Can the thrust of Ion Thruster be increased at the cost of speed?

[gggnano]

TL;DR Summary

slower xenon intake, ions passing through medium, using 'top quarks'?

Based on the little I know ion thrusters seem extremely impractical on earth, even because speeds over 20,000-30,000+km/h will result in immense friction and fire, so why not cut the high speed somehow and increase thrust which is abysmall? For example: the ions passing through a medium?? Wider nozzle? I'm sure these have been thought ad nauseum already and won't work but it's just so frustrating to be locked with such a high speed and almost no thrust.

 

[FactChecker]

My understanding is that the whole idea is to have a propulsion system that requires very little mass but can spit out a tiny stream at enormous velocities. The low mass allows us to put it in space with less rocket power. The tiny stream at enormous velocities allows it to accelerate a spaceship to high speeds over a long time. Without that tradeoff, I don't know if there is any advantage to ion propulsion.

 

[Benjies]

You are referring to propulsion systems that are casually referred to as "deep space engines". These engines are not used for anything on Earth as the thrust/weight makes them unsuitable to move mass on earth when compared to chemical propulsion systems.

In electric propulsion, your ions are always passing through "a medium", as you put it- it's just a question of what medium you put your propellant through. Electrostatic, Electromagnetic, or thermal mediums.

Thermal-medium thrusters are still applied in space, and meet some of the criteria you are looking for (sacrificed efficiency for higher thrust/weight). These engines, however, still aren't operated on Earth to produce useful work or move mass.

Nozzles do not impact your propulsion unit. Think about the function of a nozzle: expand propellant and increase velocity. The temperatures seen within electric propulsion units is so high that the system can no longer be modeled as a solid, liquid, or gas- rather, it is a 'plasma'. Some "other thing". Not sure we know how to expand plasma flows: the propulsive unit spits stuff out at incredibly high velocities, and we call it good at that, without a nozzle. If you want to have more leverage over your propellant flow, try identifying inefficiencies with your anode/cathode (electromagnetic or electrostatic), or your resistor (electrothermal).

 

[Filip Larsen]

In addition to what has already been mentioned, I just want to spell it out directly: From the rocket equation it follows that the higher the exhaust velocity the less reaction mass you need to achieve a given delta-v, or equivalently, for the same reaction mass you get more delta-V. Since available delta-V is always a major mission design parameter having high efficiency is a real plus. Thus, as had been mentioned, once in orbit with no drag or gravitational losses to overcome, high efficiency is almost always preferable to high thrust.

Regarding engine designs that allow a wider range of possible thrust levels take a look at the VASIMR design. It is able to variably produce more thrust at the cost of lower efficiency. As I understand it, it will however require more power to operate compared to, say, an ion engine with same thrust.

 

[gggnano]

So I had more time today and found this fantastic paper from NASA, long story short: No.

https://descanso.jpl.nasa.gov/SciTechBook/series1/Goebel_02_Chap2_thruster.pdf

Have a look at page 23, the equation basically:

Thrust in mN = 1.65*amperes*Squareroot(Volts)

So assuming insane amperage of say 1000 and a Tesla-coil-kind of voltage of 500,000v results in N: 1166.7

Forgetting about the thousand Newton thrust this is over 1 MW power which is a lot, apperantly thousands of solar panels or tens of thousands USD spent for diesel generators not to mention how neither can lift its own weight with just 1000 N thrust.

The 1.65 number comes from the Xenon and you can hardly manipulate it as Xenon is close to perfect for the purpose, you can get Hydrogen but the thrust will be much lower...or you can evaporate a rare heavier metal and get 2 times higher at best which will be far more expensive and dangerous. You can use Iodine with similar results btw so Iodine is the only preferred choice over Xenon given its abundance, colorization, storage, still lower thrust. Yet, even iodine can kill you so: safety first.

I am curious yet if heat itself cannot help as in with a cold gas thruster? 2000 celcius or so?

 

[gggnano]

Just to elaborate on the obvious: I am using 1000 N, 1 kN as the thrust where things are "about to get interesting" as you're entering human-level of flight at this point albeit you will need several thousand at least for realistic scenario (aka anything other than a child flying on small helicopter).

 

[Astronuc]

In order to levitate, one needs thrust or lift equal to the weight of the mass being levitated. In order to move, one needs an excess of thrust to overcome accelerate or overcome drag in the atmosphere. If one wants to get up out of the atmosphere, one needs as much thrust as possible to accelerate to speed (ultimately escape or orbital velocity) within the thermo-mechanical limits (e.g., tensile/shear/bending strength/resistance or stiffness) of the vehicle.

Usually for electric or electromagnetic propulsion (e.g., ion or plasma), one is looking for very high specific impulse, in order to maximize thrust while minimizing mass flow rate (to minimize the propellant required for a mission), which leaves the velocity (or change in velocity/momentum) of the propellant flow as a key variable/factor. That higher the energy or power, the lower the mass flow rate for a given thrust. But there is only so much energy that can be transferred through physical system which is usually constrained by strength of materials, creep or melting point. One can build a bigger engine, but one has to build a bigger energy/power system and ostensibly a power conversion/transfer system. The bonus comes from combining energy generation into the propellant, e.g., a fusion reactor, in which the propellant products of fusion, but then one would have to find a way to separate products from useful (energy producing) reactants. That's a huge challenge.

 

[Astronuc]

The 1.65 number comes from the Xenon and you can hardly manipulate it as Xenon is close to perfect for the purpose, you can get Hydrogen but the thrust will be much lower...or you can evaporate a rare heavier metal and get 2 times higher at best which will be far more expensive and dangerous. You can use Iodine with similar results btw so Iodine is the only preferred choice over Xenon given its abundance, colorization, storage, still lower thrust. Yet, even iodine can kill you so: safety first.

What is the reason folks use Xe, as opposed to some other element or inert gas?

I am curious yet if heat itself cannot help as in with a cold gas thruster? 2000 celcius or so?

Cold gas or hot gas (2000°C)?

Electrode/grid erosion/degradation is an issue.

 

[gggnano]

^ Tantalum-Hafnium-Carbide has close to 4000 C melting point so I suspect 2000 C can be handled yet given the weight of the gas with earth's gravity...I've no idea. Which is why I am giving twice lower than it's melting point.

As to Xenon: it's the heaviest non-radioactive gas, non-corrosive gas, hardly reactive too even at high temperatures. Yet if you're even after more insane speed but even more abysmal thrust Hydrogen or Helium should work better...can't give you Hydrogen's numbers right away since I wasn't looking for speed but thrust only yet it won't surprise me if you can get orders of magnitude higher speed with hydrogen.

 

[Benjies]

Something else worth considering is the absurd amount of money, time, and expertise thrown at this subject. The tens-if-not-hundreds of PhD scientists who have evaluated (for their full time job, even) how to squeeze as much performance as possible out of these propulsion units have a common-sense and reasonable answer as to why systems in propulsion work the way they work now.

Take a chemical engine example. We've been building rocket engines for almost a century- how sizable do you think the specific impulse increase, or performance increase, has been for launch engines? Specific impulse increased by 24% from the Rocketdyne F1 (Saturn V) to the SpaceX Raptor (Starship). Thrust/weight has increased by 53%, but we're getting to a point of diminishing returns on staged combustion engines- there's simply not enough innovative space available with the engine to enable the engine to rise beyond its conventional uses. They can't get a staged combustion chemical engine to enable a single-stage-to-orbit design, and they can't get an electric engine to do anything useful in Earth's atmosphere.

 

[Drakkith]

As to Xenon: it's the heaviest non-radioactive gas, non-corrosive gas, hardly reactive too even at high temperatures. Yet if you're even after more insane speed but even more abysmal thrust Hydrogen or Helium should work better...can't give you Hydrogen's numbers right away since I wasn't looking for speed but thrust only yet it won't surprise me if you can get orders of magnitude higher speed with hydrogen.

Yes, but now you've reduced the thrust by a factor of around 12 by switching to hydrogen, along with possibly increasing the size, mass, and complexity of the fuel system, as hydrogen has to remain cryogenic (and even then still leaks out over time). Note that despite what you may believe, thrust is still very important in ion engines. The Dawn spacecraft operated its ion engines for 5.9 years, 54% of its time spent in outer space, before running out of fuel. With required thrust times measured in months (it took 270 days for a 1.81 km/s delta V change), slashing the thrust by more than an order of magnitude simply won't do. You won't be able to change course in time to do anything.

Also, you keep saying 'speed' when talking about these fuels. I assume you're referring to the exhaust velocity of the propellant?

 

[Astronuc]

As to Xenon: it's the heaviest non-radioactive gas, non-corrosive gas, hardly reactive too even at high temperatures. Yet if you're even after more insane speed but even more abysmal thrust Hydrogen or Helium should work better...can't give you Hydrogen's numbers right away since I wasn't looking for speed but thrust only yet it won't surprise me if you can get orders of magnitude higher speed with hydrogen.

Yes, it's the heaviest non-radioactive gas, non-corrosive gas, hardly reactive too even at high temperatures.

Furthermore, it has the lowest work function (ionization potential) of the non-radioactive noble gases, about half that of He. Radon has a lower ioniization potential, but it is radioactive.
https://chem.libretexts.org/Bookshe...le_Gases/1Group_18:_Properties_of_Nobel_Gases

 

[Flyboy]

Yes, you can increase the thrust by using heavier propellant atoms, but after a certain point, it's more effective to use some other form of electric propulsion. Pulsed plasma, Hall Effect thruster, hell, even arcjet. There's a well understood correlation between specific impulse and thrust. Increase one, and the other diminishes accordingly.
Unless you want to ride atop a ball of nuclear fire (or many in succession). If you go into nuclear reactions like active fission or fusion, things get more... interesting. It's possible to have high thrust and high specific impulse with, say, pulsed nuclear propulsion or a nuclear saltwater rocket. But that's in the realm of "what's the minimum safe distance for testing this? behind the Moon?" territory.