[ExI] Spacecraft (was MM)

Keith Henson hkeithhenson at gmail.com
Mon Jan 3 17:47:26 UTC 2011


On Mon, Jan 3, 2011 at 5:00 AM,  Samantha Atkins <sjatkins at mac.com> wrote:
>
> 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.

Ah.  The Hubble, which has been up for 20 years and is based on
technology at least 10 years before that, has a pointing accuracy of 7
milliarcseconds.  A milliarcsecond is about 5 x 10^-9 radians, so 7
would be about 35 x 10-9 rad.  At the end of a 36,000,000 m radius,
the error would be ~1.3 m

>>> 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.

It would be a ten fold scale up of the largest CW laser, then buy them
in the thousands.  Of course it will *never* be done unless some group
or government decides to do it.
>
>> 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 don't think so.  I have seen the video at the Beamed Energy
Propulsion Conference a year ago and as I recall, the laser was fixed
and the vehicle self centered in the beam.

> I expect that even only using laser propulsion starting around 300 km up would still have some such issues.

Laser propulsion of this kind has been proposed for a considerable
time without (as far as I know) finding such problems.  There were
very similar proposals to use lasers from space to power aircraft back
in the mid to late 70s.
>
>
>>> 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.

36,000 km.

A vehicle moving at upwards of 2 km/sec isn't going to deviate much
from the expected path.  It's also a cooperative target that would be
telling the control system acceleration details and location.
>
> 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.

It's a topic of considerable interest.  Short high intensity pulses
work better.  Google "laser ablation" "space debris"  A CW laser is
not as efficient, but small objects would just be vaporized in a few
seconds.  https://e-reports-ext.llnl.gov/pdf/245817.pdf  It's possible
to absorb MW per square meter into a flowing gas stream, but the
radiation equilibrium is probably above the boiling point of aluminum,
but probably less than reentry temperature.

>>  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

about a meter

>> 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.

Yep.  This is in the context of building 200 GW a year so the feedback
isn't excessive.

> 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.

You can bootstrap, especially with the LEO to GEO stage.

>>> 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?

One part in a thousand roughly.
>>
>>> 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?

Talk to Spike about the control problem.  But the acceleration is
modest, around a g, and while the velocity is high, the angular
velocity tracking the vehicle is low and you get feedback in less than
1/10th of a second.

>  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.

With ablation you have to wait for the previous vapor cloud to get out
of the way, typically about a ms.

> 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.
>
It might not even cost a lot if laser diodes could be used directly
through the window.  A 3 by 3 array of absorber channels would take
under 10 kW to heat.
>>
>>> 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?

At least a thousand.  Maybe 3% hot spares.
>>
>>> 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.

It's not likely to be an energy solution for the US so you don't need
to be concerned.  It's more likely to be something the Chinese do.

Keith




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