Why does rockets have stages

The earliest multi-stage rocket designs used serial staging — Wernher von Braun and his team in Germany during the Second World War considered adding an upper stage to the A-4 to create the larger A-9 and A that would be able to cover a much larger distance. A more familiar example of serial staging is my favourite rocket, the Saturn V. With each stage that fell away, retrorockets on the spent stage and ullage motors on the next stage helped put distance between the structures so staging would be clean and safe.

The Titan II that launched the Gemini mission did things a little differently. Also a serial staging rocket, it used hot staging. A closer look at the rocket reveals holes in the body! The holes were for the exhaust of the second stage burning. The space shuttle, we know, had boosters on the side of its main tank and not stages that sat one on top of the other.

The shuttle used a kind of staging called parallel staging. Parallel staging has small booster stages strapped to a central sustainer. All the engines ignite at launch and the strapped-on boosters fall away when their propellant is spent. The sustainer keeps burning to put the payload into orbit.

With the shuttle, solid rocket boosters are the stages that fall away from the main sustainer, the external tank that fed the main engines. Some rockets use a mix of these two kinds of staging, like the Titan III. If everything goes well, the first prototype might fly within a few years, so stay tuned to this story.

Another idea which has had some testing is the idea of a nuclear rocket. Unlike a chemical rocket, which burns fuel, and blasts it out the back for thrust, a nuclear rocket would carry a reactor on board. It would heat up some kind of working fuel, like liquid hydrogen, and then blast it out the back for propulsion. NASA did some tests a few decades ago with a nuclear thermal rocket called NERVA, and found that they could sustain high levels of thrust for very long periods of time.

Their final prototype, provided continuous thrust for over 2 hours, including 28 minutes at full power. NASA calculated that a nuclear-powered rocket would be roughly twice as efficient as a traditional chemical rocket. It would have a specific impulse of more than seconds. But flying a nuclear rocket into space comes with a significant downside. Rockets explode.

But earlier this year, researchers at Harvard finally created some in the lab. They used a tiny vice to squeeze hydrogen atoms with more force than the pressures at the center of the Earth. It took an enormous amount of energy to squeeze hydrogen together that tightly, but in theory, once crafted, it should be relatively stable.

When you ignite it, you get that energy back. If used as a rocket fuel, it would provide a specific impulse of seconds. Compare that to the mere from chemical rockets.

A rocket powered by metallic hydrogen would easily get to orbit with a single stage, and travel efficiently to other planets. Single Stage to Orbit rockets would be awesome. Science fiction has foretold it. That said, at the end of the day, whatever gets the most amount of payload into orbit for the lowest price is the most interesting rocket system. The engine mostly just ignites and controls the combustion, while the fuel tank needs to be huge.

Long I'll analyse the very optimal case, in which the energy spent is the minimum energy needed to get your rocket in orbit. Real cases go waaay above this and it's not even practical to do energy balance. In order to launch a single stage rocket, there's a specific size that minimizes the needed energy. So, if the rocket is too small, the little amount of fuel you can fit into it does not contain enough energy to lift the hull.

If the rocket is too big, then you're just wasting energy because it's unnecessarily heavy. That means you have an optimal size which minimizes the energy of a single-stage rocket. Once you find it, the amount of fuel associated to that size is the very minimum you'll need to get your rocket on orbit. Now consider a rocket of that same size, but with 2 stages. Launch weight would be lower if you had a fixed fuel load, and only one engine and fuel tank.

However the specific impulse applied to the payload would be lower. So the trick is to try to reduce the deadweight structural mass as the fuel is consumed. Another compromise system is to have jetisonable external fuel tanks, like the shuttle, which are thrown away once their fuel is consumed. The mass of most of the tank and structure is now overkill and waste. It would be nice to be able to jettison that extra mass so that the fuel left can accelerate only the payload. That's what a multi-stage rocket does.

It jettisons the mass of initial stages so that the remaining fuel and thrust can accelerate much smaller mass to a much higher velocity than it would have been able to if there was only one stage. Another benefit is that you can use rocket motors that are tuned for different velocities.

In the initial stage you need maximum thrust and the rocket is not moving as fast. In the later stages you want high efficiency motors, not necessarily high thrust. To get very high velocities it requires less overall fuel and mass with multiple stages.

The reason rocketeers stage models is to enable the uppermost stage to attain a very high altitude. This is accomplished by dropping mass throughout the burn so the top stage can be very light and coast a long way upward. There are two methods of staging rocket motor. They describe the way the upper stage s are ignited, and will be described in this article. The easiest method is called "direct" staging, where the lower stage motor ignites the upper motor. Most of this article will describe "direct" staging.

The second method is called indirect staging. In this method, the upper stage motor is ignited by some other device not the lower stage. The device used to ignite the motor is a separate ignition system that is carried on the rocket. This method was discussed in Apogee e-zine newsletter 91 Oct 28, Indirect staging is used on rockets that are larger than a D engine; because the lack of large special booster engines required for the direct-staging technique.

In direct staging, the lower "booster" stage motor ignites the top motor in the rocket. From the modeler's perspective, direct staging is simple and cheap. You don't need any complicated electronics or launch support equipment.

The reason it works is explained by the physical make-up of the rocket motors. The drawing in Figure 2 shows a cut-a-way drawing of a typical black-powder propellant rocket engine. The typical rocket engine has a special slow-burning propellant that burns after the propellant is consumed. This is called the "delay grain. The delay also spews out a lot of smoke while it burns, which makes the rocket easier to track as it ascends into the air.

In the special "Booster" stage motor, there is no delay grain, nor ejection charge. It only contains the fast-burning propellant. When the propellant burns upward toward the top of the motor, it throws a lot of heat and burning particles forward as it finishes its burn see Figure 3.

The hot gasses and the burning particles go forward into the nozzle of the upper stage. There is so much heat that comes out of the booster motor, that the top stage instantly ignites see Figure 4. It doesn't need a separate starter because the heat from the lower motor supplies the energy to get it to start burning.

Really, the black-powder propellant "booster" motor is exactly the same as an upper stage motor. The only difference is that the booster motor does not contain the special delay composition. If you look into the front end of a booster motor, you'll see the hard black surface of the propellant.

In comparison, a regular rocket motor that has a delay will have a top end that is capped with grayish looking clay see Figure 2. By not having a delay element incorporated in the booster motor, we get ignition of the top stage nearly instantaneously after all the propellant in the booster has burned out.

There is a good safety reason for this. If there were a delay between burnout of the booster, and ignition of the upper stage, the model could arc over. So it might not be pointed vertically when it ignites. This is a serious safety hazard and should be avoided. To start, the two motors in the rocket have to use "Black Powder" propellant.

Because black powder motors burn linear; from the nozzle end toward the front end. This is important, particularly for the special booster motor. The propellant itself becomes a bulkhead; which is needed to hold the pressure inside the rocket engine. Without internal pressure, thrust wouldn't be created. In a booster stage motor, the propellant that hasn't burned yet becomes the bulkhead that holds the pressure inside the motor.

As it burns, the bulkhead becomes thinner and thinner. When the flame nears the top, bulkhead becomes so thin that it can't hold the pressure and the bulkhead bursts. This is what throws the hot gasses and the burning chunks of propellant forward see Figure 3. In a composite propellant motor, the actual propellant is soft and rubbery. It can't hold back any internal pressure. That means it can't be used as its own bulkhead like the rock-hard black powder propellant.

Composite propellant motors always need to have a solid bulkhead made from another material to hold the internal pressure of the motor. This solid bulkhead prevents composite propellant motors from being able to be used as "direct" staging booster motors. Another problem associated with composite propellant motors is that they require high pressure to sustain the burning process.

You've probably seen a CATO of a composite motor that ruptures right at ignition. When this happens, the motor snuffs out.

The remaining propellant doesn't burn. It just falls to the ground as a chunk of rubber. What this means is that even if the propellant could act like a structural bulkhead, as soon as the flame reached the front end and broke through; it would immediately snuff out.

The likelihood of hot gasses and burning chunks being thrown forward out the top is greatly reduced. The reason the top stage must have a black powder propellant rocket engine for direct staging to work is because the flammable substance inside the upper motor must be near the nozzle. The gasses and the burning chunks eject forward from the booster motor have to come into contact with the propellant of the top stage.

In a black powder motor, the propellant is right inside the nozzle opening. Compare this to a composite propellant motor. Here, the propellant has a hole right through the middle -- from the nozzle all the way to the forward bulkhead. So there is less of a likelihood that the gasses are going to make it up into the motor. You'd think it would be possible to ignite because the gasses are so hot. But it doesn't. An example is easily illustrated by blowing through a straw.

If the straw is open all the way through, you can easily blow air through it.