PDA

View Full Version : how to make mega thrust in ion propulsion?



bonzelite
2005-Oct-19, 07:39 AM
i read the recent HDLT news about the new gens of ion thrusters.

in light of that, how can the thrust to weight ratio be increased A LOT? for example, if you had an ion propelled car and a 700hp corvette at the dragstrip, the corvette would win the race because it is faster off the line. the ion engine, despite having more "thrust" of the particles out the back, would barely move a sheet of paper if such paper were placed near the ion exhaust. and the ion car would sit there as the corvette ran low 11s on the 1/4 mile.

given that ion propulsion can push particles out the back at tremendous velocities, how do we translate that into neck-snapping g-force, felt upon the craft itself, from rest? as in a drag racing car? or F-16? what would it take to have an ion engine replace a jet engine, in other words? is it just a matter of upping the voltage? current density? more ions per unit of volume?

Van Rijn
2005-Oct-19, 08:48 AM
There may be some practical problems scaling up an ion drive to higher power densities, but the key thing you need is power.

In a rocket, to put it simply, you throw something in one direction to go in the other direction. The faster you throw it, the more energy it takes. There are a many details, but fundamentally, that is what you are left with. The advantage of an ion drive over a chemical rocket is that, given the power source, it can "throw stuff" faster. Depending on the mission, that can be an advantage.

The problem is that the power source is also mass you need to accelerate. So you probably need a nuclear reactor for a decent amount of energy. And the net acceleration can be maybe 1/10000 of a G or so. To get better acceleration you would need, to start, a really low mass/high density energy source. We don't have that today.

publiusr
2005-Oct-19, 08:32 PM
Lets take a look at the JIMO concept. No EELV could launch the thing, and in order to start it properly you needed to have an upper stage to kick it out of Earth-Moon so you could pull the rods--otherwise you just spend a good chunk of your useful life on Earth escape doing loop-de-loops. Jeff Bell--who I don't always agree with--wrote a good article on the Myth Of Low Thrust Propulsion.

Gullible Jones
2005-Oct-20, 12:08 AM
You could use a VSIMR - low gear to kick out of orbit, high for interplanetary cruising.

Alternatively, you could use a pulsed plasma rocket, and increase the pulse speed when you needed more thrust.

Also... how about a fission rocket with a choke? I'm thinking you could use the Z pinch effect to increase the specific impulse at the expense of thrust. Put a hard pinch on the plasma during cruising, and reduce it when higher accelerations are needed... I'm not sure that would work though - and there might be a better way to choke the exhaust than with the Z pinch.

genebujold
2005-Oct-20, 02:09 AM
i read the recent HDLT news about the new gens of ion thrusters.

in light of that, how can the thrust to weight ratio be increased A LOT? for example, if you had an ion propelled car and a 700hp corvette at the dragstrip, the corvette would win the race because it is faster off the line. the ion engine, despite having more "thrust" of the particles out the back, would barely move a sheet of paper if such paper were placed near the ion exhaust. and the ion car would sit there as the corvette ran low 11s on the 1/4 mile.

given that ion propulsion can push particles out the back at tremendous velocities, how do we translate that into neck-snapping g-force, felt upon the craft itself, from rest? as in a drag racing car? or F-16? what would it take to have an ion engine replace a jet engine, in other words? is it just a matter of upping the voltage? current density? more ions per unit of volume?

You don't.

The beauty of the ion engine is that it is many times more efficient than chemical or even nuclear rockets. Put simply, the ratio of total mass to total impulse is orders of magnitude lower.

The problem is that the specific impulse (the "neck-snapping" quality you mentioned) is very low. It's the ion engine's ability to fire at "full thrust" for very long periods of time (ie, years, if necessary) that makes it so attractive for long-term work such as keeping satellites positioned and moving stuff from LOE to Lunar, Mars, or even Pluto orbits. Because the propellant comes out at such incredibly high velocities, it requires exceedingly little of it to generate the same amount of thrust as a chemical rocket engine.

Don't worry, though - one of these days someone will invent one that's ten times as efficient and produces thousands of times the thrust.

Damburger
2005-Oct-20, 02:07 PM
Its not impossible for an ion engine to produce thrusts you would find in a chemical rocket, it just takes a hideous ammount of energy to do so. The advantage of ion engines is that they use less propellant so your craft weighs less. You negate that advantage by strapping Three Mile Island to it.

publiusr
2005-Oct-20, 07:35 PM
That is very true. Try to spiril it out of our deep gravity well, and youd be better off with one huge chemical rocket and be done with it.

Nuclear Thermal is the way to go. Not nuclear electric.

genebujold
2005-Oct-21, 01:13 AM
That is very true. Try to spiril it out of our deep gravity well, and youd be better off with one huge chemical rocket and be done with it.

Nuclear Thermal is the way to go. Not nuclear electric.

It all depends upon the application.

Gullible Jones
2005-Oct-21, 10:34 PM
There's also the MPD thruster. (http://en.wikipedia.org/wiki/Magnetoplasmadynamic_thruster) Sucks up lots of energy, but gives you good thrust and excellent specific impulse.

(A nuclear-thermal rocket with one of those things as an "afterburner" for interplanetary cruising could work very well, methinks.)

robert1701
2006-Mar-27, 03:17 AM
Is it a good thing to have so much power? :confused:

Say you have enormous power at your disposal and you can throw a lot of particles out of the back of your spaceship. :)

What would this do to the guy behind you? :think:

What about the planet you just left? Will it be fried by what's coming out of your exhaust? :doh:

Nicolas
2006-Mar-27, 09:28 AM
to have a significant influence on the planet behind you, you'd need a crazy amount of energy in your exhaust.

That amount of energy means you'll leave the planet very fast,so there will be only very limited time for the planet to suffer from your exhaust.

So that ain't an issue, unless you make rockets of say the size of a large country (which we'll probably never do).

About the man behind you: he'll be about as bad off as when he would be standing directly behind any current rocket: totally fried.

antoniseb
2006-Mar-27, 12:57 PM
Its not impossible for an ion engine to produce thrusts you would find in a chemical rocket, it just takes a hideous ammount of energy to do so.

It is pretty easy, and I think very useful, to calculate how much energy it takes to accelerate an ion powered probe up to various speeds, and how long it will take to get up to that speed.

If you have taken Physics One in High School or College, you can do this calculation. If you are a smart kid who hasn't taken Physics One, you can still figure it out. It requires only simple algebra, and these simple Newtonian approximations:

- The change in momentum of the space craft for each ion sent out the back is the same as the momentum of the ion but in the opposite direction. (change in velocity is proportional to the velocity of the ion).

- The energy required to accelerate the ion to the exhaust speed is proportional to the square of the velocity. (Thus it takes four times the energy to go twice as fast).

A side note about efficiency: the Deep Space One probe was almost 100% efficient in its use of energy being converted to thrust.

Damburger
2006-Mar-27, 01:30 PM
It is pretty easy, and I think very useful, to calculate how much energy it takes to accelerate an ion powered probe up to various speeds, and how long it will take to get up to that speed.


The thread title is about thrust though, not delta-v.

antoniseb
2006-Mar-27, 02:37 PM
The thread title is about thrust though, not delta-v.
Thrust being delta-v per unit time. Still easy to calculate, and worth doing if this topic interests you.

Ara Pacis
2006-Mar-29, 06:00 AM
Wouldn't an exhaust plume disperse fairly rapidly in space? I know Zubrin says this would mitigate nuclear exhaust in his Nuclear Salt-Water Rocket. Would it still hold for ion propulsion?

antoniseb
2006-Mar-29, 01:39 PM
The exhaust from anything we'll build this century shouldn't be a problem. If we ever start blasting reaction mass out the back at .99c we'll potentially be causing some problems elsewhere. I've mentioned before that tailgating is strongly discouraged.

Ara Pacis
2006-Mar-30, 05:59 AM
But if we expel reaction mass at .99c it will exceed the escape speed of the earth, the sun and the galaxy. Unless someone is within a few km of your six, it probably won't matter.