[ExI] Spacecraft (was MM)

[Samantha Atkins]

On Jan 1, 2011, at 5:39 PM, Keith Henson wrote:

> On Sat, Jan 1, 2011 at 1:19 PM,  Samantha Atkins <sjatkins at mac.com> wrote:
> 
>> On Jan 1, 2011, at 2:40 AM, Keith Henson wrote:
>> 
>>> On Fri, Dec 31, 2010 at 11:07 PM,  Samantha Atkins <sjatkins at mac.com> wrote:

<snip>

> 
>>>> 
>>>>> Based on
>>>>> Jordin Kare's work, this takes a flotilla of mirrors in GEO.  Current
>>>>> space technology is good enough to keep the pointing error down to .7
>>>>> meters at that distance while tracking the vehicle.  The lasers don't
>>>>> need to be on the equator so they can be placed where there is grid
>>>>> power.  They need to be 30-40 deg to the east of the lunch point.
>>>>> 
>>>> 
>>>> Uh huh.  What is the max distance you are speaking of?
>>> 
>>> Around one sixth of the circumference 40,000/6, 6,666 km.
>> 
>> That amounts to about 0.002 MOA tracking a rocket through atmosphere.
> 
> MOA?
> 

Miinute of arc.  

>> If we can do that then we can shoot down any old missile, any time with perfect accuracy.
> 
> The possibility of the laser beam going off target for some reason is
> why you want a long path to the east over water.
> 
> But yes, this transport method does have some rather obvious military
> applications.  A 6 GW laser beam delivers the energy of 1.5 tons of
> TNT per second.
> 

What I was attempting to point out is that we obviously do not have this kind of ability today nor, as far as a quick scan showed, is it expected any time soon.


> snip
> 
>> The current record for a small test vehicle climbing an admittedly low power beam is measured in the hundreds of feet.
> 
> The ones that have gone up a few hundred feed are not related at all
> to this kind of setup.  They only work in the atmosphere.  This works
> best outside.
> 

But you have a very long tracking path in conditions containing many possible sorts of turbulent and perturbation from ideal paths.  One of the challenges of the earth side experiment was in dealing with these to keep the beam properly centered.  I expect that even only using laser propulsion starting around 300 km up would still have some such issues.


>> A power beam that strong would bring issues of whether it would propel or melt the nozzles.  If the beam got a bit off center then it could be a real danger to the rocket itself which presumably is not of a high melting point alloy such as the nozzles would be.
> 
> The current thoughts on the design has the laser beam going through a
> sapphire window filled with cold flowing 10-20 bar hydrogen.  6 GW
> sounds like a lot, but it is absorbed over close to 1000 square
> meters.

So I am tracking a target approximately 32 m across up to 6000 km away with a mirror system that moves according to where the target "should" be rather than where it perhaps actually is due to the unexpected and/or incalculable.   

One nice thing about these big laser beams to orbit is they could accidentally or on purpose de-orbit a lot of space junk that has accumulated there.  :)  Hmmm.  Perhaps a decent feasibility test is to do some such target shooting.    Of course there would me a major international uproar over such. 

>  So that's 6 MW per square meter.  That's in the range of what
> happens inside the fire box of a coal fired power plant.  Thought
> about on a smaller scale, it's 600 W per square cm.  It's not hard to
> imagine a 1 cm square hole dumping 600 watts of heat into a flowing
> stream of hydrogen and heating the gas to 3000 deg K.  Regen cooling
> keeps the nozzle from getting too hot.
> 

Yes, I believe that part can work in principle.  I am worried by the required accuracy under real conditions though.

>> The aiming is by no means trivial.
> 
> I didn't mean to give the impression it was.  However, the pointing
> accuracy of Hubble is less than a meter from GEO to the trajectory
> path.  Tracking is slow traversing about 8 deg in 900 sec.
> 
>> Nor is the amount of power needed by the lasers.
> 
> It's a huge consideration.  At 50% overall, the grid draw would be 12
> GW.  On the other hand, Three Gorges is 22 GW.


So if the average SBSS produces 5 GW it will take nearly all the output of three of them to run this sort of launch pattern.   Is the 12 GW the minimum necessary for using this type of launch on this size of payload?  The answer changes the payoff and initial cost times considerably.  

> 
>> How do the orbital mirrors station keep reflecting that intense a power beam?
> 
> It's not particularly intense.  The mirrors in GEO are 30 meters across.

How much of the power beam is hitting each one?  

> 
>> What is the required station keeping and mirror adjustment speed?
> 
> You can compensate for the light pressure by orbiting 4 km inside GEO.
> Tracking is as above, slow.

Slow tracking gives no room for any perturbations in flight path, right?  Using ablative laser launch there almost certainly will be perturbations.  You are boiling off material which changes the effective beam strength in what seems to me a rather chaotic roiling pattern.   But the description of the window etc above may be of help in handling this problem by directing the superheated hydrogen.   It would be great to see a ground based demonstration of such an engine in a controlled smaller environment.    One of the objectives would be understanding the likely turbulence. 


> 
>> What kind of lasers do you have in mind for this application.  This site, http://www.rp-photonics.com/high_power_lasers.html, doesn't lead me thing multi GW lasers are particularly straightforward especially no for such sustained high precision power levels.
> 
> I don't understand why you think high precision power levels are required.


You don't?  You want a combined 6 GW I believe.  Even if you use a lot of lower powered lasers you have spread the problem out over many many beams to arrive on target.  How many beams are you thinking of?  

> 
>> The most powerful ground based lasers I could find were anti-missile lasers that seemed to top out at 10 MW or so.  These were not atmosphere compensated.  How much power will you lose to atmosphere compensation?  I understand thus far that atmospheric self-focusing only works in narrow power ranges defined by the type of laser used, atmospheric conditions and amount of atmosphere to be traversed.   All of this doesn't lead me to belief this is so straightforward.
> 
> I really don't like arguments from authority, but Dr. Jordin Kare
> http://en.wikipedia.org/wiki/Jordin_Kare knows far more about this
> than I do.  However, the proposal does not use power levels where you
> get atmospheric distortions.  Clouds at the laser end will be a
> problem.
> 


What I can find from Jordin Kare hasn't set my mind at ease on these questions.

- samantha