Why do we need multistage rockets

[Cookie Monster]

If energy source was Nuclear fuel , we won't need multi stage Rockets the unlimited supply of fuel from Miniature Nuclear power plant would give you enough power to fly the whole rocket intact into outer space

The fuel source we use is outdated 100 year old tech being refined and reapplied

Nuclear fuel and the use of miniature nuclear power plant to power the rocket will have two possibilities...

1) Fission: Many nations use nuclear fission based power plants to generate electricity. Given enough time/effort/money...it could be possible to develop an extremely miniaturized one to fit on a rocket...but there's obviously the major risk of nuclear radiation if something goes wrong with the rocket causing it to explode or fall back to earth. Also a fission based nuclear reactor works by heating up water...this steam then spins a turbine, the spinning motion of the turbine generates electricity. How would this approach contribute to lifting off the rocket? In order to lift off and accelerate the total mass of the rocket, force must be applied in the opposite direction(towards the earth in this case). There would have to be some other mechanism involved in order to use fissile material to generate force.

2) Fusion: this is in the realm of sci fi for now. We don't have a practical working Fusion reactor(that produces a net surplus) even on a large scale let alone miniaturizing it to fit in a rocket. This option would be safe as opposed to the first one in terms of radiation.

...so neither is practical. Using conventional fuel still makes the most sense. Though companies like SpaceX are introducing new techniques like landing the first stage back instead of discarding it(along with the engines) to be destroyed.

Another option can be(if possible) is to use a jet with an engine(or combination of engines) that can perform at subsonic, supersonic, and hypersonic speeds. This way the jet can take off from a runway(with the payload)...reach supersonic then hypersonic speeds...launch cargo...and land back on earth.

Anyways I digress...this is not the subject matter of this thread so I'll stop here.