change in velocity, (𝑣 − 𝑢) can be written as ∆𝑣, so the impulse becomes:

𝑰 = 𝒎∆𝒗 We can see that the impulse, or change in momentum, is equal to the mass of the object multiplied by its change in velocity, with the units 𝑘𝑔𝑚𝑠67 The specific impulse is the impulse generated per unit mass of propellant, mass is measured in kg, so specific impulse is given by:

𝐼'( =𝐼𝑚#

𝑚# is the mass of fuel used as oppose to the mass of the thruster, which is 𝑚. This gives specific impulse the units of velocity 𝑚𝑠67 . However it is more commonly given with the unit seconds (𝑠). This is because in the 70s scientists on opposite sides of the Atlantic Ocean were using different systems of measurement. American scientists would use lb and ft, whereas European scientists were using kg and m. By dividing their 𝐼'(, by the acceleration due to gravity at sea level (in their chosen units), both scientists would end up with specific impulse in seconds, a unit everyone agreed on. I would also like to look briefly at how specific impulse is calculated experimentally, so that later on you will understand where the values have come from. To measure the specific impulse we first must look at Newton’s second law, 𝐹 = 𝑚𝑎. Acceleration is the rate of change of velocity and so Newton’s law can be written as:

𝐹 = 𝑚Δ𝑣Δ𝑡

If we then multiply up by Δ𝑡 we get:

𝐹∆𝑡 = 𝑚∆𝑣

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You should recognise 𝑚∆𝑣 from the equation for calculating the impulse. And so:

𝐹∆𝑡 = 𝐼 The impulse can be found using calculus by setting limits where Δ𝑡 ↦ 0 , giving the integral:

𝐼 = 𝐹 𝑑𝑡

Therefore if we plot a graph of 𝐹 against 𝑡, the area under the graph is equal to the impulse. Thus we can calculate the specific impulse by dividing this area by the mass of propellant burnt.

To give an example, a typical liquid chemical propulsion rocket only tends to generate a maximum specific impulse of around 300-400s [3,4,5].

Magnetoplasmadynamic Thrusters (MPDTs)

Overview

The structure of the thruster consists of a thin, central cathode surrounded by a cylindrical anode which forms the discharge chamber. At the back of the discharge chamber there is an insulator plate, through which the propellant is

injected. MPDTs can be operated using a variety of propellants, the most common are hydrogen, argon or lithium [7,8,9]. After the propellant is injected it is ionised by a beam of high-speed electrons flowing from the anode to the cathode. The charged ions are then accelerated by the Lorentz force. The force acting on the particles means there is an equal and opposite force which acts on the thruster, causing the thruster to accelerate in the opposite direction. The Lorentz Force, which is the principle behind MPDTs is a force that accelerates a charged particle as it moves through perpendicular electric and magnetic fields. In an MPDT the magnetic field can be provided in one of two ways, the two designs are referred to as self-field thrusters and applied-field thrusters. The high current flowing down the cathode creates the magnetic field for the self-field variation. Whereas with the applied-field system, the plasma is accelerated by an external magnetic field [6,7]. Other variations of MPDTs include steady-state and quasi-steady state thrusters. Steady state thrusters fire continuously, much like traditional chemical propulsion, quasi-steady thrusters fire in short, repeated bursts of around one thousandth of a second. This makes it much easier to operate the thrusters at a higher power, as the power level does not need to be sustained continuously.

Hall Effect MPDTs (HETs)

Hall Effect Thrusters are the most common form of Magnetoplasmadynamic thruster. The Hall Effect is a result of the Lorentz force, which I mentioned earlier. The Lorentz force acts on a charge moving through a magnetic field.

We can explain the Hall Effect by looking at the flow of electrons through a wire in a

02468101214

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Force/N

Time/s

B

F

e-

(Fig. 3) The Lorentz force (F) acting on an electron moving through a perpendicular magnetic field (B)

(Fig. 1) A visualisation of the sort of graph that you might plot to calculate the total impulse, please note that this graph is not supposed to represent actual experimental values.

(Fig. 2) The basic design of an MPDT. (Jahn & Choueiri, 2002)

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magnetic field. As the electrons flow, they are deflected to one edge of the wire by the Lorentz force, as a result this side becomes negatively charged and therefore the opposite side becomes positively charged. This causes a potential difference to exist across the wire. The nature of this potential difference, the orientation of the positive and negative terminals, opposes the magnetic field. The attraction experienced by the electron towards the positive charge is cancelled out by the Lorentz force exerted as a result of the magnetic field. Hence the electrons are able to continue flowing through the wire. This perpendicular potential difference is called a Hall current [14]. In HETs, the Hall current exists as electrons circulating around a central electromagnet solenoid. The electrons are first produced downstream by a hollow cathode, they are then caught by a radial magnetic field where they orbit and ionise the propellant that is injected from the anode. The charged ions experience a force from the electric field, and are accelerated out of the thruster. The propellant ions are not affected by the magnetic field as they have much more mass than an electron. As the positively charged ions are ejected they tend to pull electrons with them, electrons are highly mobile. As a result the thruster remains electrically neutral [15].

The most common type of Hall effect thrusters are axisymmetric with a magnetic coil at the centre of a cavity, which is surrounded by a second magnetic coil. The coils form an electromagnet that generates the radial magnetic field.i

i During my research I thought that perhaps nuclear reactors could provide enough power. After investigating this a little further I found that this is in fact being considered and there are a number of articles published on this matter. However, this is not the focus of my EPQ.

HETs have already proven to be extremely practical with several Russian satellites operating an HET system [19].

Performance and Application of MPDTs

The main advantage of the MPDT is that of all the forms of electric propulsion, MPDTs can produce the highest thrust values, generally 10-100mN [8,10,11], (but with the potential to produce as much as 100-200N [7,11]). MPDTs can also maintain the high specific impulses that are associated with electric propulsion. Generally MPDTs produce specific impulses in the range of 1000-5000s [16,17,18,40]. However the major advantages of the MPDT have also hindered its progress. Producing high thrust values requires a large amount of power, power that cannot yet be generated on board a spacecraft. In fact, very little testing has been conducted on steady-state MPDTs which demand very high powers indeed. Instead much of the research uses quasi-steady state thrusters. Testing in this way can be carried out at powers of up to 6MW, whereas the highest power at which a steady-state MPDT has been tested is just 600kW [9,10]. Although MPDTs can operate at lower powers, their efficiency drops significantly and so to maintain a good efficiency they must be operated at high powers. MPDTs also have the advantage of being able to carry a large payload; typically just 10% of the mass of a chemical rocket is cargo, whereas a 40kg MPDT was tested using less than 130g of hydrazine propellant [11]. I believe MPDTs have a huge amount of potential, however due to the issue previously mentioned where the efficiency drops at low powers, there is only one recorded instance of an MPDT being tested in space. The Japanese EPEX experiment was launched on March 18 1995. Over the few days which were allocated for the experiment the EPEX MPDT fired over 40,000 times (it was a quasi-steady state variation). According to Toki, Shimizu and Kuriki, the results from the experiment in space agreed well with performance that was measured during laboratory testing. Indicating that MPDTs are a very viable option for the future of space exploration. In my opinion MPDTs are one of the most promising forms of EM propulsion, despite the current lack of practical systems for generating enough poweri. This is because so much time and money is put into energy research

(Fig. 4) Axisymmetrical Hall Effect Thruster. (Komurasaki Koizumi Laboratry)

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nowadays it cannot be long before a compact, efficient system is developed to operate on board spacecraft and generate large amounts of power. The advantages of MPDTs, mainly their high thrust values in comparison to other forms of EM propulsion, make them prime candidates for the main propulsion method on long-distance space missions.

Lifetime Limiting Factors of MPDTs

To a large extent, the cathode is the limiting factor in the performance and lifetime of most propulsion systems. The cathode is crucial as it generates the electrons that not only ionise the propellant but also that neutralise the plasma discharge. Early thrusters often used tungsten filaments as the cathode, however due to the high work function of tungsten, these cathodes required a lot of power, this meant that tungsten cathodes operated at very high temperatures and they would typically evaporate rapidly with a lifetime of just hundreds of hours [13a]. To be of any use and to be able to satisfy my first criterion the thruster must have a lifetime significantly longer that hundreds of hours. Fortunately a new type of hollow cathode were later developed which significantly extends the lifetime of most forms of EM propulsion to the thousands of hours required in my criteria [39,13b.]. Developments in using a mixture of lithium and barium propellants has proven to help solve the problem of cathode erosion, further increasing the potential lifetime of MPDTs [54,55].

Electrostatic Ion Thrusters (EITs)

Overview

Electrostatic ion thrusters (EITs) operate using electrostatic attraction. In essence, charged ions are accelerated towards an oppositely charged plate where they pass through small holes and are ejected from the thruster before being neutralised by an electron gun. Similarly to MPDTs, the neutral gas atoms are injected at the back of the discharge chamber where they are ionised by electrons that are ejected from the cathode. However unlike other forms of EM propulsion, after being ionised, the positive ions drift, by diffusion, toward the electrostatic grids at the downstream end of the thruster. Once they enter these grids, the ions are accelerated by the potential difference between the positive and negatively charged plates. The positive

plate acts to remove electrons from the discharge chamber. After leaving the thruster, the charged plasma stream is neutralised by an electron gun that fires electrons away from the thruster into the plasma beam. This prevents the thruster from becoming charged, which would cause the thruster to attract the ions back towards itself. The electrons from the electron gun are prevented from re-entering the thruster by the large negative charge on the downstream accelerator plate. The magnets surrounding the discharge chamber redirect any electrons that drift towards the chamber walls. This increases the chances of the gaseous atoms being ionised. Towards the end of the life of the thruster the potential difference across the grids drops. This means that electron backstreaming is possible, leading to corrosion of the electrostatic grids and eventual failure of the thruster [21,22,57].

Performance and Application of EITs

Currently the primary application for EITs is as attitude control on board satellites. Such thruster produce thrust values of between 1mN and 100mN [22,23] and with specific impulses of between 2600 and 3000s [22,23,41]. EITs are among the most commonly used forms of electric propulsion, having been tested on numerous satellite systems in LEO as well as other spacecraft such as the NSTAR EIT which was used on board the Deep Space 1 probe.

(Fig. 5) Electrostatic ion thruster. (Oona Räisänen)

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Lifetime Limiting Factors of EITs

The lifetime of an EIT is limited primarily by erosion of the accelerator grid. As the grid becomes more and more worn it’s effectiveness decreases and it no longer accelerates the ions by such a large amount, resulting in a decrease in thrust. Additionally, as the accelerator plates erode they produce a smaller potential difference and so there is more electron backstreaming, accelerating the end of the life for the thruster. There are a number of ways in which the accelerator grid is eroded. One of which is by electron backstreaming. The electrons that are produced by the electron gun downstream of the accelerator grids are attracted back towards the discharge chamber by the positive plate, however this is largely reduced by the large potential difference on the negative accelerator grid, which repels the negative electrons. Towards the end of the thruster’s life this potential drops and more electron backstreaming occurs [26]. During a ground test conducted on NASA’s NSTAR ion thruster, it was found that after an operational period of 30,000 hours the accelerator grid was almost completely destroyed [12]. This is a result of a current of secondary ions generated downstream of the discharge chamber. They are generated when charge is exchanged, in the form of an electroni between ions in the plasma propellant beam leaving the discharge chamber and the neutral plasma exhaust. This exchange results in a fast moving, neutral atom in the plasma propellant beam and a slow moving ion in the plasma exhaust. These slow moving ions are attracted to the negatively charged accelerator grid, which is used to accelerate the plasma propellant beam. Most ions hit the grid with enough energy to erode it over time [13a.].

Pulsed Plasma Thruster

Overview

Pulsed plasma thrusters (PPTs) are possibly the most basic form of electric propulsion, they were also the first form of electric propulsion to be flown in space, on the soviet probes Zond 2 and 3. I ii n a PPT energy is stored in a capacitor, the capacitor is then discharged to produce an arc of electricity with an extremely high voltage. The arc causes the solid

i Note, this is a different process to electron backstreaming.

propellant to sublime and ionises the gaseous atoms to form plasma. As the ions move between two charged plates the circuit is completed by the electric arc, allowing a current to flow. This produces a magnetic field and so the same Lorentz force as described in the section on HETs accelerates the charged ions to a very high velocity, generating an impulse and hence a force.

The arc is repeatedly sparked, the pulsing usually occurs at a very high frequency producing an approximately constant thrust. Consequently the thrust can be varied by changing the size of the capacitor discharge or by increasing the frequency of the discharge. As the solid propellant sublimes more propellant is moved toward the discharge chamber by a feed spring. There are also PPT variants that use gas propellant.

Performance and Application of PPTs

A typical pulse will eject something in the region of 10-8 kg of propellant per pulse at a frequency of 5000 Hz. This propellant is ejected at a velocity that has been shown to produce a specific impulse of up to 16,600 s in a single pulse. This is much higher than the specific impulse produced by any other form of propulsion I have researched. However, due to the large amount of energy lost during ablation, mainly as heat, the specific impulse is drastically reduced and in practice PPTs are in fact some of the worst performing thrusters, generating specific impulses in the region of just 800-1600s

[28,29,44]. Similarly to other forms of ion engine, PPTs generate low thrusts, in the region of 10-3 N [25,27]. Currently the fairly high specific impulse and simplicity of PPTs makes them ideal for use on board satellites. Most commonly they are used for attitude and altitude adjustments. The most important feature for an ACS is a small impulse bit, the airbus systems used for three decades until the late nineties had minimum impulse bits of magnitude 0.1 Ns, however

(Fig. 6) A basic PPT (David Darling)

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PPTs can generate impulse bits of magnitude just 10 𝜇Ns [28,29].

Lifetime Limiting Factors of PPTs

The limiting factor in the lifetime of a PPT is the capacitor lifetime. However the lifetime of the capacitor is often much longer than that of the mission and so is not of particular concern and in fact, the propellant will often run out before the capacitor is spent. Meaning there is very little limiting the lifetime of these thrusters [30,31]. As an example of the longevity we only have to look to NASAs EO-1 spacecraft, which was launched in 2000. Designed to fly for a year and a half EO-1 continues to collect data 14 years on[47,48,49].

Solar Sails

Overview

Solar sails are a form of propulsion that relies upon solar wind pressure to generate thrust. Most commonly the designs feature a flat, square sail with four booms extending to each of the corners of the sail. The sail is attached to the boom at various points along the length of each boom. This configuration is highly scalable as more attachment points can simply be added when a large sail is required. Alternative designs, where the boom is attached only at the corners of the sail, resembling a parachute or at the centre where the four booms intersect, are not as scalable because the tension greatly increases when the sail becomes large

The sail itself is constructed from a very thin film such as aluminised Mylar. Mylar is a film commonly used in the electronics industry and is readily available [32,33].

There are however, several other solar sail designs, mainly a heliogyro configuration and a disc sail. The three designs each require very different mechanisms to control their stability.

Square sails rely on smaller sails at the tips of each of the booms; these can be individually controlled to produce the desired steering effect. Heliogyro sails behave much like a helicopter, where the blades spin to maintain stability, however do not confuse the spinning motion to maintain stability in a solar sail with the spinning motion which produces the thrust on a helicopter. Since space is a vacuum, thrust cannot be generated in this way. Finally, the disc sail maintains stability by moving the centre of mass relative to the centre of pressure. The solar sail is accelerated as light hits the solar sail and is reflected, though some light is absorbed and re-emitted. To accelerate the solar sail requires however a change in the momentum of the photon. Traditionally photons have no mass and so should have no momentum. However because photons travel at such high speeds they experience the effects of relativity. Relativity increases the mass of a moving object. Although we do not notice this at everyday speeds, photons move at the speed of light. When taking into account relativity, there are two masses, the rest mass 𝑚@, which is the mass of an object at rest with respect to the reference frame in which the object is being observed, and the inertial mass 𝑚, which is the mass given to a moving object. Photons have no rest mass, however relativity gives the photon an inertial mass and so momentum. We can calculate the momentum, 𝑝 , of a photon using the full version of Einstein’s famous equation 𝐸 = 𝑚𝑐C which is 𝐸C =(𝑚@𝑐C)C + (𝑝𝑐)C . By setting the rest mass equal to 0 and rearranging the equation we get:

𝑝 =𝐸𝑐

Using Planck’s equationi we can express this as:

𝑝 =ℎ𝑓𝑐=ℎ𝜆

i Planck’s equation is 𝐸 = ℎ𝑓 where 𝑓 is the frequency of the light.

(Fig. 7) A solar sail designed for NASA’s ST9 mission (L’ Garde inc.)

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Where 𝜆 is the wavelength of the photon and ℎ is Planck’s constant.

Performance and Application of Solar Sails

Unlike other forms of spacecraft propulsion where the performance can be measured in terms of thrust and specific impulse, solar sails do not produce any thrust themselves, and have no propellant. Consequently these are not characteristics that can be measured in solar sails. Instead, solar sail performance can be measured in terms of three other parameters. Sail Parameters:

• 𝜎 – The Greek letter sigma represents a parameter called the areal density. This is calculated by dividing the total mass of the sail by its area, and is usually expressed in g m-2.

• 𝑎I – The characteristic acceleration is a parameter that can be used to compare the acceleration of different solar sails, because the actual acceleration of a solar sail will vary depending heavily upon its location, the characteristic acceleration is defined as the acceleration a solar sail would experience at 1 AU from our Sun. 𝑎I is related to the areal density by (assuming a typical 90% efficiency)[37]:

𝑎I =8.28𝜎

• 𝜆 – The final parameter is the lightness number, and is a ratio of the maximum acceleration divided by the sun’s local gravity. At 1AU from our Sun, this is given by:

𝜆 =𝑎I5.93

I have chosen to focus on the square sail configuration, as it is generally the most popular amongst astronomical aeronautical organisations and individuals. They also have no major disadvantages over the other designs, but offer many advantages. Mainly they are the most structurally robust, and do not require any external inputs to function, such as the rotating heliogyro. Further development has also made the storage of square sails quite efficient. Their only major disadvantage is comparable mass. As the booms are absent from the heliogyro configuration [34].

Because solar sails do not carry any propellant, they effectively have an infinite specific impulse; the only factor which affects how long they can accelerate for is how long they receive light for. Obviously the further from the sun the sail is the smaller the force accelerating it becomes, as the light intensity decreases. However, for the case of deep space missions laser beams have been proposed, which could follow the sail and provide it with continuous thrust, even beyond out solar system where the light from the sun becomes much dimmer. Current solar sail designs can have 𝑎I values of 0.2-1.6 mm s-2 and areal densities of 4-20 g m-2 [35,36]. A solar sail measuring 80 x 80m will typically experience around 5 N of force at a distance of 1AU from the sun, which is higher than the thrust that is produced by most forms of electric propulsion [37].

Lifetime Limiting Factors of Solar Sails

With no moving parts or propellant, solar sails also have extremely long lifetimes, with the only limiting factor being damage from an external source. The first ever solar sail to be launched in space was as part of the IKAROS probe. The probe was launched in 2010, and has operated for a total of more than 40,400 hours [46].

Evaluation

The various alternative forms of propulsion each have their merits and drawbacks and as such are suited to a variety of applications. However my investigation principally looks for thrusters that are suited to being the primary thrust source for deep space missions. Although they are one of the most popular forms of electric propulsion on board satellites, I do not believe that PPTs can ever be as well suited to fit the criteria I have chosen as the

𝑇𝑦𝑝𝑒 𝐼'((𝑠) 𝑇ℎ𝑟𝑢𝑠𝑡(𝑚𝑁) 𝐿𝑖𝑓𝑒𝑡𝑖𝑚𝑒 ℎ𝑟𝑠

MPDT (HET)

1000-5000 [16,17,18,40]

10-100 (10,000)

[8,10,11]

1200-1300 [38,40]

(10,000-50,000) [54,55]

EIT 2600-3000 [22,23,41] 1-100 [22,23,42] 30,000 [12,43]

PPT 800-1600 [25,27,45] 1-10 [25,27,44] 122,000 [47,48,49]

Solar Sail ∞ 1000-10,000

[37] > 40,000 [46]

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three other forms of electric propulsion I have investigated. My principal reason is that they simply cannot produce anywhere near the same thrust as MPDTs, EITs or solar sails. Also, being fairly simple devices, there is little room for improvement. Compared to the other propulsion mechanisms their only advantage is their long lifetime, however this is rivalled by that of solar sails and the sole reason it has not been equalled is simply because there have been no solar sails in space long enough, with the earliest launch of a solar sail being 2010. I strongly believe that the IKAROS solar sail will go on to match the lifetime of any PPT. One of the strongest candidates for space exploration is the MPDT, which theoretically could produce hundreds of Newtons of thrust whilst still maintaining a high specific impulse. The only issue faced by MPDTs is their longevity, which is less than that of the other propulsion methods; this is generally a result of cathode erosion. However numerous solutions have been suggested which could reduce the cathode erosion, including a lithium-barium propellant [54,55]. The only other major issue faced by MPDTs is finding an on board power source, as the require vast amounts of power to function. As previously mentioned this is most likely to come in the form of a small nuclear reactor, much like the SNAP reactors tested by NASA in the 1960s, though these were unable to generate enough power to supply an MPDT. However in 2012 Russia announced developments on a nuclear reactor that is designed to provide power to a range of electric propulsion systems. They believe it will be available by 2017 and would be capable of producing 100 MW of power [50,53]. MPDTs also produce the highest specific impulse of any form of electromagnetic propulsion, making them very efficient. More recently in December 2014 NASA announced that their 1kWe nuclear space reactor had successfully passed it’s proof-of-concept testing [51,52,53]. EITs seem to occupy a kind of middle ground. In that their thrust is comparable to the actual thrust produced by MPDTs today (note it is not the same as the theoretical maximum thrust of MPDTs) but their lifetime is significantly longer. Currently EITs are preferable to MPDTs due to their longer lifetime, however with the recent improved understanding of how the propellant affects the erosion of the cathode I believe that MPDTs will soon be able to outperform EITs in every respect.

To this extent I believe that of the three forms of electromagnetic propulsion that I have investigated, MPDTs are the most promising for the future of space exploration. Not only do they have the highest specific impulse, which is the most important factor in economical long-duration missions, they also currently produce thrust to match that of any other form of electromagnetic propulsion and could theoretically produce thrust one hundred times greater. Furthermore, with the rapid rate of development in nuclear and propellant technology I believe that it will not be long until their lifetime is extended significantly. The huge advantage of solar sails is of course their infinite specific impulse. As they do not have a propellant to run out of solar sails can feasibly produce thrust forever. The thrust they produce is also quite significant, with a 80m solar sail producing as much as 5N of thrust. In comparison, the International Space Station measures 108.5m x 72.8m, so in terms of spacecraft an 80m solar sail is not too large. Designs have been proposed for solar sails that are several kilometres across that would produce several hundreds of Newtons of thrust. Due to the necessity for light however, solar sails can only be operated within a certain range of the sun before they become ineffective. Technologies involving lasers directed at the sail have been proposed but are in reality a long way from completion. I believe that in the future, when these laser technologies are realised, solar sail could well be the propulsion system of the future.

Conclusion As a result of my research, I believe that the future of space exploration will be reliant on new, more efficient technologies that allow us to travel for longer to deeper parts of space. The distinct advantage that MPDTs and solar sails provide us with it the ability to accelerate spacecraft for thousands of hours, unlike chemical propulsion which last just minutes. Over the many months or years that this acceleration is applied the spacecraft is able to reach vast speeds, which are unimaginable with chemical propulsion. Allowing us to travel to deep space faster and for a lower cost than ever before. Ultimately this will allow us to increase the number of space missions being launched and allow us to collect an incredible amount of information about our solar system and beyond. Being lightweight makes solar sail extremely cheap to put into space, and once they are there

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solar sails fly free, requiring no propellant. They can also continue to operate for as long as needed and with no moving parts and very few electrical systems there is little to go wrong. Solar sails can even be made to tack towards the sun meaning that once a solar sail mission is complete it can be returned to earth for alterations or repairs and reuse In my opinion, in an era where efficiency and energy usage are some of the biggest problems faced by man, solar sails, which require no propellant whatsoever, are the most promising form of propulsion. Their functionality and simplicity is unrivalled, and their performance is unbeaten. MPDTs have the potential to become very effective means of propelling spacecraft, and are considerably more compact than solar sails. However currently they are in the early development stage and so the time of using MPDTs as the primary source of thrust is a long way off. MPDTs have the potential to rival solar sails and I believe that the future of space exploration will include solar sails and MPDTs working alongside each other in use for different applications.

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Glossary ACS Attitude Control System Attitude the orientation of an object with respect to an inertial frame of reference or a celestial body. Anode a negatively charged electrode. Cathode a positively charged electrode. Discharge Chamber the section in a rocket or thruster where the thrust is produced, the products formed here are ejected through the exhaust. Electrode a conductor through which electricity enters or leaves an object, substance, or region. Impulse a force acting briefly on a body and producing a finite change in momentum. Impulse Bit the change in momentum delivered per pulse. Momentum the quantity of motion of a moving body, measured as a product of its mass and velocity. Newtons the SI unit for force Payload the part of a spaceship’s load from which revenue is derived; passengers and cargo. Specific Impulse a measure of the efficiency of rocket and jet engines. It represents the force with respect to the mass of propellant used per unit time. In a zero gravity atmosphere such as space it has the units seconds. Thrust the propulsive force of a jet or rocket engine, measured in Newtons. Thrusting Lifetime the duration of time for which the thruster is capable of producing thrust, some components may last longer than this. The lifetime can be limited by factors such as the amount of propellant or the lifetime of individual components.