Dawn Mission, pre-launch disscusion Options

[centsworth_II]

How does this slip change the mission profile...

According to Emily's article, all the mission objectives will be met and
"...the Earth-to-Vesta cruise duration is shortened by two months with
the two-month delay to a September launch, resulting in an arrival date
at Vesta that is the same -- October 2011 -- regardless of whether Dawn
launches in July or September."

[gpurcell]

Thanks, missed that when I was reading the article.

[Jason W Barnes]

According to Emily's article, all the mission objectives will be met and
"...the Earth-to-Vesta cruise duration is shortened by two months with
the two-month delay to a September launch, resulting in an arrival date
at Vesta that is the same -- October 2011 -- regardless of whether Dawn
launches in July or September."

Great -- but how? What precisely is the mission design / trajectory change that allows this to happen? Will there still be a Mars flyby/gravity assist? Will they burn more fuel? Less? Will it hamper extended mission capabilities? Why isn't this information available somewhere -- I've looked and can find nothing.

[ugordan]

I think I read somewhere that the actual ion thrust period starts some 60-80 days after launch so there's plenty of margin there. Part of the additional margin might be supplied by the Delta II launch vehicle, too. Ion engines are weak but they sure provide greater flexibility than chemical ones.

[edstrick]

ONCE they have had operational experience in space, they will know the REAL flight power output levels of the solar panels and the REAL specific inpulse <pounds (force) of thrust for so many seconds per pound (mass) of propellant> of the ion rockets.

You HAVE to be able to fly the mission with an unfavorible launch during the worst part of the launch window, large launch errors, poor solar panel performance, and poor ion drive performance.

If everything is nominal, you are going to have flight reserve capability. or be able to handle 'Uh-oh"'s that develop during the mission.

If things work generally better than nominal, ooooh!

[elakdawalla]

The fact that those solar panels and ion engines need to be tested out in flight to determine their actual performace margins is the reason that the timelines, post-launch, are somewhat vague. The basic plan will be the same -- Mars flyby followed by Vesta and then Ceres -- regardless of the launch date, but I've been told that the Vesta arrival date, for instance, can change by as much as a couple of months from the nominal one if the engines perform significantly better than their design requirements. They'll just have to wait and see how fast a ship they've got once it's set sail! This is also the reason they're being cagy about whether they'll be able to do any other asteroid flybys, however distant. Until they know how well their engines are performing, it's kind of hard to figure out which asteroids will get close enough to the trajectory, and when, for them to be observed.

--Emily

[nprev]

OT here, but I find it intriguing that the engines provide so much flexibility in terms of arrival times despite their incredibly small delta-v in comparison to chemical propellants. It seems that even minor improvements in this technology will reap serious benefits for UMSF.

Pity that Xe has the highest atomic weight of the stable noble gases. I know I'd be shot for asking this in some circles, but wouldn't it be nice to use radon with its big, fat average atomic weight of 86 vs. Xe's 54...? (That stuff has to be good for something, after all... )

[kwp]

wouldn't it be nice to use radon with its big, fat average atomic weight of 86 vs. Xe's 54...?

True. Pity, though, about that 3 day half-life...

Mercury (the element, not the planet) is nicely volatile, relatively easy to ionize and weights in
at a nice, fat 200 atomic mass units. Unfortunately, though, it's tendency to contaminate surfaces has
precluded its use in ion thrusters.

-Kevin

[edstrick]

" incredibly small delta-v in comparison to chemical "

Ion rockets have enormously large delta-v capability because of their high specific impulse <bounce per ounce>. The killer is that they have horrendously low thrust... the force is small, but they difference is they keep going.. and going.. and going..........................

I wonder about the use <and cost per pound> of Krypton or even Argon. You get less bounce per ounce, but JIMO, for example, somebody said, was going to use more or less the entire world's supply of xenon. If simply lobbing something on an escape trajectory cost less, it might be cost effective to use more of a lower atomic weight ion. What are current tradeoffs on different gasses for ion rockets?

[hendric]

You get less bounce per ounce, but JIMO, for example, somebody said, was going to use more or less the entire world's supply of xenon.

I think later on we found some more information that showed the magnitude was off by a factor of 10^4 or so. So there's plenty of Xenon gas for space exploration (at least for now).

[The Messenger]

Pity that Xe has the highest atomic weight of the stable noble gases. I know I'd be shot for asking this in some circles, but wouldn't it be nice to use radon with its big, fat average atomic weight of 86 vs. Xe's 54...? (That stuff has to be good for something, after all... )

I have been told hydrogen would actually be the most efficient ion fuel - If it could be stored more efficiently at high density. I think it has to do with the efficiency of acceleration mechanism - the amps per unit of thrust ratio. I'll try to find out more...

[ugordan]

I have been told hydrogen would actually be the most efficient ion fuel

The way the rocket equation works, the most important thing in being efficient is the speed of the exhaust, not mass expelled per second. Just as chemical rockets try to achieve high temperatures and low molecular weight of their exhaust, the same would probably apply to ion engines. The trouble here would be as you say in the acceleration mechanism. Practical difficulties using a low molecular mass gas likely greatly offset the theoretical advantages it would have.

[nprev]

Hmm. Thanks for the enlightenment & corrections, all.

So, then, the ideal fuel would be an intermediate weight element that is particularly amenible to electromagnetic acceleration & magnetic vector control? Would something like Fe ions be the best (disregarding the horrendous difficulties of vaporizing the stuff)?

[Greg Hullender]

All other things being equal, the specific impulse of an ion drive using Xenon is about 11.5 times worse than one using hydrogen, BUT the thrust is 11.5 times better. Since even with Xenon (worse Isp), such a thruster has 10x the Isp of a conventional rocket BUT even with Xenon (better thrust) the thrust is so low as to barely be usable, it seems clear why it's currently the fuel of choice for ion drives.

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.

Since the mass ratio varies EXPONENTIALLY with the specific impulse, and exp(11.5) = 100,000, this does say that we could do fantastic things if we could just get enough energy to make a hydrogen ion engine practical. (And solve some other technical problems, of course.) :-)

--Greg

[edstrick]

"...I think later on we found some more information that showed the magnitude was off by a factor of 10^4 or so..."

What's 4 decimal places between friends...?