The Dipole Drive: A New Concept for Space Propulsion
I almost believe in this ... but no.<p>It&#x27;s just a parallel plate capacitor, with holes in the plates that allow the surrounding plasma to pass trough. The question is:<p><i>Why a plate capacitor with holes in the plates inside a diluted plasma is not a perpetual mobile?</i><p>Let&#x27;s assume that initially the device is with speed 0 inside a huge bath of plasma that is also at speed 0. We charge the two plates of the capacitor instantly and magically. In this model and the calculations the ions never hit the plates because they have holes, so the plates never discharge. There is some energy in the electric field of the capacitor.<p>Now the device is operational and it starts to move, the ions also move. Now the devices and ions have some kinetic energy.<p>* If we believe the whole idea the ions just go away, and the plates never discharge, and new plasma enters the device, so it continues to accelerate forever. So the total energy of the system increase and increase, that is something impossible.<p>* If we don&#x27;t believe the whole idea, the problem is that the ions accumulate outside the plates in some kind of ion cloud, even if they never hit the plates. Initially the effect is small, but eventually the charge of the ion clouds compensate the charge of the plates and the plasma inside the device feels no net force, so the device moves at a constant speed. Essentially, some part of the energy that was stored in the electric field was transformed into kinetic energy, but there is only a finite amount of it, so the device must reach a maximal velocity.<p>---<p>There is some complications, because in the Solar system the plasma is not static, because there is solar wind. So, just imagine that you pick a reference frame (i.e. starship) that is moving at the same speed of the solar wind. In this reference frame (i.e. starship) initially the plasma looks static and the giant capacitor is moving. When it is turn on, there is some energy in the electric field and some kinetic energy. After a while, the charges are redistributed, and the speed of the giant capacitor changes, but only a finite amount, it can&#x27;t continue accelerating forever. If you see the scene from the normal reference frame (i.e. the Sun) the device is initially static and after a while it is moving with a cloud of ions around it, but the total change in speed is the same.<p>I guess there is some drag, and the final speed in the starship reference frame is 0 and the final speed in the Sun reference frame is the same of the speed of the solar wind.<p>---<p>About other comments:<p>* I agree that the momentum transferred to the protons and electrons is different. With some assumptions the ratio is 43, but in a moving device in a moving solar wind the calculation is more complicated. The exit speed of the electrons is 100 times faster than the solar wind, so it makes almost no difference. The exit speed of the proton is roughly the speed of the solar wind, so the calculation must be fixed, but there is a net effect that is no 0. (Until the ions cloud accumulate enough charge.)<p>* In an ion thruster the first step is to ionize the gas. Here they start with an ionized gas. The difference is that in a ion thruster they separate the positive and negative field.ions, so they waste some energy in the separation and then they accelerate only the positive ions. The separation of the ions is an active process, not just a static electric<p>* Hat tip for sandworm101 for the comment <a href="https:&#x2F;&#x2F;news.ycombinator.com&#x2F;user?id=sandworm101" rel="nofollow">https:&#x2F;&#x2F;news.ycombinator.com&#x2F;user?id=sandworm101</a><p>---<p>About the article:<p>&gt; <i>The acceleration of the electrons is a form of drag, which is provided for by loss of spacecraft kinetic energy. It therefore could, in principle be used to generate electric power, partially compensating for the power consumed to accelerate the protons.</i><p>This makes no sense. The acceleration of the electrons create some thrust in the wrong direction, but it&#x27;s not like a drag, it&#x27;s like a thruster in the wrong direction.<p>&gt; <i>To see what the performance of a dipole drive might be, let us work an example, assuming a 500 W power source to drive the system. The electron current negates about 2% of the thrust (1&#x2F;43rd) produced by the proton current. The maximum possible jet power is thus about 490 Wj. Assuming additional inefficiencies, we will round this down to 400 Wj, for a total system electrical to jet power efficiency of 0.8. The relationship of jet power (P) to mass flow (m) and exhaust velocity (c) is given by: P = mc^2&#x2F;2</i><p>I&#x27;m not sure that mixing thrust and energy is a good idea, but anyway they assume that the efficiency is 80% and use the usual formula to get the speed. The problem is that assuming that only a 20% of the energy will be wasted is optimism, not a real calculation. How much energy is lost due to the collision of the ions and the plates? How much to recharge the plates? How much is lost in real drag? How much is lost as heat? ... There are a lots and lots of factors, and then you have to consider the nasty ions clouds.