Fuzzy Dunlop

Firstly, it's likely that the X-37 has more than one purpose - the USAF loves its multi-role vehicles. The most obvious use is reconnaissance. Traditional spy satellites have two issues. The first is that they are predictable, they tend to overfly the same area at the same time every couple of days, so if you're smart you can often hide your activities. You can get around this by altering the orbit, but you have limited amount of fuel, so you can't do this too often (most observed satellites do this 4 or 5 times during their mission). The second issue is that they're staggeringly expensive, a single KH-11 is belived to cost about $4.5 billion - as a single piece of equipment only a Nimitz class supercarrier is more expensive. So there is a clear case for re-use, so you can change orbit frequently and once you run out of fuel you can just land, gas up and re-lauch - without having to shell out billions in new optics. Now the X-37 is far too small for a fully fledged KH-11 (think a heavily upgraded Hubble) but it's cargo bay could accomodate the optics of ORS-1 (a small reconnaissance satellite the USA recently launched to provide intelligence in Afghanistan) which is still quite useful. During the first X-37 mission USA-212 the craft frequently shifted orbits, and all of it's orbits were precisely in sync with the earth's rotation so that the X-37 was in exactly the same place, at the same time of day, every 2 or 3 days. That's a clear pattern that indicates earth observation was a mission objective. Of course there are likely other uses - the USAF has launched a number of small experimental sattelites, including reconnaissance sattelites operating in unusual spectra, that the X-37 could be useful in prototyping.

Think of it like a hybrid rocket engine. The air is rammed through a block of solid fuel. You can even use coal... I was thinking about that. Could you have a precooled jet engine which used kerosene? Obviously it wouldn't be able to near liquify the air, but is that a complete deal breaker?

Ramjets are vastly more simple, and thus cheaper, than anything with a turbine can be. So I think for disposable uses like missiles (which is the major use for ramjets currently) they will stay in fashion.

In any liquid rocket engine you have to figure out a way to get the fuel into the combustion chamber. In a large design that means you need to force a vast amount of fuel (the Space Shuttle burned 535,000 gallons of fuel - about an Olympic swimming pool full - in 8 minutes) against an incredibly high pressure (200 atmospheres for the Space Shuttle). That requires some enormously powerful pumps. In fact these turbopumps are generally the most important and expensive component of any design - sometimes launch vehicles get called "a turbopump with a rocket attached". Basically they work a bit like a jet engine, only instead of burning fuel to accelerate air, they burn fuel to accelerate more fuel. This generates some serious power. The most powerful rocket engine ever built is the Russian RD-170, originally built for the Soviet shuttle programme and now used in the Ukrainian Zenit rocket. This design has 4 separate combustion chambers gathered around a common turbopump, which produces 190 MW - enough to power Salt Lake City.

I know the Indian PSLV (Polar Satellite Launch Vehicle) vectors its thrust by injecting a solution of Strontium perchlorate into the nozzel of the solid first stage. This was stored in strap on tanks like the ones on the Titan SRBs.

I'm betting It has something to do with the steering. On the Titans with SRBs the core engine wasn't ignited at launch so that couldn't be used for thrust vectoring.

The radioactivity of the exhaust isn't an issue for two reasons. 1) The exhaust is moving so fast that it won't stick around long enough to cause any issues. 2) The exhaust is moving so fast that most of the nuclei in it will have more kinetic energy than your average alpha particle. Anything that gets in the way of the exhaust will be utterly obliterated.

Reaction wheels are unlimited in the sense that you have unlimited pointing ability provided you have no external torque. RCS dosen't give you that.

1) Basically because Kerbin is much smaller than earth LKO is a much smaller, tighter circle than LEO. Because the orbit is tighter you get higher centifugal forces at the same speed. So around Kerbin you don't need to go as fast for the centrifugal force to cancel out gravity as you would around earth. 2) I think that figure might be the longditude of the ascending node. edit: just realised thats probably not a very helpful answer - the longditude of the ascending node is basically the angle between the node and an arbitrary line pointing away from earth, through the equator towards a fixed reference direction. Basically Iridium has 6 orbital planes, which each cross the equator at 30 degrees to each other. The longditude of the ascending node defines which plane is which. All the planes have the same angle to the equator i.e. inclination

Yep light can bend space, in fact if you have enough light in the same place it can warp space so much it produces a black hole, called a Kugelblitz

Why not just cut the solar pannels, the microwaves and the battery and just focus the sun onto the propellent with a big curved mirror. AKA a solar thermal rocket.

For Low explosives like gunpowder and rocket fuel yes. For High explosives like TNT and C4 no.

With the ruggidized medium wheels I seem to have an issue where on certain sections of terrain they give almost no grip - then on the next section they work fine. And there seems to be no logic to which sections have this behaviour so I think it might be a bug.

Using current technology we can't go that fast at all, voyager is acctually travelling slower than earth orbits the sun (hence it acctually gets closer to use for a few months of the year). NASA's Solar Probe+ will exceed 200 km/s, but that's using the Helios style sundiving strategy (only much much closer)

I belive the LISA pathfinder spacecraft is planned to use FEEP based attitude control

Well obviously if you drop in from an orbit on the edge of Earth SOI you're going to hit the atmosphere at near escape velocity. But from LEO you should be mostly OK, according to a previous XKCD what if that's barely enough to sear a steak. There's another factor here in that dropping straight down is, for the same speed, much worse than entering at a shallow angle. If you just graze the atmosphere you can kill a lot of your velocity in the very thin upper layers and hence the peak tempuratures aren't nearly as high.

Well the Challanger SRB is the only stage of any launch vehicle ever to directly kill its riders (though a Soyuz has come very close) so in that sense they are empirically more dangerous. Also you can turn liquid engines off, which is especially useful on the pad. The Space Shuttle did this on several occasions. With solids you might not know about a fault until you launch it, at which point it's too late. Another thing is that when solids fail they tend to explode spectacularly, which liquid engines generally don't do. Falcon 9 has survived a engine failure on it's first stage, if the same thing happened on a Delta 2 strap on the entire rocket would be toast. Also solids tend to cause more vibrations than liquids, which can shake around vital equipment.

At times like this I hate Lancashire and it's clouds (which have the additional disadvantage of reflecting all the light from Manchester). I've only seen one so far. Also, my neck hurts.

Firstly SpaceX use Kerosene in the upper stage because they have experience with it in the lower stage. Kerolox engines are completely different from hydrolox engines, so SpaceX would have to spend huge amounts of time and money developing another powerplant. They would also have to pay for the expense of maintaining two different production lines. Liquid hydrogen also requires a lot of extra pad infrastructure that SpaceX would have to build and maintain. Liquid hydrogen has some disadvantages despite its high ISP. It's low tempurature and density requires huge insulated tanks, which can be more expensive to manufacture. Also the Zenit rocket is fairly modern and that uses Kerolox on all stages.

Well K^2 gave the example of Diamond to Graphite, and while it's true that Graphite is more stable (under normal conditions) than Diamond, it's not by that much. Then energy released when Diamond is converted into Graphite is relatively tiny. Converting from Metallic Hydrogen to regular would liberate a massive amount of energy, so it's slightly harder to see how it would remain stable. (Though there are lots of stable compounds that do give out a huge amount of energy when they decompose - eg TNT)

The hydrogen is allowed to boil off and escape through pressure relief vents because otherwise the pressure in the tank will rise until it explodes. It's much easier to keep it liquid by keeping it cold than by keeping it under pressure.

It's the same reason a rifle bullet is spun - to keep it pointing in the same direction. Think of a bike, balancing on it when it's still is difficult, but when the wheels are spinning they try to hold themselves upright. The upper stage of the Delta 2 rocket is a solid motor with no thrust vectoring. It can't control its direction so instead it is just pointed where it needs to go and then spun up to keep it heading that way. After it's burned out the payload can release weights to slow the spin down, like an ice skater throwing her arms out. The spirit rover travelled to mars spinning at about 2 revolutions per minute. While the rover was in transit the engineers wanted to keep the comunication antenna pointed at earth and the solar pannels at the sun. They could have used RCS thrusters to keep it steady, but that takes fuel, they could have used reaction wheels, but they are complex and heavy. So the simplest option was just to keep it slowly spinning while pointed the right way.

This is utterly fantastic!

Sphereical re-entry means you can't produce any lift. Lift is good because you can stay in the upper atmosphere for longer and slow down more gently. Pure ballistic entry with a spherical capsule give g-forces which are brutal on astronaut bodies. That's not too far off the soyuz capsule design.

The landing gear probably won't be motorised because that would make the wheels sort of redundant. I think they may well introduce steerable gear though.