View source for Electric Propulsion
== '''Summary''' == The Enterprise will use electrical propulsion as it's main driving force. The electricity will come from [[Nuclear Power|Nuclear reactors]] located in the secondary hull or the nacelles.<br /> Electrical drives are not new technology. They have been available for many years. However, lack of a suitable mission and specially, lack of an adequate power source has hampered their development. Although electrical propulsion is not very powerful, it is exceptionally efficient, and can be applied for very long periods. So despite tiny accelerations, the Enterprise will eventually reach very high speeds.<br /> This page covers the characteristics of the electrical propulsion systems required for the Enterprise to complete it's various [[Mission|missions]]. It also covers limitations of current technologies, and the research required to develop electrical propulsion to the level required by the Enterprise.<br /> '''Propulsion design spreadsheet'''<br /> https://docs.google.com/spreadsheet/ccc?key=0Am4pITAox8pIdGJaM2t6RzNfUXFvUmZ3SEk0OTRJY0E#gid=0 <br /><br /> [[File:BTE_Power_diagram_260_MW.jpg|500px]]<br /> :::Enterprise main power train and coolant diagram<br /> [[File:Power_train2.jpg]]<br /> :::Main power train model.<br /> == '''Theory''' == '''Propulsion equation'''. This is the equation that links the electrical power, the propulsive force and the ISP.<br /> '''Pe = F x ISP *9,81 / 2n or Pe = F x Ev /2n''' Pe = electric power (Watts) F = Engine force (Newtons) ISP = Specific impulse (Seconds) n = efficiency of electric engine 9,81 conversion factor Ev = Exhaust velocity (m/s) '''Specific impulse'''. <br /> '''ISP = Ev / 9,81''' ISP = Specific impulse (Seconds) Ev = Exhaust velocity (m/s) '''Fuel consumption'''<br /> '''m = F/Ev''' m = Fuel consumption (kg/s) F = Force (Newtons) Ev = Exhaust velocity (m/s) '''Velocity'''<br /> '''V = Ev x ln(Mo/Mf)''' Ev = Exhaust velocity (m/s) V = ship final velocity (m/s) ln = natural logarithm Mo = Initial mass of the ship (kg) Mf = Final mass of the ship (kg) == '''Key performance parameters''' == '''Propulsive force (Thrust)'''<br /> The propulsive force is the force the engines produce to push the ship. This is factor of the available electrical energy, the engine efficiency and the propulsive system ISP and ejection velocity. For the Enterprise: Available electrical energy = 2,5 GWe<br /> ISP = 5000<br /> Efficiency (n) = 80%<br /> Thrust = 2 500 000 000 W x 2 x 0,8 / (5000*9,81) = 84 000 N = 8 300 kg = 8,3 Tons '''Orbital calculations'''<br /> 90 day transit times is the goal for the trip to Mars. However, due to orbital mechanics, the return trip may be longer. See reference 2.2. == '''Materials''' == == '''Technologies''' == The Electric Propulsion systems can be characterized into the following classes: ===Electrothermal Propulsion Systems=== 1. Resistojets '''2. Arcjets''' :An arcjet heat a propellant using an electric arc rather than a chemical reaction. It is therefore a thermal engine. The ISP from an arcjet can be higher than for a chemical rocket, but remains at around 500s, one order of magnitude less than what is required for the Enterprise. http://www.nasa.gov/offices/oce/llis/0736.html 3. Microwave & ECR thrusters ===Electromagnetic Propulsion Systems=== '''1. Magnetoplasmadynamic (MPD) Thrusters''' :[[File:200_kW_MPD_thruster.jpg]] :A gas is ionised, turned into a plasma and fed into a acceleration chamber, where the interaction between an electrical current in the plasma and the magnetic field produced by electromagnets pushes the plasma up to high speeds. Vasimir is an application of this principle. :Experimental thrusters in the 200 kW range were tested and proved successful in the 1990[https://docs.google.com/file/d/16c7Am3Z9zScLBx6PP9bXyh_N1VitLVAu0bLM26N0fuHao-2H4eYyioo_tXaq/edit?usp=drive_web]. Efficiency was about 50%. Wikipedia reference[http://en.wikipedia.org/wiki/Magnetoplasmadynamic_thruster] :Electrode erosion is a problem for unmanned missions, but for the Enterprise the electrodes could be replaced by the crew. One of the interesting aspects of this technology is the high thrust density, theoretically in the 100 000 N/m2 range. At such densities, a single unit about 2m wide could provide the thrust required for the Enterprise. It seems unlikely thought that a single unit could be built to handle the power required and it is more likely that a dozen or more units in parallel would be required. The optimum solution might be to match one reactor, one unit. <br /> :This is one of the best candidates for the Enterprise propulsion. However, since it is more efficient at larger sizes, the lack of a suitable power source to test the principle in space has hampered the development of this technology.<br /> :'''VASIMR<br />''' :[[File:vasimir.jpg|300px]]----[[File:Vasimir_operation.jpg|300px]] :The Vasimr (Variable Specific Impulse Magnetoplasma rocket) engine, as per 2011, has an optimum specific impulse of 5000. Required power is 200 kW, with an efficiency of 60%, for a thrust of 6 N. 14 000 engines would be required for the Enterprise. The fuel is argon but other gases can be used. The concept should be scalable up to 500N per unit, bringing the number of engines down to 168. Operating voltage 600V DC<sub>1.4</sub>. (Sizing information required) <br /> '''2. Pulsed Plasma Thrusters''' : A material is transformed into a burst of plasma by a short lived electric arc (think of a spark plug), and the plasma is accelerated by the electric field between an anode and a cathode. This is a simple but inefficient type of thruster that, at 10% efficiency, is not suitable for the Enterprise. <br /> :http://en.wikipedia.org/wiki/Pulsed_plasma_thruster<br /> :However, in the following paper : http://alfven.princeton.edu/papers/tem_jpc2002.pdf a proposal is made for a much more powerful version, that, although still only 50% efficient, might be further upgraded to provide the required thrust. The proposed fuel would be lithium. The design is very simple, and might be very light.<br /> === '''Electrostatic Propulsion Systems''' === '''1. Hall effect<br />''' :[[File:hall1.jpg]]--[[File:hall2.jpg]] :The Hall effect thruster is a (mostly) Russian technology. Over 200 units have been flown. Engine performances are comparable to ion grid. Hall effect thrusters are physically smaller than ion grid thrusters. This is a distinct advantage for the Enterprise configuration. Wikipedia cites efficiencies up to 75%. However, thrust densities are quite low and tens of thousands of units would be required for the Enterprise. . <br /> ''' 2.Ion grid<br />''' :The ion grid thruster is a mature technology that has performances very close to the BTE mission requirements. <br /> :The current NASA model is the NEXT thruster. . The engine thrust is very small, at 0,2 N per motor, with 6,9 kW and 70% efficiency. To supply the 84 000 N required by the Enterprise 400 000 thrusters would be required. The fuel is Xenon gas. With a size of about 600mm wide per unit, about 80 000 m2 are required to mount the thrusters, a surface of 280m x 280m.<br /> :The Hipep Ion engine has an efficiency of 80% and similar characteristics than the NEXT. The HIPEP is rectanglar and can be assembled in tight grids. The model tested was 600mm x 1200mm (approx, to be confirmed). 124 000 HIPEP thrusters would be required, for an area of about 90 000 m2.<br /> <br /> '''2. Dual stage 4 grid''' <br /> :This evolution of the Ion thruster has its origins in Fusion research[https://docs.google.com/file/d/1MRQYOpWpfoVTpu177AeY1O1Lxu7Jos-8oKQ1FzbvIUFBaVvLaSZMd8rgkdhq/edit?usp=drive_web]. These thrusters were originally created as ion sources for fusion research, with very high thrust densities. The 4 grid thruster provides higher ISP, less angular dispersion for higher thruster efficiency and a much higher output per area of thruster. The test results suggested efficiencies around 70% or more with a thrust density of 60N/m2. The thrusters could fit in 2000m2, so all the thrusters could be mounted on the tail of the nacelles in an area about 7m high by 300m wide. :So this is an excellent candidate for the Enterprise drive, possibly better than Vasimir. It might allow quite quick travel times to Jupiter as well, using a higher ISP propellant, and longer accelerations. The travel time to Jupiter or Saturn would likely be cut in half. <br /> '''4. Colloidal Accelerators & FEEP''' <br /> :This is a technology in the very early stages of developement. The fuel is composed of tiny droplets of semi conductors, encased in a shell of protein. The propulsion method uses electric fields to accelerate the particles. Efficiencies may be very high. Tho envisioned market is micro satellites, but it might be possible to 'print out' large boards of these micro thrusters using micropressor production technologies and eventually reach the required thrust (with millions of thrusters).<br /> <br /> '''5. NanoFET'''<br /> <br /> [[File:nanofet.jpg]] :NanoFET propulsion is an interesting new propulsion method that promises high thrust densities and efficiency in the range of 90% or more. It has the advantage that it does not involve heat or ionisation, so it's much less damaging to the equipment and does not lose power to radiation. It probably requires no cooling either, making it much simpler than other drives.<br /> :The thrust densities of the designs being worked on today at the university of Michigan should be in the range of 10N/m2, for low ISP of less than 1000s. This would require 8000 m2 on the Enterprise, about 10 times less than for Hall effect thrusters.<br /> :The ISP can be tuned over a wide range. An ISP of about 6 000s would be just right for the GEN1 trip to Mars, and higer ISP in the range of 20 000s would be great for mission aborts and for trips to the outer planets.<br /> :The fuel would be carbon, in the form of nanotubes. It could be stored dry or in suspension in a carrying oil.[http://en.wikipedia.org/wiki/N…..n_thruster].<br /> :In other words, the drives takes carbon dust and accelerate it to very high velocities using simple electric fields between charged plates. The components are built from microchips, with millions of tiny electric launchers 'tubes'. No heat, no mess, no superconducting magnets. Best of all, it is highly scalable and can be build right now at small scale, with exactly the same efficiency as future large drives. :With a thrust density of 100 N/m2 (likely achievable at higher ISP values), an ISP of 6 000s and an efficiency of 90-95%, this would fit nicely into the back of the Enterprise radiators (about 800 m2) and provide the best propulsion system possible while staying within the bounds of today's technology.<br /> One major deal breakers may be the requirements of handling the carbon nanotubes and getting them to the drives at a sufficient rate.<br /> === Solar wind propulsion === :This is completely different technology, that replaces thrust propulsion with a mini magnetosphere, that interacts with solar wind to provide propulsion and radiation protection. === Propellants === :'''Hydrogen'''<br /> :Hydrogen is an effective propellant, very appropriate for high ISP thrusters. <br /> :'''Xenon'''<br /> :Xenon is an inert gas that is relatively easy to ionise and denser that other inert gases. It is the best choice for the fuel of most types of ion engines and may be used as well as an ionizing agent in the coolant system, to provide the required plasma for the MHD generator. Xenon is quite expensive, at about 20$ per liter (6g). At current prices, if Xenon was the sole fuel of the Enterprise, the 25 000 tons of Xenon required would cost over 80 billion dollars. A possible source of Xenon would be the oxygen plants required for the fuel of the rockets used to carry up the materials for the Enterprise into space. There are about 45 billions tons of Xenon in Earth's atmosphere, allowing for about 2 millions trips on the scale of the Enterprise trip to Mars. Due to cost and availability concerns, argon may be a better choice that Xenon.<br /> :'''Argon'''<br /> :Argon is a inert gas that composes almost 1% of Earths atmosphere. It can be used instead of Xenon as propellant for the Enterprise. It should also be possible to use it as the ionising agent in the cooling system for the MHD generator operation. The Vasimir engine uses argon as propellant.<br /> :'''Water'''<br /> :Water <br /> :'''Helium'''<br /> :Helium is <br /> :'''Liquid metals'''<br /> :Sodium, Lithium, lead, lead bismuth, mercury, :Liquid salts :'''Nanofluids'''<br /> :Nanoparticles in suspension in a carrier fluid can have interesting propulsive properties.<br /> == '''BTE project design''' == '''Google docs spreadsheet for Propulsion system design'''<br /> :https://docs.google.com/spreadsheet/ccc?key=0Am4pITAox8pIdGJaM2t6RzNfUXFvUmZ3SEk0OTRJY0E#gid=0<br /> Type of engine chosen<br /> Alternative Fuel<br /> Alternative [[File:Power_train2.jpg]]<br /> A version of the nuclear power train for the Enterprise<br /> Red: Nuclear reactor<br /> Blue: Magnetohydrodynamic power generator<br /> Green: Turbine/compressor combo<br /> Dark Green: Turboalternator<br /> The coolant leaves the turbine to go to the radiators. It returns from the radiators, goes through the compressor and re-enters in the reactor.<br /> The figure at right is to scale. == '''Future research''' == Large ion thrusters have not been studied much because of the lack of a suitable power source. So powerful ion engines require the development of space nuclear reactors. <br /> Increased efficiencies are an important goal. However, for the typical Enterprise mission, increased ISP is not an absolute requirement. The Vasimir thruster offers an attractive alternative since the ISP can be tuned to fit the mission, for maximum efficiency. However, this may also be true for most ion engines, although for a narrower range.<br /> The physical size of the engines is a problem, since the numbers required for most types of electric engines will have difficulty fitting in the space available. This might impact the physical aspect of the GEN1 Enterprise. == '''References''' == === Electrical propulsion === A good overview of the choices. Jordan[https://docs.google.com/file/d/15FUPqG-HaK9WxHAHhsIFAUTrvxx8_p6M1brdCZkp0r5oVtaPbAIJcdh5x0_E/edit]<br /> Another good overview of electric propulsion, very systematic and complete. Jahn, Choueiri[https://docs.google.com/file/d/14hTRuxjuqgX79BlEEQCFqZ8afYKESFMmDgWVVyUn8jFiVXTNYXljxm1KVcGU/edit]<br /> Analysis of continuous electric propulsion for Mars mission[http://erps.spacegrant.org/uploads/images/images/iepc_articledownload_1988-2007/2005index/219.pdf]<br /> 1.4- MW class electric propulsion system design; http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120001636_2012001192.pdf<br /> 1.5- NanoFET, Thruster on a chip, nanoparticle fuel, electric acceleration, 100 to 10 000 ISP, 90% efficiency; http://www.engin.umich.edu/newscenter/pubs/engineer/engineerfeatures/thruster-on-a-chip<br /> 1.6- NEXT, ion engine, 4190 ISP, 6,9 kW, 0,236 N; http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080047732_2008047267.pdf 1.7- 20 kW ion engine http://www.alta-space.com/hiper/publications/SP2010_paper.pdf 1.8- Mini magnetosphere propulsion:<br /> https://engineering.dartmouth.edu/~d76205x/research/Shielding/docs/Winglee_00.pdf<br /> 1.9- Vasimir, lots of good info <br /> http://nextbigfuture.com/2007/11/vasimr-engines-plus-200-mw-of-nuclear.html === Constant and variable thrust orbits === The excellent Wikipedia page on this subject.[http://en.wikipedia.org/wiki/Delta-v_budget]<br /> Vasimir orbits, including the use of variable ISP. This is the famous 39 days to Mars paper[https://docs.google.com/file/d/16LbymGypNk5RnGnNVxMRS_3eHEq2Myv6OdzUBsJIyK0Rc3plxVHfDT3XIAiA/edit?usp=drive_web].<br /> === 3- Low energy orbits === 3.1- Excellent review and explanation of quick orbits[https://docs.google.com/file/d/1jH35MOq0aa1ua07c7pwK3n_fDJk15_W4BUYMvPTEETFgtztMUSLciRYZ_d0X/edit?usp=drive_web]<br />
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