# Tag Info

31

The piece that you are asking about not only can be done, but it has been done. SMART-1 was launched in to GTO in 2003 and entered orbit around the Moon in 2004, using only an Ion engine to do so. It gradually reduced its orbit around the Moon, eventually colliding in to it. The trick is to use the Lagrange points to give one more time to do a proper orbit.

26

Xenon is the heaviest non-radioactive elemental inert gas. The added mass allows for denser packing at less pressure. The mass is one of the limiting factors, so having a more dense gas helps tremendously. The limiting factor relates to the mass of the propellant. Essentially, a heavier mass allows for more momentum to come from the overall system. The mass ...

26

You can't do it. It's impossible. Each thruster provides thrust, but each thruster has mass, as do the power sources needed to power them and the tanks to store their fuel. No currently existing ion thruster is able to produce anywhere near that much thrust for its mass, and more significantly, even the best power sources (even speculative ones or those at ...

22

While the ISP on Ion thrusters is awesome, the overall thrust is pretty low. Thus the transit time from LEO to GEO would be quite long and slow. In some cases this matters. If it takes an extra year to get in service, that is a year of lost service while in transit. In fact a critique of the Falcon 9's ability to do dual launches is that only the smaller ...

22

Does it have any additional thrusters? Not to thrust towards its targets. For that, it's 100% ion thruster propelled. It does also have a set of 12 MR-103G variable thrust (0.9 N maximum) RCS (Reaction Control System) hydrazine monopropellant thrusters that launched with only 46 kg of propellants (read: total thrust of its RCS doesn't provide the spacecraft ...

22

Assuming you mean "quite small" in terms of mass as well as thrust output. Fundamentally, current ion drives are limited by the amount of power available to them - it takes many, many kilowatts of input power to provide tiny amounts of thrust. As you know from the answer you linked, the Dawn spacecraft is powered by 3 NSTAR ion engines which ...

22

The expression $v_e = \sqrt{\frac{2Vq}{m}}$ is a non-relativistic approximation. This is quite valid when the exhaust velocity is small compared to the speed of light, which is the case for ion thrusters made to date (exhaust velocity is on the order of $10^{-4}c$). A more precise expression is $${v_e}^2\left(1+\frac{2Vq}{mc^2}\right) = \frac{2Vq}m$$ No ...

20

In an ion thruster, particles are accelerated because of their electrical charge. The force acting on them is proportional to the charge (and the external field applied, which we can treat as fixed for a specific engine design). Naturally, the heavier the particle is, the less it is accelerated by this force. An extended particle we can describe as a ...

18

Here's how you can work it out. First, thrust in kilo-Newtons (kN) divided by mass in metric tons yields acceleration in meters-per-second-per-second. Divide by 10 to get acceleration in approximate Earth surface gravities (9.81 is the real factor). Dawn uses its thrusters only one at a time (they aren't pointed the same direction), and a single NSTAR ...

18

(Top edit: The Question asserts "Xenon and krypton are popular despite their heavy mass" and asks about exploring H or He ion propellants for improved Isp. This answer shows that lighter is not better for ion thrusters, because Isp is not the proper measure of a power-limited situation. Hence, although lighter atoms have been explored for other reasons, ...

16

Ion engines balance two different kinds of efficiency: $I_{sp}$ which is basically "reaction mass efficiency". For that you want the highest possible exhaust velocity, which will be helped by a higher voltage. Energy efficiency for which you want the exhaust velocity to be in the same range as the $\Delta v$ required for your mission. With a given ...

15

A conventional thruster with two liquid propellants requires energy too. But it is chemical energy stored in the propellants. Ion thrusters use no chemical energy at all, all the energy of the ion beam is from the electrical energy used by the thruster. In fact, a conventional rocket engine with a lot more thrust than a ion thruster uses a lot more of ...

13

The rule you have for the total $\Delta V$ of a low-thrust spiral is an upper limit arrived at as you let the thrust go to zero. However that takes an infinite amount of time. The total $\Delta V$ of a spiral with non-zero thrust is less, and the time is finite. But it is a good rule of thumb for quick calculations when trying to establish feasibility. ...

13

I can't give a precise answer to your primary question besides "Extremely unlikely", but here are some facts on cosmic rays that might help coming to a conclusive answer: Current models are able to describe the distribution of energies and ion masses rather well. What we do not know precisely is the source of this radiation. There are plenty possible ...

13

While this seems like a good idea at first, you very quickly run into the main problem with ion engines: their tiny thrust. Let's compare a typical ion engine from the Dawn mission and an upper stage commonly used for interplanetary injections, the Centaur Upper Stage with its RL10 Hydrogen-Oxygen engine. Ion Engine Thrust: 90 mN Mass: 8.2 kg RL10 C-1 ...

12

While their ISP is insanely good, ion thrusters have miserable T/W ratio, less than 1/1000th of unity; you simply cannot take off from any large body with them, no matter how many of them you use. The primary limiting factor, I think, is power consumption. The thrusters used on Dawn and DS1 require 24kW per newton of thrust. You'd need 1.21 gigawatts to ...

12

Leaving aside the power consumption problems (which have been well discussed in other answers) and returning to the part of the question where you asked: Are they [Ion Thrusters] fundamentally limited in some way? Is there reason to hope what limits them now could be overcome at some point so their high Isp could be used for higher acceleration on ...

12

Dawn and Deep Space 1 both use the NSTAR ion engine - I got my stats from a mix of sources so there may be small differences between the engines used on the two spacecraft, but they seem to be pretty similar. Dawn has 3 redundant NSTAR thrusters (not intended to be used together); DS1 has 1. Thruster mass is 8.2kg, power processing unit and control unit ...

12

No, there are no planned missions using an Ion drive to the outer solar system. The reason is something that you haven't taken in to account. Sunlight drops significantly as one goes further from the Sun. One might be able to get the continuous acceleration you indicated to get to the Asteroid belt, but to go much further then that would require large solar ...

12

I am assuming you mean by propulsion by the CubeSat itself. Not at the moment! Mostly because of the throughput (thruster lifetime) constraint on small Electric Propulsion (EP) thrusters designed for CubeSats. Right now the leading CubeSat EP thruster is the BIT-3 (this is the thruster that will be used to go to the moon on my answer to your original ...

11

A single-use, disposable ion drive can be made no larger than it needs to be, thus uses no more fuel than it needs to. A reusable ion drive tug, first of all, has to take its payload to destination, and then come back; due to the exponential increase in fuel requirements for linear increases in delta-v, this can be more than twice as much fuel as a one-way ...

11

TL; DR: Trajectory optimization for continuous thrust is difficult and this field is very active in research. 2021 clarifications: Methodology For the least amount of fuel, the best is the thrust the least amount of time as possible and only when it's extremely efficient ($\eta \geq 0.98$). But that also implies that it will take an incredibly long amount ...

11

The thing to notice about this type of drive is that the ions encounter the positively charged accelerating grid first. The ions that provide the thrust to the rocket are positively charged as well, so they will be repelled and contained within the plasma chamber. The ions' only means of reaching the interior of the accelerating grid is diffusion, meaning an ...

11

I'll first try to independently reproduce your calculation: The thrust force is the transfer of momentum per unit time: $$F = \frac{dp}{dt}.$$ Assuming ions accelerate by mutual coulomb repulsion with the device without any interference, the magnitude of the momentum transferred to the device for each ion is equal to the ion's momentum, which we can get ...

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TLDR: Yes, but probably no, well maybe. Fundamentally, the second law of thermodynamics is about entropy: If you create it, part of the energy budget of a running engine has to be spent on getting rid of it. Combustion processes create entropy as a result of creating heat, and the rest follows. To the extent that fuel cells (and human cells) avoid ...

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ESA is working on such an engine. an ESA-led team has built and fired an electric thruster to ingest scarce air molecules from the top of the atmosphere for propellant, opening the way to satellites flying in very low orbits for years on end The molecules collected by the intake designed by QuinteScience in Poland are given electric charges so that ...

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Knowing that the thrust is proportional to the grid voltage Almost correct! All else equal, the thrust will be proportional to the ion velocity, which will vary as the square root of their kinetic energy, which is proportional to the acceleration voltage. why don't they just eject some electrons from the grid to increase the electrical potential? Well to ...

10

Optimal low-thrust trajectories for interplanetary missions are complicated to calculate. It's a little too broad of a topic for a Q&A. In fact, I spent an entire internship at NASA Johnson Space Center working on this problem (building of the work of many others years before and hence), and wrote my first (non-conference) academic paper on exactly this ...

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