The Rocket Equation’s never-ending tyranny


A Soyuz spacecraft docking with the International Space Station. As the picture makes plain, typical human-occupied spacecraft orbits are very close to Earth; SpaceShipTwo wouldn’t get even this high. NASA photo.

So the latest Scaled Composites/Virgin Galactic SpaceShipTwo test flight has ended badly, killing one of its two test pilots.

Meanwhile Orbital Sciences, another private space firm, suffered an expensive accident this week as well when one of its (unpeopled) Antares cargo rockets blew up seconds into an ISS resupply mission. Of the three main private human space travel players, only Elon Musk’s SpaceX has survived the week unscathed.

These are testing times for human space travel. When Scaled Composites’ SpaceShipOne won the Ansari X Prize for private spaceflight a decade ago, it gave the field a huge shot in the arm. There was a feeling that private enterprise might triumph where government investment was faltering.

Scaled Composites/Virgin Galactic is a notable pioneer, the first organisation ever to send people to space with a hybrid engine. SpaceShipOne literally burnt rubber, while SpaceShipTwo had recently switched to a new design burning, essentially, nylon. Hybrids don’t pack quite as much energy into each gram of fuel as the traditional rockets that have powered the Soyuz (kerosene-oxygen) and Saturn (kerosene-oxygen/hydrogen-oxygen) launch vehicles. However, hybrids are simpler, which seems like a useful advantage.

But hybrids may just not work at scale. Virgin Galactic was once scheduled to start taking passengers up as early as 2010, and there was talk of doing it by this year, and last week the schedule said 2015, and that’s not going to happen now either. (And in a sense they were not going to space anyway: their sub-orbital flights have recently been planned to fall well short of the Kármán line, 100 kilometres up, which is generally treated as the start of “space”.) With the only existing SpaceShipTwo craft destroyed, the project may chose to substantially change their design. Or Virgin may walk away from what remains a risky and expensive exercise.

All this should remind us that space travel is hard. The route that Scaled Composites has been taking is particularly difficult. No-one really knows for sure whether hybrids can be made to work. There’s an interesting argument that Scaled Composites’ high point came when it won the X Prize. But all that is detail. Space travel is hard.

Space travel is hard because of the stresses involved, and because of physics and chemistry – especially the physics of what’s called “the rocket equation“. The equation basically specifies all your constraints: gravity is what it is, and fuel weighs a lot, and you can’t make it much lighter, and you need to take quite a lot of it almost up to low-Earth orbit, so there’s not a lot of room for anything else. Plus you have to slow down from orbital speeds to land by using atmospheric compression (often described as “friction”), which is pretty damned scary.

In a sense we’re lucky that we can get to space at all. The rocket equation tells you that if the Earth’s radius were just a little bit bigger  – say, 10,000 kilometres – then no rocket that could be built would get us to space. Instead, the Earth’s radius is 6700 kilometres and we can get to space – but only just, with tiny payloads. It’s frustrating, but it’s inescapable with our current technologies. The astronaut Don Pettit has described it, accurately, as “The Tyranny of the Rocket Equation“.

If you are old enough to remember the space program of the early Shuttle days, you probably assumed that space travel would get easier and easier. That turns out to be what might be called the Continuous Progress Error, common in futurology: we’d got from Kitty Hawk to Vostok 1 in just 58 years, and to the moon in just eight more, so space-travel-as-normal and trips through the solar system were just a matter of time. Hence Kubrick and Clarke’s 1968 vision of 2001, with Pan Am flights to orbit and missions to Jupiter (or Saturn, if you were reading Clarke’s book). The tyranny of the rocket equation explains why that future isn’t happening now or in the near future. Progress only happens until it runs into fundamental resource constraints. In the same way, people in the 1960s and 1970s wrongly expected Concorde to usher in an era of commonplace supersonic passenger flight.

And so it’s now more than four decades since we sent anyone outside near-Earth orbit. Solar system exploration has adopted a new paradigm, the remarkable little remote probes that are now busily exploring the Saturnian system, and preparing to land on comet 67P/Churyumov-Gerasimenko, and digging little holes in the surface of Mars, and that will next year give us our first good look at the ex-planet Pluto.


Rosetta in orbit around 67P/Churyumov-Gerasimenko – cool, if not the space exploration future we expected.

Getting people to space is not getting easier very fast, and it will probably stay hard for some time.

Space travel actually has three ugly problems: it’s hard to get to, the radiation and micrometeorites are often a bastard, and there’s nothing to do there anyway, because space is really big and pretty much empty, apart from the aforementioned radiation and micrometeorites. That’s why SpaceShipTwo ended up being a millionaires’ tourism venture; tourism is one of the few ways space might pay for itself.

Space travel’s three ugly problems are not well understood, partly due to the influence of sci-fi movies where all three are presumed to have been fixed or, often, not to even exist. If you want to understand the three, though, it’s not that hard. For the getting-there problem, in particular, I recommend Don Pettit’s little rocket equation essay or Piers Bizony’s book How To Build Your Own Spaceship, not actually a how-to volume despite its title, but a wonderful treatise on the details of beating the rocket equation, full of tales of physics, chemistry, engineering, politics and economics. Coping with the rocket equation is such a tough challenge that the other two ugly problems rarely come into focus.

Bizony opens his book by quoting the singer Billy Bragg joking that when his parents told him “soon man will be on the moon”, he didn’t realise they meant just one man. But that’s the way space travel looks for the moment: not many people, going not very far, and the whole enterprise advancing not very fast.

About David Walker

David Walker runs editorial consultancy Shorewalker DMS (, editing and advising business and government on reports and other editorial content. David has previously edited Acuity magazine and the award-winning INTHEBLACK business magazine, been chief operating officer of online publisher WorkDay Media, held policy and communications roles at the Committee for Economic Development of Australia and the Business Council of Australia and run the website for online finance start-up eChoice. He has qualifications in law and corporate finance. He has written on economics, business and public policy from Melbourne, Adelaide and the Canberra Press Gallery.
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7 years ago

So unless there is a major breakthrough very soon, the chances of us moving on to another planet after we’ve stuffed this one are looking very remote. A sobering thought.
Thanks for the insights David.

7 years ago
Reply to  David Walker

So really, bet on staying here on the surface of Earth, and act accordingly.

A very informative post David. With a slightly disturbing summary considering how we’re treating the place.

steve from brisbane
7 years ago

Apart from the rocket engine dangers, am I the only person who thought the “feathering wing” design of these spaceplanes always looked like an accident waiting to happen? I expected that it would be the cause of the first crash, but now, given a large amount of bad publicity coming out about previous warnings of safety issues being ignored, I wouldn’t be surprised if the project dies completely….

Steve from Brisbane
7 years ago


Virgin Galactic’s space plane broke apart in mid-air seconds after its re-entry system deployed prematurely in an accident on Friday that killed one of its pilots and left another seriously injured, US crash investigators have said.

Not a bad bit of prescience from a blogger from Brisbane, no?

7 years ago

As far as I can tell, the 4th problem is that putting people into space is a strange fantasy of a limited number of groups, notably dictators, nationalists, baby-boomers still thinking how wonderful the moon landing was, and perhaps a few others. A lot of the rest of the world is happy to live with robots and so on which are vastly cheaper and thus can get more scientifically interesting things done for the same amount (I seem to remember this includes the main physics organization in the US).

Tim Macknay
Tim Macknay
7 years ago

So unless there is a major breakthrough very soon, the chances of us moving on to another planet after we’ve stuffed this one are looking very remote. A sobering thought.

I suspect it will take a lot more than just one breaktrhough.

With present technology, actually living in any extraterrestrial location we can conceivably reach would be akin to life imprisonment inside a nuclear submarine, or at best, perhaps an antarctic base. I ain’t rushing out to buy a ticket.

7 years ago
Reply to  Tim Macknay

I don’t think it’ll ever be “us” in the individual sense. It’ll be “us” in the humanity sense. A lucky few will get to go and start life anew and the rest will be left behind to rot.

Tim Macknay
Tim Macknay
7 years ago
Reply to  desipis

I’d assumed that was self evident. My comment about not wanting a ticket was rhetorical. I don’t expect that anyone, “lucky” or otherwise, is likely to live permanently in space in my lifetime. People will probably continue to visit space intermittently, as they do at present, but that will be it. Barring multiple major breakthroughs, of course.