The hard working but unloved multi-stage rocket

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.

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.

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.

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.

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.

Related articles

• SpaceX Scores in California Launch, Heads Toward Reusable Rockets (businessweek.com)
• Orbital Completes First Commercial Supply Mission to ISS (parabolicarc.com)
• Orbital Signs Launch Agreement with Skybox Imaging (virtual-strategy.com)
• Can Quiet, Efficient ‘Space Elevators’ Really Work? (2eyeswatching.com)
• ‘Space Elevators’ Could Provide Safe & Efficient Cosmic Travel, Researchers Say (mukeshbalani.wordpress.com)
• Getting into Space (thisishowwefixthings.wordpress.com)