Ion-engine, low trust? -Conservation of energy, newtons law

[Sereneti]

Hi

In RL ion-engines do have a low trust.
but, where did the energy go?

If i throw a stone with 10 joule in one direction, i got an impuls of 10 joule in the other direction.

A propeller with 1kw give the air the energy of 10 joule (i ingore the loos from the motor...) , and get an impulse from 10 joule in the other direction. -> generate a lot of thrust

If a ion-engine have the power of 1kw, the ion engine generate much less thrust as an propeller in air...

Where did the energy gone?
ar all of the energy wasted to ionice the Xenon?

 

[Superfluous J]

If you think ions work poorly, try your propeller in space! 1kw of energy into the propeller and you get NO MOVEMENT AT ALL.

In space, you throw Xenon (or any propellant) one way, you move the other way according to the law of conservation of momentum. Doesn't matter if it took 1kw or 1.21gw to throw that propellant.

To answer the actual question: The energy (that isn't used to toss the propellant out the back) turns to heat and radiates away. At least, that's all I can figure

[eddiew]

13 minutes ago, 5thHorseman said:

If you think ions work poorly, try your propeller in space! 1kw of energy into the propeller and you get NO MOVEMENT AT ALL.

Well technically your ship would spin the other way to the propeller, just slower because of the higher mass on that end of the axle 

*edit* Speculation for the OP: ionising Xenon is probably pretty hard. It's a noble gas and it does not want to have an electron stripped off it.

Edited May 12, 2016 by eddiew

[andrewas]

The propeller moves a lot of mass backwards relatively slowly. The ion engine moves a tiny mass backwards very quickly. Kinetic energy varies with the square of the velocity, so it takes more energy to generate the same impulse with a faster exhaust.

[Sereneti]

1 hour ago, eddiew said:

Well technically your ship would spin the other way to the propeller, just slower because of the higher mass on that end of the axle 

*edit* Speculation for the OP: ionising Xenon is probably pretty hard. It's a noble gas and it does not want to have an electron stripped off it.

ok, that is logical...
i hadnt think that ionising of xenon is so hard...

---

 

 

[PB666]

4 hours ago, Sereneti said:

Hi

In RL ion-engines do have a low trust.
but, where did the energy go?

If i throw a stone with 10 joule in one direction, i got an impuls of 10 joule in the other direction.

A propeller with 1kw give the air the energy of 10 joule (i ingore the loos from the motor...) , and get an impulse from 10 joule in the other direction. -> generate a lot of thrust

If a ion-engine have the power of 1kw, the ion engine generate much less thrust as an propeller in air...

Where did the energy gone?
ar all of the energy wasted to ionice the Xenon?

 

kN = ^{2* efficiency * kW} / _{ISP * g}

kN = kilonewtons
kW = kilowatts - the power output of the solar panels or nuclear reactor than can be used in the ION drive.
ISP * g = exhaust velocity.

The energy goes into the energy of the ejected plasma, which generally ranges from 2000 m/s to 1000000 m/s

Edited May 12, 2016 by PB666
Repair and define units

[Lukaszenko]

As was mentioned, most of the energy goes into moving the exhaust (which is moving FAST) as opposed to the actual spacecraft. Since energy is exponentially related to speed, to throw the exhaust out the back 2x fast takes 4x as much energy, and yet your spacecraft only accelerates 2x faster.  And if you think that's inefficient, take a look at a photonic rocket: 300 MW per Newton of thrust. 

[Sereneti]

5 minutes ago, Lukaszenko said:

As was mentioned, most of the energy goes into moving the exhaust (which is moving FAST) as opposed to the actual spacecraft. Since energy is exponentially related to speed, to throw the exhaust out the back 2x fast takes 4x as much energy, and yet your spacecraft only accelerates 2x faster.  And if you think that's inefficient, take a look at a photonic rocket: 300 MW per Newton of thrust. 

but newtons third law:
„Actioni contrariam semper et aequalem esse reactionem: sive corporum duorum actiones in se mutuo semper esse aequales et in partes contrarias dirigi.“

If i trow something with X joules away, i get of X joules back...


in this case is not importan of the mass i throw away, only the energy i use to trow something away...
 

 

10 minutes ago, Lukaszenko said:

As was mentioned, most of the energy goes into moving the exhaust (which is moving FAST) as opposed to the actual spacecraft. Since energy is exponentially related to speed, to throw the exhaust out the back 2x fast takes 4x as much energy, and yet your spacecraft only accelerates 2x faster.  And if you think that's inefficient, take a look at a photonic rocket: 300 MW per Newton of thrust. 

the photon-rocket had to generate the "mass" bevore it can be trown away....
 

[eddiew]

39 minutes ago, Sereneti said:

i hadnt think that ionising of xenon is so hard...

There's quite a good illustration of it here: http://www.iun.edu/~cpanhd/C101webnotes/modern-atomic-theory/ionization-energy.html

Noble gasses are pretty tough to ionise because they're already at their most stable state. Asking them to lose an electron requires them to be unstable. Lithium and sodium on the other hand, are relatively eager to dump electrons - but they're also dangerously reactive and you wouldn't really want to carry them on your space ship

I suspect (haven't researched) that Xenon is used because it is 100%  un-reactive, and the heaviest non-radioactive noble gas, meaning you can carry a bigger weight of eject-able mass in a lower pressure container without worrying about it turning into something else or bonding with the container during the trip.

 

[Sereneti]

2 minutes ago, eddiew said:

I suspect (haven't researched) that Xenon is used because it is 100%  un-reactive, and the heaviest non-radioactive noble gas, meaning you can carry a bigger weight of eject-able mass in a lower pressure container without worrying about it turning into something else or bonding with the container during the trip.

 

At first they tried to use Hg , but it had react with the electronic...
(Hg , liquid, light to ionise, and a lot of mass/volume)
 

[eddiew]

3 minutes ago, Sereneti said:

At first they tried to use Hg , but it had react with the electronic...
(Hg , liquid, light to ionise, and a lot of mass/volume)

And highly toxic and rather reactive. Probably do better just burning the stuff  

[Sereneti]

8 minutes ago, eddiew said:

And highly toxic and rather reactive. Probably do better just burning the stuff  

now they use Xeon...

and yes, the energy they used to ionise the xenon is waste.
 

[PB666]

20 minutes ago, eddiew said:

There's quite a good illustration of it here: http://www.iun.edu/~cpanhd/C101webnotes/modern-atomic-theory/ionization-energy.html

Noble gasses are pretty tough to ionise because they're already at their most stable state. Asking them to lose an electron requires them to be unstable. Lithium and sodium on the other hand, are relatively eager to dump electrons - but they're also dangerously reactive and you wouldn't really want to carry them on your space ship

I suspect (haven't researched) that Xenon is used because it is 100%  un-reactive, and the heaviest non-radioactive noble gas, meaning you can carry a bigger weight of eject-able mass in a lower pressure container without worrying about it turning into something else or bonding with the container during the trip.

 

Solid magnesium make be a better choice in the future, it does not need container, its relatively stable in space, and youncan produce a decent ISP with it. In space it is relatively easybtomfind, argon can be found aroud the outplanets in more abundance, zenon is almost impossible to find. Once in space you blow the fairings off the magnesium an when you need fuel have a robot gompick it up and shove intonthe feul tank.

[Bill Phil]

F=DeltaP/DeltaT

The DeltaP is just very small over normal timescales. 

P = momentum

You're shooting small amounts of gas very fast. But the total change in momentum is still small. 

[QuesoExplosivo]

2 hours ago, Sereneti said:

but newtons third law:
„Actioni contrariam semper et aequalem esse reactionem: sive corporum duorum actiones in se mutuo semper esse aequales et in partes contrarias dirigi.“

If i trow something with X joules away, i get of X joules back...


in this case is not importan of the mass i throw away, only the energy i use to trow something away...

You're confusing force with energy. Conservation of momentum is the relevant principle here.

[RizzoTheRat]

4 hours ago, eddiew said:

Well technically your ship would spin the other way to the propeller, just slower because of the higher mass on that end of the axle 

So you're saying we should use contra rotating props on spacecraft then?

[KSK]

2 hours ago, eddiew said:

There's quite a good illustration of it here: http://www.iun.edu/~cpanhd/C101webnotes/modern-atomic-theory/ionization-energy.html

Noble gasses are pretty tough to ionise because they're already at their most stable state. Asking them to lose an electron requires them to be unstable. Lithium and sodium on the other hand, are relatively eager to dump electrons - but they're also dangerously reactive and you wouldn't really want to carry them on your space ship

I suspect (haven't researched) that Xenon is used because it is 100%  un-reactive, and the heaviest non-radioactive noble gas, meaning you can carry a bigger weight of eject-able mass in a lower pressure container without worrying about it turning into something else or bonding with the container during the trip.

 

Xenon is used because it's the largest non radioactive noble gas, not the heaviest. A bit pedantic but it's size makes it relatively easy to ionise (compared to the other noble gases anyway) since it involves removing an electron from a higher energy orbital - again compared to smaller noble gases.

For an efficient propellant you want your exhaust gases to be as light as possible, so xenon is a lousy choice from that point of view. However, the exhaust velocity (and thus ISP) of an ion thruster is so high anyway that using a less efficient but less power hungry propellant starts looking like a good idea.

 

 

Edited May 12, 2016 by KSK

[Sereneti]

7 minutes ago, QuesoExplosivo said:

You're confusing force with energy. Conservation of momentum is the relevant principle here.

hm...
ok, thank you
 

i think its interresting how much energy (%) from the whole system is wasted to ionise the Xenon ...
 

3 minutes ago, RizzoTheRat said:

So you're saying we should use contra rotating props on spacecraft then?

Why not
it looks cool.
 

[wumpus]

2 hours ago, PB666 said:

Solid magnesium make be a better choice in the future, it does not need container, its relatively stable in space, and youncan produce a decent ISP with it. In space it is relatively easybtomfind, argon can be found aroud the outplanets in more abundance, zenon is almost impossible to find. Once in space you blow the fairings off the magnesium an when you need fuel have a robot gompick it up and shove intonthe feul tank.

I'm more a fan of argon for such things.  Xenon is preferred (presumably over magnesium) to being a noble gas, but so is argon.  But xenon is impossibly rare while argon is more common than carbon dioxide (I seriously hope this remains true).  The reason you prefer xenon is that the extra weight (it has about three times the mass) means that you get three times the momentum for about 75% of the ionization energy (expect to want to put a lot more energy into adding momentum when using argon).

I've heard blasting off of bits of PVC is useful for much smaller satellites, not sure if it is feasible for microsats or not.

[PB666]

19 minutes ago, wumpus said:

I'm more a fan of argon for such things.  Xenon is preferred (presumably over magnesium) to being a noble gas, but so is argon.  But xenon is impossibly rare while argon is more common than carbon dioxide (I seriously hope this remains true).  The reason you prefer xenon is that the extra weight (it has about three times the mass) means that you get three times the momentum for about 75% of the ionization energy (expect to want to put a lot more energy into adding momentum when using argon).

I've heard blasting off of bits of PVC is useful for much smaller satellites, not sure if it is feasible for microsats or not.

Mass is mass, it does not matter what form it comes in, and the ionization energy is not that big of an issue when you are talking about High ISP drives. But getting an electron off of Magnesium is really easy. The saving grace of Xenon is that it more easily compresses, it also has a higher liquid density.

[Bill Phil]

Actually, Newton's laws have little to do with energy, and more to do with momentum. 

[Lukaszenko]

20 hours ago, Sereneti said:

the photon-rocket had to generate the "mass" bevore it can be trown away....
 

It does not, it only has to throw away photons. A flashlight can be a photon rocket, for example. 

[PB666]

E = m c ^ 2 . , N = 300 MJ/sec 

N * sec / c ^ 2 =  300,000,000 m

Yep, if the energy is not gifted to you by some star  then for sure you have to convert mass into energy to getthrust. Even so the star converts mass to energy to produce light and the ship converts star light to mass to get power and the power is converted back to photons which are massless. 

 

[magnemoe]

22 hours ago, PB666 said:

Mass is mass, it does not matter what form it comes in, and the ionization energy is not that big of an issue when you are talking about High ISP drives. But getting an electron off of Magnesium is really easy. The saving grace of Xenon is that it more easily compresses, it also has a higher liquid density.

You have to vaporize the magnesium too and avoid it from deposit in the engine or other places, this is probably why they uses gasses. 

[fredinno]

On 5/12/2016 at 6:16 AM, eddiew said:

There's quite a good illustration of it here: http://www.iun.edu/~cpanhd/C101webnotes/modern-atomic-theory/ionization-energy.html

Noble gasses are pretty tough to ionise because they're already at their most stable state. Asking them to lose an electron requires them to be unstable. Lithium and sodium on the other hand, are relatively eager to dump electrons - but they're also dangerously reactive and you wouldn't really want to carry them on your space ship

I suspect (haven't researched) that Xenon is used because it is 100%  un-reactive, and the heaviest non-radioactive noble gas, meaning you can carry a bigger weight of eject-able mass in a lower pressure container without worrying about it turning into something else or bonding with the container during the trip.

 

Looking at that, Aluminum looks like a decent idea (its melting point probably makes it a meh idea though)

However, Gallium seems like another good idea, being lighter than Xenon, and being able to melt and be put through an engine easily.

IN that same vein, Beryllium Hydride seems even better. You'd get amazing ISP for a energy cost slightly lower than Xenon.

Unfortunately, Beryllium costs an arm and a leg, so...

How about compounds? CO looks like a great idea, with a Xenon-level ionization energy, a gas at STP, and a great ISP, though the oxygen's reactivity and Carbon soot probably kills the idea.

Is there a Aluminum or Magnesium compound that is a Gas? That would be great for an ION drive....

On 5/12/2016 at 8:57 AM, wumpus said:

I'm more a fan of argon for such things.  Xenon is preferred (presumably over magnesium) to being a noble gas, but so is argon.  But xenon is impossibly rare while argon is more common than carbon dioxide (I seriously hope this remains true).  The reason you prefer xenon is that the extra weight (it has about three times the mass) means that you get three times the momentum for about 75% of the ionization energy (expect to want to put a lot more energy into adding momentum when using argon).

I've heard blasting off of bits of PVC is useful for much smaller satellites, not sure if it is feasible for microsats or not.

Argon's only big advantage seems to be being common, and a somewhat higher ISP.

BTW, how much energy would you need to get 100kN (around RL10 level) ION drive, with a 4000s ISP?