[meteorite-list] Dawn Journal - November 30, 2010 stupid question

[Göran Axelsson]

Hi Steve!

You still don't get it. The hydrazine isn't there for it's energy, it is 
there as mass easily ejected by the rocket nozzle. There is no need for 
beaming energy to a spacecraft as there is no easy way to store it and 
you could collect it from the sun via solar panels. Actually, during 
most of it's journey, Dawn is producing more energy than it can use with 
it's solar panels.

Your other idea of burning hydrogen close to the sun is also flawed. 
Even in the upper atmosphere of the sun the gas is so thin it is almost 
pure vacuum. (Seems to be 500 atoms per cm3 close to the 3000000 C 
corona. 1 gram of hydrogen contains 600000000000000000000000 atoms of 
hydrogen , it would take 1200000 cubic kilometers of solar wind to 
collect 1 gram of hydrogen close to the sun.)
But if you collected some hydrogen, what are you going to burn it with? 
Oxygen? If you bring oxygen then with just 12% more mass you could bring 
the hydrogen too. The oxygen to hydrogen weight ratio is 16 to 2.  And 
beside that, chemical rockets are a waste of mass, electric ion powered 
is the way to go, just as Dawn.
But if you managed to do grazing passes of the sun to collect energy and 
boost your speed you will run into another problem. When you reach a 
certain speed you would break free from the gravity of the sun, but 
before that your orbit would take you further out just like a comet. The 
closer to the escape speed the longer time it would take for each round 
trip. It would take a looooong time to reach escape velocity if you only 
accelerate close to the sun.

Just as Dawn, the more it accelerate away from the sun, the higher up 
from the sun it goes and contrary to what most people would think, Dawn 
actually moves slower!

And by the way, to reach light speed requires infinite amount of energy 
so it's not possible, at least with all the physical laws discovered so far.

/Göran


Steve Dunklee wrote:
> Thankyou for your explaination! In a future mission energy can be added to the probe with lasers masers or any other high energy beam to reduce the amount of hydrazine needed  and allow craft to gain energy in areas far from the sun . It would be cheaper to beam it up after launch rather than launch the energy up. I would still like to see NASA or the ESA launch a small bussard ram jet using the hydrogen near the sun as fuel to try to reach near light speed . Every approach of the sun in the right orbit would speed it up by gravity and then use the ions collected near the sun to speed it up even more. It could be done with a small relatively inexpensive probe. The first country to achieve light speed would be similar to the first man on the moon! CHEERS Steve
>
> On Sat Dec 4th, 2010 5:14 PM EST Göran Axelsson wrote:
>
>   
>> The reaction wheels is powered by electricity from the solar panels. The reason (as I understands it) to power down the reaction wheels is to minimize the wear on bearings and motors.
>> Using the attitude jets for turning is just another way of aiming the craft. What they have done is simply slowed down the speed it turns. A small acceleration and a small brake takes less fuel than a larger acceleration and a harder brake. It just takes longer time.
>>
>> For a solar powered spacecraft energy is free, reaction mass is expensive. The hydrazine and xenon is basically the same thing, to xenon energy is added from solar power while hydrazine contains it in it's chemical structure.
>>
>> /Göran
>>
>> Steve Dunklee wrote:
>>     
>>> This might be a stupid question but I was wondering if a future craft similar to this could use the ion propulsion unit to power the reaction wheels. And if the rection wheels or gyros are powered up to full speed before the launch if it could reduce the amount of energy neeeded? In addition a high powered maser could be used for attitude adjustment from earth or space withought using the energy aboard the craft. If designed correctly the craft could be repowered for attitude adjustment indefinately from earth instead of having to launch the energy from earth. Cheers Steve
>>>
>>>   
>>>       
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