Does rocket propellant (solid or liquid) get added to the rocket during the final preparation of the launch (count down) in all cases? If so, why is it filled at the final instant?
The bottom line is, rockets are fueled when it is the least dangerous and most cost effective to do so. Let me start by talking about some of the major types of rocket fuels, and when they are typically fueled. For the sake of simplicity, I'm making no difference between fuel and oxidizer, even though they are technically different.
Solid fueled rockets are usually filled when manufactured. Putting the fuel in can be quite dangerous, and needs to be done under exceptionally controlled circumstances. Furthermore, X-rays often need to be done on the fuel to ensure an even batch, cracking can cause serious issues during launch, up to and including total launch failure. Thus, putting it in when you build the rocket is usually the best path forward.
Cryogenic fuels, such as are used by Falcon, Saturn V, Soyuz, the Space Shuttle, etc. These need to be loaded in at the last minute, because they will boil off of the rocket, and can potentially cause damage to the rocket if left for too long.
All other liquid fuels are highly toxic, and as a result, require special precautions when operating around them. Thus, they are usually not loaded until the last minute. The exception for this might lay in military grade missiles, which need to be fueled and ready to go at a moment's notice. These are usually placed in some silo, allowing for greater environmental control than a rocket exposed to the elements.
Spacecraft fuels are typically loaded at the launch site, as many of them are toxic as well, and virtually all of them are explosive. They are usually loaded prior to the countdown commencing, however, but something like a week or two before, depending on how things work exactly. The primary exception is Xenon used for ion engines.
For more information, see Wikipedia.
No. Two examples:
Solid rockets are "fueled" (not really the right term) during manufacture. The grain of a solid rocket motor contains a mix of fuel and oxidizer. Once manufactured it is integrated into the rocket. No additional propellant is added during the countdown. Lots of rockets use solid rockets, e.g. the shuttle. Most ballistic missiles are entirely solid motors, specifically because by the time they fueled the nuclear war would be over. Pegasus is also entirely solid (see #2 below regarding HAPS).
Rockets with storable propellants are loaded earlier than final countdown. Typically during launch integration or even by the propulsion system vendor. Examples of storable propellant systems:
- Hydrazine for monopropellant thrusters like the Pegasus HAPS module)
- Xenon Hall thrusters, where the non-cryogenic propellant is stored in pressure vessels. Not suitable for launch of course because they are low thrust, but they are used on satellites that sit on rockets.
Cryogenics (liquid hydrogen, liquid oxygen) are loaded during final countdown because they boil-off and thermally soak the structure causing potentially nasty things like icing and getting components below their operating temperatures.
For LOX/RP rockets that use a mix of storable (RP) and cryogenic (LOX) propellants, the RP is often loaded well before launch. I know for example, that the RP on an Atlas V is often loaded during the dry-run for the roll-out to the pad, and once loaded stays on the rocket. It's only the LOX that's loaded during the final countdown.