Page Text: These are big picture sort of posts, but as someone who knows the details, I’d like to hear what you think.
All best,
on Wednesday 2008.07.16 at 19:36 | Reply
gravityloss
Nice, can’t comment much right now, I’m leaving for a few weeks but I’ll do something when I get back…
You might want to check Jon Goff’s analysis about colonization, comparing it to Salt Lake City…
engstudent
great site – I noticed yourre a fan of New Space.
I loved the X-33/Venturestar idea wish they couldve at least flown it and demonstrated some of the tech.
Im kinda stuck with the impression that commercial launchers arent ready for primetime and NASA still has to trail blaze for a few more decades.
Great blog though, do you follow NSF for news and discussions?
on Saturday 2009.07.18 at 10:23 | Reply
Andrew W
I’ve spent some time hunting around spaceflight blogs for discussion of a launch system that makes sense to me but perhaps isn’t feasible as I haven’t found anything about it, perhaps you have?
The idea similar to the microwave powered lightship using LH2 as the propellant.
The craft launches and climbs through the atmosphere using an air breathing engine, perhaps a turbo rocket, when it gets to high altitude, rather than use a microwave beam directly to heat the propellant, collect the beam via a large rectenna on the top surfaces of the craft, then use that energy to heat the propellant, either with a carbon arc or by turning the electricity back into microwaves.
The microwave beam could also be used as the craft climbs through the atmosphere to accelerate H2 before it enters the combustion chamber of the engine, thus increasing exhaust velocity.
As the microwave beam is collected over a large surface area precise focusing of the beam, as is required in the lightship concept, wouldn’t be necessary.
In principle, with the combination of air breathing and H2 propellant the system should be able to achieve a specific impulse of close to a thousand seconds, making SSTO possible.
given geosynchronous SPS’s are supposed to be able to focus microwave beams on to rectenna’s 10km across over about 30,000km, a target (say) 30 metres across from a distance of say 300km doesn’t seem unreasonable.
The ground hardware shouldn’t be too hard or expensive to build and power requirements could be supplied from an existing national grid.
Cheers,
on Tuesday 2009.07.21 at 20:53 | Reply
gravityloss
This requires huge infrastructure in the form of space microwave transmitters, if it’s on top of the craft.
“As the microwave beam is collected over a large surface area precise focusing of the beam, as is required in the lightship concept, wouldn’t be necessary.”
I think the space microwave concepts have low power per area – not sufficient to power a multiple hundred megawatt or gigawatt class rocket with a few meters of collecting area.
You WANT high power per area for a beam rocket.
And the power system could be heavy, if you have to electrically transfer and possibly transform something the order of a gigawatt.
on Wednesday 2009.07.22 at 20:33 | Reply
Andrew W
Ground based transmitters would be used as such a craft could fly inverted once it had climbed through the atmosphere (as the shuttle does).
Beam densities of 50-100MW/ sq m, with total utilised power of around 50,000 MW should be about right for something the size of a delta winged 767.
Getting high beam density may be why this idea isn’t being pursued with much vigour, mircowaves are hard to focus, my problem is my maths ain’t that hot so I’ll need to learn a few equations to know for sure.
Rectennas are fairly light.
The engine discussed here, Scaled up 10,000 fold might be the sort of thing needed, 22 kg thrust at high Isp from something that fits in a shoe box sounds encouraging: http://www.waynesthisandthat.com/mpd.htm
With enough power and with hydrogen propellant the Isp of this system can apparently be pushed as high as 15,000 seconds, you wouldn’t need anything like those exhaust velocities to get propellant ratios way below 50%.
Obviously getting those propellant ratios down makes a huge difference to how heavy the engines and other on board systems can be.
Regards