[ExI] Spacecraft (was MM)

Keith Henson hkeithhenson at gmail.com
Sun Jan 2 01:39:37 UTC 2011


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

>>> Using the standard 1 MW/kg gives 300 GW for a 300 ton vehicle,  50 GW for a 50 ton vehicle.  Lasers are generally 10% power efficient so 10x the output power is needed to drive them.  What is the joke?
>>
>> 1MW/kg is what you need to boost against 1 g.  The trick here is to
>> get up high burning hydrogen and air with a substantial vertical and
>> horizontal velocity before the laser takes over powering propulsion.
>> Then you use a *long* acceleration to reach orbital velocity.  See
>> figure 4 here http://www.theoildrum.com/node/5485 for a typical
>> trajectory.
>>
>> And laser diodes are now 50% efficient with an ongoing development
>> project projected to reach 85%.  This is monochromatic rather than
>> coherent but the light can be converted to coherent at a loss of 10%
>> or less.
>
>
> I don't think you can quote laser diode efficiency when talking about these very high powered lasers without talking about the pumping methods, light and heat damage to components and so on.

I was mainly making the point that your number of 10% efficient is way
out of date.  Light damage is mainly a problem with ablation
propulsion, heat you just design to deal with.

6 GW would not be one laser, but more than a thousand 1-5 MW units.
The largest continuous laser is a truck mounted military one rated at
105 kW.   Internally it is a number of 15 kW units.

>>>
>>>> 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?

> 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.

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.

> 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 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.

> 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.

> 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.

> 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.

> 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.

> 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.

Keith




More information about the extropy-chat mailing list