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I've been thinking about Airship to Orbit again recently. I don't think that the original proposal will fly without the development of lightweight nuclear fusion (say, Polywell), but we've talked about beamed propulsion before in that context. At 40km altitude, the horizon is ~700km away; at 20km, it's ~500km.
So, that's the length we can accelerate over if we only have one station (which, given its large size, can serve as a power source using concentrated solar power). If we accelerate at 1m/s^2, we'll reach 1km/s by the end of the track. If we increase that to 9m/s^2, we'll reach 3km/s by the horizon, if all of that goes into horizontal flight. Of course, we need to be accelerating upwards at the same time...
My thoughts have been to use a first stage airship to launch a rocket at that point, but now I'm wondering if we can sustain a heavier than air craft, getting it to a high (4-5km/s) velocity and altitude (100-150km) that can transition to rocket power at that point for the final trip to orbit.
The development plan I'm imagining would start with the station, followed by drop tests of the vehicle. Next step will be moving to a rocket powered suborbital vehicle, and then testing out the beamed power system. Finally, integrate the two, and keep going higher and faster until we reach orbit...
For the actual beamed power, I'm thinking we could use either laser ablation, laser thermal, or beaming electrical power which can then be used by a hybrid rocket system. Although, the combined laser-particle beam system proposed by GW in this thread might work well...
Use what is abundant and build to last
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Sounds fairly reasonable. There are a couple questions that pop up, to me:
1. Why not accelerate all the way to orbit? If you can accelerate at 10 m/s^2, couldn't you just accelerate for a full ~2500 km, end up a thousand or two km above the surface of the Earth, and then be in orbit? If beamed propulsion makes sense to use half of the way, it probably makes sense to use it for the entire way to orbit?
2. For beamed propulsion, I'd like to bring up my idea for using focused sodium vapor lamps as a cheaper and more efficient way to create a beam
-Josh
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1. If you can, then do so. But I think you'll be over the horizon at that point...
2. Using saltwater? Or Ammonia?
I envisage this as being launched above international waters (I'm thinking of the Atlantic), to avoid as many entanglements with states as possible - if you're launching from an airship, you can launch anywhere. The ocean is wide enough that the entire launch can take place over it. Given that it's a single stage, though, you can choose where you land (as long as it's along your flightpath).
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If you launch in a straight line, you will never be over the horizon, you'll just be rather far from the surface of the Earth. And I'm thinking that you would use Sodium Vapor lamps on Earth to generate the light which you will then focus. On the craft you could either use Hydrogen seeded with sodium or ablative methods as you wish.
-Josh
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Well, the horizon at 300km altitude is ~2000km, so yeah, you should be still within range. If your beam reaches that far, it ought to be possible to boost into orbit almost entirely under beam power.
Which, of course, brings up why you're putting your beam station in the stratosphere. That is partly to avoid clouds and other atmosopheric interference, and partly to avoid red tape and other bureaucratic interference...
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If your beam is starting from the stratosphere it presumably capable of going more or less to infinity. Focusing is somewhat of an issue over the longer distance, I'm sure, but in all honesty I don't think another order of magnitude of distance makes too much of a difference.
-Josh
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