[ExI] Spacecraft (was MM)

[Samantha Atkins]

On Dec 31, 2010, at 9:48 AM, Keith Henson wrote:

> On Fri, Dec 31, 2010 at 12:32 AM,  Samantha Atkins <sjatkins at mac.com> wrote:
> 
>> On Dec 30, 2010, at 3:18 PM, Keith Henson wrote:
>>> 
>>> And 20 years ago, that was the right conclusion.  Now we have a path,
>>> even if it kind of expensive, to 9+km/sec exhaust velocity.  That
>>> means mass ratio 3 rockets to LEO and even better LEO to GEO.
>> 
>> I must have missed it.  Please give details, links, etc.  How expensive?  How large a payload? What technologies?
> 
> Context is SBSP, 200 GW of new power per year, one million tons of
> parts going up per year.  That's about 125 tons per hour delivered to
> GEO.


> 
> The SSTO vehicle is an evolution of the Skylon design
> http://www.astronautix.com/lvs/skylon.htm swapping lox for payload and
> a sapphire window between the engines with 10-20 bar hydrogen and a
> deep channel heat absorber behind it.  The flow of cold hydrogen keeps
> the window and the front surface of the heat absorber cool.  The
> absorber is described here:
> http://www.freepatentsonline.com/4033118.pdf
> 

A Skylon only delivers about 12 tons per trip to LEO.  They were designed for no less than 200 launch lifetimes.  And they were designed for two launch windows per day equatorial.     I don't see how you get from that to 125 tons / hr to LEO, much less GEO.  


> One part is fixed by physics and the Earth's gravity field.  The
> minimum horizontal boost acceleration after getting out of the
> atmosphere with substantial vertical velocity has to be slightly more
> than a g to achieve orbit before running into the atmosphere.  You
> want to use the minimum acceleration you can at the highest exhaust
> velocity you have energy for.  This keeps down the laser power, which
> is huge anyway.
> 
> This takes 15-20 minutes and only in the last third do you get up to
> the full 3000 deg K and 9.8 km/sec.  The average (for this size
> vehicle and 6 GW) is 8.5 km/sec, but the first 2 km/sec in air
> breathing mode has an equivalent exhaust velocity of 10.5 km/sec.  So
> about 1/3 of takeoff mass (300 tons) gets to orbit.  The vehicle mass
> is about 50 tons leaving 50 tons for the LEO to GEO stage.

That is a much much larger craft than a Skylon.  Do you have links for it?  Note that an Ares V launch (super heavy launcher) can only put about 63 tons into GEO.  So this craft capability seems like serious magic to me. 

> 
> So the payload at GEO per load needs to be 1/4 to 1/3 of 125 tons.
> Again using laser heated hydrogen 35 tons of a 50 ton second stage
> will get there.  With some care in the design, it can all be used for
> power satellite construction.
> 
> The long acceleration means the lasers must track the vehicle over a
> substantial fraction of the circumference of the earth.  

Wait, you are using lasers to provide thrust to this big honking lift vehicle?   I presume you are aware we have only tested this for very very small vehicles and never to high altitudes.    This is in no way near term tech for a vehicle of this size.  Or do you intend to user laser propulsion only for the LEO to GEO phase?   

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?


> 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?  

> There are (I think) only four locations where there is an equatorial
> launch site with thousands of km of water to the east.  The US has one
> set, China has a better one.
> 
> The lasers are the big ticket item.  At $10/watt, $60 B.  

That is the cost for 6 GW.  

> The rest,
> vehicles, mirrors, ground infrastructure, R&D, etc might bring it up
> to $100 B--which is a fraction of the expected profits per year from
> selling that many power satellites.
> 
> I don't expect it to be done by the US.  China, maybe.

It don't expect it to be done by anyone in this manner  from the above description.  I don't see how to make such a vehicle or operate that kind of laser propulsion system at such a scale.  

- samantha