question: How do multistage rockets work? And how do the rockets not just immediately fall apart upon launch?
comment: A multistage rocket is, in ELI5 terms, a bunch of small rockets stacked up, instead of one big rocket. This is more efficient because as each rocket burns out it can be discarded, which means less weight for the next stage to push. They stay together on launch because they are firmly attached to each other, and because the forces of the launch actually are pushing them together, rather than apart. When a stage is discarded it is jettisoned with explosive charges that rather violently disconnect it from the rest of the rocket stack.
passages: Multistage rocket
  
  A multistage rocket, or step rocket, is a launch vehicle that uses two or more rocket "stages", each of which contains its own engines and propellant. A "tandem" or "serial" stage is mounted on top of another stage; a "parallel" stage is attached alongside another stage. The result is effectively two or more rockets stacked on top of or attached next to each other. Two-stage rockets are quite common, but rockets with as many as five separate stages have been successfully launched.
   Section::::Separation events.
  
  Separation of each portion of a multistage rocket introduces additional risk into the success of the launch mission. Reducing the number of separation events results in a reduction in complexity. 
  
  Separation events occur when stages or strap-on boosters separate after use, when the payload fairing separates prior to orbital insertion, or when used, a launch escape system which separates after the early phase of a launch. Pyrotechnic fasteners or pneumatic systems are typically used to separate rocket stages.
  
  Section::::See also.
  
  BULLET::::- Multistage rocket
  
  BULLET::::- Three-stage-to-orbit
  
  BULLET::::- Two-stage-to-orbit
  
  BULLET::::- Single-stage-to-orbit
  
  BULLET::::- Adapter
  
  BULLET::::- Reusable launch system
   Section::::History and development.
   Section::::Assembly.
  
  Each individual stage is generally assembled at its manufacturing site and shipped to the launch site; the term "vehicle assembly" refers to the mating of all rocket stage(s) and the spacecraft payload into a single assembly known as a space vehicle. Single-stage vehicles (suborbital), and multistage vehicles on the smaller end of the size range, can usually be assembled directly on the launch pad by lifting the stage(s) and spacecraft vertically in place by means of a crane.
   Section::::Upper stages.
   Section::::Optimal staging and restricted staging.:Restricted.
   Section::::Tandem vs parallel staging design.
   After comparing the three equations for the dimensionless quantities, it is easy to see that they are not independent of each other, and in fact, the initial to final mass ratio can be rewritten in terms of structural ratio and payload ratio:
  
  These performance ratios can also be used as references for how efficient a rocket system will be when performing optimizations and comparing varying configurations for a mission.
  
  Section::::Component selection and sizing.
   Refinements in rocket design were made over the next few hundred years, at least on paper. In 1591 a Belgian, Jean Beavie, described and sketched the important idea of multistage rockets. Multistaging, placing two or more pockets of fuel in line and firing them in step fashion, is the practical answer to the problem of escaping earth's gravitational attraction.
   In serial or tandem staging schemes, the first stage is at the bottom and is usually the largest, the second stage and subsequent upper stages are above it, usually decreasing in size. In parallel staging schemes solid or liquid rocket boosters are used to assist with launch. These are sometimes referred to as "stage 0". In the typical case, the first-stage and booster engines fire to propel the entire rocket upwards. When the boosters run out of fuel, they are detached from the rest of the rocket (usually with some kind of small explosive charge) and fall away. The first stage then burns to completion and falls off. This leaves a smaller rocket, with the second stage on the bottom, which then fires. Known in rocketry circles as staging, this process is repeated until the desired final velocity is achieved. In some cases with serial staging, the upper stage ignites "before" the separation—the interstage ring is designed with this in mind, and the thrust is used to help positively separate the two vehicles.
   A multistage rocket is required to reach orbital speed. Single-stage-to-orbit designs are sought, but have not yet been demonstrated.
  
  Section::::Performance.
  
  The reason multi-stage rockets are required is the limitation the laws of physics place on the maximum velocity achievable by a rocket of given fueled-to-dry mass ratio. This relation is given by the classical rocket equation:
  
  where:
   Restricted rocket staging is based on the simplified assumption that each of the stages of the rocket system have the same specific impulse, structural ratio, and payload ratio, the only difference being the total mass of each increasing stage is less than that of the previous stage. Although this assumption may not be the ideal approach to yielding an efficient or optimal system, it greatly simplifies the equations for determining the burnout velocities, burnout times, burnout altitudes, and mass of each stage. This would make for a better approach to a conceptual design in a situation where a basic understanding of the system behavior is preferential to a detailed, accurate design. 
   By jettisoning stages when they run out of propellant, the mass of the remaining rocket is decreased. Each successive stage can also be optimized for its specific operating conditions, such as decreased atmospheric pressure at higher altitudes. This "staging" allows the thrust of the remaining stages to more easily accelerate the rocket to its final speed and height. 
   When comparing one rocket with another, it is impractical to directly compare the rocket's certain trait with the same trait of another because their individual attributes are often not independent of one another. For this reason, dimensionless ratios have been designed to enable a more meaningful comparison between rockets. The first is the initial to final mass ratio, which is the ratio between the rocket stage's full initial mass and the rocket stage's final mass once all of its fuel has been consumed. The equation for this ratio is:
   In contrast, vehicles such as the Russian Soyuz rocket and the SpaceX Falcon 9 are assembled horizontally in a processing hangar, transported horizontally, and then brought upright at the pad.
  
  Section::::Passivation and space debris.
  
  Spent upper stages of launch vehicles are a significant source of space debris for many years after use, and occasionally, large debris fields created from the breakup of a single upper stage while in orbit.
   The delta v required to reach low Earth orbit (or the required velocity of a sufficiently heavy suborbital payload) requires a wet to dry mass ratio larger than can realistically be achieved in a single rocket stage. The multistage rocket overcomes this limit by splitting the delta-v into fractions. As each lower stage drops off and the succeeding stage fires, the rest of the rocket is still traveling near the burnout speed. Each lower stage's dry mass includes the propellant in the upper stages, and each succeeding upper stage has reduced its dry mass by discarding the useless dry mass of the spent lower stages.
   Most quantitative approaches to the design of the rocket system's performance are focused on tandem staging, but the approach can be easily modified to include parallel staging. To begin with, the different stages of the rocket should be clearly defined. Continuing with the previous example, the end of the first stage which is sometimes referred to as 'stage 0', can be defined as when the side boosters separate from the main rocket. From there, the final mass of stage one can be considered the sum of the empty mass of stage one, the mass of stage two (the main rocket and the remaining unburned fuel) and the mass of the payload.
   Another performance metric to keep in mind when designing each rocket stage in a mission is the burn time, which is the amount of time the rocket engine will last before it has exhausted all of its propellant. For most non-final stages, thrust and specific impulse can be assumed constant, which allows the equation for burn time to be written as:
  
  Where formula_12 and formula_13 are the initial and final masses of the rocket stage respectively. In conjunction with the burnout time, the burnout height and velocity are obtained using the same values, and are found by these two equations:
   An alternative to scale is to make the discarded stages practically reusable: this is the goal of the SpaceX reusable launch system development program and its Falcon 9, Falcon Heavy and BFR vehicles. A similar approach is being pursued by Blue Origin.
  
  Section::::See also.
  
  BULLET::::- Aerospike engine
  
  BULLET::::- AVATAR (spacecraft)
  
  BULLET::::- Bristol Spaceplanes
  
  BULLET::::- HOTOL
  
  BULLET::::- Kankoh-maru
  
  BULLET::::- Launch loop
  
  BULLET::::- Mass fraction
  
  BULLET::::- Orbital ring
  
  BULLET::::- Roton
  
  BULLET::::- Scramjet
  
  BULLET::::- Space Elevator
  
  BULLET::::- Spacecraft propulsion
  
  BULLET::::- Two-stage-to-orbit
  
  BULLET::::- Three-stage-to-orbit
  
  BULLET::::- VentureStar
  
  BULLET::::- X-30
  
  BULLET::::- X-33
  
  BULLET::::- X-43A
  
  BULLET::::- XS-1
  
  Section::::Further reading.
   Where formula_17 is the empty mass of the stage, formula_18 is the mass of the propellant, and formula_19 is the mass of the payload. 
  
  The second dimensionless performance quantity is the structural ratio, which is the ratio between the empty mass of the stage, and the combined empty mass and propellant mass as shown in this equation:
  
  The last major dimensionless performance quantity is the payload ratio, which is the ratio between the payload mass and the combined mass of the empty rocket stage and the propellant:
   A further advantage is that each stage can use a different type of rocket engine, each tuned for its particular operating conditions. Thus the lower-stage engines are designed for use at atmospheric pressure, while the upper stages can use engines suited to near vacuum conditions. Lower stages tend to require more structure than upper as they need to bear their own weight plus that of the stages above them. Optimizing the structure of each stage decreases the weight of the total vehicle and provides further advantage.
   High-altitude and space-bound upper stages are designed to operate with little or no atmospheric pressure. This allows the use of lower pressure combustion chambers and engine nozzles with optimal vacuum expansion ratios. Some upper stages, especially those using hypergolic propellants like Delta-K or Ariane 5 ES second stage, are pressure fed, which eliminates the need for complex turbopumps. Other upper stages, such as the Centaur or DCSS, use liquid hydrogen expander cycle engines, or gas generator cycle engines like the Ariane 5 ECA's HM7B or the S-IVB's J-2. These stages are usually tasked with completing orbital injection and accelerating payloads into higher energy orbits such as GTO or to escape velocity. Upper stages, such as Fregat, used primarily to bring payloads from low Earth orbit to GTO or beyond are sometimes referred to as space tugs.
   The attitude control rocket motors for satellites and space probes are often very small, 25mm (1 inch) or so in diameter, and mounted in groups that point in four directions (within a plane).
  
  The rocket is fired when the computer sends direct current through a small electromagnet that opens the poppet valve. The firing is often very brief, a few milliseconds, and — if operated in air — would sound like a pebble thrown against a metal trash can; if on for long, it would make a piercing hiss.
   The earliest experiments with multistage rockets in Europe were made in 1551 by Austrian Conrad Haas (1509–1576), the arsenal master of the town of Hermannstadt, Transylvania (now Sibiu/Hermannstadt, Romania). This concept was developed independently by at least four individuals:
  
  BULLET::::- Kazimieras Simonavičius of the Polish–Lithuanian Commonwealth (1600–1651)
  
  BULLET::::- the Russian Konstantin Tsiolkovsky (1857–1935)
  
  BULLET::::- the American Robert Goddard (1882–1945)
  
  BULLET::::- the German Hermann Oberth (1894–1989), born in Hermannstadt, Transylvania
   After the fall of Srirangapatna, 600 launchers, 700 serviceable rockets, and 9,000 empty rockets were found. Some of the rockets had pierced cylinders to allow them to act like incendiaries, while some had iron points or steel blades bound to the bamboo. These blades caused the rockets to become very unstable towards the end of their flight, causing the blades to spin around like flying scythes, cutting down all in their path.
  
  Section::::William Congreve.
  
presuppositions: Due to the way they are constructed, multistage rocks should not be able to sustain liftoff.  Due to the way they are built, multistage rockets should not be able to sustain flight.  Due to their construction, multistage rockets should be incapable of initiating and maintaining liftoff successfully.
corrections: Multistage rockets are firmly attached to each other, and the forces of the launch are pushing them together. Multistage rockets are firmly attached to each other, and the forces of the launch are pushing them together. Multistage rockets are firmly attached to each other, and the forces of the launch are pushing them together.
raw_presuppositions: Due to the way they are constructed, multistage rocks should not be able to sustain liftoff.  Due to the way they are built, multistage rockets should not be able to sustain flight.  Due to their construction, multistage rockets should be incapable of initiating and maintaining liftoff successfully. Multistage r0ckets need to be flimsily attached to each other.
raw_corrections: Multistage rockets are firmly attached to each other, and the forces of the launch are pushing them together. Multistage rockets are firmly attached to each other, and the forces of the launch are pushing them together. Multistage rockets are firmly attached to each other, and the forces of the launch are pushing them together. Multistage rockets are firmly attached to each other, and the forces of the launch are pushing them together.