[ExI] Spacecraft (was MM)

[spike]

... On Behalf Of Keith Henson
...
>>>
>>>> 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

Ja, there are two different things being discussed here, actually three:
tracking objects thru atmosphere, tracking a moving object and Hubble
boresight accuracy.  The Hubble is indeed an impressive control system, but
it cannot track moving objects very competently.  It is really really good
at doing what it was designed to do, fix on a dim object and stay right on
it for long periods.  But it isn't nimble, doesn't need to be for that
application.  One can steer a battleship with a canoe paddle, if one is
patient and has a few days to get it done.

For anything that moves or depends on an optical feedback, an arc second of
accuracy is probably still out of our reach, but there is plenty of useful
stuff we could do with arc-second class ground based tracking.  

 ...

>> 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...Keith

There are ways to do stuff like this using fast steering secondary mirrors,
adaptable aperture, lostsa cool notions for laser propulsion that were
actually developed from a weapons program, Airborne Laser:

http://en.wikipedia.org/wiki/Boeing_YAL-1

Booeing was the prime contractor for this, but most of the pointing control
and accuracy infrastructure was subcontracted to Lockheed Missiles and Space
Company, in Sunnyvale Taxifornia, and also the Lockheed Advanced
Technologies Center in Palo Alto, using Lockheed Martin control technology
and Lockheed Martin control engineers, who incidently worked for Lockheed
Martin.  Of course we used a Booeing product to carry our control system
aloft, so those lads deserve credit where credit is due, but the control
system was pure LMCo, and it is WICKED cool, do let me assure you, clever as
all hell.

Looks to me like we could adapt the accuracy infrastructure of the ABL to
fly at about 10 to 12 km altitude and provide second stage ablative boost
assist, from about 20 km to about 80-ish km altitude.  So first stage mostly
solid propulsion, second stage ABL ablative boost, third stage throwaway
H2/LOX?  We would need to get tricky with our optical feedback loops, but I
think this is doable with current control law.

spike