SpaceX's Grasshopper Successfully Flew 325 Metres
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2 months ago they hovered at 250m: http://www.youtube.com/watch?v=xUvbh-Z8Abk
6 months ago they hovered at the height of a 12-story (40m) building: http://www.youtube.com/watch?v=Zz-NYeH-CEY
These guys are on a very fast learning curve. These near-ground tests are the hardest.
Elon said that they will be using a fully reusable first stage in 2015, only 18 months away: http://allthingsd.com/20130530/tesla-ceo-and-spacex-founder-...
[http://en.wikipedia.org/wiki/SpaceX_reusable_rocket_launchin...]
During the Corona spy satellite days, they used to catch the parachuting pods with a plane+hook. http://upload.wikimedia.org/wikipedia/commons/7/76/Keyhole_c...
(I specified American because the Soviets and Russians have historically lagged behind here, and it looks like that's still the case. I don't know what the story is for other ICBM-equipped powers. The Soviets compensated for their lesser accuracy by fielding larger bombs.)
Also keep in mind that the major factor limiting ICBM accuracy these days is the need to be completely self contained, since they're intended to operate in the middle of World War III, and no external navigation sources can be counted on. Slap a JDAM guidance system on the warhead and your ICBM can deliver it to within whatever square meter you're most interested in.
ICBMs armed with conventional bombs have been considered, since accuracy can be enough for that to work now. As far as I know, the only reasons these haven't been deployed are cost (using up an entire ICBM to deliver a chemical bomb seems wasteful) and, probably more importantly, the fact that there's no way to tell whether a given ICBM is carrying a nuclear warhead or not, thus the strong possibility that using such a weapon could trigger a nuclear war by accident. See:
The CEP figure represents the radius of a circle where there is a 50% chance of the bomb landing within that circle. The CEP figure is basically 2/3 of a standard deviation.
In any case, accuracy is gated by navigational accuracy, not so much guidance, as long as your initial aim isn't too bad. ICBMs have a 100m CEP because they're using inertial navigation all the way. Switch to GPS and you'll have no trouble doing better. In the context of SpaceX, getting the rocket to within a few hundred meters of the landing pad is perfectly fine initially, because it can then maneuver to the pad more precisely.
It's like the difference between an ultralight and a 747. Both are impressive, but on completely different scales.
Not to say a 100% flight record wouldn't be fantastic, but I'd really enjoy seeing the trend of doubling height to continue, or even greater.
Look at the Apollo program, for example. I doubt anyone would say they weren't trying as hard as they could, and yet the only failure I'd call "spectacular" was the very dumb and useless failure of Apollo 1. There were lots of other failures, of course. Apollo 11 had persistent trouble with the computer during landing, nearly leading to an abort. Apollo 12 got hit by lightning on launch, twice, and the mission was only saved by quick thinking. Apollo 13, in addition to the famous explosion that happened partway through the mission, lost an engine on launch. On later investigation, it turned out that the engine loss was due to massive oscillations in the rocket that would have torn it apart within a few more seconds if the engine hadn't fortuitously shut down. Apollo 14 had docking trouble, had to have its computer reprogrammed by the crew in flight to work around a faulty switch, and finally had a radar failure, all of which threatened the mission's success if not the astronauts' lives. Apollo 15's first stage didn't shut off when it was supposed to, nearly requiring a launch abort. Apollo 16 looks to be the first one that didn't encounter major problems.
I wouldn't be surprised if SpaceX didn't have a lot of similar failures, which are big deals to them but don't look like anything other than "success" to outside observers.
We have tried engineering more complex re-usable lifting body type designs, only the shuttle ended up flying, and it wasn't cost effective.
Let's do some very rough numbers to get a ballpark...
Say terminal velocity is 100 m/s, which is fairly fast, but a rocket is a skinny streamlined thing. (It could fall slower, maybe even 50 m/s, on its side or with parachutes, but parachutes are heavy and complicated, and falling sideways may be difficult to control, but seems quite possible.)
Say an F9 "1.1" first stage weighs 28,000 kg. The landing components mostly are already there: the engines are already gimballed and the additional software weighs nothing. Unless you're so awesome you can land in a cradle--which is not impossible--you need some landing gear, so let's add 3000kg, for a dry weight of 31,000 kg.
Amongst all its engines, our F9 has some 300,000kg of thrust. Just one engine is around 33,000kg thrust, so we'll probably only use one or two.
Two engines will get us easily about 10m/s slower per second, so we need to retro-fire for minimum 10 seconds. You probably want some margin over that; let's reserve 20 seconds fuel.
F9 1.1 stage 1 will have a burn time of 185s; our retro burn is only at about 25% capacity, so 20s of two engines is worth, oh, 5s of normal burn time. Say about 3% of the fuel.
Of course, it's not that simple, because (1) we've made the stage heavier with the landing gear, which requires more fuel and (2) to avoid losing performance we'll want to add enough fuel to add our extra burn time, which also requires more fuel, and (3) we need more fuel to lift all that fuel and mass we added. The rocket equation strikes again!
(Of course, it may not be that bad, because usually first stages have plenty of margin, and most payloads are not max. Maybe if you encounter a really heavy payload you just agree to throw away the rocket that time; use an old one or something.)
Most of my assumptions are pessimistic (I think) but in the real world they may turn out optimistic. I would guess you're looking at 3-5% of the fuel load for a recoverable VTOL stage. A lot depends on details, as with rockets weight scales very badly against performance.
They have lately been working on parachute-only recovery, apparently having decided that the mass/complexity penalty of parachutes was better than that of retro-fire fuel. They've lost several vehicles already to minor parachute problems. Parachutes aren't easy.
SpaceX, working at a different scale (though not a different flight regime, at least for the first stage) still thinks retro-rocket landing to be the answer. I imagine an Armadillo-size rocket is less likely to have its various parts crushed at a parachute-speed descent than an F9 stage. Armadillo does occasionally get a STIG back in one piece, and parachute-into-water F9 stages are known to be completely trashed. Dry weight would seem to make a difference here.
Parachutes have lots of trouble. The rate of descent they achieve is still fairly fast, so you need something to brake the descent at the end.
This is why various American capsules splashed into the ocean, as well as the Shuttle boosters, because the water acts as a cushion. However, dunking your delicate rocket in salt water is a bad idea. The American capsules weren't reused, and the Shuttle boosters required enough refurbishment afterwards as to make it not really worth recovering them.
You can land on land, but then you need some seriously hefty landing gear, or retro-rockets. The Soviets use retro-rockets fired at the last moment to recover their capsules on land. A rocket has problems that a capsule does not, though, namely that you need to keep it from tipping over once it lands! You can make a capsule fat, but a rocket really needs to be tall and thin, and any sort of horizontal velocity will have it falling on its side. You also have very little control over where the rocket comes to a rest. Again, tolerable for a capsule, but dropping your rocket into a forest is not going to be conducive to reusability.
You can add wings, like the Shuttle did, and land horizontally. Trouble with that is that wings are extremely heavy. Each pound of wing in the Shuttle was a pound of payload it couldn't carry. If you try to fit every stage of a rocket with wings so it can come back and land, you'll end up not having much excess capacity left over to actually carry a payload. Wings are great for aircraft, when you can use them to efficiently fight gravity for the whole flight. But they suck for spacecraft, where they don't help at all on the way up, and are just dead weight for most of the flight.
So landing a rocket vertically on its engines isn't really all that bad in the end. Inelegant, perhaps, but it has a lot of advantages. You get absolute control over where the rocket lands. You can land it as softly as you like. You can ensure that it touches down with no horizontal velocity so as not to tip over. You use the same engines that you used for the launch, so there's no extra weight there. The only real downsides are the need for some sort of landing gear, and the need to carry a bit of extra fuel. Neither one is all that bad.
When considering the stability, don't make the common mistake of comparing it to balancing a pencil. When the rocket turns, the motors turn as well, so it is not inherently unstable the way you might think. Rather, the stability is neutral: if you perturb the rocket, it will just stay in its new position, not returning to how it was before, but neither does it run farther away. A rocket that's slightly tilted will stay slightly tilted (gradually accelerating sideways, of course), it won't tilt over further the way a pencil does.
Note that Robert Goddard made this same error. He built his first rocket with the engines at the top, believing this would help with stability. So if you were thinking that way, don't feel bad: the father of modern rocketry did the same thing!
Could be that the weight savings in fuel aren't enough for that choice, or that there are other good reasons not to (added complexity, potential non-reusable components)
[1] Although, aeronautics is famously non-intuitive, so ymmv.
I've studied physics in undergrad but I still made the mistake you describe in your last paragraph. The Grasshopper intuitively feels dynamically like an inverted pendulum but even harder because all you have are rockets at the bottom and you can adjust their rotation (I assume?).
On the other hand, there is no fixed pivot point around which you get torque, is this what the difference comes down to?
Do you happen to have any pointers to some articles that discuss this, and how difficult it is to balance? How does the wind affect things? What are the biggest challenges?
The inverted pendulum case (as approximated when balancing a pencil on its tip) can be reduced to a single object resting on the surface of a frictionless sphere, under the influence of gravity. At the precise top of the sphere, it can stay stationary. Perturb it even slightly, and it begins to accelerate away.
With the rocket, the object is essentially resting on a flat frictionless plane. If you tilt it a bit, then it begins to slide away, but the acceleration is always proportional to the initial perturbation. This is because the rocket always remains in the same orientation, and so the force always comes from the same direction. That, in turn, is because the rocket engine moves along with the rest of the rocket, rather than being a fixed point in space.
Think about it another way: without external support, and ignoring the influence of the atmosphere, a rocket is in free fall. The fact that it's accelerating to counter the force of gravity doesn't change that fact. Again ignoring the atmosphere, a rocket firing with 1g of thrust counteracting gravity behaves the same as a rocket firing with 1g of thrust in empty space. The second case is obviously not unstable (what would it be unstable with respect to?) and so the first case isn't either.
Adding the atmosphere back in, I could be way off here, but I think what little effect it would have at low speeds would be beneficial. Since the goal is not to move, the drag from the atmosphere will help make sure that happens. Since the rocket is more or less of a uniform shape, that drag shouldn't end up applying much of a torque either.
Edit: forgot to mention, I'm afraid I don't have pointers to articles. I've collected this stuff through various readings and thought over the years and don't have anything specific to point to. I think I first learned about Goddard's pendulum mistake in high school physics, for example.
The most important factor in reusability is making sure that the vehicle gets back to someplace convenient without any significant damage. If you crash-land using parachutes somewhere in the ocean or in a desert, that's not going to happen. So you need a controlled landing at a prepared site. The overhead necessary to return to the original landing site is generally more than made up by the fact that you get the rocket back exactly where you need it within a matter of hours and without any damage other than the normal wear and tear caused by every flight.