I was watching some videos of russian ICBMs and they have weak thrust and flames which propels them slowly out of the silo, and then a lot of thrust and flame accelerates it into the air, like a second, more powerful motor kicked in. Why is it like that?Do these rockets use solid rocket fuel?
It is not that they have a weak thrust at the beginning, but that something that massive takes time to get moving. The thrust on those rockets are incredibly powerful, but when something weighs a hundreds tons, you can't expect it to take off like a high performance sports car. What you usually see coming out of the silo is the same motor propelling the missile. It is just that when the motor is still in the silo, the thrust is confined to a certain area inside that you cannot see. When it finally exits the silo, it is no longer confined and makes a huge blast as it spreads out all around. It was the same motor, just not confined to a silo anymore. It should be noted though that some of these ICBM's do indeed contain several motors just like the space shuttle. In some cases you would be right, that these missiles have an initial motor to blast them out of the silo and then a main motor to blast them into the atmosphere. In things like subs, the missile does not contain a launching motor, but is surrounded by a pocket of air. When it is launched, the force of the air rising blasts the missile to the surface and once it breaks the water line, the motor kicks in and it is launched. For an example of this, scroll to about 2:40 in the video below and watch. And yes, most modern ICBM's use solid fuel because it is safer and easier to store. One of the problem with liquid fuel is that it needed to be kept cold. It would often boil off due to the surrounding temperature and cause ice to form on everything. This meant that the ICBM's could not be kept fueled and had to be fueled before launch causing a delay in response time.
