Putting this week’s space tragedies into perspective. It’s ROCKET science, folks

I started writing this post after the Orbital Sciences rocket explosion earlier this week. Unmanned, nothing hurt except pride and stock price. I was going to begin with a joke about how the sound of a rocket blowing up is spelt.

Buzz Aldrin descends to the lunar surface, 20 July 1969, illuminated by light reflecting from the regolith. Photo:NASA.
Rocket science in action. Buzz Aldrin descends to the lunar surface, 20 July 1969. Michael Collins, in his autobiography, Carrying The Fire, figured the mission had a 50/50 chance. Public domain, NASA.

Then news came of the Virgin Galactic tragedy during test flight on Friday, and it didn’t seem right. Appalling news and certainly not something to joke about. In such moments we have first to think of the family, friends, and colleagues of those affected.

To me both accidents underscore the fact that rocket science is – well, rocket science. It’s gained that repute for a reason. We’ve long fantasised about space-flight becoming as routine as jumping into the car. But the laws of physics – particularly, the way the energy curves rise – tell me that reality is otherwise, especially when we think of going into orbit or beyond. While it’s tempting to put the Antares booster failure down to use of left over Soviet moon rocket motors from the 1960s, the fact remains that all rockets, and especially those required to boost something into orbit or beyond, are high-tech engineering that push the limits of materials physics. And complex systems, inevitably, fail in complex ways.

The problem is the energy curve, which is exponential. On the face of it, rockets are simple – so simple that medieval technology could produce them. Something burns in a combustion chamber, hot gases rush out a nozzle at one end and push the rocket in the other, thanks to the Third Law of our friend Sir Isaac Newton.

The penultimate firework - JATO units, seen here thrusting a B-47 into the skies. Public domain, via Wikipedia.
The anti-penultimate firework – JATO units, seen here thrusting a B-47 into the skies in the mid-1950s. Public domain, via Wikipedia.

Unfortunately it doesn’t scale up well. The Royal Arsenal, at Woolwich, was able to make fireworks into battlefield weapons by the turn of the nineteenth century, the ‘Congreves’ fired at Fort McHenry to produce ‘the rockets’ red glare’ of the US national anthem. But powder rockets were limited by chemistry. Other chemical reactions – liquid oxygen and an oil fraction, for instance – offered more energy, and by the first decades of the twentieth century, engineers were working on liquid fuelled rockets.

The problem engineers hit was mass ratio, the difference between the mass of an empty and fuelled rocket. This is everything in rocketry. The equation is R = (Mpt/Me) + 1, where R is the ratio, M is mass in kg, Mpt is propellant mass, and Me is empty rocket mass. It’s important because of the other rocket equation, Δv = Ve*lnR, where Δv is total change-of-velocity capacity, Ve is exhaust velocity and R is mass ratio. Ln is the natural logarithm of X, the exponent to which the transcendental number e (2.7182818…) has to be raised to equal X, which in this equation is R. See what I mean about rocket science? 

OK, enough geek porn. The point being that the lighter the rocket vs fuel mass, at all times, the better off you are. A rocket with half-empty tanks is lugging wasted mass, and that is a killer when it comes to the energy needed to reach orbit.

Atlas booster with Mercury MA-9 atop. NASA, public domain.
Atlas booster with Mercury MA-9. NASA, public domain.

That’s why Convair’s SM-65 Atlas booster, developed from 1951, was an aluminium balloon that dropped two engines on the way up. It’s why the Saturn V Moon rocket had three stages, renewing that mass-ratio every time it dropped an empty stage.

Achieving this wasn’t easy. In theory, a chemical rocket is simple – a tank of fuel (let’s say kerosene), a tank of oxidiser (let’s say liquid oxygen). Pump both into a combustion chamber, ignite them, and off you go. Actually, problems begin at once. Liquid oxygen (LOX) is super-cold – a light blue liquid that boils at 90.19 degrees Kelvin (-297.3 degrees Fahrenheit, -183 degrees Celsius). You have to pump it into the rocket at the last minute, because it’ll boil off fairly soon even in the best-insulated tank. You have to keep LOX away from its fuel (kerosine or liquid hydrogen, typically) until it’s needed. But sealing joints can be difficult. Early seals used Ulmer leather, which LOX tended to saturate and ignite. Today various exotic compounds are used. Duct tape is NOT among them.

Wait, there’s more. Kerosine and oxygen burns at 3670 degrees K – (3396.8 deg C, 6146.3 deg F). Even titanium melts at 1668 deg C. How do you stop your motor melting? One answer is to make a thin double-wall chamber and nozzle and use your LOX (or liquid hydrogen in a LOX/LH rocket) as a coolant, before sending it into the combustion chamber. Of course, you have to get the rate of flow right to make sure your cryo-liquid provides enough cooling – but relate that to the flows needed to burn properly in the motor.

That takes a ton of geekery. And there’s the fact that being super-cooled on one side and super-hot on the other is a Bad Thing in terms of metallurgy, one of many reasons why big rocket motors have reliable firing times, without maintenance, of minutes.

Apollo 12 lifting off. The SIV stage is the one just clear of the tower. Moments after this photo was taken, spacecraft and tower were hit by lightning. Photo: NASA http://www.hq.nasa.gov/ alsj/a12/ ap12-KSC-69PC-672.jpg
Apollo 12 lifting off. Moments after this photo was taken, spacecraft and tower were hit by lightning. Photo: NASA http://www.hq.nasa.gov/ alsj/a12/ ap12-KSC-69PC-672.jpg

That’s without considering pressures. Combustion chamber pressures in the F-1 motor that launched men to the Moon topped 70 megapascals – 1,015 psi, or around 69 times the pressure of the air you and I happily breathe at sea level. That makes ‘thin-walled’ a moot term with the only answer being – you’ve guessed it – more mass, even if you do something mathematically clever with curves and ribs to increase relative thickness.

The next problem is firing. Asymmetric combustion can cause shock waves strong enough to destroy the motor. Rocketdyne made their huge F-1 burn properly – and launch men to the Moon on the back of 750,000+ kg of thrust per motor – with tests that included igniting black powder charges inside the combustion chamber during engine firings.

Since the mid-twentieth century, developments in chemistry have offered ways of building solid fuel rockets that approach liquid-fuel energies without the mechanical complexity. But once lit, they can’t be stopped. That worried Space Shuttle launch controllers, who envisaged chunks of burning SRB crashing through the windows of the Cape control centre if there was an emergency abort-and-fly-back.

F-1 motor firing on test. Public domain, via Wikipedia.
F-1 motor firing on test. Public domain, via Wikipedia.

Wait, there’s even more to rocketry. All the thrust is at the bottom of the stack – like trying to loft a pencil balanced on your finger. One answer is to add gyroscopes (more mass). What about control vanes in the rocket blast, or wings? The former have to stand up to metal-melting blasts. The latter add drag during the initial launch phase and (naturally) mass.

The current approach is to pivot the motors, though this adds mass – and then, how do you instruct that system? Yup – gadgets that add even more mass.

By any measure, the science demands expensive and exotic materials, expensively machined to miniscule tolerances, because the engineering parameters are completely unforgiving. A near-invisible scrap of loose metal in a valve – even an over-tightened screw with a slight burr over which a wire passes and abrades – might be enough to kill a system. That’s why rocketry is so expensive. It’s why I doubt we’ll get ‘gas-and-go’ car reliability for orbital rocket launchers. To do that, we need a different technology.

For me the amazing thing isn’t that rockets fail. It’s that they don’t. Much. They are incredibly complex machines – and they do stuff that, in an everyday sense, is not ordinarily possible. To me that underscores the tremendous skill and work that goes into any launch. Very, very talented people work like Trojans, doing very, very smart things, to make sure. I salute them. They are tweaking the nose of physics, as we currently understand it. And most times, it works.

See why it’s called ‘rocket science’?

Copyright © Matthew Wright 2014


10 thoughts on “Putting this week’s space tragedies into perspective. It’s ROCKET science, folks

  1. Wow. I knew it was ‘rocket science’ but when you start drilling down into the details you really understand what that means. You’re right, it won’t ever be a buggy ride in the park with the current constraints. That will take a science fiction style leap to something else. And a lot of people willing to put their giant brains and their lives on the line to test the possibilities, as the Apollo program people did. What makes me think it’s possible (apart from too much sf in the formative years) is a tiny detail from the movie, Apollo 13. It was the slide rules. We walk around today with more computing power in our pockets than they had in the whole of nasa. But the did it anyway. They sent those boys to the moon and back with slide rules. We can do anything!

    1. Yes – absolutely. I think it’ll require some lateral thinking and a re-conceptualisation. The approach we have today is the one Von Braun used for the V2. Oh yes, the computing…we forget that back then ‘computers’ were roomfuls of gear that had to be custom programmed, every time – with less grunt than a modern phone, and that for everyday math they used slide rules! I still own a slide rule, incidentally…

      1. I’m pretty sure my husband does too. He’s only a few years older than me but it was those crucial years that went from slide rule for leaving maths to calculator. By the way, he requested me to double like your FB post about the broccoli for trick or treaters. I think you two would get on…

  2. Ahhh. Nothing like a little geek porn the first day of NaNoWriMo! Well, this side of the date line.

    It fascinates me the way too many people take for granted technological marvels. My grandfather was an electrician in rural Georgia who would bring electrical power to farmers who had never seen an electric light before — except maybe on a rare town trip. That was in the 1930s, only a long lifetime ago.

    The truth is that more people are killed in traffic accidents every day here in the States than have been killed in human space-flight related accidents, ever, including all countries so engaged.

    It isn’t that spaceflight is all that safe. It is, as you rightly point out, that some very very smart people work extremely hard to MAKE it work.

    1. Yes – we have to put the scale of the accidents and injuries in due proportion! As I see it, spaceflight is still the ‘get on top of the log and float downriver’ equivalent to shipping, relatively speaking. Every so often, the log rolls over. Eventually someone will invent an outrigger, and so on. My main concern, of course, is the energies required – exponentially higher than anything earth-bound, even high-performance jets don’t really approach some of the temperatures, pressures and forces involved. To me that says that we won’t ever make spaceflight routine and low-risk without some kind of lateral break-through. Certainly in materials science – there is good reason to think carbon might solve a lot of the issues. Also in propulsion. In that sense, I’m wondering about the Skylon project. Pushing the barrow out yet further there’s also the possibility that fusion systems might offer a cheaper or safer way of getting off-planet. Once up there, of course, low-thrust ion drives are probably the answer, but there’s that annoying issue of getting to orbit first…

  3. I did a summer at NASA’s Marshall Space Flight Center, and it was an internship that involved tours of Space X, Boeing, ULA, etc. Ever since then, I’ve kept my eye on the private space game, because what I saw at Space X scared me. Just so much unprofessionalism and people working on zero hours of sleep and trying to get stuff done really quickly and really cheap. Barely skirting the edges of safety to save 90% on cost. A friend of mine went back there two years later and said it had improved greatly but…man, it is a MIRACLE that Space X didn’t have any huge failures before 2012.

    As you dissected, rocket science is difficult in the hands of those who wield it responsibly. It’s near impossible in the hands of those who don’t respect it. I never visited Orbital or Scaled Composites in person, so I’m not sure if they were on the super-clean-super-professional side like Boeing, or on the eating-froyo-next-to-flight-hardware side like Space X. Either way, I hope that the private companies can learn from NASA’s history instead of making their own stupid mistakes.

    That’s not to say that NASA is better – NASA has probably grown TOO risk adverse – and I do believe the private industry will help manned space travel in the long run. But I’m not surprised by any of this. I just thought Space X would be the first.

    1. Interesting! Am very jealous you got to tour the cool stuff! 🙂 I guess there’s a balance. i read an interesting piece by one of the Shuttle former mission controllers the other day about the sheer number of things that can go wrong. I guess it’s a balancing act. And it’s also a key point. NASA is viewed as an expensive over bloated government effort because of that risk averse nature. Which is fine to cut back and instead use the lean private sector model. Until something goes wrong. And this IS rocket science, with all that this implies!

  4. Given the private sector’s stellar safety record with nuclear power plants, I’m sure there is absolutely nothing to worry about.

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