The How and Why of Rockets’ Staging

By Amy Shira Teitel | February 17, 2018 9:57 am
Apollo 11 staging; that's the first stage falling away before the second stage lit. NASA.

Apollo 11 staging; that’s the first stage falling away before the second stage lit. NASA.

When we talk about spaceflight — modern or vintage, manned or unmanned, orbital or deep space — launch vehicles all serve the same purpose: overcome gravity and get the payload off the Earth. Whatever the mission, it starts with a rocket launch. Even, because I can hear you asking about it, payloads that were launched from the payload bay of the space shuttle; that payload got to orbit via a shuttle launch. And because all rockets harness the same technology, they all share one common element. Whether they’re big or small, they all go through staging. 

Every rocket that has ever put anything into space has been a multistage rocket. Every payload has needed an extra kick of thrust to overcome the weight of the rocket and payload to get it into space. It’s a tricky balancing act. It’s not just the payload that has mass we need to cancel out with thrust against the downward force of gravity. The rocket’s body has mass as well, as does the fuel on board. The payload is typically the smallest portion of mass on a launch. The propellants — the fuel and oxidizer — weigh the most.

This is where staging comes in handy. Because the propellant is heaviest, as it burns the mass of the whole rocket goes down. That means that the rocket structure becomes dead weight, which needs more fuel to keep lifting it, which means the overall mass goes up… it compounds from there. Staging is a way of getting rid of dead weight so the energy of the burning engine is transferred to the payload so it gets into orbit! Getting rid of dead weight is done via staging, the process of discarding spent rocket stages, and there are a few ways this can be done.

Serial Staging

Serial staging uses stages attached one on top of another. The first stage ignites at launch and burns through its fuel until its propellants are gone. Now spent, the useless first stage falls and the second stage takes over. This is a staging manoeuver. Depending on the rocket, the second stage could get the payload into orbit or a third or fourth stage will ultimately be the one to deliver it to space. It depends on the individual rocket and mission.

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-10 that would be able to cover a much larger distance. A more familiar example of serial staging is my favourite rocket, the Saturn V. It had three stages — the S-IC (first), the S-II (second), and the S-IVB (third) — that were separated by explosive bolts. 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! That’s because the second stage fired before the first stage fell away. The holes were for the exhaust of the second stage burning.

Parallel Staging

There’s another way rockets can stage. 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. This rocket used a two-stage Titan II as the sustainer and added two solid rocket stages as boosters that fell away once they were done, much like the SRBs on the shuttle.

Stage-and-a-half

Then there’s another kind of staging that’s weird and pretty uncommon called stage-and-a-half, but we’ve all seen it in action. The Atlas D that launched John Glenn in 1962 as well as the other three Mercury astronaut who followed in his orbiting footprints used this kind of staging.

Stage-and-a-half has a main core that acts like a sustainer stage and a booster stage that falls away during the flight. In the case of the Atlas D, the booster isn’t on either side of the sustainer, it’s sort of like a skirt that sits around the sustainer engine.

This odd setup was born of mid-century necessity. Engineers had already launched multistage rockets, but there were issues with the upper stages not firing on time and an unfortunate propensity for those rockets to blow up — it was the early 1960s and everyone in the space game was still figuring it out. To make sure the engines all ignited properly, it made sense to Atlas designers to have all engines ignite while the rocket was still on the launch pad. Dropping the booster that was also sort of part of the main stage was how it dropped dead weight in flight, making the rocket light enough to put a Mercury capsule into orbit.

Single Stage to Orbit

Of course, all these methods require the stages fire on time and detach cleanly, and with so many moving parts it opens the door for compilations. The unicorn of rockets is the single stage to orbit, a simpler technology to do the same job of putting payloads into space. One company taking on this challenge is the ARCA Space Corporation whose Haas 2CA rocket might make the first SSTO flight as early as the end of 2018. Until this becomes reliable, we’ll keep having various staging manoeuvers as our paylods leave the Earth.

 

Sources: MA-6 Mission Report; Universe Today; NASA; Historic Spacecraft; ESA; Russian Space Web.

 

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  • OWilson

    Excellent article, thanks!

    I’m looking forward to a future article on how we safely got all those 14 astronauts launched safely off the Moon, without the massive launch pad infrastructure we need here on Earth!

    • Andrew Worth

      14?

      • OWilson

        How many was it?

        Pleae don’t keep us in suspense! :)

        • Andrew Worth

          Wasn’t it Apollo’s 11, 12, 14, 15, 16 & 17 with two landing crew on each?

          • OWilson

            So it was an even dozen?

            Thanks for the correction!

          • http://www.mazepath.com/uncleal/EquivPrinFail.pdf Uncle Al

            Add one crew fried on the pad for being in a pure oxygen atmosphere at 760 torr plus spark. A pure oxygen atmosphere at 150 torr is no smarter. Absent inert fill, oxygen is absorbed and alveoli collapse. Breathing is hard work. Bootstrapping breathing is suffocating.

  • http://www.mazepath.com/uncleal/EquivPrinFail.pdf Uncle Al

    We thus doubt Elon Musk’s BFR as formulated will efficiently escape Earth’s deep gravitational well. Good engineering exploits all tricks. Shallow gravitational wells – moon, Mars – are more forgiving.

    Consider moon surface to Mars. BFR loaded and fueled, launched with boosters. Discard boosters, light the candle. More than enough fuel remains to tail-land on Mars…though staging on Phobos or Deimos base is a better fallback. Don’t haul a whale when a shark will do

  • http://www.mazepath.com/uncleal/EquivPrinFail.pdf Uncle Al

    I have been intellectually inappropriately touched.Because the propellant is heaviest, as it burns the mass of the whole rocket goes down. That means that the rocket structure becomes dead weight, which needs more fuel to keep lifting it, which means the overall mass goes up

    Cheese and rice, lady! The Starflight Handbook, Eugene Mallove and Gregory Matloff, 1989 (30 years old), p. 41, “The Staging Principle.” Read the whole of Chapter 3.

    • Andrew Worth

      Wow, we agree on something, I guess the author meant the dry mass fraction goes up.

      • http://www.mazepath.com/uncleal/EquivPrinFail.pdf Uncle Al

        The statement is unsupportable. It straddles the line between broad ignorance and frank stupidity.

  • ObeyMyBrain

    Looking at the Skylon wiki page it sounds like it would be single stage to LEO but use an upper stage to get payload to GTO

  • Maximilian Derzweite

    While getting rid of the empty tank is an important part of staging, another and potentially more important reason is to switch out the engine for one that is best suitable for the situation the spacecraft is in:
    at Liftoff you need high thrust atmosphere optimized engines, but efficiency is not as important; later in flight you want to use higly efficient engines optimized for vacuum flight to get the required speed to stay in orbit, but the thrust can be way lower since the first stage already bought you the time in space.

    • http://www.mazepath.com/uncleal/EquivPrinFail.pdf Uncle Al

      A de Laval nozzle’s skirt depends on ambient pressure if you want maximum efficiency. They’ve tried two-part sliding skirts going from atmosphere to vacuum. One obtains another Bell Boeing V-22 Osprey.

  • Andrew Worth

    Every rocket that has ever put anything into space has been a multistage rocket. Every payload has needed an extra kick of thrust to overcome the weight of the rocket and payload to get it into space.

    No, many single stage rockets can get to space, that only requires a rocket be capable of a velocity of at least 1.5 km/s (the Kármán line, at an altitude of 100 km (62 mi) above sea level, is conventionally used as the start of outer space) Redstone, which put Alan Shepard into space on a suborbital hop was single-stage, the challenge is getting into orbit which requires a rocket achieve over 8 km/s.

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