Dawn Mission, pre-launch disscusion Options

[ugordan]

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....

Try this: Tsiolkovsky rocket equation. Notice the delta V is proportional to exhaust velocity with fixed mass ratios, there is no time variable in the equation.

[nprev]

Simple yet powerful; thanks, UG!

[abalone]

And it has been discussed in detail how much more energy it takes to ionize hydrogen than xenon

The choice of xenon over hydrogen has very little to do with the ionisation energy, 15.4 eV vs 12eV is inconsequential when compared to the energy needed to accelerate the ions. It also needs to be remembered that if one looks at ionisation energies/ mole and lets assume they are roughly equal for H2 and Xe. Then the ionisation energy/unit of mass is 65X as high for H2 as Xe because of their different atomic weight. The ionisation energy / gram if far more important in calculating efficient use of power than ionisation energies/ mole

what can be done feasibly with our current technology.

The limit of current technology has as much to to with the power source as anything else, that was the point I was trying to make.

All the early work with ion thrust was done with caesium and mercury before they switched to xenon. So what have they all got in common? High atomic mass and the ability to be handled as fluids at a convenient temperature. They were after maximum thrust per kW. Cesium and mercury were abandoned because of electrode erosion. Maybe hydogen has a similar electrode erosion problem as well.

(impulse)...It reins supreme in terms of efficiency

If that were our primary consideration then we should design a thruster that runs on nuclear energy and uses a high efficiency light source to expel photons at the speed of light. Our impulse goes up to 3x 10*7. Maybe that will be possible one day but to get any meaningful kind of thrust we are going to need an awful lot of power.

delta V is proportional to exhaust velocity with fixed mass ratios, there is no time variable in the equation

Time constraints are however a practical consideration as I have outline above, no scientist wants to expire before his experiment in complete

[elakdawalla]

The Kennedy Media Archive now has photos of Dawn being de-stacked and returned indoors.

--Emily

[ugordan]

Time constraints are however a practical consideration as I have outline above, no scientist wants to expire before his experiment in complete

Agreed, but this has nothing to do with what the rocket equation says and yet you were replying to my remark on it. Yes, thrust is important in certain regimes, but in other regimes the actual efficiency of the engine is more important. That's the gist of what the other Doug is trying to say I think.

[punkboi]

The Kennedy Media Archive now has photos of Dawn being de-stacked and returned indoors.

--Emily

The unfortunate sight of seeing Dawn back in its processing facility is made up for by the photos of Phoenix now in place at pad 17-A.

[abalone]

Agreed, but this has nothing to do with what the rocket equation says and yet you were replying to my remark on it. Yes, thrust is important in certain regimes, but in other regimes the actual efficiency of the engine is more important. That's the gist of what the other Doug is trying to say I think.

I did wrap the reply to your post with that to Doug's and I suppose it should have been kept separate.

I cant resist a comment on your last post and that it to say that "efficiency" is not the same as "impulse". I would venture to say that the hydrogen Shuttle main engines may in fact be more efficient than ion thrusters. Efficiency in engineering terms is the fraction of the input energy converted to the desired output energy. For the Shuttle main engines this is approx 50%. I cant find any accurate figures for ion thrusters but it would surprise me if it were much higher than this. Again the barrier we come to is the limitations on the power sources used today.

[rlorenz]

I cant resist a comment on your last post and that it to say that "efficiency" is not the same as "impulse". I would venture to say that the hydrogen Shuttle main engines may in fact be more efficient than ion thrusters. Efficiency in engineering terms is the fraction of the input energy converted to the desired output energy. For the Shuttle main engines this is approx 50%. I cant find any accurate figures for ion thrusters but it would surprise me if it were higher than this. Again the barrier we come to is the limitations on the power sources used today.

Ion thrusters have a couple of efficiency parameters. The electrical efficiency (beam power, or
half mdot v squared, divided by power in) is often 90% or more. You waste a little bit in ionizing
the stuff in the first place. There is also a propellant utilization efficiency - again typically over 90% -
some propellant leaks out un-ionized, and some must be expended in a neutralizer to balance the
charge flux.

Of course, there is the question of propulsive efficiency - ion thrusters are great at putting power
efficiently into the kinetic energy of the beam, which is not the same as adding kinetic energy to
the vehicle. That efficiency is maximized when the vehicle speed approaches the exhaust
velocity....

[abalone]

Found some information.
Thrust efficiency appears to range from about 40% to 75% depending on Impulse with about 60-65% at Isp 2500
http://www.engin.umich.edu/dept/aero/space...IAA-96-2973.pdf

[punkboi]

Now that Phoenix is safely on its way, September 26 is being eyed as the new launch date for Dawn...

[tedstryk]

Slightly OT, but I have reprocessed Hubble's 1994 Vesta FOC image (well, a series of images, but only in one band and of one face). While there are certainly equally good (if not better) images, most of them are in the infrared or visible spectrum, while this is in the near UV. And I don't care what the silly scientists say, if you look at this one closely, it is obvious that Vesta is really a giant sculpture of a smiling pig.

[Juramike]

Leaning Jowler?

http://en.wikipedia.org/wiki/Pass_the_Pigs

[Phil Stooke]

Nice, Ted! I haven't seen much done with FOC. Also I forgot to tell you how great the Miranda dark side pics were.
Phil

[tedstryk]

Thanks. I also made this composite of the "Big-3"



Ceres and Vesta are HST FOC images, Pallas is from WFPC/2 (It is a super-resolution view, but the images are not from the PC chip).

[Phil Stooke]

Nice one of Ceres. I'll have to compare it with the color set that appeared recently.

Phil