OK, this isn't complete, but I'm getting way behind, so I'm posting what I have so far.
Airship to Orbit
The technology is disclosed in U.S. Patent Application Publication 2007/0205330, filed by Hubbard Aerospace, LLC, published on 6 September 2007 (it's attached to this post as a PDF, too). The premise here is that a semi-rigid dirigible type craft can be taken into orbit. The craft does not rely solely on lifting gases for lift, nor does it use conventional propeller-type drive, but uses a combination of lifting gas, lifting body shape, and turbojet engines that can draw air for combustion from reservoirs within the craft itself when outside air is too rarefied or simply unavailable.
When I first saw this, I thought it was iffy, but the more I read, the more I believe that it might work. The initial problems I saw with it were:
1. It's a blimp. As it goes higher into the atmosphere, there's going to be less and less air outside it to provide buoyancy, and it's going to have to travel fairly quickly in the rarefied atmosphere to substitute lift from its body for the lost buoyancy. Can it really get to the required speed?
2. It's a blimp. To achieve low Earth orbit (LEO), you need to get moving pretty darn fast. Like on the order of thousands of miles per hour. Like over 17000 miles per hour. Can conventional turbofan or turbojet engines get something moving that rapidly?
3. It's a blimp. Assuming it does make it to LEO, it's going to be hitting atmosphere at very high speed (again, over 17000 miles per hour) and encounter very hot temperatures. The temperatures reentry vehicles experience on their external surfaces can be hotter than the surface of the sun. Can the fabric of the blimp take the temperatures that it will encounter?
4. It's a blimp that uses hydrogen as its lifting gas. I'm sorry, but when I see blimp and hydrogen in the same sentence, I think of the Hindenburg. For those who don't remember what the Hindenburg was, it was a huge German dirigible (a.k.a. Zeppelin) that in 1937 crossed the Atlantic to a New Jersey port where it exploded and came down in a fiery mass that killed a lot of people and made a very long-lasting impression that hydrogen in blimps is bad. Knee-jerk reaction: hydrogen BAD.
So, let's take a look at these four issues. The first one that I looked into was the reentry problem. Yes, that presupposes that the craft achieves LEO, but a failure on reentry would probably be the most catastrophic, as we saw with the Columbia disaster earlier this century. For those that don't remember, a failed tile, less than six inches square, on the leading edge of one wing led to the complete destruction of the Shuttle during reentry and the deaths of all seven astronauts aboard. Note: an interesting independent analysis came to a very different conclusion about what caused the destruction of Columbia.
Back to the blimp reentry issue. The heat of reentry is generated not from friction but from the dramatic shock wave produced by the object entering the atmosphere at hypersonic speeds. For an orbital body, its weight must be regarded as the centripetal force keeping it in orbit, but the orbital speed must be high enough to counteract that weight to keep the body from falling. Thus, F=mg=mV^2/R, in which mass cancels itself out so that V=SQRRT(gR). g is the acceleration of gravity and R is the distance from the center of the Earth, which in this case is (RE + h), where RE is the radius of the Earth and h is the altitude of the satellite. The gist is that orbital speed is independent of mass - no matter how big or small an object is, it has to maintain the same speed to stay in orbit at a given altitude. A typical LEO is and altitude of around 120 miles. Using the radius of Earth as 3800 miles and the acceleration of gravity as 32.2 ft/s^2 = 32.2((3600s/hr)^2)/5280 mi/hr^2 = 79000 mi/hr^2, we get V=SQRRT((79000)(3920)) = 17600 mi/hr or around Mach 20. In layman's terms, that is really friggin' fast. This is many times faster than the fastest aircraft we know we have produced - the Blackbird SR-71 reconnaissance plane, which could do better than Mach 3. So this blimp is supposed to hit atmosphere at Mach 20 and is supposed to experience a smoother reentry than, say, a Space Shuttle, which has a metal airframe and metal skin.
