Dawn Mission, pre-launch disscusion Options

[AndyG]

To work this out, note that for a given energy (and assuming just one charge on the ion), thrust varies with the square-root of the mass of the ion, while specific impulse varies inversely with the thrust. Xenon has an atomic mass of 132 (not 54 -- that's the atomic number), and sqrt(132) = 11.5.

You beat me to it. But can we assume one charge per ion? Surely not: it's easier to knock a few electrons off a Xenon atom than it is to ionise Hydrogen.

Andy G

[TheChemist]

.. it's easier to knock a few electrons off a Xenon atom than it is to ionise Hydrogen.
Andy G

Actually not.
It is easier to knock one e from Xe than H (first ionization potential for Xe is 12.1 < 13.6 for H) , but you need ~21 and ~32 eV to kick the second and the third e from Xe+ and Xe2+.

[AndyG]

Thanks, TheChemist - you're quite right. I withdraw the remark I made earlier!

Andy

[Orlin Denkov]

The last update by Marc Rayman, apparently, first on Space Daily:
Stand Down At Dawn Launch Pad

[djellison]

As seen on TPS's blog yesterday.


Doug

[Greg Hullender]

Chemist: Perhaps you can confirm something for me. I was surprised that the ionization energy for hydrogen was so close to the energy for Xenon, but then I thought that perpaps a larger problem would be breaking the H2 molecular bond. However, the enthalpy of H2 seems to be only about 217 KJ/mole -- just a fraction of the 1312 KJ/mole for ionization.

http://www.stanford.edu/~cantwell/AA283_Co...mochemistry.pdf

(See equation A1.33).

Am I reading this correctly? That seems awfully small somehow.

Now, contrast that with the energy required to accelerate 1 mole of Xenon to 30 kps. I come up with just under 60,000 KJ/mole. So the ionization cost seems to be just a couple of percent -- asssuming 100% efficiency, of course.

--Greg

[TheChemist]

Greg,

Xe is a monoatomic molecule, while hydrogen is a diatomic molecule H2.
The value of 1312 KJ/mol is for the ionization potential of a hydrogen atom, H.

H --> H+ plus e-

To use hydrogen for fuel, I guess you would have first to break H2 to atomic H (I have not looked into the specifics of ion engines so this is off the top of my head).
I hope this helps.

[Juramike]

The ionization potential for molecular hydrogen is 15.4 eV/molecule (pay for ref here):

(Handy website for energy unit conversions: http://www.volker-quaschning.de/datserv/fa...n/index_e.html)

H2 --> [H2]+ + e-

15.4 eV/molecule x 6.022E23 molecules/mol x 1.602E-22 kJ/eV = 1486 kJ/mol.

Your typical MS system does the "ping and fling" thing to ionize molecules, then accelerate them. You wouldn't necessarily need to break apart molecular hydrogen in order to accelerate them for thrust.

-Mike

[Greg Hullender]

Thanks guys. I suppose I'm being greedy and wanting to ionize both hydrogen atoms, but it's clearly enough to just ionize the molecule.

Other than the thrust problem, of course, I suspect there are erosion issues. I note that some earlier systems used Cesium (VERY easy to ionize) but the Cesium corroded the equipment too fast. However the thrust problem appears to be serious enough that they don't consider using Argon or even Krypton as a fuel.

--Greg

[nprev]

The way the rocket equation works, the most important thing in being efficient is the speed of the exhaust, not mass expelled per second.

You know, I've heard about this equation since reading Heinlein, but for some reason have never seen it. Would you be so kind as to post it, UG? Tired of being ignorant, here....

[abalone]

I have enjoyed the dicussion on ion engines and different fuel choices and impulse outcomes. The use of hydrogen gas as a fuel would certainly kick the impulse of an ion engine through the roof and the ionisation of hydrogen is not really a problem.

In chemical engines hydrogen gives high impulse for two reasons
1. the energy content (combustion)/ unit of mass is very high, 5X that of petroleum fuels
2. transforming heat from combustion to kinetic energy is very efficient with very small molecules

This is also the reason that hydrogen engines run fuel rich, the addition of so many small molecules (H2)improves the impulse even though they are not burnt

Unfortunatetly neither of these can help us with ion engines. We can not extract the energy of combustion that can potentially be extracted in chemical engines and they do not rely on this mechanism of transforming heat into kinetic energy either.

In ion thrusters the limiting factor is the amount of energy available. This electrical energy is converted into kinetic energy, so here is where the difficult decisions need to be made.

Lets assume we have a choice of accelerating 1 unit of mass of fuel to 30000m/s typical in todays ion thrusters or to to 100,000m/s in some engine in our dreams. Great we have just increased our impulse from 3000 to 10,000. To infinity and beyond as Buzz would say.

Problem, you need 10X the energy to do that, (remember Ek= 1/2 mv*2) or with the same energy supply we can only do that to 1/10 unit of fuel. Unfortunately 1/10 of the fuel accelerated to 3X to velocity will only give you 1/3 of the thrust. Remember that thrust comes from consevation of momentum P=m.v (so 1/10 * 3= 1/3)

Damn that inconvenient maths.
We have just gone from a mouse's fart in thrust to a crickets fart but the fuel will last almost for ever

So Xenon is chosen quite deliberately to give the maximum thrust from the available power, same reason that solid rockets give the majority of the thrust at launch in chemical rockets


So don't forget that famous rocket engineers saying without the impulse you wont get anywhere but in the end its the thrust that makes the bacon
PS I don't know if its famous rocket engineers saying, I just made that up

[dvandorn]

We all have to remember that the use of rocket propulsion for spacecraft falls into two very distinct categories.

To launch from a planetary surface or out from a low orbit requires sufficient thrust to overcome the relentless pull of the body deep into whose gravity well you are located. In this situation, impulse takes a back seat to thrust; you need to have enough thrust to counterbalance gravity, or else you're never going anywhere. The Saturn V needed to develop more than seven million pounds of thrust not primarily because of the required specific impulse, but because the entire fueled rocket weighed more than six million pounds. It needed the short-term thrust required to counter-balance the extreme weight of the vehicle and get it moving out of the bottom of Earth's gravity well.

However, once you are in a trajectory that does not intersect any other solar system body, specific impulse reigns supreme. Now you're in the world of Newtonian physics, in which your spacecraft continues to move along a vector, losing little speed, and doesn't need to fight to keep from falling back down into a gravity well. As long as you work *with* gravity and not against it, the thrust needed to overwhelm the effects of a gravity well just isn't needed. Now is the time when you want to max out the specific impulse of your engine, since even a very low thrust engine can apply a considerable amount of acceleration to your spacecraft if you let it continue to run for weeks or months.

So, the decision becomes pretty simple -- you need high-thrust engines while working in and close to gravity wells, and you don't end up caring as much about the efficiency (i.e., specific impulse) of your engines. But once you get out into space, you're better off using low-thrust, high-impulse engines. It's not necessarily intuitive, but that's how it works.

-the other Doug

[abalone]

specific impulse reigns supreme.
-the other Doug

No, wrong. The specific impulse can be 1,000,000 but that will not get you to your destination unless the thrust is enough in the time you have available. Noone is going to wait 10 years to get to Vesta. It is a compromise between thrust and specific impulse for the powers source available. The principle is exactly the same for ion engines as for chemical engines, the only difference is a few orders of magnitude.

[abalone]

If specific impulse reigns supreme then why do they use Xenon instead of hydrogen

[dvandorn]

It reins supreme in terms of efficiency. I never said it got you there faster, it just gets you there with minimum spacecraft weight per m/sec of delta-V. And I dispute that no one will wait ten years to get to Vesta -- though this isn't a method you'd use when you transport people. For people, who need to dash from one radiation-safe spot to the next, you need to opt for speed and deal with the incredible hit you take in performance.

And it has been discussed in detail how much more energy it takes to ionize hydrogen than xenon... we're also talking about what can be done feasibly with our current technology.

-the other Doug