The hard working but unloved multi-stage rocket

Cutaway drawings showing three multi-stage rockets

Cutaway drawings showing three multi-stage rockets (Photo credit: Wikipedia)

If you’ve ever read about rocket technology, particularly the issues involved in how much fuel is needed to get somewhere, you quickly run into a stark reality.  The payload, the part of the rocket that you want to get somewhere, is inevitably a tiny portion of the size and weight of the rocket.

The rest is made up of the rocket engine, the fuel, and the fuel tank or tanks.  The further and faster that you want the rocket to go, the bigger the rocket engine needs to be and the more fuel you need.  Not only that, additional fuel and bigger engines adds more mass, which in turn requires even more fuel and even bigger engines.

To illustrate what I’m talking about, take a look at this diagram (courtesy of Wikipedia) showing the V2 rocket: one of the earliest military rockets.

500px-V-2_rocket_diagram_(with_English_labels).svg

Most of the V2 is fuel and rocket engine.  The actual payload, the warhead, is a tiny component of the structure.  And it’s worth noting that the V2 generally wasn’t even a suborbital rocket.  It had just enough range to fly from France to Britain.  As soon as we try to design rockets to get into orbit, or go even further, to get a usable payload anywhere, they become gigantic.

English: The description of Saturn V second st...

English: The description of Saturn V second stage (S-II) (Photo credit: Wikipedia)

The designers of missile weapons systems in the late 40s and early 50s came up with an idea.  Why carry all of the weight of the rocket around for the whole trip?  If you jettisoned parts of the rocket, you saved the weight of their fuel tanks.  And if the smaller remaining mass could be propelled by smaller engines, you could dump the large engines you needed at takeoff.  This innovation resulted in rockets that could be much smaller.

It was a brilliant development.  The multi-stage rocket was born.  It opened up the development of intercontinental ballistic missiles (admittedly a mixed blessing).  Everyone was pretty happy with it for missile technology since missiles are essentially throwaway weapons.

However, everyone, left, right, and center, seems to hate it for spacecraft.

Why exactly is this?  I know that I myself have always had the feeling that the multi-stage system was somehow unseemly, wasteful, temporary.  It gave me the feeling that we were only barely getting it done, that the technology was working, but fragile and primitive.

I don’t think I’m unique in this.  Science fiction largely ignores the multi-stage system.  Even the hardest science fiction novels almost always have the spacecraft as a self contained system.  Even when the ship is a generational ship, taking decades or centuries to get to its destination, there is almost never any mention of disposable stages.  Even in scenarios where they clearly would be a benefit.

And of course the space industry has been on a quest for decades to find a single stage to orbit system.  Most have given up on this, but I suspect they’re not happy about it.  We all seem to want to make the multi-stage system go away.

Propellants, Explosives and Rocket Motor Estab...

(Photo credit: Wikipedia)

We have excuses of course.  The multi-stage system is expensive.  A reusable single stage system would be more efficient.  Of course, every one that anyone has tried to design so far has been anything but that, but we try anyway.  What was the US Space Shuttle but an attempt to get as close to a single stage system as we could?  Never mind that the Shuttle was actually more expensive to operate than the multi-stage capsule systems.

Another excuse is that the multi-stage system leaves junk lying around, in oceans and in near Earth orbit.  There’s definitely something to this, although it doesn’t really have to be so.  It’s just the way we’ve always done it before.  Stages could be designed to be retrieved (as the Shuttle boosters were) or finish with just enough unspent fuel to de-orbit and burn up on reentry.

The real issue, I think, is that we want space travel to be like something we’ve done for thousands of years: sea travel.  Traveling on the sea involves ships.  The ships need supplies from land, but keep their shape throughout their travels.  We want space travel to be like that.  Again, think of all the science fiction stories, including Star Trek, where naval ranks are used, and the spacecraft is referred to as a ship.

The problem, of course, is that space travel hasn’t shown itself to be anything like sea travel.  But we seem to have a problem accepting this.  At least the west does.  The east has always seemed to be a little more at peace with going into space on top of a multistage missile.  Indeed, it was the Soviet Union who pioneered this paradigm.

Space Elevator GEO Station

Space Elevator GEO Station (Photo credit: FlyingSinger)

There are other paradigms that scientists and science fiction have investigated, such as beamed propulsion, solar sails, and ion drives, and many of these ideas do hold promise, but not for getting out of a deep gravity well like the Earth’s surface.  Space elevators are an interesting idea that might someday provide an economical way to get into orbit, but the engineering scale and challenges are daunting.

This has led some people to the “big dumb booster” philosophy.  Instead of resisting the multistage system, why not embrace it, build the cheapest brute force stages possible, and focus innovation energy elsewhere?  Maybe the most effective way forward is to find more economical and quicker ways to build these stages.

We might someday land on Mars and later take off using a booster stage that we manufactured while we were there.  The booster might even be manufactured by robots that had been sent on ahead.  The ability to quickly manufacture a booster stage might become the “killer app” technology of the space age.

Even if we’re eventually able to build more efficient space drives that allow us to dispense with staging for interplanetary travel, they might again come into the picture for interstellar exploration.  For a rocket to accelerate to relativistic velocities and decelerate at its destination, even with a 100% efficient E=mc2 drive, the mass to payload ratio is something like 40,000 to 1.

The idea that we wouldn’t use stages in these scenarios isn’t realistic.  For generation ships, if stages could take decades or centuries off of the trip, they would almost certainly be used.

All of this is to say that, in the absence of some magical technology like a warp drive, staging will almost certainly be a feature of future space travel.  Space travel will not be like ocean voyages.  We need to let go of that notion so that it keeps us open to all options.

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13 Responses to The hard working but unloved multi-stage rocket

  1. Ignostic Atheist says:

    The funny thing about a space elevator is that the diameter of the cable would need to be much thicker at the middle than the ends. Makes you wonder how you’ll climb it.

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  2. The idea of launching from an orbital platform appeals to me, since supposedly most of the fuel is used on escaping orbit. This would seem to solve many of the problems with rockets, but raises another problem. How to get the stuff into orbit in the first place?

    I really like the idea of a space elevator, but don’t know enough to say if it’s viable or not.

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    • Brett says:

      It’s not possible to build a space elevator on Earth with any materials we have available. Nanotubes individually come close to the tensile strength necessary for it, but they lose that when bundled together – and you need more than tensile strength for a space elevator. It’s going to be battered heavily by winds and faced with temperature changes acting differently on different segments of it.

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    • I like the idea of a space elevator too, but I’m also cognizant of Brett’s points. We don’t yet have, or know if it’s possible to have, a material that could be compact and light enough to be useful as cabling, but strong enough not to break.

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  3. Brett says:

    I’m sympathetic to the Big Dumb Booster idea myself, although it does feel “wasteful” even when it saves money. Maybe it’s just a matter of changing your perspective on things – nobody really cares that most packaging is only used once, for example. And if the upper stages are in space, then you might be able to use them for other things if you have in-space infrastructure.

    If I recall correctly, though, the actual rockets themselves don’t tend to be the expensive part of a space launch. It’s everything else – the ground operations, etc.

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  4. Nice post, SAP.

    I always imagined really big ships like generational ships etc being built in orbit, which would indeed be essentially a kind of staging on a grand scale.

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    • Thanks DM.

      I think you’re right, those kinds of ships would definitely be assembled in orbit. But I also think there is a good chance they’ll burn through some stages (also assembled in orbit) accelerating to their interstellar cruising speed, and more stages decelerating back down to interplanetary speeds at the destination. (Leaving some spent stages flying through the universe at perhaps tens of thousands of kilometers per second.)

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      • Good point. I hadn’t thought about it like that. It’s not just escaping from earth’s gravity well, it’s the need to accelerate up to very high speeds.

        They’ll still need to carry massive rockets with them for deceleration. I guess that means there could be a second multi-stage deceleration phase at the end of the journey.

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  5. Steve Morris says:

    Interesting article. I think we all share the same frustration. The ship is a metaphor for freedom, and the core principle of a multi-stage rocket is that it’s an irreversible journey,

    If you are planning interstellar travel I think you have to drop the notion of using rockets and fossil fuels to get there. There just isn’t enough energy density. That leads you to some kind of nuclear or antimatter energy source, or something even more exotic. That fuel would be relatively light compared with the mass of the engine, etc. Since you will need the engine in order to decelerate, I’m not sure there’s so much benefit in a multi-stage design.

    An alternative would be a nanoscale ship powered by a laser in Earth orbit or similar.

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    • A metaphor for freedom. That’s a good way of putting it. It also explains why fuel and other supplies rarely come up in space operas.

      I definitely agree that chemical rockets are out for interstellar travel, but given the astronomical energy requirements for getting to another star in anything less than a century, I think even an antimatter rocket would end up needing to be staged.

      On nanoscale ships, I could see the payload of a multistage system being nanoscale. Smaller payloads means smaller and less numerous stages.

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